854 

14 CFR Ch. I (1–1–24 Edition) 

§ 171.311 

Wind Velocity: The ground equipment shall 

remain within monitor limits with wind 
velocities of up to 70 knots from such di-
rections that the velocity component 
perpendicular to runway centerline does 
not exceed 35 knots. The ground equip-
ment shall withstand winds up to 100 
knots from any direction without dam-
age. 

Hail Stones: 1.25 centimeters (

1

⁄

2

inch) diame-

ter. 

Rain: Provide required coverage with rain 

falling at a rate of 50 millimeters (2 
inches) per hour, through a distance of 9 
kilometers (5 nautical miles) and with 
rain falling at the rate of 25 millimeters 
(1 inch) per hour for the additional 28 kil-
ometers (15 nautical miles). 

Ice Loading: Encased in 1.25 centimeters (

1

⁄

2

 

inch) radial thickness of clear ice. 

Antenna Radome De-Icing: Down to 

¥

6 

°

C (20 

°

F) and wind up to 35 knots. 

(d) The transmitter frequencies of an 

MLS must be in accordance with the 
frequency plan approved by the FAA. 

(e) The DME component listed in 

paragraph (a)(4) of this section must 
comply with the minimum standard 
performance requirements specified in 
subpart G of this part. 

(f) The marker beacon components 

listed in paragraph (b)(4) of this section 
must comply with the minimum stand-
ard performance requirements specified 
in subpart H of this part. 

§ 171.311 Signal format requirements. 

The signals radiated by the MLS 

must conform to the signal format in 
which angle guidance functions and 
data functions are transmitted sequen-
tially on the same C-band frequency. 
Each function is identified by a unique 
digital code which initializes the air-
borne receiver for proper processing. 

The signal format must meet the fol-
lowing minimum requirements: 

(a) 

Frequency assignment. 

The ground 

components (except DME/Marker Bea-
con) must operate on a single fre-
quency assignment or channel, using 
time division multiplexing. These com-
ponents must be capable of operating 
on any one of the 200 channels spaced 
300 KHz apart with center frequencies 
from 5031.0 MHz to 5090.7 MHz and with 
channel numbering as shown in Table 
1a. The operating radio frequencies of 
all ground components must not vary 
by more than 

±

10 KHz from the as-

signed frequency. Any one transmitter 
frequency must not vary more than 

±

50 

Hz in any one second period. The MLS 
angle/data and DME equipment must 
operate on one of the paired channels 
as shown in Table 1b. 

T

ABLE

1a—F

REQUENCY

C

HANNEL

P

LAN

 

Channel No. 

Fre-

quency 

(MHz) 

500 ...........................................................................

5031.0 

501 ...........................................................................

5031.3 

502 ...........................................................................

5031.6 

503 ...........................................................................

5031.9 

504 ...........................................................................

5032.2 

505 ...........................................................................

5032.5 

506 ...........................................................................

5032.8 

507 ...........................................................................

5033.1 

508 ...........................................................................

5033.4 

509 ...........................................................................

5033.7 

510 ...........................................................................

5034.0 

511 ...........................................................................

5034.3 

*

*

*

*

* 

598 ...........................................................................

5060.4 

599 ...........................................................................

5060.7 

600 ...........................................................................

5061.0 

601 ...........................................................................

5061.3 

*

*

*

*

* 

698 ...........................................................................

5090.4 

699 ...........................................................................

5090.7 

T

ABLE

1b—C

HANNELS

 

Channel pairing 

DME parameters 

DME No. 

VHF freq. 

MHz 

MLS angle 

freq. MHz 

MLS Ch. 

No. 

Interrogation 

Reply 

Freq. 

MHz 

Pulse codes 

Freq. 

MHz 

Pulse codes 

µ

s 

DME/N 

µ

s 

DME/P Mode 

IA 

µ

s 

FA 

µ

s 

* 1X ....................

....................

....................

..............

1025 

12  ..............

..............

962 

12 

** 1Y ..................

....................

....................

..............

1025 

36  ..............

..............

1088 

30 

* 2X ....................

....................

....................

..............

1026 

12  ..............

..............

963 

12 

** 2Y ..................

....................

....................

..............

1026 

36  ..............

..............

1089 

30 

* 3X ....................

....................

....................

..............

1027 

12  ..............

..............

964 

12 

** 3Y ..................

....................

....................

..............

1027 

36  ..............

..............

1090 

30 

* 4X ....................

....................

....................

..............

1028 

12  ..............

..............

965 

12 

** 4Y ..................

....................

....................

..............

1028 

36  ..............

..............

1091 

30 

* 5X ....................

....................

....................

..............

1029 

12  ..............

..............

966 

12 

855 

Federal Aviation Administration, DOT 

§ 171.311 

T

ABLE

1b—C

HANNELS

—Continued 

Channel pairing 

DME parameters 

DME No. 

VHF freq. 

MHz 

MLS angle 

freq. MHz 

MLS Ch. 

No. 

Interrogation 

Reply 

Freq. 

MHz 

Pulse codes 

Freq. 

MHz 

Pulse codes 

µ

s 

DME/N 

µ

s 

DME/P Mode 

IA 

µ

s 

FA 

µ

s 

** 5Y ..................

....................

....................

..............

1029 

36  ..............

..............

1092 

30 

* 6X ....................

....................

....................

..............

1030 

12  ..............

..............

967 

12 

** 6Y ..................

....................

....................

..............

1030 

36  ..............

..............

1093 

30 

* 7X ....................

....................

....................

..............

1031 

12  ..............

..............

968 

12 

** 7Y ..................

....................

....................

..............

1031 

36  ..............

..............

1094 

30 

* 8X ....................

....................

....................

..............

1032 

12  ..............

..............

969 

12 

** 8Y ..................

....................

....................

..............

1032 

36  ..............

..............

1095 

30 

* 9X ....................

....................

....................

..............

1033 

12  ..............

..............

970 

12 

** 9Y ..................

....................

....................

..............

1033 

36  ..............

..............

1096 

30 

* 10X ..................

....................

....................

..............

1034 

12  ..............

..............

971 

12 

** 10Y ................

....................

....................

..............

1034 

36  ..............

..............

1097 

30 

* 11X ..................

....................

....................

..............

1035 

12  ..............

..............

972 

12 

** 11Y ................

....................

....................

..............

1035 

36  ..............

..............

1098 

30 

* 12X ..................

....................

....................

..............

1036 

12  ..............

..............

973 

12 

** 12Y ................

....................

....................

..............

1036 

36  ..............

..............

1099 

30 

* 13X ..................

....................

....................

..............

1037 

12  ..............

..............

974 

12 

** 13Y ................

....................

....................

..............

1037 

36  ..............

..............

1100 

30 

* 14X ..................

....................

....................

..............

1038 

12  ..............

..............

975 

12 

** 14Y ................

....................

....................

..............

1038 

36  ..............

..............

1101 

30 

* 15X ..................

....................

....................

..............

1039 

12  ..............

..............

976 

12 

** 15Y ................

....................

....................

..............

1039 

36  ..............

..............

1102 

30 

* 16X ..................

....................

....................

..............

1040 

12  ..............

..............

977 

12 

** 16Y ................

....................

....................

..............

1040 

36  ..............

..............

1103 

30 

S

17X .................

108.00  ....................

..............

1041 

12  ..............

..............

978 

12 

17Y ....................

108.05 

5043.0 

540 

1041 

36 

36 

42 

1104 

30 

17Z ....................

....................

5043.3 

541 

1041  ..............

21 

27 

1104 

15 

18X ....................

108.10 

5031.0 

500 

1042 

12 

12 

18 

979 

12 

18W ...................

....................

5031.3 

501 

1042  ..............

24 

30 

979 

24 

18Y ....................

108.15 

5043.6 

542 

1042 

36 

36 

42 

1105 

30 

18Z ....................

....................

5043.9 

543 

1042  ..............

21 

27 

1105 

15 

19X ....................

108.20  ....................

..............

1043 

12  ..............

..............

980 

12 

19Y ....................

108.25 

5044.2 

544 

1043 

36 

36 

42 

1106 

30 

19Z ....................

....................

5044.5 

545 

1043  ..............

21 

27 

1106 

15 

20X ....................

108.30 

5031.6 

502 

1044 

12 

12 

18 

981 

12 

20W ...................

....................

5031.9 

503 

1044  ..............

24 

30 

981 

24 

20Y ....................

108.35 

5044.8 

546 

1044 

36 

36 

42 

1107 

30 

20Z ....................

....................

5045.1 

547 

1044  ..............

21 

27 

1107 

15 

21X ....................

108.40  ....................

..............

1045 

12  ..............

..............

982 

12 

21Y ....................

108.45 

5045.4 

548 

1045 

36 

36 

42 

1108 

30 

21Z ....................

....................

5045.7 

549 

1045  ..............

21 

27 

1108 

15 

22X ....................

108.50 

5032.2 

504 

1046 

12 

12 

18 

983 

12 

22W ...................

....................

5032.5 

505 

1046  ..............

24 

30 

983 

24 

22Y ....................

108.55 

5046.0 

550 

1046 

36 

36 

42 

1109 

30 

22Z ....................

....................

5046.3 

551 

1046  ..............

21 

27 

1109 

15 

23X ....................

108.60  ....................

..............

1047 

12  ..............

..............

984 

12 

23Y ....................

108.65 

5046.6 

552 

1047 

36 

36 

42 

1110 

30 

23Z ....................

....................

5046.9 

553 

1047  ..............

21 

27 

1110 

15 

24X ....................

108.70 

5032.8 

506 

1048 

12 

12 

18 

985 

12 

24W ...................

....................

5033.1 

507 

1048  ..............

24 

30 

985 

24 

24Y ....................

108.75 

5047.2 

554 

1048 

36 

36 

42 

1111 

30 

24Z ....................

....................

5047.5 

555 

1048  ..............

21 

27 

1111 

15 

25X ....................

108.80  ....................

..............

1049 

12  ..............

..............

986 

12 

25Y ....................

108.85 

5047.8 

556 

1049 

36 

36 

42 

1112 

30 

25Z ....................

....................

5048.1 

557 

1049  ..............

21 

27 

1112 

15 

26X ....................

108.90 

5033.4 

508 

1050 

12 

12 

18 

987 

12 

26W ...................

....................

5033.7 

509 

1050  ..............

24 

30 

987 

24 

26Y ....................

108.95 

5048.4 

558 

1050 

36 

36 

42 

1113 

30 

26Z ....................

....................

5048.7 

559 

1050  ..............

21 

27 

1113 

15 

27X ....................

109.00  ....................

..............

1051 

12  ..............

..............

988 

12 

27Y ....................

109.05 

5049.0 

560 

1051 

36 

36 

42 

1114 

30 

27Z ....................

....................

5049.3 

561 

1051  ..............

21 

27 

1114 

15 

28X ....................

109.10 

5034.0 

510 

1052 

12 

12 

18 

989 

12 

28W ...................

....................

5034.3 

511 

1052  ..............

24 

30 

989 

24 

28Y ....................

109.15 

5049.6 

562 

1052 

36 

36 

42 

1115 

30 

28Z ....................

....................

5049.9 

563 

1052  ..............

21 

27 

1115 

15 

856 

14 CFR Ch. I (1–1–24 Edition) 

§ 171.311 

T

ABLE

1b—C

HANNELS

—Continued 

Channel pairing 

DME parameters 

DME No. 

VHF freq. 

MHz 

MLS angle 

freq. MHz 

MLS Ch. 

No. 

Interrogation 

Reply 

Freq. 

MHz 

Pulse codes 

Freq. 

MHz 

Pulse codes 

µ

s 

DME/N 

µ

s 

DME/P Mode 

IA 

µ

s 

FA 

µ

s 

29X ....................

109.20  ....................

..............

1053 

12  ..............

..............

990 

12 

29Y ....................

109.25 

5050.2 

564 

1053 

36 

36 

42 

1116 

30 

29Z ....................

....................

5050.5 

565 

1043  ..............

21 

27 

1116 

15 

30X ....................

109.30 

5034.6 

512 

1054 

12 

12 

18 

991 

12 

30W ...................

....................

5034.9 

513 

1054  ..............

24 

30 

991 

24 

30Y ....................

109.35 

5050.8 

566 

1054 

36 

36 

42 

1117 

30 

30Z ....................

....................

5051.1 

567 

1054  ..............

21 

27 

1117 

15 

31X ....................

109.40  ....................

..............

1055 

12  ..............

..............

992 

12 

31Y ....................

109.45 

5051.4 

568 

1055 

36 

36 

42 

1118 

30 

31Z ....................

....................

5051.7 

569 

1055  ..............

21 

27 

1118 

15 

32X ....................

109.50 

5035.2 

514 

1056 

12 

12 

18 

993 

12 

32W ...................

....................

5035.5 

515 

1056  ..............

24 

30 

993 

24 

32Y ....................

109.55 

5052.0 

570 

1056 

36 

36 

42 

1119 

30 

32Z ....................

....................

5052.3 

571 

1056  ..............

21 

27 

1119 

15 

33X ....................

109.60  ....................

..............

1057 

12  ..............

..............

994 

12 

33Y ....................

109.65 

5052.6 

572 

1057 

36 

36 

42 

1120 

30 

33Z ....................

....................

5052.9 

573 

1057  ..............

21 

27 

1120 

15 

34X ....................

109.70 

5035.8 

516 

1058 

12 

12 

18 

995 

12 

34W ...................

....................

5036.1 

517 

1058  ..............

24 

30 

995 

24 

34Y ....................

109.75 

5053.2 

574 

1058 

36 

36 

42 

1121 

30 

34Z ....................

....................

5053.5 

575 

1058  ..............

21 

27 

1121 

15 

35X ....................

109.80  ....................

..............

1059 

12  ..............

..............

996 

12 

35Y ....................

109.85 

5053.8 

576 

1059 

36 

36 

42 

1122 

30 

35Z ....................

....................

5054.1 

577 

1059  ..............

21 

27 

1122 

15 

36X ....................

109.90 

5036.4 

518 

1060 

12 

12 

18 

997 

12 

36W ...................

....................

5036.7 

519 

1060  ..............

24 

30 

997 

24 

36Y ....................

109.95 

5054.4 

578 

1060 

36 

36 

42 

1123 

30 

36Z ....................

....................

5054.7 

579 

1060  ..............

21 

27 

1123 

15 

37X ....................

110.00  ....................

..............

1061 

12  ..............

..............

998 

12 

37Y ....................

110.05 

5055.0 

580 

1061 

36 

36 

42 

1124 

30 

37Z ....................

....................

5055.3 

581 

1061  ..............

21 

27 

1124 

15 

38X ....................

110.10 

5037.0 

520 

1062 

12 

12 

18 

999 

12 

38W ...................

....................

5037.3 

521 

1062  ..............

24 

30 

999 

24 

38Y ....................

110.15 

5055.6 

582 

1062 

36 

36 

42 

1125 

30 

38Z ....................

....................

5055.9 

583 

1062  ..............

21 

27 

1125 

15 

39X ....................

110.20  ....................

..............

1063 

12  ..............

..............

1000 

12 

39Y ....................

110.25 

5056.2 

584 

1063 

36 

36 

42 

1126 

30 

39Z ....................

....................

5056.5 

585 

1063  ..............

21 

27 

1126 

15 

40X ....................

110.30 

5037.6 

522 

1064 

12 

12 

18 

1001 

12 

40W ...................

....................

5037.9 

523 

1064  ..............

24 

30 

1001 

24 

40Y ....................

110.35 

5056.8 

586 

1064 

36 

36 

42 

1127 

30 

40Z ....................

....................

5057.1 

587 

1064  ..............

21 

27 

1127 

15 

41X ....................

110.40  ....................

..............

1065 

12  ..............

..............

1002 

12 

41Y ....................

110.45 

5057.4 

588 

1065 

36 

36 

42 

1128 

30 

41Z ....................

....................

5057.7 

589 

1065  ..............

21 

27 

1128 

15 

42X ....................

110.50 

5038.2 

524 

1066 

12 

12 

18 

1003 

12 

42W ...................

....................

5038.5 

525 

1066  ..............

24 

30 

1003 

24 

42Y ....................

110.55 

5058.0 

590 

1066 

36 

36 

42 

1129 

30 

42Z ....................

....................

5058.3 

591 

1066  ..............

21 

27 

1129 

15 

43X ....................

110.60  ....................

..............

1067 

12  ..............

..............

1004 

12 

43Y ....................

110.65 

5058.6 

592 

1067 

36 

36 

42 

1130 

30 

43Z ....................

....................

5058.9 

593 

1067  ..............

21 

27 

1130 

15 

44X ....................

110.70 

5038.8 

526 

1068 

12 

12 

18 

1005 

12 

44W ...................

....................

5039.1 

527 

1068  ..............

24 

30 

1005 

24 

44Y ....................

110.75 

5059.2 

594 

1068 

36 

36 

42 

1131 

30 

44Z ....................

....................

5059.5 

595 

1068  ..............

21 

27 

1131 

15 

45X ....................

110.80  ....................

..............

1069 

12  ..............

..............

1006 

12 

45Y ....................

110.85 

5059.8 

596 

1069 

36 

36 

42 

1132 

30 

45Z ....................

....................

5060.1 

597 

1069  ..............

21 

27 

1132 

15 

46X ....................

110.90 

5039.4 

528 

1070 

12 

12 

18 

1007 

12 

46W ...................

....................

5039.7 

529 

1070  ..............

24 

30 

1007 

24 

46Y ....................

110.95 

5060.4 

598 

1070 

36 

36 

42 

1133 

30 

46Z ....................

....................

5060.7 

599 

1070  ..............

21 

27 

1133 

15 

47X ....................

111.00  ....................

..............

1071 

12  ..............

..............

1008 

12 

47Y ....................

111.05 

5061.0 

600 

1071 

36 

36 

42 

1134 

30 

857 

Federal Aviation Administration, DOT 

§ 171.311 

T

ABLE

1b—C

HANNELS

—Continued 

Channel pairing 

DME parameters 

DME No. 

VHF freq. 

MHz 

MLS angle 

freq. MHz 

MLS Ch. 

No. 

Interrogation 

Reply 

Freq. 

MHz 

Pulse codes 

Freq. 

MHz 

Pulse codes 

µ

s 

DME/N 

µ

s 

DME/P Mode 

IA 

µ

s 

FA 

µ

s 

47Z ....................

....................

5061.3 

601 

1071  ..............

21 

27 

1134 

15 

48X ....................

111.10 

5040.0 

530 

1072 

12 

12 

18 

1009 

12 

48W ...................

....................

5040.3 

531 

1072  ..............

24 

30 

1009 

24 

48Y ....................

111.15 

5061.6 

602 

1072 

36 

36 

42 

1135 

30 

48Z ....................

....................

5061.9 

603 

1072  ..............

21 

27 

1135 

15 

49X ....................

111.20  ....................

..............

1073 

12  ..............

..............

1010 

12 

49Y ....................

111.25 

5062.2 

604 

1073 

36 

36 

42 

1136 

30 

49Z ....................

....................

5062.5 

605 

1073  ..............

21 

27 

1136 

15 

50X ....................

111.30 

5040.6 

532 

1074 

12 

12 

18 

1011 

12 

50W ...................

....................

5040.9 

533 

1074  ..............

24 

30 

1011 

24 

50Y ....................

111.35 

5062.8 

606 

1074 

36 

36 

42 

1137 

30 

50Z ....................

....................

5063.1 

607 

1074  ..............

21 

27 

1137 

15 

51X ....................

111.40  ....................

..............

1075 

12  ..............

..............

1012 

12 

51Y ....................

111.45 

5063.4 

608 

1075 

36 

36 

42 

1138 

30 

51Z ....................

....................

5063.7 

609 

1075  ..............

21 

27 

1138 

15 

52X ....................

111.50 

5041.2 

534 

1076 

12 

12 

18 

1013 

12 

52W ...................

....................

5041.5 

535 

1076  ..............

24 

30 

1013 

24 

52Y ....................

111.55 

5064.0 

610 

1076 

36 

36 

42 

1139 

30 

52Z ....................

....................

5064.3 

611 

1076  ..............

21 

27 

1139 

15 

53X ....................

111.60  ....................

..............

1077 

12  ..............

..............

1014 

12 

53Y ....................

111.65 

5064.6 

612 

1077 

36 

36 

42 

1140 

30 

53Z ....................

....................

5064.9 

613 

1077  ..............

21 

27 

1140 

15 

54X ....................

111.70 

5041.8 

536 

1078 

12 

12 

18 

1015 

12 

54W ...................

....................

5042.1 

537 

1078  ..............

24 

30 

1015 

24 

54Y ....................

111.75 

5065.2 

614 

1078 

36 

36 

42 

1141 

30 

54Z ....................

....................

5065.5 

615 

1078  ..............

21 

27 

1141 

15 

55X ....................

111.80  ....................

..............

1079 

12  ..............

..............

1016 

12 

55Y ....................

111.85 

5065.8 

616 

1079 

36 

36 

42 

1142 

30 

55Z ....................

....................

5066.1 

617 

1079  ..............

21 

27 

1142 

15 

56X ....................

111.90 

5042.4 

538 

1080 

12 

12 

18 

1017 

12 

56W ...................

....................

5042.7 

539 

1080  ..............

24 

30 

1017 

24 

56Y ....................

111.95 

5066.4 

618 

1080 

36 

36 

42 

1143 

30 

56Z ....................

....................

5066.7 

619 

1080  ..............

21 

27 

1143 

15 

57X ....................

112.00  ....................

..............

1081 

12  ..............

..............

1018 

12 

57Y ....................

112.05  ....................

..............

1081 

36  ..............

..............

1144 

30 

58X ....................

112.10  ....................

..............

1082 

12  ..............

..............

1019 

12 

58Y ....................

112.15  ....................

..............

1082 

36  ..............

..............

1145 

30 

59X ....................

112.20  ....................

..............

1083 

12  ..............

..............

1020 

12 

59Y ....................

122.25  ....................

..............

1083 

36  ..............

..............

1146 

30 

** 60X ................

....................

....................

..............

1084 

12  ..............

..............

1021 

12 

** 60Y ................

....................

....................

..............

1084 

36  ..............

..............

1147 

30 

** 61X ................

....................

....................

..............

1085 

12  ..............

..............

1022 

12 

** 61Y ................

....................

....................

..............

1085 

36  ..............

..............

1148 

30 

** 62X ................

....................

....................

..............

1086 

12  ..............

..............

1023 

12 

** 62Y ................

....................

....................

..............

1086 

36  ..............

..............

1149 

30 

** 63X ................

....................

....................

..............

1037 

12  ..............

..............

1024 

12 

** 63Y ................

....................

....................

..............

1087 

36  ..............

..............

1150 

30 

** 64X ................

....................

....................

..............

1088 

12  ..............

..............

1151 

12 

** 64Y ................

....................

....................

..............

1088 

36  ..............

..............

1025 

30 

** 65X ................

....................

....................

..............

1089 

12  ..............

..............

1152 

12 

** 65Y ................

....................

....................

..............

1089 

36  ..............

..............

1026 

30 

** 66X ................

....................

....................

..............

1090 

12  ..............

..............

1153 

12 

** 66Y ................

....................

....................

..............

1090 

36  ..............

..............

1027 

30 

** 67X ................

....................

....................

..............

1091 

12  ..............

..............

1154 

12 

** 67Y ................

....................

....................

..............

1091 

36  ..............

..............

1028 

30 

** 68X ................

....................

....................

..............

1092 

12  ..............

..............

1155 

12 

** 68Y ................

....................

....................

..............

1092 

36  ..............

..............

1029 

30 

** 69X ................

....................

....................

..............

1093 

12  ..............

..............

1156 

12 

** 69Y ................

....................

....................

..............

1093 

36  ..............

..............

1030 

30 

70X ....................

112.30  ....................

..............

1094 

12  ..............

..............

1157 

12 

** 70Y ................

112.35  ....................

..............

1094 

36  ..............

..............

1031 

30 

71X ....................

112.40  ....................

..............

1095 

12  ..............

..............

1158 

12 

** 71Y ................

112.45  ....................

..............

1095 

36  ..............

..............

1032 

30 

72X ....................

112.50  ....................

..............

1096 

12  ..............

..............

1159 

12 

** 72Y ................

112.55  ....................

..............

1096 

36  ..............

..............

1033 

30 

858 

14 CFR Ch. I (1–1–24 Edition) 

§ 171.311 

T

ABLE

1b—C

HANNELS

—Continued 

Channel pairing 

DME parameters 

DME No. 

VHF freq. 

MHz 

MLS angle 

freq. MHz 

MLS Ch. 

No. 

Interrogation 

Reply 

Freq. 

MHz 

Pulse codes 

Freq. 

MHz 

Pulse codes 

µ

s 

DME/N 

µ

s 

DME/P Mode 

IA 

µ

s 

FA 

µ

s 

73X ....................

112.60  ....................

..............

1097 

12  ..............

..............

1160 

12 

** 73Y ................

112.65  ....................

..............

1097 

36  ..............

..............

1034 

30 

74X ....................

112.70  ....................

..............

1098 

12  ..............

..............

1161 

12 

** 74Y ................

112.75  ....................

..............

1098 

36  ..............

..............

1035 

30 

75X ....................

112.80  ....................

..............

1099 

12  ..............

..............

1162 

12 

** 75Y ................

112.85  ....................

..............

1099 

36  ..............

..............

1036 

30 

76X ....................

112.90  ....................

..............

1100 

12  ..............

..............

1163 

12 

** 76Y ................

112.95  ....................

..............

1100 

36  ..............

..............

1037 

30 

77X ....................

113.00  ....................

..............

1101 

12  ..............

..............

1164 

12 

** 77Y ................

113.05  ....................

..............

1101 

36  ..............

..............

1038 

30 

78X ....................

113.10  ....................

..............

1102 

12  ..............

..............

1165 

12 

** 78Y ................

113.15  ....................

..............

1102 

36  ..............

..............

1039 

30 

79X ....................

113.20  ....................

..............

1103 

12  ..............

..............

1166 

12 

** 79Y ................

113.25  ....................

..............

1103 

36  ..............

..............

1040 

30 

80X ....................

113.30  ....................

..............

1104 

12  ..............

..............

1167 

12 

80Y ....................

113.35 

5067.0 

620 

1104 

36 

36 

42 

1041 

30 

80Z ....................

....................

5067.3 

621 

1104  ..............

21 

27 

1041 

15 

81X ....................

113.40  ....................

..............

1105 

12  ..............

..............

1168 

12 

81Y ....................

113.45 

5067.6 

622 

1105 

36 

36 

42 

1042 

30 

81Z ....................

....................

5067.9 

623 

1005  ..............

21 

27 

1042 

15 

82X ....................

113.50  ....................

..............

1106 

12  ..............

..............

1169 

12 

82Y ....................

113.55 

5068.2 

624 

1106 

36 

36 

42 

1043 

30 

82Z ....................

....................

5068.5 

625 

1106  ..............

21 

27 

1043 

15 

83X ....................

113.60  ....................

..............

1107 

12  ..............

..............

1170 

12 

83Y ....................

113.65 

5068.8 

626 

1107 

36 

36 

42 

1044 

30 

83Z ....................

....................

5069.1 

627 

1107  ..............

21 

27 

1044 

15 

84X ....................

113.70  ....................

..............

1108 

12  ..............

..............

1171 

12 

84Y ....................

113.75 

5069.4 

628 

1108 

36 

36 

42 

1045 

30 

84Z ....................

....................

6069.7 

629 

1108  ..............

21 

27 

1045 

15 

85X ....................

113.80  ....................

..............

1109 

12  ..............

..............

1172 

12 

85Y ....................

113.85 

5070.0 

630 

1109 

36 

36 

42 

1046 

30 

85Z ....................

....................

5070.3 

631 

1109  ..............

21 

27 

1046 

15 

86X ....................

113.90  ....................

..............

1110 

12  ..............

..............

1173 

12 

86Y ....................

113.95 

5070.6 

632 

1110 

36 

36 

42 

1047 

30 

86Z ....................

....................

5070.9 

633 

1110  ..............

21 

27 

1047 

15 

87X ....................

114.00  ....................

..............

1111 

12  ..............

..............

1174 

12 

87Y ....................

114.05 

5071.2 

634 

1111 

36 

36 

42 

1048 

30 

87Z ....................

....................

5071.5 

635 

1111  ..............

21 

27 

1048 

15 

88X ....................

114.10  ....................

..............

1112 

12  ..............

..............

1175 

12 

88Y ....................

114.15 

5071.8 

636 

1112 

36 

36 

42 

1049 

30 

88Z ....................

....................

5072.1 

637 

1112  ..............

21 

27 

1049 

15 

89X ....................

114.20  ....................

..............

1113 

12  ..............

..............

1176 

12 

89Y ....................

114.25 

5072.4 

638 

1113 

36 

36 

42 

1050 

30 

89Z ....................

....................

5072.7 

639 

1113  ..............

21 

27 

1050 

15 

90X ....................

114.30  ....................

..............

1114 

12  ..............

..............

1177 

12 

90Y ....................

114.35 

5073.0 

640 

1114 

36 

36 

42 

1051 

30 

90Z ....................

....................

5073.3 

641 

1114  ..............

21 

27 

1051 

15 

91X ....................

114.40  ....................

..............

1115 

12  ..............

..............

1178 

12 

91Y ....................

114.45 

5073.6 

642 

1115 

36 

36 

42 

1052 

30 

91Z ....................

....................

5073.9 

643 

1115  ..............

21 

27 

1052 

15 

92X ....................

114.50  ....................

..............

1116 

12  ..............

..............

1179 

12 

92Y ....................

114.55 

5074.2 

644 

1116 

36 

36 

42 

1053 

30 

92Z ....................

....................

5074.5 

645 

1116  ..............

21 

27 

1053 

15 

93X ....................

114.60  ....................

..............

1117 

12  ..............

..............

1180 

12 

93Y ....................

114.65 

5074.8 

646 

1117 

36 

36 

42 

1054 

30 

93Z ....................

....................

5075.1 

647 

1117  ..............

21 

27 

1054 

15 

94X ....................

114.70  ....................

..............

1118 

12  ..............

..............

1181 

12 

94Y ....................

114.75 

5075.4 

648 

1118 

36 

36 

42 

1055 

30 

94Z ....................

....................

5075.7 

649 

1118  ..............

21 

27 

1055 

15 

95X ....................

114.80  ....................

..............

1119 

12  ..............

..............

1182 

12 

95Y ....................

114.85 

5076.0 

650 

1119 

36 

36 

42 

1056 

30 

95Z ....................

....................

5076.3 

651 

1119  ..............

21 

27 

1056 

15 

96X ....................

114.90  ....................

..............

1120 

12  ..............

..............

1183 

12 

96Y ....................

114.95 

5076.6 

652 

1120 

36 

36 

42 

1057 

30 

96Z ....................

....................

5076.9 

653 

1120  ..............

21 

27 

1057 

15 

859 

Federal Aviation Administration, DOT 

§ 171.311 

T

ABLE

1b—C

HANNELS

—Continued 

Channel pairing 

DME parameters 

DME No. 

VHF freq. 

MHz 

MLS angle 

freq. MHz 

MLS Ch. 

No. 

Interrogation 

Reply 

Freq. 

MHz 

Pulse codes 

Freq. 

MHz 

Pulse codes 

µ

s 

DME/N 

µ

s 

DME/P Mode 

IA 

µ

s 

FA 

µ

s 

97X ....................

115.00  ....................

..............

1121 

12  ..............

..............

1184 

12 

97Y ....................

115.05 

5077.2 

654 

1121 

36 

36 

42 

1058 

30 

97Z ....................

....................

5077.5 

655 

1121  ..............

21 

27 

1058 

15 

98X ....................

115.10  ....................

..............

1122 

12  ..............

..............

1185 

12 

98Y ....................

115.15 

5077.8 

656 

1122 

36 

36 

42 

1059 

30 

98Z ....................

....................

5078.1 

657 

1122  ..............

21 

27 

1059 

15 

99X ....................

115.20  ....................

..............

1123 

12  ..............

..............

1186 

12 

99Y ....................

115.25 

5078.4 

658 

1123 

36 

36 

42 

1060 

30 

99Z ....................

....................

5078.7 

659 

1123  ..............

21 

27 

1060 

15 

100X ..................

115.30  ....................

..............

1124 

12  ..............

..............

1187 

12 

100Y ..................

115.35 

5079.0 

660 

1124 

36 

36 

42 

1061 

30 

100Z ..................

....................

5079.3 

661 

1124  ..............

21 

27 

1061 

15 

101X ..................

115.40  ....................

..............

1125 

12  ..............

..............

1188 

12 

101Y ..................

115.45 

5079.6 

662 

1125 

36 

36 

42 

1062 

30 

101Z ..................

....................

5079.9 

663 

1125  ..............

21 

27 

1062 

15 

102X ..................

115.50  ....................

..............

1126 

12  ..............

..............

1189 

12 

102Y ..................

115.55 

5080.2 

664 

1126 

36 

36 

42 

1063 

30 

102Z ..................

....................

5080.5 

665 

1126  ..............

21 

27 

1063 

15 

103X ..................

115.60  ....................

..............

1127 

12  ..............

..............

1190 

12 

103Y ..................

115.65 

5080.B 

666 

1127 

36 

36 

42 

1064 

30 

103Z ..................

....................

5081.1 

667 

1127  ..............

21 

27 

1064 

19 

104X ..................

115.70  ....................

..............

1128 

12  ..............

..............

1191 

12 

104Y ..................

115.75 

5081.4 

668 

1128 

36 

36 

42 

1065 

30 

104Z ..................

....................

5081.7 

669 

1128  ..............

21 

27 

1065 

19 

105X ..................

115.80  ....................

..............

1129 

12  ..............

..............

1192 

12 

105Y ..................

115.85 

5082.0 

670 

1129 

36 

36 

42 

1066 

30 

105Z ..................

....................

5082.3 

671 

1129  ..............

21 

27 

1066 

15 

106X ..................

115.90  ....................

..............

1130 

12  ..............

..............

1193 

12 

106Y ..................

115.95 

5082.6 

672 

1130 

36 

36 

42 

1067 

30 

106Z ..................

....................

5082.9 

673 

1130  ..............

21 

27 

1067 

15 

107X ..................

116.00  ....................

..............

1131 

12  ..............

..............

1194 

12 

107Y ..................

116.05 

5083.2 

674 

1131 

36 

36 

42 

1068 

30 

107Z ..................

....................

5083.5 

675 

1131  ..............

21 

27 

1068 

15 

108X ..................

116.10 

508  ..............

1132 

12  ..............

..............

1195 

12 

108Y ..................

116.15 

5083.8 

676 

1132 

36 

36 

42 

1069 

30 

108Z ..................

....................

5084.1 

677 

1132  ..............

21 

27 

1069 

15 

109X ..................

116.20  ....................

..............

1133 

12  ..............

..............

1196 

12 

109Y ..................

116.25 

5084.4 

678 

1133 

36 

36 

42 

1070 

30 

109Z ..................

....................

5084.7 

679 

1133  ..............

21 

27 

1070 

15 

110X ..................

116.30  ....................

..............

1134 

12  ..............

..............

1197 

12 

110Y ..................

116.35 

5085.0 

680 

1134 

36 

36 

42 

1071 

30 

110Z ..................

....................

5085.3 

681 

1134  ..............

21 

27 

1071 

15 

111X ..................

116.40  ....................

..............

1135 

12  ..............

..............

1198 

12 

111Y ..................

116.45 

5086.6 

682 

1135 

36 

36 

42 

1072 

30 

111Z ..................

....................

5085.9 

683 

1135  ..............

21 

27 

1072 

15 

112X ..................

116.50  ....................

..............

1136 

12  ..............

..............

1199 

12 

112Y ..................

116.55 

5086.2 

684 

1136 

36 

36 

42 

1073 

30 

112Z ..................

....................

5086.5 

685 

1136  ..............

21 

27 

1073 

15 

113X ..................

116.60  ....................

..............

1137 

12  ..............

..............

1200 

12 

113Y ..................

116.65 

5086.8 

686 

1137 

36 

36 

42 

1074 

30 

113Z ..................

....................

5087.1 

687 

1137  ..............

21 

27 

1074 

15 

114X ..................

116.70  ....................

..............

1138 

12  ..............

..............

1201 

12 

114Y ..................

116.75 

5087.4 

688 

1138 

36 

36 

42 

1075 

30 

114Z ..................

....................

5087.7 

689 

1138  ..............

21 

27 

1075 

15 

115X ..................

116.80  ....................

..............

1139 

12  ..............

..............

1202 

12 

115Y ..................

116.85 

5088.0 

690 

1139 

36 

36 

42 

1076 

30 

115Z ..................

....................

5088.3 

691 

1139  ..............

21 

27 

1076 

15 

116X ..................

116.90  ....................

..............

1140 

12  ..............

..............

1203 

12 

116Y ..................

116.95 

5088.6 

692 

1140 

36 

36 

42 

1077 

30 

116Z ..................

....................

5088.9 

693 

1140  ..............

21 

27 

1077 

15 

117X ..................

117.00  ....................

..............

1141 

12  ..............

..............

1204 

12 

117Y ..................

117.05 

5089.2 

694 

1141 

36 

36 

42 

1078 

30 

117Z ..................

....................

5089.5 

695 

1141  ..............

21 

27 

1078 

15 

118X ..................

117.10  ....................

..............

1142 

12  ..............

..............

12.5 

12 

118Y ..................

117.15 

5089.8 

696 

1142 

36 

36 

42 

1079 

30 

860 

14 CFR Ch. I (1–1–24 Edition) 

§ 171.311 

T

ABLE

1b—C

HANNELS

—Continued 

Channel pairing 

DME parameters 

DME No. 

VHF freq. 

MHz 

MLS angle 

freq. MHz 

MLS Ch. 

No. 

Interrogation 

Reply 

Freq. 

MHz 

Pulse codes 

Freq. 

MHz 

Pulse codes 

µ

s 

DME/N 

µ

s 

DME/P Mode 

IA 

µ

s 

FA 

µ

s 

118Z ..................

....................

5090.1 

697 

1142  ..............

21 

27 

1079 

12 

119X ..................

117.20  ....................

..............

1143 

12  ..............

..............

1206 

12 

119Y ..................

117.25 

5090.4 

698 

1143 

36 

36 

42 

1080 

30 

119Z ..................

....................

5090.7 

699 

1143  ..............

21 

27 

1080 

15 

120X ..................

117.30  ....................

..............

1144 

12  ..............

..............

1207 

12 

120Y ..................

117.35  ....................

..............

1144 

36  ..............

..............

1081 

30 

121X ..................

117.40  ....................

..............

1145 

12  ..............

..............

1208 

12 

121Y ..................

117.45  ....................

..............

1145 

36  ..............

..............

1082 

30 

122X ..................

117.50  ....................

..............

1146 

12  ..............

..............

1209 

12 

122Y ..................

117.55  ....................

..............

1146 

36  ..............

..............

1083 

30 

123X ..................

117.60  ....................

..............

1147 

12  ..............

..............

1210 

12 

123Y ..................

117.65  ....................

..............

1147 

36  ..............

..............

1084 

30 

124X ..................

117.70  ....................

..............

1148 

12  ..............

..............

1211 

12 

** 124Y ..............

117.75  ....................

..............

1148 

36  ..............

..............

1085 

30 

125X ..................

117.80  ....................

..............

1149 

12  ..............

..............

1212 

12 

** 125Y ..............

117.85  ....................

..............

1149 

36  ..............

..............

1086 

30 

126X ..................

117.90  ....................

..............

1150 

12  ..............

..............

1213 

12 

** 126Y ..............

117.95  ....................

..............

1150 

36  ..............

..............

1087 

30 

Notes: 
* These channels are reserved exclusively for national allotments. 
** These channels may be used for national allotment on a secondary basis. The primary reason for reserving these channels 

is to provide protection for the secondary Surveillance Radar (SSR) system. 

S

108.0 MHz is not scheduled for assignment to ILS service. The associated DME operating channel No. 17X may be as-

signed to the emergency service. 

(b) 

Polarization. 

(1) The radio fre-

quency emissions from all ground 
equipment must be nominally 
vertically polarized. Any horizontally 
polarized radio frequency emission 
component from the ground equipment 
must not have incorrectly coded angle 
information such that the limits speci-
fied in paragraphs (b) (2) and (3) of this 
section are exceeded. 

(2) Rotation of the receiving antenna 

thirty degrees from the vertically po-
larized position must not cause the 
path following error to exceed the al-
lowed error at that location. 

(c) 

Modulation requirements. 

Each 

function transmitter must be capable 

of DPSK and continuous wave (CW) 
modulations of the RF carrier which 
have the following characteristics. 

(1) DPSK. The DPSK signal must 

have the following characteristics: 

bit rate ..............................

15.625 KHz 

bit length ...........................

64 microseconds 

logic ‘‘0’’ ............................

no phase transition 

logic ‘‘1’’ ............................

phase transition 

phase transition ................

less than 10 microseconds 

phase tolerance ................

±

10 degrees 

The phase shall advance (or retard) 
monotonically throughout the transi-
tion region. Amplitude modulation 
during the phase transition period 
shall not be used. 

861 

Federal Aviation Administration, DOT 

§ 171.311 

(2) CW. The CW pulse transmissions 

and the CW angle transmissions as may 
be required in the signal format of any 
function must have characteristics 
such that the requirements of para-
graph (d) of this section are met. 

(d) 

Radio frequency signal spectrum. 

The transmitted signal must be such 
that during the transmission time, the 
mean power density above a height of 
600 meters (2000 feet) does not exceed 

¥

100.5 dBW/m

2

for angle guidance and 

¥

95.5 dBW/m

2

for data, as measured in 

a 150 KHz bandwidth centered at a fre-
quency of 840 KHz or more from the as-
signed frequency. 

(e) 

Synchronization. 

Synchronization 

between the azimuth and elevation 
components is required and, in split- 
site configurations, would normally be 
accomplished by landline interconnec-
tions. Synchronization monitoring 
must be provided to preclude function 
overlap. 

(f) 

Transmission rates. 

Angle guidance 

and data signals must be transmitted 
at the following average repetition 
rates: 

Function 

Average 

data rate 

(Hertz) 

Approach Azimuth .................................................

13 

±

0.5 

High Rate Approach Azimuth ................................

1

39 

±

1.5 

Approach Elevation ...............................................

39 

±

1.5 

Back Azimuth .........................................................

6.5 

±

0.25 

Basic Data .............................................................

(

2

) 

Auxiliary Data ........................................................

(

3

) 

1

The higher rate is recommended for azimuth scanning an-

tennas with beamwidths greater than two degrees. It should 
be noted that the time available in the signal format for addi-
tional functions is limited when the higher rate is used. 

2

Refer to Table 8a. 

3

Refer to Table 8c. 

(g) 

Transmission sequences. 

Sequences 

of angle transmissions which will gen-
erate the required repetition rates are 
shown in Figures 2 and 3. 

862 

14 CFR Ch. I (1–1–24 Edition) 

§ 171.311 

863 

Federal Aviation Administration, DOT 

§ 171.311 

(h) 

TDM cycle. 

The time periods be-

tween angle transmission sequences 
must be varied so that exact repeti-
tions do not occur within periods of 
less than 0.5 second in order to protect 
against synchronous interference. One 

such combination of sequences is 
shown in Figure 4 which forms a full 
multiplex cycle. Data may be trans-
mitted during suitable open times 
within or between the sequences. 

864 

14 CFR Ch. I (1–1–24 Edition) 

§ 171.311 

(i) 

Function Formats (General). 

Each 

angle function must contain the fol-
lowing elements: a preamble; sector 
signals; and a TO and FRO angle scan 

organized as shown in Figure 5a. Each 
data function must contain a preamble 
and a data transmission period orga-
nized as shown in Figure 5b. 

(1) 

Preamble format. 

The transmitted 

angle and date functions must use the 
preamble format shown in Figure 6. 
This format consists of a carrier acqui-
sition period of unmodulated CW trans-

mission followed by a receiver synchro-
nization code and a function identifica-
tion code. The preamble timing must 
be in accordance with Table 2. 

865 

Federal Aviation Administration, DOT 

§ 171.311 

(i) 

Digital codes. 

The coding used in 

the preamble for receiver synchroni-
zation is a Barker code logic 11101. The 
time of the last phase transition mid-
point in the code shall be the receiver 
reference time (see Table 2). The func-
tion identification codes must be as 
shown in Table 3. The last two bits (I

11

 

and I

12

) of the code are parity bits 

obeying the equations: 

I

6

+ I

7

+ I

8

+ I

9

+ I

10

+ I

11

= Even 

I

6

+ I

8

+ I

10

+ I

12

= Even 

(ii) 

Data modulation. 

The digital code 

portions of the preamble must be 
DPSK modulated in accordance with 
§ 171.311(c)(1) and must be transmitted 
throughout the function coverage vol-
ume. 

(2) 

Angle function formats. 

The timing 

of the angle transmissions must be in 
accordance with Tables 4a, 4b, and 5. 
The actual timing of the TO and FRO 
scans must be as required to meet the 
accuracy requirements of §§ 171.313 and 
171.317. 

(i) Preamble. Must be in accordance 

with requirements of § 171.311(i)(1). 

T

ABLE

2—P

REAMBLE

T

IMING

1

 

Event 

Event time slot begins at— 

15.625 kHz 

clock pulse 

(number) 

Time (milli-

seconds) 

Carrier acquisition: 

(CW transmission) ...............

0 

0 

Receiver reference time code: 

I

1

= 1 ....................................

13 

0 .832 

I

2

= 1 ....................................

14 

0 .896 

I

3

= 1 ....................................

15 

0 .960 

I

4

= 0 ....................................

16 

1 .024 

I

5

= 1 ....................................

17 

2

1 .088 

Function identification: 

I

6

...........................................

18 

1 .152 

I

7

...........................................

19 

1 .216 

I

8

...........................................

20 

1 .280 

I

9

...........................................

21 

1 .344 

I

10

(see table 1) ....................

22 

1 .408 

I

11

.........................................

23 

1 .472 

I

12

.........................................

24 

1 .536 

T

ABLE

2—P

REAMBLE

T

IMING

1

—Continued 

Event 

Event time slot begins at— 

15.625 kHz 

clock pulse 

(number) 

Time (milli-

seconds) 

END PREAMBLE .................

25 

1 .600 

1

Applies to all functions transmitted. 

2

Reference time for receiver synchronization for all function 

timing. 

T

ABLE

3—F

UNCTION

I

DENTIFICATION

C

ODES

 

Function 

Code 

I

6

 

I

7

 

I

8

 

I

9

 

I

10

 

I

11

 

I

12

 

Approach azimuth ....................

0 

0 

1 

1 

0 

0 

1 

High rate approach azimuth .....

0 

0 

1 

0 

1 

0 

0 

Approach elevation ..................

1 

1 

0 

0 

0 

0 

1 

Back azimuth ............................

1 

0 

0 

1 

0 

0 

1 

Basic data 1 .............................

0 

1 

0 

1 

0 

0 

0 

Basic data 2 .............................

0 

1 

1 

1 

1 

0 

0 

Basic data 3 .............................

1 

0 

1 

0 

0 

0 

0 

Basic data 4 .............................

1 

0 

0 

0 

1 

0 

0 

Basic data 5 .............................

1 

1 

0 

1 

1 

0 

0 

Dasic data 6 .............................

0 

0 

0 

1 

1 

0 

1 

Auxiliary data A ........................

1 

1 

1 

0 

0 

1 

0 

Auxiliary data B ........................

1 

0 

1 

0 

1 

1 

1 

Auxiliary data C ........................

1 

1 

1 

1 

0 

0 

0 

(ii) 

Sector signals. 

In all azimuth for-

mats, sector signals must be trans-
mitted to provide Morse Code identi-
fication, airborne antenna selection, 
and system test signals. These signals 
are not required in the elevation for-
mats. In addition, if the signal from an 
installed ground component results in 
a valid indication in an area where no 
valid guidance should exist, OCI signals 
must be radiated as provided for in the 
signal format (see Tables 4a, 4b, and 5). 
The sector signals are defined as fol-
lows: 

(A) 

Morse Code. 

DPSK transmissions 

that will permit Morse Code facility 
identification in the aircraft by a four 
letter code starting with the letter 
‘‘M’’ must be included in all azimuth 
functions. They must be transmitted 
and repeated at approximately equal 
intervals, not less than six times per 

866 

14 CFR Ch. I (1–1–24 Edition) 

§ 171.311 

minute, during which time the ground 
subsystem is available for operational 
use. When the transmissions of the 
ground subsystem are not available, 
the identification signal must be sup-
pressed. The audible tone in the air-
craft is started by setting the Morse 
Code bit to logic ‘‘1’’ and stopped by a 
logic ‘‘0’’ (see Tables 4a and 4b). The 
identification code characteristics 
must conform to the following: the dot 
must be between 0.13 and 0.16 second in 
duration, and the dash between 0.39 and 
0.48 second. The duration between dots 
and/or dashes must be one dot plus or 
minus 10%. The duration between char-
acters (letters) must not be less than 
three dots. When back azimuth is pro-
vided, the code shall be transmitted by 
the approach azimuth and back azi-
muth within plus or minus 0.08 seconds. 

(B) 

Airborne antenna selection. 

A sig-

nal for airborne antenna selection shall 
be transmitted as a ‘‘zero’’ DPSK sig-
nal lasting for a six-bit period (see Ta-
bles 4a and 4b). 

T

ABLE

4a—A

PPROACH

A

ZIMUTH

F

UNCTION

 

TIMING

 

Event 

Event time slot 

begins at— 

15.625 

kHz clock 

pulse 

(number) 

Time 

(milli-

sec-

onds) 

Preamble ...............................................

0 

0 

Morse code ............................................

25 

1 .600 

Antenna select .......................................

26 

1 .664 

Rear OCI ...............................................

32 

2 .048 

Left OCI .................................................

34 

2 .176 

Right OCI ...............................................

36 

2 .304 

To test ...................................................

38 

2 .432 

To scan

1

................................................

40 

2 .560 

Pause ....................................................

................

8 .760 

Midscan point ........................................

................

9 .060 

FRO scan

1

............................................

................

9 .360 

FRO test ................................................

................

15 .560 

End Function (Airborne) ........................

................

15 .688 

End guard time; end function (ground)

................

15 .900 

AA

1

The actual commencement and completion of the TO 

and the FRO scan transmissions are dependent on the 
amount of proportional guidance provided. The time slots pro-
vided shall accommodate a maximum scan of plus or minus 
62.0 degrees. Scan timing shall be compatible with accuracy 
requirements. 

T

ABLE

4b—H

IGH

R

ATE

A

PPROACH

A

ZIMUTH AND

 

B

ACK

A

ZIMUTH

F

UNCTION

T

IMING

 

Event 

Event time slot 

begins at— 

15.625 

kHz clock 

pulse 

(number) 

Time 

(milli-

sec-

onds) 

Preamble ...............................................

0 

0 

Morse Code ...........................................

25 

1 .600 

Antenna select .......................................

26 

1 .664 

Rear OCI ...............................................

32 

2 .048 

Left OCI .................................................

34 

2 .176 

Right OCI ...............................................

36 

2 .304 

To test ...................................................

38 

2 .432 

To scan

1

................................................

40 

2 .560 

Pause ....................................................

................

6 .760 

Midscan point ........................................

................

7 .060 

FRO scan

1

............................................

................

7 .360 

FRO test pulse ......................................

................

11 .560 

End function (airborne) ..........................

................

11 .688 

End guard time; end function (ground)

................

11 .900 

1

The actual commencement and completion of the TO and 

the FRO scan transmissions are dependent on the amount of 
proportional guidance provided. The time slots provided will 
accommodate a maximum scan of plus or minus 42.0 de-
grees. Scan timing shall be compatible with accuracy 
requirements. 

(C) 

OCI. 

Where OCI pulses are used, 

they must be: (1) greater than any 
guidance signal in the OCI sector; (2) at 
least 5 dB less than the level of the 
scanning beam within the proportional 
guidance sector; and (3) for azimuth 
functions with clearance signals, at 
least 5 dB less than the level of the left 
(right) clearance pulses within the left 
(right) clearance sector. 

T

ABLE

5—A

PPROACH

E

LEVATION

F

UNCTION

 

T

IMING

 

Event 

Event time slot 

begins at: 

15.625 

kHz clock 

pluse 

(number) 

Time 

(milli-

sec-

onds) 

Preamble ...............................................

0 

0 

Processor pause ....................................

25 

1 .600 

OCI ........................................................

27 

1 .728 

To scan

1

................................................

29 

1 .856 

Pause ....................................................

................

3 .406 

Midscan point ........................................

................

3 .606 

FRO scan

1

............................................

................

3 .806 

End function (airborne) ..........................

................

5 .356 

End guard time; end function (ground)

................

5 .600 

1

The actual commencement and completion of the TO and 

FRO scan transmissions are dependent upon the amount of 
proportional guidance provided. The time slots provided will 
accommodate a maximum scan of 

¥

1.5 degrees to + 29.5 

degrees. Scan timing shall be compatible with accuracy 
requirements. 

The duration of each pulse measured 

at the half amplitude point shall be at 
least 100 microseconds, and the rise and 

867 

Federal Aviation Administration, DOT 

§ 171.311 

fall times shall be less then 10 micro-
seconds. It shall be permissible to se-
quentially transmit two pulses in each 
out-of-coverage indication time slot. 
Where pulse pairs are used, the dura-
tion of each pulse shall be at least 50 
microseconds, and the rise and fall 
times shall be less then 10 microsec-
onds. The transmission of out-of-cov-
erage indication pulses radiated from 
antennas with overlapping coverage 
patterns shall be separated by at least 
10 microseconds. 

N

OTE

: If desired, two pulses may be sequen-

tially transmitted in each OCI time slot. 
Where pulse pairs are used, the duration of 
each pulse must be 45 (

±

5) microseconds and 

the rise and fall times must be less than 10 
microseconds. 

(D) 

System test. 

Time slots are pro-

vided in Tables 4a and 4b to allow radi-
ation of TO and FRO test pulses. How-
ever, radiation of these pulses is not re-
quired since the characteristics of 
these pulses have not yet been stand-
ardized. 

(iii) 

Angle encoding. 

The encoding 

must be as follows: 

(A) 

General. 

Azimuth and elevation 

angles are encoded by scanning a nar-
row beam between the limits of the 

proportional coverage sector first in 
one direction (the TO scan) and then in 
the opposite direction (the FRO scan). 
Angular information must be encoded 
by the amount of time separation be-
tween the beam centers of the TO and 
FRO scanning beam pulses. The TO and 
FRO transmissions must be symmetri-
cally disposed about the midscan point 
listed in Tables 4a, 4b, 5, and 7. The 
midscan point and the center of the 
time interval between the TO and FRO 
scan transmissions must coincide with 
a tolerance of 

±

10 microseconds. Angu-

lar coding must be linear with angle 
and properly decoded using the for-
mula: 

θ

=

V

2

T

t

0

−

(

)

where: 

q 

= Receiver angle in degrees. 

V = Scan velocity in degrees per micro-

second. 

T

0

= Time separation in microseconds be-

tween TO and FRO beam centers cor-
responding to zero degrees. 

t = Time separation in microseconds between 

TO and FRO beam centers. 

The timing requirements are listed in 
Table 6 and illustrated in Figure 7. 

868 

14 CFR Ch. I (1–1–24 Edition) 

§ 171.311 

(B) 

Azimuth angle encoding. 

Each 

guidance angle transmitted must con-
sist of a clockwise TO scan followed by 
a counterclockwise FRO scan as viewed 
from above the antenna. For approach 
azimuth functions, increasing angle 
values must be in the direction of the 
TO scan; for the back azimuth func-
tion, increasing angle values must be 
in the direction of the FRO scan. The 
antenna has a narrow beam in the 
plane of the scan direction and a broad 
beam in the orthogonal plane which 
fills the vertical coverage. 

(C) 

Elevation angle encoding. 

The radi-

ation from elevation equipment must 
produce a beam which scans from the 
horizon up to the highest elevation 
angle and then scans back down to the 
horizon. The antenna has a narrow 
beam in the plane of the scan direction 
and a broad beam in the orthogonal 
plane which fills the horizontal cov-

erage. Elevation angles are defined 
from the horizontal plane containing 
the antenna phase center; positive an-
gles are above the horizontal and zero 
angle is along the horizontal. 

(iv) 

Clearance guidance. 

The timing of 

the clearance pulses must be in accord-
ance with Figure 8. For azimuth ele-
ments with proportional coverage of 
less than 

±

40 degrees (

±

20 degrees for 

back azimuth), clearance guidance in-
formation must be provided by trans-
mitting pulses in a TO and FRO format 
adjacent to the stop/start times of the 
scanning beam signal. The fly-right 
clearance pulses must represent posi-
tive angles and the fly-left clearance 
pulses must represent negative angles. 
The duration of each clearance pulse 
must be 50 microseconds with a toler-
ance of 

±

5 microseconds. The trans-

mitter switching time between the 
clearance pulses and the scanning 

869 

Federal Aviation Administration, DOT 

§ 171.311 

beam transmissions must not exceed 10 
microseconds. The rise time at the 
edge of each clearance pulse must be 
less than 10 microseconds. Within the 
fly-right clearance guidance section, 
the fly-right clearance guidance signal 
shall exceed scanning beam antenna 
sidelobes and other guidance and OCI 
signals by at least 5 dB; within the fly- 
left clearance guidance sector, the fly 
left clearance guidance signal shall ex-
ceed scanning beam antenna sidelobes 
and all other guidance and OCI signals 
by at least 5 dB; within the propor-
tional guidance sector, the clearance 
guidance signals shall be at least 5dB 
below the proportional guidance signal. 
Optionally, clearance guidance may be 
provided by scanning throughout the 
approach guidance sector. For angles 
outside the approach azimuth propor-
tional coverage limits as set in Basic 
Data Word One (Basic Data Word 5 for 
back azimuth), proper decode and dis-
play of clearance guidance must occur 
to the limits of the guidance region. 

Where used, clearance pulses shall be 
transmitted adjacent to the scanning 
beam signals at the edges of propor-
tional coverage as shown in Figure 8. 
The proportional coverage boundary 
shall be established at one beamwidth 
inside the scan start/stop angles, such 
that the transition between scanning 
beam and clearance signals occurs out-
side the proportional coverage sector. 
When clearance pulses are provided in 
conjunction with a narrow beamwidth 
(e.g., one degree) scanning antenna, the 
scanning beam antenna shall radiate 
for 15 microseconds while stationary at 
the scan start/stop angles. 

(3) 

Data function format. 

Basic data 

words provide equipment characteris-
tics and certain siting information. 
Basic data words must be transmitted 
from an antenna located at the ap-
proach azimuth or back azimuth site 
which provides coverage throughout 
the appropriate sector. Data function 
timing must be in accordance with 
Table 7a. 

T

ABLE

6—A

NGLE

S

CAN

T

IMING

C

ONSTANTS

 

Function 

Max 

value of 

t

(usec) 

T

o

(usec) 

V(deg/ 

usec) 

T

m

 

(usec) 

Pause 

time 

(usec) 

T

t

(usec) 

Approach azimuth .....................................................................

13,000 

6,800 

0.02 

7,972 

600 

13,128 

High rate approach azimuth ......................................................

9,000 

4,800 

0.02 

5,972 

600 

9,128 

Approach elevation ...................................................................

3,500 

3,350 

0.02 

2,518 

400 

N/A 

Back azimuth .............................................................................

9,000 

4,800 

¥

0.02 

5,972 

600 

9,128 

T

ABLE

7a—B

ASIC

D

ATA

F

UNCTION

T

IMING

 

Event 

Event time slot 

begins at:

1

 

15.625 

kHz clock 

pulse 

(number) 

Time 

(milli-

sec-

onds) 

Preamble ...............................................

0 

0 

Data transmission (bits I

13

–I

30

) ..............

25 

1 .600 

Parity transmission (bits I

31

–I

32

) ............

43 

2 .752 

End function (airborne) ..........................

45 

2 .880 

End guard time: end function (ground)

................

3 .100 

1

The previous event time slot ends at this time. 

T

ABLE

7b—A

UXILIARY

D

ATA

F

UNCTION

T

IMING

— 

(D

IGITAL

) 

Event 

Event time slot 

begins at: 

15.625 

kHz clock 

pulse 

(number) 

Time 

(milli-

sec-

onds) 

Preamble ...............................................

0 

0 

Address transmission (bits I

13

–I

20

) ........

25 

1 .600 

Data transmission: (bits I

21

–I

69

) .............

33 

2 .112 

T

ABLE

7b—A

UXILIARY

D

ATA

F

UNCTION

T

IMING

— 

(D

IGITAL

)—Continued 

Event 

Event time slot 

begins at: 

15.625 

kHz clock 

pulse 

(number) 

Time 

(milli-

sec-

onds) 

Parity transmission (bits I

70

–I

76

) ............

82 

5 .248 

End function (airborne) ..........................

89 

5 .696 

End guard time; end function (ground)

................

5 .900 

T

ABLE

7c—A

UXILIARY

D

ATA

F

UNCTION

T

IMING

— 

(A

LPHANUMERIC

) 

Event 

Event time slot 

begins at: 

15.615 

kHz clock 

pulse 

(number) 

Time 

(milli-

sec-

onds) 

Preamble ...............................................

0 

0 

Address transmission (bits I

13

–I

20

) ........

25 

1 .600 

Data transmission: (bits I

21

–I

76

..............

33 

2 .112 

End function (airborne) ..........................

89 

5 .696 

870 

14 CFR Ch. I (1–1–24 Edition) 

§ 171.311 

T

ABLE

7c—A

UXILIARY

D

ATA

F

UNCTION

T

IMING

— 

(A

LPHANUMERIC

)—Continued 

Event 

Event time slot 

begins at: 

15.615 

kHz clock 

pulse 

(number) 

Time 

(milli-

sec-

onds) 

End guard time; (end function ground)

................

5 .900 

(i) 

Preamble. 

Must be in accordance 

with requirements of § 171.311(i)(1). 

(ii) 

Data transmissions. 

Basic data 

must be transmitted using DPSK mod-
ulation. The content and repetition 
rate of each basic data word must be in 
accordance with Table 8a. For data 
containing digital information, binary 
number 1 must represent the lower 
range limit with increments in binary 
steps to the upper range limit shown in 
Table 8a. Data containing digital infor-
mation shall be transmitted with the 
least significant bit first. 

(j) 

Basic Data word requirements. 

Basic 

Data shall consist of the items speci-
fied in Table 8a. Basic Data word con-
tents shall be defined as follows: 

(1) 

Approach azimuth to threshold dis-

tance 

shall represent the minimum dis-

tance between the Approach Azimuth 
antenna phase center and the vertical 
plane perpendicular to the centerline 
which contains the landing threshold. 

(2) 

Approach azimuth proportional cov-

erage limit 

shall represent the limit of 

the sector in which proportional ap-
proach azimuth guidance is trans-
mitted. 

(3) 

Clearance signal type 

shall rep-

resent the type of clearance when used. 
Pulse clearance is that which is in ac-
cordance with § 171.311 (i) (2) (iv). Scan-
ning Beam (SB) clearance indicates 
that the proportional guidance sector 
is limited by the proportional coverage 
limits set in basic data. 

871 

Federal Aviation Administration, DOT 

§ 171.311 

872 

14 CFR Ch. I (1–1–24 Edition) 

§ 171.311 

T

ABLE

8a—B

ASIC

D

ATA

W

ORDS

 

Data bit 

# 

Data item definition 

LSB 

value 

Data bit 

value 

Basic Data Word No. 1 

1 

Preamble .........................

N/A  1 

2 

..........................................

............

1 

3 

..........................................

............

1 

4 

..........................................

............

0 

5 

..........................................

............

1 

6 

..........................................

............

0 

7 

..........................................

............

1 

8 

..........................................

............

0 

9 

..........................................

............

1 

10 

..........................................

............

0 

11 

..........................................

............

0 

12 

..........................................

............

0 

13 

Approach azimuth to 

threshold distance 
(Om

¥

630m).

100m  100m 

14 

..........................................

............

200m 

15 

..........................................

............

400m 

16 

..........................................

............

800m 

17 

..........................................

............

1600m 

18 

..........................................

............

3200m 

19 

Approach azimuth propor-

tional coverage limit 
(negative limit) (0

° 

to 

¥

62

°

).

2

° 

¥

2

° 

20 

..........................................

............

¥

4

° 

21 

..........................................

............

¥

8

° 

22 

..........................................

............

¥

16

° 

23 

..........................................

............

¥

32

° 

24 

Approach azimuth propor-

tional coverage limit 
(positive limit) (0

° 

to + 

62

°

).

2

° 

2

° 

25 

..........................................

............

4

° 

26 

..........................................

............

8

° 

27 

..........................................

............

16

° 

28 

..........................................

............

32

° 

29 

Clearance signal type ......

N/A  0 = pulse; 1 

= SB 

30 

Spare ...............................

............

Transmit 

zero 

31 

Parity: (13 + 14 + 15. . . 

+ 30 + 31 = odd).

N/A  N/A 

32 

Parity: (14 + 16 + 18. . . 

+ 30 + 32 = odd).

N/A  N/A 

Note 1: Transmit throughout the Approach Azimuth guidance 

sector at intervals of 1.0 seconds or less. 

Note 2: The all zero state of the data field represents the 

lower limit of the absolute value of the coded parameter 
unless otherwise noted. 

Basic Data Word No. 2 

1 

Preamble .........................

N/A  1 

2 

..........................................

............

1 

3 

..........................................

............

1 

4 

..........................................

............

0 

5 

..........................................

............

1 

6 

..........................................

............

0 

7 

..........................................

............

1 

8 

..........................................

............

1 

9 

..........................................

............

1 

10 

..........................................

............

1 

11 

..........................................

............

0 

12 

..........................................

............

0 

13 

Minimum glide path (2.0

° 

to 14.7

°

).

0.1

° 

0.1

° 

14 

..........................................

............

0.2

° 

15 

..........................................

............

0.4

° 

16 

..........................................

............

0.8

° 

T

ABLE

8a—B

ASIC

D

ATA

W

ORDS

—Continued 

Data bit 

# 

Data item definition 

LSB 

value 

Data bit 

value 

17 

..........................................

............

1.6

° 

18 

..........................................

............

3.2

° 

19 

..........................................

............

6.4

° 

20 

Back azimuth status ........

............

see note 4 

21 

DME status ......................

............

see note 6 

22 

..........................................

............

23 

Approach azimuth status

............

see note 4 

24 

Approach azimuth status

............

see note 4 

25 

Spare ...............................

............

Transmit 

zero 

26 

......do ...............................

............

Do. 

27 

......do ...............................

............

Do. 

28 

......do ...............................

............

Do. 

29 

......do ...............................

............

Do. 

30 

......do ...............................

............

Do. 

31 

Parity: (13 + 14 + 15. . . 

+ 30 + 31) = odd).

N/A  N/A 

32 

Parity: (14 + 16 + 18. . . 

+ 30 + 32 = odd).

N/A  N/A 

Note 1: Transmit throughout the Approach Azimuth guidance 

sector at intervals of 0.16 seconds or less. 

Note 2: The all zero state of the data field represents the 

lower limit of the absolute range of the coded parameter 
unless otherwise noted. 

Basic Data Word No. 3 

1 

Preamble .........................

N/A  1 

2 

..........................................

............

1 

3 

..........................................

............

1 

4 

..........................................

............

0 

5 

..........................................

............

1 

6 

..........................................

............

1 

7 

..........................................

............

0 

8 

..........................................

............

1 

9 

..........................................

............

0 

10 

..........................................

............

0 

11 

..........................................

............

0 

12 

..........................................

............

0 

13 

Approach azimuth beam-

width (0.5

°

¥

4.0

°

) See 

note 7.

0.5

° 

0.5

° 

14 

..........................................

............

1.0

° 

15 

..........................................

............

2.0

° 

16 

Approach elevation 

beamwidth (0.5

° 

to 

2.5

°

) See note 7.

0.5

° 

0.5

° 

17 

..........................................

............

1.0

° 

18 

Note: values greater than 

2.5

° 

are invalid.

............

2.0

° 

19 

DME distance (Om to 

6387.5m.

12.5m  12.5m 

20 

..........................................

............

25.0m 

21 

..........................................

............

50.0m 

22 

..........................................

............

100.0m 

23 

..........................................

............

200.0m 

24 

..........................................

............

400.0m 

25 

..........................................

............

800.0m 

26 

..........................................

............

1600.0m 

27 

..........................................

............

3200.0m 

28 

Spare ...............................

............

Transmit 

zero 

29 

......do ...............................

............

Do. 

30 

......do ...............................

............

Do. 

31 

Parity: (13 + 14 + 15. . . 

+ 30 + 31 = odd).

............

32 

Parity: (14 + 16 + 18. . . 

+ 30 + 32 = odd).

N/A  N/A 

873 

Federal Aviation Administration, DOT 

§ 171.311 

T

ABLE

8a—B

ASIC

D

ATA

W

ORDS

—Continued 

Data bit 

# 

Data item definition 

LSB 

value 

Data bit 

value 

Note 1: Transmit throughout the Approach Azimuth guidance 

sector at intervals of 1.0 seconds or less. 

Note 2: The all zero state of the data field represents the 

lower limit of the absolute range of the coded parameter 
unless otherwise noted. 

Basic Data Word No. 4 

1 

Preamble .........................

N/A  1 

2 

..........................................

............

1 

3 

..........................................

............

1 

4 

..........................................

............

0 

5 

..........................................

............

1 

6 

..........................................

............

1 

7 

..........................................

............

0 

8 

..........................................

............

0 

9 

..........................................

............

0 

10 

..........................................

............

1 

11 

..........................................

............

0 

12 

..........................................

............

0 

13 

Approach azimuth mag-

netic orientation (0

° 

to 

359

°

).

1

° 

1

° 

14 

..........................................

............

2

° 

15 

..........................................

............

4

° 

16 

..........................................

............

8

° 

17 

..........................................

............

16

° 

18 

..........................................

............

32

° 

19 

..........................................

............

64

° 

20 

..........................................

............

128

° 

21 

..........................................

............

256

° 

22 

Back azimuth magnetic 

orientation (0

° 

to 359

°

).

1

° 

1

° 

23 

..........................................

............

2

° 

24 

..........................................

............

4

° 

25 

..........................................

............

8

° 

26 

..........................................

............

16

° 

27 

..........................................

............

32

° 

28 

..........................................

............

64

° 

29 

..........................................

............

128

° 

30 

..........................................

............

256

° 

31 

Parity: (13 + 14 + 15. . . 

+ 30 + 31 = odd).

N/A  N/A 

32 

Parity: (14 + 16 + 18. . . 

+ 30 + 32 = odd).

N/A  N/A 

Note 1: Transmit at intervals of 1.0 second or less through-

out the Approach Azimuth guidance sector, except when 
Back Azimuth guidance is provided. See Note 8. 

Note 2: The all zero state of the data field represents the 

lower limit of the absolute range of the coded parameter 
unless otherwise noted. 

Basic Data Word No. 5 

1 

Preamble .........................

N/A  1 

2 

..........................................

............

1 

3 

..........................................

............

1 

4 

..........................................

............

0 

5 

..........................................

............

1 

6 

..........................................

............

1 

7 

..........................................

............

1 

8 

..........................................

............

0 

9 

..........................................

............

1 

10 

..........................................

............

1 

11 

..........................................

............

0 

12 

..........................................

............

0 

13 

Back azimuth proportional 

coverage negative limit 
(0

° 

to 

¥

42

°

).

2

° 

¥

2

° 

14 

..........................................

............

¥

4

° 

15 

..........................................

............

¥

8

° 

T

ABLE

8a—B

ASIC

D

ATA

W

ORDS

—Continued 

Data bit 

# 

Data item definition 

LSB 

value 

Data bit 

value 

16 

..........................................

............

¥

16

° 

17 

..........................................

............

¥

32

° 

18 

Back azimuth proportional 

coverage positive limit 
(0

° 

to + 42

°

).

2

° 

2

° 

19 

..........................................

............

4

° 

20 

..........................................

............

8

° 

21 

..........................................

............

16

° 

22 

..........................................

............

32

° 

23 

Back azimuth beamwidth 

(0.5

° 

to 4.0

°

) See note 

7.

0.5

° 

0.5

° 

24 

..........................................

............

1.0

° 

25 

..........................................

............

2.0

° 

26 

Back azimuth status ........

............

See Note 10 

27 

......do ...............................

............

Do. 

28 

......do ...............................

............

Do. 

29 

......do ...............................

............

Do. 

30 

......do ...............................

............

Do. 

31 

Parity: (13 + 14 + 15. . . 

+ 30 + 31 = odd).

N/A  N/A 

32 

Parity: (14 + 16 + 18. . . 

+ 30 + 32 = odd).

N/A  N/A 

Note 1: Transmit only when Back Azimuth guidance is pro-

vided. See note 9. 

Note 2: The all zero state of the data filed represents the 

lower limit of the absolute range of the coded parameter 
unless otherwise noted. 

Basic Data Word No. 6 

1 

Preamble .........................

N/A  1 

2 

..........................................

............

1 

3 

..........................................

............

1 

4 

..........................................

............

0 

5 

..........................................

............

1 

6 

..........................................

............

0 

7 

..........................................

............

0 

8 

..........................................

............

0 

9 

..........................................

............

1 

10 

..........................................

............

1 

11 

..........................................

............

0 

12 

..........................................

............

1 

(13– 
30) 

MLS ground equipment 

identification (Note 3).

............

13 

Character 2 ......................

N/A  B1 

14 

..........................................

............

B2 

15 

..........................................

............

B3 

16 

..........................................

............

B4 

17 

..........................................

............

B5 

18 

..........................................

............

B6 

19 

Character 3 ......................

N/A  B1 

20 

..........................................

............

B2 

21 

..........................................

............

B3 

22 

..........................................

............

B4 

23 

..........................................

............

B5 

24 

..........................................

............

B6 

25 

Character 4 ......................

N/A  B1 

26 

..........................................

............

B2 

27 

..........................................

............

B3 

28 

..........................................

............

B4 

29 

..........................................

............

B5 

30 

..........................................

............

B6 

31 

Parity: (13 + 14 + 15. . . 

+ 30 + 31 = odd).

N/A  N/A 

32 

Parity: (14 + 16 + 18. . . 

+ 30 + 32 = odd).

N/A  N/A 

Note 1: Transmit at intervals of 1.0 second or less through-

out the Approach Azimuth guidance sector, except when Back 
Azimuth guidance is provided. See note 8. 

874 

14 CFR Ch. I (1–1–24 Edition) 

§ 171.311 

Note 3: Characters are encoded using the International Al-

phabet Number 5, (IA–5): 

Note 4: Coding for status bit: 
0 = Function not radiated, or radiated in test mode (not reli-

able for navigation). 

1 = Function radiated in normal mode (for Back Azimuth, 

this also indicates that a Back Azimuth transmission follows). 

Note 5: Date items which are not applicable to a particular 

ground equipment shall be transmitted as all zeros. 

Note 6: Coding for status bits: 

I

21

 

I

22

 

0 

0  DME transponder inoperative or not available. 

1 

0  Only IA mode or DME/N available. 

0 

0  FA mode, Standard 1, available. 

1 

1  FA mode, Standard 2, available. 

Note 7: The value coded shall be the actual beamwidth (as 

defined in § 171.311 (j)(9) rounded to the nearest 0.5 degree. 

Note 8: When back Azimuth guidance is provided, Data 

Words 4 and 6 shall be transmitted at intervals of 1.33 sec-
onds or less throughout the Approach Azimuth coverage and 
4 seconds or less throughout the Back Azimuth coverage. 

Note 9: When Back Azimuth guidance is provided, Data 

Word 5 shall be transmitted at an interval of 1.33 seconds or 
less throughout the Back Azimuth coverage sector and 4 sec-
onds or less throughout the Approach Azimuth coverage sec-
tor. 

Note 10: Coding for status bit: 
0 = Function not radiated, or radiated in test mode (not reli-

able for navigation). 

1 = Function radiated in normal mode. 

(4) 

Minimum glidepath 

the lowest 

angle of descent along the zero degree 
azimuth that is consistent with pub-
lished approach procedures and obsta-
cle clearance criteria. 

(5) 

Back azimuth status 

shall represent 

the operational status of the Back Azi-
muth equipment. 

(6) 

DME status 

shall represent the 

operational status of the DME equip-
ment. 

(7) 

Approach azimuth status 

shall rep-

resent the operational status of the ap-
proach azimuth equipment. 

(8) 

Approach elevation status 

shall rep-

resent the operational status of the ap-
proach elevation equipment. 

(9) 

Beamwidth 

the width of the scan-

ning beam main lobe measured at the 

¥

3 dB points and defined in angular 

units on the antenna boresight, in the 
horizontal plane for the azimuth func-
tion and in the vertical plane for the 
elevation function. 

(10) 

DME distance 

shall represent the 

minimum distance between the DME 
antenna phase center and the vertical 
plane perpendicular to the runway cen-
terline which contains the MLS datum 
point. 

(11) 

Approach azimuth magnetic ori-

entation 

shall represent the angle meas-

ured in the horizontal plane clockwise 
from Magnetic North to the zero-de-
gree angle guidance radial originating 
from the approach azimuth antenna 
phase center. The vertex of the meas-

ured angle shall be at the approach azi-
muth antenna phase center. 

N

OTE

: For example, this data item would 

be encoded 090 for an approach azimuth an-
tenna serving runway 27 (assuming the mag-
netic heading is 270 degrees) when sited such 
that the zero degree radial is parallel to cen-
terline. 

(12) 

Back azimuth magnetic orientation 

shall represent the angle measured in 
the horizontal plane clockwise from 
Magnetic North to the zero-degree 
angle guidance radial originating from 
the Back Azimuth antenna. The vertex 
of the measured angle shall be at the 
Back Azimuth antenna phase center. 

N

OTE

: For example, this data item would 

be encoded 270 for a Back Azimuth Antenna 
serving runway 27 (assuming the magnetic 
heading is 270 degrees) when sited such that 
the zero degree radial is parallel to center-
line. 

(13) 

Back azimuth proportional cov-

erage limit 

shall represent the limit of 

the sector in which proportional back 
azimuth guidance is transmitted. 

(14) 

MLS ground equipment identifica-

tion 

shall represent the last three char-

acters of the system identification 
specified in § 171.311(i)(2). The char-
acters shall be encoded in accordance 
with International Alphabet No. 5 (IA– 
5) using bits b

1

through b

6

. 

N

OTE

: Bit b

7

of this code may be recon-

structed in the airborne receiver by taking 
the complement of bit b

6

. 

(k) 

Residual radiation. 

The residual 

radiation of a transmitter associated 
with an MLS function during time in-
tervals when it should not be transmit-
ting shall not adversely affect the re-
ception of any other function. The re-
sidual radiation of an MLS function at 
times when another function is radi-
ating shall be at least 70 dB below the 
level provided when transmitting. 

(l) 

Symmetrical scanning. 

The TO and 

FRO scan transmissions shall be sym-
metrically disposed about the mid-scan 
point listed in Tables 4a, 4b and 5. The 
mid-scan point and the center of the 
time interval between the TO and FRO 
scan shall coincide with a tolerance of 
plus or minus 10 microseconds. 

(m) 

Auxiliary data

—(1) 

Addresses. 

Three function identification codes are 
reserved to indicate transmission of 
Auxiliary Data A, Auxiliary Data B, 

875 

Federal Aviation Administration, DOT 

§ 171.313 

and Auxiliary Data C. Auxiliary Data 
A contents are specified below, Auxil-
iary Data B contents are reserved for 
future use, and Auxiliary Data C con-
tents are reserved for national use. The 
address codes of the auxiliary data 
words shall be as shown in Table 8b. 

(2) 

Organization and timing. 

The orga-

nization and timing of digital auxiliary 
data must be as specified in Table 7b. 
Data containing digital information 
must be transmitted with the least sig-
nificant bit first. Alphanumeric data 
characters must be encoded in accord-
ance with the 7-unit code character set 
as defined by the American National 
Standard Code for Information Inter-
change (ASCII). An even parity bit is 
added to each character. Alphanumeric 
data must be transmitted in the order 
in which they are to be read. The serial 
transmission of a character must be 
with the lower order bit transmitted 
first and the parity bit transmitted 
last. The timing for alphanumeric aux-
iliary data must be as shown in Table 
7c. 

(3) 

Auxiliary Data A content: 

The data 

items specified in Table 8c are defined 
as follows: 

(i) 

Approach azimuth antenna offset 

shall represent the minimum distance 
between the Approach Azimuth an-
tenna phase center and the vertical 
plane containing the runway center-
line. 

(ii) 

Approach azimuth to MLS datum 

point distance 

shall represent the min-

imum distance between the Approach 
Azimuth antenna phase center and the 
vertical plane perpendicular to the cen-
terline which contains the MLS datum 
point. 

(iii) 

Approach azimuth alignment with 

runway centerline 

shall represent the 

minimum angle between the approach 
azimuth antenna zero-degree guidance 
plane and the runway certerline. 

(iv) 

Approach azimuth antenna coordi-

nate system 

shall represent the coordi-

nate system (planar or conical) of the 
angle data transmitted by the ap-
proach azimuth antenna. 

(v) 

Approach elevation antenna offset 

shall represent the minimum distance 
between the elevation antenna phase 
center and the vertical plane con-
taining the runway centerline. 

(vi) 

MLS datum point to threshold dis-

tance 

shall represent the distance 

measured along the runway centerline 
from the MLS datum point to the run-
way threshold. 

(vii) 

Approach elevation antenna 

height 

shall represent the height of the 

elevation antenna phase center rel-
ative to the height of the MLS datum 
point. 

(viii) 

DME offset 

shall represent the 

minimum distance between the DME 
antenna phase center and the vertical 
plane containing the runway center-
line. 

(ix) 

DME to MLS datum point distance 

shall represent the minimum distance 
between the DME antenna phase center 
and the vertical plane perpendicular to 
the centerline which contains the MLS 
datum point. 

(x) 

Back azimuth antenna offset 

shall 

represent the minimum distance be-
tween the back azimuth antenna phase 
center and the vertical plane con-
taining the runway centerline. 

(xi) 

Back azimuth to MLS datum point 

distance 

shall represent the minimum 

distance between the Back Azimuth 
antenna and the vertical plane perpen-
dicular to the centerline which con-
tains the MLS datum point. 

(xii) 

Back azimuth antenna alignment 

with runway centerline 

shall represent 

the minimum angle between the back 
azimuth antenna zero-degree guidance 
plane and the runway centerline. 

§ 171.313 Azimuth performance re-

quirements. 

This section prescribes the perform-

ance requirements for the azimuth 
equipment of the MLS as follows: 

(a) 

Approach azimuth coverage require-

ments. 

The approach azimuth equip-

ment must provide guidance informa-
tion in at least the following volume of 
space (see Figure 9): 

T

ABLE

8b—A

UXILIARY

D

ATA

W

ORD

A

DDRESS

 

C

ODES

 

No. 

I

13

 

I

14

 

I

15

 

I

16

 

I

17

 

I

18

 

I

19

 

I

20

 

1. 

0 

0 

0 

0 

0 

1 

1 

1 

2. 

0 

0 

0 

0 

1 

0 

1 

0 

3. 

0 

0 

0 

0 

1 

1 

0 

1 

4. 

0 

0 

0 

1 

0 

0 

1 

1 

5. 

0 

0 

0 

1 

0 

1 

0 

0 

6. 

0 

0 

0 

1 

1 

0 

0 

1 

7. 

0 

0 

0 

1 

1 

1 

1 

0 

8. 

0 

0 

1 

0 

0 

0 

1 

0