background image

AIM 

6/17/21 

removing the 

NA

 (Alternate Minimums Not 

Authorized) symbol from select RNAV (GPS) and 
GPS approach procedures so they may be used by 
approach approved WAAS receivers at alternate 
airports. Some approach procedures will still require 
the 

NA

 for other reasons, such as no weather 

reporting, so it cannot be removed from all 
procedures. Since every procedure must be individu-
ally evaluated, removal of the 

NA

 from RNAV 

(GPS) and GPS procedures will take some time. 

NOTE

 

Properly trained and approved, as required, TSO-C145() 
and TSO-C146() equipped users (WAAS users) with and 
using approved baro-VNAV equipment  may plan for 
LNAV/VNAV DA at an alternate airport. Specifically 
authorized WAAS users with and using approved 
baro-VNAV equipment may also plan for RNP 0.3 DA at the 
alternate airport as long as the pilot has verified RNP 
availability through an approved prediction program. 

d.  Flying Procedures with WAAS 

1. 

WAAS receivers support all basic GPS 

approach functions and provide additional capabilit-
ies. One of the major improvements is the ability to 
generate glide path guidance, independent of ground 
equipment or barometric aiding. This eliminates 
several problems such as hot and cold temperature 
effects, incorrect altimeter setting, or lack of a local 
altimeter source. It also allows approach procedures 
to be built without the cost of installing ground 
stations at each airport or runway. Some approach 
certified receivers may only generate a glide path 
with performance similar to Baro

VNAV and are 

only approved to fly the LNAV/VNAV line of minima 
on the RNAV (GPS) approach charts. Receivers with 
additional capability (including faster update rates 
and smaller integrity limits) are approved to fly the 
LPV line of minima. The lateral integrity changes 
dramatically from the 0.3 NM (556 meter) limit for 
GPS, LNAV, and LNAV/VNAV approach mode, to 
40 meters for LPV. It also provides vertical integrity 
monitoring, which bounds the vertical error to 50 
meters for LNAV/VNAV and LPVs with minima of 
250’ or above, and bounds the vertical error to 35 
meters for LPVs with minima below 250’. 

2. 

When an approach procedure is selected and 

active, the receiver will notify the pilot of the most 
accurate level of service supported by the combina-
tion of the WAAS signal, the receiver, and the 
selected approach, using the naming conventions on 

the minima lines of the selected approach procedure. 
For example, if an approach is published with LPV 
minima and the receiver is only certified for 
LNAV/VNAV, the equipment would indicate 
“LNAV/VNAV available,” even though the WAAS 
signal would support LPV. If flying an existing 
LNAV/VNAV procedure with no LPV minima, the 
receiver will notify the pilot “LNAV/VNAV 
available,” even if the receiver is certified for LPV 
and the signal supports LPV. If the signal does not 
support vertical guidance on procedures with LPV 
and/or LNAV/VNAV minima, the receiver annunci-
ation will read “LNAV available.” On lateral only 
procedures with LP and LNAV minima the receiver 
will indicate “LP available” or “LNAV available” 
based on the level of lateral service available. Once 
the level of service notification has been given, the 
receiver will operate in this mode for the duration of 
the approach procedure, unless that level of service 
becomes unavailable. The receiver cannot change 
back to a more accurate level of service until the next 
time an approach is activated. 

NOTE

 

Receivers do not “fail down” to lower levels of service 
once the approach has been activated. If only the 
vertical off flag appears, the pilot may elect to use the 
LNAV minima if the rules under which the flight is 
operating allow changing the type of approach being flown 
after commencing the procedure. If the lateral integrity 
limit is exceeded on an LP approach, a missed approach 
will be necessary since there is no way to reset the lateral 
alarm limit while the approach is active. 

3. 

Another additional feature of WAAS receiv-

ers is the ability to exclude a bad GPS signal and 
continue operating normally. This is normally 
accomplished by the WAAS correction information. 
Outside WAAS coverage or when WAAS is not 
available, it is accomplished through a receiver 
algorithm called FDE. In most cases this operation 
will be invisible to the pilot since the receiver will 
continue to operate with other available satellites 
after excluding the “bad” signal. This capability 
increases the reliability of navigation. 

4. 

Both lateral and vertical scaling for the 

LNAV/VNAV and LPV approach procedures are 
different than the linear scaling of basic GPS. When 
the complete published procedure is flown, 

±

1 NM 

linear scaling is provided until two (2) NM prior to the 
FAF, where the sensitivity increases to be similar to 
the angular scaling of an ILS. There are two differ-
ences in the WAAS scaling and ILS:  1) on long final 

1

1

36 

Navigation Aids