four times that the VOR or localizer coded identification is transmitted. When either the VOR or the DME is
inoperative, it is important to recognize which identifier is retained for the operative facility. A single coded
identification with a repetition interval of approximately 30 seconds indicates that the DME is operative.
g. Aircraft equipment which provides for automatic DME selection assures reception of azimuth and distance
information from a common source when designated VOR/DME, VORTAC and ILS/DME navigation facilities
are selected. Pilots are cautioned to disregard any distance displays from automatically selected DME equipment
when VOR or ILS facilities, which do not have the DME feature installed, are being used for position
determination.
1-1-8. NAVAID Service Volumes
a. The FAA publishes Standard Service Volumes (SSVs) for most NAVAIDs. The SSV is a three-dimensional
volume within which the FAA ensures that a signal can be received with adequate signal strength and course
quality, and is free from interference from other NAVAIDs on similar frequencies (e.g., co-channel or
adjacent-channel interference). However, the SSV signal protection does not include potential blockage from
terrain or obstructions. The SSV is principally intended for off-route navigation, such as proceeding direct to
or from a VOR when not on a published instrument procedure or route. Navigation on published instrument
procedures (e.g., approaches or departures) or routes (e.g., Victor routes) may use NAVAIDs outside of the SSV,
when Extended Service Volume (ESV) is approved, since adequate signal strength, course quality, and freedom
from interference are verified by the FAA prior to the publishing of the instrument procedure or route.
NOTE-
A conical area directly above the NAVAID is generally not usable for navigation.

b. A NAVAID will have service volume restrictions if it does not conform to signal strength and course quality
standards throughout the published SSV. Service volume restrictions are first published in Notices to Air
Missions (NOTAMs) and then with the alphabetical listing of the NAVAIDs in the Chart Supplement. Service
volume restrictions do not generally apply to published instrument procedures or routes unless published in
NOTAMs for the affected instrument procedure or route.
c. VOR/DME/TACAN Standard Service Volumes (SSV).
1. The three original SSVs are shown in FIG 1-1-1 and are designated with three classes of NAVAIDs:
Terminal (T), Low (L), and High (H). The usable distance of the NAVAID depends on the altitude Above the
Transmitter Height (ATH) for each class. The lower edge of the usable distance when below 1,000 feet ATH is
shown in FIG 1-1-2 for Terminal NAVAIDs and in FIG 1-1-3 for Low and High NAVAIDs.

1-1-6

Navigation Aids