Navigation Aids 1 − 1 − 6 tions 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 Airmen (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. 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. 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 depar- tures) 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. A conical area directly above the NAVAID is generally not usable for navigation. 8/15/19 3/15/07 12/31/20 AIM  7110.65R CHG 2 AIM NOTE b. A NAVAID will have service volume restric- c. VOR/DME/TACAN Standard Service Volumes (SSV). FIG 1 − 1 − 1 Original Standard Service Volumes