Antenna height above average terrain
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In telecommunication, antenna height above average terrain is the antenna height above the average terrain elevations from 3.2 to 16 kilometers (2 to 10 miles) from the antenna for the eight directions spaced evenly for each 45° of azimuth starting with true north.
Note: In general, a different antenna height above average terrain will be determined in each direction from the antenna. The average of these eight heights is the antenna height above average terrain. In some cases, such as seashore, fewer than eight directions may be used.
Source: from Federal Standard 1037C and from MIL-STD-188
