DHAAT ( Direction Height Above Average Terrain ) is really a subset of the standard HAAT calculation. To be used correctly, one must know the elevation at the given location and the antenna height. DHAAT is part of the Short Spacing calculation.

"The directional height of the antenna above average terrain ( DHAAT ) is calculated from the average of the antenna heights above average terrain from 3 to 16 km ( 2 to 10 mi. ) from the proposed site along a radial extending in the direction of the existing station and the radials 15 degrees to either side of that radial. -- FCC 47 CFR 90.621"


HAAT ( Height Above Average Terrain ). See the FCC's definition of HAAT that discusses the prescribed technique for its calculation. In order to conduct this calucation a dataset of elevations ( e.g., USGS NED ) at specific latitude/longitude points is required as well as some basic ellipsoid calcuations to compute bearing and distance along a radial.

Short Spacing

Normally co-channel stations/transmitters operating in close proximity will experience material harmful interference from one another. This is addressed in bulk by the FCC where any two co-channel stations must be separated geographically by some minimum distance. The FCC also often perscribes a maximum distance ( or greater ) between any two co-channel stations where interference protection is not given.

For example, per FCC 47 CFR 90.621, some 800 MHz licensees that are co-channeled can be as close as 88 km to one another, but a specific set of criteria must be met. This criteria is call "Short Spacing" where the DHAAT and the power of transmitter is considered.

FCC Glossary of Terms

The ULS Glossary contains a list of terms commonly used in the Universal Licensing System (ULS) in regards to filing applications or reviewing licenses. Some formal "legal" definitions often used the FCC rules are described in 47 CFR 90.7.