Originally Posted by
agg_karan
My assumption has no technical backing but AID is always subtracted from the measured reading.
The AID 80ft compensation would not be an adjustment of the displayed value by 80ft.
The speed of the signal in the coaxial cable between the instrument and antenna is, depending on the type of cable used, 60-80% of the speed of the signal in air - a difference called the 'velocity factor.' In addition the signal has to travel the AID distance twice, outwards and inwards.
The 80ft AID means that the cable installed between the instruments and the antennas is always 80ft in length and never cut to length. The certification requirements for the cable would specify a velocity factor. The instrument therefore 'knows' to subtract (2 x known 80ft x known speed of light x known cable velocity factor) from the measured delay. I would guess the subtraction is done in terms of time as it would reduce software complexity and the number of code paths, especially if the same instrument could also output in meters.
So for an 80ft AID the actual displayed value would differ from the 'raw' value by 96-128ft, allowing for cable velocity factors of 60-80%.