THE LAST CLUE
If you were heading towards someone on the ground who was using a PRC-112 and you had the requisite on board equipment (ARS6) you would not only get both range and bearing to him but you could talk to him – and he could talk you in. This would be very handy so you could be confident that you were heading for the right spot, etc – presumably it would normally be the handling pilot (the driver) who would be talking directly to the person on the ground with the PRC-112.
The on board equipment interfaces with the aircraft’s voice intercom system.
To talk to the PRC-112, the handling pilot would, I believe, select the emergency position on his intercom.
In this case, the handling pilot (Flt Lt Cook) had his intercom set to emergency.
It is surely understandable that a digital readout of range provided by an intrinsically accurate system together with a confident voice giving encouragement (saying something like “I can hear you approaching – you’re doing just fine …”) could outweigh one’s visual judgment when the person on the ground was not in the expected/ planned position.
Far from being in any way careless (nor indeed negligent) the crew were, IMHO, showing caution/ apprehension/ prudence in engaging in a cruise climb that would given them some extra height (as opposed to aborting the exercise) and, had they actually been the bit further out they thought they were, would have put them high enough at the turning point/ waypoint/ RV (at the planned position of the PRC-112) to get a second opinion from the Mac TACAN/ DME (to which the TACAN CU was set) and so save something from a test if conditions on the ground would have prevented visual confirmation of the performance – and perhaps confirm their reservations as to the actual position of the ground equipment.