For the bench technician

How good is this transponder conversion the FAA won't let us have?
Because it NEVER has to be adjusted (no more reason for biennials).
Because it gives us plane-to-plane collision avoidance, for free.

B. Keith Peshak
keith.peshak@gtwn.net

The following is the final test observations taken during the construction of the Monarch-Air AIS-P conversion of an AT-150 S/N 26537 obtained after factory overhaul from Narco Avionics.

The only adjustment remaining in the transponder is the 9.4 Vdc power supply voltage linear regulator (which could be replaced by one standard tolerance and one selected resistors, which would eliminate ALL adjustments).

Transponder current demand has been reduced to about 750 mA from the 14 Vdc power supply (10.5 watts) at idle! GPS current demand is 290 mA. With the transponder operating in AIS-P only, the total current demand is 1100 mA. This goes up to 1240 mA at 100 ppm ATCRBS reply rate.

sensitivity from -71 dbm to 0 dbm
P2 suppression from -71 dbm to -3 dbm

AOC sensitivity (this could be changed at will by changing one selected resistor either R431 or R432).

PPM	% reply
100-1000	100
1100	92
1200	84
1300	78
1400	73
1500	70
1600	66
1700	62
1800	60
1900	58

Transponder ATCRBS P3-F1 separation

Closer to the 3.00 microsecond "wait" spec than any transponder I have ever had on the bench, during which any received P4 pulse is ignored (said "normal" transponder design is not correctable, and does not necessarily have the "ignore the P4" feature, which is required to operate correctly with TCAS)!

Transponder ATCRBS pulse and separation

450 nanoseconds out of 1450 nanoseconds is the fed ATCRBS spec, they just don't train us where to measure it (peak, -1db, -3db, 50% point, ...).
Closer than monostables can be adjusted.

Transponder AIS-P pulse.
500 nanoseconds is the fed mode S spec, they just don't say where to measure it (peak, -1db, -3db, 50% point, ...).

Message preamble

The pulse train is intended to be 1/2us pulse, 1/2us gap, 1/2us pulse, 2us gap, 1/2us pulse, 1/2us gap, 1/2us pulse, 3us gap (if the MSB of the DF# is a logic one [it is]) or 3 1/2us gap (if the MSB of the DF# is a logic zero [it isn't]).

The first part of the Mode S AIS-P message

The DF# is pinout wired as 11111. A logic 1 is a 10 pair (bucket), a logic 0 is a 01 pair (bucket). The first five bits are the DF# MSB to LSB are 11111, so 1010101010. Following is the data of the message.

A better look at each pulse in the data stream

The long pulse should be 1.0us

The short pulse should be 0.5us

The long 1.0us gap and short 0.5us pulse and gap

Transponder ATCRBS RF F1-F2 pulse separation

Mode A reply

Mode C reply

Mode S AIS-P squitter

If the detector is operating under square law, that brings us down 0.79 db on the last pulse, reference the first pulse.