RADAR TEST SET
THEORY OF OPERATION - Continued
The transponder correctly observed the Test Set's interrogation and replied, but the Test Set observed
excessive signal power from other than the direction the Test Set was aimed. This could be due to
reflection of the transponder signal, or due to a reply from a second transponder not within the Test
Set's target area. The Test Set will indicate this by displaying the message "FAULT=ANGLE".
If the message "FAULT=ANGLE" occurs, the Test Set operator should change position relative to the aircraft
antenna, or attempt to move the location of reflective objects near the aircraft. If a fail with "no replies" or "low
replies" occurs, the operator should change position and / or move closer to the aircraft antenna to determine
if there is multipath or if there was a problem with the transponder being tested.
MTL MEASUREMENT METHOD
Minimum Triggering Level (MTL) is normally defined as the transponder's input signal level required to
achieve 90% reply rate. When testing in an over-the-air test environment, there is the possibility of replies lost
to interference or ambient noise, which can produce MTL measurement errors. Minimizing the number of
interrogations during the test is desired to limit Mode 4 code exposure and reduce interference to other
systems. With a limited number of samples, missing one pulse makes a significant difference in the reply
percentage as the reply rate is effectively averaged by the number of samples.
In the presence of interference, the test signal received by the transponder may need to be significantly above
MTL to keep the replies lost to interference below 10%. The solution to this situation is to reduce the reply
threshold to around 80%. This allows twice as many errors, reducing the effects of interference and the
limited averaging. In the absence of interference, the measured MTL will be slightly low, but typical
transponders rapidly transition from 90% to 10% replies in a 1 to 2 dB window.