GTX327 "R" Reply indication (question)

My GTX-327 transponder does not seem to show the "R" reply symbol often (something like once every few minutes). I though I had a problem with my antenna/wiring, and double checked everything. Borrowed a friend's transponder, same thing.

Tonight I went out and asked ATC to give me a transponder check. He gave me my own VFR code, and flew for about a 1/2 hour. In that time, I caught a glimpse of the "R" reply symbol maybe only twice. ATC subsequently told me they tracked me all along.

My question is this: Is the "R" indication not equivalent to the interrogation/reply light on the tube transponders? Should I not be seeing this every few seconds, as the radar antenna sweeps me?

Given that a second unit behaves the same way in my setup, as well as in my friend's plane, do other people see this same behaviour as described above? Inquiring mind wants to know.

Alfio,

My GTX327 functions the same. Mater of fact I thought it was broken because I never really noticed the indicator coming on. I really have to stare at the unit for some time to catch it coming on. I also checked mine with a local tower (40 miles away) and they said the XPNDR was functioning properly.

I thought the R was TCAS interrogation from another plane, not ATC. No?

N130WN said:

I thought the R was TCAS interrogation from another plane, not ATC. No?

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According to the GTX-327 Pilot's Guide:
http://befc-wichita.org/Aircraft/Pilot_Guides_and_Manuals/GTX%20327%20Transponder%20Pilot%20Guide.pdf

The "R" is an indication that the transponder is replying to interrogations (mode A and C). There is no mention of TCAS. I would assume TCAS is just spoofing the interrogation signal of the secondary radar to get its reply.

Glad to hear others are seeing the same behavior as we are. Software bug?

Txponder

Lycosaurus said:

My GTX-327 transponder does not seem to show the "R" reply symbol often (something like once every few minutes). I though I had a problem with my antenna/wiring, and double checked everything. Borrowed a friend's transponder, same thing.

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Alfio, I see the same thing with mine. The return symbol flashes only very sporadically.......
(off the topic)..Last fall ATC could not get anything other than a primary target on me.....into the shop and started to trouble shoot. Transponder put on the test bench and checked out fine....all coax was good.......changed antanae and went flying and the unit worked fine. I am still not sure that was the problem though..... I have the 90deg soldered joint where the coax enters the transponder and think that it may be that....if it acts up again I will change to different connection (It is really a chinzy connection which could be subject to vibration)......anyway till it quits again I'll keep my fingers crossed....

Marc DeGirolamo said:

..... I have the 90deg soldered joint where the coax enters the transponder and ....

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My tray came with a blind-mate to BNC connector. Did not have to solder anything. Looks like there are two options out there, solder or BNC. I got my -327 from Steinair. You may be able to just get the connector, and then crimp a BNC on your cable.

I went flying yesterday afternoon, and climbed to 8500 ft to do some leaning tests. Saw the transponder reply indication quite active up there (say every couple or few seconds). Not very consistent though. The flight area was about 25 miles from the airport. As I descended the reply indications diminished. Hmmm... Maybe I was getting more interrogations from TCAS up there (or maybe too many interrogations from TCAS when I was at a lower altitude, overloading the housekeeping uP). I'll have to follow this more closely next time.

Called Garmin

Called Garmin Tech. Support this morning, and related my findings. He told me that Garmin is aware of this behavior and that it is normal. He mentioned that the R indication only shows up if the unit is interrogated at higher than a certain rate. He said that a normal secondary radar scan may not show an R indication.

While he did not have the technical knowledge of why this is, he promised to ask the design engineers and report back to me by email. I'll report back if I get an update.

Edit:

Well, as promised, Garmin's tech support replied to my request ... R ;-)

Alfio,

Very interesting information. I had no idea the XPNDR was being interrogated and replying at such a high rate. I assume at greater distances from the interrogation source and at lower altitudes, the XPNDR misses more of the interrogations than at a higher altitude therefore the respond indicator appears to be more erratic at lower altitudes, I also observed this behavior in my airplane. Thanks for following up on this and posting your findings.