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Correlation between weekend flyers and Mode A transponder use

Doesn’t that mean level unverified ie they haven’t confirmed the level rather than really unknown?

I’ve often heard, “[…] indicating 2500ft, altitude unverified”. So I’ve always taken “level unknown” to mean that they don’t have any indication of the altitude.

EIKH Kilrush

do you mean targets without altitude information?

Yes; the TAS6xx boxes display such traffic.

Normally one does see altitude info

but sometimes not. Next time I fly I will get a pic.

On my system I don’t see squawks or aircraft regs. On a TAS 6xx box connected to an Avidyne MFD via RS232, one can see both and I have seen this, on a flight in a Cessna 400.

IIRC, “unverified” is the required ATC wording if the target is either secondary only OR has not confirmed his altitude to the ATC unit.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

not that we have many UK ATCOs writing here because of the Official Secrets Act

Are ATC procedures secret in the UK?

ESKC (Uppsala/Sundbro), Sweden

bookworm wrote:

To the best of my knowledge, all TCAS/TAS interrogations are Mode C. You have little need to know another aircraft’s squawk.

It makes sense that most TCAS/TAS would only (mostly) do Mode C interrogations, since you do not particularly care for an aircraft’s squawk.

Peter, could it be that neither Mode A interrogations nor Mode C interrogations work well for you?

To be able to detect traffic the TAS605 only needs to detect an interrogation reply. It doesn’t matter if it’s a Mode-A reply or Mode-C reply. My guess is that only the latency of the reply and the signal azimuth is needed to detect traffic. The actual data of the reply is not important. If your receiver/antenna is weak you might be able to detect traffic but fail to receive and decode complete Mode A and Mode C replies.

What I am saying is that just because your TAS detects traffic without any altitude information doesn’t necessarily imply that you properly receive Mode A replies. (If I understand correctly, you do not have any means of displaying the squawk of detected traffic. So you do not know that you detect squawk codes.)
And the reason that you see more altitude less traffic on weekends could then be that on weekends older aircraft are flying. And older transponders and antennas might mean that their transponder replies are weaker. Which in turn makes it difficult for your equipment to properly decode their replies.

I’ve found a good explanation of Mode A and Mode C here: http://www.airsport-corp.com/modec.htm

From that site I’ve copied an image that shows the difference between a Mode A interrogation and a Mode C interrogation.

And taken this interesting piece of information about the interrogation replies:

There is no difference between a Mode A and Mode C reply. If a Mode A interrogation is issued, a Mode A reply is expected, and the pulses will be decoded that way. Likewise, if a Mode C request is made, the data received from the aircraft will be calculated as altitude.

ESTL

A few of things:

1) I have never seen a target which suddenly lost or suddenly acquired the altitude readout. This suggests it isn’t some marginal issue in the TAS605 installation.

2) On a local low level (say below 3000ft) flight in my area, the Mode A targets are much more numerous at weekends as a % of transponding targets.

3) In the UK, above say 3000ft, and above cloud at any altitude, practically all traffic ever seen visually is transponding and is visible as Mode C (I can’t tell Mode S).

4) The only known defect in my installation is a very late warning from 12 o’clock – of the order of 1-2nm, against the advertised 15nm. This improves to c. 3-5nm against bizjets. I am not the only TAS6xx owner with this problem, however. It makes no difference whether the target is above or below, which tends to argue against the culprit being the TB20GT roof curvature. It might be the TAS6xx gets upset by prop modulation of the signal?

IMHO, 2) and 3) are “GA social” factors, because there is no technical explanation.

My system is set to display only targets within plus or minus 3000ft of me, and this obviously cannot be applied to Mode A ones, which I disregard if I am above say 3000ft or above cloud.

And the reason that you see more altitude less traffic on weekends could then be that on weekends older aircraft are flying. And older transponders and antennas might mean that their transponder replies are weaker. Which in turn makes it difficult for your equipment to properly decode their replies.

Yes; possible, but see my point 1) above.

No input thus far from avionics shops however, on whether Mode A-only installations correlate with older aircraft. One would expect they might, but Mode C, and its virtually mandatory use for a lot of flying in the UK, has been around for an awfully long time. One syndicate member told me recently about Mode S, which they are installing (about £3k) because they find it practically impossible to fly abroad without it.

So, yeah, very likely that a lot of GA is hanging on “outside the system” as long as they can, and these are mostly weekend flyers. Nice of them to stay invisible…

Administrator
Shoreham EGKA, United Kingdom

TAS and TCAS systems interrogate on mode A and C, and some on mode S as well. The own interrogation is the first selectivity, using a directive beam (the same as it receives). Timing of the reply is used for distance, and the timing difference on the antenna’s for direction in both transmit and receive.

Passive systems like ZAON uses the less reliable signal strenth method. Where there can be quite some difference in power output of different transponder. Larger aircraft typically have a more powerfull transponder and would show nearer then a small aircraft at the same distance.

It seems this mode A only is really a UK thing. There is one transponder manufacturer who is producing very poor quality transponder. I seem all kinds of issues of these. I don’t think these account for Peters issues. Although at low cost, more of these might fly during the weekend.

In the Netherlands and German it is very rare to see an non mode S aircraft during avionics testing. Some very old ones don’t have a transponder at all, which limits the possibilities for flying in the Netherlands quite severe.

In Scandivia I come across a mix of mode C and S, and it seems mode S is upcoming aswell. I guess mainly to easily travel outside Scandivia.

The Avidyne TAS systems shows non mode C targets upto 12.000 Ft of your aircraft. Above 12.000 Ft these get supressed.

JP-Avionics
EHMZ

Peter wrote:

Are there really so many planes which have specific old Mode A – only transponders? Or do many pilots turn off Mode C?

I don’t believe I’ve never seen a plane with a Mode A transponder, so I doubt it’s the former. Then again I’ve rarely seen anybody switch off Mode C. I did see a guy turn off the transponder entirely in Italy, for reasons that weren’t clear to me. With Mode S you obviously have a much stronger motivation (you’re broadcasting a traceable tail number) to turn off transponder functions, and for that reason I think diddling with the transponder output will become more commonplace as time goes on.

Silvaire wrote:

and for that reason I think diddling with the transponder output will become more commonplace as time goes on.

I don’t. Screwing with your transponder makes it almost certain you will be prosecuted for an infringement. And I am afraid I don’t buy the rampant privacy argument – very few people really care. Most prefer to be able to see themselves on FR24.

EGTK Oxford

JasonC wrote:

And I am afraid I don’t buy the rampant privacy argument – very few people really care. Most prefer to be able to see themselves on FR24.

Relatively few people have a brain, but some do, and will.

Jesse wrote:

TAS and TCAS systems interrogate on mode A and C,…

That contradicts the FAA guidance:

Mode C Surveillance (p17)
TCAS uses a modified Mode C interrogation known as the Mode C Only All Call to interrogate nearby Mode A/C transponders. The nominal interrogation rate for these
transponders is once per second. Since TCAS does not use Mode A interrogations, the Mode A transponder codes of nearby aircraft are not known to TCAS.

But they may be simplifying it.

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