Peter wrote:
An active TCAS system uses returns from its own transmission only. It could not measure the distance otherwise.
But passive systems (such as the Zaon units) can also determine range. I guess by measuring the time delay between the target’s response to the triggering interrogation on 1030 MHz…
Peter wrote:
Doesn’t the altitude come back in the same (longer) packet as the squawk?
It doesn’t. The “modes” are actually different interrogations. In response to a mode A interrogation, the transponder will return only the squawk. In response to a mode C interrogation, it will return only the pressure altitude. Radar stations alternate between mode A and C interrogations.
I actually very rarely see low level traffic that is anything other than showing altitude. Maybe your system has a problem Peter.
But passive systems (such as the Zaon units) can also determine range. I guess by measuring the time delay between the target’s response to the triggering interrogation on 1030 MHz…
I think the two Zaon products used purely signal strength.
I actually very rarely see low level traffic that is anything other than showing altitude. Maybe your system has a problem Peter.
I see very few Mode A targets on weekdays… hence the subject of the thread.
However, there are still lots around, judging from the % of traffic which e.g. Farnborough Radar calls as “level unknown”. That is exactly where I would expect people to turn off Mode C.
even one of my flight examiners deselected it because he “didn’t like to be watched”
With the extremely low turnover in the UK PPL community, probably 75% the pilots flying today shared your instructor (or one of his similarly minded colleagues).
Mode A make more harm than good because ATC are forced to give traffic information (or even avoidance) about such traffic which may be vertically separated by several thousand feet. I am not sure how smart it is to fly underneath a TMA transponding mode A.
I don’t think they are too bothered about that.
Out of the four aircraft I fly regularly from a local flying school. Only two have mode C transponders. One has mods A and the other has no transponder at all.
By all accounts it’s a simple cost versus benefit issue. They can see no benefit in upgrading to mode S and it’s going to cost 3500 pounds per aircraft.
I see a benefit in ultimately going Mode-S and ADS-B – but it’s different if you’re an owner than if you’re in a club I suspect. For example, if I’m being met by someone at an airport, or I want to let friends/family know where I am I can just give them a link to FR24 and they’ll see what’s going on real-time.
Peter wrote:
However, there are still lots around, judging from the % of traffic which e.g. Farnborough Radar calls as “level unknown”. That is exactly where I would expect people to turn off Mode C.
Doesn’t that mean level unverified ie they haven’t confirmed the level rather than really unknown?
IIRC, they would then use “unverified”. I am sure an ATCO will clarify this soon… not that we have many UK ATCOs writing here because of the Official Secrets Act 
JasonC wrote:
I actually very rarely see low level traffic that is anything other than showing altitude.
I have seen the same on our equipment. Traffic almost always shows on our screens with a height difference. It’s rare to see a target with no numbers alongside, although it does happen, which also tends to say that TCAS CAN see mode A returns, so they are not invisible.
JasonC wrote:
Doesn’t that mean level unverified ie they haven’t confirmed the level rather than really unknown?
I have always thought that’s what is often meant, but I take Peter’s point that they would probably say unverified rather than unknown in that case.
Peter wrote:
On Sunday I did a little scenic local, Shoreham, around the Isle of Wight, and back. Loads of Mode A targets.Are there really so many planes which have specific old Mode A – only transponders? Or do many pilots turn off Mode C?
Rather than “Mode A targets” do you mean targets without altitude information? To the best of my knowledge, all TCAS/TAS interrogations are Mode C. You have little need to know another aircraft’s squawk.
