How does ATC radar derive altitude from transponder altitude?

That belongs here but you may be right. Historically 200ft error was allowed but not any more. Now the TXP-reported pressure altitude is taken directly.

Peter wrote:

That belongs here but you may be right. Historically 200ft error was allowed but not any more. Now the TXP-reported pressure altitude is taken directly.

Are you 100% sure of this? For confirmation, when you say “pressure altitude” do you mean the FL displayed on a Mode S transponder or that FL modified to altitude on the local QNH?

Regards, SD..

Peter wrote:

Now the TXP-reported pressure altitude is taken directly.

Now why would that be? My understanding is that even with a transponder reporting altitude ATC will not take that as reliable unless they have radio communication with and altitude confirmation from the pilot. If that is so why would a presumably unconfirmed altitude from a presumably non-communicating pilot be considered definitive enough for reporting/prosecution?

You need to read the linked thread

That’s the way it is…

A transponder radiates pressure altitude i.e. flight level. The radar server (the computer system which drives ATC screens) then uses local QNH to correct that into altitude. So in that screenshot above the target is showing “A017” (1700ft altitude).

My guess is that not many will try the “my altitude encoder was off” defence is because they are then admitting to having a known unairworthy aircraft and will then get done for that Remember that these actions don’t go to a court where you could defend yourself.

Peter wrote:

A transponder radiates pressure altitude i.e. flight level. The radar server (the computer system which drives ATC screens) then uses local QNH to correct that into altitude.

OK,thanks. So that guy could be talking to Farnborough or London Information and using their QNH. If I recall Tranponders have to be within something like 200ft (I think +/-185ft?) tolerance? So this guy could happily have 1450ft on his altimeter or even 1400ft and still he shows up as an infringer of that airspace.

skydriller wrote:

something like 200ft (I think +/-185ft?) tolerance

Only when your altitude is “validated” and only when validated by the same unit whose CAS belong to.

Speaking to London Info, mode c not validated.

Speaking to Farnborough, mode c validated for Farnborough. So the Farnborough ATCO will now that the transponder reads a bit high. But the London TC controlers are at Swanwick. Different unit. So they have to take the mode C reading at face value.

That the way it is setup. Of course it is not the most logical and efficient from a GA point of view.

So that guy could be talking to Farnborough or London Information and using their QNH. If I recall Tranponders have to be within something like 200ft (I think +/-185ft?) tolerance? So this guy could happily have 1450ft on his altimeter or even 1400ft and still he shows up as an infringer of that airspace.

Yes he could be flying with the wrong QNH.

The theory is that when flying a plane near CAS, you are supposed to obtain the QNH from the owner of that CAS. Of course nobody does this, unless they happen to be talking to them anyway, but almost no VFR traffic routinely talks to Gatwick… You could listen to their ATIS, I suppose.

My understanding, and this was explicitly stated on the Gasco infringments course, is that no transponder error allowance is made. If your altitude return, corrected by the radar server’s local QNH, is at all into CAS, then you are infringing, and you will be reported. So if say some reg says you are allowed a 200ft txp error (the figure which has been widely assumed in GA for many years – I don’t know what reg applies) then you cannot fly above 2300ft under a 2500ft CAS. And most traffic under 2500ft CAS will be at 2300ft The alternative is to make sure your transponder (whatever the altitude source is) is spot on and allow at least 200ft. Allowing 100ft (like people had been doing since for ever) is no good because “2400” about actually be 2500.

I’ve also been told that with a Mode S box the altitude is returned to nearest 25ft and if you are 25ft into CAS then you will be reported for that too.

I have never read any account of the actual radar implementation, and it is most likely secret. There must be some nontrivial aspects to it e.g. what if the QNH varies across the whole bit of CAS?

How much pressure variation could there be across a single piece of CAS? I would assume that the QNH is measured at the runway since that is where it matters, so one would need to examine the pressure variation from the runway to the CAS extremes. How many millibars would be needed to produce say a 70kt wind? @bookworm will know how to work that out, if he’s still about.

But the London TC controlers are at Swanwick. Different unit. So they have to take the mode C reading at face value.

I would find that surprising, that a Gatwick controller would be seeing Axxx targets based on Swanwick QNH.

AFAIK, above the transition altitude, targets are shown at flight level, and then the Mode C altitude is taken at face value, with no QNH correction.

Peter wrote:

There must be some nontrivial aspects to it e.g. what if the QNH varies across the whole bit of CAS?

I very much doubt it has non-trivial aspects, it’ll be a simple correction. The same QNH is used throughout the piece of airspace that belongs to whoever’s advertising that QNH, pure and simple. If there’s more than 1mb difference in some CTA somewhere, no one will be flying (at least VFR) because of the gale force winds (Hurricane Sandy – an extremely destructive storm with a precipitously steep pressure gradient – would have only had about 1mb difference over the distance from one side of Manchester’s TMA to the other – so there’s absolutely no point having to take into account changing QNH over the breadth of a CTA or even TMA).

Most Mode-S boxes can show the pressure altitude they are reporting. It’s probably a good idea to make that show on the display by default, that way it’s easy to check against your altimeter by periodically setting it to 1013 and making sure the transponder and altimeter read the same. The Garmin GTX335 with Garmin’s encoder will show the pressure altitude with a 10ft resolution, which is more than enough to be able to ensure your transponder is transmitting the right pressure altitude.

Personally I always listen to the ATIS of any controlled airspace I’ll be flying under before I get there (even if it means unsquelching the radio). Or you can ask London Info for the Gatwick (or wherever) QNH, after all that’s what FISOs are there for :-) In particular don’t use the regional QNH if you’re anywhere near CAS – it’s always a few mb low (the idea I guess is for safer terrain clearance margin if everyone on the regional QNH is flying slightly too high – but with the unintended consequence that you’ll probably bust controlled airspace if you’re using the regional QNH and fly, say, 200ft below the base of CAS based on your altimeter). Personally I don’t use regional QNH, I’ll always get the QNH from the nearest ATIS.

Really all the mess of different pressure settings – QFE and regional QNH needs to go away – and pilots to be taught to use just the applicable QNH only, and the lowest flight level moved up to FL180 (which I believe is going to happen eventually).

Peter wrote:

I’ve also been told that with a Mode S box the altitude is returned to nearest 25ft and if you are 25ft into CAS then you will be reported for that too.

Mode S boxes allow for 25 ft reporting, but you don’t have to have an encoder with 25 ft resolution. I expect that new installations would have such encoders, but upgraded installations might not. When we upgraded our aircraft from mode C to mode S we simply swapped the old King transponder with a plug-compatible Trig transponder and kept the old encoder with 100 ft resolution.