How does ATC radar derive altitude from transponder altitude?

Mode C is just another Mode A where the “transponder code” corresponds to a level with 100 ft resolution.

Here is how it works: Mode_A_and_Mode_C_Codes_pdf

Peter wrote:

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.

I thougt I read somewhere that all the airport/airfiled under the London TMA use the same QNH (presumably Heathrow’s one) to alleviate this issue. But just checking METAR now, I see most at 1021 and Biggin Hill, Stanstead and Southend at 1022.
The LTMA is defined in ENR2.1 but it doesn’t say which QNH is applicable.

Good point wether the Gatwick approach controllers (London TC, sat at Swanwick) and the Gatwick tower controllers use the same QNH. I would hope so :)

It looks like Mode-C then does not have any kind of error detection (not even a parity bit). Let’s say you’re flying at 1500’ pressure altitude (Mode-C reply code 0001 1101 0000), and interference happens to cause a single bit error (received as 0011 1101 0000 instead), this will mean the receiver sees 4000’ instead.

Presumably there’s some sanity checking in the receiver’s end (e.g. rejecting Mode-C replies where pulses are not at consistent amplitudes, or rejecting unexpected altitudes – e.g. it’s clearly a bad readout if an aircraft is at 1500’, then suddenly reports 4000’ and then 1500’ again), but who knows if CAIT still isn’t on a hair trigger for this :-)

Mode-S does have error detection codes as part of the transmission, though ( http://web.mit.edu/6.933/www/Fall2000/mode-s/ecc.html )

But a Mode-S transponder still radiates altitude using Mode-C, right? If not, older secondary radar installations would not be able to receive altitudes from “pure” Mode-S transponders.

Sure, but I suspect in a lot of places, the SSR equipment is new enough to be doing Mode-S. (Certainly the zero-tolerance-on-CAS-busts places).

Mode-S does have error detection codes as part of the transmission

Where is this stuff added?

Say you have a KEA130A encoding altimeter, or one of the very popular encoder boxes like here which are used by most light GA, feeding a Mode S box like a GTX330. The error detection bits must be added by the transponder, so whatever duff gray code you get from the encoder will error-check ok.

This is a reason why I got rid of that Ameri-King box in preference for the (relatively inexpensive and minuscule) Garmin GAE12 encoder. I really couldn’t trust the 20 year old Ameri-King box, and it adds a lot of wiring.

alioth wrote:

The same QNH is used throughout the piece of airspace that belongs to whoever’s advertising that QNH, pure and simple.

Where I fly there might be ten or more separate altimeter settings available from FAA approved ATIS or AWOS stations around a given airspace area, different controllers and altimeter settings apply to adjoining airspace areas and so on. That’s why 200 ft is typically used as the margin of error, as well as the accuracy limitations of the aircraft and ground based equipment.

This is a reason why I got rid of that Ameri-King box in preference for the (relatively inexpensive and minuscule) Garmin GAE12 encoder. I really couldn’t trust the 20 year old Ameri-King box, and it adds a lot of wiring.

Sure, though not all users have that option. For example King autopilot users are STC-bound to use the KEA130A encoding altimeter (or whatever). Well… in theory It’s actually funny (not funny) to observe that most KEA130A overhauls ($3k) are triggered not by the altimeter going off spec but by the encoder producing duff codes, like FL510 for a TB20. Awfully tempting to fit a separate gray code box…

That’s why 200 ft is typically used as the margin of error, as well as the accuracy limitations of the aircraft and ground based equipment.

That would make sense, though AFAICT this old allowance is not used in the UK anymore. Your current altitude value has to be below CAS, end of discussion

I really (honestly ) don’t want to get back into that infringments discussion, notwithstanding its pretty civilised nature compared to elsewhere in the UK, but the cases where somebody got away with a bust with an off spec transponder involved a formal Part M company altimeter test and a report to the CAA by a (gosh) “licensed engineer”, so I reckon 500 quid…