Transponder and pressure

[psmd0311]

Just wanted to know what report is seen on the screen of the controllers.

As far as i know the transponder gives an altitude at standard, 2992. So if actual pressure is 3032 and an A/C is flying at 10,000 feet does atc see the plane at 9,700 feet because of what the transponder is reporting at 2992. Or is there a program in the system that adjusts to what the pilots see and the controller will also see 10,000?

thanks for your help!

[7000]

Not sure about American systems (but I imagine it's very similar) but in the UK, everything operating above the transition altitude (usually 3000ft, but 6000ft in the London TMA) all the Mode C radar readouts are given with 1013mb (ISA standard pressure) as the reference. This is because the transponder will ALWAYS transmit Mode C information ref 1013mb.
Below the transition altitude (at certain larger ATSUs) the height information transmitted from the transponder is still with ref to 1013, but the ground equipment at the ATSU will calculate the height with reference to the appropriate aerodrome QNH. For example, in the case of the London TMA this will be the Heathrow QNH.
All this prevents any errors of Mode C data if the pilot were to select an incorrect pressure setting on his equipment.

[GOLF_BRAVO_ZULU]

The transponder picks the altitude, or used to, off the encoding altimeter. It then records what that instrument is indicating. Above the Transition Altitude, it will be set on QNE anyway.

[Regular Cappuccino]

Transponders report the a/c's level with reference to standard pressure (1013.25 mb / hectopascals / whatever) regardless of the QNH set by the crew.
In the UK, ATC radars will display by default, the level reported by the transponder as a FL. Thus in the case you quote, in the UK the a/c would be flying at FL100 (10,000ft) on Standard, and the radar would display '100'. If it was flying for some reason at 10,000ft on a local QNH, the radar would display whatever the FL equivalent of that altitude was.
Approach radars however, generally have the ability to convert the a/c's FL to an altitude when it's flying at or below the Transition Altitude (where I work - 4000ft, but others vary). The local QNH is fed into the system driving the display and as the descending a/c passes through the TA the displayed Mode C label changes from (for example) '053' to 'A40' (indicating 'Altitude rather than FL). Climbing a/c will be displayed as an altitude until they pass the TA, when the display will change to a 3 digit FL (eg. 055).
Hope this helps
RC

[psmd0311]

Thanks for all of your replies.

The reason i am asking is because i got a call from atc when i landed today. I was told to descend to 10,000. We had traffic converging, but 1000 feet below us. I am sure i didn’t bust 10,000.

but when i landed, atc said they checked the tapes and i was down to 9,700. The other A/C was at 9,200. Giving just 500 feet clearance and all this trouble.

After the conversation with atc i was thinking how i could have screwed up.

Anyway the local pressure was 3020, which i had set in. So i thought maybe the 280 feet difference with the pressure would show atc i was at 9,700 feet, when i was showing i was at 10,000 doing nothing wrong.

I with an approach control in the US. Transition atl isnt till 18,000. So does anyone know how their radar works?

Anyway it was just a thought, trying to work out what went wrong and how to get it resolved.


thanks again

[SM4 Pirate]

We usually apply a +/- 200 feet to a mode C readout for and accurate level. Outside that we consider you unverfied and ask you to stop squawk mode C. At all times we consider your airborne gear actually working; unless you say otherwise.

Mode C broadcasts on 'pressure altitude'. The pressure altitude is pressure corrected at our end for consistent application. This is done either a) automatically based on sensors or b) manually entered correction. In case b, it done once by the appropriate system person, not by an individual controller.

It is very possible that you were at A100; the readout was A097 and everything is normal...? Well except your mode C was out of 'tollerance'. Doesn't mean you busted your level; or you were within your 100ft tollerance of height lock and you mode C was 200ft off...

However, some systems get mode S/ADS-B data and that is revealing your 'true' altitude; ie what's on you altimeter (effectively) in this case they may be able to 'prove' your error.

That said; this is the reason we use 1000ft; if everything lines up you should still have 400ft sep; how tall is the 'pocket of air' taken by an aircraft? (About 80 feet max). So what's the problem...

[Spodman]

I've seen some other posts on this subject recently and am surprised that many don't seem to grasp the concept. Reality Cap has described it nicely but some don't seem to have got it.the transition altitude (usually 3000ft, but 6000ft in the London TMA)This means that if an aircraft is maintaining or reporting passing or descending/climbing to maintain 10,000 FT the altimeter must be set to 1013.2/29.92, you arein the standard pressure levels.Anyway the local pressure was 3020, which i had set in.Rules vary, and those in Oz are barely rational, but those in the US seem more or less optimum to me. When you are passing the lowest available flight level (depends on QNH, default FL180 in the US) on descent you set QNH. When you are climbing through the highest available altitude (18,000 ft in the US) set 1013.2.
Whichever way you do it ATC will receive the level you are actually at (rather than that displayed to you) corrected by the forecast QNH value in the radar machine innards.

You should have been on 29.92