Radar display pressure setting for mode C

[cpl4hire]

Aircraft Altitude encoders for mode C use the Standard pressure of 1013 mb as their reference. Is the "adjustment" to convert this to Altitude with respect to the current QNH done automatically by the display equipment or does it have to be set manually by ATC ?

[NeoDude]

I believe it is done automatically by the equipment, depending on what QNH is set on the ADIS. The equipment can also automatically switch between FL and altitude and regional pressure settings depending on where the contact is.

I stand to be corrected though...

[eastern wiseguy]

Neo Dude is correct.

[Spitoon]

Depends very much on equipment and systems used. Once you get outside UK NATS things can be rather different.

[cpl4hire]

Thanks for the replies, could anyone answer the following:-
Is that common on all radar installations old & new ?
Can the pressure setting ever be wrong (due to data feed failure etc.) and is the failure flagged to the Controller ?

[Spitoon]

As above - it is not common in the sense that all systems work in that way or in the same way. Older systems are less likely to offer the facility or a suitable interface, more modern systems are no doubt designed to incorporate the facility although I suspect that some may use a proprietary interface.

As with any data system, it is possible for data to be wrong - GIGO. I suspect that radar display systems will use whatever they are fed with - within the usual sensible bounds - on the assumption that the data is validated downstram of the originating interface. So if a radar display is fed with a pressure value that is between, say, 950 and 1050 hPa (the usual bounds used), it will not know if the value is right or wrong. I have seen a system which deemed the 'current' value expired after 40 mins or thereabouts and to flag this to the controller but I can't recall which company made it. I don't think radar displays usually continually 'handshake' with whatever system is supplying the pressure data so a failure of the comms feed or data source would not be likely to be detected.

[beamwidth]

In Ireland,using the Thales Eurocat, the QNH is manually input into the system at one position and updates everywhere, with the info coming from the Met office Metar.
As with spitoons comment above, if there is an unusually high or low pressure you get a warning and a confirmation message. This also applies if there pressure changes by 3 hPa or more from one update to the next.

[cpl4hire]

Thanks Spitoon & Beamwidth,

For the explanation of the "arrangements" at your end.