Squawk Ident on the ground will not have any effect on Swanwick, as a transponder should not be replying to SSR or Mode S all-call interrogations when on the ground(*). As such, none of the Heathrow radars can see aircraft when they are on the stand and taxiing around the airport. There is no way therefore that a squawk ident done on the ground will have an effect at Swanwick, unless the aircraft has a fault and is replying when it shouldn't and the Heathrow ER radar is providing an SSR track - but even then I would imagine some airport filtering is applied at the centre.
The only affected user would be the Heathrow Tower which is using multilateration as part of the A-SMGSC and a squawk ident could be detected by multilat system from Mode S squitter and roll-call replies that are permitted when on the ground.
*Just to clarify, not all aircraft have this capability. It requires a weight-on-wheels type switch mechanism to determine the aircraft state. Smaller airframes and rotorcraft are likely not be be equipped, and rely on pilot action to put the transponder in a state to reply. However, from vast majority of EGLL traffic, switch mechanism will automate the transponder response state.
Last edited by Radarspod; 16th January 2008 at 09:58.
Reason: clarification