if an aircraft is providing responce to SSR interogations on the ground, then it is operating incorrectly. The pilot will move the transponder from standby to an operational state (can't remember the switch settings, something like active or tx/rx or suchlike) at some time either at the gate or on pushback, the transponder is now prepared to reply. But until it takes of and the suppression signal from the wow switch is removed, the transponder will not respond to Mode A/C interrogations or Mode S all-call. If they did, the two radars at Heathrow will be tracking all of the aircraft around the airfield, which would cause all kindsof problems. The aircraft should only be appearing on the ATM displays at EGLL and to any other radar users once the aircraft is airborne (Mulitlat feeding ASMGCS is different)
If some SSR tracks are getting through to H23 or H10 radars and sent to Swanwick, this is only because of faulty aircraft transponder suppression, not normality.
I'm currently sitting in the Eurocae working group that defines the MOPS for transponders, and we happened to discuss this very subject this morning!
