One reason for this is that the transmitter of transponder 1 would blow up the receiver of transponder 2 and vice-versa, since they are operating on exactly the same frequency
Mmmm. Transponders transmit on 1090Mhz but receive on 1030Mhz so can't see any blow up caused by that.
Anyway our £5 dongles that do listen on 1090 seem to cope just fine 
I think NAT's issue re LPAT is garbling though that happens to some degree anyway. I wish they would at least try both transponder and LPAT ADS-B out together in their ongoing trail and see if it 'just works' in real life. . It would save a lot of us shelling out to upgrade our MODE S to add ADS-B.
I'd like to understand how NATS can tell if it is one of two devices transmitting anyway.
As Ive done quite a lot of the Flying with LPAT on behalf of AOPA and NATS perhaps I can input some thoughts.
Not using the Aircraft Transponder during flying of the LPAT was defined by the CAA to avoid interference, garble and FRUIT, from the LPAT with the conventional Mode-S and A/C Transponder. It has not been determined scientifically that the LPAT would cause problems with the conventional ground interrogations/response but a precaution that was implemented as part of the CAA authorisation to conduct the trials.
In my particular case the need to switch of my Transponder is a more of a practical issue with that fact that I also have an ADS-B enabled transponder hence during the flying trails we needed to be confident that the ground tracking of the LPAT and receipt by an LPAT in another aircraft was from the LPAT transmissions. In addition we needed to be certain that any onboard transponder was not flooding the Rx of the LPAT and affecting range performance of the LPAT Rx.
Overall the trials of the LPAT are being done in an environment to determine the performance of the LPAT and therefore eliminate any possible effect of a working transponder in the same aircraft. In addition the CAA authorisation for the LPAT is for only one working transponder in the aircraft undertaking the trials.
Having said all that Im sure I have had both operating together, pure accident, and not had any reports from ATC that I have disappeared from their radar :-) but nothing formally tried and perhaps we need to include an LPAT flight with an active transponder to determine if LPAT performance is affected on the RX side. As you suggest a well designed Rx should be able to cope with a nearby Tx on the same frequency within limits as my onboard PCAS as well as other existing traffic warning devices demonstrate.
Ref the discussion on the use of two active Transponders in the same aircraft i.e. Conventional and/or LPAT. This would produce interesting results if they both would respond to ground interrogations, the LPAT does not respond to conventional Ground Interrogations, as the Radar Display Trackers would see two response from a target. At the moment the ADS-B position data from the Uncertified GPS enable Transponders and LPAT, is not sent to any of the Radar Tracking software but only recorded so in this safety conscious environment it can be determine if the accuracy, reliability and integrity of the ADS-B transmissions from uncertified GPS is sufficient for presentation to ATC either on its own or as part of a combined Conventional Transponder, ADS-B enable environment. In the future - lots of work still to do and part of the overall European Trials of uncertified ADS-B enabled transponders, including LPAT, I'm sure this is the direction that it will go in Europe and is already part of the overall surveillance environment in other countries where conventional SSR ground Stations are challenged by terrain and other factors.
All of this is part of a European Project EVA which is looking at the use of Electronic Conspicuity devices to assist the VFR pilot. You can read more of this in the AOPA magazine.
Merry Christmas and a Happy New Year..
Alan
