Hi Ian,
Sorry, this was supposed to come before Alan, Chris and Steve's replies and your update but decided to post it anyway in case it helps others.
Yes, that (your second last post) about sums it up, though it's not quite as straightforward.
For many months non-directional bearingless transponders have been referred to as mode A/C in all posts and it was the aim of PAW to try and provide some warning of these. Keith's news post is the first time I have seen these being called bearingless mode S.
As I explained above, the transmissions from all transponders, whether mode A, C or S are 'bearingless', because they contain no position location information (except altitude in mode C or S), which is why it takes a very expensive ground radar system or 100K commercial TCAS to work out where they are. Even then, the accuracy of that information is dependent on a number of factors including geography and the density and type of radar stations. What Lee has been trying to do is provide us with warning of the presence of these aircraft by extrapolating from the basic information that transponders provide. In an ideal world, we would like to know the location of all transponder equipped aircraft, but this of course gets much harder the less information the transponder provides, viz Mode S is 'easier' than mode C, which is significantly easier than mode A (which only transmits a squawk code).
It is important to note that, contrary to popular belief,
transponders are 'passive' devices, which ONLY transmit in response to a ground radar or TCAS interrogation.Most recent mode S transponders (the newest and most technically advanced versions of transponder) are
capable of
being 'adapted' to also work as more precise
active ADSB transmitters by providing a suitable gps position input as Alan and Chris describe, but very few have actually been 'converted' principally due to cost, though as Chris says, this is now much cheaper and easier for permit aircraft as cheaper non-certified gps sources (including PAW) can be used to do this.
As you are aware, ADSB then transmits
accurate positional data, making it 'relatively' easy to display their position on screen ( if you can do the magic software bit). This therefore is the 'easy bit' hence it is already fully implemented.
Since getting this part working, Lee has done considerable work to develop the programme to provide
reliable information for transponder contacts (unreliable information simply discredits the system and leads people to distrust it), whilst at the same time being heavily committed to developing the replacement RF Bridge for PAW's own 'mini' version of ADSB. I and others in the Engineering sub group have been assisting him with the mode S testing and the new Mode S and audio alerts are the result.
Mode S transponders have co-located ADSB transmitter, which periodically transmits ADSB packets which contain data of interest to PAW such as Aircraft code, height and position. ADSB packets can also be transmitted without a Mode S transponder.
Why would PAW need to try and figure the threat distance from a Mode S transponders Bearingless transmission when it can also see it's ADSB packet? Which is why I am puzzled by Keith referring to them in his news update.
Most Mode S transponders do not transmit ADSB for the reasons explained above. If they do, PAW uses the ADSB signal and effectively 'ignores' the mode S one. But there are an awful lot of mode S transponders out there not transmitting ADSB which would otherwise remain invisible.
Ground stations, air traffic etc, obtain positional info from mode S only aircraft via the primary radar.
Not sure that is correct. My understanding is they use ADSB transmission if available, and verify it is bonafide against the Primary return. Otherwise they would not have needed to upgrade the radars?
Not the case Ian - According to reports no UK ATC units currently use ADSB! Scary or what!!
In practice ATC units now use very expensive Secondary Surveillance Radar (SSR) to determine the location of transponder equipped aircraft (search Wikipedia).
Alan/Chris/Steve, Sorry to cross post - it obviously took me a lot longer to type out and check such a complicated reply on a mini iPad! Especially as every time I went to post another one of you had got in before me!!
Best regards
Peter
