Thanks Peter; I tried to simplify the short squitter version of the transponder but I did not know PAW cannot use this information. I assume it just becomes, to PAW, another 'plain' Mode S transponder and the list should look like this instead:
1. A transponder will reply to interrogation from ATC or TCAS. This reply (squawk) can be used by PAW to generate a proximate alert based on the strength of the transmission. The reply to interrogation contains no other information that can be used by PAW.
2. A squitter is a thingy that transmits various bits of information about the aircraft and it does this periodically i.e without interrogation
3. A short squitter only transmits a Hex code
4. An extended squitter has GPS and other good stuff added to the short squitter and, so, 'becomes' ADSB
5. The early Mode S transponders did not have a squitter and relied on being interrogated. PAW can only generate a warning when ATC interrogate the transponder because there is no squit for it to see, only a squawk. This will be a 'signal strength' warning.
6. The next line of Mode S transponders had a short squitter, so sent out information without being interrogated. However, because it is only a Hex code PAW cannot generate a warning based on the squitter signal and, so, acts like No.5.
7. The latest Mode S transponders have an extended squitter and we can plug a GPS into them and chuck out ADSB signals. Both Mode S and ADSB transmit without interrogation through the extended squitter. PAW will give us a Mode S warning ('signal strength') if there is no GPS info on the squit and a full ADSB warning if GPS is plugged into the transponder.
We now know why one of the aircraft was dark and it seems the earlier firmware of the Funke transponders is either No.5 or No. 6. The supposition is there was no ATC or TCAS around to interrogate the Mode S transponder and, therefore, no trigger for a Mode S reply for PAW to use.
There is still some question as to the Funke extended squitter and whether it does squit as it should i.e. ALL the time, or whether it only squits when interrogated. Hopefully this is not the case or else it just becomes another plain transponder but with a fancy reply. However, Rob's report on a Pingbuddy seeing him would suggest this is NOT the case. As he could not see the other aircraft (singular) this shows there was no ATC interrogation but as he was picked up on the Pingbuddy it suggests the Funke transponder is working properly i.e. sending out the extended squitter message periodically and without being asked.
