Sorry if I seem to be only telling 'bits' but obviously company confidentiality forbids indiscretion.
Absolutely right. To clarify, I'm not a NATS employee and I'm only looking for information that's in the public domain.
I'm trying to work out how to respond to the CAA's Regulatory Impact Assessment that would have aircraft operators spending hundreds of millions of pounds on Mode S related avionics. The cost benefit analysis, dating from 1998, hangs on overcoming two major issues, Mode A code shortages and RF congestion in the 1090 MHz region. The claimed benefit appears to be very sensitive to how close the current system is to saturation, and I'm trying to put together a picture of how accurate 1998 CBA will be.
Part answer to 6) for UK is that because NAS FLAT tracks aircraft the UK does have the ability to use specific codes for more than one aircraft. So it is quite possible to have several aircraft on the same (preferably one of the sector discreet codes) code, but with the TDB displaying the correct callsign etc. But we do not do that.
This intrigues me. What is "NAS FLAT"? Are you saying that in the current system, the same Mode A code could be used for more than one aircraft in different parts of the UK, but the feature is not used?