question for all. How do the aircraft work the different sites. I would assume with altitude they hit many sites and would get hetrodine such as the ground units get with multiple site reception. can anyone give some insight how the aviation units are not hampered.
Heterodyne is not really an issue with P25 simulcast, as the timing is extremely precise. Naturally this would only apply to simulcast cells, not to the standalone sites.
The aircraft will affiliate with whichever site has the strongest RSSI, so it would roam just as a ground unit would. Obviously while at altitude it would certainly "see" more sites than a ground unit. Still, the same principle applies. The radio will choose the site with the strongest signal and will lock onto that site until the RSSI drops below a predetermined value, at which point it checks the adjacent sites again for a stronger control channel.
The radio doesn't care if a site is a standalone site, or a simulcast cell (which is a virtual "site" compromised of multiple physical locations known as subsites), it'll find the best one and that's the one it'll operate on.
Digital is a lot more forgiving when it comes to simulcasting, in particular with LSM/CQPSK modulation, so it wouldn't be a factor.
PS - This applies to ground units also. They are not affected by heterodyne on this system. Scanners have a tough time with simulcasting, and are negatively affected by multi-path (signals from multiple transmitters on the same frequency hitting the receiver all at once), but the actual radios on the system are designed to handle it just fine.
