Fun fact everyone here probably already knows, but I’m throwing it out there just for funsies. It’s pretty common knowledge that all the ADS-B data piped into the FAA’s air traffic data system (I can’t remember the official name, and it’s probably changed since I was briefed) is added to all the other air traffic surveillance data sources (PSR, SSR, etc.) with the track being displayed to controllers representing the weighted average of the differing data, the weighting of each varying based on some algorithmic sorcery. That’s not the fun fact, though. The fun fact is based on how and when the FAA systems leverage the MLAT calculations for these aircraft.
Remember when they invented ADS-B back in the 90’s? Remember how those were the good ole days of pre-9-11, when with few exceptions, bad stuff generally happened over somewhere else, because we were good to go here in the US? So, pre-9-11 the FAA culture, and the company line was essentially something to the effect of “Safety of Flight Over Rules EVERYTHING!”. While in many ways, that’s still the company line, or at least it is when things like 737 Max debacles are not happening. One of the aspects of our society nowadays has forced the hand of the FAA, and they must now say “Safety of Flight Over Rules EVERYTHING…except national security”.
Back when the system was being developed up in the Capstone project in Alaska, and to a limited extent parts of Colorado, the FAA still believed that civil aviation was the global equalizer that no matter what, everyone could agree on. Because of this culture, I would be surprised if the question of signal validity or nefarious use of the ADS-B system was ever raised. If so, I know for a fact it would have been shot down in an instant as not aligning with their safety-of-flight says HELL NO" mantra.
Wanna guess how often those issues were raised after the 9/11 attacks and the subsequent realization that there are many in this world who would LOVE to mess with a Superpower’s air traffic surveillance system? Much to the frustration of the new DHS leadership, by the time the FAA thought to apply even basic encryption protocols to their new and rapidly integrating ADS-B surveillance system, it was too late to act, or at least that was what they claimed. “The users can’t be asked to foot the bill for a complete rebuild of a system they just only recently agreed to equip for”. In other words, the airline executives didn’t want to have ask the government for another hand-out so soon since the last one, bad timing. So what to do? Enter in, smart people.
I would think that the computing power required to do this would be absolutely off the charts, but I am not a smart person. Soon after 9/11 occurred, the FAA could only take DHS and DoD fist pounding for so long. Someone was going to either spoof ghost aircraft into the NAS surveillance system and create utter chaos using a €2500 ($3000) software defined radio, and they were actually trying to shut it, ADS-B, off. DoD agreed to take custody of enough domestic PSR systems before the FAA mothballed them all (a plan since revamped), but what about unencrypted data flying through the air?
Every single ADS-B transmission which includes the aircraft’s GNSS/ INS calculated position, as well as a handful of other obscure data packets no one cares about, is validated in real time, as it’s piped in, using MLAT to cross reference the position. If the position falls within whatever is considored an expected or acceptable position delta, the ADS-B data is allowed to continue into the greater NAS surveillance monster. However, if the signal is found to differ from the calculated MLAT position beyond that acceptable difference, the ADS-B signal is diverted to a holding ‘bucket’, and retained for some duration of time in the event there is further scrutiny. This eventually led to the FAA’s Aircraft HEXCODE Blacklist for repeat offenders. They found that the vast majority of these flagged positional data packets weren’t the bad guys trying to reap havoc on the US. It was shoddy ADS-B Out system components and installation, mostly from the humble General Aviation community.
Anyway, fun fact that took longer to work through then I originally thought. If you want, you can print out this book I just wrote and I’ll sign it so you can impress all your friends!!!