Not a lot of continuing interest in this issue on this board, but I did say I would investigate and report back. Bottom line is, I have been unable to find an engineering analysis that led to the restricted distribution of TIS-B traffic data. But, I have talked to several knowledgeable technical people including a guy named John Collins who is an expert on the subject. They assure me that the reason traffic is not broadcast to everybody with ADS-B In is that there is insufficient bandwidth to do so. It is not politics, coercion, meanness, or social engineering. It is practical engineering considerations.
I can believe this when I hear that the 1090 MHz band is shared by transponders, surveillance radars, TCAS, ADS-B, and other services. I’m told it is already nearly oversubscribed in busy airspace like the LA Basin. That apparently is the reason the FAA opted to go with a second ADS-B frequency, 978 MHz in addition. That band is less congested, but 20% of it is already occupied by FIS-B weather data, which is broadcast to everybody. When you think about it, yes, there is a whole lot of weather data to be broadcast, but its repetition rate is on the order of minutes. By contrast, the repetition rate for traffic data is on the order of 5-10 seconds. You can do your own assumptions and calculations (I couldn’t find the “official” ones), but when I do that I come out with too many messages, not enough bandwidth.
Another reason I can believe the bandwidth limitation answer is all the other things the ADS-B design does to reduce the amount of data that has to be broadcast. Most ADS-B ground stations broadcast into geographic sectors (usually four quadrants, but the one at my home field looks like it might be only two or three). So traffic data is broadcast in a particular sector only if there’s an aircraft with an ADS-B Out transmitter in that sector that indicates it wants to receive the data. Further, they only broadcast on one frequency or the other, not both, and the frequency selected is the one that the aircraft’s ADS-B Out transmitter says the aircraft is in a position to receive. Additionally, the other service, ADS-R that retransmits on the other frequency, only does so if an aircraft needs it, and the way you say you need it is, again, by the ADS-B Out transmitter saying what frequency the aircraft needs to receive.
Near as I can tell, the conspiracy theory that the FAA was intentionally withholding traffic data originated with an editorial by Flying Magazine editor Robert Goyer a couple years ago. He called it “social engineering” and said that if only the FAA had “left well enough alone” traffic would have been broadcast to everybody. That’s just completely false. A broadcast to everybody would have required a completely different design. Of course it could be done. Lots of things can be done for enough money. I suppose the FAA could have broadcast TV pictures of controllers’ radar screens to aircraft in 1965. They didn’t, but not because they were mean and evil and wanted to shut down General Aviation.
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Originally Posted by MEdwards
