In article ,
"Adrian Jansen" wrote:

You might want to consider the implications of WAAS. I have no direct
experience, but the principle is that you take an external signal and use it
to 'correct' the GPS location to another place - hopefully more accurate.
But what is to stop you sending bogus 'corrections' and making the GPS think
its somewhere else entirely ? The WAAS signals are much easier to generate
than the original GPS satellite signals. Sounds an easy way to cheat to me.

But how far can the WAAS signal "correct" the GPS one?

I assume the principle is that the transmitter knows exactly where it
really is, so if GPS says it is somewhere else then the difference is an
error which will apply to all other GPS receivers in the area. Since
the GPS error is likely to be on the order of 5m - 10m, and very very
unlikely to be more than a couple of hundred meters, I would expect the
system to be designed to correct the GPS positions by no more than a few
hundred meters.

Another characteristic is that the correction is the same not only for a
reasonably large geographic area, but that the necessary correction
changes quite slowly, over a period of many minutes. If I was designing
the system, I expect I would also take advantage of that to reduce the
amount of information that need to be transmitted.

So I would expect a maximum correction possible of, say, less than a km,
and rapid changes to the correction to be either impossible or else
rather unusual and therefore suspicious.

I don't see how you could use that to make any significant difference to
a glider flight.

-- Bruce