>As for RAIM, my Garmin GNS430 (spiffy aviation GPS) has RAIM. Luckily
>I've never actually seen the RAIM warning flag. My understanding of
>RAIM matches what's been said before, position information is
>heuristically computed and when an anomalous position/speed occurs,
>the flag is raised. Sudden changes in position, altitude, speed, etc.
>would set off the flag, taking under consideration that an airplane
>would generally not invoke an implulse accelleration :)
I don't think that's how RAIM works. My understanding is that it uses
extra satellites (anything over 4 in a 3D fix) to compute an
overdetermined solution by least squares. Then it looks to see if one
or more of the raw satellite measurements are inconsistent with this
fix by more than a certain amount. If so, it marks this satellite as
unhealthy, notifies the user, and drops it from the fix. I believe 6
is the minimum number of satellites needed for RAIM in a certified
aviation receiver.
This is nothing more than standard statistical processing when you
detect and eliminate outliers in your raw data.
There are bits in the broadcast ephemeris that allow a satellite to
mark itself as unhealthy so that receivers will ignore it. There is
also supposed to be a mechanism that will cause a satellite to switch
to a "nonstandard" code sequence when it detects an internal fault so
that ordinary receivers will stop receiving it.
The problem is when a satellite "silently" fails, without marking
itself as unhealthy and/or changing the PRN code.
This happened some years ago when one GPS satellite suddenly jumped
way off in its timing for just a few frames. This was enough,
however, to screw up the many CDMA base stations in North America that
were all using that one satellite for timing.
In those days, it was common for GPS receivers used for stationary
timing to track just one satellite at a time, as this reduced the
small-scale timing jitters. But it left you completely vulnerable
to the failure of the satellite you were tracking.
There's a common theme here -- redundancy is good. And I think that's
the only reasonable approach to solving the original problem of GPS
timing integrity: have lots of GPS receivers in lots of different
places all comparing results.
Phil
