[BC] GPS recommendations

[Burt I. Weiner]

Satellite propagation 101:

Almost 30 years ago I had this brilliant idea that a WWV type of signal at 10 MHz should be put up on a satellite so that it could be used as a standard calibrating signal by all who could receive it.  A friend of mine who was in charge of a rather large uplink facility and very knowledgeable in this stuff offered me a channel (for free) to do this. (He already knew what was going to happen.)  As a test I got my best 10 MHz oscillator and stuffed it into the baseband of a modulator.  Voilla, I had a nice reference signal as received back at home.  He said, "Lets try a 'speriment (expriment)." The speriment was to take the output of the 10 MHz oscillator as applied to the modulator, feed it into the X axis of a scope and take the 10 MHz as received from the satellite and feed it into the Y axis of the scope and look a 1:1 Lissajou pattern.  Amazing, the phase shift was terrible.  It was flopping all over the place, much worse than the propagation as normally received from any of the WWV's.  As an additional speriment, we de-modulated the modulator feeding the uplink, recovered the 10 MHz signal and looked at the phase shift just through the modulator.  While it had about 90 degrees of phase shift, it was quite stable.

Regarding "Map Snapping"...  I've never heard this term before, but I discovered it several years ago.  I had just bought a new Magellan 4250 GPS receiver and was surprised at how accurate the maps were and how I always seemed to be on the road.  That is until a few days later when I was on the way home and using the GPS while driving my normal route.  It wanted me to stay on the freeway and get off at the next offramp.  Well, I got off at the earlier offramp, almost paralleling the freeway.  I noticed that the GPS still had me on the freeway for several blocks until the freeway and the road I was on diverged.  All of a sudden the GPS jumped to put me on the road I was actually on.  I quickly discovered that while it's good at getting me to where I need to go, it's certainly nowhere near as accurate as my Delorme running TopoQuads under GPS.

Burt