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From: Mike Maurice (no email)
Date: Mon Oct 03 2005 - 19:34:42 EDT
I thought some of you might find the following article of interest,
if only to make your education concerning GPS more complete and on
the off chance that you might want to venture above 80 North or South.
http://www.gpsworld.com/gpsworld/article/articleDetail.jsp?id=72174
A few excerpts.
Several unique phenomena affect GPS performance at high latitude,
where the satellite constellation changes dramatically due to the
inclination of the GPS satellite orbits. No satellites pass directly
overhead as one approaches the North Pole. Instead, all satellites
appear closer to the horizon, and at very high latitudes (greater
than 80 degrees) satellite visibility increases to 12 or more at
low-elevation angles. This produces a reduced geometry as well as a
high-potential multipath environment.
The northernmost settlement in Canada and in the world, Alert lies
450 nautical miles from the North Pole. Figure 1 shows elevation
angles for latitude 82 degrees north. As many as 15 or 16 satellites
may be visible during the day, but satellite elevations rarely rise
above 45 degrees. The resulting position dilution of precision (PDOP)
varies from a very respectable 1.5 to 2.0 to as high as 5.5 with
numerous spikes for short periods of time.
Regards,
Mike
Capt. Mike Maurice
Tualatin(Portland), Oregon
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