Chris Hastie wrote:
Since GPS satellites are not geostationary, does it matter which bit
of the sky the unit can see? My office window faces north, which
clearly would be no good for geostationary satellites from the
northern hemisphere, but how about GPS?
The view to the sky does matter with GPS as well to some degree, but
obviously not as much as with geostationary satellites. GPS satellites
orbit the earth at an inclination of about 55 degrees, which means that
no GPS satellite is ever further up north than latitude 55N (or south of
latitude 55S).
If you run a program that draws a skyplot of the GPS satellites visible
at your location over time (24-48 hours should be enough), you'll see
that a circle shaped area forms on the plot, where satellites are never
seen. This void is north of your location on the northern hemisphere and
south on the southern hemisphere. The closer to the poles you are, the
larger and closer to the center of the sky the void is. And vice versa,
the closer to the equator you are, the smaller and more to the north
and/or south edge the void is.
Here are links to some plots, these two are from the northern hemisphere:
http://www.accd.edu/sac/earthsci/rudnicki/2170/skyplot.gif
http://www.spahni.com/common/fun/gps/GPS_Data_SkyPlot.gif
This is from Antarctica, but the plot would look the same on the north
pole as well:
http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap5/fig22-86.jpg
If you look at the plots, you'll notice that the north side is not a
total loss. You'll get coverage from the partial view to the west and
east and might even receive some satellites from the north that are on
the other side of the pole. As well as some satellites directly
overhead, depending on how far up north your location is. But if you
have a choice, the side facing the equator is generally better than the
side facing a pole.
Thanks for that. Another question to show my ignorance of GPS. You said
elsewhere that a view of 4 satellites was needed. I've often wondered
why so for time keeping? I can see why for positioning, but I can't help
thinking with a static unit surely it should be able to work out the
time from one providing you accurately tell it where it is?
General purpose GPS receivers, like the Garmin 18, can't be told to
operate in a position hold mode, so they'll always want at least 3-4
satellites to achive a position lock as well as time. But many timing
GPS receivers, like the Motorola Oncore VP/UT, can be told to enter a
position hold mode, in which they'll assume that they are held static at
a predetermined location. In this mode receiving just one satellite at a
time indeed is enough for timing applications. That's how I run my
Oncore VP with an inside antenna and usually about 1-4 satellites being
received simultaneously without any problems.
Tapio
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