... they don't fly over the North Pole.
As I write this, today is the 29th Jan 2003. Here's a plot of GPS satellites that have orbited overhead over the last 8 hours:
Its like a time exposure of the Garmin Etrex satellite view.
Actually it's a bit of C# .NET code I'm playing with.
I parse the NMEA $GPGSV frame and plot the results. I get a similar plot as the Etrex - so my code must be close.
Big grey blob - strong signal. Black - current position. Small blob - no signal.
I was bemused to see the "void": the near circular area that satellites fear to tread. I assumed, initially, a bug in my code.
My latitude is 52ºN. The hole appears just above the origin... must be a maths bug...
No, not a bug. Its those satellites! Their "Inclination" orbital parameter is close my latitude of 52ºN.
Put crudely - they don't fly over the North Pole.
On Jan 2003 these satellites were near overhead some point in the day:
| Vehicle | GPS ID | Inclination |
| GPS BII-08 | 21 | 56.1871 |
| GPS BIIA-10 | 23 | 56.4006 |
| GPS BIIA-26 | 10 | 56.2101 |
Orbital period 718 minutes.
I seem to be slowly building up a "x-ray" of my house with the GPS struggling to get signals.
Weak signals are seen where item obscure: cold water header tank ENE elevation 40º. PC monitor 3 ft west of the GPS. Stronger signals are directly south where there is a window.
This maps the GPS satellite number to the vehicle number.
http://www.city.yokohama.jp/yhspot/ysc/NAVIGATN.TXT
and this emulates a view from space:
http://liftoff.msfc.nasa.gov/RealTime/JTrack/3D/JTrack3D.html
I have a Bluetooth GPS too!