First, I had heard that the GPS satellites’ orbits take them from between around 60degN and 60degS. So I was surprised to find when I was over 60degN last weekend that the diagram showing the satellite positions included some near the northern horizon. I had expected all the visible birds to be in the “southern” half of the diagram. Or are the satellites so high that I am looking across the Pole at ones that are 180 degrees around the world from me?
Second, a couple of times recently, there has been an exclamation mark after the number of one of the satellites, like this: “28!” Does this mean the receiver is surprised to get number 28, or what?
The satellites are 11,000 miles up, so my guess is that your guess is pretty much correct - not 180 degrees, but but what your receiver perceives as “north-pole-ward”, and in fact across the north pole and below 60 degrees on that side of the world.
You can go here http://liftoff.msfc.nasa.gov/realtime/JTrack/ and see the real time orbits of the GPS constallation, as well as all other known satellites. Scroll down and choose “J-Track 3D”.
The GPS orbits appear to be inclined at least 45 degrees or so.
According to this site the current NAVSTAR-GPS satelites are at orbits with a 55° inclination. (There was an early block of satelites at 60°, but it’s been discontinued.)
I’m not sure how to interpret the ‘orbit distance’ that is given as 20200km, but I assume it’s the radius of the orbit. That would mean that the satelites are at a little more than an earth-radius up. There is no way that you could see one of these ‘over the north pole’.
My only idea is that maybe the Garmin12 can use GLONASS satellites. (I’ve got a Garmin12 so I ought to know, but I don’t.) The GLONASS satellites are at a slightly lower orbit and at 64.8° inclination. (cite)
Yet again, as I have a Garmin12, I ought to know about this - but I don’t. The display is a bit buggy, and could it not be that there is a second satellite ‘hidden’ behind 28, so that it only shows a part of the number?
(geeky note: Did you know that you can upgrade the firmware of the Garmin12? All you need is a serial cable (with an odd connector) and a PC. - The latest firmware is 4.58 and can be downloaded from Garmin!) )
I disagree with your analysis. If the radius of the satellite’s orbit is 20200 km, and the radius of the earth is 6371 km, then the angle between me and a satellite on my northern horizon is cos[sup]-1/sup=72deg.
If I’m at 60degN, the satellite on my northern horizon is at 48degN, on the opposite meridian. Conversely, I should be able to see “far-side” satellites down to 53degN.
No, but it just occurs to me it can use LORAN. I wonder how that shows up on the display.
Yes, I think you’ve got it! Probably there’s a “2” hidden behind the “28” and the right hand edge looked like a “!”
I stand corrected. I somehow managed to interpret the radius as a diameter…
Now, that I refuse to believe (at least until I see some evidence:))! I can’t see why they would bother to include an antenna for 100kHz in such a small handheld device, especially at that price!
And if it used LORAN it would work better under tree cover etc, where mine is next to useless.
You can get them through the first ShareHardware project ever!
And depending on your PDA of choice you can get real-time live updates of your position together with maps in your PDA! (RealMaps worked great with my Psion 3c/5mx, but not as well with my current Revo.)