[Wildlifewriter]

Do you ever look look at the "Sky View" display on your GPS?

Most units have a section which shows the signal strength from various satellites in view - and some have a little "chart" which shows where the sats are in the sky, relative to your current position. Have you noticed something strange about it?

I have.

Now, most GPS users just take all of this satellite stuff for granted. They're up there, we're down here, this shows the position. Hike completed, or cache found - job done. But it's not that simple...

There are thirty American nav satellites in operation at the moment. (Plus some which are out of service, under test or just loafing around.) On the GPS display, the positions of these seem to be random. They change from hour to hour, but - seen from one place - there's no obvious pattern

The pattern IS there. These GPS satellites are in orbit around the Earth, at an altitude of some 20,500 Km. (10,900 Nautical miles). The orbits are inclined to the equator at an angle of 55º. Like this...



Of course, that diagram doesn't help much because the Earth is a sphere. If we look at the orbits on a projected map, the whole system looks something like...



Notice something interesting: each satellite track (in a rather fetching pink) passes over a different part of the surface - but none of them pass over the North or South Poles. This is a result of the inclined orbits - and it means that none of the satellites are overhead at much more than 60º North, or South.

I don't know about you, but I'm still having some difficulty in visualizing what all this means , as seen from the ground. To help, I set a little program running on my PDA, which records the track of each GPS satellite over a 12-hour period. (Because the sats orbit twice in one day, it gives a picture of the whole constellation as it's seen from here.) This is the result...



... which is kinda interesting. The tracks show that in our part of the world (my latitude is 54º 33' N) there is an area of the sky where GPS satellites are NEVER seen.

I didn't know that...

... And I'll bet YOU didn't, either.


-Wlw.

[HighlandNick]

The recorded track that your last PDA pic gives indicates the actual track that the satellite takes. If you could see it, looking up, (a bit like a contrail left by a jet) then that would show its path.
That doesn't mean that the satellite isn't in view, as it can "see" a large patch of the ground from where it is but from the side slightly.

Presumably the orbits of the satellites (as set by the US military) allows them to concentrate their view of the surface for military purposes, rather than civil ones. Perhaps, due to the "Special Relationship" they don't need to look at us (UK and NI) too closely as they won't have cruise missiles targeted on Belfast??

In any case, there doesn't seem to be much point in having too many satellites orbiting over the poles, as there is little need for them here. Even with the 55 degree inclined orbit, there will still be at least four satellites visible (but lower on the horizon) even at the poles. In this way, satellite coverage is maximised in areas of high population, below say 60 degrees north / south.
There is a good webpage - http://www.kowoma.de/en/gps/orbits.htm that shows some GPS satellite orbits, and from here, you can go to a Java applet on the NASA site that shows the J - orbital path that satellites take. This applet can be speeded up to show the path of any satellite.

[Wildlifewriter]

[HighlandNick]

[Billy Twigger]

You guys need to stay in more!

[Naefearjustbeer]

I managed to glean Blah Blah Blah GPS Blah Blah PDA Blah Blah SKY Blah GAP Blah Blah Blah from that lot
_________________
www.naefearjustbeer.co.uk

[Wildlifewriter]

[HighlandNick]

A couple of Spirograph images that show the same hole from precessing curves:

[Billy Twigger]

[HighlandNick]

Hah Hah
Try Googling for:

gyroscope precessing

[Guest]

A satellite directly overhead (90 degrees elevation) is completely useless for navigation as one of the major error components in the system is directly proportional to the tangent (tan) of the angle of elevation, since tan 90 = infinity you get infinate positional error. Running alongside each satellite orbit are large strips of the earth's surface where the signal is usable so the strips of a satellite orbiting at 60 degrees latitude extend to the poles.

[Billy Twigger]

[HighlandNick]

[Wildlifewriter]

[HighlandNick]