Satellites

[+Jhwk]

Just a quick question I had this morning. Wouldn't a stationary satellite system, fixed at a point above the earth's surface provide an overall increase in accuracy and GPSR design simplicity over ones that are hurling around the planet in geostationary orbit? Wouldn't it be easier to design a GPSr based simply on the time distance calculations versus time, distance, arc, deflection, and every other calculation that I can't think of?

 

need more math...

[+embra]

This is more a guess than an authoritative response...but I think the geosynchronus (sp?) orbit requires a position above the equator (or close to it). I could well be wrong on that. But if true, that would severly limit the triangulation possibilities that these seemingly crazy orbits present, as well as the places that one could use a GPSr.

 

I watch for other guesses and better informed responses on this.

[+Klemmer]

Simple answer:

In order to be stationary with respect to the earth's surface, the satellite has to be in geosynchronous orbit (about 25,000 miles above earth). That is A LOT further than GPS satellites are now. Harder to get the satellites up there, more power to broadcast down to earth, bigger antennas on earth... etc..... You know those TV satellite antennas on your or your neighbor's roof? Imagine walking around with one? Not practical.

 

Embra is also correct. Geosychronous orbits HAVE to be above the equator. That's another problem.

Edited December 22, 2005 by Klemmer & TeddyBearMama

[+geognerd]

My comments also will fall into the guess category. Check out the quote below from this website:

It should also be clear that it is not possible to orbit a satellite which is stationary over a point which is not on the equator. This limitation is not serious, however, since most of the earth's surface is visible from geostationary orbit. In fact, a single geostationary satellite can see 42 percent of the earth's surface and a constellation of geostationary satellites—like the one Clarke suggested—can see all of the earth's surface between 81° S and 81° N.

 

Of course, the advantage of a satellite in a geostationary orbit is that it remains stationary relative to the earth's surface. This makes it an ideal orbit for communications since it will not be necessary to track the satellite to determine where to point an antenna. However, there are some disadvantages. Perhaps the first is the long distance between the satellite and the ground. With sufficient power or a large enough antenna, though, this limitation can be overcome.

 

The fact that there is only one geostationary orbit presents a more serious limitation. Just as in putting beads on a loop of string, there are only so many slots into which geostationary satellites can be placed. The primary limitation here is spacing satellites along the geostationary belt so that the limited frequencies allocated to this purpose don't result in interference between satellites on uplink or downlink. Of course, we also want to make sure the satellites aren't close enough to run into one another since they will have some small movement.

So, in summary, I guess there would be crowding and power issues if the GPS satellites were geostationary, and incomplete coverage of the earth.

 

Edited to add that I believe the system is set up so that at minimum 4 satellites are visible at any time at any place around the earth. A constellation of satellites orbiting around the earth (not stationary) would guarantee the aformentioned coverage is met temporally and spatially.

Edited December 22, 2005 by geognerd

[+Sputnik 57]

My equally uniformed guess has to do with altitude and reception of signals. A circular geosynchronous orbit is at an altitude of 22,240 miles above the surface of the earth. GPS satellites orbit at 12,600 miles up.

 

Edit: I type a lot slower than Klemmer and Geonerd.

Edited December 22, 2005 by Sputnik 57

[+blindleader]

So, in summary,

And the number one reason not to use geosynchronous orbits is that the satellite geometry would be horrible to the point of being useless 100% of the time. You think 50ft EPE is bad now, think about operating in the hundreds of feet of error at best. And if you live at a low latitude, think in terms of many miles of error and approaching infinity as you approach the equator.

[vr12]

All geostationary satelites are located in equator plane which means atleast three things:

1. You will not be able to resolve between south/north hemisphere.

2. latitude resolution at the equator would be horrible, my guess is >100 km

3. GPS sattelites at north/south pole will be too low to see.

Edited December 22, 2005 by vr12

[+TotemLake]

You also have to think of the economies of hardware. It's easier to use orbiting satellites versus stationiary satellites and you need fewer of them to do the same thing than stationary satellites.

 

Take at what it took to get the Iridium (satellite phones) system up. The economies dictated a minimum of low orbiting satellites to cover 99% of the world 99% of the time. Originally designed with 77 satellites in mind, it was pared down to 66 satellites reducing hardware and operational costs to the tune of between $300,000-$500,000 per satellite excluding any launching costs and associated insurance.

 

The point is, the fewer birds in the air, the less hardware and operational costs are endured. Not bad for a 50 foot EPE eh?

[+Jhwk]

so, basically, you are saying I can't have a bird, or series of birds parked over the US? Like the weather satellites?

[+TotemLake]

There aren't that many covering the USA. The two locations for our weather satellites are over the Atlantic and over the Pacific.

2 MTSATs

2 GOES

 

There are other satellites, but they are used to cover the rest of the Earth for long distance advanced forecasts.

 

For some software to download so you can see how the satellites are stationed and where they will orbit over next, go here.

 

=-=-edited to modify my first statement-=-=

Edited December 23, 2005 by TotemLake

[peter]

so, basically, you are saying I can't have a bird, or series of birds parked over the US? Like the weather satellites?

That is correct. There isn't any way to position a satellite to stay above any place on the earth's surface other than points along the equator. There's really no such thing as a "stationary satellite" since satellites need to orbit around the center of the earth to avoid falling down. Even in orbit they're still continually falling toward the earth as they move, but the curved surface of the earth is dropping away under them just as fast so they remain in orbit.

 

If the orbital radius is large enough (about 26000 miles) then it takes the satellite exactly 24 hours to orbit once so it makes one orbit per day. If that orbit is over the equator then the satellite remains over the same spot on earth and *appears* stationary although it's still in orbit - this is called a geostationary orbit. You can also have an orbit at that radius but inclined at an angle to the equator, called a geosynchronous orbit. In that case the satellite will stay over parts of the earth with about the same longitude but it'll go between the northern and southern hemispheres as it orbits.

 

The Japanese are using this type of orbit with their version of 'WAAS' satellites. By having three geosynchronous satellites all at about the longitude of Japan and in an inclined orbit they make sure that at least one is always pretty high in the sky above Japan.

[+Jhwk]

Thanks to all, especially peter. great explanation for a satellite noob like me

 

closing thread.

Edited December 23, 2005 by Jhwk