gps satellites

Is there an advantage, other than probably cost, to having the GPS satellites at 12,000 miles rather than in geostationary orbit? Would it work either way?

Geostationary can only apply to equatorial orbits. With GPS satellites it makes more sense to have them in a polar orbit so that they can cover the entire Earth's surface over time. Also I presume that putting a satellite at the relatively large distance required for a stationary orbit (22,000 miles) would significantly reduce the precision with which the distance could be measured (GPS doesn't use triangulation, it works by measuring the distance between satellite and receiver).

2009-01-28 10:18:02 by zuestra99

3D Animation of a GPS satellite in orbit. Created in Rhino and Bryce

Seems that no matter how perfectly positioned these GPS satellites are they would drift out of position and that would throw off the computations of our GPS units in cars etc.

The GPS satellites orbit the earth twice a day. It isn't necessary for them to stay in perfect position. The crucial point is that each satellite should "know" where it is. Tracking stations around the world allow the orbits of the satellites to be calculated, that information is sent to the satellites and the satellites, in turn, broadcast that data to the GPS receiver on the ground. When a GPS receiver picks up broadcasts from 4 or more satellites, it can determine its position relative to those satellites and hence derive its geographical location.

2008-05-16 08:14:10 by Peter D

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The satellites wander around and are stabilized with short "burns" of rocket fuel. How many meters do they wander before the ground station orders them to move? I would also strongly suspect (correct me if I am wrong) that the absolute position can be corrected in software to some extent. but with accuracies on the ground of 5 meters or so, I would think the satellites would have to be pretty stable. Yet on the other hand it's hard to imagine those things being nailed to the sky.

That, is government information that they generally do not give out. It really doesn't matter since their information is corrected by ground control. For our needs, they are "nailed" to the sky. With government code, their accuracy can be given in small factions of an inch. The "dithering" that the signals use, however, is to keep the locations of what the government kept as inaccurate to be just that, inaccurate. Sorry, but your need to know isn't high enough for more than that.

2007-10-22 18:40:43 by Barney

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I need to make a model or hands on demonstration that I could use to demonstrate the basics of satellites and gps positioning systems.

I've learned the fundamentals of the positioning system, and the system of satellites, but I'm struggling to think of an effective way to present it.

Thanks

Put out a large area shallow pan. Mark three separated spots on the bottom of the pan to indicate the location of your satellites. Ideally, each satellite spot would be located on one of the points of an isoceles triangle. Mark a fourth spot among the three satellite spots as your target spot, the place locating the position you want to fix via your simulated GPS.

Fill the pan with water so as to cover its entire area with, say, 1 inch of water. Put a device to sit on the bottom of the pan where the target spot is marked so that the device sticks out of the water.

Stick a finger in the water over the first "satellite" spot to create a ripple. The ripples represent the GPS signals from that satellite. Time the time t1 it takes for the ripple to travel from the first spot to the target. Repeat this with the other two satellite spots to get t2 and t3.

To calibrate the velocity V of a ripple, measure the distance between one of the satellite spots and the target, using a meter or yard stick. Then V = S/t; where S is the measured distance and t is the time it took the ripple to get to the target. In a real GPS V = c, the speed of light, so this step isn't necessary for a real GPS case.

Given the wave velocity is the same V for all the ripples, you can calculate the distance between the target device and each of the spots where you dipped your finger to create the ripples. Thus, you'll have s1 = Vt1, s2 = Vt2, and s3 = Vt3.

Repeat these procedures until you have, say, N = 30 calculated sets for s1, s2, and s3. Let S1 = sum(s1)/N, S2 = sum(s2)/N, and S3 = sum(s3)/N be the average distances to the target from the respective dipping points representing three satellites. This is an important step to minimize measuring errors.

Plot each of these three average distances on a piece of paper as radii around each of the corresponding three dipping points. Where the three radii intersect is the calculated position of your target device. You can sort-of prove this result by moving the target around and redoing the experiment to show your experimental results correspond with the actual position of the device on the pan.

Check your calculated "GPS" results with the actual postion as measured with a meter rod or yard stick; it'll be interesting to see how accurate your calculated "GPS" measures were. Speculate on where the errors might have come from. Might GPS have the same kinds of error?

And there you have it...that's how GPS works. By ranging off three or more satellites, you can fix a position on Earth's surface quite accurately...down to about 1 m error using the best GPS receivers. And, if you add more satellites, the ranging error becomes less because there are more radii to intersect.

2008-10-09 10:54:55 by Rick S

Can the North Koreans or the Iranians use our satellites for GPS use?

Would the kind civilians use be of any use by these countries for military use?

Yes they can use the satelites just like anyone else however the United States has codes which increase the accuracy of any co-ordinates for military applications. Only the U.S. and whoever it wishes to give these codes to has them. So North Korea can use the codes for military applications but they won't be anywhere near as accurate as the U.S. accuracy and if the U.S. wish to in a time of war they can completely disable, or throw off the civilian codes. The USAF runs and maintains world GPS.

2009-06-08 05:08:38 by Mr Average Joe