Another Reader question about GPS Satellite positioning, from PeterD:
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.
No related posts.
They don’t drift away because they can calculate their position very accurately using atomic clock!
they can calculate their position, and when they move out of the position they should be in, the use engines to put themselves back into the correct position
I believe they do have small thruster jets on them that allow controllers to make small orbital adjustments when needed.
since the earth is a little less than 25000 miles in circumference the satellite is positioned at about 1100 miles from earth surface, the necessary speed computed to maintain it at the exact speed of the earths rotation at that height and the satellite speed set to maintain position at that point. try this link for further information.
There are a few sites on the earth that have been set up to monitor and track the satellites. Their positional information is calculated and then sent to them to update the onboard computers.
A complete explanation can be found at
You can also find more information about GPS devices at
the ground stations monitor the locations of the satellites and adjust them as needed.
teh satellites transmit data that describe their orbits. gps receivers use this information.
It doesn’t matter if the satellites drift or not — what matters is that the GPS units know what the drift is. The satellites broadcast their exact position all the time. Actually, they broadcast the numbers for an equation that can be used to calculate the position. Then your car GPS unit calculates the equation to find out how far away the satellite is. When this is done for 3 or 4 satellites, the car unit can figure out where the car is and how fast it’s going.
The GPS ground station figures out the orbit equations for the satellites, several times a day. Then they send the equations up to the satellites by radio, and the satellites broadcast the equations to the whole world.
The Block II GPS satellites have been deployed:
* In 6 nearly circular orbital planes.
* With 4 satellites equally spaced within the plane (the 3 in-orbit spares are spread across different orbital planes).
* In orbital planes at an inclination of 55 degrees.
* At altitudes of approximately 20200km above the earth.
As the GPS satellites are in nearly circular orbits, at an altitude of approximately 20200km above the earth, this has a number of immediate effects which make the prediction of satellite location comparatively easy:
* Their orbital period is approximately 11hrs 58mins, so that each satellite makes two revolutions in one sidereal day (the period taken for the earth to complete one rotation about its axis with respect to the stars).
* At the end of a sidereal day (approximately 23hrs 56mins in length) the satellites are again over the same position on earth.
* Reckoned in terms of a solar day (24hrs in length), the satellites are in the same position in the sky about four (4) minutes earlier each day.
* The orbit groundtrack approximately repeats each day, except that there is a small drift of the orbital plane to the west (-0.03 per day). [end quotes]
The orbits are well known and tracked, and it is the combination of signals form several satellites that allows the user the accuracy of data.
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.
They stay in orbit around the earth. Just like the moon.