- Consumer Electronics & Personal Gadgets
G P S global positioning satellites. How do they work.
What Is a G.P.S. Satellite
G.P.S stands for Global-Positioning-System .
There are about 33 GPS satellites in orbit at this time. Some of satellites have been in operation over 20 years.
At least 24 of them are in service at all times. The others are able to be turned on in the event one fails.
They are in a grid called a GPS constellation, surrounding the Earth in equally spaced orbital planes. From almost any place on earth you are in line of sight with at least four satellites. They are in a Geosynchronous orbit.
So what is a Geosynchronous orbit?
At first it seems simple. The satellite is orbiting the earth in a way that if you could see it, would look like it was always in the same place in the sky.
Sounds simple right? Guess again. The math is mind boggling.
The satellite orbits at about 12,000 miles from the earths surface. It travels at a speed that keeps it orbiting with the planet, and in the same spot in relation to a given spot on the surface. So instead of going around the planet like the moon. It goes around at the same speed the planet turns. So it orbits . But always appears to be in the same place when observed from the ground. Imagine trying to figure out the math to accomplish that.
Just a side note. The satellite that broadcasts your satellite TV service is also in a Geosynchronous orbit. But at about 22,300 miles out in space. So imagine if you will, a straight line 22,300 miles long, from your satellite dish to the satellite. Now if you move the bottom of that line 1/8 of an inch. The the other end moves hundreds of miles. That is why your dish moving just a bit can mess up your TV watching.
Another Side Note
There are over 13,000 satellites orbiting this planet. Many are no longer functioning. They will eventually burn up on the way back through the atmosphere.
How Do They Know Where I Am?
For your GPS receiver to work ,it must have contact with at least three satellites.
Each satellite sends a signal at the exact same time. Because the satellites are at different distances from your GPS receiver, they take different amounts of time to reach the receiver.
This is also called triangulation.
The computer in your receiver is programmed to know what satellite and where that satellite is located. So by calculating the time it takes the signal to get there from each satellite , the receiver can figure out where it is,and display it along with a map of the location. Once again , mind-boggling math.
The standard GPS most of us use , is accurate to within 20 meters. Of course there are systems that are accurate to one meter. But those require the help of a set ground location to help.
I have heard that the military has units accurate to one meter without ground support. I have no data to back up or deny this .
So that is the basics of GPS systems. Thanks for reading, and happy travels.