how come satelites scale for so many GPS units out there?
Vladimir Ghetau asked:
Hi ,
Hi ,
I am really wondering about this, and couldn’t find any answer, or maybe I wasn’t searching for the right thing.
With all these GPS units being sold nowadays, how come the satellites keep handling all these units questioning “what’s my position now”, how come they scale so well? I need a point to start understanding this.
Cheers!
any number of gps receivers can listen to a gps satellite.
It’s only a one-way communication, like a radio broadcast. The GPS satellite doesn’t ‘tell’ each GPS individually where it is. The satellites are continually broadcasting their position and time and the GPS unit works it out its own location by receiving signals from four or more satellites and calculating how long each signal has taken to arrive. It then solves an equation to find out its location – the only location where those particular signals would have arrived at that particular time.
All the satellites do is broadcast a signal for anyone and everyone to listen to. The satellites do not send signals to individual GPS units.
It just like your local radio station. Any number of people can listen on their own radios.
.
Because the satellites don’t listen. The satellites do only one thing the whole day long: They send their own position and time out into the world, as well as position data about all other satellites (Almanac). All GPS satellites send also on the same frequencies, but like all low-flying satellites, this carrier frequency is shifted by Doppler effect.
The satellites are not smart at all, they don’t know anything. They don’t even know their own position, they need a ground station for telling them where they are. A GPS satellite is nothing else but an atomic clock with a small computer to produce a low-speed digital data stream (50 bit/s, slower than even the oldest modems in history), with the information you need, and a sender to send this data to the receiver. The 50 bit/s data is combined with a code sequence at a much higher data rate, so multiple bits (called “chips” for such codes) describe one bit of data.
The receiver does the magic: It receives the signals from at least four satellites, their current orbital data and time data and uses them for calculating the distances to the four satellites, by first calculating the current position of the GPS satellite from the orbital data the satellite told the receiver, and then calculates the distance to the satellite by analyzing the signal. Each signal of a satellite is synchronized with the atomic clock in the satellite. at the moment your receiver receives the signal, it was traveling from the satellite to your location at the speed of light, so by knowing the travel time of the signal, you can know your distance to the satellite (in the moment it send the signal to you). Now, your receiver has no atomic clock to tell how late it should be, and it does not need it. That is the point where the forth satellite comes into play. With the four signals from the satellites and their four positions and time information, your receiver can calculate your position by using a special mathematical trick. You could imagine this trick as a smarter way to play with the informations from the satellites and try different combinations of the data. The receiver does a set number of iterations/tries to get the best solution signal travel time fitting for the received signals – and that solution also tells the receiver where it most be relative to the four satellites. From the process of calculating the position, the receiver can also estimate how accurate the solution probably is – an important information for GPS receivers inside aircraft.
As others have said, the GPS units do not ask the satellites anything or send any signals up to the satellites at all. They only listen to the satellites and use information contained in the satellite signals to calculate where they are on their own.
The GPS system depends on almost unbelievably accurate clocks in each satellite that create a signal accurate to a tiny fraction of a second so that the remarkable computer in the receiver can create a 3 dimensional position of latitude, longitude and altitude from the timing of several satellites – the more the merrier. The satellites only send, the receivers only receive.