How does GPS Work?

Global Positioning System satellites circle the earth in the same orbit, twice a day and transmit signals down to the stations located on mother earth. The information retrieved from these signals is taken and then triangulation is used to accurately calculate the user’s precise location. The GPS receiver will then compare the time when the signal was transmitted by the satellite to the time the signal was received. This time difference will then tell the receiver how far away the satellite actually is. With these distance measurements from a couple of the satellites, the receiver will then be able to determine the user’s exact position and will then display it on an electronic map in the unit.

It is important to note that these GPS receivers only receive information and don’t transmit signals in any way. For unobstructed transmittance of signals, receivers are required to have an unobstructed view of the sky, so units are generally placed outdoors and tend to perform very poorly if placed near tall buildings or within forests. GPS operations are highly dependent on accurate time references that are generally provided by atomic clocks at the U.S. Naval Observatory. Each and every GPS satellite will have an atomic clock placed on board.

So, we already know that these satellites transmit information that indicates the current time and its current location. All these GPS satellites synchronize operations so that any repeating signals can be transmitted at the very same instant. These signals move at the speed of light and arrive at the receivers end at slightly varied times, as some satellites are farther away than the others. the distance to the satellites is calculated by estimating the time taken for the signals to reach the GPS receiver. Once the receiver has estimated the distance of at least 4 of these satellites, it can then calculate their positions in three dimensions (latitude and longitude and even the altitude). Once the receiver is locked on to the signals of at least three of these satellites, it can then calculate a 2 d position (longitude and latitude) and can also track movements. Once the position is determined, the unit can then calculate other factors like the speed, trip distance, track, distance to the destinations, sunrise time and sunset time, and so on.

Today, there are at least 24 functional satellites doing the rounds at all times. The GPS satellites that are operated by the U.S. Air Force orbit the earth with a period of 12 hours. Ground stations can also precisely track each satellite’s orbit.

So, if you’ve heard of GPS and have asked yourself the question "How does GPS work?’ many times, then this is juts a brief explanation.

How does GPS determine Positions?
A GPS receiver generally ‘knows’ the current location of the GPS satellites, since this information is usually included in the transmissions made by the satellites. By estimating the distance to the satellites, the GPS receiver also ‘knows’ that it is located on the surface of an imaginary sphere that is centered at the satellite. It will then determine the size of several of these spheres, one for each of the satellites. The receiver will then locate where the spheres intersect.

Accuracy of GPS
The accuracy of a position that id determined by the GPS receiver depends entirely on the type of receiver. Most hand-held units have an accuracy of about 10 to 20 meters. Other types of GPS receivers use a special method called the DGPS or Differential GPS in order to obtain accuracy that is much higher. This DGPS generally requires an extra receiver that is fixed at a nearby known location. Observations made by this stationary receiver are generally used to correct positions that are recorded by roving units, and this helps to produce an accuracy that is much greater than 1 meter.

When the GPS system was first created, timing errors were initially inserted into the transmissions in order to limit accuracy of all those non-military receivers to about 100 meters or so. This part of the GPS Operations, known as Selective Availability, was eliminated in the month of May 2000.