GPS

What is GPS?
GPS stands for Global Positioning System, which is a system that allows suitable receivers to find their position anywhere on the earth's surface.
The vehicle positions shown on the mTrak website have been found using the GPS receivers in the mTrak units.

GPS consists of three components:-

• A constellation of around 24 satellites, each orbiting at around 20,000 km altitude, transmitting signals to earth.
• Ground stations to control the satellites.
• The receiver inside your mTrak unit (or smart phone, or car Sat Nav, or one of many other electronic items)

The signals from the satellites say very accurately where the satellite is, and what time it is.

How does it work?
A GPS receiver works out where it is by receiving signals from several satellites. Although the signals take less than 0.1 s to get from the satellites to the receiver, the receiver accurately measures how much longer the signals take to arrive from each satellite than from the others.

Knowing that the signals travel at the speed of light, the time differences allow the receiver to work out how much closer it is to each satellite compared to the others. With some clever mathematics, and the information of where the satellites are, the receiver's position in 3 dimensions is calculated. The time and the speed the receiver is travelling at are also worked out.

Incorrect explanations
Many explanations of how GPS receivers work are incorrect, and they say that the receivers know how long the signal takes to arrive, because the receivers know what time it is, and when the signals were transmitted. The clock in a standard GPS receiver isn't accurate enough for that. The receivers can only measure time difference, not absolute time, accurately enough to work out position.

A predecessor of GPS, the DECCA system was a two dimensional version, with ground-based transmitters. The earliest DECCA systems only worked out the time differences, and left the user to work out the position. The charts to help with that had curves overlaid, called hyperbolae, which were lines of all the points a certain distance further from one DECCA transmitter than another. The principal is the same as GPS, in that difference in distance from the transmitters is what is measured. If it was distance from each transmitter that could be measured, the DECCA charts would have had circles not hyperbolae drawn on them.