How does my Navigation System know where I am?

Satellite navigation systems are such a common sight in modern cars that many of us take them for granted. Although they are simple to use, the technology behind these devices is far from simple.

The system that underpins all Sat Nav devices is a system of navigational satellites. There are currently two popular global systems – one maintained by the US (GPS) and one by Russia (GLONASS) – as well as a number of regional systems. The US system consists of 24 (actually 27, three of which serve as backups) satellites which orbit the globe at an altitude of approximately 19,300 km. Each satellite circles the Earth twice every 24 hours. The key to this system is the positioning of the satellites; at any time, four of the satellites in the system should have line-of-sight to any point on the surface of the Earth. Each satellite sends out a series of time signals, each perfectly synchronised with all the other satellites. A GPS receiver picks up four signals, although it can function with three.

Since the four satellites are different distances away from the receiver, their time signals will arrive at slightly different times. Since the receiver “knows” how fast the signal is travelling (the speed of light) and how great the difference is between the time given in the signal and the time at the receiver’s end, it can determine how long the signal took to arrive. The receiving unit can thus determine the distance between the transmitter and receiver.

Knowing the distance from any one satellite is not enough, since there are a number of points on the earth’s surface all equally distant from the satellite; in essence, this is the intersection of a sphere, the radius of which is the distance between the satellite and the receiver, and the surface of the earth. However, by comparing the distance from multiple satellites, the receiver’s processor can locate the point where three or four receivers’ spheres intersect. The receiver can thus derive its latitude, longitude and altitude. This process is called trilateration and a brief, 2D example of it is given below:

Identifying the receiver’s exact position is only part of the problem for a navigation system, but the next step is somewhat simpler. The system’s memory stores a set of maps, periodically updated. By comparing its coordinates to the coordinates on the map, the system locates the receiver’s position relative to roads, buildings and terrain features.

Although these principles are simple, several factors complicate matters. The first is the processing power necessary to perform trilateration calculations continuously. The second is the accuracy necessary to measure the minor differences in time between signals. Each satellite is equipped with a highly precise atomic clock, but the receiver carries only a normal quartz clock. Receivers compensate for this with a further calculation. If the receiver has a signal from four satellites, it can calculate the only time value that would cause all four spheres to intersect. The receiver thus continually sets its own clock to the time on the satellites’ atomic clocks.

The final problem is a simple technical one – not all signals travel at the same speed through the atmosphere. Weather conditions can cause subtle variations in signal speed, and obstacles such as tall buildings can deflect the signal. In order to counteract these discrepancies, differential GPS signals check their coordinates not only against satellites but against the coordinates reported by other systems in the area.

Rashed is a seasoned guest blogger who enjoys guest posting on business and technology related topics. Rashed is currently guest posting on behalf of RAM tracking who specialise in vehicle tracking software.

About Jakk: Jakk Ogden is a professional self-employed blogger and the founder / owner of Technology Blogged. 22, with a love for good writing, you'll find me playing 'Drag Racing' on my HTC One X and rocking a pair of Grado headphones. If you love technology, be sure to subscribe to my feed for unique editorials. Find me on Google+. View author profile.