Quezi

Around the world, making an artistic GPS track (graphic by Oliver Belikan - CC-BY)

GPS, the Global Positioning System, can determine your location anywhere on the earth. In good conditions the accuracy is just a few meters. How can GPS do this?

The GPS system uses a constellation of over 24 satellites orbiting the earth. The orbits are arranged so that several satellites are “in view” at any time, from any point on earth. Each satellite broadcasts a continuous signal which precisely encodes the time (from an on-board atomic clock), plus a mathematical description of the satellite’s orbit together with information on approximately where all the other satellites will be located.

The GPS receiver looks at the information received from four or more satellites, and examines the precise time at which each signal was received. From this it can calculate a unique three-dimensional position from which the measured delays would be expected.

The accuracy of the determined position depends on several factors:

• Accuracy of the satellite’s atomic clock
• Accuracy of the receiver’s (corrected) clock
• Accuracy of the coded description of the satellite’s orbit
• Tropospheric and ionospheric signal path length variations
• Calculation precision
• Multi-path distortion

These sources of error limit the horizontal accuracy to about 15 meters, but in practice some of the errors can be adjusted for or will cancel each other out and a modern civilian GPS system may achieve a horizontal accuracy of about 5 meters in good conditions, and vertical (altitude) accuracy of about 10 meters. These are 95% figures, which means that 5% of the time the reading could be less accurate. Going indoors, or even under tree cover, will greatly reduce the accuracy.

It’s possible to increase accuracy further, for example for surveying purposes, but this increases the complexity and cost of the receiver. Techniques to improve accuracy include precisely measuring the phase of the arriving signals, and comparing the satellite signals with an earth-based signal broadcast from a known nearby point (called differential GPS).

Before a GPS receiver can calculate its position, it needs to get a “lock” on the satellite signals. It must search for them by varying the time slots in which it is searching for the signals, and by varying the frequency at which it is searching for the signals. This can take from 30 seconds to 20 minutes. After the receiver has a lock it can extract meaningful information from the signals it is receiving.

If the startup time is too long, it can be reduced using Assisted GPS, which requires a local source of precise time or of fairly precise location. With this information, the GPS receiver knows roughly how to lock on to the signal and can do so faster. A convenient means of providing assisted GPS is from a nearby cellphone tower that broadcasts the co-ordinates of its location.

The GPS system is operated by the United States Air Force. Originally it provided degraded accuracy for civilian applications, but since 2000 has allowed civilian users to obtain the same full accuracy as military users.

GLONASS is a similar Russian-operated system. China has a regional system, and other countries are planning regional systems to enhance coverage of their part of the world. The Galileo system is being developed by European countries and other interested countries and is intended to be operational by 2013. It is intended to offer improved signal strength and greater precision.