GPS Network & Ground System

- summary or overview about the GPS satellite system and network, detailing the system aspects including ground stations.

In order to run the GPS satellite system, a comprehensive worldwide network is required.

The fully operational GPS satellite system consists of a constellation of 24 operational satellites with a few more in orbit as spares in case of the failure of one. The GPS satellites are in one of six orbits. These are in planes that are inclined at approximately 55 degrees to the equatorial plane and there are four satellites in each orbit. This arrangement provides the earth user with a view of between five and eight satellites at any time from any point on the Earth. When four satellites are visible, sufficient information is available to be able to calculate the position on Earth.

GPS satellite orbits
GPS satellite orbits

GPS control network

The GPS satellites need to be monitored and controlled from the ground and it is necessary to be in contact with each satellite for most of the time to be able to maintain the level of performance required. To achieve this there is a master station located at Falcon Air Force Base, Colorado Springs, USA. However there are other remote stations are located on Hawaii, Ascension Island, Diego Garcia and at Kwajalein. Using all these stations the satellites can be tracked and monitored for 92% of the time. This results in two 1.5 hour periods each day when the satellite is out of contact with the ground stations.

Using the network of ground stations the performance of the GPS satellites is monitored very closely. The information that is received at the remote stations is passed to the main operational centre at Colorado Springs and the received information is assessed. Parameters such as the orbit and clock performance are monitored and actions taken to reposition the satellite if it is drifting even very slightly out of its orbit, or any adjust the clock if necessary or more usually provide data to it indicating its error. This information is passed to three uplink stations co-located with the downlink monitoring stations at Ascension Island, Diego Garcia and Kwejalein.

GPS operation

GPS operates by a process of triangulation. Each GPS satellite transmits information about the time, and its position. By comparing the signals received from four satellites the receiver is able to deduce how long it has taken for the signals to arrive and from knowledge of the position of the satellites it can calculate its own position.

The GPS satellites transmit two signals on different frequencies. One is at 1575.42 MHz and the other at 1227.6 MHz. These provide two services, one known as course acquisition (C/A) and the other is a precision (P) signal. The precision signal is only available for the military, but the C/A elements of GPS are open to commercial use, although initially a random "wobble" was put onto this to degrade its accuracy for civilian use. This facility known as Selective Availability (S/A) was discontinued in May 2000.

Both signals are transmitted using direct sequence spread spectrum (DSSS), and this enables all the satellites to use the same frequency. They can be separated in the GPS receiver by the fact that they use different orthogonal spreading codes, and this works in exactly the same way as the CDMA cell phone systems. The spreading codes are accurately aligned to GPS time to enable decoding of the signals to be facilitated.

The coarse acquisition signal at 1.5 GHz uses a 1.023 MHz spreading or chip code, while the precision signal is transmitted at 1.2 GHz using a 10.23 MHz code. This precision signal is encrypted and uses a higher power level. Not only does this assist in providing a higher level of accuracy, it also improves the reception in buildings.

All the GPS satellites continually transmit information. This includes what are termed ephemeris data, almanac data, satellite health information, and clock correction data. Correction parameters for the ionosphere and troposphere are also transmitted as these have a small but significant effect on signals even at these frequencies.

The ephemeris data is information that enables the precise orbit of the GPS satellite to be calculated. The almanac data gives the approximate position of all the satellites in the constellation and from this the GPS receiver is able to discover which satellites are in view. Although each satellite contains an atomic clock, they all drift to a small extent and as a result details of the clock offsets are transmitted. It is found that it is more effective to measure the error and transmit this data than maintain the clock exactly on time.

The Global Positioning System, GPS, has been in use for a number of years now and has proved to be very successful, with the GPS technical aspects being well understood by the various companies designing, manufacturing and selling GPS or satnav systems.

By Ian Poole

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