GPS Satellites

- summary or overview of the GPS satellites, the satellite orbits, and function of the satellites themselves.

The GPS satellites form the basis of the overall global positioning system. Their development took place over many years, and they presented many technical challenges in terms of obtaining the correct level of accuracy.

Several blocks of GPS satellites have been launched over the years, each set providing updates to the previous ones as the system has been progressively updated.

GPS satellite basics

The GPS satellites have been under constant improvement since the first ones were launched in 1978. Although the first satellite was launched in 1978, it took until 1994 before a full constellation of 24 satellites was achieved.

Each satellite is built to last approximately 10 years, although the detail below show this varies considerably according to the different phases. This lifetime is not governed so much by the reliability of the electronics, but more by other actors such as battery life and the amount of fuel that can be carried for keeping the satellite exactly in the right orbit and travelling at the required velocity.

Each GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended. The transmitter power of a satellite is about 50 watts or less.

Satellite Block Launch Period Successful Launches Design Life
Launch Mass
H x W x L (cm)
I 1978 - 1985 10 5 759  
II 1989 - 1990 9 7.5 1660  
IIA 1990 - 1997 19 7.5 1816  
IIR 1997 - 2004 12 10 2032 152 x 193 x 191
IIR-M 2005 - 2009 8      
IIF 2010 - 2011 1 (11 in prep) 15   244 x 197 x 197
IIIA 2014 onwards 12 planned      

Summary of GPS Satellites

GPS satellite orbits

The GPS satellites orbit the earth in a Medium Earth Orbit, MEO. The mean distance from the centre of the Earth is 26560 km (the mean radius of the Earth is 6360 km) and this means that the orbit altitude of the satellites is around 20 200 km.

The GPS satellites travel with a speed of about 3.9 km /s relative to the Earth as a whole, as opposed to relative to a fixed point on its surface. They have an orbit time of 12 hours (sidereal time) which equates to about 11 hours 58 minutes "Earth" time. This means that each satellite reaches a given position four minutes earlier each day (as it orbits the earth twice a day).

The satellite orbits are arranged on six planes. The inclination of the angles of the planes towards the equator is 55° and these planes are rotated by 60° against each other. This gives complete coverage of the globe. This means that the orbits range from 55° north to 55° degrees south. It is worth noting that Block I satellites had an inclination of 63° against the equator.

GPS satellite orbits
GPS satellite orbits

Within each orbit, there are at least four satellites. The system was designed for four satellites in each slot, but additional satellites are in orbit to act as "hot" spares in case of failure. In this way, when a satellite fails, and other one can be quickly put into its position to fill the gap.

Inclination of GPS satellite planes

The arrangement of the inclination of the satellite orbits at 55° to the equator has been decided upon to avoid too many satellites being over the polar regions at any time. The orbits run far enough north and south to ensure sufficient polar coverage. However it also ensures improved coverage in areas where there are more users.

GPS satellite orbit angles
GPS satellite orbit angles

An additional advantage is that it provides for a more stable constellation - factors disturbing orbits like the solar wind and gravitation fields have almost equal effects on all of the satellites using this arrangement.

The disadvantage for the polar regions is that at no time are any satellites directly above the users. This can lead to a small but predictable loss of precision.

By Ian Poole

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