The GPS system can be devided into three basic segments. The space segment will be discussed below, the control segment will be explained on a separate page:
- Space segment (satellites)
- Control segment (control stations)
- User segment (GPS receiver)
The space segment consists of at least 24 satellites. The first of the satellites was brought to its orbit as early as 1978. During the years the satellites became more and more sophisticated and meanwhile five different types of these satellites exist (Block I, Block II, Block IIA, Block IIR und Block IIF).
Block I Satellites
From 1978 to 1985 11 Block I satellites were launched from California,
each having a weight of 845 kg. None of those still operates today. Their
lifespan was supposed to be 4.5 years and all of them exceeded this lifespan
about another 5 years. The oldest of the satellites, in the beginning
designed as prototype for the testing of the system, has been operating
for 13 years.
All signals of the Block I satellites were accessible for civil users. Solar panels served as power supply with a power of 400 W. During the satellite’s way through the earth shadow, nickel-cadmium batteries served as reserve. Steering thrusters are operated with Hydrazine.
Further information about the Block I satellites can be obtained here, however with the satellites being out of operation the most recent available information is from 1996.
Block II Satellites
Block II satellites weigh more than 1500 kg, which is about twice the
weight of Block I satellites. The first of these satellites was launched
in 1998 from Cape Canaveral. They have a wingspan of approximately 5.1
m and are constructed for a service life of 7.5 years. A total of 9 Block
II and 18 Block IIA satellites were launched till September 1996.
Although the satellites are still in six different orbits, each with the same angle to the equator, the newer Block IIA satellites have a slightly different constellation in space. In 1990 the first Block IIA satellite (A for "advanced") was launched. Further information about the Block II satellites can be obtained here. The status of the total system can be found here.
In September 2005, the first satellite of a new generation (IIR-M, replacement, modernized) was successfully launched. The satellites of this type has the capacity to implement a second civil signal (L2C) and a new military signal with a new code (M-code on L1M and L2M). The satellite weighs 2 tons and costs $ 75 million.
Block II and Block IIA satellites are equipped with two rubidium and two cesium atomic clocks with a clock stability of at least 10 - 13 s. From the base frequency of the atomic clocks (10.23 MHz) all other requencies that are required for the GPS-satellite are derived. The newer satellites of Block IIR and IIR-M are equipped with three rubidium atomic clocks. Their extreme precision of ± 1 second in 1 million years is absolutely necessary for the functioning of the system. For an explanation please refer to the chapter “determination of a position”.
Starting with the new Block IIR satellites only the so-called C/A-signal
(Coarse/Acquisition) is accessible for civil use. The power supply and
the propulsion system are the same as for the Block I satellites. However
the solar panels have a higher capacity of 750 W.
Initially, the satellites of the Block IIR generation should be brought to their orbit in groups of three by space shuttles. But after the challenger catastrophe in 1986 it was decided to take the satellites to the orbit in pairs with Delta rockets.
Paradoxically, the first two satellites launched with a Delta rocket
were lost when the rocket had to be destroyed due to a malfunction shortly
after the lift off. This was the first malfunction ever of a rocket of
Block II satellites have a couple of further features which are not related to the GPS system. For example they are equipped with sensors capable of detecting atomic explosions.
The last of the Block IIa satellite lauched so far was brought to space on January 30, 2001 from Cape Canaveral. The launch of one Block II satellite costs about $ 50 Mio. which points up the high investment costs the system implicates. The enormous budget is only granted by the US congress because the system can be used by the military as well as for civil tasks.
Actual information about the status of Block II satellites can be obtained here or here.
The next generation (Block IIF) is planned to provide a third frequency for civil use (L5), allowing position determinations with even higher precision. This Block IIF satellites may be equipped with hydrogen maser clocks instead of atomic clocks due to their even higher precision.
The radiated signal power of the satellites is only about 50 W. For comparison:
television satellites like ASTRA satellites irradiate with a power of
100 W, but focused on Europe, and still a dish antenna with a diameter
of at least 50 cm is required for a good reception, whereas GPS antenna
typically are only an inch across. However television satellites have
to manage a considerably higher data transfer rate than GPS satellites.
Due to their high frequency, GPS signals cannot permeate stone or water. Even a thick foliation in forests may attenuate the signal to an extent that some (mainly older) GPS receivers have difficulties in receiving the signal. However, GPS works in any weather including a thick cloud cover. Problems may only arise in very heavy snowfall.