Satellite fact sheet
GPS satellites are the constellation of 24 satellites managed by the US Department of Defence. They orbit at 10,988 nautical miles (about 20,200 km) above the Earth's surface. These satellites are launched into orbit using rockets known as EELVs (Evolved Expendable Launch Vehicles). The particular EELVs used are the Delta IV and Atlas V.
The current generation of satellites is the Block IIF type. These weigh 1,700 kilograms. When ready for launch, they are roughly similar in size to a medium sized van, at 2.5 x 2.10 x 3.5 metres. When in orbit, the four solar panels arrays unfold giving a diameter of 21 metres.
The constellation of satellites is known as the "Space Segment" of the Global Positioning System. The "Ground Segment" refers to the monitoring and control stations and the "User Segment" refers to the receiving equipment that lets people use GPS.
The Russian system Glonass consists of 21 satellites in 3 orbital planes. Some commercial GPS receivers also read information from the Glonass satellites.
In May of 2000, under direction from President Clinton, civilian GPS accuracy was greatly increased by the removal of "Selective Availability". This was a built-in random error generator incorporated in the radio signals from GPS satellites. Its purpose was to prevent hostile powers from using GPS to the same accuracy that the US military could. The military now has other ways of selectively controlling availability of GPS. Switching off Selective Availability allowed a ten-fold increase in the precision of civilian navigation, at a stroke.
Because of the high altitude of GPS satellites (around 11,000 miles), between 5 and 11 of them will always be at least 5 degrees above the horizon at any one time, anywhere on the globe. The satellites are on 6 orbits tilted at 55 degrees to the Equator. GPS coverage is therefore better nearer to the Equator, because the satellites never pass over points with a latitude greater than 55 degrees North or South. Newcastle in the UK is about 55 degrees North.
In contrast, Low Earth Orbits are just above the Earth's atmosphere and are typically between 100 and 800 miles in altitude. Orbiting at this altitude, an object takes only about 90 minutes to completely circle the Earth, travelling at around 17,000 miles per hour. Low Earth Orbit is used by manned vehicles such as the space shuttle and the International Space Station. It is also the type of orbit used by weather and remote sensing satellites.
Low Earth Orbit satellites can sometimes be seen at night with the naked eye. They appear as star-sized points of light moving across the sky (not flashing like aircraft lights, not as fast as meteors - "shooting stars"). The light is sunlight reflected from the satellites' solar panel array while the satellite is not in the Earth's shadow. Magazines and web sites publish the predicted times and locations of sightings of objects such as the International Space Station and the bright Iridium satellites.
Most communications satellites, used for relaying international phone calls and television signals, are in geostationary orbits. These are higher than GPS orbits, around 22,000 miles. Geostationary satellites orbit at the Equator, at the same rate that the Earth rotates, meaning that they stay above the same point on the Earth's surface all the time. This is why satellite TV dishes are in fixed positions.