The United States vehicle GPS tracking systems design calls for a total of at least 24 and up to 32 solar powered radio transmitters, forming a constellation such that several are “visible” from any point on Earth at any given time. The first one was launched on February 22, 1978. In mid-1994 all 24 were broadcasting. The minimum “constellation” of 24 includes three “spares.” As many as 28 have been up and working at one time. The newest GPS satellites (designated as Block IIR) are at a “middle altitude” of about 11,000 nautical miles (nm), or roughly 20,400 kilometers (km) or 12,700 statute miles above the Earth’s surface. This puts them above the standard orbital height of the space shuttle, most other satellites, and the enormous amount of space junk that has accumulated. They are also well above Earth’s air, where they are safe from the effects of atmospheric drag. When GPS satellites “die” they are sent to orbits about 600 miles further out. Car tracking GPS satellites are below the geostationary satellites, usually used for communications and sending TV, telephone, and other signals back to Earth-based fixed antennas. These satellites are 35, 763 km (or 19,299 nm or 22,223 sm) above the Earth, where they hang over the equator relaying signals from and to ground-based stations.
The NAVSTAR satellites are neither polar nor equatorial, but slice the Earth’s latitudes at about 55°, executing a single revolution every 12 hours. Further, although each satellite is in a 12 hour orbit, an observer on Earth will see it rise and set about 4 minutes earlier each day.5 There are four or five satellites in slots in each of six distinct orbital planes ( vehicle GPS locator ) set 60 degrees apart. The orbits are almost exactly circular. The combination of the Earth’s rotational speed and the satellites’ orbits produces a wide variety of tracks across the Earth’s surface.
You are looking down on the Earth, directly at the equator and at a (north-south) meridian that passes through Lexington, Kentucky.The number of each satellite is shown near its track; the number marks the point where the satellite is at the end of the two-hour period. vehicle GPS locator satellites move at a speed of 3.87 km/sec (8,653 miles per hour). The Block IIR satellites weigh about 1077 kilograms (somewhat more than a ton) and have a length of about 11.6 meters (about 38 feet) with the solar panels extended. Those panels generate about 1100 watts of power. The radio on board broadcasts with about 40 watts of power. (Compare that with your “clear channel” FM station with 50,000 watts.) The radio frequency used for the civilian GPS signal is called “GPS L1” and is at 1575. 42 megaHertz (MHz). Space buffs might want to know that they usually get into orbit on top of Boeing Delta II rockets fired from the Kennedy Space-flight Center at Cape Canaveral in Florida. Each satellite has on board four atomic clocks (either cesium or rubidium) that keep time to within a billionth of a second or so, allowing users on the ground to determine the current time to within about 40 billionths of a second. Each satellite is worth about $65 million and has a design life of 10 years.