To obtain a precise position, GPS satellites continuously transmit radio signals. The satellites are in orbit around the Earth. GPS satellites receive signals from a large number of GPS receivers on the Earth, which is called the network. The satellites orbit the Earth at approximately the same speed, but the Earth is in motion, so the locations of the satellites change with time. The global network of GPS satellites consists of 24 orbits, each 24 hours apart. Each orbit consists of 24 satellites that move at a slightly higher speed than the Earth, or about 7,500 miles (12,000 km) per hour. This higher speed allows the GPS receiver to track the satellites quickly. The space shuttle makes multiple trips around the Earth during the launch and the space shuttle's velocity is approximately 7,000 mph (11,230 km/h) which allows for much less latency for the signal to arrive at the satellite. At any given location on the Earth, a GPS receiver will typically be able to determine its position within 60 to 100 meters.
In 1998, the U.S. government certified L5-only receivers for use in certain navigation and landing applications. The certification was withdrawn in 2001, because they found there were too many L5-only receivers on the market, and the range of error was too high. Rather than provide re-certification in 2002, the government intended to start a dialogue with manufacturers to create and deploy L5+ receivers to the market. Although the U.S. government was going to meet the demand for these receivers by providing L5+ receivers, the demand only grew. The government has not yet responded with a solution.
The signals from GPS satellites are transmitted on two known frequencies: L1 (1575.42 MHz) and L2 (1227.6 MHz). L1 serves as the primary, or primary carrier, and is on the lower frequency side of the L-band spectrum. L2 is the secondary carrier, and is on the higher frequency side. The difference between the frequencies of the two signals is called the L1/L2 frequency ratio. The L1 frequency is referred to as the L1 frequency (also known as the L1C), and the L2 frequency is referred to as the L2 frequency (also known as the L2C). The L1C/L2C frequency ratio is 1.023 MHz. 827ec27edc