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Since the first GPS satellite was launched by the
Department of Defense in 1978, the radio navigation system has grown to 24 operational
satellites in six circular orbits 20,200 km (10,900 nm) above the earth.
GPS provides two levels of service -- Standard Position Service (SPS) for general public
use and Precise Position Service (PPS) for Department of Defense use. SPS accuracy is
intentionally degraded to protect national security interests. The process called
Selective Availability (SA), controls the availability of the systems full capabilities.
The SPS accuracy specifications include the effects of SA. GPS provides accuracy for
position. The accuracy is with respect to geographic, or geodetic coordinates of the
earth, regardless of time of day or weather conditions. SPS coverage is continuous and
worldwide but is limited to using only 6 satellites at a time.
GPS operates on two frequencies defined as L1, 1575.42 and L2, 1227.6 MHz. GPS
antennas are typically available in active or passive type. Active antennas have the
amplifier built into the antenna; passive antennas rely on the receiver to provide the
amplification. |
