The main components of the GPS system are Space, Control, and User segments. The GPS operational constellation is made up of twenty-four satellites that orbit Earth at the altitude of ~20,000 km (Figure 9.2). The satellites are placed in six nearly circular orbital planes, inclined at 55 degrees with respect to the equatorial plane, with nominally four satellites in each plane. This configuration assures the simultaneous visibility of five to eight satellites at any point on Earth. The constellation establishes the Space Segment.
GPS satellites are powered by solar energy. During the solar eclipses, they use the backup batteries carried onboard. Because the satellites tend to drift from their assigned orbital positions, primarily due to orbit perturbations caused by Earth, the Moon, and planets' gravitational pull, solar radiation pressure, and so forth, they have to be constantly monitored by the Control Segment to determine their exact location in space. In order to keep the satellites as close as possible to the predesigned orbits, each satellite is equipped with small rocket boosters that can be fired when the orbit correction is needed. The Control Segment consists of 11 monitoring stations, each checking the exact altitude, position, speed, and overall health of the orbiting satellites 24 hours a day. In 2005 the National Geospatial-Intelligence Agency (NGA) added six more stations to the initial network of five. Based on these observations, the position coordinates and clock bias, drift, and drift rate can be predicted for each satellite, and then broadcast to each satellite for retransmission back to the users. The satellite position is parameterized in terms of predicted ephemeris, expressed in a
Earth-Centered-Earth-Fixed (ECEF) reference frame, known as the World Geodetic System 1984 (WGS84) (www.wgs84.com). The clock parameters are provided in the form of polynomial coefficients and, together with the predicted ephemeris, are broadcast to the users in the GPS navigation message. The accuracy of the predicted orbit is typically several meters.
Since the launch of the first GPS Block I satellite in 1978, the system evolved through several spacecraft designs, focused primarily on increased design life, extended autonomous operation time, better frequency standards (clocks), and the provision for accuracy manipulation, and was controlled by the DOD. The current constellation, as of February 2008, consists of 31 Block II/ IIA/IIR/IIR-M satellites. Information about the current status of the constellation can be found, for example, at the U.S. Naval Observatory (USNO) Web site (http://tycho.usno.navy.mil/gpscurr.html). The User Segment, including all GPS equipment used in a variety of civilian and military applications, establishes the third component of the GPS system (Figure 9.3).
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