Some major component of GPS

GPS might be divided up in the following way:

The Earth

The first major component of GPS is Earth itself: its mass and its surface, and the space immediately above. The mass of the Earth holds the satellites in orbit. From the point of view of physics, each satellite is trying to fly by the Earth at four kilometers per second. The Earth’s gravity pulls on the satellite vertically so it falls. The trajectory of its fall is a track that is parallel to the curve of the Earth’s surface. The surface of the Earth is studded with little “monuments”– carefully positioned metal or stone markers–whose coordinates are known quite accurately. These lie in the “numerical graticule” which we all agree forms the basis for geographic position. Measurements in the units of the graticule, and based on the positions of the monuments, allow us to determine the position of any object we choose on the surface of the Earth.

Earth-Circling Satellites

The United States GPS 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 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.

Ground-Based Stations

While the GPS satellites are free from drag by the atmosphere, their tracks are influenced by the gravitational effects of the moon and sun, and by the solar wind. Further, they are crammed with electronics. Thus, both their tracks and their innards require monitoring. This is accomplished by four ground-based stations near the equator, located on Ascension Island in the South Atlantic, at Diego Garcia in the Indian Ocean, and on Kwajalein Atoll, and in Hawaii, both in the Pacific, plus the master control station (MCS) at Schriever (formerly Falcon) Air Force Base near Colorado Springs, Colorado. A sixth station is planned to begin operation at Cape Canaveral, Florida. Each satellite passes over at least one monitoring station twice a day. Information developed by the monitoring station is transmitted back to the satellite, which in turn rebroadcasts it to GPS receivers ( electronic tracker ). 

Receivers

This is the part of the system with which you will become most familiar. In its most basic form, the satellite receiver consists of

• an antenna (whose position the receiver reports),

• electronics to receive the satellite signals,

• a microcomputer to process the data that determines the antenna position, and to record position values,

• controls to provide user input to the receiver, and

• a screen to display information.

More elaborate units have computer memory to store position data points and the velocity of the antenna. This information may be uploaded into a personal computer or workstation, and then installed in GIS software database. Another elaboration on the basic GPS unit is the ability to receive data from and transmit data to other GPS Tracking Device –a technique called “realtime differential GPS” that may be used to considerably increase the accuracy of position finding.

More GPS tracking solutions at http://www.jimilab.com/ .