The GPS OCS consists of the master control station (MCS), located at Falcon Air Force Base in Colorado Springs, Colorado; remote monitoring stations, located in Hawaii, Diego Garcia, Ascension Island, and Kwajalein; and uplink antennas, located at three of the four remote monitor stations and at the MCS._The four remote monitor stations contribute to satellite control by tracking each GPS satellite in orbit, monitoring its navigational signal, and relaying this information to the MCS. The four stations can track and monitor the whereabouts of each GPS satellite 20 to 21 hours per day. Land-based and space-based communications connect the remote monitoring stations with the MCS.
GPS user equipment varies widely in cost and complexity, depending on the receiver design and application. Receiver sets, which currently vary in price from approximately $135 or less to $30,000, can range from fairly simple devices that provide only basic positioning information to complex multichannel units that track all satellites in view and perform a variety of functions. Most GPS receivers consist of three basic components: (1) an antenna, which receives the signal and, in some cases, has anti-jamming capabilities; (2) a receiver-processor unit, which converts the radio signal to a useable navigation solution; and (3) a control/display unit, which displays the positioning information and provides an interface for receiver control.
The GPS relies on the principle of “pseudoranging” to provide accurate positioning, velocity, and timing information. Each satellite in orbit transmits a continuous radio signal with a unique code that includes data about the satellite's position and the exact time the coded transmission was initiated, as kept by the on-board atomic clocks. A pseudorange measurement is created by measuring the distance between a user's receiver and a satellite by subtracting the time the signal was sent by the satellite from the time it was received by the user.
In general, a user's three-dimensional position can be determined by simultaneously measuring the ranges from a user's receiver to three satellites.. However, because the GPS satellites and receiver clocks are not perfectly synchronized, observations from a fourth satellite are needed to eliminate the receiver clock bias that is common to all the pseudorange measurements. Figure 1-2 illustrates the GPS pseudoranging concept.
Instead of transmitting one code on one radio signal (as described above), each satellite actually transmits two distinct spread spectrum signals that contain two different codes, the coarse acquisition (C/A) code and the precision (P) code. The C/A code is broadcast on the L-band carrier signal (known as L1), whose frequency is centered at 1575.42 MHz. The P-code is broadcast on the L1 carrier in phase quadrature with the C/A carrier and on a second carrier frequency (designated as L2), which is centered at 1227.60 MHz.
The L1 C/A-code provides free positioning capability to civilian and commercial users all over the world and is known as the Standard Positioning Service (SPS). The P-code is normally encrypted using National Security Agency cryptographic techniques, and decryption capability is available only to the military and other authorized users as determined by the U.S. Department of Defense. The encryption process, known as anti-spoofing (A-S), denies unauthorized access to the P-code and also significantly improves a receiver's ability to resist locking onto mimicked GPS signals, which could provide incorrect positioning information to a GPS user. P-code availability on both the L1 and L2 carrier signals through decryption capability provides authorized users with more accurate positioning and is known as the Precise Positioning Service (PPS).
The accuracy of GPS is degraded for users of the SPS through a process known as selective availability (SA). SA is a deliberate degradation in GPS accuracy accomplished by intentionally varying the precise time of the clocks on board the satellites, which introduces errors into the GPS signal, and by providing incorrect orbital positioning data in the GPS navigation message. SA is normally set to a level that will provide 100-meter (2 drms) positioning accuracy to users of the SPS.2 The March 1996 Presidential Policy Directive on GPS states that it is the intention of the U.S. government to “discontinue the use of
SPS accuracy is normally represented using a horizontal 2 drms measurement, or twice the root mean square radial distance error. Normally, 2 drms can be graphically represented as a circle about the true position containing approximately 95 percent of the position determinations.