GPS

The receiver does not need a precise clock, but does need to have a clock with good short-term stability and receive signals from four satellites in order to find its own latitude, longitude, elevation, and the precise time. The receiver computes the distances to the four satellites by the differences between local time and the time the satellite signals were sent and it then decodes the satellites' locations from their radio signals and an internal database. The receiver should now be located at the intersection of four spheres, one around each satellite, with a radius equal to the time delay between the satellite and the receiver multiplied by the speed of the radio signals. The intersection point gives the precise location of the receiver. If elevation information is not required, only signals from three satellites are needed.

In reality, the four spheres rarely intersect. There are several causes: The initial local time was a guess; due to the relatively unprecise clock of the receiver; the radio signals move more slowly as they pass through ionosphere; or the receiver may be moving. The receiver then applies an offset to the local time, and therefore to the spheres' radii, so that the spheres finally do intersect in one point. Once the receiver is roughly localized, most receivers mathematically correct for the ionospheric delay. The delay varies with the angle to the satellite, which changes the distance that the radio signal travels through the ionosphere. Some receivers attempt to fit the spheres to a directed line segment, because most receivers move.

The receiver contains a mathematical model to account for these influences, and the satellites also broadcast some related information which helps the receiver in estimating the correct speed of propagation. High-end receiver/antenna systems make use of both L1 and L2 frequencies to aid in the determination of atmospheric delays. Because certain delay sources, such as the ionosphere, affect the speed of radio waves based on their frequencies, dual frequency receivers can actually measure the effects on the signals.

In order to measure the time delay between satellite and receiver, the satellite sends a repeating 1,023 bit long pseudo random sequence; the receiver knows the seed of the sequence, constructs an identical sequence and shifts it until the two sequences match.

Different satellites use different sequences, which lets them all broadcast on the same frequencies while still allowing receivers to distinguish between satellites. This is an application of Code Division Multiple Access, CDMA.
There are two frequencies in use: 1575.42 MHz (referred to as L1), and 1227.60 MHz (L2). The L1 signal carries a publicly usable coarse-acquisition (C/A) code as well as an encrypted P(Y) code. The L2 signal usually carries only the P(Y) code. The keys required to directly use the P(Y) code are tightly controlled by the U.S. government and are generally provided only for military use.

A minor detail is that the atomic clocks on the satellites are set to "GPS time", which is the number of seconds since midnight, January 5, 1980. It is ahead of UTC because it doesn't follow leap seconds. Receivers thus apply a clock correction factor, (which is periodically transmitted along with the other data), and optionally adjust for a local time zone in order to display the correct time. The clocks on the satellites are also affected by both special, and general relativity, which causes them to run at a slightly faster rate than do clocks on the Earth's surface. This amounts to a discrepancy of around 38 microseconds per day, which is corrected by electronics on each satellite. This offset is a dramatic test of the theory of relativity in a real-world system; the offset of which is measured is exactly that predicted by theory, within the limits of accuracy of measurement.

The accuracy of GPS can be improved in a number of ways:

• Using a network of fixed ground based reference stations. These stations broadcast the difference between the location given by GPS and their real location, and clients can then correct their position by the same amount. This method is called Differential GPS or DGPS. The accuracy of DGPS without degradation of GPS is less than a meter. DGPS was especially useful when GPS was still degraded (via the "Selective Availability" described below), since DGPS could nevertheless provide 5-10 meter accuracy. The DGPS network has been mainly developed by the Finnish and Swedish maritime administrations in order to improve safety in the archipelago between the two countries.
• Exploitation of DGPS for Guidance Enhancement (EDGE) is an effort to integrate DGPS into precision guided munitions such as the Joint Direct Attack Munition (JDAM).
• The Wide-Area Augmentation System (WAAS). This uses a number of additional satellites to transmit correction data, including information on ionospheric delays, individual satellite clock drift, and suchlike. Although only a few WAAS satellites are currently available (in 2002), it is hoped that eventually WAAS will provide sufficient reliability and accuracy that it can be used for critical applications such as GPS-based instrument approaches in aviation (landing an airplane in conditions of little or no visibility).
• A Local-Area Augmentation System (LAAS). This is similar to WAAS, in that similar correction data is used. But in this case, the correction data is transmitted from a local source, typically at an airport or another location where accurate positioning is needed. This correction data is typically useful for only about a thirty to fifty kilometer radius around the transmitter.
• Wide Area GPS Enhancement (WAGE) is an attempt to improve GPS accuracy by providing more accurate satellite clock and ephemeris (orbital) data to specially-equipped receivers.

Applications

The systems is used by countless civilians as well, who can use the GPS's Standard Positioning Service worldwide free of charge. Low cost GPS receivers (price $100 to $200) are widely available. The system is used as a navigation aid in aeroplanes, ships and cars. Hand held devices are used by mountain climbers and hikers. Glider pilots use the logged signal to verify their arrival at turnpoints in competitions.

In the past, the civilian signal was degraded, and a more accurate Precise Positioning Service was available only to the United States military and other, mostly government users. However, on May 1, 2000, US President Bill Clinton announced that this "Selective Availability" would be turned off, and so now all users enjoy nearly the same level of access, allowing a precision of position determination of less than 20 meters. For military purposes, "Selective Deniability" may still be used to, in effect, jam civilian GPS units in a war zone or global alert while still allowing military units to have full functionality.

Military (and selected civilian) users still enjoy some technical advantages which can give quicker satellite lock and increased accuracy. Commercial GPS receivers are also required to have limits on the velocities and altitudes at which they will report fix coordinates; this is to prevent them from being used to create improvised cruise missiles or ballistic missiles.

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