GPS The best known satellite navigation system is the United States' Global Positioning System (GPS), and as of 2006 the GPS is the only fully functional satellite navigation system. This consists of 24 to 27 satellites that orbit in six different planes. The exact number of satellites varies as satellites are replenished when older ones are retired. They orbit at an altitude of approximately 20,000 km with an inclination of 55 degrees. The satellites are tracked by a world-wide network of monitor stations. The tracking data is sent to a master control station that continuously updates position and clock estimates for each satellite. The updated data is then uplinked to the satellite via one of several ground antennas.

A GPS tracking system uses GPS to determine the location of a vehicle, person, or pet and to record the position at regular intervals in order to create a track file or log of activities. The recorded data can be stored within the tracking unit, or it may be transmitted to a central location, or Internet-connected computer, using a cellular modem, 2-way radio, or satellite. This allows the data to be reported in real-time, using either web browser based tools or customized software.

Geocaching The availability of hand-held GPS receivers for a cost of about $90 and up has led to recreational applications including Geocaching. Geocaching involves using a hand-held GPS unit to travel to a specific longitude and latitude to search for objects hidden by other Geocachers. This popular activity often includes walking or hiking to natural locations.

Calculating positions GPS allows receivers to accurately calculate their distance from the GPS satellites. The receivers do this by measuring the time delay between when the satellite sent the signal and the local time when the signal was received. This delay, multiplied by the speed of light, gives the distance to that satellite. The receiver also calculates the position of the satellite based on information periodically sent in the same signal. By comparing the two, position and range, the receiver can discover its own location.

To calculate its position, a receiver first needs to know the precise time. To do this, it uses an internal crystal oscillator-based clock that is continually updated by the signals being sent in L1 from various satellites. At that point the receiver identifies the visible satellites by the distinct pattern in their C/A codes. It then looks up the ephemeris data for each satellite, which was captured from the NM and stored in memory. This data is used in a formula that calculates the precise location of the satellites at that point in time.

Finally the receiver must calculate the time delay to each satellite. To do this, it produces an identical C/A sequence from a known seed number. The time delay is calculated by increasingly delaying the local signal and comparing it to the one received from the satellite; at some point the two signals will match up, and that delay is the time needed for the signal to reach the receiver. The delay is generally between 65 and 85 milliseconds. The distance to that satellite can then be calculated directly, the so-called pseudorange.

The receiver now has two key pieces of information: an accurate estimate of the position of the satellite, and an accurate measurement of the distance to that satellite. This tells the receiver that it lies on the surface of an imaginary sphere whose radius is that distance. To calculate the precise position, at least four such measurements are taken simultaneously. This places the receiver at the intersection of the four imaginary spheres. Since the C/A pattern repeats every millisecond, it can only be used to place the user within 300 kilometers (180 mi). Thus the multiple measurements are also needed to determine whether the receiver has lined up its internal C/A code properly, or is "one off".

The calculation of the position of the satellite, and thus the time delay and range to it, all depend on the accuracy of the local clock. The satellites themselves are equipped with extremely accurate atomic clocks, but this is not economically feasible for a receiver. Instead, the system takes redundant measurements to re-capture the correct clock information.

To understand how this works, consider a local clock that is off by .1 microseconds, or about 30 meters (100 ft) when converted to distance. When the position is calculated using this clock, the range measurements to each of the satellites will read 30 meters too long. In this case the four spheres will not overlap at a point, instead each sphere will intersect at a different point, resulting in several potential positions about 30 meters apart. The receiver then uses a mathematical technique to calculate the clock error that would produce this offset, in this case .1 microseconds, adjusts the range measurements by this amount, and then updates the internal clock to make it more accurate.

This technique can be applied with any four satellites. Commercial receivers therefore attempt to "tune in" to as many satellites as possible, and repeatedly make this correction. In doing so, clock errors can be reduced almost to zero. In practice, anywhere from six to ten measurements are taken in order to round out errors, and civilian receivers generally have 10 to 12 channels in total.

Calculating a position with the P(Y) signal is generally similar in concept, assuming one can decrypt it. The encryption is essentially a safety mechanism; if a signal can be successfully decrypted, it is reasonable to assume it is a real signal being sent by a GPS satellite. In comparison, the C/A signal can be generated fairly easily, allowing an unscrupulous user to send out their own fake signal, which would be difficult to distinguish from the original. Mathematical techniques can be used here as well, making spoofing of the C/A signal a very difficult prospect for any modern receiver equipped with some sort of RAIM system.

(source: wikipedia.org)

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