GPS principles

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The basic principle of GPS navigation system to measure the known location of the satellite to the user the distance between the receiver and then integrated multi-satellite data can know the exact location of the receiver. To achieve this purpose, the satellite's position can be based on-board clock time recorded in the satellite ephemeris identified. The user to the satellite's distance from the satellite signal transmitted through the records to the user the experience of time, and then multiplied by the speed of light received (due to the interference of the atmosphere ionosphere, this distance is not the user and the real distance between the satellites, but pseudorange (PR): When the GPS satellites to work, it will continue to use 1 and 0 binary symbol composed of pseudo-random number (PN for short) launched navigation message. GPS pseudo-code system uses a total of two, namely civilian C / A code and the military of the P (Y) code. C / A code frequency 1.023MHz, repeat cycle of a millisecond, the code distance one microsecond, equivalent to 300m; P code frequency of 10.23MHz, repeat the cycle of 266.4 days, Code spacing of 0.1 microseconds, equivalent to 30m. The P code Y code is formed on the basis of confidentiality better performance. navigation message includes satellite ephemeris, work status, clock corrections, ionospheric delay correction, atmospheric refraction correction and other information. It is in the solution from the satellite signal modulation up to 50b / s modulation in the carrier frequency on the launch. navigation data included in each of the main frame 5 sub-frames per frame size 6s. the first three frames of all 10 code; repeat every 30 seconds, updated hourly. After the two were 15000b. navigation message in the contents of the main remote codes, code conversion, the first data blocks 1,2,3, including the most important was ephemeris data. When the user receives the navigation data, the extraction of the satellite time and do it with their own clock compared satellite and the user will be aware of the distance, and then use the navigation data in the satellite ephemeris data, calculate the location of the satellite launch when the message the user in WGS-84 geodetic coordinate system position, velocity and other information can be learned.
GPS satellite navigation system can be seen part of the role is to constantly launch navigation message. However, users accept the machine using the onboard clock with the clock can not always synchronized, so in addition to the user's three-dimensional coordinates x, y, z, but also introduce a Δt is the time between the satellite and receiver differential as unknown, then four equations out of these four unknown solution. So if you want to know the location of the receiver, at least four satellites can receive the signal.
GPS receiver can receive the exact timing can be used to nanosecond time information; for forecasting the next few months in which broad locations satellite ephemeris; used to calculate the required positioning satellite coordinates of the broadcast ephemeris a precision of several meters to tens of meters (each satellite is different at any time change); and GPS system information, such as satellite status.
GPS receiver of the code can be measured from the satellite to the receiver, as with the receiver clock error of satellite and atmospheric propagation errors, it is called pseudorange. 0A code measured on the pseudo-range as UA code pseudorange, the accuracy is about 20 meters, measured on the P code pseudo-range as P code pseudo-range, precision is about 2 meters.
GPS receiver to receive satellite signals, decode or use other technology, the information on modulation in the carrier removed, you can restore the carrier. Strictly speaking, the carrier phase should be known as the carrier beat phase, which is received by the Doppler frequency shift effects of the satellite signal carrier phase and the receiver local oscillation signal phase difference. Generally determined by the receiver clock time measurement epoch, to keep track of the satellite signal, you can record changes in the value of the phase, but the start time of observation and satellite receiver initial phase of the oscillator is not known, since epoch beginning phase integer not know about that ambiguity, only as a parameter in the Data Processing Solution. Phase observations are accurate to the millimeter, but only solved ambiguity, only the relative positioning, and continuous observation for some time to use the phase observations, and to achieve better than the meter-level positioning accuracy of only phase observations can be used.
By positioning mode, GPS positioning into a single point positioning and relative positioning (differential positioning). Single point positioning is only one receiver according to the observation data to determine the way the receiver position, it can only use pseudorange measurements can be used for rough travel, etc. navigation. Relative positioning (differential position) is based on two or more receivers of observational data to determine the relative position between the observation points the way, both can be used pseudorange phase measurements can be applied, geodetic or engineering survey shall be phase observations using the relative positioning.
In the GPS observables includes satellite and receiver clock error, atmospheric propagation delay, multipath effects, etc. error, even when in the position calculation by the satellite broadcast ephemeris error, and during most of the public when the relative positioning error was offset or reduced, thus positioning accuracy will be greatly enhanced, dual-frequency receiver can view the two frequency ionospheric measurement error of the atmosphere to offset a major part in the high precision, when the distance between the receiver (there is significant difference between the atmosphere ) should be used in dual-frequency receiver.

Relativity for the GPS provides the necessary amendments
Global Positioning System GPS satellite timing signals to provide latitude, longitude and altitude information, accurate distance measurement requires precise clock. Therefore, we must use precise GPS receivers relativistic effect.
Accuracy within 30 meters in GPS receiver means that it has made use of the relativistic effect. University of Washington physicist Clifford M. Will explained in detail, said: "If you do not take into account relativistic effects, satellite clock on the Earth's clock is not synchronized." Relativity that fast moving objects with the passage of time slower than static. Will calculated for each GPS satellite approximately 1.4 million kilometers per hour across the distance, which means its on-board atomic clocks on Earth every day the clock slower than 7 microseconds.
The gravitational force exerted on time more relativistic effects. About 2 million meters of altitude, GPS satellites by the gravitational pull by approximately one quarter of the ground. The result is that on-board clock speed of 45 microseconds per day, GPS should be included in a total deviation of 38 microseconds. Ashby explained: "If there is no satellite frequency compensation of 11 km per day will increase the error." (This effect is in fact more complex, because the satellite along an eccentric orbit, sometimes closer from Earth, sometimes Also very near.)

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