3.5 gps
GPS principles
The basic principle of GPS navigation system for measuring the known position 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, the satellite position can be based on the clock board recorded in the satellite ephemeris identified. The remote user of the satellite signal transmitted from the satellite to the user documents the experience of time, and then multiplied by the speed of light received (due to interference from the ionosphere atmosphere this distance is not the user and the real distance between the satellites, but pseudo (PR): When the GPS satellites to work, it will continue to use 1 and 0 symbol composed of binary pseudo-random number (PN for short) has launched navigation message. GPS pseudo-code system uses a total of two, namely C Civil / A code and the military P (Y) code. C / A code 1.023MHz frequency, the repetition cycle of a millisecond, the remote code microsecond equivalent to 300m; P code frequency 10.23MHz, repeating the cycle of 266.4 days, the Code spacing of 0.1 microseconds, equivalent to 30m. PY code is the code formed on the basis of better performance of confidentiality. navigation message includes satellite ephemeris, professional status, clock corrections, ionospheric delay correction, atmospheric refraction correction and other information. It is in the solution of the modulation satellite signal to 50b / s modulation of the carrier frequency on the pitch. navigation data included in each major part of 5 sub-frames per image size 6s. The first three images of the entire Code 10, repeat every 30 seconds, updated hourly. After the two were 15000b. navigation message in the content of the main remote codes, code conversion, data blocks of the first 1,2,3, including the most important was the ephemeris. When the user receives the data navigation, retrieval of satellite time and do it their own clock over satellite and the user be aware of the distance, and then use the navigation data in the satellite ephemeris data, calculate the position of the satellite launch when the message to the user in WGS-84 geodetic coordinate system position, velocity and other information can be drawn.
GPS satellite navigation can be seen in part the role is to constantly launch navigation message. However, users accept the machine using the onboard clock with the clock can not always synchronized, in addition to the user in three-dimensional coordinates x, y, z, but also At introduce is the time between the satellite and receiver differential as unknown, then these four equations with four unknowns 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 date can not be used to nanosecond time information, to forecast the coming months in which large ephemeris satellite offices used to calculate the position coordinates necessary satellite broadcasting ephemeris accuracy of a few meters to several tens of meters (each satellite is different any time change), and GPS information, such as satellite status.
code GPS receiver can be measured from the satellite and receiver as the receiver of the satellite clock error and error propagation in the atmosphere, it is called pseudo. 0A measured code on pseudorange as a pseudo-code CPU, the accuracy is about 20 meters, measured on the P-code pseudorange as a pseudo code P-scope, accuracy is about 2 meters.
GPS receiver for receiving satellite signals, decode or use other technologies, information on the modulation of the carrier removed, you can restore the carrier. Strictly speaking, the carrier phase must be known by the name of the carrier beat phase, which is received by the effects of Doppler frequency shift of the satellite signal carrier phase and the receiver local oscillation phase difference signal. Generally determined by the time the receiver clock time measure, to keep track of the satellite signal, you can save the changes the value of the phase, but the starting time phase of observation and satellite receiver initial oscillator is not known, since the early phase whole time unaware of this ambiguity, only as a parameter in the solution of data processing. Phase observations are correct the millimeter, but only solved ambiguity, that the relative positioning and continuous observation for some time to use the phase observations, and to increase the positioning accuracy level meters phase observations can be used.
In positioning mode, GPS positioning into a single point positioning and relative positioning (differential positioning). the point positioning is only one receiver, according observational data to determine how the receiver position, he can not use measures of pseudo-distance can be used for travel Gross, navigation etc.. The relative positioning (differential position) is based on two or more receivers of observational data to determine the relative position between observation points the way, both can be used in phase measurements of pseudo-distance can be applied geodesic and engineering are the phase observations using the ratio positioning.
In the observable GPS satellite and includes the receiver clock error, propagation delay in the atmosphere, the multiple effects, error, etc., even if in the position calculation by the satellite ephemeris error, and for most of the public when the relative positioning error has been canceled or reduced, the positioning accuracy will be greatly improved, a dual frequency receiver can display the measurement error of two-frequency ionospheric atmosphere to offset the bulk of high precision, when the distance between the receiver (there is a significant difference between the atmosphere) must be used in dual-frequency receiver.
Relativity for the GPS provides the necessary changes
Global Positioning System GPS synchronization satellite to provide the latitude, longitude and altitude, the accurate measurement of distance requires precise clock. Therefore, we must use of precise GPS receivers relativistic effect.
Accuracy within 30 meters in the 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 clock of the Earth is not synchronized. "Relativity objects moving quickly with the passage of time slower than static. Will be calculated for each GPS satellite of about 1.4 million km / h through the distance, meaning that its board atomic clocks on Earth each day clock slower than 7 microseconds.
The gravitational force exerted over time relativistic effects. About 2 million meters of altitude, the GPS satellites by the attraction gravity of about one quarter of the earth. The result is that the clock speed of 45 microseconds flight per day, GPS should be included in a gap total of 38 microseconds. Ashby said: "If there is no satellite frequency compensation of 11 km per day increases the error." (This effect is actually more complex, because the satellite along an eccentric orbit, sometimes closer to Earth, sometimes very close.)
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