GPS from getting started to giving up (XI), differential GPS

One 、 principle

Difference GPS By eliminating or reducing GPS The measurement error is thus improved GPS Method of positioning accuracy .

Difference GPS The basic principle of eliminating or reducing measurement errors is to use the correlation of errors . Take satellite clock error for example . If we know the actual exact position of a receiver , Subtract from its measured position , The error term can be obtained , This error term includes the satellite clock error . If another receiver also receives the same satellite signal nearby to locate , It can reduce the error term in the calculation , This eliminates the satellite clock error , Because for these two receivers , Their satellite clock error is the same , This is the correlation of errors .

        Of course , For both receivers , Not all errors are the same , But within a certain range , There are still some errors that are similar . such as Ionospheric delay , If two receivers are close , It can be approximately regarded as the same ; Empathy , Tropospheric delay 、 This correlation also exists in satellite ephemeris . Other errors are multipath 、 The receiver noise is irrelevant , So such an error cannot pass the difference GPS To eliminate , contrary , They also cause the superposition of errors .

        In this difference method , We call the receiver with known location as reference station or base station , Those whose location is unknown are called mobile stations , The error correction value is broadcast by the reference station , The mobile station uses the error correction value to correct the position .

From difference GPS The working principle of is easy to see , The closer the mobile station is to the base station , The correction effect will be better . We call the distance between the mobile station and the reference station Baseline distance , The length of Baseline length .
 

Two 、 classification

• Classification by baseline distance ： Local area 、 Area 、 wide area

  1. Local area is general 10km~100km.
  2. The wide area can cover the whole continent or the whole world .
  3. The area is between local area and wide area .
• The target parameters corrected by difference are different , Can be divided into ： Position difference 、 Pseudo range difference 、 Carrier phase difference

1. Position difference ： The base station broadcasts the difference between the position calculated according to its own pseudo range observation value and the known self position , The mobile station calculates the position according to its pseudo range observation value, and then adds the received difference broadcast by the base station to get its final position . This method requires that the position coordinates calculated by the mobile station and the reference station respectively contain the same error term , It also requires that the mobile station and the reference station adopt the same positioning algorithm and the same set of satellite measurements , It's hard to achieve , Poor performance , So I don't use much .
2. Pseudo range difference ： What the base station broadcasts is the difference between its own pseudo range observation value and the known distance from itself to the satellite , The mobile station uses its own pseudo range observation value plus the received pseudo range difference broadcast by the base station to obtain its own final pseudo range , And then used for position calculation . This method does not require the mobile station and the reference station to adopt the same positioning algorithm , Its accuracy can reach decimeter level .
3. Carrier phase difference ： Similar to pseudo range difference , Only the base station broadcasts the carrier phase difference correction , There is no need to solve the integer ambiguity in the difference correction term , Just keep its value unchanged ; The mobile station still needs to solve the problem of weekly ambiguity . The accuracy of this method can reach the millimeter level at most .

• According to the positioning results, it can be divided into ： Absolute positioning 、 Relative positioning

  1. Absolute positioning requires knowing the precise position of the reference station .
  2. Relative positioning seeks the position of the mobile station relative to the base station , At this time, the absolute position of the base station may not be known , The base station can directly broadcast the observed values to the mobile station , Instead of broadcasting differential corrections .

• Divide by difference ： Single difference 、 Double difference 、 Three bad

1. Single difference ： It is the difference between the measured values of two different receivers on the same satellite . It can eliminate the satellite clock error . In the case of a short baseline , It can also basically eliminate the atmospheric delay error . The error of satellite ephemeris is basically eliminated . But the mean square deviation of the measurement noise is expanded to the original root sign 2 times .

2. Double difference ： The difference between the measured values of two satellites by two receivers at the same time , Equivalent to the difference between two single differences , It can eliminate the clock error of the receiver . This is because the receiver clock difference is the same for different satellites . The mean square deviation of double difference noise is single difference Radical sign 2 times .

3. Three bad ： The difference between the measured values of two satellites by two receivers at two times , Equivalent to the difference between two double differences , Double difference cycle ambiguity can be eliminated . This is because when the receiver locks a satellite signal , It keeps the integer ambiguity in the carrier phase measurement of the satellite signal unchanged . Three difference noise mean square deviation is the root of double difference 2 times . 

• According to the movement state ： Static positioning 、 Dynamic positioning

  1. Static positioning ： The measured value can be collected continuously for a long time , And do post-test processing , To improve accuracy .
  2. Dynamic positioning ： It usually meets the requirements of real-time .

• According to the real-time requirements ： real-time processing 、 Post test processing

3、 ... and 、 Further improvement measures

• The site selection of the reference station is generally open 、 The ground reflection is weak 、 Higher ground , And choose choke antenna and high-quality receiver , To suppress the multipath effect and its own error as much as possible .
• The reference station broadcasts the pseudo range correction value after deducting the influence of its own clock error , To avoid adding error to the user receiver .
• In addition to broadcasting pseudo range correction values, the reference station , The rate of change of the pseudo range correction value is also broadcast , To facilitate the use of the user receiver .
• Large baseline , The local ionosphere is used for the reference station and the mobile station respectively 、 Tropospheric model , It can improve the positioning accuracy .
• If a region is covered by multiple reference stations , The weighted average can be corrected separately , It can reduce the error , Improve accuracy .
• Wide area differential GPS The system is usually responsible for the error of satellite ephemeris 、 Satellite clock error 、 Ionospheric delay, etc. respectively provide differential correction .