In the positioning measurement of satellite navigation system, two problems are mainly solved: the position of the satellite at the observation time and the spatial distance from the station to the satellite at the observation time.
First, the principle of satellite positioning
(A) the principle of triangulation
The principle of triangulation is adopted in satellite positioning. The position is determined by the ranging signal emitted by the satellite, that is, the satellite in the ether sky is a known point, and the distance from the satellite to the ground point is measured, and then the position of the receiving equipment on the earth surface or in the air is determined by the distance.
Satellites constantly send orbit information including satellite position and accurate time information generated by atomic clock carried by satellites, and at the same time transmit ranging signals. Beidou receiver (with its own clock and radio signal receiver) receives the signal from the satellite and measures the spatial distance from the satellite to the receiver. At this time, the receiver is located at a certain point of the arc where the spherical surface with the observation satellite as the center and the space distance from the observation satellite to the receiver as the radius intersects the earth surface.
By analogy, we can determine the intersection point between the spherical surface and the earth's surface with the second and third observation satellites as the center and the space distance from the satellite to the receiver as the radius. The point where three arcs intersect on the earth's surface is the position of the receiver.
Because the quartz clock used in the receiver has a large error compared with the atomic clock carried by the satellite, it is necessary to receive the observation values of four satellites and calculate the time difference between the satellite clock and the receiver clock, so as to calculate the accurate propagation time and finally the accurate propagation distance.
(2) the principle of distance measurement
Distance measurement is calculated by the following formula:
D=c？ t
D is the distance from the satellite to the receiver;
-c is the propagation speed of electromagnetic waves in the atmosphere;
-t is the signal propagation time from the satellite to the receiver;
(3) Error sources that affect the positioning results.
1. Satellite correlation error
Satellite clock error, that is, satellite atomic clock error;
Satellite ephemeris error, also known as satellite orbit error;
Relativistic effect, that is, the relative error caused by the different States of satellite clock and receiver;
2. The transmission path produces errors.
Ionospheric delay, that is, when the radio signal passes through the ionosphere, the path of the signal will be bent and the propagation speed will change, thus causing deviation in the measured distance.
Tropospheric delay, that is, when the signal passes through the troposphere, the propagation path of the signal will bend, which makes the measurement distance deviate;
Multipath effect, that is, the satellite signal (reflected wave) reflected by the reflector around the station enters the receiving antenna, which interferes with the signal directly from the satellite (direct wave) and makes the observation value deviate;
3. Receiver correlation error
Receiver clock error, that is, the error between the quartz clock used by the receiver and the satellite standard time;
Receiver position error, that is, the error of receiver antenna phase center relative to the center of station mark;
The phase center deviation of the receiving antenna, that is, the phase center of the receiving antenna should be consistent with its geometric center in theory, but it changes with the intensity and direction of signal input during observation.
Second, the satellite positioning method
(A) according to the recipient's status classification
1. Static positioning
In an observation process, the waiting point of the receiver has no perceptible movement relative to the surrounding fixed points, or the movement is too slow to be ignored, so the method to determine the position of the waiting point is called static positioning.
2. Dynamic positioning
In an observation, the waiting point of the receiver has obvious movement relative to the surrounding fixed points, so the method to determine the position of the waiting point is called dynamic positioning.
The difference between static positioning and dynamic positioning is mainly whether the position of a fixed point is regarded as a constant in the established mathematical model during observation.
(two) according to whether the recipient has a reference standard classification.
1. Single point positioning
Single point positioning is a method to independently determine the fixed absolute position of the receiver in the coordinate system, which is also called absolute positioning. Single point positioning requires only one receiver.
2. Relative positioning
Relative positioning is to use two or more receivers, which are placed at both ends of the baseline, to observe the same satellite synchronously to determine the relative position of the fixed point. Relative positioning can eliminate/weaken [h 1] errors such as satellite clock error, ephemeris error and signal propagation error. Data difference through each receiver.
3. Differential positioning
Belongs to a kind of relative positioning. A receiver should be placed on the reference station for observation. According to the precise coordinates known by the reference station and the coordinates calculated by the receiver, the correction number of the real coordinates and the coordinates obtained by positioning can be calculated, and the reference station will send this data in real time. When the user receiver carries out positioning observation, it also receives the correction number sent by the reference station and corrects its positioning result, thus improving the positioning accuracy.
(three) according to the nature of satellite positioning observation information classification
The signal received by the receiver includes navigation message, pseudo-random code, carrier wave and other information.
1. pseudorange positioning
Positioning is achieved by measuring propagation time with pseudo-random code.
2. Carrier phase positioning
Positioning is achieved by carrier phase measurement.
3. Doppler positioning
The relative motion between satellite and receiver produces Doppler frequency shift, and positioning is realized by measuring Doppler frequency shift.
Third, RTK technology.
RTK technology, that is, carrier phase difference technology, is a real-time difference method to process the carrier phase observations of two stations. The carrier phase collected by the reference station is sent to the user receiver for difference and coordinate calculation.
RTK can obtain centimeter-level precision positioning in real time in the field, and it is a new commonly used satellite positioning measurement technology.
3RTK technology
Network RTK technology: also known as multi-reference station RTK, the basic principle is to sparsely and evenly lay out the network of reference stations in a large area, and then learn from the principles and methods of multi-reference station wide-area differential positioning technology and local differential positioning technology to eliminate or weaken various system errors, so as to obtain high-precision positioning.
Network RTK effectively optimizes the limitations of traditional RTK technology, such as limited working distance, low efficiency and insufficient power supply of a single reference station, by using CORS base station system, and has the advantages of wide application range, high precision and single-machine operation in the field.