Main errors of GPS
measurement
In GPS measurement, the main errors affecting the observation accuracy can be divided into the following three categories:
I. Errors related to GPS satellites
The errors related to GPS satellites mainly include satellite orbit errors and satellite clock errors.
1. Satellite Clock Offset
Since the position of the satellite is a function of time, GPS observations are based on precise timing, while The information corresponding to the satellite position is transmitted to the receiver through the encoded information of the satellite signal. In GPS positioning, whether it is code phase observation or carrier phase observation, both the satellite clock and the receiver clock are required to maintain strict synchronization. In fact, although GPS satellites are equipped with high-precision atomic clocks (rubidium clock and cesium clock), there are still inevitable deviations and drifts between them and the ideal GPS time. The total amount of this deviation is within about 1 ms.
The deviation of the satellite clock can generally be determined by the main control station of the satellite through continuous monitoring of the running state of the satellite clock, and provided to the receiver through the satellite’s navigation message. After the clock difference is corrected, the synchronization difference between the satellites can be kept within 20ns.
In relative positioning, the satellite clock error can be eliminated by calculating the difference (or difference) of the observations.
2. Satellite Orbit Deviation
It is difficult to estimate and process the satellite orbit deviation, mainly because the satellite is subject to a variety of cameras during operation. Due to the complex influence of power, it is difficult to fully and reliably measure this force through ground monitoring stations, and to grasp their law of action. At present, satellite orbit information is obtained through navigation messages.
It should be said that satellite orbit error is one of the main error sources of current GPS measurements. The longer the measured baseline length, the greater the effect of this error.
In GPS positioning measurement, there are the following methods to deal with satellite orbit errors:
1) Ignore orbit errors
This method is based on the satellite orbit information obtained from the navigation message, and no longer considers the actual error of the satellite orbit, so it is widely used in real-time single-point positioning work with low precision.
2) The orbit improvement method is used to process the observation data
This method is to introduce the correction parameters that characterize the satellite orbit deviation in the data processing, and assume These parameters are constant for a short time, and they are solved together with other known variables.
3) Difference of synchronous observations
This method is to use the synchronous observations of the same satellite at two or more observation stations together Ask for difference. In order to reduce the influence of satellite orbit errors. Since the influence of the position error of the same satellite on the synchronous observations of different observation stations has the nature of systematic error, the above method of calculating the difference can significantly reduce the influence of the satellite orbit error, especially when the baseline is short, its effectiveness is even less. obvious.
This method is extremely important for accurate relative positioning.
II. Errors related to satellite signal propagation
The errors related to satellite signals mainly include atmospheric refraction error and multipath effect
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1. Influence of ionospheric refraction
Like other electromagnetic wave signals of GPS satellite signals, when they pass through the ionosphere, they will be affected by this Due to the influence of the dispersion characteristics of the medium, the propagation path of the signal changes. Ionospheric refraction has the least effect on the signal propagation path when the GPS satellite is in the zenith direction, and has the greatest effect when the satellite is close to the horizon.
In order to reduce the influence of the ionosphere, the following measures are usually used in GPS positioning
(1) Using dual-frequency observation
Because the influence of the ionosphere is a function of the signal frequency, electromagnetic wave signals of different frequencies are used for observation. The effect can then be determined more and the observations corrected. Therefore, GPS receivers with dual frequencies are widely used in precise positioning measurements. It should be clearly pointed out, however, that observations should be avoided as much as possible at noon of solar radiation or during periods of anomalous sunspot activity. Especially in precise positioning measurement.
(2) Correction by using ionospheric model
For single-frequency GPS receivers, in order to reduce the influence of telegraph houses, navigation telegrams are generally used. The provided ionospheric model, or other suitable ionospheric model, corrects the observations, but this model is still being perfected, and the current model correction efficiency is about 75 percent.
(3) Use the synchronous observation value to find the difference
This method is to use two or more receivers to make the synchronous observation of the same satellite. Calculate the difference to reduce the influence of ionospheric refraction, especially when the distance between the observation stations is relatively short (<20km), because the satellite signals have similar paths to each observation station, and the conditions of the medium they pass through are similar. The difference between the synchronous observations of the same satellite signal can significantly weaken the effect of ionospheric refraction, and the residual will not exceed 0.000001. The significance of this approach is particularly evident for single-frequency GPS receivers.
2. Influence of tropospheric refraction
The influence of tropospheric refraction on observed values can be divided into dry and wet components. The dry component is mainly related to the humidity and pressure of the atmosphere, while the wet component is mainly related to the atmospheric humidity on the signal propagation path. The influence of the dry component can be calculated from the atmospheric data on the ground; the wet component cannot be accurately measured at present. For transport short baselines (<50km), the effect of the wet component is small.
About the influence of tropospheric refraction, there are generally the following methods:
(1) When the positioning accuracy requirement is not high, the influence may not be considered.
(2) The tropospheric model is used for correction;
(3) The method of difference is obtained by observation. Similar to the influence of the ionosphere, when the observation stations are not far apart (<20km), since the paths of the signals passing through the troposphere are similar and the physical properties of the troposphere are similar, the difference between the synchronous observations of the same satellite can be significantly weakened. The effect of tropospheric refraction.
3. Influence of multi-path effect
Multi-path effect, also known as multi-path error, refers to the fact that the receiver antenna does not directly receive the signal transmitted by the satellite. , it is also possible to receive satellite signals reflected by the ground objects around the antenna one or more times. The superposition of the signals will cause the position of the measurement reference point (phase center point) to change, resulting in errors in the observation. Reflecting surfaces vary in nature and are difficult to control. According to the experimental data, in the general reflection environment, the influence of multipath effect on the pseudorange of code measurement can reach the meter level, and the influence on the pseudorange of phase measurement can reach the centimeter level. In a high-reflection environment, not only will its influence increase significantly, but it will often lead to loss of lock on the received satellite signal and cycle slips in the carrier phase observations. Therefore, in precision GPS navigation and surveying, the influence of multipath effects cannot be ignored.
The current measures to reduce the influence of multipath effects are:
(1) The environment where the receiver antenna is placed should avoid strong reflective surfaces, Such as water surface = flat and smooth ground and flat building surface.
(2) Choose an antenna with appropriate shape and good shielding, etc.
(3) Properly extend the observation time to weaken the periodic influence of the multipath effect.
(4) The circuit design of the GPS receiver is improved to reduce the influence of multipath effects.
III. Errors related to receiving equipment
The errors related to GPS receiver equipment mainly include observation error, receiver clock error, antenna phase center Integer uncertainty effects of errors and carrier phase observations.
1. Observation error
Observation error includes the resolution error of the observation and the placement error of the receiver antenna relative to the station.
According to experience, it is generally believed that the resolution error of observation is about 1 percent of the signal wavelength. Therefore, it is known that the resolution error of the carrier phase is not smaller than that of the code phase. Since this error is an accidental error, it is possible to appropriately increase the observed amount, which will significantly weaken its influence.
The placement error of the receiver antenna relative to the center of the observation station is mainly the error of the placement and alignment of the antenna and the error of measuring the height of the antenna. Minimize the effect of this error.
2. The clock error of the receiver
Although the GPS receiver has a high-precision quartz clock, its daily frequency stability can reach -11 of 10 However, the impact on the carrier phase observation is still not negligible.
The more effective way to deal with the receiver clock error is to regard the receiver clock errors at each observation time as being correlated, thus establishing a clock error model and expressing it in the form of a time polynomial, Then, it is solved uniformly in the adjustment calculation of the observation quantity, and the coefficients of the polynomial are obtained, and thus the clock error correction of the receiver is also obtained.
3. Whole cycle unknowns of carrier phase observation
The most precise observation method currently used in carrier phase observation, because the receiver can only measure The carrier phase is not a fractional part of the whole cycle, and the number of whole cycles of the open wave phase cannot be directly measured, so there is the problem of the uncertainty of the whole cycle.
In addition, during the observation process, the cycle slip occurred due to the loss of lock of the satellite signal. From the loss of lock of the satellite signal to the re-lock of the signal, it has no effect on the fractional part of the non-integer cycle of the carrier phase, which is still the same as before the loss of lock, but the number of whole cycles is interrupted and no longer continuous, so the impact of cycle slip on the observation Similar to the influence of the unknowns of the whole week, in the data processing of precise positioning, the unknowns of the whole week and the cycle slip are the key issues.
4. The deviation of the phase center position of the antenna
In GPS positioning, the observation value is based on the phase center position of the receiver antenna, so the antenna The phase center of is theoretically consistent with its geometric center. However, in fact, the phase center position of the antenna varies with the strength and direction of the signal input, that is, the instantaneous position of the phase center during observation (called the apparent phase center) will be different from the theoretical center position of the unit , the effect of the deviation of the antenna phase center on the relative positioning results, depending on the performance of the antenna, can reach several millimeters to several centimeters. So for precise relative positioning, this effect cannot be ignored.
In practice, if the same type of antenna is used to simultaneously observe the same group of satellites on two or more observation stations not far apart, the difference between the observed values can be calculated to weaken the satellites. The effect of phase center offset. It should be mentioned that, when arranging the antennas of each observation station, all are oriented according to the azimuth marks attached to the antennas, so that they point to the magnetic north pole according to the compass.