In GPS measurement, the main errors affecting the accuracy of observation can be divided into the following three categories:
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First, the error related to GPS satellites
The errors associated with GPS satellites mainly include satellite orbit errors and satellite clock errors.
Satellite clock
Since the position of the satellite is a function of time, the GPS observations are based on precise time measurements, and the information corresponding to the satellite position is transmitted to the receiver via the encoded information of the satellite signals. In GPS positioning, both the code phase observation and the carrier phase observation require that the satellite clock and the receiver clock be strictly synchronized. In fact, although GPS satellites are equipped with high-precision atomic clocks (cuckoo clocks and cuckoo clocks), there are still unavoidable deviations and drifts between them and ideal GPS. The total amount of such deviations is within about 1 ms.
This deviation from the satellite clock can generally be determined by the satellite's master station through continuous monitoring of the satellite clock's operational status and provided to the receiver via the satellite's navigation message. After the correction of the clock difference, the synchronization difference between the satellites can be kept within 20 ns.
In relative positioning, the satellite clock can be eliminated by observing the difference (or difference).
2. Satellite orbital deviation
It is difficult to estimate the orbital deviation of the satellite. The main reason is that the satellite is subject to the complex effects of multiple perturbations during operation, and it is difficult to fully and reliably measure the force through the ground monitoring station and master their effects. Regularity, at present, satellite orbit information is obtained by navigating the message.
It should be said that satellite orbit error is one of the main sources of error in current GPS measurements. The longer the measured baseline length, the greater the impact of this error.
In GPS positioning measurements, there are straightforward methods for dealing with satellite orbit errors:
1) Ignore the orbital error
This method takes the satellite orbit information obtained from the navigation message as the standard, and does not consider the actual error of the satellite orbit. Therefore, it is widely used in real-time single-point positioning with low precision.
2) Using the orbit improvement method to process observation data
In this method, the correction parameters that characterize the satellite orbital deviation are introduced in the data processing, and it is assumed that these parameters are constant in a short time, and are solved together with other knowledgeable numbers.
3) Synchronous observation value difference
This method utilizes the difference in the simultaneous observations of the same satellite along with two or more observatories. To reduce the impact of satellite orbit error. Due to the influence of the position error of the same satellite on the simultaneous observation of different observation stations, it has the nature of systematic error. Therefore, the method of the above difference can significantly reduce the influence of satellite orbit error, especially when the baseline is short, its utility is less. obvious.
This method is extremely important for the relative positioning of precision.
Second, the error related to satellite signal propagation
Errors related to satellite signals mainly include atmospheric refraction errors and multipath effects.
1. The effect of ionospheric refraction
Like other electromagnetic wave signals of GPS satellite signals, when they pass through the ionosphere, they will be affected by the dispersion characteristics of this medium, and the propagation path of their signals will change. When the GPS satellite is in the zenith direction, the ionospheric refraction has the least impact on the signal propagation path, and when the satellite approaches the horizon, it has the greatest impact.
In order to reduce the influence of the ionosphere, the following measures are usually adopted in GPS positioning.
(1) Using dual frequency observation
Since the influence of the ionosphere is a function of the signal frequency, observations are made using electromagnetic wave signals of different frequencies. It is possible to determine its impact more and correct the observations. Therefore, a GPS receiver with dual frequency is widely used in measurement in precision positioning. However, it should be clearly stated that observations should be avoided as much as possible during the noon of solar radiation or during the anomalous period of sunspot activity. Especially in precision positioning measurements.
(2) Correction using the ionospheric model
For single-frequency GPS receivers, in order to reduce the influence of the electric house, the ionospheric model provided by the navigation message or other suitable ionospheric model is usually used to correct the observation, but this model is still being improved. The current model correction efficiency is about 75%.
(3) Using differential observations to find the difference
This method uses two or more receivers to make a difference in the simultaneous observation of the same satellite to reduce the influence of ionospheric refraction, especially when the distance between the stations is relatively close (<20km) due to the arrival of satellite signals. The paths of the observing stations are similar, and the mediums passing through are similar. Therefore, by comparing the synchronized observations of the same satellite signals by the observing stations, the influence of the ionospheric refraction can be significantly attenuated, and the residual will not exceed 0.000001. For single-frequency GPS receivers, the significance of this approach is particularly significant.
2. Influence of tropospheric refraction
The influence of tropospheric refraction on the observed values ​​can be divided into dry and wet components. The dry component is mainly related to the humidity and pressure of the atmosphere, and the wet component is mainly related to the atmospheric humidity on the signal propagation path. The effect on the dry component can be calculated from the atmospheric data on the ground; the wet component is currently not accurately determined. For transporting short baselines (<50km), the effect of the wet component is small.
Regarding the influence of tropospheric refraction, there are generally the following treatment methods:
(1) When the positioning accuracy is not high, the influence may not be considered.
(2) Correction using the tropospheric model;
(3) A method of measuring the difference by observation. Similar to the influence of the ionosphere, when the observation stations are not far apart (<20km), the physical characteristics of the troposphere are similar because the signals pass through the troposphere, so the difference between the synchronized observations of the same satellite can be significantly weakened. The effect of tropospheric refraction.
3. Multipath effect
Multipath effect, also known as multipath error, means that the receiver antenna may receive satellite signals that are reflected one or more times through the antenna around the antenna in addition to the signal received by the satellite. The signal superposition will cause the measurement reference point. The change in the position of the (phase center point) causes the measurement to produce an error, and this error varies depending on the nature of the reflection surface around the antenna and is difficult to control. According to the experimental data, in the general reflection environment, the influence of the multipath effect on the code pseudo range can reach the meter level, and the influence on the phase pseudo range can reach the centimeter level. In a highly reflective environment, not only will its impact increase significantly, but it will often result in loss of lock on the received satellite signals and cycle slips in the carrier phase measurements. Therefore, in precision GPS navigation and measurement, the effects of multipath effects are not negligible.
The current measures to mitigate the effects of multipath effects are:
(1) The environment in which the receiver antenna is placed should avoid strong reflective surfaces such as water surface = flat and smooth ground and flat building surface.
(2) Select an antenna that is suitable in shape and shielded well.
(3) Appropriately extend the observation time and weaken the cyclical effects of multipath effects.
(4) Improve the circuit design of the GPS receiver to attenuate the effects of multipath effects.
Third, the receiving equipment related errors
The errors associated with GPS receiver equipment mainly include observation errors, receiver clock errors, antenna phase center errors, and full-cycle uncertainty of carrier phase observations.
Observation error
The observation error includes the resolution error of the observation and the placement error of the receiver antenna relative to the measurement site.
According to experience, it is generally believed that the resolution error of the observation is about 1% of the signal wavelength. Therefore, it is known that the resolution error of the carrier phase is not smaller than the code phase. Since this error is an accidental error, the observation can be appropriately increased, and the influence thereof will be significantly weakened.
The placement error of the receiver antenna relative to the center of the observation station is mainly the error of the antenna and the error of the alignment and the measurement of the height of the antenna. In the precision positioning work, it must be carefully and carefully operated to minimize this error. influences.
2. The clock difference of the receiver
Although the GPS receiver has a high-precision quartz clock, its daily frequency stability can reach 10 to 11 squares, but the influence on carrier phase observation is still not negligible.
A more effective way to deal with the receiver clock error is to consider the receiver clocks at each observation time as correlated, thereby establishing a clock difference model and expressing it as a form of time polynomial, then adjusting the observations. In the calculation, the solution is solved uniformly, and the coefficients of the polynomial are obtained, so that the clock difference of the receiver is also corrected.
3. The whole week unknown of carrier phase observation
The most sophisticated observation method currently used in carrier phase observation, since the receiver can only measure the fractional part of the carrier phase rather than the whole week, and cannot directly measure the whole number of open-wave phase, there is a problem of uncertainty throughout the week.
In addition, during the observation process, the cycle slip occurs due to the loss of the satellite signal. From the loss of the satellite signal to the re-locking of the signal, there is no influence on the fractional part of the carrier phase that is not a whole week, and it is still consistent with the loss before the lock, but the whole number of weeks is interrupted and not continuous, so the effect of the cycle slip on the observation Similar to the effect of the whole week's unknowns, the whole week's unknowns and cycle slips are critical issues in precision-positioned data processing.
4. Antenna phase center position deviation
In GPS positioning, the observation is based on the phase center position of the receiver antenna, so the phase center of the antenna is theoretically consistent with its geometric center. However, in fact, the phase center position of the antenna varies with the intensity and direction of the signal input, that is, the instantaneous position of the phase center at the time of observation (referred to as the center of the apparent phase) is different from the theoretical center position of the unit. The influence of the deviation of the phase center of the antenna on the relative positioning result may be several millimeters to several centimeters depending on the performance of the antenna. So for precise relative positioning, this effect cannot be ignored.
In practice, if the same type of antenna is used and the same group of satellites are simultaneously observed at two or more observing stations not far apart, the difference between the observations can be used to weaken the influence of the phase center offset. It should be mentioned that when the antennas of each observatory are placed, they are oriented according to the azimuth attached to the antenna so that they are directed to the magnetic north pole according to the compass.
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