Summary of Error Causes and Corresponding Solutions of RTK Surveying and Positioning

The error of RTK measurement and positioning mainly comes from the following stages: GPS satellite broadcast signal, satellite signal propagation and the ground receiving the identification satellite signal.

1.Conversion Parameter Influence

During RTK surveying, it is required to calculate conversion parameters first, that is, WGS-84 coordinate system shall be converted into the local coordinate system. The accuracy of parameters directly affects the accuracy of surveying. The factors affecting the conversion parameters include: the accuracy and the distribution of conversion control point, different results caused by varied ways in RTK surveying like when using conversion parameters, the base RTK receiver shall not be moved, only after which that it is allowed to measure control points. Otherwise, systematic error will be generated, causing that the results obtained may be wrong. Besides, the methods for parameters confirmation include four-parameter and seven-parameter.

2.Satellite Signal Influence
For RTK, it is required that at least 5 same satellite signals shall be received for both the base and the rover at the same time. When more satellites can be received and the distribution is good, the RTK signal will be better. Otherwise, the survey accuracy will be poor or even cannot be solved.

3. Influence of Data Link Transmission of RTK Base Station
Transmission of wireless signals is of great importance during RTK surveying because the observed data and known data are required to be transmitted through wireless transmission from a base RTK receiver to a rover RTK receiver. 

However, the wireless data link signals are easy to be blocked by mountains and buildings in the transmission process and may become abnormal by interference from other electromagnetic waves. Therefore, the stability of the RTK surveying signals do matter to the receiving status of the rover RTK’s radio.

4. Influence of Working Mode of Rover RTK Receiver

Usually, there are two working modes of the rover RTK receiver: using poles or tripods. By using poles, although it is convenient, the receiver is easy to shake. As a result, the accuracy fluctuation is a little large. However, by using tripods, it is a little bit more complicated, but the accuracy is higher, especially for the situation requiring that the receiver shall be fixed when measuring control points.

5.Selection of Operation Time
In order for RTK to receive enough satellites, rainy days and weather with thicker clouds should be avoided. Meanwhile, to reduce the influence of troposphere and ionosphere, the time around 14:00 shall be avoided.

6.Selection for Erection Position of Base RTK Receiver
In order to ensure the reception of enough satellite signals and can emit a strong radio data link, the base RTK receiver shall be under the environment without large area of shelter and wireless interference that affects the communication of data link. Therefore, the base RTK receiver shall be set in the wide-open sites like the roof of a building or at a high altitude, and it shall be avoided to be operated in such places where there is TVs, radio transmission towers, airports, high-tension cable and large area of water, etc.

7.Selection of Rover RTK Receiver Operation
In addition to the topographic survey and stakeout, for the rover RTK receiver, like base RTK receiver, it shall be operated by avoiding the influences of satellite signals and data link and multi-path effects.

Solutions:
1. Multi-path error can be reduced by the following measures: 
A:①The surveying station shall be far away from a large area of placid water. There are ideal sites like shrubs, grasses and other ground vegetation where the energy of microwave signal can be absorbed well. Besides, due to poor ability of reflection, tillage land and other rough ground can be selected to be a surveying station.

②The surveying station shall not be set in hillside, valley and basin in case that the reflection signals can be absorbed by the antenna over its multi-path suppression board, resulting in multi-path effect. 

③The surveying station shall be far away from the high buildings. Furthermore, cars shall not be parked near the surveying station.

B: ① Set multi-path suppression board in the antenna.
② The receiving antenna shall have a strong inhibition effect on the reflection signals with different polarization characteristics.

2.To weaken the side effects of electromagnetic radiation, the following measures shall be followed with to away from interference sources: if there are radio transmitter nearby, the surveying station shall be set over 200 meter away from the radio transmitter; if there is high-voltage line, 50 meters over far away which is allowed. The most effective way to reduce the electrical noise of antenna in the surveying station of base RTK receiver is to continuously monitor the cycle skipping and signal-to-noise ratio of visible satellites.

3. In RTK mode, the difference between the effective radius of the rover and base is not too far apart(generally less than 20km). Owing to the similar path for the signals go through the troposphere, it can significantly weaken the influence of the troposphere refraction by calculate the difference of the synchronous observation value for the same satellite, which is widely used in precise survey of relative positioning.

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