GNSS RTK Fix in Urban Canyon
A surveyor is using a GNSS RTK system to locate property corners in a downtown area with tall buildings. The RTK radio link is established, and the system reports a 'fixed' solution with horizontal precision (HRMS) of 0.014 m and vertical precision (VRMS) of 0.025 m. The surveyor measures a corner and records the position. However, the surveyor notices that the number of satellites used is 12, but the PDOP is 4.5. The sky is partially obstructed by buildings, and the surveyor is standing near a glass facade. The surveyor repeats the measurement after 10 minutes and gets a position that differs by 0.030 m horizontally and 0.050 m vertically. The surveyor needs to achieve 0.02 m horizontal accuracy for the corner. What should the surveyor do?
1. Assess the reliability of the RTK fixed solution based on the given information.
Read the worked answer
The RTK solution is 'fixed', which means the ambiguities are resolved as integers. However, a fixed solution can still be wrong if the ambiguity resolution is incorrect due to multipath or poor satellite geometry. The PDOP of 4.5 is relatively high (good geometry is typically <3). The number of satellites (12) is adequate, but the high PDOP indicates that the satellites are clustered in the sky, likely due to obstruction from buildings. The proximity to a glass facade can cause multipath. The repeat measurement difference of 0.030 m horizontal and 0.050 m vertical indicates that the solution is not repeatable at the 0.02 m level. The reported HRMS of 0.014 m may be optimistic because it does not account for multipath or systematic errors. Therefore, the fixed solution is suspect.
2. Recommend actions to improve the accuracy and verify the corner location.
Read the worked answer
The surveyor should: (1) Move away from the glass facade to reduce multipath. (2) Wait for a better satellite geometry (lower PDOP) or use a different satellite constellation (e.g., enable GLONASS, Galileo, BeiDou). (3) Use a ground plane or choke ring antenna if available. (4) Re-observe the point at a different time (e.g., after 1-2 hours) to allow satellite geometry change. (5) Use a total station to tie to nearby control points if possible. (6) Check the RTK solution by reinitializing and repeating the measurement multiple times. (7) If the required accuracy is 0.02 m, the observed repeatability of 0.03 m is insufficient; the surveyor should not accept the point until the repeatability is within tolerance. (8) Consider using a local base station or VRS with a shorter baseline to reduce atmospheric errors. (9) Document the conditions and note that the point may need to be re-measured.