Fundamental in the determination of GNSS solutions is resolving the correct number of full cycles of the carrier signal (so-called fixing ambiguities) in order to determine the distances from the satellites to the receiver. These distances contain errors caused by inaccuracies in the satellite clock and by the ionosphere and troposphere. When a base station is used, these errors and errors in the satellite orbits are nearly identical to both the rover and base station receivers when the baseline distance is short. By removing these common errors through RTK processing, centimeter level accurate vectors can be calculated between the base station and the rover.
Multipath, the reflection of GNSS signals from nearby objects and structures create their own indirect measurements from the satellites to the GNSS receiver and is the most critical source of inaccuracy in precision GNSS applications. The worst case is when the receiver doesn’t see the direct signal at all; e.g., the satellite is behind a building, but it’s still receiving the signal reflected off of the nearby structure. Such indirect signals are usually strong, unhelpful and misleading.
The other aspect impacting the veracity of a fixed solution is when there are weak GNSS signals. Frequently, weak signals are due to their penetration directly through tree canopy. While the TRIUMPH-LS can’t move the obstacles that are creating multipath out of the way, its sophisticated engineering is designed to handle even the weakest signals like no other system with its RTK Verification System (patent pending).
When located in difficult environments and under tree canopy, all GNSS receivers are prone to give bad fixed solutions that may be appear to be acceptable if they are not verified. Existing methods to verify GNSS solutions include “dumping” the receiver, turning it upside down to cause the RTK engines to reset, and re-observing the point at a later time.
The TRIUMPH-LS automates these processes with its built-in software features of Verify and Validate. Verify automatically resets the RTK engines after every fixed epoch is collected in the first step of its process. Epochs are sorted by distance and placed into groups during the first step. Once a group has built up a set level of confidence the RTK engines are allowed to collect the remaining epochs without resetting. If epochs fall too far away from the best selected group from the first step, they are rejected and the RTK engines are reset. Validation is the final step of the process. With this feature enabled the RTK engines will reset one final time at the end of the observation and collect 10 additional epochs. Allowing sufficient time between the first step and the final validation step will guarantee a bad solution is not allowed to be accepted. From extensive testing of these features in the worst of multipath environments, a bad solution has yet to be accepted when the Verify and Validate features are used and 120 epochs are collected.
Matt Johnson, PLS