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1. Introduction to RTK
Real-time Kinematics (RTK) is to calculate position of user with cm-level accuracy by resolving integer ambiguities contained in carrier phase measurements.
The set of RTK system consists of a reference station with a known position, data communication link for broadcasting corrections to users, and users who want to calculate their positions accurately using double difference carrier phase measurements for resolving integer ambiguities.
Unlike the DGNSS using pseudorange measurements, which achieves m-level positioning accuracy, RTK suffers much performance degradation due to spatial and temporal decorrelation since it provides much accurate positioning results. Therefore, users should be located within 10km from the RTK reference station in order to achieve typical RTK positioning performance.
2. Compact Network RTK
At the GNSS Lab. in the SNU, we proposed Compact RTK in order to reduce time latency errors while broadcasting corrections. Compact RTK Message, which is suitable for low-rate data link, is allocated to RTCM MT #4081 and is under consideration for the standardization.
On the other hand, Network RTK is a widely used method to reduce spatial decorrelation errors by using measurements from multiple reference stations surrounding users. There are 3 famous method to implement Network RTK, which are VRS(Virtual Reference Station, Trimble), FKP(Flaechen korrektur parameter, Geo++), and MAC(Master-Auxiliary Concept, Leica).
Figure 1. RTK vs. Compact Network RTK
3. Performance of Compact Network RTK and Future works
Recently, we are working on Compact Network RTK which is a research on combining Compact RTK and Network RTK. By combining two methods, we can reduce not only temporal decorrelation errors but also spatial decorrelation errors. Therefore, low-rate data link can be used to Network RTK system in the mean of Compact RTK.
We have tested Compact Network RTK with real GPS data from CORS in the US. We selected some reference stations in the state of North Carolina as in the figure 2. PITT reference station was chosen as a master station, and other stations such as HILB, NCAS, NCLI, and NCFA are selected as auxiliary stations. For performance evaluation, RALR reference station was chosen as a user which is located 27 km apart from the master station.
Figure 2. System Configuration for Performance Evaluation
Compact RTK can be applied to all of the conventioanl Network RTK methods and in this test, we applied it to the MAC, FKP and VRS. As a result, user can achieve 6cm horizontal errors (2DRMS) and 7.99cm vertical errors (95%) under 500~700 bps data link. The results are shown in the figure 3.
Figure 3. Positioning Results of MAC based Network RTK and MAC based Compact Network RTK
The positioning result of a method combining Compact RTK and 3 different Network RTK methods are shown in the table blow. With the Compact RTK, we can reduce the speed of data link by 60%, however, positioning degradation was only 1mm.
Figure 4. Positioning Results of Compact Network RTK under low-rate data link
Test results shown so far has been discussed in the RTCM SC-104 committee and introduced in the magazine called GPS World internet edition on September 14, 2009. Currently, protocols of Compact RTK are allocated to the RTCM MT 4081 for research and we are working on for the standardization of the Compact RTK.
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