Dynamic Test Result of the Compact RTK Under a Harsh Communication Environment of Land Transportation

Byungwoon Park, Donghwan Yoon, Junesol Song, Changdon Kee, Seungwoo Seo, Junpyo Park

Abstract:	GNSS is one of the core technologies to find a position of each land transportation vehicle and to guide its route. A decimeter accuracy is necessary to provide the ITS (Intelligent Transportation System) where-in-lane-level service such as “lane control”, “autonomous vehicle control” and “precise navigation”, and RTK (Real Time Kinematics) has been suggested as a strong candidate technique for these services. RTK is initially designed for the static positioning in surveying, and the environment of land transportation is very harsh to the current RTK application. The Nottingham Geospatial Institute (NGI) has classified many obstacles to providing a high accuracy positioning solution in real-world as the loss of the fixed integer ambiguity during satellite line-of-sight outages and the fragility of the data communications service that delivers the real-time correction information. Communication-disconnects occasionally occur to the vehicles on the road and old age of correction caused by the disconnection do harm to the RTK position accuracy. General correction message for the RTK is based on RTCM (Radio Technical Commission for Marine Services) version 3 MT (Message Type) 1004 which mainly consists of pseudorange and carrier-phase measurement. These observables include the distance from the satellite to the RS (Reference Station) and clock biases in receiver and satellite which changes so fast that the message should be transmitted as soon as possible to keep the rover’s accurate position robustly. Moreover, its non-linear variation is hard to predict, so several second latency on the road can be fatal to the vehicle's RTK position. On the other hand, Compact RTK technique proposed to implement RTK in an extremely low rate data-link can be a good method to compensate the timelatency and to solve the data cut-off problem in land transportation. Compact RTK protocol, recently assigned in RTCM SC-104 proprietary message MT 4081, consists of PRC (Pseudo Range Correction) and CPC (Carrier Phase Correction) instead of code and carrier observations. These terms vary slowly and linearly, so that 500bps is large enough to keep the RTK accuracy without any degradation. A previous study had compared the latency compensation performance between the general RTK and the Compact RTK on zero and non-zero baseline static tests. According to its result, the error increment of the Compact RTK in 15 second latency was horizontally 0.73cm and vertically 2.54cm (RMS), while those of the general RTK were 2.38cm and 4.04cm. The rover receiver configuration did not permit 20 second or order correction for its RTK operation, so the MT1004 for the general RTK did not help to maintain the RTK fixed mode, but the receiver using MT4081 for the Compact RTK kept its RTK fixed position with only 8cm accuracy degradation. This paper shows how the Compact RTK contributes to the continuous and robust RTK solution of a dynamic vehicle under a harsh communication environment. We constructed a test-bed in a parking lot 0.12km apart from the reference station in SNU (Seoul National University). Velocity of the rover van in the parking lot was 20km/h, and two Trimble NetR9 receivers were connected to the same antenna on the roof. The reference station in SNU sent the RTK corrections in the format of MT1004 and MT4081 respectively, and the rover receivers were fed the corrections 15 and 30 second after the transmission. The rover test module compensated for the latency using one dimensional extrapolation algorithm and then converted MT4081 into MT1004 to feed it to one receiver, while the other receiver for the general RTK applied the old MT1004 message directly. Tracking to the same routes 3 times, the rover logged the real-time processed results for both the Compact and the general RTK results, and finally the results were compared to the post- processed position. Similar to the previous static test result, the error increment of the MT 4081 was below 5cm, which is far smaller than that of MT1004. Moreover, the Compact RTK kept its RTK fixed solution continuously through all the dynamic test with all the 30 second latency. Therefore, Compact RTK can be a solution to compensate for a long latency (i.e. 30sec) under a harsh communication environment not to lose the fixed ambiguity of RTK position. If we use Compact RTK for the land transportation, we can expect smaller data, less computing power, simpler algorithm, but more robust system to the time delay.
Published in:	Proceedings of the ION 2015 Pacific PNT Meeting April 20 - 23, 2015 Marriott Waikiki Beach Resort & Spa Honolulu, Hawaii
Pages:	599 - 605
Cite this article:	Park, Byungwoon, Yoon, Donghwan, Song, Junesol, Kee, Changdon, Seo, Seungwoo, Park, Junpyo, "Dynamic Test Result of the Compact RTK Under a Harsh Communication Environment of Land Transportation," Proceedings of the ION 2015 Pacific PNT Meeting, Honolulu, Hawaii, April 2015, pp. 599-605.
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