Performance Evaluation and A New Disaster Prevention System of Precise Point Positioning at Sea

Eiko Saito, Nobuaki Kubo, Kazumasa Shimoda

Abstract:	Sea buoys enable the observation of wave heights and tide levels. For this reason, they served as an effective tool in informing the tsunami warning system during the Great East Japan Earthquake. However, drawbacks of sea buoys include high cost and long period of construction, difficulty of ensuring power supply, and difficulty of maintenance because they have to be built on the sea. Sea buoys are constructed such that they receive positioning correction data from reference stations on the land through RTK-GNSS. If ships are also utilized as sea buoys, the number of ocean observation points could be increased to achieve more extensive and accurate analysis of the sea state. Although previous research on tsunami detection using RTK-GNSS positioning of ships was presented at ION 2012, the study had limitations due to the use of reference stations for reception of correction data. If ships could observe sea level fluctuations autonomously without using reference stations, they could be extended in terms of disaster prevention and sea monitoring. In our research, we focused on the use of Precise Point Positioning (PPP) on ships. We compared the navigation performance of current PPP onboard the 6185 GT ship GINGA MARU under two navigational conditions: traveling and anchored. Moreover, we propose a disaster prevention system using anchored ships instead of sea buoys. Firstly, we analyzed the performance of PPP to determine whether PPP can be used practically at sea. We used Trimble PPP receivers, which receive the correction information of precise GNSS orbit and clock from the satellite based augmentation system OmniSTAR, and used the satellite navigation systems GPS, QZSS, GLONASS, and BEIDOU. The duration of the experiment was approximately 40 days and included ocean areas of Japan, Australia, and Singapore. Positioning accuracy was investigated separately for each of two navigational states: traveling and anchored. Performance evaluation of PPP at sea was dependent on the availability of PPP, which was checked for accuracy by comparing with RTK-GNSS solutions. For the accuracy evaluation, we used the results after convergence of PPP, which took about 30 min on average. According to the experimental results of 40 days, the availability of PPP was greater than 95%. The remaining 5% of results were stand-alone positioning. Because no antenna was installed at the top of the radar mast and funnel of the ship, GNSS signals were sometimes obstructed by these structures, especially when anchored, as the ship was constantly swinging and changing direction. Since it is impossible to analyze RTK on the ocean when far from the shore, we compared PPP solutions with RTK-GNSS post-processed solutions when the ship was in Tokyo Bay. The nearest reference station, at a baseline distance of about 10 km, was used to determine the fixed RTK-GNSS solutions. In this analysis, RTK fixed positions were regarded as the true positions of the ship. According to experimental results over the duration of 24 hours, positioning accuracy on board was 2 cm (standard deviation) in the horizontal direction and 6 cm (standard deviation) in the vertical direction. The vertical position was slightly less accurate than the horizontal position when compared with the normal RTK solutions, but this level of accuracy was considered adequate for use in precise applications as mentioned earlier. However, deviations were also observed over time between PPP solutions and RTK fixed solutions, with approximately 20 cm bias in the horizontal direction and approximately 10 cm bias in the vertical direction. These errors are likely due to differences in the positioning methods of PPP and RTK, as each method uses different reference stations. We continue to analyze the data for further understanding of these deviations. Secondly, we analyzed alternatives for data interpolation of sea buoys and their application toward disaster prevention and mitigation using ships at anchor. It is quite important that ships observe as much sea data as possible, rather than sea buoys. Sea buoys typically record measures of waves, tides, tsunami, wind direction, wind speed, water temperature, flow direction, flow rate, temperature, and pressure and are equipped with satellite communication equipment to transmit data to the shore. Merchant ships are generally equipped with weather observation equipment, which enable them to make the same observations as sea buoys. With proper data transmission tools, ships at anchor in the bay could easily replace sea buoys for data measurement and transmission. In this study, we provide an overview of a system for notifying sailors of abnormal ocean conditions using precise positioning information from PPP aboard ships. Our proposed system is to transmit sea level fluctuation data from the onboard measurement device to other ships. If the device detects an abnormal fluctuation in sea level, the results are reported to the duty officer, who shares the abnormal fluctuation data with the entire ship via the ship’s alarm or communication system. An abnormal fluctuation in sea level can be detected using measurements of wind, wave, and sea surface height displacement derived from the PPP results. This is the same method implemented by tsunami monitoring systems in Japan, which use Global Positioning System (GPS) measurements of sea buoys. Based on our test results, it is possible for PPP aboard ships to detect a tsunami correctly. A large number of ships are anchored in Bay. Disaster prevention and mitigation systems that can be carried out by ships is necessary not only to protect assets on coastal land, but also to protect valuable assets of ships and sailors themselves.
Published in:	Proceedings of the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2016) September 12 - 16, 2016 Oregon Convention Center Portland, Oregon
Pages:	3412 - 3432
Cite this article:	Saito, Eiko, Kubo, Nobuaki, Shimoda, Kazumasa, "Performance Evaluation and A New Disaster Prevention System of Precise Point Positioning at Sea," Proceedings of the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2016), Portland, Oregon, September 2016, pp. 3412-3432. https://doi.org/10.33012/2016.14809
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