Differential navigation for UAV platforms with mobile reference station

Differential navigation for UAV platforms with mobile reference station NAWRAT ALEKSANDER, KOZAK KAMIL, DANIEC KRZYSZTOF, KOTERAS ROMAN Department of Automatic Control and Robotics, Silesian University of Technology, ul. Akademicka 16 44-100 Gliwice, Poland, e-mail: aleksander.nawrat@polsl.pl, kamil.kozak@polsl.pl, krzysztof.daniec@polsl.pl, roman.koteras@polsl.pl Abstract Rapid technology progress results in the construction of an increasingly smaller autonomous platforms, what leads to the need of very precise navigation systems. This paper presents steps of development of Mobile Differential Global Positioning System. It was designed to cooperate with autonomous vehicles of small size. Created mobile reference station allows to decrease position estimation error of low cost GPS receiver down to few centimeters. Key Words GPS, DGPS, reference station, differential correction, Unmanned Autonomous Vehicle, UAV. 1. Introduction Incredibly rapid science and technology progress results in the construction of an increasingly smaller autonomous platforms. In consequence, there appears a problem of the precise designation of their geographic location[3][4]. Already in the first half of the eighties of the twentieth century, many institutions, research centers and equipment manufacturers have joined to the research, which main purpose was to increase the position accuracy determined by the satellite navigation systems. Systems of this type are used in the navy, aviation, and for many years are commonly used in typical urban applications. Coupled with a GPS receiver (Global Positioning System), that provides the geographical location and with electronic topographic maps, electronic systems became an invaluable tool for navigation[1][6]. Nowadays GPS receivers are commonly used in everyday life but only a small percentage of users realizes that estimated position include serious errors. The precision of GPS receiver is not sufficient, position estimation error defined as the distance between real position and estimated one, vary from zero up to few hundred meters (Fig. 1), depend on GPS receiver manufacturer. Comparing to the size of unmanned objects (approximately 0.5-2 m length) ISSN: 1792-4863 465 ISBN: 978-960-474-231-8

accuracy of GPS is insufficient. It is even unacceptable from the standpoint of using the measurement results in control algorithms for autonomous objects. Even for the higher class receivers, the accuracy is very small. This problem does not really on the quality of the receiver, but the inaccuracy of the satellite position in orbit around the Earth. Fig. 1 GPS Position Estimation error Where: A- Precise receiver position. B- Radius of possible GPS estimation error. 2. Problem definition Differential GPS correction is already commonly used to minimize the GPS position estimation error, however only small percentage of Earth area is covered by reference stations that calculates and generates corrections for public use. Navigation algorithms designed for small in size flying UAV platforms are very difficult to develop and cannot correctly work without precise position estimation. On the areas covered by public DGPS (Differential Global Positioning System) reference stations, position estimation error can be lowered down to few centimeter. Such a precision is sufficient for navigation. Problem appears when UAV have to operate in region not covered by DGPS correction. Solution to described situation can be obtained in few different ways. First one is to substitute GPS with another positioning system designed to operate in the area of interest. Second one is to create new DGPS reference station that covers desired area. As described earlier, GPS position estimation without differential correction includes error that may lead to the dangerous situation such as UAV crush. Solution that implements GPS independent navigation system is possible to realize and in some situations may be desired over GPS navigation. However it involves construction of a local infrastructure of some kind, that will allow to relatively localize UAV platform. Last method uses existing GPS navigation infrastructure and introduces new source of differential correction. Question is, is it possible to create Mobile DGPS reference station that will allow to produce differential correction in any desired geographical location. 3. Differential GPS One of possible solution to lower position estimation error is to use precise, expensive GPS receivers that operates in the band L1/L2/L5. However, use of such class of measuring devices makes autonomous platform extremely expensive and less profitable from an industrial point of view. Another, more cost-effective solution is to introduce a so-called differential correction DGPS, ISSN: 1792-4863 466 ISBN: 978-960-474-231-8

Fig. 2. The idea of differential measurements in the satellite navigation systems ( Fig. 2 C ) in order to eliminate correlated measurement errors. All calculation results and corrections are sent to GPS receivers by some local transmitter for example radio line ( Fig. 2 D) is to use synchronous time measurements made by at least two receivers of the system ( Fig. 2 B). Fig. 2 A and Fig. 2 Differential GPS Where: A - Low cost GPS receivers B - Wireless transmition C - DGPS Reference Station D - GPS Satellites in range By applying the differential correction method the influence of errors can be, if not completely, then at least largely reduced. It ISSN: 1792-4863 467 ISBN: 978-960-474-231-8

is assumed that the propagation conditions of signals between the satellite and differential station are the same as between the satellite and the receiver user. This leads to increase the accuracy of position estimation. Correction is most efficient if the distance between receiver and reference station is in range 150-300 km. By definition, the reference station is an electronic device of exactly known geographical location, that receives signals from navigation satellites and calculates differential corrections. Differential correction, is a difference between precisely known geographic position and estimated position. In the next step calculated differential correction is sent to another receivers. Position estimation error of low cost GPS receivers with applied differential correction can be decreased from 50-100 meters (depend on GPS producer) down to few centimeters. During research process different methods of differential correction estimation were tested. The method, which proved to be the best and is widely used by differential GPS stations, lies in the fact, that the receiver on differential station is measuring pseudo distance for all visible at the moment satellites. Navigation messages sent by satellites contains information about its current location in orbit. Taking into account fact that position of the station is precisely known (determined by geodetic measurements) there is possibility of calculate the precise distance. The difference between distance calculated by GPS receiver and distance calculated by reference station, becomes the differential correction (PRC pseudo-range correction) for the i-th satellite. Correction determined in this way for the i-th satellite is used to increase by the GPS receiver to improve accuracy of its estimated position. Assuming that user and reference station are located in an area, where the same satellites are visible, it is possible to precisely correct distance calculation for every satellite. If GPS receiver is in range of satellites not visible by reference station they are neglected in position estimation. 4. Mobile DGPS reference station Project of mobile DGPS reference station was developed and tested in few possible configurations and steps. Public reference systems very often work and send differential corrections with use of Ethernet, GPRS or other kind of wireless media [2][5][6]. All of enumerated solutions are hard or expensive to use in embedded control system that works onboard of UAV physical platform. Range of Ethernet connection based on radio modules is not sufficient and is no stable. Reliability and range of GPRS is sufficient, however it works only on the areas covered by GSM network. First main assumption was to establish stable long distance connection that will allow to constantly transmit calculated correction. A lot of radio modules that guarantee long range stable connection are available on the market. With use of them DGPS correction can be easily transmitted to the object. In the first step Fig. 3 DGPS signal, from public reference stations, transmitted over Ethernet was received by standard PC and retransmitted by chosen media to the UAV. Solution worked properly and was stable even on long range connection. However few disadvantages could be enumerated. Internet connection was required, access to public reference station was obligatory. Only small percent of Earth is covered by public differential correction stations. ISSN: 1792-4863 468 ISBN: 978-960-474-231-8

Fig. 3 DGPS correction retransmission. On second step corrections received from public reference were substituted by corrections generated by precise GPS platform OEM4-G2L of Novatel company Fig. 4. The only disadvantage of such a solution was that precise geographical position of GPS receiver antenna, that was used as a source of correction, had to be known. In last step both solutions were merged together in order to create DGPS corrections outside of urban areas. Standard PC with connected OEM4-G2L GPS and GPRS modem established its precise position with use of DGPS correction from Ethernet or with use of numeric topographic map and educated operator that is able to mark on the map precise location of GPS antenna. When position is fixed GPRS is turned off and OEM4-G2L GPS is reprogrammed to generate corrections. Such a solution allows to generate DGPS correction to any number of UAV objects working in range. Fig. 4 DGPS Reference station 5. Results of tests Different tests were performed to check correctness of implemented algorithms or in order to validate data provided by GPS receiver manufacturers or to check fact stated in literature. Main fact that was proven by tests confirms the information that low cost GPS receivers are very inaccurate. As test showed position estimation from GPS receivers, from different manufactures, includes highest errors when they are not moving. So it is similar to the situation when unmanned object is standing still and not moving. Tests of low cost GPS receivers provides maximal position estimation error equal to 104 meters Fig. 5 This proves that navigation of UAV based on GPS position estimation only is definitely not sufficient. Even position estimation from precise GPS OEM4-G2L receiver varied in time and the maximal error was equal to 15 meters Fig. 6. ISSN: 1792-4863 469 ISBN: 978-960-474-231-8

Fig. 5 Position Estimation Error of Low cost GPS receiver. Tests performed with OEM4-G2L GPS working as a source of DGPS correction and a low cost GPS as a main source of position estimation proved that differential correction algorithm works properly. Maximal position estimation error after 10 hours of tests, was equal to two meters. Such a result is sufficient for UAV navigation. And can be successfully used in future projects. Fig. 6 Position Estimation Error of OEM4-G2L GPS receiver. Fig. 7 Position Estimation Error of low cost GPS receiver with DGPS correction from OEM4-G2L GPS receiver 6. Conclusions Constant and unstoppable evolution of technology leads to minimization of electronic devices. Small in size flying UAV platforms are much harder to control, they are very unstable in the air. However they are desired by the market. Minimization of flying objects leads to totally new problems that need to be solved in order to successfully accomplish development process. One of them is UAV navigation, and precise designation of object geographic location. Different methods can be used to estimate object position inside buildings, on the ground outside but it's hard to find out anything else than Global Positioning System (GPS) that works in open air. Present GPS navigation system is a result of research of many institutions, research centers and equipment manufacturers that joined together in the first half of the eighties of the twentieth century, in order to increase the position accuracy determined by the navigation satellite systems receivers. However obtained results as test showed what was described in chapter 5 ISSN: 1792-4863 470 ISBN: 978-960-474-231-8

are not sufficient for precise small in size UAV navigation. One of possible methods to improve GPS position estimation is differential correction, described in detail in chapter 3. It allows, as was proven by performed test Fig. 7, to lower position estimation error down to one meter for low cost GPS receiver. Taking into account fact that DGPS reference stations don't cover all area of possible UAV operations and in areas where corrections are generated it's hard to provide them to flying object, some additional steps had to be performed. As a result, as described in chapter 3, solution called Mobile DGPS reference station was developed and tested. It allows to minimize cost of DGPS correction reception in areas covered by public reference stations. It allows to establish stable and long range connection with multiple flying UAV objects and provide them with DGPS correction generated locally. Project will be extended in the future to be more autonomous and more precise. One of possible extensions is correction of height determination. Error of height measurement with use of GPS receivers of any kind is up to 200m. Successful achievement of this goal will allow to use created reference system in any geographical location and generate differential correction for UAV's during missions. Symposium, 1996., IEEE 1996, ISBN: 0-7803-3085-4, p. 169-173. [2] RTCM Recommended Standards for Differential GNSS (Global Navigation Satellite Systems) Service, Special Committee N. 104, Radio Technical Commission for Maritime Services, 1998. [3] Ya Yang Hui, Cheng Xhiping, Xu Shanjia, Wan Shisong, An unmanned air vehicle (UAV) GPS location and navigation system, Microwave and Millimeter Wave Technology, 2008. ICMMT 2008. International Conference, 1998, p. 472-475. [4] Ben Yun, Kemao Peng, Chen, Enhancement of GPS Signals for Automatic Control of a UAV Helicopter System, Control and Automation, ICCA 2007. IEEE International, p. 1185-1189. [5] Soares, M.G., Malheiro, B., Restivo, F.J., An internet DGPS service for precise outdoor navigation, Emerging Technologies and Factory Automation, 2003, Proceedings. ETFA '03. IEEE Conference, p. 512-518 vol.1 [6] Chuck Gilbert, Differential Global Positioning System (DGPS) Broadcast Standard For Marine Navigation, http://www.ccg-gcc.gc.cddgps/main_e.htm, 2002. [7] Hofmann-Wellenhof, B., Lichtenegger, H., and Collins, J., GPS: Theory and Practice, Springer-Verlag Wien, 1994. Acknowledgment This work has been supported by Ministry of Science and Higher Education funds in the years 2009-2010 as development project OR00013409. 7. Literature [1] Gupta, S.K., Foong Sui Jin, Khoo Aik Leong, An intelligent Small Area DGPS, Position Location and Navigation ISSN: 1792-4863 471 ISBN: 978-960-474-231-8