Integrated Navigation GPS/BeiDou/INS Performance in Two Hemispheres

Rapid development of the BeiDou satellite navigation system over the past decade has made the new GNSS ready for evaluation of some of its capabilities. This article describes a series of road trials in China and Australia using a new multi-sensor integrated navigation system that fuses GPS, BeiDou and MEMS inertial sensors. Preliminary test results indicate that this integrated system can continuously provide navigation solutions in all the test environments, using inertial measurements to bridge GNSS outages. An analysis of various distributions of satellites in the constellations reveals the influence of the two GNSS systems’ satellite geometry on real-time positioning.

YONG LI ue to the huge success of GPS The Chinese BeiDou system (BDS) UNIVERSITY OF NEW SOUTH WALES in both military and civil appli- began providing regional positioning GANG SUN cations, several other GNSSs services in December 2011. With 16 sat- NANJING UNIVERSITY OF SCIENCE AND D have been developed, built, and ellites launched into orbit by the end of TECHNOLOGY operated in the last few decades. GNSS, the following year, BDS now can provide WEI JIANG regional, and augmentation systems are positioning, timing and short message UNIVERSITY OF NEW SOUTH WALES comprise a growing family that also communication services in China as includes GLONASS, Galileo, BeiDou, well as adjacent Asian and Pacific areas. and Japan’s Quasi-Zenith Satellite Sys- On December 27, 2012, China released tem (QZSS). New members, such as the first version of a complete interface the Global Indian Navigation System control document (ICD) for the BeiDou (GINS), are preparing to join in next system’s B1 Open Service signal-in- decade. space. Multiple GNSSs not only provide As a new member of the GNSS com- users with multiple choices as to which munity, BeiDou is experiencing rapid system or systems to employ, but also growth with strong support from both improve accuracy and reliability of the the Chinese government and numerous GNSS receivers by using the compatible domestic BeiDou receiver manufactur- and interoperable receiving techniques. ers. The progress of BeiDou development www.insidegnss.com NOVEMBER/DECEMBER 2013 InsideGNSS 57 INTEGRATED NAVIGATION

has also attracted a lot of attention from man filter, unscented Kalman filter, and signal challenged environments such as governmental, academic and industrial particle filter — have been adopted to urban canyons. By using the odometer sectors outside of mainland China. enhance the performance. Furthermore, data and an advanced data fusion algo- GNSSs are widely used for land vehi- auxiliary sensors such as odometers or rithm, GNSS outages can be seamlessly cle position and navigation. But it is dif- magnetometers have been evaluated to bridged with inertial measurements and ficult for GNSS-only positioning systems possibly overcome performance limita- therefore provide continuous navigation. to obtain a continuous solution when in tions during GNSS outages. In the field tests, the GPS/BeiDou/ GNSS signal–challenged environments. Here we used a low-cost GPS/Bei- MEMS receiver kept valid solutions The most popular method is to use a self- Dou/MEMS solution for real-time posi- during GNSS outages spanning more contained system that is immune to sig- tion, velocity, and attitude estimation. than 300 seconds. These features make nal jamming to assist the GNSS, such as Integration with the BeiDou module was the integrated navigation system suitable an inertial navigation system (INS). intended to enhance the overall naviga- for a wide range of applications, includ- For operations in such situations, tion performance of the system com- ing: vehicle and personal navigation and most land and aerial vehicle navigation pared with the use of GPS only. Tech- tracking, vehicle orientation reference, applications preferentially incorporate nical details of the GPS/BeiDou/MEMS autonomous machine guidance, aerial an inertial measurement unit (IMU) solution are included in the Manufactur- and land mobile mapping, unmanned based on a microelectromechanical sys- ers section near the end of this article. aerial vehicles, and research on multi- tem (MEMS) form factor. Cars manufactured since 1996 are sensor navigation. This article evaluates positioning generally equipped with the On-Board performance in the northern and south- Diagnostics (OBD)-II interface, which Analysis of BeiDou ern hemispheres using a new GPS/Bei- enables vehicle owner or a repair tech- Constellation’s Global Dou/MEMS integrated navigation sys- nician to access the state of health infor- Coverage tem. Road tests were conducted in real mation for various vehicle sub-systems, BeiDou is one of the four main GNSSs in time in Nanjing, China, and Sydney, including the vehicle speed and odom- the world, which will provide position- Australia, under various conditions: eter readings. The OBD-II hardware ing, navigation, and timing (PNT) ser- open sky view, urban downtown areas, interface is the female 16-pin (2x8) J1962 vice for global users by 2020. According tunnels and underground parking. We connector. to the development plan, BeiDou began analyzed the results with BeiDou sat- The OBD-II interface, which can providing PNT services for Asia-Pacific ellite visibility of the two cities respec- usually be found underneath the dash- region this year. tively. board on the driver’s side of a vehicle, As of December 2012, 14 usable Bei- has four different protocols that manu- dou navigation satellites were on orbit, Integrated Navigation facturers can use for their cars. The including 5 GEO (geosynchronous Hardware Platform OBD-II protocol implemented in the Earth orbit) satellites, 5 IGSO (inclined MEMS sensors usually have large drift device in our tests is currently compat- geosynchronous orbit) satellites and 4 errors that prevent them from being ible with the ISO 9141-2 standard widely MEO (medium Earth orbit) satellites. used in inertial-only mode. Therefore, used in European and Asian cars. Geographic coverage is an impor- the performance of the GNSS/MEMS- The GNSS/INS receiver implements tant indicator for assessing constella- INS system degrades quickly during the OBD-II protocol on the board to tion design. Figures 1 and 2, respectively, GNSS signal outages. access the odometer speed of the vehicle, show the average satellite visibility and Some non-linear integration tech- which improves the navigation perfor- position dilution of precision (PDOP) niques — such as the extended Kal- mance during GNSS outages in GNSS values over a 24-hour period for BDS.

11 70 70 10 12 50 9 50 10 30 8 30 10 7 10 8 6 -10 -10 5 6 -30 4 -30 4 -50 3 -50 -70 2 -70 2 -150 -100 -50 050 100 150 -150 -100 -50 050 100 150

FIGURE 1 Average number of visible satellites of Beidou constellation over FIGURE 2 Average PDOP value of BeiDou constellation over a 24 hour a 24 hour period (above a five degree elevation cutoff angle) period (above a five degree elevation cutoff angle)

58 InsideGNSS NOVEMBER/DECEMBER 2013 www.insidegnss.com 5 0 -5

East error/m -10 500 1000 1500 2000 2500 3000 3500 4000 10 0 10 North error/m 500 1000 1500 2000 2500 3000 3500 4000 0 -10

Up error/m -20 500 1000 1500 2000 2500 3000 3500 4000 Time/s

FIGURE 3 Three different antennas and GPS/BeiDou/MEMS receiver for FIGURE 4 Static positioning error of three antennas on ENU direction: AGA, the antenna evaluation GPS antenna; MGA, magnetic GPS antenna; BDA, BeiDou antenna

Reflecting BDS’s current phase of Upgrading from the first-generation, between MGA and BDA positioning development, which concentrates the the second-generation BDS designs results are 0.05 meter in north-south, satellites over China and adjacent coun- in the passive positioning technol- 2.3 meter in east-west, and 1.6 meter in tries, more than eight satellites can be ogy, which is same as the GPS. With- up-down direction. seen at many locations in the Asia- out doubt, it also supports all services It’s interesting to note that the AGA Oceania region, where the BDS PDOP including a short message service (SMS). has the largest error on east and up is as small as 2 (with the elevation cut- Therefore, all antennas for BDS must be directions, but less error on north direc- off angle set as five degrees), providing compatible with these two generations tion. Even so, the positioning perfor- assured PNT services in this area. Some of satellites. mance of the AGA is more numerically effects on the PNT services from the The GPS/BeiDou/MEMS receiver stable and its convergence speed, faster constellation configuration will be pre- used in this article integrates GPS/Bei- than the others. sented and analyzed later during our Dou module but with only one RF input. The results in Figure 4 indicate that description and evaluation of the road It is important to evaluate its position- the GPS/BeiDou/MEMS receiver is tests. ing performance with different anten- able to provide positioning service with nas, including the specially designed both the GPS antennas and the BeiDou Comparison of Antennas BeiDou antenna. A comparison of solu- antenna. However, solutions from the The GPS L1 signal frequency is tions using different types of antennas three antennas revealed some differ- 1575.42MHz, while the BeiDou B1 sig- is presented. ences. Because the comparison tests of nal is 1561.098MHz. Frequency is one of This antenna evaluation was car- the three antennas were not conducted the differences between GPS and BeiDou ried out in the University of New South at the same time, and the GPS/BeiDou/ antenna. Most antenna manufactures Wales Satellite Navigation and Position- MEMS receiver is based on satellite have designed the BeiDou antennas that ing (SNAP) Lab, in Sydney, Australia. As pseudorange processing, the differences support both B1 and L1 frequencies. shown in Figure 3, three different anten- between three antennas (less than three Another difference in the two types nas were evaluated by one-hour static meters in horizontal directions and less of antennas is the communication mode positioning test using the GPS/BeiDou/ than seven meters in vertical direction) between satellites and receiver they sup- MEMS receiver. From left to the right, are within an acceptable error range. ported. GPS is a passive, one-way com- they are a GPS antenna (AGA), general The differences between the MGA munication system. The GPS receiver magnetic GPS antenna (MGA), and the and BDA antennas in all three direc- only receives the signals from the satel- BeiDou antenna (BDA). (Only the BDA tions were small (less than two meters lites. It does not transmit any signals to antenna can track both BeiDou and GPS after convergence on a position solu- the satellites. signals. The Beidou L1 frequency falls tion). Therefore, using a BeiDou antenna In contrast, BDS has two generation into the AGA antenna’s frequency band, or GPS antenna would not significantly of satellites operating in the orbits. The but we have not yet confirmed that it can affect the positioning performance. first generation is based on the active track the Beidou signals.) The reference positioning technology, which means receiver provided a high-precision RTK Performance in Field Trials the receiver not only receives the sig- solution. Several field trials were conducted to nals from satellites but also transmits Figure 4 provides the positioning evaluate the performance of the integrat- self-information to the satellites (e.g. the errors of each antenna for the east/ ed navigation system in different scenes short message service). north/up (ENU) direction. The bias and different hemispheres. In these tri- www.insidegnss.com NOVEMBER/DECEMBER 2013 InsideGNSS 59 INTEGRATED NAVIGATION

outputting its position solution (the test route depicted in Figure 7 had a clear yellow dots diverging from the route), sky view from the highway during which which is obviously poor and jumps far the receiver could track enough satellites away from the road. to provide position fixes. The positioning In the long tunnels (Tunnel 3 and results from GPS/BeiDou and GPS-only Tunnel 4) under the lake, the GPS/Bei- are slightly different from each other. Dou module totally stopped outputting By overlaying the solutions on a Google a solution. There is a short sky view Earth image, we found that the GPS- GPS/BeiDou/MEMS configuration (left) and when passing by the junction of two- only solution (red) fits the traffic lanes GPS/MEMS configuration (right) direction tunnels and a way out. How- very well, but GPS/BeiDou solution (yel- ever, the integrated navigation system low) has a bias within two meters. These als, a GPS/BeiDou/MEMS receiver and continued to output a solution (red) in errors were mostly toward the south. a GPS/MEMS receiver were mounted on all tunnels, which reasonably bridged As we have analyzed the BeiDou the central armrest beside the driver as the GPS/BeiDou outages. antenna and GPS antennas and excluded seen in the accompanying photo. Let’s look at the performance of the the antenna contribution as the reason The solutions from these two systems system in greater detail. for the bias, it seems that this bias can were compared to reveal whether add- Comparison of Visible Satellites somehow be explained by the satellite ing BeiDou capability to the integrated between GPS and BDS. Figure 6 depicts geometry. Figure 8(a) shows the skyplot navigation system improved positioning the numbers of visible GPS/BDS space of the BeiDou constellation in Nanjing performance. Two antennas were placed vehicles (SVs) when the car was station- at the time of the experiment (around on the top of the car, a MAZDA 6 which ary. At the same time, a zero-velocity 14:00 UTC ), and Figure 8(b) shows the uses the ISO 9141-2 ODBII protocol: update (ZUPT) function was working to skyplot of GPS constellation at the same these were the general magnetic GPS eliminate the inertial sensor errors. Ten time. Quite obviously the distribution of antenna for the GPS/MEMS receiver GPS SVs and eight or nine BDS SVs were the BeiDou SVs in the current regional and the BeiDou antenna for the GPS/ available. That is a good result in light of phase of BDS is rather unsymmetrical BeiDou/MEMS receiver. the BeiDou constellation coverage seen in the North-South direction and more in Figure 1. weighted on the south, while the skyplot Test One: Round Trip Results of GPS-Only vs. GPS/BeiDou of GPS constellation is more symmetri- in Nanjing with Open Sky. The section of the road- cal than the BDS. We designed the road test route to ensure a variety of road conditions and environments, as shown in Figure 5. Section 1 is the airport highway with an open sky view. Section 2 is the city road crossing Nanjing from south to north with two short tunnels and two longer tunnels, including the longest tunnel (about 2.3 kilometers) in Nanjing under Xuanwu Lake. Section 3 is the return path crossing the urban downtown areas with tall buildings on both sides. We expected that the GNSS-only solution would be of poor quality or unavailable due to these multipath and outage scenarios. The distance of the FIGURE 5 Two-way road test in Nanjing: yellow, GPS/BeiDou positioning; red, GPS/BeiDou/MEMS entire test trajectory was approximately positioning 53 kilometers (≈33 miles). Test results show that the GPS/Bei- dou solution (yellow) keeps working in both areas of open sky view and urban canyons. When driving through the tunnels, the INS-only mode was acti- vated to update the current position. In the short tunnels (Tunnel 1 and Tunnel FIGURE 6 Visible SVs number of GPS and BDS 2), the GPS/BeiDou module continued

60 InsideGNSS NOVEMBER/DECEMBER 2013 www.insidegnss.com FIGURE 7 Solutions from GPS-only (red) and GPS/BeiDou (yellow) on airport high way FIGURE 8 BDS (a) and GPS (b) skyplot of the test in Nanjing

FIGURE 9 Performance of GPS/BeiDou/MEMS receiver in underground parking site FIGURE 10 Trajectory of Sydney road trial

GPS/BeiDou vs. GPS/BeiDou/MEMS. Test Three: Vehicle the entire trial, including when in the Referring again to Figure 5, on the way Field Test in Sydney tunnel. back to the starting point, no GNSS As Nanjing is located in the northern Figures 11 and 12 show two magni- positioning blockage appeared in the hemisphere, a corresponding vehicle fied portions of the trajectory, which are urban area. This shows that GPS/Bei- trial was conducted in one of the big road segments aligned east-west and Dou has the potential to provide more cities of the southern hemisphere, Syd- north-south, respectively. Note that the reliable and continuous solutions than ney, Australia. Similar to the road test in trajectory in Figure 11 shows an obvi- GNSS standalone. Nanjing, this trip was also designed to ous bias from the main road in the include both open sky view and urban north direction, while Figure 12 shows Test Two: view with dense trees, as well as a section an error away from the road in the west. Underground Parking with a short tunnel. Figure 13(a) shows a skyplot of the The GNSS/INS unit can continuously The trajectory is shown in Figure 10 BDS constellation over Sydney at the operate in INS-only mode, useful in with the GPS/BeiDou solution plotted time of the test (22:0 UTC), and a cor- such places as the underground parking in yellow and the GPS/BeiDou/MEMS responding skyplot of the GPS constella- lots where drivers could lose their way. solution plotted in red. We can see that tion at the same time depicted in Figure When returning from the road test, the the integrated navigation system is able 13(b). Again, the distribution of BeiDou car was driven back to the underground to provide continuous solution during SVs is unsymmetrical in both the north- parking lot (under the white roof build- ing shown in Figure 9). Beginning at the entrance to the parking lot, the GPS/Bei- Dou solutions (yellow) quite obviously began to jump far away from the actual direction of the vehicle’s path because of the extremely poor or blocked GNSS sig- nals underground. Meanwhile, the inte- grated solution (red) gave continuously accurate positions, from the entrance to FIGURE 11 Part trajectory of Sydney road test (E-W direction) where the car eventually was parked. www.insidegnss.com NOVEMBER/DECEMBER 2013 InsideGNSS 61 INTEGRATED NAVIGATION

Front view (left) and back view (right) of nAX5.2

constant offset positioning errors. How- both tests in terms of satellite visibility ever, as a GNSS under development, BDS and geometry revealed that the direction currently has only 14 operational satel- of a position bias is consistent with the lites. After it has reached full deploy- satellites’ unsymmetrical distribution. ment with more working satellites pro- This is understandable as BeiDou is not a viding global coverage, we expect to see fully deployed system, but we can expect better performance from BDS. better performance over time with more satellites. Concluding Remarks This article presented the navigation Acknowledgment performance of a new GPS/BeiDou/ The authors appreciate the generos- MEMS integrated navigation system. ity of Nanjing TXZ Technology Ltd. in We compared and evaluated the system’s allowing them to borrow the GNSS/INS performance in different scenarios in devices for these tests. TXZ Technology FIGURE 12 Part trajectory Sydney road test (N-S direction) field trials in the northern and southern specializes in GNSS and INS integration hemispheres. and its applications. south and east-west directions. Most Analysis of results show that the BDS satellites distribute in the north-west, has the ability to provide accurate and Manufacturers which cause the error to accumulate in reliable positioning solution to support The integrated GPS/MEMS receiver the north and west directions. navigation applications. But the overall and GPS/BeiDou/MEMS receiver used The distribution of GPS satellites is performance of BeiDou in these tests is for the research described in this article much better than the BDS, a situation still not as good as GPS. One probable were the Navextech nAX5.1 and nAX5.2 that is very similar with the Nanjing test. reason is that the GPS/BeiDou module from TXZ Technology, Ltd., Nanjing, All these phenomena are consistent with in GPS/BeiDou/MEMS receiver only China. The antennas tested during the the road test results. uses B1 signals, which degrades the research were the G5 antenna from Nanjing and Sydney are at similar positioning accuracy. Therefore, per- Antcom Corporation, Torrance, Cali- distances from the equator, with both formance may improve by accessing B2 fornia USA, the TUV magnetic mount located in mid-latitude area. From the and B3 signals with a more advanced GPS antenna, and TXZ’s BeiDou anten- Figure 8(b) and Figure 13(b), it can be receiver. na. The real-time RTK solution was seen more BDS satellites are distributed In urban canyon and tunnels, GPS/ obtained with the NAX-RTK system on the parts towards the equator. This BeiDou/MEMS receiver can provide manufactured by TXZ Technology. is the probable reason for the apparent accurate and robust navigation solu- The AX5.2 is hosted in a small box tions even (L127×W78×H43 millimeters) with a during GNSS simple user interface. The front panel of outages by the box (photo on left) has two LEDs on integrating the left-hand side to indicate status of GNSS a nd power supply and the antenna connec- inertial mea- tion. Two LEDs on the right side indicate surements. the operational status of the GNSS and Further the INS. On the back panel (photo on investigation right) a multi-function interface imple- of constant ments two UARTs, the OBD-II commu- error trends nication, and power supply from either FIGURE 13 BDS (a) and GPS (b) skyplot of the test in Sydney f o u n d i n the OBD-II or the vehicle 12-volt sup-

62 InsideGNSS NOVEMBER/DECEMBER 2013 www.insidegnss.com Supply voltage 7 to 12 VDC Output rate 10Hz (default) Power dissipation 2.6W Odometer OBD-II interface Solutions continued from page 42 Operating temp. -40 to +85ºC Orientation range 360º about all axes Weight 250g Pitch and roll accuracy <0.2º tributors to the GNSS position error in urban environments. The error charac- Size 127×78×43mm Yaw accuracy <0.1º teristics are quite different from those Output type RS232 and USB Position accuracy <2m RMS (with GNSS) of multipath interference and differ- COM settings 115,200 bps, 8 bits data, Velocity accuracy <0.1m/s (with GNSS) ent techniques are needed to mitigate no-parity, 1-bit stop them. Therefore, NLOS reception Output data format GPS info, inertial sensor GNSS outage span >300s should be treated as a separate error raw data, PVA solution source by the GNSS community. TABLE 1. nAX5.2 specifications Additional Resources GNSS specifications Further discussion of NLOS and mul- Channel 50 channels, L1 Signal strength -144dBm(Acquire), tipath mitigation techniques, together -159dBm (Tracking) with a list of references, may be found Position accuracy <5m(Horizontal), Power dissipation [email protected](Average) in a paper by P. D. Groves et alia, “A <10m(Vertical) Portfolio Approach to NLOS and Cold start 35s Speed accuracy <1m/s Multipath Mitigation in Dense Urban Hot start 2s Operating temp. -30 to +70ºC Areas,” Proceedings of ION GNSS+ Accelerometer specifications 2013, which is also available from UCL Discovery. Range ±18g Temp. sensitivity ±120ppm/ºC Initial bias ±50mg (1G) Nonlinearity 0.1% PAUL D GROVES, Bias stability ±0.2mg (1σ) UNIVERSITY COLLEGE Gyroscope specifications LONDON (UCL) Dr. Paul D. Groves is Range ±350º/s Temp. sensitivity ±250ppm/ºC a Lecturer (Academic Initial bias ±2º/s (1 ) Nonlinearity 0.1% σ Faculty Member) at Bias stability ±0.007º/s (1σ) University College London, where he leads TABLE 2. Sensor specifications research into robust positioning and navigation within the Space Geodesy and Navigation Labo- ply. A micro-USB virtual COM on the current research interests include multisensor ratory. With colleague Dr. Ziyi Jiang, he recently panel facilitates communication with a integration of GPS, INS, and Locata, attitude completed an investigation of new GNSS NLOS computer via a standard USB port. An determination, GPS receiver technologies, and and multipath mitigation techniques. Groves SMA connector on the panel is used to optimal estimation/filtering theory and applica- recently published the greatly expanded second tions. connect an external GPS antenna. Table edition of his book, Principles of GNSS, Inertial, Gang Sun is a Ph.D. can- 1 and 2 provide the detailed specifica- and Multisensor Integrated Navigation Systems didate at the Department (Artech House). The first edition was reviewed tions of nAX5.2 and the sensors. of Precision Instrument, by Demoz Gebre-Egziabher in the September/ Reference School of Mechanical Engineering, Nanjing October 2008 issue of Inside GNSS. His most [1] China Satellite Navigation Office.BeiDou Nav- University of Science and recent article for Inside GNSS was The PNT Boom: igation Satellite System Signal In Space Interface Technology. His research Future Trends in Integrated Navigation in the Control Document Open Service Signal B1I (Ver- interests include sensor calibration, attitude esti- March-April 2013 issue. Previously, he authored sion 1.0). 2012. pp 1-77. mation system, aircraft navigation and GPS/INS an article on deep integration of GNSS/inertial [2] Wikipedia. http://en.wikipedia.org/wiki/On- applications. board_diagnostics#OBD-II. Accessed June 2013. systems in the January/February 2008 issue. Wei Jiang is a Ph.D. can- Authors didate at the School of Civil and Environment Yong Li is a senior Engineering in Univer- research fellow at the sity of New South Wales, School of Civil and Envi- Australia. Her current ronment Engineering, research interest is the University of New Locata /GNSS/INS multi-sensor integration, par- South Wales (UNSW), ticularly the implementation of new navigation Sydney, Australia. He and data fusion algorithms. obtained a Ph.D. in aerospace flight dynamics. His www.insidegnss.com NOVEMBER/DECEMBER 2013 InsideGNSS 63