Messages Evaluation for BDSBAS Single-Frequency Service
Hongwen Wang, Kun Fang, Beihang University, Beijing, China Zhiqiang Dan, Xiao Li, Beijing Hualong Tong Science & Technology Co., Ltd, Beijing, China Kai Guo, Zhipeng Wang, Beihang University, Beijing, China Yanbo Zhu, Aviation Data Communication Corporation, Beijing, China
Location: Beacon B
The 55th satellite of BeiDou Navigation Satellite System (BDS), namely the 3rd Geosynchronous Earth Orbit (GEO) satellite of BDS-3, was successfully launched on June 23, 2020. After the in-orbit payload testing, this satellite started to provide open services of positioning, navigation and timing from July 2020, which indicates the completion of the space segment in the BeiDou Satellite-Based Augmentation System (BDSBAS) composed of three GEO satellites, i.e., PRN 130, 143 and 144. By providing single-frequency (SF) service currently, as well as the planned dual-frequency multi-constellation (DFMC) service in future, the BDSBAS can achieve APV-I and CAT-I capabilities in China and the surrounding areas. Currently, it is under testing and certificated by Civil Aviation Administration of China (CAAC). According to the scheme for civil aviation application verification and evaluation, BDSBAS signal characteristics need to meet the requirements of International Civil Aviation Organization (ICAO) Standards and Recommended Practices (SARPs). The BDSBAS operational performance needs to be compatible with international aviation industry standards. As for BDSBAS messages broadcast by three GEO satellites, it is necessary to evaluate the message format, parameters and service performance.
This paper focuses on analyzing BDSBAS data collected from August to November in 2021. Nearly 8 million data points were processed to investigate: (1) whether the BDSBAS GEO satellites could broadcast augmentation messages with the contents and formats meeting the requirements described in Appendix A of DO-229E; (2) whether the user differential range error (UDRE) and grid ionospheric vertical error (GIVE) decoded from BDSBAS messages could envelop the corresponding differences respectively induced by satellite and ionospheric errors; and (3) whether BDSBAS could be available in 95% or better of time in Chinese mainland and at the test site. All the three months test data were collected by a Septentrio PolaRx5S receiver operational in CNS/ATM Laboratory. A NovAtel 702 antenna, which was updated to an 850 antenna later, connecting to the receiver is installed on the top of New Main Building at Beihang University in Beijing, China. The test items refer to WAAS performance analysis report.
Firstly, the basic formats of BDSBAS messages received were evaluated. Message type 2~4 may transmit alert information for users when BDSBAS corrections are no longer bounded by UDREs. The weighting of the satellite corresponding to the increased UDRE after alerts will be reduced. The satellite can even be excluded from the navigation solution, thus the availability may be affected dramatically. In addition, all message types have their specific maximum update and timeout intervals. Alerts or handover operations can interrupt normal BDSBAS message stream, which leads to the late transmission of the original messages. This paper counts message numbers with alerts, abnormal update intervals and other format anomalies for BDSBAS messages in four months. Results show that the BDSBAS adopts the transmission mode of fixed time sequence. The transmission cycle of a set of messages is 240 seconds. There are no alert messages broadcast by GEO satellites, which has little or no impacts on the BDSBAS availability and user safety. Almost all message types transmitted by the GEO satellite of PRN 130, the primary source of augmentation information, can be broadcast on time. Only a few messages are late due to the testing messages and null messages. However, the messages decoded from GEO satellites of PRN 143 and 144 are abnormal in many days, which causes that the type and content do not match standards. The abnormal messages are mainly due to wrong decoding by the receiver after cross check with other data streams.
Then, the UDREs and GIVEs decoded in message type 2~4 and 26 from the BDSBAS GEO with the PRN of 130 were evaluated. They are essential integrity parameters that make a significant difference to the integrity performance (the probability of integrity risk) in the user segment. The UDRE mapped from BDSBAS message type 2~4 is used to monitor the satellite residual error, which is equal to the difference between the raw pseudorange and the calculated reference range. The UDRE must envelop the error caused by satellite ephemeris and clock corrections with a certain probability of 99.9% (3.29 sigma). Similarly, the GIVE mapped from BDSBAS message type 26 is used to monitor the ionospheric grid point residual error and must envelop the difference between the vertical ionospheric delay interpolated from the ionospheric grid points and the calculated reference delay with a certain probability of 99.9%. This paper calculated the range residual errors for 32 GPS satellites and ionospheric residual errors for 117 BDSBAS grid points at New Main Building during four months. The results show that UDREs and GIVEs are sensitive to the corrected errors, however, they cannot perfectly satisfy the envelop requirement with a probability better than 99.9%. The number of the monitored GPS satellites is about 10 per epoch and the number of available ionospheric grid points is about 85, which may be due to the limited number of ground monitoring stations of BDSBAS so that the original measurement for ionospheric monitoring is not adequate.
Finally, the availabilities of BDSBAS in the coverage area and at the test site were evaluated based on valid messages. BDSBAS coverage area evaluation estimates the percent of service volume where BDSBAS can provide LPV-200 service level in China and the surrounding areas, while the test site evaluation estimates the percent of time for the same service level at New Main Building. This paper uses the received navigation, observation and augmentation data in RINEX format to calculate user protection levels (HPL and VPL) and position errors (HPE and VPE). Protection levels are calculated to analyze the integrity at a 30-second interval and 1-degree spacing for coverage area evaluation, while protection levels and position errors are calculated to analyze availability and integrity at a 1-second interval for test site evaluation. Availability contour and Stanford figure are plotted to display test results. The results show that the availability of 99.9% for BDSBAS at New Main Building can be easily achieved. There are several Hazardous Misleading Information (HMI) events even though the antenna is static and less affected by multipath. However, the availability of 95% or better in Chinese mainland has not been achieved yet, especially in the northwest and northeast border areas. This may be also due to the fewer monitoring stations equipped near border areas and in other countries, which results in inadequate capabilities for satellite and ionospheric monitoring.
The analysis in this paper indicates that BDSBAS is capable of broadcasting augmentation information continuously and stably over a long period of time within the service area, although it has not yet been announced formally and is still in the testing phase. In future, it is possible to replace the testing message type 0 and the null message type 63 with authentication messages to prevent BDSBAS signals being spoofed if the L5I authentication scheme is adopted by DFMC BDSBAS. The message evaluation process and content proposed in this paper can support the validation evaluation of BDSBAS in civil aviation applications and the development of international industrial standards.
 ICAO Annex 10, Vol. I Radio Navigation Aids, Aeronautical Telecommunications, 7th Edition, July 2018.
 RTCA. Minimum Operational Performance Standards for Global Positioning System / Satellite-Based Augmentation System Airborne Equipment. DO-229E, Washington DC, USA, 2016.
 FAA William J. Hughes Technical Center. WIDE AREA AUGMENTATION SYSTEM PERFORMANCE ANALYSIS REPORT, July 2021.
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