Archive | 2019
BeiDou 3 Signal Quality Analysis and its Impact on Users
Abstract
Nowadays, one can use four global navigation satellite systems (GNSS). Two of them are complete constellations (GPS, Glonass) and two (BeiDou, Galileo) are already providing initial services and will be finished in the near future. Additionally, satellite-based augmentation systems (SBAS) and regional systems like WAAS, EGNOS, GAGAN or QZSS complement the GNSS service. However, within all systems one can observe changes, modifications, and updates every year. This can be related to signal property changes, renewing satellites up to the implementation of completely new GNSS platforms in space. Especially, for safety critical applications using GNSS, like advanced receiver autonomous integrity monitoring (ARAIM) or ground-based augmentation systems (GBAS) the new or changed signal properties are of high interest and are crucial for integrity analysis. With the help of detailed information about the signal deformation and the received signal power it is possible to calculate realistic error bounds and consequently realistic protection level for these kinds of safety-critical applications. This paper will present signal analysis results based on observations of the new BeiDou 3 signals appeared in the last two years. After a brief introduction of the measurement facility the basic analysis of the quality of the signals in spectral and modulation domain is introduced. This covers the transition from BeiDou 2 up to BeiDou 3 including the test and validation phase in the beginning of BeiDou 3. Based on captured in-phase and quadrature (I/Q) samples we will give a detailed overview on the BeiDou 3 satellite payload characteristics including signal deformation analysis. The paper will assess how such imperfections will influence the pseudo-range measurements and consequently provide the capability for error analysis with respect to safety-critical applications. We will show the dependency of tracking biases depending on different receiver configuration. Using the German Aerospace Center ́s (DLR) precise calibrated measurement facility, we will also present an analysis of the transmitted satellite signal. Considering the measured power in relation to the boresight angle of the satellite one get a cut through the antenna pattern of the satellite and can assess the antenna symmetry properties. Examples for different satellites will be presented. INTRODUCTION The Chinese BeiDou navigation satellite system (BDS) is one of the most rapidly growing satellite navigation systems of the last years. It started 2000 with a demonstration phase with 3 geostationary test platforms. In a second step it has been expanded to a regional navigation system with 16 space crafts in total [Yang 2017]. In 2015 the Chinese have started updating their system from a regional to a global one with the launch of their first BeiDou 3 satellite. This development to a global system implies not only the expansion of the coverage but also a significant change of the provided signals and services to achieve better compatibility to existing GNSS and consequently less implementation effort at manufacturer side and therefore better competitiveness. Until mid of 2017 the information about the BeiDou 3 signals and services were very fragmentary. From official statements and supplementary information which circulated in the navigation community, draft overviews with respect to service type, frequency and spreading modulations were collected and presented in [Betz 2015, Teunissen and Montenbruck 2017]. Figure 1 shows an extract of the GNSS signal overview of [Teunissen and Montenbruck 2017] regarding BeiDou, GPS, and Galileo. Clearly visible is the change in signal provision from a regional BeiDou system (BDS-2) to a global one (BDS-3). One can observe, that a certain similarity of the new BDS-3 signal frequencies and modulation types with GPS and Galileo is targeted which emphasize the ambitions for compatibility with other GNSS. Figure 1 Signal overview regarding BeiDou 2 (regional navigation system) and BeiDou 3(global navigation system) and GPS and Galileo [Teunissen and Montenbruck 2017, page 1235] Within the last 13 months interface control documents (ICDs) regarding the B1C, B3I and B2a have been released, which provide detailed information on signal structure, modulation, coding, message types, and structure. However, still a couple of signals without detailed official information exist, which motivates the authors at least to analyze the presence and modulation type of all L-band transmissions of BDS-3. In March 2015 the first satellite BDS I1-S of BeiDou 3 was launched, followed by further four satellites, i.e. BDS M1-S, BDS M2-S, BDS I2-S and BDS M3-S. These first 5 initial BeiDou 3 satellites were some kind of test satellites for validation purposes [Betz 2015, page 262] but with operational capability. The satellites are indicated by an “S” in the end of each satellite name like BDS M2-S. The concept is similar to European Galileo implementation plan in the past starting with the installation of the so-called Galileo In-Orbit Validation Elements (GIOVE A/B) followed by In-Orbit Validation (IOV) satellites with full operational capability (first four Galileo satellites). However, the Chinese have combined the former two steps into one. The analysis that is presented in the following will show that substantial tests have been performed and obviously the analysis resulted in certain changes of the final payload design. Since 2017 China has already launched further 16 fully operational BeiDou 3 satellites. Therefore, now it seems to be the right time to analyze the present nominal state of the signal in space (SIS) behavior and to assess its usability for challenging GNSS applications, e.g. in the field of safety-critical applications. The first signals of the new BeiDou 3 system were captured on August 10, 2015. Researchers from the Joint Research Center of the European Commission (JRC) at Ispra, Italy, captured the signal at B1 frequency (which corresponds to GPS L1 and Galileo E1 band) and conducted a first signal analysis in time, spectral, and correlation domain [Bravaro 2015]. Based on their analysis they confirmed the presence of a time-multiplexed binary offset carrier (TMBOC) signal and its power sharing among its individual components as well as an additional BOC(14,2) and a legacy B1I signal (BeiDou 2) at L1-14 MHz. Researchers from JAVAD GNSS and DLR also have been busy tracking the newest BeiDou satellites at that time, [Camaron 2015] summarized their findings. Researchers at JAVAD GNSS validated not only signals on L1 but also at E5a and E5b frequencies and provided a first impression on the signal strength which users can expect on ground. The researchers of DLR focused their analysis on spectral measurements to identify signals and their modulation type on all frequency bands in the overall L-band. Figure 1 provides an overall spectral overview of the transmitted signal on 24 of October in 2015. The measured spectrum fits exactly with the expectation presented in Figure 1. Figure 2 Spectral overview of the transmitted BeiDou L-band signals of BDS I2-S on 24.10.2015 In [Xiao 2016] receiver outputs and spectra are presented for all BeiDou 3 signals in the L-band. Additionally, inphase and quadrature data have been recorded and I/Q constellation diagrams are presented. However, with the included noise no statements about signal and modulation purity as well as quality could be retrieved. Beside from research on the signal characteristics, scientists have been observed variations in the BDS resulting pseudo-ranges at user receiver level [Wu 2017, Zhou 2018]. Signal distortions are an important contribution to the signal in space range error (SISRE), hence motivating to perform very precise signal analysis to characterize qualitatively and quantitatively the amount and influence of the BeiDou 3 signal behavior. Based on the above mentioned previous publications it can be concluded that they primarily addressed the investigation of basic modulation parameters of the signals and provided an initial analysis of the transmitted modulation type, chip rate, and primary as well as secondary codes. Detailed information about nominal signal distortions or information about signal power received on ground or antenna pattern characteristics are missing. However, such information is essential for the assessment of BeiDou 3 with respect to its usability for safety-critical applications. This paper tries to fill the gaps of information on detailed and precise signal quality assessment to provide the basis for a usability analysis of BeiDou 3 regarding safety-critical applications. The analysis will be based on measurements with DLR’s high gain antenna located in Weilheim, Germany. The SIS analysis part will contain significant more findings than in previous publications and will provide in-depth analysis of signal quality and signal distortions. First, the DLR measurement facility and the necessary steps of data calibration will be briefly described. This accurate calibration is the key enabler for performing detailed measurements and allows for the compensation of the impact of the receiving system imperfections and propagation influences (e.g. ionospheric refraction). After that, we will start with basic analysis of the quality of the signal shape in spectral and modulation domain. Based on the captured I/Q samples the authors will give a detailed overview on the BeiDou 3 satellite payload characteristics including analysis of signal distortions. In order to assess the effect of such signal imperfections on safety-critical applications, the paper will assess how such errors propagate to the receiver observables. Setting out from the requirements and needs of these critical applications as well as current Minimum Operational Performance Standards (MOPS) the estimated signal characteristics and payload imperfections will be used to assess their impact on