Dual-frequency Multi-constellation Satellite-based Augmentation System Research Articles

DFMC SBAS service performance analysis of multi-GNSS based on BDS-3 in different regions

Abstract The dual-frequency multi-constellation satellite-based augmentation system (DFMC SBAS) plays a crucial role in many fields, such as civil aviation and other safety-critical applications. To analyze the performance of the third generation of the BeiDou navigation satellite system (BDS-3) dual-frequency SBAS services in global major regions and the enhancement of navigation service performance by multi-Global Navigation Satellite System DFMC SBAS, four schemes are designed to conduct experiments using user end positioning in China (CHN), Europe (EUR), and Australia (AUS), and the results are analyzed under Approach with Vertical guidance I (APV-I) and Category I Precision Approach (CAT-I). In terms of the number of enhanced satellites, user range error, positioning accuracy, and DFMC SBAS service integrity, the results show that the positioning performances of the BDS-3 DFMC SBAS in the horizontal and vertical directions are 0.60 and 1.12 m for the CHN region, 0.87 and 1.39 m for the EUR region, and 0.64 and 1.11 m for the AUS region. In the CHN and AUS regions, the horizontal and vertical accuracies are comparable and better than those in the EUR region. Under the APV-I performance index, except for stations at the edge of the service range in the CHN and AUS regions, the availability and continuity of all stations were 100% and over 99% in the EUR region. The BDS-3 + Galileo + GPS DFMC SBAS service can significantly improve the positioning accuracy compared to a single constellation. Under the APV-I performance index, the availability and continuity of all stations in the three regions reached 100%, which also satisfied the minimum requirement of CAT-I service availability (99%).

Local mitigation of higher-order ionospheric effects in DFMC SBAS and system performance evaluation

Dual-frequency multi-constellation (DFMC) satellite-based augmentation system (SBAS) is a new SBAS standard for aeronautical navigation systems. It supports aircraft navigation from the enroute to approach phases via the L1 and L5 frequencies (1575.42 and 1176.45 MHz). Although the ionosphere-free (IF) combination in the DFMC SBAS operation removes the first-order ionospheric delays in the pseudorange measurement, remaining terms including the satellite-clock offset errors and higher-order ionospheric (HOI) delays are still unaccounted for. The DFMC SBAS accuracy and integrity can be affected by the HOI effects, especially during severe ionospheric disturbances. In this work, we present the local DFMC SBAS corrections with and without the mitigation of HOI delays. We first estimate the HOI delay terms using the received pseudorange followed by separate satellite and receiver bias estimations based on the minimum sum-variance technique. The integrity terms can then be obtained. The performances of DFMC SBAS using the global navigation satellite system (GNSS) data including GPS, Galileo, and QZSS are evaluated using obtained GNSS data at stations in Thailand on the ionospheric quiet and disturbed days. The results show that with the HOI mitigation, the vertical positioning errors (VPE) on the quiet and disturbed days can be improved by 12% and 9%, whereas the vertical protection levels (VPL) are improved by 16% and 21%, respectively. In addition, we perform a preliminary assessment of DFMC SBAS based on the International Civil Aviation Organization (ICAO) requirements of two categories: Localizer Performance with Vertical guidance (LPV-200) and Category I precision approach (CAT-I) showing promising results.

Designing the Signal Quality Monitoring Algorithm Based on Chip Domain Observables for BDS B1C/B2a Signals under the Requirements of DFMC SBAS

To guarantee the integrity of a global navigation satellite system (GNSS) for safety-critical users, a satellite-based augmentation system (SBAS) makes use of the integrity monitoring architecture, of which the signal quality monitor (SQM) is an important component to address the potential risks caused by satellite-induced signal anomalies. Due to the introduction of dual-frequency multi-constellation (DFMC) techniques in 2025, the ranging uncertainty will be reduced by the elimination of first-order ionospheric delay, but the biases measured in each individual signal will be inflated by the ionosphere-free combinations. Moreover, multiple modulations of DFMC signals might introduce applicability uncertainty of a traditional SQM method that has been protecting GPS L1C/A signal only. Thus, higher requirements are put forward for future SQM methods in detection sensitivity and modulation independence. This paper first proposes a design methodology for the SQM algorithm for BDS B1C/B2a signals, which could be easily extended to the DF combinations of other GNSS core constellations. Then, by comparing the performances of SQM baseline algorithms based on traditional multi-correlator and emerging chip domain observables (CDOs), respectively, the superiority of CDO-based SQM is declared. Detailed design iterations are further discussed, including the algorithm practicalization with optimizing code-phase bin length and lowering sampling frequency, as well as the metric simplification, to promote the overall performance while preserving a lower implementation complexity. Ultimately, a CDO-based SQM algorithm for BDS B1C/B2a signals is reached, which would be considered as an effective candidate in new generation DFMC SBASs.

Preliminary performance analysis of a prototype DFMC SBAS service over Australia and Asia-Pacific

The next-generation of satellite-based augmentation system (SBAS) will employ a dual-frequency multi-constellation (DFMC) service providing several advantages over the classical SBAS L1 service. In September 2017, a two-year SBAS test-bed was initiated by Australia and New Zealand in preparation for building an operational system. This system includes DFMC SBAS in addition to the legacy SBAS L1 service and real-time precise point positioning (PPP) service over the Asia-Pacific region. In this paper, we focus on the new DFMC SBAS, and first discuss its positioning and integrity monitoring (IM) algorithms. Next, its performance is tested and analyzed using real DFMC SBAS data, restricting focus to LPV-200 in aviation. Based on real SBAS messages from the geostationary (GEO) satellite Inmarsat-4F1, testing was performed first at two known stations with actual GNSS observations, and next at simulated grids covering the entire Asia-Pacific region. A sensitivity analysis is performed, investigating the contribution of the error sources and the impact of changing the satellite mask angle. Experimental results of 12 consecutive days from September 14 to 25, 2018 of the new SBAS products show that the dual-frequency correction (DFC) residual errors have the dominant contribution in the values of the protection levels needed in IM, whereas other error sources such as the airborne receiver errors, the tropospheric and the ionosphere residual errors have secondary impacts. The accuracy of the positions was found to be at the sub-meter to meter level and was always bounded by the protection levels (PLs) at the known stations. The simulated PLs were less than the alert limit of LPV-200 during the test period in most of the areas of interest, indicating the availability of IM using DFMC SBAS in the main areas of the Asia-pacific region.

Generation of DFMC SBAS corrections for BDS-3 satellites and improved positioning performances

The next-generation satellite-based augmentation system (SBAS) will follow the dual-frequency multi-constellation (DFMC) protocol and broadcast satellite clock-ephemeris corrections and integrity information via GPS L5-like signals to improve the accuracy and integrity for GNSS dual-frequency users. BeiDou Navigation Satellite System (BDS)-3 B1C/B2a dual-frequency users will obtain a higher positioning accuracy using DFMC clock-ephemeris corrections. This paper focuses on an approach for the generation of DFMC SBAS corrections and its preliminary results. Analyzing the signal-in-space ranging error characteristics of BDS-3 satellites reveals that it is impossible to obtain orbits that are more accurate than the broadcast orbits using regionally distributed SBAS monitoring stations. Therefore, a novel DFMC SBAS correction generation approach for BDS-3 satellites is proposed in which only clock offset corrections are calculated. The effectiveness of the proposed approach is validated using measurements from mainland China. By using clock corrections only, B1C/B2a dual-frequency users can obtain the best accuracy. The results of the BDS-3 B1C/B2a dual-frequency positioning experiment show that the user equivalent ranging error is reduced from 0.59 m to 0.37 m with clock corrections but is reduced to only 0.43 m with the addition of clock-ephemeris corrections. With clock corrections only, the 95% positioning errors are reduced from 3.57 m to 1.53 m and from 3.79 m to 2.24 m in the horizontal and vertical directions, respectively, indicating a 48% reduction in positioning errors compared to users without SBAS corrections; however, the error reduction is only 40% with clock-ephemeris corrections.

Estimation method of SBAS dual-frequency range error integrity parameter

The development of a dual-frequency multi-constellation satellite-based augmentation system (DFMC SBAS) is in progress worldwide. The broadcasted dual-frequency range error (DFRE) integrity parameter reflects the effects of satellite ephemeris and clock corrections. A user uses the DFRE to calculate the protection level and then determines whether the DFMC SBAS service satisfies the requirements of the current flight phase. However, the calculation of the DFRE has not been reported. Herein, a DFRE estimation method is proposed based on the projection method. Using the ephemeris–clock covariance matrix of each satellite, the maximal projection direction was solved, and the projection of the covariance matrix on this direction was used as the DFRE to form an envelope for the maximal corrected error. Results show that the DFRE can form an envelope of the maximal corrected error with a set probability, and the integrity performance in the user segment satisfies the Category I precision approach requirement.

Computation of satellite clock–ephemeris corrections using a priori knowledge for satellite-based augmentation system

The dual-frequency multi-constellation (DFMC) satellite-based augmentation system (SBAS) should be able to provide a vertical protection level in the range of 10---12 m, which is sufficient to support Category I precision approach operations. Because of the limited data rate of the SBAS signal and the need to augment 91 satellites simultaneously, DFMC SBAS does not broadcast equivalent terms for fast pseudorange corrections in legacy L1-only SBAS. An analysis of the wide area augmentation system (WAAS) suggests that the range error after applying long-term satellite error corrections, known as satellite clock---ephemeris (SCE) corrections in DFMC SBAS, is not accurate enough for Category I operations. Thus, it is necessary to improve the accuracy of these SCE corrections. With the construction of DFMC SBAS, there is an opportunity to upgrade the SCE correction algorithm. Based on the fact that the Global Navigation Satellite System SCE parameter-fitting error constitutes the majority of the SCE error, we propose a method of computing the SCE corrections by combining a priori knowledge of the SCE parameter-fitting error with real-time measurements. A comparison of the user range error after applying WAAS SCE corrections and fast corrections indicates that the proposed method extends the period when a satellite is augmented by 22.9 % and reduces the root mean square error by 27 %.