Citation: Wang, X.; Cui, X.; Liu, G.;
Lu, M. Designing the Signal Quality
Monitoring Algorithm Based on Chip
Domain Observables for BDS
B1C/B2a Signals under the
Requirements of DFMC SBAS. Remote
Sens. 2023, 15, 1008. https://doi.org/
10.3390/rs15041008
Academic Editors:
Damian Wierzbicki and
Kamil Krasuski
Received: 16 December 2022
Revised: 10 February 2023
Accepted: 10 February 2023
Published: 12 February 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Article
Designing the Signal Quality Monitoring Algorithm Based on
Chip Domain Observables for BDS B1C/B2a Signals under the
Requirements of DFMC SBAS
Xiang Wang
1,2
, Xiaowei Cui
1,
*, Gang Liu
1
and Mingquan Lu
1,3
1
Department of Electronic Engineering, Tsinghua University, Beijing 100084, China
2
State Key Laboratory of Geo-Information Engineering, Xi’an 710054, China
3
Beijing National Research Center for Information Science and Technology, Tsinghua University,
Beijing 100084, China
* Correspondence: cxw2005@tsinghua.edu.cn
Abstract:
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 architec-
ture, 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 superi-
ority 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.
Keywords:
satellite-based augmentation system (SBAS); BeiDou navigation satellite system (BDS);
BeiDou satellite-based augmentation system (BDSBAS); signal quality monitor (SQM); dual-frequency
multi-constellation (DFMC); chip domain observable (CDO); integrity
1. Introduction
The integrity of global navigation satellite systems (GNSSs), e.g., GPS, GLONASS,
Galileo, and BDS, is an important performance indicator to safety-critical users, such as
civil aircrafts in approaches. If a GNSS constellation failed to provide the expected nominal
service but the users were not notified in time, an incident of loss of integrity would occur.
Conventionally, the open service performances of individual GNSS core constellations
can hardly meet the requirements of International Civil Aviation Organization (ICAO) for
approaches with vertical guidance (APV) [
1
]. In order to enhance GNSS integrity for civil
aviation users, several kinds of augmentation systems have been developed, one of which is
called a satellite-based augmentation system (SBAS) that is intended to serve civil aviation
in Category-I (CAT-I) precision approaches (PAs) [
2
]. Several SBASs have been operational
for years, such as the wide-area augmentation system (WAAS) covering North America
and the European geostationary navigation overlay service (EGNOS) covering Europe
Remote Sens. 2023, 15, 1008. https://doi.org/10.3390/rs15041008 https://www.mdpi.com/journal/remotesensing
