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Du et al. Satell Navig (2021) 2:3 Page 13 of 22

the failure modes with their assumed probabilities, is the for PPP in urban environments are discussed. Moreo-
focus of this paper. However, to examine the prior proba- ver, some open issues, on which more R&D (research
bilities of diferent types of faults requires comprehensive and development) eforts should focus, regarding PPP
studies, which are beyond the scope of this review paper. vulnerabilities and integrity for ITS applications are
PPP integrity at both system-level and user-level are identifed.
still rudimentary. More investigations should be made Tis study will assist in designing a representative
with respect to the following: (1) system-level (i.e. net- threat model as needed for integrity monitoring of
work-end) integrity information needs to be provided the PPP solutions and in developing relevant integ-
along with the correction products similar to those pro- rity monitoring procedures and algorithms. How-
vided by SBAS; (2) careful assessment of the probabilities ever, the probabilities of many of these threats are still
of PPP threats, as mentioned earlier; (3) quality control under investigation and require comprehensive studies
and integrity monitoring schemes for PPP applications which are outside the scope of this paper and will be
in diferent environments, including open sky, suburban addressed in future work.
and urban areas; (4) improvement of integrity monitor-
ing algorithms corresponding to diferent PPP models
or techniques, e.g. dual-frequency IF PPP and uncom- Abbreviations
ITS: Intelligent transport system; GNSS: Global Navigation Satellite System;
bined PPP, and foat-PPP and fxed-PPP; (5) the integ- GPS: Global Positioning System; SPS: Standard Positioning Service; C/A:
rity of some certain aspects of PPP processing, such as Coarse/acquisition; RTK: Real‑time kinematic; NRTK: Network RTK; SBAS: Satel‑
multi-GNSS PPP, multi- or single-frequency PPP; (6) lite based augmentation system; PPP: Precise point positioning; AR: Ambiguity
resolution; INS: Inertial navigation system; IGS: International GNSS Service;
efcient recursive integrity monitoring algorithms to PCO: Phase centre ofset; PCV: Phase centre variation; NLOS: Non‑line‑of‑
address historical faults, utilising predicted states or time sight; UPD: Uncalibrated phase delay; FCB: Fractional‑cycle bias; IRC: Integer
updates; (7) integrity for an integrated system of PPP and recovery clock; DSC: Decoupled satellite clock; CORS: Continuously Operating
Reference Station; RA: Regional augmentation; FMEA: Failure modes and
other techniques or data sources, e.g. INS, LiDAR (Light efects analysis; FTA: Fault tree analysis; NSC: Non‑standard code; UI: Under‑
Detection and Ranging) and maps; (8) integrity of other investigation; RTX: Real Time eXtended; EOP: Earth orientation parameter; P LOI :
critical states for ITS applications, such as velocity and Probability of loss of integrity; NSC: Non‑standard code; MGEX: Multi‑GNSS
experiment; iGMAS: International GNSS Monitoring and Assessment Service;
altitude/heading (Reid et al. 2019; Binjammaz et al. 2013). IF: Ionosphere‑free; GRAPHIC: Group and phase ionospheric calibration; TEC:
Total electron content; ROTI: Rate of TEC Index; TECU: TEC unit; AC: Analysis
Concluding remarks centres; RTS: Real‑time service; RMS: Root mean square; GLONASS: Global‑
naya Navigazionnaya Sputnikovaya Sistema; DCB: Diferential code bias;
ITS applications require high accuracy and high integ- TGD: Time group delay; ISB: Inter‑system bias; IFB: Inter‑frequency bias; ICAO:
rity positioning. PPP as a high precision positioning International Civil Aviation Organization; RTCA : Radio Technical Commission
technique attracts much attention for ITS applications for Aeronautics; AL: Alert limit; HAL: Horizontal AL; VAL: Vertical AL; TTA : Time
to alert; IR: Integrity risk; PL: Protection level; HPL: Horizontal PL; VPL: Vertical
due to its fexibility and low cost; however, it is sub- PL; GBAS: Ground Based Augmentation System; SIS: Signal in space; URA : User
ject to a variety of threats and faults. To improve posi- rang accuracy; RAIM: Receiver Autonomous Integrity Monitoring; FDE: Fault
tioning reliability and provide integrity for PPP it is detection and exclusion; ARAIM: Advanced RAIM; MHSS: Multiple Hypothesis
Solution Separation; ISO: International Organization for Standardization; PAS:
vital to investigate and analyse all potential failures of Publicly available specifcation; SOTIF: Safety of the intended function; ADAS:
PPP and to study corresponding integrity monitoring Advanced driver‑assistance system; RTCM‑SSR: Radio Technical Commission
approaches. for Maritime Services‑State Space Representation; CNES: Centre National
D’Etudes Spatiales; QZSS: Quasi‑Zenith Satellite System; P FA : Probability of false
In this review paper an overview of vulnerabilities in alert; LiDAR: Light detection and ranging; R&D: Research and Development;
GNSS PPP that might impact its integrity in ITS appli- GPST: GPS time; GRG : Groupe de Recherche de Géodésie Spatiale; PE: Position‑
cations is given. Failure modes and efects analysis and ing errors; OCT: Observation consistency test; LOM: Local overall model.
fault tree analysis are investigated. Tey are classifed Acknowledgements
into diferent groups according to their sources. Te The authors highly appreciate the support of Dr. Denis Laurichesse and CNES
cause and characteristics, impacts, fault models and for providing PPP‑Wizard software (version 1.4.2). We acknowledge the IGS,
CNES and CODE (Centre for Orbit Determination in Europe) for providing
some probabilities of these threats are summarised, the high‑rate GNSS observations, satellite orbit and clock products, and DCB
with discussion on some major threats. Tis paper also products. We thank NovAtel Inc. for granting us permission to use their online
presents a brief review of the research on integrity of information. We would also like to thank Dr. Kan Wang at Curtin University,
Australia for her helpful remarks and the anonymous reviewers for their valu‑
PPP, with a focus on ITS scenarios. Requirements, chal- able comments and suggestions.
lenges, and existing methods of integrity monitoring