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

anywhere in the world (Zumberge et al. 1997). However, aviation cannot be applied directly for ITS applications
PPP can also be augmented by a regional reference sta- due to diferent requirements and the challenging urban
tion network or integrated with SBAS services (Heßel- environment (Zhu et al. 2018). Moreover, considering the
barth and Wanninger 2013; Wübbena et al. 2005). Hence diferent types of observations and models of positioning
PPP is more fexible than the diferential GNSS position- techniques, it is especially difcult to provide integrity
ing techniques, making it an attractive technique for for PPP users.
many precise positioning applications. With the devel- Knowledge of the characteristics of PPP vulnerabilities
opment of new GNSS signals, new GNSS constellations, and their mitigation methods is essential to improving
and infrastructures, PPP with real-time Ambiguity Reso- the reliability and integrity of PPP. Tis paper extends
lution (AR) is an attractive alternative to the diferential the work of Imparato et al. (2018b), which provided an
GNSS positioning techniques (Collins 2008; Ge et al. overview of vulnerabilities in RTK and SBAS. Te goal
2008; Laurichesse and Mercier 2007). Te advent of is to review potential faults and threats in GNSS PPP
dual-frequency mass-market GNSS chipsets with carrier- as well as the research developments and key issues of
phase measurement capability further enhances the PPP PPP integrity monitoring, focusing on the challenges of
technique for autonomous driving applications (de Groot its application in ITS. Section A brief overview of PPP
et al. 2018; Murrian et al. 2016; xAUTO technology characteristics gives a brief introduction to the PPP tech-
2017). Moreover, the integration of PPP with other tech- nique, including its concept, models, and implementa-
nologies, such as an Inertial Navigation System (INS), tions. Section Vulnerabilities and integrity fault analysis
can shorten the convergence/reconvergence time of the in PPP lists the potential failure modes of PPP, with the
PPP solution and improve the positioning availability, demonstrations of fault analysis methods and detailed
making PPP more applicable, even in an urban environ- discussion of the main vulnerabilities of GNSS PPP. Sec-
ment (Gao et al. 2017; Zhang and Gao 2008). tion GNSS integrity concept and approaches reviews the
One of the key issues for ITS technology is safety, status of PPP integrity research and identifes some open
which cannot be assured without reliable and trustwor- research issues concerned with PPP vulnerabilities and
thy positioning. However, due to the weak GNSS satellite integrity, with a focus on urban scenarios. A summary is
signals, GNSS measurements are vulnerable to a number given in section Integrity of PPP in ITS context.
of threats and faults caused by satellites and/or receiver
problems, as well as the environment, particularly in A brief overview of PPP characteristics
urban areas where ITS technology is most in demand PPP is a high precision positioning technique which can
(Ioannides et al. 2016; Martins 2014; Tomas et al. 2011). be performed with a single GNSS receiver, utilising the
Furthermore, unlike diferential GNSS positioning meth- undiferenced measurements of both code and carrier-
ods such as RTK and NRTK, PPP only relies on the meas- phase. Te PPP technique requires the precise orbit and
urements from the user receiver. Many error sources, clock information of satellites (via so-called “data prod-
such as satellite clock ofsets, initial satellite and receiver ucts”) to achieve high positioning accuracy. Real-time
phase biases and so on, cannot be eliminated or mitigated orbit and clock products are expressed as the correc-
in undiferenced processing (Bisnath and Gao 2009). tions to broadcast ephemeris messages and are dissemi-
As a result, PPP is more afected by such errors. On the nated via the Internet or broadcast by satellites (either
other hand, the carrier-phase cycle slip and outlier edit- SBAS or GNSS satellites, in the case of the latter as for
ing for PPP is more challenging than for the diferen- the planned High Precision Service of Galileo naviga-
tial positioning methods (Kouba et al. 2017). Although tion satellite system) (Fernandez-Hernandez et al. 2018;
GNSS threats have been investigated in many studies, Heßelbarth and Wanninger 2013; the International GNSS
and monitoring systems are increasingly being deployed Service (IGS) 2019; Weber et al. 2007). In addition, the
(Bhatti and Ochieng 2007; Martins 2014; Ochieng et al. observations are corrected for the errors due to relativ-
2003; Tomas et al. 2011; Tombre et al. 2017), few refer ity, satellite and receiver Phase Centre Ofsets (PCO)
explicitly to the PPP technique. and Phase Centre Variations (PCV) (Schmid et al. 2005),
ITS applications require high levels of integrity, which phase wind-up (Wu et al. 1993), troposphere (dry com-
is one of the most important performance indicators ponent), Earth tides, ocean tide loading, and various
(Zhu et al. 2018). Integrity is concerned with how much hardware delays or biases, using appropriate models.
we can trust the positioning results in the cases of both Other error sources including ionospheric efects, mul-
nominal and faulted conditions. In recent years, the issue tipath, Non-Line-of-Sight (NLOS) errors, and cycle slips
of integrity for land transportation and/or high accuracy remain the most challenging for real-time ITS applica-
positioning has attracted more attention. However, the tions. Tese errors contribute to most of the fault inci-
classical integrity concept and algorithms developed for dents for the PPP implemented in urban environments.