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Session A2: Land-Based Applications

GNSS Navigation Message Authentication with the Trimble RTX GNSS Correction Service
Markus Brandl, Ralf Schmid, Ulrich Weinbach, Christian Pagels, Ramzi El Khayat and Stefan Junker, Trimble Inc.
Date/Time: Wednesday, Sep. 18, 5:08 p.m.

The Trimble RTX correction service provides state-of-the-art PPP augmentation services in terms of accuracy and convergence time. The Trimble RTX technology is based on the estimation of precise satellite orbits, satellite clocks, satellite biases and ionosphere models computed from the data of a worldwide network of GNSS reference stations. The corrections are transmitted to the user via L-band signals from geostationary satellites or the internet. The system supports all current GNSS: GPS, GLONASS, Galileo, BeiDou, and QZSS. An accuracy of 2.5 cm (95%) is achieved after a convergence time of less than 1 minute by using regional ionospheric and tropospheric models. Since 2019, the service also provides integrity information required for safety-critical and autonomous applications.
GNSS spoofing is a substantial threat to GNSS-based autonomous applications. Both academic research experiments and real-world spoofing attacks could demonstrate the potential impact on positioning. The easy availability of low-cost spoofing devices increases the likelihood of attacks giving rise to a serious safety concern, especially for autonomous applications. Several countermeasures are required in order to protect GNSS position and timing solutions from spoofing. Navigation message authentication (NMA) is one important defense method. With the launch of the Galileo Open Service Navigation Message Authentication (Galileo OSNMA), this problem is addressed for the Galileo system to protect the Galileo navigation messages. NMA is also discussed for next-generation BeiDou satellites. Authentication of the GPS L1C navigation message and spreading codes are proposed, known as GPS Chimera. These existing and planned improvements of the GNSS space segment promise to provide increased security for GNSS positioning and timing. However, it will require several years or even decades until the GNSS constellations are fully upgraded with such security features. In addition, legacy signals like, e.g. the important GPS L1CA signal will not benefit at all from these developments.

To counteract spoofing attacks/risks using today’s signals, an alternative approach is required. With the addition of the GNSS navigation message authentication feature to the Trimble RTX correction service, this demand is addressed. The globally distributed Trimble RTX tracking network consisting of approximately 120 stations is able to track and monitor signals of all four GNSS constellations and therefore perfectly suited as the backbone for a global GNSS NMA service. GNSS NMA data is produced using state-of-the-art cryptographic algorithms in the highly-redundant Trimble RTX control centers. Besides the authentication of the navigation messages, also their integrity is monitored in real-time. This enables the Trimble RTX correction service to protect users from both non-authentic navigation data (resulting from spoofing attacks/activities) and from authentic, but faulty navigation messages generated by the GNSS system ground controls. In addition, imperfection in data integrity protection mechanisms of navigation data, like in the GPS LNAV parity check, can fail, in particular under challenging tracking conditions. These rare, but existing failures can be reliably detected using NMA. The paper will outline the system architecture of the new Trimble RTX NMA based on the Trimble RTX ground infrastructure, the navigation data integrity checks, the NMA message generation, the broadcast of the cryptographic authentication data via low-bandwidth geostationary communication satellites, and the usage of the authentication data in the GNSS positioning solution.

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