Vulnerabilities

Interference. GNSS core signals are transmitted from Medium Earth Orbit constellations at 20 000km above the earth and thus reach users receivers as weak signals. Although the spread spectrum nature of these signals provides some natural protection to interference, the earth interference power to space signal power ratio is frequently unfavourable and this has to be addressed as a first type of GNSS vulnerability.

Space weather. When aiming to provide precision approach services, GNSS solutions such as SBAS and GBAS do provide additional corrections to core constellation signals, in particular through modelling and correcting of the ionospheric layer disturbances impacting the core constellation signals. These ionospheric disturbances are highly dependent upon the solar activity, and therefore Space weather represents a second type of GNSS vulnerability.

Cyber security. Finally, as other information carrying systems, a third type of vulnerability is system intrusion which has to be addressed through appropriate cyber security layers.

Mitigations

Interference impact may be practically addressed by States though several actions lines such as e.g. : enforcing an efficient spectrum policy and regulations against unlawful interference sources, designing a State intervention policy against illegal jammers, designing a State cooperation process to limit the impact of e.g. Defense jamming activities over communities using GNSS services.

Space weather impact has mostly to be addressed through GNSS system design. This involves several ranges of activities such as collecting and analysing data, defining worst cases, defining system mitigation strategies (such as algorithm tuning, scintillation reduction, geo ranging deployment, etc…).

Cyber security policy has to be designed with respect to a set of agreed threats and vulnerabilities, which are usually addressed at restricted State level, and then system mitigations are requested to be implemented at system level.

Alternative means

When GNSS is used as a primary means of navigation or time stamping source, the implementation of typical mitigations as above described allows GNSS to support a high percentage of availability. When additional availability is required (e.g. for Safety of life applications, critical applications, etc..) it is usually requested to maintain or deploy additional service layers which are typically expected to either replace GNSS in the case of mitigation or system failure, or to provide an alternative degraded service, but still meeting the main mission objectives (e.g. for aviation safely landing aircraft following a GNSS loss).

These alternatives layers/policies are often quoted as Alternative Position Navigation Timing (A-PNT) solutions. Typical solutions implemented as of today for aviation include the use of ground beacons networks (ILS, VOR, NDB, DMEs) for continuing navigation in case of loss of, or too important degradation of, GNSS services. Alternative source of datation through atomic clock transmission over Long Waves High Power transmitters supporting ATC or radar data critical time stamping are today implemented within some European States.

Benoît ROTURIER graduated as an Aeronautical Telecommunications engineer from Ecole Nationale de l’Aviation Civile (ENAC), Toulouse in 1985 and obtained a PhD in Electronics from Institut National Polytechnique de Toulouse, in 1995. He also qualified as an Instrument Flight Rules pilot in 1993.  He is now acting as the program manager for satellite navigation systems implementation within Direction des Services de Navigation Aérienne (DSNA). He is also involved in international standardization activities as the chairperson of the International Civil Aviation Organization (ICAO) Navigation Systems Panel (NSP), and French representative of the Performance Based Navigation Study Group (PBNSG).