GPS Denied Navigation

Solutions to Navigating in GNSS Denied Environments

The jamming and spoofing of Global Navigation Satellite Systems (GNSSs) has increased in scope and visibility over the past 15 years. Constellations like the US Global Positioning System (GPS), the European Union’s Galileo, the People’s Republic of China’s Beidou and Russia’s GLONASS are all vulnerable.

GNSS disruption typically uses two tactics: Position, Navigation and Timing (PNT) signals transmitted by these constellations can be denied (jamming) or manipulated (spoofing). Deliberate PNT signal disruption is relatively easy to achieve because the signal is so weak by the time it reaches Earth. This is because the signal must travel thousands of kilometres from the satellite in space.

Expert Timing

Positioning, Navigation & Timing

Timing is indispensable for navigation and one of the reasons why GNSS jamming and spoofing can cause havoc is because they affect the timing information embedded in a GNSS signal. It may be prudent to rely upon an alternative timing source to avoid an overreliance on PNT signals.

Inertial Navigation Systems

Several mechanisms can be employed to mitigate the dangers of GNSS jamming and spoofing by either greatly reducing, or eliminating, reliance on these satellite navigation systems.

Military ships, vehicles, aircraft and even personnel can use Inertial Navigation Systems (INSs). INSs do not depend on external radio signals, unlike GNSS, to determine position and other navigation information. An inertial navigation system achieves this by relying on gyroscopes and accelerometers. These devices can sense movement and direction and hence are of great value in assisting navigation.

INS have been available since the Second World War and were particularly important for aerial, and later, space navigation. Over the years, their sophistication has continually improved via the employment of technologies like lasers.

Atomic Clocks

An atomic clock provides accurate timing information by measuring atomic resonance. A standard approach in atomic clock design is to observe the resonance of caesium atoms cooled to near-absolute zero temperatures. Some caesium atomic clocks will lose or gain one second of time every 300 million years. Since their invention and perfection in the 1930s and 1940s, atomic clocks have tended to be large structures occupying significant space. Advances in electronics miniaturisation have paid dividends. Some atomic clocks are now small and light enough to equip space-constrained environments like warships, military vehicles and combat aircraft.

LORAN (Long Range Navigation)

Other alternative external timing sources are emerging which can be used to substitute a GNSS constellation’s PNT signal. For example, there is renewed interest in the LORAN (Long Range Navigation) system developed during the Second World War by the United States.

LORAN used a global network of radio transmitters to help airborne and maritime navigation. The locations of these transmitters were shown on maps, along with the radio frequencies they transmitted. Aircraft and ships could use LORAN signals and the position of the transmitters, to help triangulate their location. Ironically, LORAN fell into disuse partially because of the advent of GNSS services in the 1980s and 1990s.

GNSS jamming and spoofing vulnerabilities have triggered renewed interest in the system. To this end, plans are afoot in the United States, the United Kingdom and elsewhere to reevaluate LORAN as a timing source. Much LORAN infrastructure still exists around the world and portable LORAN systems have also been developed. Specifically, interest focuses on revitalising the last incarnation of LORAN, known as LORAN-C, as a potential timing source to aid navigation.

CHAYKA

It is noteworthy that the Russian military appears to have reactivated elements of its CHAYKA radio navigation system. CHAYKA is analogous to LORAN-C. Transmitters in Russia close to the Ukraine theatre of operations are thought to be in operation. These transmitters maybe providing an alternative timing source to Russian forces in Ukraine to counter the GNSS jamming which has featured in that country’s ongoing war.

Magnetism

Research has commenced looking at avant-garde navigation techniques which may enter routine use in the coming years. One approach is to sense and exploit differences in the Earth’s magnetic field which change according to one’s location on the planet. The United States’ Defence Advanced Research Projects Agency’s Advances in Magnetic Navigation initiative is one such project examining this approach.

There is unlikely to be a single technology that can be employed to outflank all GNSS spoofing and jamming. Instead, a holistic approach may be the best way to avoid these dangers by adopting several technologies like LORAN, INS and perhaps magnetic navigation. These techniques could be exploited when GNSS is unavailable and combined when it is to provide progressively sharper levels of PNT information.