Most modern military systems rely on Global Navigation Satellite Systems (GNSS) to provide communications and accurate Positioning, Navigation and Timing (PNT) information for precision operations in all types of weather, anywhere on the planet. However, just as many governments become reliant on these systems, rogue actors are introducing a proliferation of GNSS-degrading and denying devices to compromise the overwhelming advantages offered by satellite-based PNT systems.
Perhaps the best known GNSS constellation is the Global Positioning System (GPS), which provides the bulk of the PNT data that U.S. and allied forces rely on to communicate, navigate and make decisions on the modern battlefield. Yet, due to their ubiquity and our forces’ reliance on them, enemies have developed and/or have access to portable devices that can jam and spoof GPS signals.
To counter these threats, alternate sources of PNT information should be considered when GPS is not available. There are multiple options available today, and a layered approach to signal protection could deliver better results than relying on any single solution. There are also alternative signals that are available and suitable for low size, weight and power assured PNT devices. A combination of additional PNT signals and other fail-safe solutions, together with traditional GPS systems, can make PNT applications more resilient against jamming and spoofing, and suitable for operations in GPS-denied environments.
Weak signal strength
One of the vulnerabilities of GPS signals is that they are extremely weak compared to other RF signals. Since GPS satellites orbit the Earth at 12,500 miles above the surface and the average signal transmission power is in the 10-100-watt range, by the time the signal reaches the surface it is actually weaker than the natural background noise. A GPS receiver can detect this weak signal in the background noise because it “knows” which signal pattern to look for. Once the search pattern is identified and the data stream decoded, the GPS receiver can calculate the signal location.
This low signal strength also makes GPS transmissions susceptible to interference from natural phenomena, RF transmissions seeping into navigational frequency bands, intentional GPS signal jamming through small consumer-grade GPS jammers or military grade jammers.
Military GPS receivers use the encrypted Selective Availability Anti-Spoofing Module (SAASM) GPS signal to ensure that the transmission received can be trusted. Although encrypted signals are considered safe from spoofing because their authenticity is assured, they can still be jammed with minimal technical effort. For example, a one-to-10-watt jammer can deny GPS coverage for a large area, whether the signals are encrypted or not. The U.S. military’s new M-Code, which replaces SAASM, is slightly less prone to jamming, but not immune to it.
Counter jamming technology solutions
GPS jamming can be mitigated through interference, detection and mitigation technology and tactics. For example, dedicated navigation systems and unrelated technologies such as cellular systems, although designed for a different purpose, can provide useful alternative PNT data under certain circumstances. A mix of diverse technologies and platforms, such as space-based and terrestrial or microwave and long wave signal strategies can decrease the likelihood that interference will impact more than one PNT source.
Perhaps the best way to deal with interference is to prevent it from getting into the GPS receiver in the first place. Advanced active antenna technology called Controlled Radiation Pattern Antennas have multiple, narrow, steerable beams which point at the individual satellites and away from the interfering signals. Even simpler passive anti-jam antennas whose beams are focused near the zenith and away from the horizon (where most interference comes from) can be very effective countermeasures.
Often interference is temporary, so another tactic is to ignore GPS during periods of interference and “flywheel” or coast on internal sensors. Other options include using an Inertial Measurement Unit consisting of accelerometers and gyroscopes to provide accurate positioning and navigation for several minutes, or employing a precision internal oscillator, such as an atomic clock, to provide accurate time sync for hours.
Multi-frequency and multi-constellation receivers offer an added measure of protection since jamming all bands is more difficult. Current SAASM and the new M-Code signals operate in both the L1 and L2 bands, but new GPS satellites now have the L5 and L2C signals to provide more diversity. Galileo, GLONASS and Beidou signals are other diverse signals available today, though they can never be a sole solution. Additionally, new signals are now available, such as Satellite Time and Location (STL) that are 1,000 times stronger than GPS (though not as accurate), which can augment GPS during outages.
Ultimately, there is no “silver bullet” or single solution approach to monitoring, detecting and mitigating threats in order to safeguard GPS-reliant systems from interference. It will take a layered approach, combining traditional and new technologies, together with innovative solutions that meet both existing and emerging needs in any environment.
John Fischer is vice president of advanced research and development at Orolia.