The use of Global Navigation Satellite Systems (GNSS) has become ubiquitous in our lives. Today, there is hardly any human or economic activity in which one of the satellite systems, such as, the GPS (USA), GLONASS (Russia), Galileo (Europe), and BeiDou (China) is not used. Besides, countries like India are also launching their own systems like the Indian Region Navigation Satellite System (NaVIC).
With ever-increasing reliance on GNSS in every walk of life for position, navigation, and timing (PNT), within the framework of location the need for bulletproofing these systems from intentional and unintentional disruptions (jamming and spoofing) is even greater than before. Of all the satellite navigation systems, GPS (Global Positioning System) is most widely used around the world.
Since GNSS signals travel a long way from geostationary satellite constellations, orbiting at approximately 22,000 miles (35,000 km) above the Earth’s equator, they become weak by the time they reach the earth stations/receivers and are susceptible to interference. Typically, GPS signals operate in three radio frequency (RF) bands: L1 (1575.42 MHz), L2 (1227.6 MHz), and L5 (1176.45 MHz). Due to the weak nature of the signals, any form of RF interference can cause severe disruptions to position, velocity, timing, and navigation, which are critical in virtually every industry and sector. Any disruption can lead to partial or full jamming of GNSS/GPS receivers and potentially cause serious harm and economic losses.
Further, GPS signal jammers, costing as little as $40, are easily available on e-commerce websites, and are capable of crippling or jamming GNSS/ GPS receivers, intentionally or unintentionally. “You don’t have to be the target to be the victim,” said Logan Scott, Co-founder of Lonestar Aerospace and GNSS/GPS expert, at a recent webinar on jamming. “Encountering interference, you are pretty much on your own. By the time the cavalry shows up, the battle is over.”
All forms of GNSS/GPS disruptions are on the rise. In 2019, the European Organization for the Safety of Air Navigation, (Eurocontrol) received “3,500 reports of outages”. In January 2020, the Mexican government passed a federal law outlawing the use of jammers after investigations revealed that out of the 3,400 reported cargo thefts in 2019, commercially available jammers were used in 85% of the cases. In September 2020, the U.S. Maritime Administration (MARAD) issued an advisory to the shipping industry, warning of global GNSS disruptions.
In the case of military conflicts, the civilian sector bears the brunt of GNSS disruptions as collateral damage, as seen in 2011 during the “frequency war” between the U.S. and North Korea. Pyongyang was accused of repeatedly carrying out large-scale jamming using military grade mobile jammers, which affected mobile phones, timing clocks, and other wireless services as far inland as Seoul. The jamming signals originated from the North Korean border towns of Kaesong and Haeju.
Application of GNSS/GPS technology can be found in virtually everything that we do today — from ordering pizza online to air travel, from synchronizing electricity grids to global stock trading that depends on precise timestamps, and much more. Therefore, the economic implications of GNSS/GPS outages can be crippling, besides being a potential danger to life — especially in cases like a drone crash in a crowded area. According to a June 2019 report by the National Institute of Standards and Technology, the U.S. private sector economy alone benefited to the tune of $1.3 trillion between 1987 and 2017 by using GNSS/GPS technology.
It is noteworthy that while the economic benefits of the technology might be significant, losses would be astronomical in case of an extended GNSS/GPS outage. For example, in the marine sector, the immediate economic benefit of GNSS/GPS might be negligible, but in case of wide-scale disruption affecting the shipping industry, losses can be as high as $85 million per day, according to an analysis prepared by the Rand Corporation for the Homeland Security Operational Analysis Center (HSOAC).
In simple terms, jamming occurs due to transmission of high-decibel radio frequencies close to the L1, L2, and L5 frequency bands at which GNSS/GPS receivers operate. The interfering frequencies tend to overload the receivers to such an extent, that they lose the Coarse Acquisition or Code Access (C/A) lock on the satellites. According to the Royal Academy of Engineering, “signal received at ground level from the GNSS satellites is weak — it may be as low as -160dBW (1 x 10-16W) — jamming over a small area is easily achieved.” Jammers or transmitters that produce these interfering frequencies lead to the disruption of GNSS/GPS services. As a simpler analogy, try imagining the voices of people in a pub or a nightclub getting drowned out when the volume of the music is pumped up.
From the situational awareness point of view, it would be helpful to remember that intentional jamming is a means to an end and not the end itself. Therefore, placing jamming in the larger context of the PNT threat matrix helps develop a better understanding.
The large-scale adoption of GPS also means it is the common target for those who want to cause intentional disruptions. A small, 10-watt jammer can disrupt unprotected GNSS/GPS receivers. The high vulnerability is not only because of the lower power at the GPS receiver end (approximately -160dbW), but also because the Standard Positioning Service (SPS) code document for civilian GPS signals, which lays down the standards and specifications, is available in the public domain. Hence intentional disruptions can be carried out easily.
Due to the frequencies at which the various GPS signals work, unintentional interferences can happen for any number of reasons — medical equipment, cellular communication, radio transmission, and even domestic Wi-Fi, to name a few.
The first line of defense against GPS interference (jamming and spoofing) is detection. There are several ways in which jamming or spoofing can be detected, such as, by determining the signal strength, jump in values, time stamp jumps, and measuring Doppler shifts.
Signal strength: Legitimate signals from GNSS satellites are well below the surrounding background noise level. If any energy is detected in these bands, it means there is interference. Also, the signal strength from satellites does not vary much at the receiver end since the satellites are always relatively far away, whereas signals originating from a jammer (or a spoofer) can be very strong when transmitted from nearby areas. Therefore, a sudden change in signal strength (towards stronger) indicates the presence of a jammer in the vicinity.
Jump in values: To recognize fake (or spoofed signals), various measures can be taken. For example, receivers will receive both fake and real signals simultaneously. When the navigation processing gets fooled by the fake signals, range measurements will jump substantially to the new erroneous values. Step changes in the values are not possible in real physical systems, so this is an indication of interference.
Time stamp jumps: The data streams from each satellite may also indicate discontinuities as the receiver switches from tracking the real signal to the fake; the same goes for time indication. This will be especially detectable with a meaconing or playback attack because time will jump backwards when the replay starts.
Doppler shifts: Radio waves experience Doppler shifts the same way as sound waves, based on how the objects (transmitters/ receivers) move. The Doppler shifts for the real satellites will all be different as the object moves either towards or away from them, depending on their position in the sky. However, the Doppler shift caused by the object’s motion is the same for all the satellite signals because they are all arriving from the same direction. This uniformity of Doppler shifts is another indication of spoofing. But to account for Doppler shifts as the object moves around requires a fair degree of sophistication (to adjust for each individual satellite) and requires tracking the object’s course.
Considering the necessity and importance of having a functional GNSS/GPS, numerous anti-jamming techniques have been developed and the technology is rapidly evolving. Mitigating jamming by changing frequency allocation is difficult, since GNSS/GPS frequencies are in proximity to Industrial Scientific Medical (ISM) equipment. However, for some critical sectors like civil aviation, multi-GNSS (GPS + Galileo) and multi-frequency (L1+L2) systems are used. Such combinations are able to provide accurate PNT data despite interference.
Null steering is the most commonly used anti-jamming technique, using Controlled Reception Pattern Antennae (CRPA). Since the desired satellite signals and the unwanted jamming or spoofing signals normally arrive at the receiver antenna from different directions, the unwanted signals can be prevented from reaching the receiver by designing an antenna that spatially blocks them. The antenna direction should be so adjusted that maximum ‘gain’ is in the direction of the desired signals and minimum or ‘null’ gain is in the direction of the noise/unwanted signal. In a receiving antenna, ‘gain’ describes the antenna’s ability to convert radio waves coming in from a particular direction into electrical power.
Beam steering is another effective GPS anti-jamming technique that is independent of the source of the jamming signal. It receives selected signals from at least four different GPS satellites and correspondingly produces four different anti-jamming solutions in the form of four separate anti-jamming algorithms. These algorithms are decoded by GPS receivers to determine the best possible solution to counter the jamming threat. The entire process is automated.
The anti-jamming performance of GPS significantly improves when used in conjunction with Inertial Navigation Systems (INS). First, data obtained from INS is used to eliminate abnormal GPS measurements caused by interference (jamming or spoofing). Next, INS can help recapture the signals when a GPS receiver loses them because of jamming, movement, or shielding. Finally, with INS the bandwidth of the receiver tracking the loop can be relatively narrow and the dynamic and anti-jamming performance can be balanced properly. Advanced Digital Signal Processing techniques like Amplitude Domain Procession (ADP) and ADP in Frequency Domain (FADP) filtering can be deployed at the RF front end to mitigate jamming threats.
GNSS receivers are susceptible to both narrowband (Continuous Wave) and wideband jammers. Wideband interference — intentional or accidental — is most effective in jamming nearby GNSS users, as it is frequency agnostic. Wideband jammers use modulation such as FM chirp (swept Continuous Wave) and Additive White Gaussian Noise (AWGN).
In comparison, narrowband jammers operate in selected frequencies, making mitigation measures relatively simpler to implement. Therefore, it is important to have the ability to identify the presence of wideband GNSS jammers and geolocate them as they pose a greater threat to GNSS users.
In this respect, Phased Arrays Antennae can help determine the Angle of Arrival (AOA) of an RF signal source. A network of phased arrays is used to infer two or more AOA. The AOA algorithms partition the signal into multiple narrowband channels for processing. The AOA measurements from two or more stations can be used to triangulate the location of the interference source.
To exploit the wideband characteristics of the source signal, a cross-correlation method needs to be employed. The cross correlation of received signals between two distant stations will produce a distinct peak at a cross-correlation delay with the Time Difference of Arrival (TDOA) of the corresponding source.
Hence, TDOA is another sensor station’s observation that relates to the geographical location of the wideband source signal. If three or more stations are used as reference points, the combination of two or more detected TDOA measurements can be used to precisely geolocate the jammer’s position.
Companies like NovAtel provide a suite of anti-jamming solutions like GNSS Resilience and Integrity Technology (GRIT) to deal with the threat of jamming and spoofing, which is increasingly getting sophisticated. GRIT is a suite of firmware features enabling trusted situational awareness and interference mitigation tools across applications and environments. GRIT can be deployed on land, sea, and air, including on Unmanned Aerial Vehicles (UAVs). As threats to assured positioning adapt, NovAtel has evolved its solutions to continue providing the most reliable, trustworthy and resilient PNT measurements possible.
GRIT combines NovAtel’s successful Interference Toolkit (ITK) with the power of spoofing detection. Also available in GRIT is an optional functionality enabling time-tagged snapshots of analog to digital samples. The time-tagged digitized RF data allows users to characterize the RF environment and develop their own interference location algorithms.
Through situational awareness techniques like spoofing detection, time-tagged data, and interference mitigation such as anti-jam technology and digital filters, GRIT builds GNSS resilience and integrity to better protect PNT measurements. With these robust tools, GRIT joins NovAtel’s industry-leading, anti-jam technology on the hardware side like GAJT (GPS Anti-Jam Antenna Technology — GAJT-710ML, GAJT-710MS, GAJT-410ML, GAJT-410MS, and GAJT-AE-N), in providing resilient, robust, and trustworthy PNT protection.
It is worth noting that there is no single solution to mitigate GNSS/GPS interference. A combination of techniques and solutions like those offered by NovAtel’s GRIT suite need to be deployed for neutralizing jamming threats and ensuring resilient PNT.
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