GNSS Jamming
The world has four Global Navigation Satellite System (GNSS) constellations. Each of these satellites transmit Position, Navigation and Timing (PNT) signals to recipients on Earth.
GNSS receivers can be used by vessels, vehicles, aircraft, people and fixed sites. A receiver will use PNT signals transmitted by at least four satellites to compute its position via triangulation. GNSS spacecraft are equipped with atomic clocks which let each satellite transmit timing information as part of the PNT signal. The timing signal is important as the measurement of speed is the product of calculating distance over time.
A significant shortcoming of all GNSS constellations is the weakness of their satellites’ PNT signals by the time those signals reach Earth. GNSS satellites tend to be placed in Medium Earth Orbits, giving them an altitude of around 10,799 nautical miles/nm (20,000 kilometers) above Earth. The challenge for the PNT signal is that it must travel a long distance before reaching its recipient.
Any Radio Frequency (RF) electromagnetic signal is like a long-distance runner: The further the signal travels, the less strength it has when it arrives at its destination. The International Telecommunications Union (ITU) makes a range of radio frequencies across the 1.1 gigahertz/GHz to 1.6GHz waveband available for PNT signals. The ITU is the United Nations organization tasked with globally regulating the use of the radio segment of the electromagnetic spectrum.
Weak Signals
Let us suppose that the PNT signal has a frequency of 1.1GHz and a strength of 26 watts/W at the satellite’s antenna as it leaves for its journey to Earth. The antenna has a gain of 13 decibels/dB. Put simply, antenna gain is a measure of how much power the antenna can focus in one direction. Gain can be thought of as analogous to the difference between a garden hose (high gain) and a shower head (low gain). The shower head sprays water droplets in a myriad of directions, the garden hose sends water in a focused jet in a specific direction. The signal has a strength of 44.2dB which is combined with the antenna gain of 13dB giving an overall strength of 57.2dB. However, the signal must then travel through space to reach the Earth, gradually losing power as it does so across its 10,799nm journey. By the time the signal reaches the GNSS receiver on Earth, it has a strength of -135.1dB and is thus very weak.
We’re Jamming!
The weakness of a GNSS PNT signal on Earth creates opportunities for Electronic Warfare (EW) cadres. The simplest way to prevent a GNSS receiver working properly is to jam the incoming PNT signal. A rule of thumb in EW is that one always tries to jam the RF receiver rather than the transmitter simply because it is easier. As we have seen above, a signal is far stronger when it leaves the transmitter than when it reaches the receiver. One simply needs comparatively less power to jam a receiver than a transmitter.
The goal of GNSS jamming is to eclipse the power levels of the PNT signal arriving at the GNSS receiver with a more powerful jamming signal. Given that the PNT signal could be as weak as -135.1dB, comparatively little power is needed to drown out the strength of the PNT transmission with the jamming signal. Tactically, the attacker needs to be in a line-of-sight range from the GNSS receiver they are targeting.
Let us suppose the attacker is targeting a GNSS receiver on a vehicle six kilometers (3.7 miles) away. Five watts (36.99dB) of jamming power will produce a signal with a strength of -62.4dB at the GNSS receiver. This is still a weak signal, but one that is nevertheless comparatively stronger than the PNT transmission. The PNT signal could be drowned out by the jamming causing the vehicle to lose position, navigation and timing information while the jamming continues.
Remedies
GNSS jamming is a menace in peacetime as well as wartime and is witnessed on an almost daily basis in current global trouble spots like the eastern Mediterranean and the Black Sea.
Measures are being taken to mitigate the effects of GNSS jamming by using alternatives like Inertial Navigation Systems (INSs). INSs do not depend on exterior PNT signals. Likewise, GNSS systems are being designed to recognise when jamming is occurring and these hostile signals by blanking out RF reception in the direction from which the jamming is emanating.
Some GNSS systems will only accept incoming signals at specific power levels and with specific characteristics. This approach ensures that only PNT signals are received by the device. Finally, alternative radio navigation systems like LORAN (Long Range Navigation), which are comparatively harder to jam, are coming to the fore. LORAN was developed during the Second World War but has fallen into disuse because of the global GNSS uptake.
No single solution discussed above provides a silver bullet to render GNSS jamming impotent. Taken together they offer a robust response to the GNSS jamming threat.
