le Defense has led the effort against deliberate interference or jamming, most of the techniques in this session apply across industries.
This series builds on our expertise in the industry in literally writing the book on GNSS. If you’re ready to dig deeper into these concepts, then please check out our book, An Introduction to GNSS, on our website.
Assured PNT in a Contested Environment
Position, Navigation and Timing (PNT) is a fundamental enabler — not just for navigation, but also for timing. Secure communications and networks often derive precise time from GNSS, and trustworthy PNT is necessary for so many modern services. GNSS is still the only PNT system that has 24/7 global coverage with absolute position in latitude, longitude, and height. It is trustworthy when used correctly, so we need to ensure that it is available.
But the weaknesses of GNSS are that the signals are on fixed, known frequencies and the power of the signals when they reach the Earth's surface is weak. They are susceptible to being overpowered either by local interference or deliberate jamming, and as well as to being deceived by false signals or "spoofing".
The good news is that there are ways to protect your GNSS system in a contested environment. The aim is to provide actionable, reliable Position, Navigation and Timing (PNT) despite contested conditions. In Defense circles we call this "Assured PNT".
To do this, we can use multi-constellation multi-frequency GNSS, encrypted signals to deal with spoofing, open-signal spoofing detection, anti-jam antennas, other sensors together with GNSS, and situation awareness.
What Causes a Contested GNSS Environment?
Contested GNSS is not a theoretical issue but an everyday and growing problem. The openly reported problems include:
- Local jamming. For example, those used, generally illegally, by people who want to hide their location from tracking devices.
- Deliberate high-powered jamming designed to disrupt infrastructure and military operations.
- Spoofing, when your system is fooled to believe that it is in a different location or a different time.
Let’s recall the journey of the satellite signals through space to antennas, and receivers, and on to our applications. Mitigation techniques and equipment can be used to protect your position and timing at every stage of the signal’s journey. Fortunately, the mitigation techniques are additive: the more of them you use, the better the protection. As a result, we recommend a layered defense.
Building Protection in a Contested Environment
As we explained in episode three, there are many multi-constellation, multi-frequency GNSS signals to choose from. The use of as many signals as you can access improves both accuracy and reliability. This also applies to regional augmentation systems and correction services if they are available to you. There is plenty of choice of devices and frequencies, but we need to be discerning. The principles are the same for all but each GNSS has subtly different qualities suited to certain applications and regions.
Encrypted GNSS Signals
The satellites also provide encrypted GNSS signals. For government-authorized users, encrypted GNSS is used to defeat spoofing. In GPS the encrypted signals are the P(Y) Code and the new M-Code. To get the benefit of the anti-spoofing technology, a suitable military receiver must be loaded with the correct encryption key. Similar systems are in operation or planned for other GNSS.
The antenna is the "front door" of the PNT system so we need to ensure that it is secure and only lets in the desired signals. Anti-jam antennas dynamically change the shape of the gain pattern of the antenna in response to unwanted signals. Nulls (which are areas of no signal amplification) are directed towards the interference so only genuine signals from the satellites are used. This is done with multiple antenna elements on one array; these can null in the same number of directions as one less than the number of elements. So a seven-element array can null simultaneously in six directions.
Also, additional jamming