Intro to GNSS On-Demand Webinars

How do you measure a position through GPS satellites? What happens to a satellite’s signal as it travels toward the Earth? Which calculations or technologies do we use to generate the most accurate positioning possible?

As a leader in positioning and autonomy, Hexagon | NovAtel® wrote the book on GNSS. Now, we’re making that knowledge shareable far and wide in our webinar series.

Through the series, we follow a satellite’s signal through space, reception by antennas, computations by receivers, and applications across user equipment. We’ll explain how satellite infrastructure and atmospheric effects can contribute to positioning errors, and how technologies compensate for and resolve them.

We hope this series sparks your imagination! When you’re ready to learn more about GNSS technologies and its applications across industries, download our Introduction to GNSS book here.

Hexagon | NovAtel Business Development Manager for Defense and Military Peter Soar introduces the strategies and methods to protect GNSS positioning in a contested environment. Through specialized equipment and algorithms, Peter demonstrates how we can protect position, navigation, and timing (PNT) information from jamming and spoofing.





Hello, welcome to episode six of the NovAtel webinar series on An Introduction to GNSS. I’m Peter Soar, the business development manager for defense and military at NovAtel in Hexagon Autonomy and Positioning.


Here, we’ll explore techniques for delivering GNSS time and position in conditions where it is made difficult by unintentional or deliberate interference. While 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.


Anti-Jam Antennas

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 signals can be excised by advanced algorithms which discriminate by frequency as well angle of arrival. Beam-steering systems go further by pointing beams of increased gain towards known satellites; this needs knowledge of the satellite positions, and position and heading of the receiver; so it's a complex approach that needs extra hardware and different algorithms and software.


Both kinds of antenna protect against jamming; they also defeat some spoofing if the receiver is already tracking.


Spoofing Detection Algorithms

At the receiver level, we can now detect spoofing so that you can take appropriate action. Recall Episode Two when we explained the difference between jamming and spoofing. While jamming attempts to block your GNSS signals completely, spoofing seeks to deceive your receiver.


Spoofing is done with either rebroadcasted real signals or false signals from a simulator. For users of open-signal GNSS receivers there are emerging spoofing detection methods. Knowing that you are under a spoofing attack means that you can take steps to use alternative measures and use the GNSS information wisely. Types of attacks might include matched and variable power, jamming followed by spoofing, the use of software defined radios, even spoofing before the receiver starts up. NovAtel has recently demonstrated that these attacks can be detected, allowing remedial action to be taken.


Using Additional Sensors for a Robust Solution

By using additional sensors that respond differently to conditions, the solution can be made to be robust as we described in Episode 5.


Robustness comes from using complementary dissimilar (or "heterogeneous") sensors which provide position and time data and rates of change, but the way that they work is different so that they are not susceptible to the same weaknesses. While each of the elements may have drawbacks, together they can provide continuous PNT data.


The classic way is GNSS+INS. It is best to use tight coupling techniques that make best use of the advantages of the sensors. The algorithms in the GNSS receiver are aided by the INS so that the system gives a solution, even when fewer satellites are visible than are normally needed.


More sensors, such as visual techniques, RADAR, LiDAR and many others can be used; and, with advanced algorithms, sensor fusion can provide a comprehensively robust system.


Building Situation Awareness

GNSS situation awareness is needed so that you can make informed decisions on which PNT system to rely on, and those which may succumb to attack. Modern GNSS receivers can examine and report on the signals which are inaccessible to other tools as they are below thermal noise. And, as I mentioned earlier, we can now detect spoofing with open-signal receivers. Anti-jam antennas can be used to provide information on the strength and type of jamming signals, and they can give information on the direction to the jammer.


This situation awareness enables you to make informed decisions and take appropriate action; and it is an aid to being able to attribute the interference to its source.


A Layered Defense


To protect your PNT in a contested environment, we recommend a layered defense. Select the right GNSS signals to use, and use as many as you can. If you are authorized, use encrypted GNSS signals with a secure encryption key. Employ open-signal spoofing detection, as well as an anti-jam antenna. Create a robust positioning solution with complementary heterogeneous sensors. And, GNSS situation awareness also helps because understanding the environment supports you in overcoming adverse conditions.


GNSS positioning and timing is truly achievable in a contested environment; it just requires ingenuity, suitable equipment, and the right tools.