ieve 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 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 algo