Anti-Spoofing Vs. Anti-Jamming -  Know the difference

In 1970, GPS signals were invented. Since the inception of GPS, electronic warfare principles were already well defined. Commonly, antagonist’s forces can target the position, navigation, and other GPS-based data in either of the ways:

• Jamming: Jamming disrupts the GPS receiver and hinders the transmission of GPS signals. Consequently, the GPS receiver is unable to operate in case of jamming.  
• Spoofing: Spoofing also targets the GPS receiver and allows it to show a false position.

Surely, spoofing and jamming of the signals are not the same. So, the solution to these problems also differs.

Spoofing

Spoofing requires reproduction of the false GPS signals using more than one satellite. Later on, those signals are transmitted to the local GPS receiver. (See Figure 1) in case of successful spoofing the GPS receiver is unable to differentiate between the actual satellite signals and the false signals. Thus, the opponents target the desired GPS receivers and befool the enemies to show spoofed locations.  

GPS signals require encryption of binary code 1s and 0s, commonly known as Y-code. This binary code is sent on the L1 and L2 frequencies with spoofed signals. How does it deceive the military targets? It happens easily as Y-code keeps on changing 10.23 million per second. This encrypted code does not have a repetition of the sequence within the whole week. In simpler words, Y-code can create a unique sequence of 1s and 0s 6.18 trillion times per week via one satellite.                          

Opponents create Y-code only with the help of encryption keys. Otherwise, it is impossible to create a Y-code and deceive the GPS receiver. GPS signals also feature the Coarse Acquisition or C/A code. Initially, it was used to assist the Y-code but now it has been widely used in all public GPS receivers.  

Hence, the C/A code is not encrypted. It’s 1s and 0s sequence changes 1.023 million per second and gets repeated after every millisecond. Opponents can recreate and spoof the GPS receivers easily by using the C/A code. As the C/A code structure is published openly in public signal space, so, the opponents can easily capture it and use these signals to show false positions.

How to prevent spoofing?

Tracking of encrypted Y-code makes it possible to prevent spoofing. No ordinary GPS receiver can track the Y-code. The receivers that carry the Selective Availability Anti-Spoofing module (SAASM) can track the encrypted code. Another condition includes that the GPS receiver also must have valid decryption keys and modules should be protected from potential reverse engineering in case of opponent attacks.

The Novatel OEM625s are the SAASM receivers controlled by United State Defense Department. The sales and distribution of these receivers are regulated under strict principles and these are manufactured for only US government-authorized customers.

However, Multi-Constellation receivers are available for civil users. These receivers can identify multiple GNSS signals like GPS, GLONASS, Galileo, and Beidou at once. To deceive the target receiver, an enemy has to recreate and send all possible GNSS signals concurrently.  

An Inertial Measurement Unit (IMU) can assist the navigation system to prevent security lapses. As an opponent cannot falsify the gravitational field of the earth or is unable to understand the vehicle dynamics, it cannot spoof the IMU.  

Jamming

A low signal power enhances the probability of GNSS and all CDMA signals falling prey to deliberate or unintentional signal jamming. After a certain limit of the interference signal, the actual GNSS signals cannot identify from the interference signals. (See Figure 2)

The capability of the Y-code to transmit 10 times more than the C/A code can improve the jamming process, it happens when P/Y-code power is spread 10 times more than the actual frequency range. Thus, Y-code receivers can tackle the 10 times the jamming power of C/A code receivers.

The advanced and fastest signals like GPS L5, Galileo E5a, and Galileo E5b also work on the same mechanism and shows better jamming performance. However, it all depends upon the power of interference signals when interference signals are too high they can overpower the low-code rate and high-code rate. In such cases, SAASM receivers can also face jamming or disruption.

How to prevent jamming?

Fortunately, we have found several combating strategies to avoid deliberate or accidental interference. Have a look at top of them:

1. Filter out the receiver

The most basic defense against jamming is to filter out the interference signals as soon as it approaches the receiver. However, this strategy is helpful for out-of-band signals or signals that are not directly in the range of GNSS. Nonetheless, if the receiver receives an in-band signal, it may also jam the receiver, stopping all frequency transmission.   

2. Install IMU in the receiver

By aiding the receiver with the IMU, jamming can be avoided. Receivers with IMU modules are impenetrable to radio frequency interference. IMU works with the navigation solution and overpasses the gaps of seconds to minutes leading to better GNSS performance.

3. Nullify the jamming signals by an adaptive antenna

The subject of the jamming activity can nullify the interference signals with the help of an adaptive antenna. It requires multiple antenna elements installed at a fixed measure of distance. Thus, signals are processed among different antennas to determine the direction of interference signals from where they are transmitted to the targeted GPS receiver. Further, lower gains are created by changing the power of signal receiving strength among antennas. Then the direction of the nulls is transmitted toward the source of interference leading to preventing the receiver from the interference signals. (See Figure 3) Novatel’s GAJT® anti-jam antennas are the controlled reception pattern antenna (CRPA). It effectively prevents the GPS receivers from the interference of all types. Furthermore, it also works best with interference signals that match the GNSS frequency band.