What is the difference between GPS Jamming and Spoofing?

Spoofing and Jamming are two ways in which adversaries may attempt to disrupt position, navigation and time solutions derived from GPS / GNSS. Spoofing is when a GPS receiver is made to calculate a false position and jamming is when GPS signals are overpowered locally by other RF signals so that a GPS receiver can no longer operate. Both spoofing and jamming are done using foreign RF transmitters producing signals to disrupt the relatively weaker GPS signals from satellites. But spoofing and jamming are two different problems and hence the potential mitigation solutions for them also differ accordingly. Anti-spoofing and anti-jamming are the terms given to the respective countermeasures taken against these adversary signals and their transmitters.

Spoofing

GPS signals are of two types - the encrypted Military GPS signals called P(Y) code for government authorized use only and the civilian GPS signals called C/A code. As the C/A code structure is openly published in a public signal-in-space interface specification, it can be recreated by a relatively competent adversary who can then accurately generate a “spoofed” version of the GPS signal and then transmit them to capture a local GPS receiver. If the targeted GPS receiver is unable to tell the difference between the real satellite signals and the spoofed signals, it will calculate its position according to the false signals and hence display the receiver as appearing to be at a different location. Click here to learn more about GPS Spoofing.

Souce: Novatel

Anti-Spoofing

The following are a few anti-spoofing countermeasures for GPS receivers:

• The best way to protect against spoofing is to directly track the encrypted P(Y)-code GPS signals. This can only be done using GPS receivers called Selective Availability Anti-Spoofing Modules (SAASM). Only SAASMs are equipped with the decryption key for the P(Y)-code GPS signals. These modules are also usually tamper-proof to prevent reverse engineering attempts by adversaries. SAASMs are only available to government-authorized customers, and their sales and distribution are tightly controlled by the United States Department of Defense.
• For civilian applications, multi-constellation receivers that can track multiple GNSS such as GPS, GLONASS, Galileo, and BeiDou simultaneously can be effective against spoofers as they can refer to the other constellations to check whether they are receiving spoofed signals or not. An adversary would have to produce and transmit all possible GNSS signals simultaneously to spoof the target receiver which would be relatively very difficult and would require more resources.
• GPS receivers can also be integrated with an inertial measurement unit (IMU) which is a device that measures and reports the body's specific force, angular rate, and sometimes orientation of the body. IMUs use a combination of accelerometers, gyroscopes, and magnetometers to estimate the position of the receiver without signals from satellites. IMUs can be an additional measure of protection by aiding the navigation solution as an adversary cannot spoof the Earth's gravitational field or vehicle dynamics and to deceive the IMU.

Jamming

Unlike spoofing, jamming doesn't require accurate recreation of GPS signals. As GPS signals travel over a long distance to reach the receivers from the satellites, they have a low signal power. Hence, they are susceptible to interference both accidental and intentional. Jamming is when a transmitter is used to create RF signals of a higher or the same frequency as GPS signals to cause intentional interference which makes it difficult for GPS receivers to receive any signal and renders them useless. Jamming is even more problematic as GPS jammers are relatively much simpler and easier to make than spoofing devices. Even small GPS jammers that fit in one’s palm can cause jamming over a range of several meters.

Source: Novatel

GPS’ military use P(Y) code is transmitted at 10 times the code rate of the civilian C/A code providing an inherent improvement in jamming performance. This arises from the “spreading” of P/Ycode power over 10 times the frequency range, allowing Y-code receivers to handle 10 times the jamming power of C/A code receivers. Newer and faster civilian signals such as GPS L5, Galileo E5a, and Galileo E5b are also integrated with the same improvement in jamming performance. However, as GPS jammers can be placed much closer to the receivers than the satellites they are intended to receive signals from, a high enough interference can overpower both low-code rate and high-code rate signals, hence, even SAASM receivers can be jammed. Click here to learn more about GPS Jamming.

Anti-Jamming

There are a few types of mitigation strategies to overcome interference:

• Using a filtering system in the receiver can filter out as much of the interference as possible as soon as it reaches the receiver. This is especially effective for out-of-band signals or signals that do not have the same frequency as that of GPS signals. Unfortunately, this will not work against interference signals that have the same frequency as that of GPS signals or in-band signals.
• An IMU can aid the receiver in navigation when attacked by jamming signals. As IMUs are impervious to radio frequency interference, they can provide a navigation solution to bridge gaps of seconds to a few minutes in absence of GPS signal reception.
• Certain GPS receivers are integrated with an adaptive antenna array that has multiple antenna elements spaced apart at a distance. These antennas are called Controlled reception pattern antennas or CRPAs. CRPAs use signal-processing techniques to determine the direction from which an interfering signal is arriving and then adaptively change the apparent receiving strength of the antenna array by creating a lower gain (“nulls”) in the antenna receiving pattern. These nulls can then be pointed towards the source of interference, the receiver can be protected from interference arriving from that direction. CRPAs are very effective at mitigating all types of interference, including GNSS jamming signals.

Source: Novatel