What is GPS Jamming?

Jamming in the context of GNSS/GPS refers to the intentional or unintentional interference with satellite navigation signals that can degrade or completely disrupt positioning accuracy and availability.

Types of GPS Jamming:

GPS jamming can be categorized into two main types based on frequency characteristics. 

1. Out-of-band jamming affects the antenna’s Low Noise Amplifier (LNA) through overdrive and saturation, resulting in distortion of the entire digitized band. This typically manifests as a drop in Carrier-to-Noise ratio (C/N0) or complete loss of signal tracking due to increased signal power causing attenuation in the receiver’s Automatic Gain Control (AGC). A loss of just 1 bit in signal coding is equivalent to a 6dB Signal-to-Noise Ratio drop.
2. In-band jamming operates within the GNSS frequency bands and is assessed using the Spectral Separation Coefficient (SSC), which measures the overlap between the jamming signal’s Power Spectral Density (PSD) and the GNSS signal’s PSD. The impact is directly proportional to this spectral overlap, with a perfect separation coefficient of 0 indicating no interference.

GPS Jamming Characteristics and Effects:

Non-intentional jammers typically exhibit 1 to 10MHz bandwidth modulations (OFDM, PSK) and usually affect only a subset of GNSS signals. For example, partial signal jamming often occurs in the L2 band for GLONASS in the San Francisco area. In contrast, intentional jammers frequently sweep the entire band rapidly using linear chirping techniques to degrade all constellations simultaneously.

The severity of jamming effects can be ranked as follows: initial C/N0 degradation leading to measurement accuracy reduction, loss of tracking for a limited number of satellites causing solution accuracy degradation, and ultimately complete signal loss. Wide-band signals like L5 naturally benefit from increased bandwidth resistance to jamming, as does Galileo E5b at 1207MHz.

GPS Jamming Detection and Mitigation:

Modern GNSS systems incorporate sophisticated jamming detection mechanisms. The STA9100 system, for instance, uses a Wideband Interference Monitor (IFM) that performs spectral analysis of Intermediate Frequency (IF) sample streams and AGC saturation analysis. It employs a 256-point FFT algorithm applied periodically to detect wide-band jamming through cumulative averages and multiple detection metrics.

Additionally, Continuous Wave (CW) and narrow-band interference detection systems provide dedicated hardware to identify and mitigate specific types of jamming. These systems use configurable detection thresholds that must be calibrated based on noise measurements in jammer-free environments.

The Impact of GPS Jamming on Positioning Systems:

Jamming represents a significant threat vector for positioning engines, affecting both GNSS observations and correction services. The impact depends on the sophistication level of the jamming equipment, ranging from simple single-frequency spoofing devices costing around $150 to highly sophisticated multi-frequency, multi-constellation systems.