Cheap Implementation of a Narrowband GPS Jamming Mitigation Technique
Dembe C. Nenzhelele, Francois D.V. Maasdorp, Council for Scientific and Industrial Research, Pretoria, South Africa
GPS signals by nature are Direct-Sequence Spread Spectrum (DSSS). This means that a narrow bandwidth signal (50 Hz) is spread out in order to change it to a wider band signal (1.023 MHz). By so doing, this inherently makes GPS noisy and very low in power (at the time they reach the earth surface). Because of this weak signal power and vulnerability, GPS signals are susceptible to intentional and unintentional jamming. Due to that GPS signals are DSSS, they have some degree of inherent noise immunity. After despreading the received signal, the original GPS signal can be collapsed into a narrow bandwidth at the carrier frequency, whilst the jamming signals are spread due to the lack of correlation with the pseudo-random code. A portion of the spread jamming signal remains within the frequency band entering the tracking loop with the satellite signal. If the Jamming to Signal Ration (JSR) is higher than the processing gain (43 dB) of the spreading code, the signal tracking will become unstable. In general, a JSR of greater than 40dB is likely to prevent the GPS receiver from tracking the satellite signal and from estimating its own position.
This paper presents a study that was aimed at implementing a jamming signal canceller with the objective of investigating the improvement of the JSR. The canceller was implemented using a Software Defined Radio (SDR) and Gnuradio, open source software running on a Linux operating system.
The SDR used was the Ettus Universal Software Peripheral (USRP) B210 that supplied a stream of samples at 2MSPS from a Continuous Wave (CW) signal. The samples were then stored in two similar buffers. The one buffer was used as a temporary storage (B1), whilst the other buffer (B2) fed the vector to a custom built FFT block. The FFT block output then feeds the signal to a custom built Threshold detector wherein the signal power was calculated. If the signal power was above a certain set threshold (10 dB above the noise floor, in order to definitely eliminate false alarms from noise), this was evidence of the presence of a jammer signal. A jammer is absent if the signal is below the threshold detector. In this case, the signal stored in the buffer is added to the channel selector. The IQ samples that are higher than the threshold were then multiplied by zero and the resultant signal was then multiplied with the original signal in B1. After the multiplication, the inverse FFT (IFFT) was then performed on the signal before being added to a custom built channel selector. The channel selector then selects the channel that has a signal. Once the channel is selected, the signal is then sent to the TX/RX port of the B210.
The implementation of the canceller resulted in the GPS receiver being robust to jamming signal, resulting in a relatively high JSR (52.73 dB) in order to try and achieve the effects of jamming. The effort showed that it is possible to cancel out a relatively narrow band (about 1 kHz) jamming signal from a wideband GPS signal using a relatively cheap system.