GNSS receivers are intended to receive a set of signals from space spanning a range of frequencies. When paired, a GNSS receiver and antenna also need to reject all other non-GNSS frequencies to avoid interference with any relatively low-power satellite signals. The frequency ranges that the receiver needs to track are typically defined by a series of band pass filtering inside the receiver, called pass band.
GNSS interference comes in many forms and classifications such as inside receiver pass band (in-band) or outside receiver pass band (out-of-band). Interference can be Continuous Wave (CW), Narrow Band (NB) or Wide Band (WB) in the spectrum domain. It can also be modulated waveforms or white noise as well as pulsed or continuous in time.
Interference impacts a GNSS receiver in multiple ways depending on the interferer strength and type. The presence of interference can increase the receiver in-band noise and reduce receiver Signal to Noise Ratio (SNR). The presence of stronger interferences can also cause receiver compression or saturation and generate harmonics and intermodulation products that land in the pass band of GNSS receivers. The GNSS receivers under interference conditions will exhibit reduced carrier to noise ratio, noisier GNSS measurements, and eventually degraded positioning performance. In the most severe cases, the receiver will lose the ability to compute a position solution.
Before interference affects a receiver’s positioning performance, receiver tracking performance starts to degrade when interference, such as that caused by an intentional jammer, is still well below the receiver noise floor. Any additional noise generated by the jammer for example, will degrade the signal to noise ratio at the receiver.
Receiver acquisition is much more sensitive to the power level of interference than tracking. While in the presence of interference, GNSS receivers will have trouble acquiring the satellite signals with the presence of lower power interference.
Although intentional jammers are usually built as in-band interferers, there are many unintentional interference sources as well. In this article we focus on unintentional interference sources.
Unintentional interferences typically are not located inside the GNSS receiver pass band due to spectrum regulation (except for GPS L2 band which is not a protected band). Although some interferences are far away from the receiver pass band, they can still negatively impact the receiver if their higher order harmonics or intermodulation products are landed in band (e.g., LTE Band 14; channel 230 VHF transmitter) or if they happen to land in sensitive frequency regions of the receiver RF design.
The known unintentional GNSS interferences near GNSS operating frequencies are shown in the following figure (potential LSQ signals are not shown).
With the ever growing wireless communication industry, more and more wireless sources become unintentional interferences to GNSS receivers. The following table shows the 4G spectrum allocation, which can become interferences to GNSS receivers.
Table 1 Wireless 4G Spectrum Allocation
GNSS receivers are designed to be very sensitive to recover the GNSS signals below the noise floor (e.g., GPS L1CA signal power ~-120dBm to -130dBm on earth, 10-15 to 10-16W). It does not take much interference power to increase the noise level within the signal pass band.
Considering a frequency source with a transmit Effective Isotropic Radiated Power (EIRP) of 200mW (a typical mobile handset output power level), one kilometre distance away from a GNSS receiver system, the power at the GNSS system antenna will be only attenuated ~100dB, leaving the power level seen at the GNSS antenna to be 4x10-11W, which is still ~10000 times higher than GNSS signal level.
Luckily, this amount of interference power level is located outside of the receiver pass band. The series of out-band filtering, within the antenna and receiver, will further reduce the power levels within the GNSS system to mitigate the out-band interference impact.
Antenna choice is critical in GNSS system design. Selecting an antenna supporting the required signals/frequencies for the user application (no more or no less) and an antenna with the good out-band filtering (such as the Novatel Pinwheel® triple-frequency antenna) is recommended.
Modern GNSS receivers support multiple constellations within the same receiver hardware platform. This allows the receiver to take advantage of the greater signal availability in space, however, it presents challenges in terms of interference mitigation due to the increasing pass band bandwidth. Therefore, selecting a GNSS receiver with interference mitigation ability is an asset given the higher dynamic range receiver, digital filtering, in-band interference detection and removal.