How to eliminate GNSS interference signals

Alex He, Tersus GNSS      16 Jan. 2025

Following the guidance of the map software on her mobile phone, Lisa quickly found the Luckin Coffee on the corner. Click the automatic parking button on the car computer, and Jack's new energy vehicle efficiently completes the parking space search and automatic reverse parking. This is inseparable from the critical role of the GNSS system. The GNSS system has gradually become a key part of today's infrastructure. It is not only for positioning and navigation but also in various application scenarios, such as autonomous driving, unmanned equipment, robots, smart cities, IoTs +, and precision agriculture, which have penetrated every aspect of our lives. These applications are realized based on the GNSS receiver accurately receiving the telegrams broadcast by the GNSS satellite. Still, the satellite signal is very weak and easily interfered with. The noise and interference received will affect the accuracy of the solution and even make the receiver unable to work correctly.

1. Bustling and crowded world of electromagnetic

In the GNSS system, the satellite signal is modulated by spread frequency, reducing the satellite's transmitting power. After long-distance transmission, the signal reaches the receiver antenna and is very weak. The power of GNSS signals received on the surface is typically -140dBm, which is 20dBm lower than the background noise power. According to the formula 1dBm=10*log(P/1mW), it is about 100 times weaker than other radio background noise. You can't even "hear" the satellite signal with a normal antenna and receiver over the background noise. The radio frequency spectrum is filled with many existing bands, including VHF, FM radio, microwave, Bluetooth, WIFI, 3G, 4G LTE, and the increasingly widespread 5G, making the electromagnetic world even more lively.

Even though GNSS signals have their inherent frequencies, they can still be easily mixed with various RF signals. These signals are either close to the frequencies of GNSS signals or have powerful signal strengths. Some interference signal frequencies even penetrate the GNSS frequencies and mix with GNSS signals, forming interference signals that have a more severe impact and are more difficult to distinguish. For example, the center frequency of the L5 band is 1176.45MHz, which is also the frequency band used by DME (distance measurement equipment) and TACAN (tactical air navigation). These radio beacons are deployed near airports and emit high-power radio pulses, which may interfere with GNSS receivers using GPS and GALILEO signals in this band.

2. The impact of interference signals

Despite various signal interferences, professional GNSS receivers can still obtain GNSS signals from complex background noise. The performance of the GNSS system mainly depends on the availability of satellites and the accuracy of satellite positions. For higher reliability and higher accuracy, most current GNSS receivers are multi-constellations, multi-bands, or even full-constellations, full-bands. When using multi-constellations combination positioning, the fixation rate and reliability of ambiguity resolution are significantly improved, and the positioning accuracy can be improved by more than 20% compared to using a single constellation. When fewer satellite frequencies are interfered with, the redundant design can still ensure that the GNSS receiver has high accuracy. However, the positioning accuracy is difficult to guarantee when an interference source affects multiple frequencies within a GNSS frequency band or can affect the entire GNSS frequency band.

For example, a dual-frequency receiver of GPS and GLONASS has to switch to the dual-frequency RTK mode of GPS when the G2 band of GLONASS is wholly interfered with. This may reduce the accuracy of the measurement results.If both the GLONASS G2 and GPS L2 signals disappear, the receiver has to switch to a single L1 band RTK, DGPS, or even single point mode. For application scenarios that rely on RTK solutions with an accuracy of a few centimeters, falling back to non-RTK modes (such as pseudo-range code differential) is not acceptable.

3. Different forms of interference

There are many sources of interference, mainly various electronic and electrical equipment, from various ordinary equipment in our daily lives and work to multiple special equipment at airports, military facilities, signal towers, and so on. There are various classifications of interference, including human and non-human interference; there are suppressive, deceptive, and distributed three-dimensional types, among others. The interference patterns include narrowband signals, Wideband signals, continuous wave single tones, continuous wave frequency sweeps, and constant wave pulses. Among them, narrowband interference has the most significant impact on satellite signals. The Narrowband is compared with the bandwidth of the received signal, which is generally within 10% of the bandwidth of the received signal.

Anti-interference: Tersus in action

1. The principle of anti-interference

Tersus has extensive research in anti-interference, including the realization of different links such as antenna, RF, baseband. Research on anti-jamming technology using signal processing includes spatial filtering,Time domain filtering, frequency domain filtering, and joint filtering.

Spatial filtering technology mainly uses antenna arrays and adaptive algorithms to identify and filter interference.By applying different weights to different antennas, a beam can be formed in the direction of the signal incident (beamforming), or the signal can be suppressed in the interference direction to minimize the signal output power in that direction (nulling). Among them, beamforming requires more signal prior information and is more complex to implement than nulling technology. The number of interferences that spatial filtering can resist is related to the number of antennas used.

Time domain filtering technology is divided into two methods, linear and nonlinear, which have a good suppression effect on narrowband interference.It mainly uses the before and after correlation of narrowband interference signals, uses past signals to predict the received signal, and uses adaptive algorithms to filter out the interference signal from the received signal. Time domain filtering is a transverse filter based on FIR structure, which is easy to implement in engineering, but its iteration time is long, and its ability to suppress changing interference is low.

Frequency domain filtering technology transforms the time domain sampling signal into the frequency domain, analyzes the signal spectrum, and suppresses the parts that do not conform to the signal spectrum characteristics to achieve the effect of filtering out interference signals. Because the navigation signal is a Gaussian white noise signal when received, the amplitude is flat within the frequency band. When there is narrowband or single-tone interference, the interference spectrum can be easily distinguished in the frequency domain, thereby suppressing the interference.In the early days, the Fast Fourier Transform (FFT) was used to suppress narrowband interference in spread spectrum communications. Later, the Discrete Fourier Transform (DFT) technology was applied to frequency domain filtering technology.

The main idea of joint domain filtering is to combine the abovementioned techniques to achieve better filtering effects. There are mainly space-time domain filtering, space-frequency domain filtering, time-frequency domain filtering, and other technologies, but its implementation is relatively complex, and engineering implementation is difficult.

Among the several anti-interference algorithms mentioned above, spatial filtering technology requires a multi-antenna design. For measurement receivers, the antenna's phase center accuracy significantly impacts the final measurement results. The phase center formed by multiple antennas is relatively complex, and the accuracy cannot be guaranteed. In addition, multiple antennas will increase the size of the receiver, which is not conducive to manual outdoor operations. Therefore, the spatial anti-interference algorithm is not suitable for measurement receivers.Time domain filtering technology is simple to implement, but its iteration time is long. If the interference signal changes quickly, the anti-interference ability will be reduced. Joint domain filtering technology is complex and not conducive to engineering implementation.

Tersus combines the comprehensive balance of anti-interference requirements and engineering implementation, adopts frequency domain filtering anti-interference technology, combined with self-developed advanced algorithms to achieve automatic detection of interference signals and filter out. After the antenna receives the RF signal,Digital signal demodulation is performed by shifting the signal frequency band through the local oscillator and sampling with the ADC.Perform FFT calculation on the sampled signal, transform the signal into the frequency domain, detect points in the frequency domain higher than the normal signal amplitude threshold, force the points with abnormal amplitude to 0, remove the interference signal, and finally perform IFFT to restore the signal. The schematic diagram is as follows:

2. Hardware guarantee

Tersus’s engineers have been engaged in research on GNSS signal anti-interference for nearly 10 years and have accumulated rich, relevant experience. They can be said to be experts in the field of anti-interference and have relevant core technologies and algorithms. From hardware, software, and algorithms to personalized support, Tersus provides full-process GNSS signal anti-interference support.

In order to deal with interference signals of various forms and sources, all forms of interference must be considered in the entire process of GNSS receiver architecture, design, development, and use. The various components of the receiver must be coordinated. From the analog design of the RF front-end (the part of the antenna and receiver that captures and processes high-frequency signals) to converting digital signals and using various digital filters and algorithms to process these signals, all must work together to combat interference sources effectively.

Targeted receiver antenna design, entirely independently designed and developed baseband chip Antares (using mature 55nm process), powerful board design and integration capabilities, and targeted combination filters provide strong hardware support for the anti-interference ability of Tersus OEM boards.

3. Algorithms and software
‍Based on hardware, Tersus has also conducted unique research and design on algorithms and software to combat interference. The advanced frequency domain anti-interference algorithm can detect and eliminate interference signals in real-time.On the BX50M-TAP and BX50L-TAP boards, the anti-interference algorithm integrated in the board will work automatically and automatically remove interference signals that exceed the threshold.

In addition, the board will scan the signals of each frequency band in real-time.Through our TersusGNSSCenter software, users can view the scanning status of the board for signals in each frequency band, which allows users to understand the interference status of their environment in real time.Use the button in the lower right corner of the software RF signal display window to check each frequency band to view the signal conditions of GPS L1, BD B1, GLO G1, and other frequency bands, and scan and find interference sources on site.

If there is interference in a certain frequency band, you can find in the software that the waveform of the frequency band exceeds the horizontal line in the center of the screen.As shown in Figure 8 below, the interference signal was found during the actual test for the customer. The red box in the figure has a waveform exceeding the horizontal line, indicating that there is interference in the GPS L1/GAL E1 band.As shown in Figure 9 below, the interference situation of all frequency bands.

Summary: adapt, overcome, and never stop

Because of its characteristics and advantages, GNSS systems are increasingly becoming indispensable to advanced equipment, high-tech, and people's needs for a better life.The number of various electronic and electrical devices that can interfere with GNSS signals is also increasing rapidly with the needs of people's lives and production.We have to accept and adapt to the reality that we are increasingly dependent on GNSS systems, and the interference with GNSS signals is also increasing day by day.

But adapting to reality does not mean doing nothing. Tersus has ten years of anti-interference experience in design, R&D, manufacturing, and after-sales processes, from hardware, software, and algorithms to all aspects.Our rich anti-interference experience and powerful anti-interference technology and capabilities ensure that our boards and receivers can maintain high accuracy and high reliability in various complex and extreme environments, meeting our users' high-precision GNSS positioning needs in various scenarios and with different needs.

In the foreseeable future, various new interference sources will continue to emerge, which also puts higher demands on manufacturers' anti-interference capabilities. Tersus will continue to improve our anti-interference capabilities, constantly fight new interferences, optimize anti-interference processes, enhance anti-interference technology, and provide better anti-interference services.

References

[1]Yuelu Zhao, Lei Ji. Research on narrowband interference suppression algorithm for navigation satellite receivers. Electronic Science and Technology, 2016, 29(3): 53-57.

[2]Jia Qiongqiong, Wu Renbiao, Wang Wenyi, Lu Dan, Wang Lu. GNSS adaptive interference suppression algorithm for high-precision measurement. Journal of Electronics, 2018, 11: 2753-2760.

[3]Li Yuying, Wang Yanpeng, Wang Xing. Evaluation of the algorithm index for anti-narrowband interference in the intermediate frequency domain of satellite navigation receivers. Science and Technology, 2018, 25: 75-76.

[4]Bi Xiaojun, Shao Ran. Narrowband interference detection in OFDM system based on adaptive matching pursuit[J]. Journal of Harbin Engineering University, 2014, 07.

[5]Dai Rui, Wei Jiancheng. Research on frequency domain anti-narrowband interference algorithm for satellite navigation block processing. Modern Navigation, 2023, 1: 1-4.

[6]Zhang Zhaolin, Li Lu, Yao Rugui, Wang Ling. Narrowband interference detection technology based on LMS algorithm[J]. Journal of Northwestern Polytechnical University, 2016, 34(1): 92-97.

[7]Hao wang，Qing Chang，Yong Xu，Xianxu Li. Adaptive Narrow-Band Interference Suppression and Performance Evaluation Based on Code-Aided in GNSS Inter-Satellite Links.  IEEE Systems Journal，2020，14（1）：538-547.

About Tersus GNSS Inc.

Tersus GNSS is a leading Global Navigation Satellite System (GNSS) solution provider. Our offerings and services aim to make centimeter-precision positioning affordable for large-scale deployment.

Founded in 2014, we have been pioneers in design and development GNSS RTK products to better cater to the industry’s needs. Our portfolios cover GNSS RTK & PPK OEM boards, David GNSS Receiver, Oscar GNSS Receiver, MatrixRTK [GNSS CORS Systems] and inertial navigation systems.

Designed for ease of use, our solutions support multi-GNSS and provide flexible interfaces for a variety of applications, such as UAVs, surveying, mapping, precision agriculture, lane-level navigation, construction engineering, and deformation monitoring.

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