White paper
Measures against GNSS jamming (jamming wave)
February 2020 release June 2020 revise
Author
Introduction
FURUNO ELECTRIC CO., LTD. (Furuno) has developed technology embedded in the GNSS receiver to mitigate or detect the effect of "jamming (jamming wave) on GNSS signals (hereinafter referred to as jamming signals)" that may hinder GNSS signal reception.
In order for the GNSS receiver to calculate the position, time, etc., it is necessary to receive the GNSS signal using a dedicated GNSS antenna. However, since the GNSS signal broadcast from the satellite is very weak originally, if there is other unintentional noise source or signal source around the antenna, it prevails as a jamming signal, and the GNSS signal may have problem with normal reception.
Our GNSS receiver automatically mitigates the effects of such jamming signals and implements a mechanism to notify frequency and signal strength of the jammed signals in real time to the user. This kind of technology further enhances security for using GNSS signals, and greatly improves maintainability.
Background
In recent years, GNSS receivers have become widely used, mainly in mobile phones and in-vehicle products, and have become commonly used in everyday life. Under such circumstances, GNSS receivers are required to have high performance in position and time, but there are more specific applications requiring their safety.
What is specifically required in recent GNSS receivers are measures against "Jamming signals", which interfere with normal reception of satellite signals by mixing other signals (or noise) in the same frequency band as the satellite signals, and countermeasures against "Spoofing signals", which are fake satellite signals broadcasted by a malicious actor to misidentify the position and time information.
Among which the jamming signal is one of the most frequently encountered interferences in reality. This jamming signal may be intentionally broadcast by a malicious actor using a small device ex. Jammer. In other cases, similar signal used for other purpose may be mixed into the GNSS antenna and interfere with the normal operation of the GNSS receiver. The center frequency of the GNSS signal is 1575.42 MHz for GPS L1 and 1602.5625 MHz for GLONASS L1. However, if any signal accidently mixed into these frequency bands, it acts as a jamming signal and may adversely affect the reception of GNSS signals.
With the spread of GNSS, the problems caused by GNSS jamming signals have been increasing in recent years. For example, in Japan, cases of limited GNSS signal reception sensitivity in some areas have been reported1 since 2013. This is because
a part of the frequency bands of LTE (Long Term Evolution), a mobile phone call service that was rolled out throughout Japan at the same time. This LTE signal uses the 1.5 GHz band in part and is close to the 1575.42 MHz frequency band of the GPS L1 signal. Therefore, it is said that the transmitted wave had affected the GNSS antenna near the LTE mobile phone base station as a jamming signal.
In addition, at the Newark International Airport in U.S., GNSS have lost signals intermittently for several days, and there are cases that being investigated for several months2. The cause was a small jammer called PPDs (personal privacy
devices). PPDs are relatively inexpensive devices for generating GNSS jamming signals. Rather than interfering with third-party GNSS reception, this product was intended to obstruct the operation of its own GNSS receiver so that its location would not be known. In the case of the airport, it is reported that the jamming signal was from a truck with PPDs (truck driver does not want to expose their location to the employer) that mixed into the GNSS antenna installed on the airport side. Actually
1 “Effect of LTE Signal on Geodetic GNSS Observation and its Mitigation”, Journal of the Geospatial Information
Authority of Japan No. 129 Page 237-243.
2 “National PNT Advisory Board comments on Jamming the Global Positioning System - A National Security
the use of these PPDs is illegal in some countries, but still it happens because a few fully understand the illegality and scope of their impact.
By the way, this jamming signal not only causes the GNSS receiver to have poor reception, but also tends to make it difficult to identify the problem. The receiver simply detects a poor reception of the GNSS signal but hard to determine whether it is a malfunction of the antenna, or the effect of a shield around the antenna etc. Especially when the jamming signal is transient, even if an investigator arrives at the site after the problem occurs, the jamming signal itself is already gone and the cause may not be specified. In this case, even if the reception of the GNSS signal is restored, the risk remains and user will continue the operation with a great deal of anxiety.
To avoid this situation, our latest GNSS receiver focuses on two measures. One is to mitigate the effects of jamming signals. Our receivers have effective anti-jamming solutions against jamming signals from narrowband jammers which aim at specific frequencies. Therefore, even if a jammer is disguised, the effect of the jamming signal can be reduced to some extent, and it is possible to prevent signal loss. On the other hand, if this jamming signal is extremely strong or broadcasts on every frequency, unfortunately, it is impossible to completely offset the jamming signals while maintaining normal reception of GNSS. However, if the cause of the poor reception is known by a jamming signal and managed to figure out when and what frequency the jamming signal was, that information should be a great clue to solving the problem. Therefore, as a second solution against jamming, Furuno also focuses on detecting jamming signals. Our latest GNSS receiver can monitor whether or not the jamming signal is being received, the frequency of the jamming signal being received, and the strength of the jamming signal according to the message (hereinafter, sentence) transmitted in real time.
The following section describes the experimental results when simulating a jamming signal and inputting it to a receiver. The difference between the effects with and without jamming countermeasures and the effectiveness of the jamming signal detection function are discussed.
Method of verification
The antenna of the GNSS receiver for the purpose of outputting time information is usually placed on the rooftop of a building or on the user’s premises. Therefore, when a jamming signal arrives, it is expected that the source of the signal is not physically adjacent but from a remote point.
In Japan, the Radio Law prohibits transmitting strong signals, therefore it is necessary to pay specific attention when doing verification testing. For the purpose of verification, a signal generator, which is the source of the jamming signal, was connected between a GNSS simulator and a GNSS receiver, so as to create a similar
jamming situation by mixing those signals, as shown in Figure 2. GNSS signal sources can also be tested by connecting an active antenna installed in an open sky environment. However, in order to check the signal level and positioning status more accurately, we decided to use a GNSS simulator this time.
Figure 2. Block diagram of the verification method
The target timing module is GT-88, made by Furuno. Two GT-88, including one without jamming countermeasures is prepared so that the difference between the presence and absence of jamming countermeasures can be compared. The setting conditions for both receivers are default.
Before startup, turn off the signal generator and prepare a period during which the evaluated devices can obtain the correct satellite information. The experiment starts by inputting 1575.42 MHz jamming signal -100 dBm via signal generator. The jamming signal is increased by -10 dBm every 30 seconds thereafter until it finally reaches 0 dBm. The performance against jamming countermeasures is confirmed by the number of positioning satellites and sentences indicating that a jamming signal is being received from each evaluated device.
Test results
The followings are the test results of the Furuno Timing Module GT-88 with anti-jamming and the one without anti-jamming. Figure 3 shows the total number of positioning satellites, and Figures 4 and 5 show the results of counting the number of satellites by dividing them into GPS satellites and GLONASS satellites.
The black line indicates the strength of the jamming signal and corresponds to the right axis. The blue and red lines indicate the number of positioning satellites and correspond to the left axis. The result of the GT-88 with anti-jamming is shown in blue line, and the result of the GT-88 without anti-jamming is shown in red line.
Figure 3. Changes in the total number of positioning satellites when a jamming signal is enhanced (GPS + GLONASS + Galileo + QZSS)
Figure 4. Changes in the number of positioning satellites (total GPS) when a jamming signal is enhanced
Figure 5. Changes in the total number of positioning satellites (total GLONASS) when a jamming signal is enhanced
Fig. 3 shows that the number of positioning satellites remains active with anti-jamming (blue line) than without anti-jamming (red line). Specifically, when there is no jamming countermeasure, the number of satellites starts to decrease when the jamming signal strength reaches -70 dBm. On the other hand, with anti-jamming measures, the number of satellites can be maintained until the jamming signal is stronger than about -50 dBm. The improvement is brought by the jamming signal countermeasure implemented in GT-88, which means that users can benefit from this function without any particular setting.
Fig. 4 and Fig. 5 show that GLONASS can maintain a bigger number of active satellites than GPS respect to this jamming signal. This is because the jamming signal this time was 1575.42 MHz, as same as the center frequency of the GPS L1 signal, so it was particularly easy to affect the GPS signal reception. Depending on the frequency of the jamming signal, it might be better to use multiple GNSS satellites receiver in this way.
When the signal strength was increased to around 0 dBm, the amplifier connected became saturated, hindering the reception of the original GNSS signal thus it impossible to keep normal reception. In other words, although it was succeeded to reduce the jamming to some extent, all jamming signals could not be completely prevented.
signals. By analyzing the sentence from the receiver, it is possible to check whether or not a jamming signal is being received, and if so, what is the approximate frequency and how strong the jamming signal is. To be specific, the detection result is available every second from message called CRJ sentence, as shown in Table 1.
Below shows an example of the output of the CRJ sentence corresponding to the strength level of the jamming signal. The CRJ sentence is made of a GP line for examining the 1575 MHz frequency band and a GL line for examining the 1602 MHz frequency band. If no jamming signal exists, nothing will be recorded thereafter, but if a jamming signal is detected, the frequency and signal strength will be displayed as a set. The signal strength is displayed in the range of 1 to 255, and the larger the number, the stronger the jamming signal. Since the sentence is updated every second, it is possible to detect the jamming signal immediately.
Table 1. Output of CRJ sentence when jamming signal (1575.42 MHz) is generated Jamming
signal strength
Output example of jamming signal detection sentence The meaning of sentences No
jamming signal
No jamming signal has been detected.
-100 dBm
Detects medium intensity (53) jamming signals at 1575.420616 MHz. -90 dBm Detects a strong (166) jamming signal at 1575.420602 MHz. -80 dBm
Detects very strong (255) jamming signal at 1575.420596 MHz.
-70 dBm
Detects very strong (255) jamming signal at 1575.420596 MHz.
-60 dBm
Detects very strong (255) jamming signal at 1575.420596 MHz.
-50 dBm
Detects very strong (255) jamming signal at 1575.420596 MHz.
-40 dBm
While detecting a very strong jamming signal of 1575.420538 MHz, it also receives some noise.
-30 dBm
While detecting a very strong jamming signal around 1575.420MHz, it also receives some noise.
-20 dBm
-10 dBm
0 dBm
Table 1 summarized the result of jamming signal detection when different jamming signal was input. In addition, the frequency is accurately detected as around 1575.42 MHz, and it has been detected that the signal strength was increased to the upper limit that can be confirmed according to the setting of the
signal generator. Also, it proves that the jamming signal can be detected continuously even at around 0 dBm where GNSS reception is totally dead. Therefore, by constantly monitoring this sentence, the presence or absence of a jamming signal can be determined in real time, and even if the jamming signal is transient, the occurrence period and regularity can be estimated as well. In a word, it is expected to help identify the source of the jamming signal.
Based on the above results, we were able to confirm the jamming signal mitigation function and jamming signal detection function of the Furuno timing module.
Product introduction
Furuno offers to the market the GT-88, a high-precision synchronization-dedicated GNSS timing receiver module and the GF-88 series, GNSS reference frequency generators, also known as GNSS disciplined oscillators, since April 2019.
The GT-88 and GF-88 series have the new algorithm “Dynamic Satellite Selection ™” that reduces multipath timing errors in harsh environments such as urban canyons and vicinities of signal reflecting glass panels by selecting only satellite signals that have small errors for use in the positioning calculations. Regarding the GNSS constellations, we are now providing support for Europe’s Galileo Navigation Satellite System in addition to the conventional support for GPS, GLONASS and Quasi-Zenith Satellite System (QZSS). For the Quasi-Zenith Satellite System, our timing products support simultaneous reception of four of these satellites, including the Michibiki No. 3 geostationary satellite. They are also compatible with QZSS L1S signals and support the SLAS correction. They also feature anti-jamming and anti-spoofing functions, as introduced in this paper, which reduce concerns when using GNSS and greatly improve the reliability of these products. In addition, the GF-88 series supports a wide variety of Holdover capabilities.
Furuno will continue to work on application areas where high reliability is required. To make GNSS high-precision time signals safely available to everyone who needs them, including not only 5G mobile base stations but also financial institutions, securities trading firms and power grids.
High-precision synchronization-dedicated
About FURUNO ELECTRIC
Furuno Electric Co., Ltd. is a company mainly involved in the manufacture and sale of commercial electronic devices centered on sensing and information processing technology. Besides marine electronic navigation equipment, we also serve markets in healthcare, communication/GNSS solutions, disaster prevention, weather radar and monitoring solutions. With our corporate vision of realizing “navigation that is safe and secure, user-friendly, kind to people and the environment,” we aim to conduct business activities that contribute to our customers and to society as a whole. We are contributing to the realization of a highly safe and convenient society by providing dynamic management and time synchronization management solutions, using GNSS (Global Navigation Satellite System) and DSRC (Dedicated Short-Range Communication) and ITS (Intelligent Transport System) equipment.
