INVESTIGATION OVER JAMMING IN THE ASPECT OF THE CONSTRUCTION OF THE GNSS RECEIVER

Transcription

1 INVESTIGATION OVER JAMMING IN THE ASPECT OF THE CONSTRUCTION OF THE GNSS RECEIVER Andrzej Felski 1), Aleksander Nowak 2), Marta Gortad 3) 1) Polish Naval Academy, Gdynia, Poland, 2) Gdansk University of Technology, Gdansk, Poland, 3) Polish Naval Academy, Gdynia, Poland, ABSTRACT Nowadays jamming is considered as one of the biggest threats for using GPS in dynamic conditions. In spite of the legal prohibition of use them, jammers are common accessible, especially in the Internet and equally often applied, what is confirmed by many press and TV reports. The common use of GPS in many aspects of the everyday life entails the most users belief in the truth of results. It is especially important in practice of some portion of users educated only on basic level in selected technics of satellite positioning. Until recently even specialists have shared the conviction that the broadband GPS signal is not subject of disturbances in contrast to older wireless communication techniques because its power level is below natural level of noises and in addition it is randomly coded. At present reports about investigation over the specificity of the GNSS signals jamming appear in many publications. In this paper the results of experiments over the impact of jammers bought in the Internet on the typical receivers with the C/A signals are presented and discussed. Experiments proved that the process of jamming is not the zero-one process. The reaction of different types of receivers is different, for example depends on the height of satellites above the horizon. It causes that jamming of different space vessels is not the same. We can also observe correlation between jamming process and construction of the antenna. In short words, the reaction of the same receivers with different antennas are different. INTRODUCTION GNSS signal in space is very weak, so it is very vulnerable to disruption and interference. The reason can be natural disturbance to the RF signal. Usually it is man-made non-intentional but more and more often also intentional. The widespread of the use GPS in all aspects of the everyday life entails the most users belief in the truth of results, especially in the information about the user s position. Until recently we were convinced that the randomly coded, broadband GPS signal was not subject of disturbances in contrast to older wireless communication techniques. Only the possibility of the lack of signals because of the interception of satellites was allowed and possibility of accidental signal interferences or reflection in urbanized areas. Nowadays we know that these signals, so as all radio signals, can be subject to all kinds of disturbing. The devices for this are easily accessible and such incidents, especially jamming, happen very frequently. Indeed spoofing and meaconing are more sophisticated activities, however recent publications prove that this is not only the theory. The usage of such devices is fully illegal in all EU countries and in the USA however the possession and trade them is not forbidden. So such devices can be bought without difficulty in the Internet already for tens euros. It causes that usage of jammers is quite common, what corresponds to the media reports. It seems to be everyday problem at present time. This is at the bottom of so many reports about works over the investigation of the jamming specificity and methods of the detection of these events in last months. In this work chosen results of the investigation of the influence of such devices bought in the Internet on typical marine receivers with C/A signal are presented. 1. THE JAMMING THREAT AND JAMMERS The first reports about the possibility of the GPS signal disturbance appeared in the mid-90s[2]. However in 2001 in the report of the American Department of the Trade only potential possibilities of the interference of the GPS signal with broadband wireless communication systems is widely discusses. But in the report [3] is presented not only the problem of the interference, but also the problem of the intentional jammers adverts. Also media more and more often alarmed about cases of intentional disturbances of GPS signals. The special reaction called out disturbances caused by Northern Kore in the region of Seoul in In period between 2011 and 2012 in the UK in frame of SENTINEL project the hypothesis that disturbances from small, cheap and generally accessible devices were ubiquitous and caused the self-evident threat has been confirmed [1]. Nowadays offers to sell of detectors of the disturbances presence appear [4] however it is necessary to pay attention that the average user of the GPS usually is not aware of how the system works, so the more it is questionable whether he can notice the influence of disturbances.

2 Satellites are at least 20,000 km away and jammer is much closer to user s receiver. Even if its power is small its signal may exceed satellite signal in space. In the literature we can find reports that jammers with power of a few mw or similar have the range even 50 m, while the device about the power 1 W can disturb GPS devices in the radius of tens kilometers [7] [10]. Mitch points three principal types of such devices[8]: a. the device with declared range of some meters connected to the car (the lighter socket) power supply; b. the next group has range more than ten metres equipped with the internal battery and external antenna; c. the third group consists of hidden instruments, for example in the casing of the cellular phone, with own power supply, but usually without the external antenna. However our hypothesis was that the range of disturbing not only depends on the power of disturbing, and the influence of disturbing but also is a function of the construction of the receiver. In addition the geometrical relation of jammer position and the antenna of the receiver must be taken into account, so for example real range of jammer can be bigger if jammer is in the air. Many reports about jamming contain information about car moving nearby, however it means that jammer was found outside the antenna zone of the reception (antenna beam), and this means the radically less range. During reported investigation two models of the devices were used. They can disturb all band of GPS system (L1, L2, L5) and belongs to the first two groups (a and b). China EPC SPY 116 jammer purchased in Internet with the antenna and charger adapted to the socket of the car lighter. Declared by the producer time of working is up to 3 hours with the range up to 15 m. The second device was GX-40B (according to Internet information made by polish company TTS). Its range declared by the producer is 40 m and work time 16 hours. Fig. 1. Applied jammers. Left EPC SPY 116 and GX-40B (right). Source own photo. Both devices were examined with spectral analyzer type FSV (made by Rohde & Schwarz) equipped with the tube antenna. GX-40B spectrum is wider ( GHz) than EPC (only GHz) however EPC signal is stronger (-50dBm) when GX-40B has only -67 dbm. It is interesting, that both jammers cover also the mobile phone spectrum and in this band both have identical spectrum. 2. EXPERIMENTS The report about the possibility of the disturbance of the GPS signal appeared already in the half of 90 s [2] and in the last years there are a lot of press reports about such cases. In the professional journals there are also information about experiments on this field. In this report we have stressed the information about the influence of solutions made in devices on the resulted jamming. The main goal of the experiments was to show the influence of above-devices on the work of different GPS receivers. The research were passed at the closed PNA (Polish Naval Academy) area. Aside from of the use of different

3 receivers also different antennas has been used during the experiments. An intention was to confirm the supposition that the effect of jamming is different and is relative to constructional solutions of the receiver and the antenna [9]. Fig. 2. Situational picture of the experiments. The red point is the place of jammers installation. Source: own photo. In this report some details of two identical receivers R110 (Hemisphere) behavior is presented when they work with two different antennas (A-30 and MGL-3). The question of influence of the antennas on jamming process were considered earlier, for example by Kerr in [5] and [6]. Both jammers during tests were installed on the roof of one of the PNA buildings (see fig. 2) about 15 meter above ground. Receivers were installed in the car with antennas on the car roof so each of them works in the same conditions. During the test car moved on the car parking space which is visible on the picture, as well as down the street to the building. Fig. 3a. A-30 antenna Source: Fig.3b. MGL-3 antenna Source: The following measures of GPS receivers status and resistance on the jamming were applied: signal to noise ratio (SNR), the geometrical coefficient DOP, and system status which is delivered in the NMEA 0183 message $GPGGA. During experiments $GPGSV messages were registered too, what permitted to take into account, except the position and time, other information about the system status. 2. EXPERIMENTS A fundamental information in the context of GPS receiver disturbances is status of the system. It is one of the information contained in the $GPGGA message and means: 1 system works, 0 does not work. On the Fig. 4 and 5 examples of changes of the receivers status when approaching to the jammer are presented. It was supposed that in the certain moment the change of the status from the state of work to the state of failure appeared. Surprising is that identical receivers, installed at the same place, by the fact of the use of different antennas showed other reactions on disturbances. And in addition one of them, after approaching to jammer came back into the state of work, and after the further approaching was blocked again. The same reaction were observed many times during experiments. In the figures below all parameters are presented against the time of experiment. As the receivers are the

4 same, the reason of differences are the antennas. Both work with L1 band however MGL-3 (CSI-wireless) has narrow bandwidth (3 MHz) and lower LNA Gain (27dB) than A-30 (Hemisphere), which bandwidth is 20MHz and LNA Gain 34dB. Fig. 4. Status of receivers during some of experiments. Receivers R110, red line - antenna MGL-3, green line antenna A-30, jammer Spy Electronics. Fig. 5. DOP changes. R110/MGL-3 red line, R110/A-30 - green line, jammer Spy Electronics. Differences in receiver status and DOP are caused by different signal to noise level for each satellites. Some examples of this effect during the same experiment can be observe in the fig. 6. Fig. 6. Number of SVs during the same experiment. R110/MGL-3 red line, R110/A-30 - green line, jammer Spy Electronics.

5 It is clearly visible that the influence of jamming is not similar on different channels. Earlier were blocked channels in which lower S/N ratio appeared, and it usually corresponded to lower elevation of satellite. Sometimes during the experiments were observed cases when different receivers didn t use the same satellites for fixing the position. In addition it was noticed, that in many cases both marine receivers were blocked when smartphone receiver still worked. This observation were accidental and signals were not recorded during these experiments, so it should be continued. In different experiment not only GPS satellite, but INMARSAT one which transmits EGNOS correction were observed too. It was noticed, that there is no rule in this configuration. In some cases all GPS satellites were blocked, but in some cases not all GPS satellite were blocked and both EGNOS transmission were blocked. CONCLUSIONS During the referred experiments with weaker jammer usually all receivers took the same satellites to track (all available). While the stronger jammer worked in many cases they tracked different configurations of satellites. It was observed at greater distances from the jammer. Both of the jammers caused blocking of the receivers. It took place at different distances, however, as a rule, these moments didn t overlap for both receivers. The conclusion is, that influence of disturbances was different and clearly depended on type of the antenna. It is confirmed by analyzes of DOP (Dilution Of Precision). Differences in DOPs values mean that different configurations of satellites were tracked. It is worth noting here that jamming is not the zero-one process. Influence of jammer on GPS receivers depends on receiver kit, what means that the same jammer can have different range of operation and also can have different capability to elimination particular satellites resulting in different values of DOPs for variant receiver kit and in consequence accuracy also differs. The hypothesis appeared that resistance of the receivers to jamming is better if the receiver is simpler, while the influence of bandwidth and LNA Gain of the antenna is not clear. Bibliography [1] Extreme space weather: impacts on engineered systems and infrastructure, Royal Academy of Engineering, London, Available at: ( ). [2] G. L. Falen, Analysis and Simulation of Narrowband GPS Jamming Using Digital Excision Temporal Filtering, (Master's thesis) Air University, Air Force Institute of Technology, Ohio, [3] Global Navigation Space Systems: reliance and vulnerabilities. The Royal Academy of Engineering, London Available at: ( ). [4] GNSS Jamming detection, ( ) [5] T. H. Kerr, A Critical Perspective on Some Aspects of GPS Development and Use, Proceedings of 16th Digital Avionics System Conference, Vol. II, pp to , Irvine, CA, Oct [6] T. H. Kerr, Vulnerability of Recent GPS Adaptive Antenna Processing (and all STAP/SLC) to Statistically Non- Stationary Jammer Threats, Proceedings of SPIE, Session 4473: Tracking Small Targets, pp , San Diego, CA, 29 Jul.-3 Aug [7] H. Kuusniemi, M. Z. H. Bhuiyan, T. Kroger, Signal Quality Indicators and Reliability Testing for Spoof- Resistant GNSS Receiver, European Journal of Navigation vol. 11 no. 2, [8] R. H. Mitch, R. C. Dougherty, L. M. Psiaki, S. P. Powell, B. W. O Hanlon, J. A. Bhatti, T. E. Humpreys, Signal Characteristics of Civil GPS Jammers, Available at: ( ). [9] A. Nowak A., K. Naus K., M. Wąż Influence of GPS signals suppression on accuracy of fixing, TTS Technika Transportu Szynowego Nr 9/2012, str , ISSN [10] L. Scott, Spoofs, Proofs & Jamming, Inside GNSS, September/October 2012, p