1 International Symposium on GPS/GSS Otober 6-8, 1. Detetion of RF interferene to GPS using day-to-day /o differenes Ryan J. R. Thompson 1#, Jinghui Wu #, Asghar Tabatabaei Balaei 3^, and Andrew G. Dempster 4# # Shool of Surveying and Spatial Information Systems, University of ew South Wales, Australia, ^Shool of Eletrial Engineering and Teleommuniations, University of ew South Wales, Australia 1 r.thompson@student.unsw.edu.au jinghui.wu@unsw.edu.au 3 asghart@unsw.edu.au 4 a.dempster@unsw.edu.au Abstrat RF interferene is a threat to systems that rely on GSS signals. The arrier-to-noise-ratio (/o) an be used for deteting interferene but is a noisy measurement. The presented detetion tehnique uses the fat that GPS satellites appear in the same plae in the sky eah sidereal day with similar /o values exept in the presene of interferene. Analysis of these differenes in /o values shows they an be used for deteting wideband (WB) interferene down to a jammer-to-noise ratio (J/) of - 4.836dB and ontinuous-wave (W) interferene down to a J/ of -5dB. Results with benh W and WB interferene are also presented. Key words Jammer, /o, detetion, groundtrak, multipath. 1. Introdution. In the presene of RF interferene, systems that rely on the preise positioning and timing apabilities of a GSS reeiver will suffer a large degradation in performane. Where GSS is used in ritial situations suh as preision airraft landing in a ground based augmentation system (GBAS), the need to detet interferene beomes more signifiant. There are a number of different tehniques that an be used for GPS interferene detetion. In general the tehniques an be separated into 3 ategories: i) automati gain ontrol (AG) and hardware tehniques [1], ii) preorrelation tehniques that perform signal proessing on IF samples [,3], and iii) post-orrelation tehniques that analyse many GPS observables from the satellite aquisition and traking proess [4,5]. Signal proessing on the GSS IF raw data is effiiently used in some detetion tehniques. In [] a statistial inferene tehnique is used to detet W interferene based on a omparison of power spetral densities (PSD) using sliding evaluation and examination windows. In experiments this tehnique was able to detet W interferene down to -13dBm. In [3] a tehnique also used that ompared PSDs, as well as the power of the IF samples in the time-domain, to detet W and wideband interferene. This PSD-based tehnique was able to detet wideband interferene down to -17dBm experimentally. In [5] /o values from a model versus elevation were used for the interferene detetion. This /o based tehnique was able detet wideband interferene down to - 18.5dBm in 1 MHz and W interferene down to - 11dBm in experiments. The AG or the pre-orrelation (IF data) tehniques an be used for interferene detetion; however aess to the AG level and the IF data for applying the preorrelation tehniques is generally not available from ommerial GPS equipment. Moreover, espeially for those tehniques whih are IF sample based, speially designed and programmed hardware is required [6]. A noisy /o measurement affeted by environmental hanges is also hard to use in interferene detetion [3,7]. However the noisy /o measurements an be easily obtained from a GPS reeiver as it is an observable inluded in the RIEX and MEA standards. This paper presents a detetion tehnique that uses the day-to-day differenes of /o measurements ( /o) when the satellites are in the same position relative to its ground trak to redue the noise in normal /o measurements.. arrier-to-noise ratio (/o) In GPS baseband proessing the /o value is used for assessing the signal quality of traked satellites. In the absene of interferene it an be defined as [8] = log ( SR BW ) (1) 1 1 where is the power of the GPS signal, is the reeiver noise power in 1Hz, and BW is the reeiver's 3dB equivalent pre-orrelation bandwidth. Typially GPS signals arrive db below the noise floor, making the SR equal to -, resulting in a nominal /o of a GPS satellite of 43dB/Hz. In pratie the /o an be estimated from the in-phase and out of phase prompt orrelators in a GPS reeiver baseband and the estimated /o values vary from reeiver to reeiver depending on the estimation algorithms [8]..1 Effet of wideband interferene on /o Wideband interferene effetively raises the noise floor of the reeiver ausing a drop in /o. Using simulated GPS IF data the drop in /o due to wideband interferene was found to follow: 1
1 International Symposium on GPS/GSS Otober 6-8, 1. WB = S + 1 log1 () S + I where S is the signal power at the reeiver in the absene of interferene (normal thermal noise power, i.e. - 11.6dBm for a MHz bandwidth) and I is the interferene power in the reeiver bandwidth BW. For instane, a jammer with power -11.6dBm auses a 3dB drop in /o as the level of noise at the reeiver has doubled. In Figure 1, the drop in estimated /o for a GPS IF signal generated in MATLAB with an added wideband jammer of inreasing power is shown and ompared with (). Simulations math well with the theoretial results. /o (db/hz) 5 4 3 Simulation Theory 1 3 4 Wideband J/S (db) Figure 1. /o falls as wideband jammer power inreases.. Effet of W interferene on /o In [9] it was shown that as a W interferer rosses the 1kHz spetral lines of a GPS /A ode there are drops in the /o. The drop of a /o value is mainly dependent on four fators: the integration time T d, frequeny offset between the arrier and the interferer f i, interferene power J, and the value of the spetral line oeffiients j, aording to [9]: R ( ) = ross-orrelation of reeived ode and replia τ Inreasing integration time results in sharper drops in /o. The stronger the /A spetral line oeffiients that the W arrier rosses the greater the drop in /o. Observing the satellites over a longer period of time inreases the probability of having a signifiant drop in /o when the W arrier rosses a strong spetral line as the satellite s Doppler frequeny offset is hanging over time. In Figure the drops in theoretial /o obtained from (3) are shown. This theory is the foundation of the proposed interferene detetion method introdued in the following setions. However, in pratie, environmental and orbital fators also affet the /o estimations. /o (db/hz) 43 4 41 4 Doppler Offset (Hz) Figure. /o falls as the W rosses GPS spetral lines. 3. Environmental influenes on pratial /o estimates For a number of days /o values (1Hz) were reorded using a ordav R3 software GPS reeiver onneted to an antenna on top of the Eletrial Engineering building at the University of ew South Wales. A typial /o reord for a visible satellite (PR #3) is shown in Figure 3. where L n ( T R ( τ )sin ( f T d d = Ln + J ( Td j sin ( Td fi )) W = thermal noise power = noise proessing gain T d = integration duration time τ = ode delay fˆ = arrier frequeny estimate f = f fˆ f i = f i f J j = interferene power = j th spetral line oeffiient )) (3) /o (db/hz) 45 4 35 3 3.5 4 4.5 5 UT Time (s) x 1 4 Figure 3. /o values of an observable satellite. A satter plot is shown in Figure 4 of the averaged /o versus elevation for all satellites along with a 4th order polynomial fit for all satellites, as well as a fit for PR #3, over 4 hours. For some satellites there are plaes where there an be obstrutions or multipath that ause a large variation in the values of /o versus some
1 International Symposium on GPS/GSS Otober 6-8, 1. partiular elevations. Meanwhile, some satellites an behave better than others at all elevations in a partiular observing loation. PR #3 is an example of good performane whose polynomial fit is shown in Figure 4. /o (db/hz) 45 4 35 All Fit Sat 3 4 6 8 Elevation (degrees) Figure 4. Polynomial fit of /o versus elevation. Although at some elevations the residual error from the fit an be large, the pattern of the /o values repeats losely eah day. In Figure 5 some of the natural variations in the /o values are shown on two onseutive days. The plot for Day 1 appears shifted to the left as it appears in the sample plae in the sky ~4 minutes earlier than for Day 9. These kinds of artifats shown in Figure 5 ould ause false detetions with a tehnique that looks only at the time-series of /o values [3,7]. It also highlights the large variations of /o at lose elevations due to multipath. /o (db/hz) 44 4 4 Day 1 Day 9 38 1.7 1.8 1.9.1 UT Time (s) x 1 4 Figure 5. There are variations in /o that repeat eah time the satellite groundtrak repeats. A GPS satellite appears in the same plae in the sky every sidereal day, or 4 minutes earlier in UT time. The atual repeat time is different for eah satellite and hanges over time depending on its individual orbit. Previous work has exploited this property to detet the presene of multipath [1] and to redue site-speifi multipath errors [11]. 4. /o differenes for interferene detetion The /o variations typially aused by multipath an be removed by ompensating the satellites orbit shift for the /o estimates on two onseutive days. This ompensation is performed by determining the satellite s repeat time, then subtrating this repeat time from the timestamps of the /o values of the previous day. The differene is then taken between the /o values from the urrent day and the previous after ompensation. If there is any unusual behaviour an interferer may be present. The differene of the /o ( /o) values in Figure 5 are shown in Figure 6. /o (db) 1.7 1.8 1.9.1 UT Time (s) x 1 4 Figure 6. Taking the differene of /o values removes most of the effet of multipath. 5 Statistial properties of /o measurements The /o values were alulated for 9 onseutive days of data from the USW antenna. An analysis of the probability distribution shows that the /o values follow a symmetrial zero-mean distribution with a small bias. Visually the /o values resemble a Gaussian distribution when taken in small elevation ranges (e.g. 5 ) but overall is narrower with more outliers and higher kurtosis. During the observations the weather surrounding the antenna hanged with rain and thunderstorms. As the weather flutuations inreased the variability of the statistial properties of the /o values inreased. In Figure 7 the variations in the mean are shown for the /o of a total of 8 day pairs reorded over those 9 days. The trends of the mean values look similar for different satellites. However, as shown in Figure 8, the differene in standard deviation between both satellites and day pairs is greater ompared with the mean values. mean( /o)...4 #1 #9 #1 #17 4 6 8 Day Pair Figure 7. Mean of /o for different PR#. 3
1 International Symposium on GPS/GSS Otober 6-8, 1. σ( /o).7.6.5.4 #1 #9 #1 #19 determined by estimating the noise in the surrounding windows of /o values: Y 1 Z Y deision: mean(z) > mean(y) + σ(y)? /o values.3 4 6 8 Day Pair Figure 8. Standard deviation of /o for different PR#. In Figure 9 the standard deviations of the 4th order polynomial fit used in setion 3 before ompensation and the standard deviations of the /o values for PR3 are ompared. It is lear that the /o is almost free from multipath at different elevations when ompared with using an elevation polynomial fit. This allows an interferene detetion tehnique that uses /o to work at lower elevations than that whih just looks at /o. Standard deviation.5 1.5 1.5 /o /o 1 3 4 5 6 Elevation (degrees) Figure 9. A omparison of the standard deviation versus elevation for a /o polynomial fit and /o. There are a number of other environmental fators [1] that an hange the /o values and as a result inrease the bias and standard deviation of /o values. In the observation data a storm introdued a bias as large as.6167db in the mean of the /o values. As a result the variability in weather limits the detetion performane when using /o. 6. Interferene detetion using /o Two tehniques are presented for deteting interferene using /o. 6.1 Adaptive algorithm (W) An adaptive algorithm is used to filter out any bias in the measurements aused by weather and to adapt to the inrease in variane in /o at lower elevations. This detetion tehnique works by alulating the mean in a window of /o, and omparing that to a threshold where mean(z) is the absolute value of the mean of the window of /o values under test Z, mean(y) is the absolute value of largest mean value in the surrounding windows Y1 and Y, and σ(y) is the largest sigma value in the surrounding windows. As a drop in /o ours from a W the mean in Z will inrease. If there is no hange in mean and standard deviation in the surrounding windows it is reasonable to assume that the drop is aused by a W interferer. In Figure 1 the behaviour of the detetion algorithm is shown on /o with added interferene based on () with a window size of seonds. The detetion threshold rises visibly when the drop in /o starts to enter the Y windows. Amplitude (db) 4 4 /o T Z 1.3 1.4 1.5 1.6 1.7 1.8 UT Time (s) x 1 4 Figure 1. Threshold and detetion level over typial /o drops. T is the detetion threshold and Z is the power in the test ell. To examine the detetion performane of this tehnique over 4 hours interferene was added using (3) on a set of observation data and the /o values alulated for all satellites. In the simulations, an integration time of 1ms was used with a W having frequeny offset +.5MHz from L1 and different interferene power levels. The number of false alarms is alulated by omparing with the estimated /o of a data set with no added interferene. In Figure 11 the theoretial drops in /o over a 1.6 hour period of time are shown for all the visible satellites. The size and width of the dips are different as different /A spetral lines are being rossed and the rate of hange for the Doppler frequenies of the satellites are different. It an be seen that at any given time, it is atually highly likely that at least one satellite is affeted, despite effets to individual satellites being relatively infrequent. The longer the /o values are observed the greater the hane of the W rossing a strong /A line and the greater the probability of detetion from a large drop. 4
1 International Symposium on GPS/GSS Otober 6-8, 1. /o (db) 4 6 3.7 3.8 3.9 4 4.1 4. UT Time (s) x 1 4 Figure 11. Drops in /o from multiple visible satellites overlaid for a W (J/ of - db, f i =.5MHz). The results for data without and with additive interferene are shown in Figure 1 over 4 hours using different detetion window sizes. The detetion windows are slid over the data in 1 seond steps. For small window sizes (less than 5 seonds) false alarms are high but start to fall off sharply as the window length is inreased. At a window size of 1s the number of false alarms without interferene reahes zero. As the interferene power inreases the number of hits inreases rapidly. Hits 1 1 8 6 4 J/S: 1dB J/S: 5dB o Int. 4 6 8 Window size (s) Figure 1. Detetion hits over 4 hours at USW between days 3 and 4 in 1 for different interferer powers. A window size of s is a good hoie regardless of the interferene power. At this window size the maximum number of false alarms from all of the observation data /o pairs over 4 hours was 14. Taking a 6dB threshold (56 hits) the minimum detetable interferene power over 4 hours equates to a J/ of -5dB, or -135dBm assuming nominal GPS reeiver operating onditions. 6. Absolute mean hange algorithm (WB) As disussed in setion 5, if there is a large variation in weather onditions from day to day there an be a bias in the /o that looks similar to the effet of a weak wideband interferer. To determine the maximum drop over the observation period due to weather hange, the mean was measured for all the /o measurements using 1 seond windows and the values reorded for eah satellite over time. The greatest mean hange was deteted for PR1. An elevation utoff of was used as the hange in mean behaves erratially at lower elevations as multipath dominates. The greatest mean hange deteted was.6167db. In Figure 13 the behaviour of the mean in the 1s window is shown, where a large inrease was aused by the onset of stormy weather. /o (db).8.6.4. ormal Maximum 3.5 4 4.5 5 UT Time (s) x 1 4 Figure 13. The magnitude of the greatest drop in /o and typial days. To detet wideband interferene the maximum hange in mean will need to be estimated or stored, requiring a database of previous observation data. One a reasonable value is found for the maximum hanges in mean that should be expeted, interferene an be deteted using this value as a detetion threshold: Z deision: mean(z) > mean(o)? /o values where is the window length in seonds, mean(z) is the absolute value of the mean of the window of /o values under test in window Z and mean(o) is the absolute value of the maximum mean in all of the previously reorded observation data. Using the times or 3dB detetion threshold for the 1s windows this tehnique an detet interferene that reates a drop in /o of greater than 1.33dB. Using () this is equivalent to wideband interferene with a J/ of -4.836dB, or -115.436dBm assuming nominal GPS operating onditions. 7. Experiment results at USW. To test the detetion tehniques with real interferene a signal generator was used to generate a W at Hz frequeny offset from L1 and a ombiner was used to ombine it with the real satellite signals from the same antenna that was used to ollet the observation data. To determine the J/ and alibrate the power measurements a weak W was used that did not hange the AG value of the reeiver but reated a notieable peak in the power spetrum of the IF samples taken by the ordav. The power was estimated from the IF samples with and without the added W. The ordav has a 3dB bandwidth of 5
1 International Symposium on GPS/GSS Otober 6-8, 1..7MHz resulting in an effetive noise floor of - 19.69dBm. The /o values estimated by the ordav software behaved somewhat differently to what was expeted by (). The ause of this is still be to investigated but it is most likely aused by the ordav's partiular /o estimation algorithm or some hardware effet in the frontend. There was also a notieable onstant drop in the /o values in the presene of the stronger W interferene levels aused by attenuation of the inoming signal by the AG. In any ase there were still detetable drops in /o when interferene was added but it was not as sensitive as expeted. The detetion results for different W powers injeted at L1 (no offset) are shown in Table 1. Using /o W interferene ould be deteted down to -16.437dBm, or a J/ of -16.751 db. At the higher power levels (- 116.437dBm and above) the /o values were biased enough that the mean hange algorithm was able to detet the interferene immediately in all the detetion windows. For the weakest interferene power, -131.437dBm, neither the W nor WB tehnique had any detetion hits. To 'simulate' the effet of a wideband interferene soure a 3dB RF attenuator was plaed in-line with the antenna input. A drop of 1.73dB was visible in the results whih differed from the expeted 3dB, but still reated a notieable and detetable drop. This value 1.73dB was.8 times larger than the maximum /o value.6167db that was found using the 1s detetion window on the 7 days of observation data. Table 1. Jammer Experiment Results Jammer Power Time (s) Tehnique (dbm) J/ (db) W Hits WB Hits -111.437-1.751 6,94 1,53,447^ -116.437-6.751 9,488 1,59 3,59^ -11.437-11.751 6,667 533 5,391-16.437-16.751 5,55 1 617-131.437-1.751 7,31-19.69*,815 11,466^ * wideband test with 3dB attenuator. ^ hits in all windows. The W and WB detetion algorithms behave differently depending on the type and power of the interferene. To haraterise an interferene soure as W the W detetion algorithm will have detetion hits. To haraterise an interferene soure as wideband the WB tehnique should have hits in all detetion windows and the W tehnique should have no hits. onlusion A detetion tehnique is presented that is able to detet and haraterise W interferene down to a J/ of -1.751dB and WB interferene down to a J/ of -4.836dB. It only requires time stamped /o values without using IF samples. The major limiting fator on detetion performane was found to be effets of surrounding weather onditions. This tehnique has appliations for deteting interferene using existing GPS infrastruture without the need for modifying the existing hardware. Aknowledgements This researh is supported by the Australian Researh ounil (AR) grant through its Linkage Sheme (LP88191). Many thanks given to Peter Mumford for his support with the hardware used in the experimentation. Referenes [1] F. Bastide,. Maabiau, DM Akos Automati Gain ontrol (AG) as an Interferene Assessment Tool, IO GPS/GSS, (3) [] A.T. Balaei, Statistial inferene tehnique in preorrelation interferene detetion in GPS reeivers, Proeedings of IO GSS, 6, pp. 6 9. [3] L.M. Marti, Global Positioning System Interferene and Satellite Anomalous Event Monitor, Ohio University, 4. [4] A. dili and P. Enge, GPS reeiver autonomous interferene detetion, in Position Loation and avigation Symposium, 1998, 13 13. [5] M. Luo, G. Xie, D. Akos, S. Pullen, and P. Enge, Radio Frequeny Interferene Validation Testing for LAAS using the Stanford Integrity Monitor Testbed, Proeedings of the Institute of avigation ational Tehnial Meeting, Anaheim, A, 3. [6] P. J Mumford, A. T Balaei, and A. G Dempster, USW GSS Interferene Detetion Devie Proeedings of IGSS (7). [7] O. Isoz, Detetion and loalization of wide band interferene in the global positioning system L1 band using ross orrelation tehniques (9). [8] M.S. Sharawi, D.M. Akos, and D.. Aloi, GPS /o estimation in the presene of interferene and limited quantization levels, IEEE Transations on Aerospae and Eletroni Systems, vol. 43, 7, pp. 7 38. [9] A. T Balaei, A. G Dempster, and J. Barnes, A ovel Approah in Detetion and haraterization of W Interferene of GPS Signal Using Reeiver Estimation of /o, in in PLAS, 6. [1] Linlin Ge, Shaowei Han, and hris Rizos, Multipath Mitigation of ontinuous GPS Measurements Using an Adaptive Filter, GPS Solutions 4, no. (1, ): 19-3. [11] P. Axelrad, K. Larson, and B. Jones, Use of the orret satellite repeat period to haraterize and redue site-speifi multipath errors, Proeedings of the 18th International Tehnial Meeting of the Satellite Division of the Institute of avigation. [1] K. M Larson et al., Using GPS multipath to measure soil moisture flutuations: Initial results, GPS solutions 1, no. 3 (8): 173 177. 6