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A Novel Approach in Detection and Characterization of CW Interference of GPS Signal Using Receiver Estimation of C/No A. T. Balaei, A.G. Dempster and J. Barnes Cooperative Research Centre for Spatial Information System (CRC-SI) and the School of Surveying and Spatial Information Systems at The University of New South Wales ABSTRACT Narrowband interference can these primary modules, it can also affect the severely degrade the performance of GPS carrier and code tracking loops [3, 4] which receivers. Different narrowband interference results in deterioration of all the GPS suppression techniques also degrade this observables or in complete loss of lock in severe performance correspondingly. Detecting the cases. Continuous-wave (CW) interference has presence of interference and then characterizing been proved to have effects for the GPS C/A it can lead to its removal. Knowledge that can be code signal [5], related to the characteristics of useful is: the location of the source or direction the frequency spectrum of this signal. To make of arrival of the interference (spatial-directional the receiver less vulnerable to interference or to characteristics), the time specification and monitor the quality of the signal, interference can frequency and power of the interference be detected in the receiver either before the (temporal-spectral characteristics). Our focus in correlation (pre-correlation detection) or after it this work is on the second type. Using the post- (post-correlation detection). In the pre- processing capability of a software GPS receiver, correlation techniques, antenna [6], AGC [2] and CW interference is detected and characterized. ADC [7] have been used to detect and This is achieved by passing the GPS signal and characterize the RFI and in post-correlation the interference through the correlator. After techniques, the observables of the receiver that correlation, using the definition of carrier to are affected by RFI have usually been used to noise density ratio (C/No), a mathematical detect and characterize the interference. In [8], expression for C/No is given in which the using a statistical approach, it has been shown temporal and spectral parameters of interference that correlator output power shows consistent are found. Then, using the conventional performance under varying levels as well as definition of C/No as the squared mean of the types of interference and carrier phase vacillation correlator output divided by its variance, the was used as a backup indicator. In [9], correlator actual C/No is calculated. Finally comparing output is calculated in a multi-correlator receiver these two values and considering the structure of to estimate the frequency of the CW RFI. In this the GPS C/A code, the existence and paper, AGC level together with correlator output characterization of the existing interference is power is used to detect and characterize the RFI determined. but the difference is that the receiver simply uses a standard correlator. The other advantage of this KEYWORDS: GPS, Interference, C/No, algorithm is that it is capable of detecting and Correlator characterizing CW RFI even it is not close in frequency to any of the C/A code spectral lines. I. INTRODUCTION The problem is described in section 2. In Radio frequency interference (RFI) is amongst section 3, a mathematical expression for the the most disruptive events in the operation of a C/No which is introduced. We derived it in [4], GPS receiver. It affects the operation of the using the spectral analysis of both the signal and automatic gain control (AGC) and low noise the RFI in passing through the correlator. The amplifier (LNA) in the RF front-end [1, 2] and actual C/No is calculated using received I and Q depending on how much of it passes through data from a software GPS receiver in section 4. Hardware setup for the experiments is presented in section 5. The spectral parameters of RFI are compared in section 6 and finally section 7 concludes this paper. Figure 4 Tracking loop low pass filter, filters the II. PROBLEM DEFINITION data and the interference which is outside the The GPS C/A code is a Gold code with a filter bandwidth out relatively short 1-ms period (i.e., the PRN sequence repeats every 1 ms). Therefore, the C/A As the code is despread by getting multiplied code (neglecting the navigation data) has a line by the replica code in the receiver, interference is spectrum with lines 1 kHz apart [1]. In Figure 1, spread over the frequency bandwidth of the navigation data is incorporated into the code and original signal in the same way that the data is in Figure 2, noise and interference is added to the spread in the satellite (Figure 3). A low pass signal to achieve the final GPS signal received at filter is used in the tracking loops. Only the antenna. interference that is within the bandwidth of this filter remains (Figure 4). In figure 2, the amplitude of the interference is shown to be Jbefore. In passing the RFI through the correlator and the filter, the value of the amplitude of the remaining interfering signal is Jafter. The aim of Figure 1 Spreading the data over the C/A code these figures is to show that the value Jafter is spectrum bandwidth determined by the strength of the nearest line to the interference (in this example jth line). Now if we have an RFI with fixed frequency and a GPS signal with Doppler frequency that varies with satellite motion, over time the RFI coincides with several different consecutive lines in the spectrum. Each of these lines has its own unique Figure 2 Interference and background noise is effect on the remaining interference in the output added to make the final GPS signal received at of the loops. We propose to examine the effect of the antenna a series of lines, and thus calculate the frequency of the RFI. The quantity that can best reflect this Interference is assumed to be CW constant effect is the correlator output power. Carrier to noise ratio which quantifies the quality of the amplitude (CA) [10] at the frequency f i away signal is the parameter which is used for this from the band center of the code spectrum and purpose. In the following section, this value is ∆f i away from the nearest Dirac line in the parametrically drawn in the presence of CW RFI. spectrum. This line is also assumed to be the jth line of the spectrum. In other words this line is j kHz away from the band center as all the lines III. C/NO MATHEMATICAL are 1 kHz apart from each other. Now in the EXPRESSION base-band processing of the GPS receiver, this signal is passed through the correlator to be Many receivers report received signal quality acquired and tracked. In the correlator, first the as carrier-power-to-noise density ratio, denoted code is despread (Figure 3) and then the C/No. In [3], this receiver report has been navigation data is filtered out (Figure 4). characterized in terms of front-end bandwidth, discriminator design and other receiver characteristics. This characterization is done using the conventional definition of C/No as the squared mean of the correlator output divided by its variance. In [4], as a special case, assuming the interference to be CW constant amplitude Figure 3 Code is despread by getting multiplied signal, another expression is derived for C/No by the receiver code replica and interference is which is consistent with the previous results and spread in which spectral parameters of the RFI can be found. Figure 5 shows the final result of Figure 2 spectrum. The other point which is noticeable in as the received GPS signal passing through the this graph is the sinc functions occurring around correlator. each trough. The width of each sinc function is C/A Code Tracking Loop related to the integration period, as can be seen in the Eq. 1. The longer the integration period is S+N+I the narrower will be the sinc functions. ADC 46 prompt 44 Carrier Tracking Loop 42 CW I Output LPF 40 C/No (dBm) 38 OSC LPF 90 arctan 36 Q 34 LPF 32 Figure 5 Correlator (Code and Carrier Tracking Loops) 30 28 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 Eq. 1 shows the mathematical expression for the Doppler Frequency C/No in the output of the correlator. Figure 6 C/N0 calculated using the mathematical expression for satellite 1 with Doppler frequency changing from 0 kHz to 10 (Td R0 (τ ).sinc(∆f cTd )) 2 C/No = Eq. 1 kHz and CW interference at 14 kHz away from Ln N 0 + J (Td .C j .sinc(Td .∆f i )) 2 the band center at 1.57542 GHz. where: N0 is the thermal noise power IV. ACTUAL CALCULATION OF C/No Ln is the processing gain in the noise There are different techniques for estimating Td is the integration duration time the carrier power to noise density, C/N0. This τ is the signal-reference code phase estimate is important because it helps determine difference in code chips whether the code and carrier tracking loops are ˆ in lock, controlling the response of the receiver f c is the estimate of carrier frequency. to low signal to noise environments, and ∆f = f - fˆ c c c determining the signal to noise environment in order to assess or predict receiver performance. ∆f i = f i - f c In [11] is a review of different measurement J is the interference power techniques such as the narrow-to-wideband Cj is the jth spectral line coefficient power ratio method, correlator comparison R0 (τ ) is the cross correlation of the method, unnormalized discriminator output statistics method. It was found in [11] that the received C/A code and the receiver replica of the power ratio method performs better in terms of same code. noise in low signal-to-noise environments. This In Figure 6, as an example, using Eq. 1 and method is widely known and in [12], is assuming a specific environmental noise power, presented. The prompt I and Q samples over the the C/No is drawn for satellite 1 with Doppler accumulation intervalτ , are divided into M frequency changing from 0 kHz to 10 kHz and intervals. These samples are then used to CW interference at 100 kHz away from the band calculate a narrowband power, PN, over the center at 1.57542 GHz. whole accumulation interval and a wideband The deep troughs in this graph correspond to power, PW, over the interval τ / M , then the coincidence of CW RFI with the code spectral lines. It is clear from the picture that this summed over τ . These power estimates are: happens at 1 kHz spacing in the Doppler. As M M expected and explained in the previous section, there are different values for different lines. This PN = (∑ I Pi ) 2 + (∑ QPi ) 2 Eq. 2 i i difference comes from the difference between the coefficients of different lines in the code M PW = (∑ ( I Pi + QPi )) 2 2 Eq. 3 i where I Pi = 2(c / n0 )τ / M cos ϕ + wIPi and Q Pi = 2(c / n0 )τ / M sin ϕ + wQPi where wIP and wQP are normalized random noise samples from a zero mean unit variance normal (Gaussian) distribution. The narrow-to-wide power ratio, PN/W, is simply the ratio of the two power measurements. However, to reduce the noise, the measurement is averaged over n iterations. Thus, Figure 7 Hardware setup for the experiments 1 k PN ,r PN / M = ∑ k r =1 PW ,r Eq. 4 from left: Spectrum analyzer, RF signal generator, NordNav front-end, GPS signal In [12], it is shown how to derive Eq. generator. 5 from the above equations. M (c / n0τ + 1) 55 E ( PN / W ) ≈ Eq. 5 M + c / n0τ 50 where E() is the expectation operator. Actual C/No (dBm) 45 Rearranging this gives the measured carrier 40 power-to-noise density as a function of the power ratio measurement: 35 M PN / W − 1 c / n0 ≈ Eq. 6 30 τ M − PN / W 25 0 200 400 600 800 1000 1200 1400 1600 1800 Time (Sec) In [12], it is shown graphically that for Figure 8 C/N0 calculated using the power ratio c/n0>23dB there is less than 1dB estimation error technique for satellite 1 with Doppler frequency for an average time of 1 s (M=20 and k=50). For changing from 0 to 9 kHz and CW interference smaller values of C/N0 longer averaging time is at 14 kHz away from the band center at 1.57542 required. GHz V. HARDWARE SETUP FOR There are a number of points regarding this EXPERIMENTAL RESULTS figure which is addressed as follows. At the In Figure 7, the hardware setup to measure the beginning, it has a large value (50 dB), which is actual C/N0 is shown. The NordNav [13] just an initial condition and gradually converges software receiver is used to capture the IF data to to the real actual value of C/N0. Figure 9 and be analyzed and post processed and an HP8648B Figure 10 are focused on one of the troughs is used to generate the CW interference which is where RFI coincides with a code spectral line in combined with the GPS signal generated by a the actual measurement and theoretical SPIRENT GSS6560. In Figure 8, the actual calculation of C/N0 correspondingly. The first measured C/N0 using NordNav software GPS noticeable thing is the small peak in the actual receiver is illustrated. estimation right in the middle of the trough of the sinc function. We have explained this phenomenon in [4] using a DataFusion [14] software receiver. Around this peak, as interference frequency is very close to the carrier frequency, it is actually helping the C/N0. This phenomenon can not be helpful in our proposed detection-characterization method. By choosing appropriate M and K in the power ratio estimation of C/N0 ( Eq. 5), this peak can be averaged out. The only disadvantage of 45 choosing K and M such that the peak is averaged 40 out is that some of the information in the C/N0 is Actual C/No (dBm) also lost. This information fortunately is not 35 essential to this method as it is only the peak information of the lines which are used. 30 50 25 48 46 44 20 c a /N B ) A tu l C o (d m 42 40 100 200 300 400 500 600 700 800 900 1000 38 Time (sec) 36 34 32 Figure 11 C/N0 calculated using the power ratio technique for satellite 1 with Doppler frequency 30 1300 1350 1400 1450 1500 changing from -4 kHz to 4 kHz and CW Time (s econd) interference at the band center at 1.57542 GHz Figure 9 One of the troughs where RFI coincides with a code spectral line in the actual In Figure 12, C/N0 plots calculated using both measurement the parametric method and power ratio method, 45 are drawn together. The fact that for both techniques, the relationship between the values 40 of any two consecutive peaks remains the same /N B ) C o(d m is illustrated. In Figure 12, in fact we have 35 Figure 6 and Figure 8 which is flipped over and the horizontal axis is converted to Doppler 30 frequency. The reason why it should be flopped 1400 1600 1800 2000 2200 Doppler Frequency (Hz) 2400 2600 over is that in an increasing Doppler frequency scenario, as interference cross the spectral lines, Figure 10 One of the troughs where the last line which is crossed is in fact the lowest RFI coincides with a code spectral line in the frequency spectral line. theoretical measurement 55 Actual C/No Theroretical C/No VI. CHARACTERIZING THE CW RFI 50 The results in sections IV and III are compared 45 in this section to extract the spectral information C/No (dBm) of the CW RFI. As it was explained the peak 40 information is used. The fact that each C/A code has a unique pattern of spectrum is used in this 35 comparison. In Figure 11, the uniqueness of this pattern is shown. The Doppler frequency of 30 satellite 1 is changed from -4 kHz to 4 kHz. And 25 the CW RFI is placed right in the band center. 0 1000 2000 3000 4000 5000 Doppler Frequency (Hz) 6000 7000 8000 The symmetry in the picture can clearly show that coincidence of interference with a line Figure 12 C/N0 plots calculated using both the always results in the same pattern of C/N0. parametric method and power ratio method In Figure 13, the difference between the estimated C/N0 and the theoretically calculated one is shown. This difference has been calculated in the first 1000 kHz from the band center. This is in fact the difference between the 8 trough values of Figure 8 and the values of 8 troughs of an “8 trough searching window” over the whole bandwidth of the C/A code spectrum. In this work, the power of interference is The minimum difference in this experiment is at estimated by the value of AGC, which works 14 kHz which is the frequency at which the RFI relative to the background noise power. So if the is added to the GPS signal. For clear vision, only difference between the assumed and real the first 80 kHz is shown. In the Eq. 1, other than background noise power very different, then the the frequency of interference, there are three result of this technique wouldn’t be reliable. To other parameters: Signal power, background address this problem, we can calculate the noise power and interference power. The first environmental noise power by acquiring one of two are known parameters as the GPS signal has the satellite signals. been generated in a known environment condition and with a desired signal power. REFERENCES Interference power is also estimated from the AGC level in the RF front-end [2]. Using this [1] Kaplan E., (1996) “Understanding value we can have the theoretical C/No fit the GPS: Principles and Applications”, Artech actual C/No as much as possible. This will allow House. us having less difference between the two calculations at the frequency where interference [2] F. Bastide, C. Macabiau, exists (14 kHz in this experiment). DM Akos “Automatic Gain Control (AGC) as an Interference Assessment Tool”, ION GPS/GNSS, (2003) Difference between values of 8 consecuitive troughs (dBm) 100 90 [3] J. W. Betz, “Effect of Partial-Band 80 Interference on Receiver Estimation of C/N0: 70 Theory,” Proceedings of ION 2001 National 60 Technical Meeting, Institute of Navigation, 50 January 2001. 40 30 [4] A. T. Balaei, J. Barnes, A. G. Dempster, 20 “Characterization of interference effects on GPS 10 signal carrier phase error” SSC (2005) 0 10 20 30 40 50 60 70 Frequency (kHz) [5] Spilker J. and Natali F., (1996) “Interference Effects and Mitigation Figure 13 The difference between the Techniques”, Chapter 20 of ‘Global Positioning estimated C/N0 and the theoretically calculated System: Theory and Applications’, AIAA. one [6] Alison Brown, Sheryl Atterberg, and VII. SUMMARY AND FUTURE WORK Neil Gerein, NAVSYS Corporation, “Detection In this paper, a new technique for detection And Location Of GPS Interference Sources and characterization of CW interference was Using Digital Receiver Electronics” Proceedings presented. The advantage of this technique is that of ION Annual Meeting, June 2000, San Diego, it uses the post processing capability of GPS CA software receivers without any need for extra hardware. To do this, the spectral effect of CW [7] Frank Amoroso, “ Adaptive A/D RFI on the C/A code is analyzed and an Converter to Suppress CW Interference in DSPN expression for C/N0 as a measure of this effect is Speard-Spectrum Communications” IEEE parametrically drawn. Also using a technique for Transaction on Communications, October 1983 C/N0 estimation, and the post processing capability of a software GPS receiver, carrier [8] Awele Ndili, Per Enge, “GPS Receiver power to noise ratio was calculated for a piece of Autonomous Interference Detection” Presented pre-generated data containing a GPS signal of at the 1998 IEEE Position, Location and one particular satellite with a wide range of Navigation Symposium - PLANS ‘98 variation in Doppler frequency. Finally comparing these two values, the frequency of the [9] F. Bastide, E. Chatre, STNA, France; C. CW interference is calculated. Macabiau, ENAC, France “Gps Interference Detection And Identification Using Multicorrelator Receivers” ION GPS 2001 [10] Moelker D., “Interference in satellite navigation and mobile communication” Delft University Press, Mekelweg 4, 2628 CD Delft, Netherlands, 1998 [11] Paul D. Groves, “ GPS Signal-to-Noise Measurement in Weak Signal and High- Interference Environments” Journal of the Institute of Navigation. Vol. 52, No. 2, Summer 2005 [12] Van Dierendonck, A. J., GPS receivers, In: B. W. Parkinson and J. J. Spilker (Eds.), Global Positioning System: Theory and applications, Volume I, AIAA, Washington, DC, 1996, pp. 329-408 [13] DataFusion Corporation (2004) Kent Krumvieda, GPS Program Manager Data Fusion Corporation 10190 Bannock Street Suite 246 Northglenn, CO 80260 USA Tel: 720-872-2145 X303 Fax: 720-872-6418 [14] NordNav Technologies AB, Stadsgården 10, S-116 45 Stockholm, Sweden, Phone: +46 8 390 000, Fax +46 8 412 47 40

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