gps anti-jam evaluation testbed (gajet) development at...

GPS Anti-Jam Evaluation Testbed (GAJET) Development at Trial Petawawa 09-2

Scott McLelland, Michael Cowell, Michel Clénet, Mathieu Caillet, Ambighairajah Yasotharan

Defence R&D Canada – Ottawa

Technical Report DRDC Ottawa TR 2010-118

June 2010


Scott McLelland, Michael Cowell, Michel Clénet, Mathieu Caillet, Ambighairajah Yasotharan

Defence R&D Canada – Ottawa

Technical Report DRDC Ottawa TR 2010-118 June 2010

Principal Author

Original signed by Scott McLelland

Scott McLelland

Research Engineer

Approved by

Original signed by Bill Katsube

Bill Katsube

Section Head, CNEW

Approved for release by

Original signed by Brian Eatock

Brian Eatock

Chief Scientist

© Her Majesty the Queen in Right of Canada, as represented by the Minister of National Defence, 2010

© Sa Majesté la Reine (en droit du Canada), telle que représentée par le ministre de la Défense nationale, 2010

DRDC Ottawa TR 2010-118 i

Abstract ……..

GPS jamming experiments were performed at Trial Petawawa 09-2 to develop a prototype GPS Anti-Jam Evaluation Testbed (GAJET). GAJET is a customizable Controlled Reception Pattern Antenna (CRPA) used for developing electronic protection (EP) systems for GPS receivers. Various trial scenarios were performed at CFB Petawawa with up to 3 deployable jammer units (DJUs). Field trial results demonstrate that the prototype GAJET is capable of spatially nulling up to 2 simultaneous jammers using digital beamforming. The beamforming results were verified by using a GPS software receiver to calculate an accurate position using the L1 C/A-code while in a jamming environment. GAJET will be used at future field trials to support GPS electronic protection R&D efforts targeted at minimizing the vulnerability of Canadian Forces GPS assets to electronic attack (EA).

Résumé ….....

Des expériences sur le brouillage intentionnel des signaux GPS ont eu lieu lors de la campagne de mesures sur le terrain Petawawa 09-2 en vue de la mise au point d'un prototype de banc d’essai de systèmes anti-brouilleur GPS (GAJET). GAJET est un système de mise au point de systèmes de protection électronique (EP) pour les récepteurs GPS basé sur des systèmes d'antennes à diagramme de réception contrôlé (CRPA). Divers scénarios ont été testés à la BFC Petawawa à l'aide d'au plus trois unités de brouillage déployables (DJU). Les résultats de l'essai sur le terrain montrent que le prototype GAJET peut contrecarrer les effets d'au plus deux brouilleurs fonctionnant simultanément à l'aide de la formation de faisceau numérique. Les résultats issus de formation de faisceau ont été vérifiés à l'aide d'un récepteur GPS logiciel calculant une position précise à partir du code C/A L1 dans des conditions de brouillage intentionnel. GAJET servira à l'occasion de campagne de mesures ultérieures à appuyer les efforts de R et D sur la protection électronique des signaux GPS afin de réduire la vulnérabilité des équipements GPS des Forces canadiennes à l'attaque électronique (EA).

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DRDC Ottawa TR 2010-118 iii

Executive summary


S. McLelland; M. Cowell; M. Clénet; M. Caillet; A. Yasotharan; DRDC Ottawa TR 2010-118; Defence R&D Canada – Ottawa; June 2010.

Introduction: The GPS Anti-Jam Evaluation Testbed (GAJET) was developed at field trial Petawawa 09-2 held at CFB Petawawa in the fall of 2009. GAJET is a modular combination of a controlled reception pattern antenna (CRPA) and a GPS receiver where each sub-system can be independently investigated with respect to improving resistance to GPS jamming.

Results: The development of the prototype GAJET was completed at Trial Petawawa 09-2. This included the design and testing of a customized RF front-end, the integration of a data acquisition system, the development of digital beamforming algorithms, and GPS position determination using a GPS software receiver. Jamming trials were performed using up to 3 deployable jamming units (DJUs) transmitting both CW and broadband noise jamming signals at 1575.42 MHz (L1). The key results from the field trial were:

GAJET was tested against single threats and could remove a CW jammer at all of the tested transmit power levels but could remove broadband noise jammers only at low to medium power levels;

GAJET was tested against two simultaneous CW jammers and could remove the jamming when the two jammers were located close together; however, when the jammers were located far apart only low power jammers could be removed; and

GAJET was tested against three simultaneous CW jammers and these could not be removed.

For the two digital beamforming algorithms tested (“power inversion” and “auxiliary element power minimization”), successfully removing jammers was dependent on the type of algorithm and the reference antenna used.

Significance: The prototype GAJET is the first realization of a CRPA testbed at DRDC and GAJET can now be used at future field trials for GPS electronic protection (EP) R&D efforts. GAJET results at Trial Petawawa 09-2 are also the first successful in-house demonstration of digital beamforming to protect a GPS receiver. These trial results can now be used to analyze and improve the digital beamforming algorithms. Finally, the GAJET trial results are the first successful in-house use of the GSNRx GPS software receiver.

Future plans: GAJET will be used at future field trials to support GPS EP research and development efforts. GAJET currently supports research and development with L1 C/A-code GPS (2MHz BW) and in the future GAJET will be modified for L1/L2 P(Y)-code GPS (20 MHz BW). This will require widening the bandwidth of both the RF front-end and the digital beamforming algorithms, modifying the RF front-end for dual-band operation at L1 and L2, and adding playback capability of beamformed data files to a keyed military receiver (DAGR).

iv DRDC Ottawa TR 2010-118


DRDC Ottawa TR 2010-118 v

Sommaire .....


S. McLelland; M. Cowell; M. Clénet; M. Caillet; A. Yasotharan; DRDC Ottawa TR 2010-118; R & D pour la défense Canada – Ottawa; Juin 2010.

Introduction: Un prototype de banc d'essai pour l'évaluation de systèmes d’antibrouillage des signaux GPS (GAJET) a été mis au point lors de la campagne de mesures sur le terrain Petawawa 09-2, tenu à la BFC Petawawa à l'automne 2009. GAJET est une combinaison modulaire d'un système d'antennes à diagramme de réception contrôlé (CRPA) et d'un récepteur GPS dont chaque sous-système peut être étudié indépendamment en vue d'une amélioration de la résistance au brouillage intentionnel des signaux GPS.

Résultats: La mise au point de GAJET a été complétée lors de l'essai Petawawa 09-2. Ceci comprenait la conception et la mise au point d'un étage d'entrée RF personnalisé, l'intégration d'un système d'acquisition de données, la mise au point d'algorithmes de formation de faisceau numérique et la détermination de la position par un récepteur GPS logiciel. Des essais de brouillage intentionnel ont été menés à l'aide d'au plus trois unités de brouillage déployables (DJU) qui émettaient des signaux à ondes porteuses ou de bruit large bande à 1575,42 MHz (L1). Voici les principaux résultats des essais:

GAJET a été testé par rapport à des menaces simples et a réussi à supprimer un dispositif de brouillage à ondes porteuses de faible à forte puissance d'émission, mais n'a réussi à supprimer les dispositifs de brouillage à bruit large bande que pour des signaux de puissance faible à moyenne;

GAJET a été testé par rapport à deux dispositifs de brouillage à ondes porteuses agissant simultanément et a réussi à supprimer le brouillage intentionnel lorsque les deux dispositifs étaient situés à proximité; cependant, lorsque les dispositifs étaient séparés, seuls les dispositifs de brouillage de faible puissance pouvaient être supprimés;

GAJET a été testé par rapport à trois dispositifs de brouillage à ondes porteuses agissant simultanément, qui n'ont pas pu être supprimés.

Quant aux deux algorithmes de formation de faisceau testés (« inversion de puissance » et « minimisation de la puissance par éléments auxiliaires »), la réussite de la suppression des dispositifs de brouillage dépendait du type d'algorithme et de l'antenne de référence utilisée.

Importance: GAJET est la première réalisation d'un banc d'essai de CRPA à RDDC, et il peut maintenant servir durant les campagnes de mesures ultérieures sur le terrain dans le cadre des efforts de R et D sur la protection électronique des signaux GPS. Les résultats obtenus avec GAJET lors de l'exercice Petawawa 09-2 dénotent aussi la première démonstration interne réussie de la formation de faisceau numérique en vue de la protection de récepteurs GPS. Ils peuvent maintenant servir à l'analyse et à l'amélioration des algorithmes de formation de faisceau numérique. Enfin, les résultats obtenus avec GAJET lors de l'essai dénotent la première utilisation interne réussie du récepteur GPS logiciel GSNRx.

vi DRDC Ottawa TR 2010-118

Perspectives: GAJET servira à l'occasion de campagnes de mesures ultérieures sur le terrain à l'appui des efforts de R et D pour la protection électronique des signaux GPS. À l'heure actuelle, il vient à l'appui de la R et D sur les signaux GPS transmis à la fréquence L1 à l’aide du code C/A (largeur de bande de 2 MHz) et, à l'avenir, il sera modifié pour les signaux GPS transmis aux fréquence L1 et L2 à l'aide du code P/Y (largeur de bande de 20 MHz). Cela veut dire qu'il faudra élargir la bande passante de l'étage d'entrée RF et des algorithmes de formation de faisceau numérique, modifier l'étage d'entrée RF en vue d'un fonctionnement dans les deux bandes L1 et L2, puis ajouter une capacité de lecture/écriture des fichiers de données de formation de faisceau pour tester le système à l’aide d'un récepteur militaire à clés (DAGR).

DRDC Ottawa TR 2010-118 vii

Table of contents

Abstract …….. ................................................................................................................................. iRésumé …..... ................................................................................................................................... iExecutive summary ....................................................................................................................... iiiSommaire ........................................................................................................................................ vTable of contents .......................................................................................................................... viiList of figures .............................................................................................................................. viiiList of tables .................................................................................................................................. ixAcknowledgements ....................................................................................................................... xi1 Introduction............................................................................................................................... 12 System Development ................................................................................................................ 3

2.1 The RF Front-End.......................................................................................................... 32.2 Digital Beamforming..................................................................................................... 52.3 GPS Software Receiver ................................................................................................. 6

3 Field Testing at CFB Petawawa................................................................................................ 83.1 Test Setup ...................................................................................................................... 83.2 Test results................................................................................................................... 103.3 Analysis of the Trial Results ....................................................................................... 14

3.3.1 Jammer Removal........................................................................................... 143.3.2 Choice of Beamforming Reference Channel ................................................ 163.3.3 The GPS Software Receiver.......................................................................... 173.3.4 Intermediate Frequency Offset...................................................................... 18

4 Conclusions and Future Work ................................................................................................ 19References ..... ............................................................................................................................... 20

viii DRDC Ottawa TR 2010-118

List of figures

Figure 1: GAJET system-level block diagram ................................................................................ 1

Figure 2: Experimental RF front-end from Trial Petawawa 09-1 held in May 2009 (left) and the revised RF front-end used at Trial Petawawa 09-2 held in Oct 2009 (right) .......... 3

Figure 3: RF Front-end developed at Trial Petawawa 09-2 ............................................................ 3

Figure 4: Field Trial Analysis Tool (FiTAT) user interface in Matlab ........................................... 5

Figure 5: Four-element quadrifilar helix antenna array used for GAJET testing during the trial ... 5

Figure 6: GSNRx screen capture with position at CFB Petawawa ................................................. 6

Figure 7: Trial Petawawa 09-2 test set-up, inside and outside the rental truck, at CFB Petawawa....................................................................................................................... 8

Figure 8: Deployable Jammer Unit (DJU) at CFB Petawawa......................................................... 9

Figure 9: Deployable Jammer Unit (DJU) test set-up, relative to GAJET, at Trial Petawawa 09-2 ............................................................................................................................... 9

Figure 10: GSNRx output from three data captures taken at the same location during the trial: (a) baseline capture before jamming started, (b) capture during jamming without jammer removal processing and (c) capture during jamming with jammer removal processing.................................................................................................................... 10

Figure 11: Positions determined during jamming at CFB Petawawa trial 09-02 .......................... 14

Figure 12: FFT of the captured IF signal for a single deployable jammer unit scenario transmitting a CW jamming tone at <10 dBm (left) and 10 dBm (right).................... 16

DRDC Ottawa TR 2010-118 ix

List of tables

Table 1: Trial Petawawa 09-2 test results using one CW jammer for each capture ...................... 11

Table 2: Trial Petawawa 09-2 test results using two CW jammers for each capture .................... 12

Table 3: Trial Petawawa 09-2 test results using three CW jammers for each capture .................. 13

Table 4: Trial Petawawa 09-2 test results using one Additive White Gaussian Noise (AWGN) jammer for each capture.............................................................................................. 13

Table 5: Effects of reference channel choice on digital beamforming of a jammed data capture ......................................................................................................................... 17

x DRDC Ottawa TR 2010-118


DRDC Ottawa TR 2010-118 xi

Acknowledgements

The authors would like to acknowledge Mr. Jeff Bird (DRDC-Ottawa), Dr. Mark Petovello and Dr. Cillian O’Driscoll (both of the University of Calgary) for their invaluable assistance with the GPS software receiver (GSNRx).

The authors would also like to acknowledge Mr. Collin Wilson for his contribution to the early development of GAJET.

The authors would also like to acknowledge Ms. Janice Lang for the photography while on trial at CFB Petawawa.

The authors would also like to acknowledge Rene Apps, Pierre Richer and Mike Boyle for their contributions to the preparations and execution of the trial.

xii DRDC Ottawa TR 2010-118


DRDC Ottawa TR 2010-118 1

1 Introduction

The jamming resistance of a GPS receiver can be increased by using a Controlled Reception Pattern Antenna (CRPA) consisting of an antenna array that uses adaptive beamforming to steer nulls in the direction of jammers. The outputs from each array element are weighted and summed to remove the jammers while preserving the GPS signals. CRPAs using this spatial adaptive nulling technique have been successfully used to protect GPS receivers from jamming. This additional protection allows a GPS receiver to operate much closer to jammers before it is affected by the jamming.

Existing GPS CRPA systems are both costly and bulky. It is therefore desirable to reduce both the cost and size of these systems while simultaneously improving anti-jamming performance, if possible. The GPS Anti-Jam Evaluation Testbed (GAJET) was designed to enable research and development in these areas. GAJET is a CRPA where each hardware and software component can be individually modified resulting in a customizable CRPA that can be used to test improvements to any sub-system or any combinations of sub-systems.

A system-level block diagram of GAJET is shown in Figure 1. It consists of a four-element antenna array followed by a radio frequency (RF) front-end for each element that down-converts the received signals to four intermediate frequency (IF) signals. The IF signals are digitized by a 14-bit GaGe Octopus acquisition card and transferred, in the form of a single data file for the signal captured from each antenna element, to the hard drive of a computer. Each of these IF data files are used by a digital beamforming algorithm, implemented in Matlab, to remove the jammer(s) using spatial nulling by weighting and summing the individual IF data files from each antenna element. The result is a single data file that contains only the desired GPS signals without the jammers that were present when the GPS signal was received and captured. This processed data file is then used by a post-mission GPS software receiver, GSNRx, to provide a position solution.

Antenna Array

Channel 1

Channel 3

Channel 5

Channel 7

Custom RF Front-End

1575.42 MHz Digitizer +Hard Drive

GPS Software ReceiverDigital Beamforming21.4 MHz

Figure 1: GAJET system-level block diagram

Initial development work on GAJET was performed at field trial Imperial Hammer (held in Sardinia, Italy in the fall of 2008) as well as at trial Petawawa 09-1 (held at CFB Petawawa in May 2009). Trial Imperial Hammer was used to investigate three different antenna arrays whereas the trial Petawawa 09-1 was to perform initial experiments with various front-end architectures, the GaGe digitizer and the GSNRx software. Trial Petawawa 09-2, held at CFB Petawawa from

2 DRDC Ottawa TR 2010-118

Sep. 28 to Oct. 2 2009, built on the earlier trial results with a goal to develop a working GAJET prototype capable of providing a GPS position (using L1 C/A-code) in environments both with and without jamming.

This report describes the results of Trial Petawawa 09-2 that led to the development of a working GAJET prototype for L1 C/A-code GPS signals. The first section describes the development of the GAJET hardware and software components. The second section describes the field trial experiments and the resulting performance of the prototype GAJET system. The last section provides a summary of the GAJET development along with the future work planned for the testbed.


2 System Development

Prior to Trial Petawawa 09-2 the only sub-system shown in Figure 1 not requiring development was the antenna array. All of the other sub-systems required varying degrees of further development, which is described in this section.

2.1 The RF Front-End

Trial Petawawa 09-1 was used to experiment with various front-end architectures in conjunction with the GaGe digitizer; however, the RF front-end test set-up shown in Figure 2 (left) was large and somewhat unreliable due to the large number of connections. Nevertheless, the Trial Petawawa 09-1 set-up provided a large degree of flexibility to change the architecture in the field and the results from these initial experiments led to the simpler prototype RF front-end, shown in Figure 2 (right), used at Trial Petawawa 09-2.

Figure 2: Experimental RF front-end from Trial Petawawa 09-1 held in May 2009 (left) and the revised RF front-end used at Trial Petawawa 09-2 held in Oct 2009 (right)

LNA

LNA

LNA

LNA

LNA

LNA

LNA

LNA

40 MHz BPF

100 MHz BPF

100 MHz BPF

100 MHz BPF

IF Filter

IF Filter

IF Filter

IF Filter

GaGe Cardinside serverCh 7

Ch 5

Ch 3

Ch 1

RF Front End

Agilent Signal Generator LO Power Splitters

Antenna Array

Figure 3: RF Front-end developed at Trial Petawawa 09-2


The RF front-end architecture used at Trial Petawawa 09-2 is shown in Figure 3. The RF frequency of the front-end is L1 GPS at 1575.42 MHz. Two RFMD 3866LNAs provide RF gain and are followed by an image-noise filter which prevents image foldover noise from increasing the front-end noise figure (NF) by 3 dB during downconversion by the passive Mini-Circuits ZFM-2000+ mixer. The 21.4 MHz IF output of the mixer is passed through anti-aliasing filters (Mini-Circuits SBP-21.4+) and then digitized by the GaGe card IF acquisition system.

The total gain of the front-end is estimated to be around 50 dB and its dynamic range can be estimated as follows. The GaGe acquisition card is capable of accepting maximum input voltages ranging from 100 mV to 2 V which correspond to -16 dBm and +10 dBm, respectively, into 50 ohms. Allowing 10 dBm of headroom, the maximum power into the GaGe analog-to-digital converter (ADC) is about -25 dBm for a 100 mV input or 0 dBm for a 2 V input. The 14-bit GaGe card specification for signal to noise ratio (SNR) is 68 dB, hence the quantization noise floor of the ADC is about -16 - 68 = -84 dBm for a 100 mV input or, for a 2 V input, the noise floor is at 10 - 68 = -58 dBm. Therefore, the dynamic ranges for the GaGe card for the two input voltages, are -84 to -25 dBm for a 100 mV range and -58 to 0 dBm for a 2 V range. Since the gain of the RF front-end is fixed at about 50 dB, the dynamic range of the front-end is determined by the dynamic range of the GaGe card less 50 dB. For the 100 mV range, this corresponds to an estimated RF front-end dynamic range of -134 to -75 dBm. This range was sufficient for the initial development tests at Trial Petawawa 09-2.

Image noise is the noise that is downconverted by the unwanted sideband of the mixer. The IF of 21.4 MHz is achieved by the difference of the 1575.42 L1 RF signal and the LO tone at 1554.02 MHz; however, the mixer also downconverts the noise around 1554.02-21.4 = 1532.62 MHz to 21.4 MHz and this noise increases the NF of the front-end by 3 dB. This image noise foldover is prevented using an RF filter before the mixer that passes the desired signal at L1 but attenuates the noise around 1532.62 MHz by at least 20 dB. Two different image noise filters were tested at Trial Petawawa 09-2. A 40 MHz wide K&L bandpass filter (BPF) was used on channel 01 whereas the other three channels used Mini-Circuits 100 MHz wide BPFs. As expected, channel 01 performed better than the other channels because the 40 MHz passband did not include 1532.62 MHz. More satellites could be tracked on channel 01 and these signals had higher carrier to noise ratios (C/No) than those from satellites captured on the other channels. The passband of the 100 MHz BPFs included both mixer bands and thus resulted in a higher front-end NF on the other channels. These test results confirmed that a narrow bandwidth image noise filter should be used to minimize the noise figure of the front end.

The local oscillator signal of 1554.02 MHz was generated using an Agilent E4438C signal generator. A high quality signal generator was chosen over a lower cost phase-locked oscillator chip to minimize any corruption of GAJET experiments due to phase noise from the oscillator.

The IF filters were 7 MHz BPFs centred at 21.4 MHz. These anti-aliasing filters were originally chosen to help reduce the extra noise caused by subsampling with the GaGe card; however, subsampling was not used at Trial Petawawa 09-2. Nevertheless, the filters remove any unwanted signals downconverted from the mixer that might be aliased to baseband from higher Nyquist zones during analog to digital conversion.


2.2 Digital Beamforming

The captured data files from the GaGe digitizer are used as inputs to the digital beamforming algorithms in Matlab. These algorithms are implemented in a Matlab routine called Field Trial Analysis Tool (FiTAT) which is shown in Figure 4. FiTAT computes the optimum weighting of the data files to spatially null the jammers and construct the processed IF output signal which should contain only GPS signals. Two different algorithms, namely power inversion and reference-auxiliary element power minimization, may be chosen. It is also possible to select the antenna element used as the reference in the algorithms. GAJET uses antenna arrays with four elements and thus the two digital beamforming algorithms are theoretically capable of spatially nulling up to 3 jammers (one less than the number of elements in the array). The antenna array used at Trial Petawawa 09-2 is a four-element quadrifilar helix array, which is shown in Figure 5.

Figure 4: Field Trial Analysis Tool (FiTAT) user interface in Matlab

Figure 5: Four-element quadrifilar helix antenna array used for GAJET testing during the trial


In addition to the processed output data file, FiTAT can also output the calculated weights, the jammer to signal ratio (J/S) and frequency spectrums of the pre- and post-processed data files.

The power inversion algorithm [1] is derived from the minimum variance distortionless response (MVDR) beamformer. The power inversion method steers a null toward received strong signals (with a signal to noise ratio > 0) without requiring information, such as power and direction, about the unwanted signals. The depth of the null is proportional to the signal to noise ratio.

The reference-auxiliary element power minimization algorithm [2] uses a reference element to receive the desired signal and the auxiliary elements to steer a null toward unwanted signals.

2.3 GPS Software Receiver

Verification of the acquired and processed data captures is accomplished using GNSRx, a post-mission GNSS software receiver developed at the University of Calgary. A screen capture of GSNRx is shown in Figure 6. A software receiver offers numerous advantages compared to using a physical receiver in GAJET. Firstly, the software receiver does not have its own RF/IF front-end so GAJET RF/IF front-end experiments can be performed without having to consider the contribution from similar electronics in a physical GPS receiver. Secondly, the software receiver operating parameters can be easily changed allowing the GPS receiver parameters to be part of GAJET CRPA experiments. Finally, the processed data from Matlab can be directly used by the GPS software receiver whereas if a physical GPS receiver was used the processed data file from Matlab would have to be ‘played-back’ using a digital-to-analog converter and then upconverted to L1 for input to the physical GPS receiver.

Figure 6: GSNRx screen capture with position at CFB Petawawa


The 14-bit GaGe acquisition card has 2 GS (4 GB) of on-board memory that was nominally used with a 50 MS/s sampling rate. The data files are transferred to the hard drive for use with the GSNRx software. The system is capable of capturing 40s of data with a single channel 50 MS/s capture using all of the 2 GS memory, or 10s of data with a four channel 50 MS/s capture sharing the 2 GS memory. GSNRx can post-process these data captures in less than 5 minutes.

A small DOS utility called “Batch_GSNRx” was developed by DRDC to allow GSNRx to be run in batch mode in order to verify multiple processed files from FiTAT without the user having to set-up and initiate each GSNRx run.


3 Field Testing at CFB Petawawa

3.1 Test Setup

The GAJET electronics were set up inside a rental truck on the range at CFB Petawawa with the antenna arrays located just outside the truck. Pictures of this test setup are shown in Figure 7.

Three deployable jamming units (DJUs) were used (numbered 2, 4 and 6). The three DJUs were arranged to one side of the GAJET test set-up. A picture of a DJU at CFB Petawawa is shown in Figure 8 and the DJU arrangement around GAJET is shown in Figure 9. The angular separation between DJU 2 and 4 was 92° and between DJU 4 and 6 was 18°.

The smallest possible GaGe card reference voltage was used for all tests (100 mV) so that the ADC quantization noise was minimized. A sampling rate of 50 MS/s was used for all tests, as well as the four-element quadrifilar helix antenna array previously shown in Figure 5.

Figure 7: Trial Petawawa 09-2 test set-up, inside and outside the rental truck, at CFB Petawawa


Figure 8: Deployable Jammer Unit (DJU) at CFB Petawawa

Figure 9: Deployable Jammer Unit (DJU) test set-up, relative to GAJET, at Trial Petawawa 09-2


3.2 Test results

Baseline data was acquired with and without jamming to validate the proper operation of the GAJET prototype. These results, shown in Figure 10, demonstrated that GAJET could provide a correct latitude and longitude position with no jamming present (Figure 10a) as well as when jamming was present (Figure 10c). The position error between jamming and no jamming is 49.8m.

(a) Reference position data before jamming started

(b) During jamming without jamming removal processing (latitude and longitude are zero thus no position)

(c) During jamming with processing (calculated latitude and longitude is 49.8m from reference position)

Figure 10: GSNRx output from three data captures taken at the same location during the trial: (a) baseline capture before jamming started, (b) capture during jamming without jammer

removal processing and (c) capture during jamming with jammer removal processing


Data was then acquired during various jamming scenarios. These jammed data captures were then processed using the two digital beamforming algorithms (power inversion and power minimization) to attempt to null the jammers and calculate a position. These results are presented in Tables 1 to 4. For both beamforming algorithms, channel 01 was used as the reference channel.

The positions calculated during jamming are plotted in Figure 11 along with the true position of GAJET. These calculated positions are cold start, 10 second acquisitions with no a priori estimate of time, location or satellites in view. After a satellite was acquired, GSNRx could look up a pre-loaded ephemeris for that satellite so that the position could be calculated without waiting for an entire 30 second frame of GPS data. The positions calculated during jamming (after nulling) are roughly within a 50m diameter circle surrounding the true position of GAJET.

Each Table also includes the reference position, calculated before jamming started, so that the accuracy of the position calculated after digital beamforming can be verified.

Table 1: Trial Petawawa 09-2 test results using one CW jammer for each capture

Scenario DJU #2 DJU #4 DJU #6 Beamforming GPS Time (s) Latitude (deg) Longitude (deg) Height (m)

OFF OFF OFF None 406343.15 45.94559213 77.33092059 87.645

OFF OFF OFF Power Inversion

No DJUs Start of testing

OFF OFF OFF Power Minimization 406343.15 45.94534085 77.33062335 101.043

OFF 40 dBm CW OFF None 408718.35 45.94531183 77.33054685 114.206

OFF 40 dBm CW OFF Power Inversion 408718.35 45.94527057 77.33052093 129.167

OFF 40 dBm CW OFF Power Minimization 408718.35 45.9452995 77.33057463 104.75

OFF 30 dBm CW OFF None


OFF 30 dBm CW OFF Power Minimization











OFF 10 dBm CW OFF Power Inversion

OFF 10 dBm CW OFF Power Minimization

27 dBm CW OFF OFF None

27 dBm CW OFF OFF Power Inversion 413316.75 45.94545015 77.33054249 97.422

27 dBm CW OFF OFF Power Minimization

OFF OFF 27 dBm CW None

OFF OFF 27 dBm CW Power Inversion 414599.1 45.945448 77.33058575 93.295

1 CW DJU Increasing Tx Power (effective radiated power)

OFF OFF 27 dBm CW Power Minimization 414599.1 45.94542858 77.33061589 91.23


Table 2: Trial Petawawa 09-2 test results using two CW jammers for each capture


OFF OFF OFF None 406343.15 45.94559213 77.33092059 87.645




OFF 40 dBm CW 27 dBm CW None

OFF 40 dBm CW 27 dBm CW Power Inversion 414848.5 45.94532068 77.33053536 118.254

OFF 40 dBm CW 27 dBm CW Power Minimization



OFF 30 dBm CW 17 dBm CW Power Minimization 415096.6 45.94538846 77.33063031 95.188



OFF 20 dBm CW 7 dBm CW Power Minimization 415256.5 45.945168 77.3305194 144.115


OFF 10 dBm CW 3 dBm CW Power Inversion

OFF 10 dBm CW 3 dBm CW Power Minimization 415437 45.94535572 77.33063923 98.339

27 dBm CW 40 dBm CW OFF None

27 dBm CW 40 dBm CW OFF Power Inversion 413577.6 45.94529138 77.33043883 112.81

27 dBm CW 40 dBm CW OFF Power Minimization


17 dBm CW 30 dBm CW OFF Power Inversion







2 CW DJUs

Increasing Tx Power (effective radiated power)



Table 3: Trial Petawawa 09-2 test results using three CW jammers for each capture


OFF OFF OFF None 406343.15 45.94559213 77.33092059 87.645




27 dBm CW 40 dBm CW 27 dBm CW None

27 dBm CW 40 dBm CW 27 dBm CW Power Inversion

27 dBm CW 40 dBm CW 27 dBm CW Power Minimization









3 CW DJUs



Table 4: Trial Petawawa 09-2 test results using one Additive White Gaussian Noise (AWGN) jammer for each capture


OFF OFF OFF None 406343.15 45.94559213 77.33092059 87.645




OFF 40 dBm AWGN OFF None 417015.05 45.94543856 77.33066491 91.053

OFF 40 dBm AWGN OFF Power Inversion 417015.05 45.94543941 77.3305312 93.608

OFF 40 dBm AWGN OFF Power Minimization 417015.05 45.94546204 77.33063124 86.66

OFF 30 dBm AWGN OFF None


OFF 30 dBm AWGN OFF Power Minimization 417187.5 45.94539721 77.33065619 95.743



OFF 20 dBm AWGN OFF Power Minimization


OFF 10 dBm AWGN OFF Power Inversion







1 AWGN DJU

2 MHz BW




Figure 11: Positions determined during jamming at CFB Petawawa trial 09-02

3.3 Analysis of the Trial Results

The field trial results shown in Figure 10, Figure 11 and Tables 1 to 4 demonstrate that the GAJET prototype can successfully receive GPS signals in some types of jamming environments and provide an accurate position to the user within 10 seconds and from a cold start. The GAJET RF front-end and GaGe card were successfully used to capture and record the jamming scenarios. The digital beamforming algorithms that were tested are capable of steering nulls to attenuate some of the jamming signals and the GPS software receiver was successfully used to acquire, track and process the GPS signals in the processed data captures. Detailed analysis of the trial results is presented in the following sections.

3.3.1 Jammer Removal

Tables 1 to 4 present the results of jamming scenarios that have been processed using the two digital beamforming algorithms (power inversion and power minimization) available in FiTAT. In each table the position calculated at the start of testing, without jamming, is shown. This reference position is used to validate the accuracy of positions calculated after the digital beamforming removes the jammers. The two digital beamforming algorithms were also applied to the reference data captures, which contained no jamming, and only one of the algorithms (power minimization) resulted in an accurate position compared to the reference position calculated


without processing. The other beamforming algorithm, power inversion, removed the ability of GSNRx to calculate a position using digital beamforming.

The field trial results shown in Tables 1 to 4 show that positions can be calculated while GAJET is jammed. The key findings are:

GAJET could remove a single L1 CW jammer up to a transmit power of 0 dBm for all three jammer orientations relative to GAJET;

GAJET could remove two simultaneous L1 CW jammers up to a transmit power of 3 dBm if the two jammers were close together; however, only up to -27 dBm if the two jammers were far apart;

GAJET could not remove three simultaneous L1 CW jammers, even at low transmit powers; and

GAJET could remove single broadband jammer (transmitting AWGN) up to a transmit power of -20 dBm, which is less than when a single CW jammer was used.

The trial scenario results using a single DJU transmitting a CW tone on L1, shown in Table 1, demonstrated that GAJET cannot calculate a position with transmitted powers of -30 dBm or higher without jammer removal processing. In these scenarios at least one of the two beamforming algorithms successfully removed the jammer and enabled a position to be calculated; however, power minimization was not able to remove the lowest power jammer of -30 dBm. No position was determined when 10 dBm was transmitted but, as seen in the FFTs shown in Figure 12, the GaGe card was saturated and that is the cause of GAJET’s inability to calculate a position. Therefore, it can be concluded that GAJET was able to determine a position, using at least one algorithm, for all single jammer scenarios with received powers within the digitization limits the GaGe card. The single jammer results also indicate that DJU position does not affect the ability of GAJET to calculate a position with a single CW jammer.

The trial scenario results using two simultaneous DJUs transmitting CW tones on L1 are shown in Table 2. These results demonstrated that GAJET can calculate a position for all transmit powers when DJUs 4 and 6 were transmitting; however, only for lower transmitted powers when DJUs 2 and 4 were transmitting. It can be seen from the test set-up in Figure 9 that the difference between these two scenarios is that the angular separation of DJUs 4 and 6, as seen from GAJET, is small (18°) whereas the angular separation of DJUs 2 and 4 is large (92°). It is expected that the beamforming algorithms would be better at nulling emitters that are angularly close compared to those spaced further apart. This appears to be confirmed by these trial scenario results although more investigation is necessary to fully explain the performance in the two-jammer environment.

The trial scenario results using three simultaneous DJUs transmitting CW tones on L1, shown in Table 3, demonstrated that GAJET cannot calculate a position for any scenario. The reason for this is unknown at this time and requires further investigation.

The trial scenario results using a single DJU transmitting 2 MHz wide additive white Gaussian noise (AWGN), centred at L1, are shown in Table 4. These results demonstrated that GAJET can calculate a position with very similar performance to the scenarios with a single CW jammer, except that a lower AWGN jamming power was required to overcome the digital beamforming algorithms and jam the processed data captures.


Figure 12: FFT of the captured IF signal for a single deployable jammer unit scenario transmitting a CW jamming tone at <10 dBm (left) and 10 dBm (right)

3.3.2 Choice of Beamforming Reference Channel

All of the beamforming results in Tables 1 to 4 use channel 01 as the reference antenna element for the processing algorithms. The GAJET antenna array elements are not symmetric about element 01 therefore the effect of choosing a different reference channel (and hence a different antenna element in the array) was also investigated.

A scenario from Trial Petawawa 09-2 that did not produce a position with channel 01 as the reference was re-processed four times each with a different channel, and hence a different antenna element, as the reference. These results are shown in Table 5 and show that using channel 03 as the reference for both beamforming algorithms can remove the jammers; however, when the other channels are used GAJET remains jammed after processing.

It can be concluded that for the type of antennas and beamforming algorithms used in GAJET at Trial Petawawa 09-2 the choice of reference antenna does impact the ability to calculate a position during jamming. Furthermore, the scenario used in Table 5 includes two simultaneous jamming signals from the larger angularly separated DJU 2 and 4 where, as shown in Table 2, GAJET was previously unable to calculate a position for any scenario using channel 01 as the reference. Therefore, it can also be concluded that GAJET can calculate a position for two simultaneous CW jammers with large angular spacing; however, for the antennas and beamforming algorithms used a Trial Petawawa 09-2, the correct choice of reference channel is necessary to successfully calculate a position.


Table 5: Effects of reference channel choice on digital beamforming of a jammed data capture

AlgorithmReferenceChannel

GPS Time (s) Latitude (deg) Longitude (deg) Height (m)

No processing No position 1 No position 3 413810.150 45.945370214 77.330580154 102.520 5 No position

Power Inversion

7 No position 1 No position 3 413810.150 45.945450475 77.330785347 99.241 5 No position

Power Minimization

7 No position Scenario: DJUs #2 & #4 with CW L1 jammers; 17 dBm and 30 dBm transmit powers, respectively

3.3.3 The GPS Software Receiver

GSNRx was used at Trial Petawawa 09-2 to demonstrate its ability to replace a physical GPS receiver. GSNRx was used to verify that GPS signals could be captured with GAJET and recorded using GaGe acquisition card sampling rates from 5 MS/s to 125 MS/s. The optimum sampling rate was found to be 50 MS/s (at an IF of 21.4 MHz). At 2.3 IF, this sampling rate is low enough to maximize the length of the data capture (which is about 10 seconds for a four-channel simultaneous capture) while satisfying Nyquist. The field trials verified that 7-8 seconds of capture time is required for GSNRx to synchronize with the navigation message (i.e. obtain a ‘nav sync’) and hence display GPS time from the received GPS signal and also display a position when the current satellite ephemeris is used to aid the software receiver.

The ephemeris data was logged separately using a NovAtel GPS receiver and then converted to a file format compatible with GSNRx. Alternatively, ephemeris data downloaded from the Internet (when available the following day) was also converted to a file format compatible with GSNRx. Both methods proved to be equally useable and it was verified that 10 second data captures are sufficient for GAJET experiments. Using a separate ephemeris data file eliminates the need for 30 additional seconds of data capture so that GSNRx would have enough data capture time to extract the ephemeris from the data capture itself.

Data capture experiments at Trial Petawawa 09-2 using 5 MS/s resulted in data captures lasting over 6 minutes, and ephemeris data was extracted directly using GSNRx; however, there is a noise penalty because 5 MS/s is under-sampling the 21.4 MHz IF. Therefore, the majority of GAJET testing at Trial Petawawa 09-2 sampled at 50 MS/s and used a external ephemeris file to allow GSNRx to calculate a position with the 10 second data captures. As previously stated, this ephemeris data can be generated using logged data from another GPS receiver or can be downloaded from the Internet; however, experiments at DRDC just after Trial Petawawa 09-2 demonstrated a more convenient method where a single channel data capture using GAJET, without jamming, is performed at 50 MS/s. Since this data capture is a single channel capture, 40 seconds of capture time is possible which is enough for GSNRx to extract the ephemeris data and save that ephemeris data to a data file. This ephemeris data file is then used with subsequent four channel, 10 second long data captures. This procedure is repeated every 2 hours as new


ephemeris data is uploaded to the satellites and it can be completed solely using the equipment within GAJET. This procedure was chosen as the standard operating procedure for future GAJET trials.

3.3.4 Intermediate Frequency Offset

During early testing at Trial Petawawa 09-2 (before the results shown in Tables 1 to 4), it was not possible to achieve Navigation sub-frame synchronization (‘nav sync’) and this prevented GSNRx from calculating a position. This problem was solved during the trial by changing the Standard Tracking Options file parameters “Initial DLL BW” and “Assisted DLL BW” to 4.0 Hz and 2.0 Hz, respectively, from 2.0 Hz and 0.05 Hz, respectively. Although successful, this solution was not the most desirable as it added unnecessary noise to the GPS processing loops. The field trial data captures were analyzed by Jeff Bird at DRDC and by Dr. Petovello and Dr. O’Driscoll at the University of Calgary. They noted that by comparing code and carrier Doppler measurements from GSNRx it appeared that the IF data capture was offset by -2385 Hz from what GSNRx was expecting (21.4 MHz). Therefore, changing the IF GSNRx was expecting to 21.397615 MHz instead of 21.4 MHz allowed ‘nav synch’ with the original DLL BW settings of 2.0 Hz and 0.05 Hz, which improved the carrier to noise ratio (C/No) of the tracked satellites.

This IF offset was later proven to be caused by the fact that the GaGe card digitizer clock was not synchronized with the Agilent signal generators that provided the LO to the RF front-end. GAJET testing at DRDC just after Trial Petawawa 09-2 demonstrated that synchronizing the GaGe card with the 10 MHz reference output of the Agilent signal generator removed the IF offset. Synchronizing the digitizer with the LO was therefore included as standard operating procedure for future GAJET trials.


4 Conclusions and Future Work

Trial Petawawa 09-2 was used to develop and test a prototype GPS Anti-Jam Evaluation Testbed (GAJET). A suitable RF front-end architecture was developed and then successfully integrated with an acquisition card, digital beamforming algorithms, and a GPS software receiver to produce a system capable of GPS electronic protection (EP) experiments. GPS jamming trials with the GAJET prototype revealed that at least one of the two digital beamforming algorithms can remove up to two simultaneous jammers and allow the GPS software receiver (GSNRx) to calculate a position despite being in an electronic attack (EA) environment.

Based on the trial results, a few modifications are recommended for the GAJET prototype to improve its performance in future GPS anti-jamming field trials:

Use 2 MHz L1 SAW filters for the image noise filters in each channel to reduce the noise figure of the RF front-end;

Synchronize the GaGe acquisition card with the 10 MHz reference output of the signal generator to prevent any IF offsets from occurring in the recorded data files; and

Generate the required ephemeris data for GSNRx using 40 second, single-channel GAJET data captures every 2 hours.

With these minor modifications GAJET can be used at future field trials to support L1 C/A-code GPS electronic protection (EP) R&D efforts.

An analysis of the digital beamforming results at Trial Petawawa 09-2 point to a number of items requiring further investigation. Depending on the threat scenarios (i.e. the number of jammers, the jammer type, and the transmit power, location and orientation relative to GAJET) sometimes both beamforming algorithms removed the jammers; however, sometimes only one of them was successful. It is unknown why a given algorithm was successful for a particular threat scenario and not for others and this should be investigated. Similarly, the choice of reference antenna element also determined if an algorthim was successful or not and this should also be investigated. Finally, the beamforming algorithms were not able to remove three simultaneous jammers despite using a four element antenna array. With a four element array GAJET should theoretically be capable of removing three simultaneous jammers (one less than the number of elements).

In the future, GAJET will be upgraded to operate on both the L1 and L2 GPS bands with a bandwidth of 20 MHz on each band. This will allow GPS electronic protection (EP) experiments using the P(Y)-code. These upgrades will require broadening the bandwidth of the RF front-end and the digital beamforming algorithms. To capture signals simultaneously in both the L1 and L2 GPS bands the RF front-end and the digitizer will also have to be modified. Furthermore, P(Y)-code testing will required GAJET to ‘play-back’ the beamformed data captures to a military GPS receiver, such as a keyed DAGR. This will require digital-to-analog conversion of the beamformed IF data file followed by upconversion back up to L1 and L2 for input to the DAGR receiver. Finally, the GAJET data captures will need to be longer, perhaps up to 15 minutes long, to allow a DAGR to acquire and track both the C/A and P(Y) codes and calculate a position using the beamformed data capture.


References .....

[1] R. T. Compton Jr., “The Power-Inversion Adaptive Array: Concept and Performance,” IEEETrans. Aerosp. Electron. Syst., 15(6), pp. 803–814, 1979.

[2] A. Gecan, “Power Minimization Techniques for GPS Null Steering Antenna,” Proceedings of the 8th ION conference GPS, pp. 861-868, Sep 12-15 1995.

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GPS jamming experiments were performed at Trial Petawawa 09-2 to develop a prototype GPS Anti-Jam Evaluation Testbed (GAJET). GAJET is a customizable Controlled Reception Pattern Antenna (CRPA) used for developing electronic protection (EP) systems for GPS receivers. Various trial scenarios were performed at CFB Petawawa with up to 3 deployable jammer units (DJUs). Field trial results demonstrate that the prototype GAJET is capable of spatially nulling up to 2 simultaneous jammers using digital beamforming. The beamforming results were verified by using a GPS software receiver to calculate an accurate position using the L1 C/A-code while in a jamming environment. GAJET will be used at future field trials to support GPS electronic protection R&D efforts targeted at minimizing the vulnerability of Canadian Forces GPS assets to electronic attack (EA).

Des expériences sur le brouillage intentionnel des signaux GPS ont eu lieu lors de la campagne de mesures sur le terrain Petawawa 09-2 en vue de la mise au point d'un prototype de banc d’essai de systèmes anti-brouilleur GPS (GAJET). GAJET est un système de mise au point de systèmes de protection électronique (EP) pour les récepteurs GPS basé sur des systèmes d'antennes à diagramme de réception contrôlé (CRPA). Divers scénarios ont été testés à la BFC Petawawa à l'aide d'au plus trois unités de brouillage déployables (DJU). Les résultats de l'essai sur le terrain montrent que le prototype GAJET peut contrecarrer les effets d'au plus deux brouilleurs fonctionnant simultanément à l'aide de la formation de faisceau numérique. Les résultats issus de formation de faisceau ont été vérifiés à l'aide d'un récepteur GPS logiciel calculant une position précise à partir du code C/A L1 dans des conditions de brouillage intentionnel. GAJET servira à l'occasion de campagne de mesures ultérieures à appuyer les efforts de R et D sur la protection électronique des signaux GPS afin de réduire la vulnérabilité des équipements GPS des Forces canadiennes à l'attaque électronique (EA).

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GPS; NAVWAR; CRPA; Anti-jamming; Electronic Protection; Testbed; Beamforming; Nulling; CFB Petawawa; Trial

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