Passive radar on moving platforms for maritime and coastalsurveillance

Philipp Wojaczeka, Fabiola Coloneb, Diego Cristallinia,Pierfrancesco Lombardob, Daniel O’Hagana

aFraunhofer Institute for High Frequency Physics and Radar Techniques FHRFraunhoferstr. 20, 53343 Wachtberg, Germany;

bUniversity of Rome “La Sapienza”, Via Eudossiana 18, 00184 Roma, Italy

ABSTRACT

This contribution presents the application of passive radar for maritime and coastal surveillance. The focus ison the application of passive radar on moving platforms for target detection. Its potential on moving targetdetection and surveillance applications will be presented with results of real data analysis.

Keywords: Passive radar, PCL, clutter suppression, moving target indication, detection

1. INTRODUCTION

For several years the radar research society has been focusing its attention towards passive radar, especiallytowards passive radar on moving platforms. A passive radar, also known as passive coherent location (PCL), isa special bi- or multistatic radar system, which exploits already existing transmitter infrastructure in order todetect and localize non-cooperative, moving targets. The probably most appealing advantage is the fact, thatthe passive radar system does not emit any signals, as it uses the emissions of e.g. broadcast or communicationtransmitters as illuminating signal sources. This fact enables a covert and in principle non-detectable operation,e.g. for reconnaissance. Furthermore, due to the lack of the adversary’s knowledge about the passive radarlocation, it is very difficult to jam a passive radar system. Mounting a passive radar on a moving platform offersthe radar operator new opportunities and possibilities. A moving platform could be e.g. a small boat, a smallaircraft, or even an unmanned aerial vehicle (UAV). Due to the flexibility of the moving platform the range ofaction is increased compared to a stationary passive radar system. Therefore, it can serve as a gap filler forterrain hidden to a stationary passive radar, and it is possible to detect moving targets (PCL-MTI) at an earlierstage.In the context of maritime operations different scenarios can be imagined. An appealing scenario is for examplethe exploitation of a small boat or a UAV as carrier platform which moves along a coast line. Equipped with apassive radar system it allows to covertly observe movement distant to the coastline, e.g. of non-cooperative smalltargets approaching a harbor1,2 in order to increase the safety of harbors and other safety-critical infrastructure.A further possibility is to do covertly ground imaging by passive synthetic-aperture radar (PCL-SAR).3,4 Theseimages can be analyzed e.g. in order to detect structures and man-made objects, and can be exploited for terrainreconnaissance. By that, PCL-SAR supports the military planning.One of the most exploited transmitters of opportunity are Digital Video Broadcasting – Terrestrial (DVB-T)transmitters, due to the manifold advantages, such as generally high transmit power level (in the order of tens ofkilowatts), constant bandwidth leading to constant range resolution, and almost world-wide availability. DVB-Tis a terrestrial transmission, therefore the reception and surveillance is generally limited to on land and to coastalareas. However, for surveillance of coastal areas it is an important and meaningful tool; for surveillance of seaone can think of using satellite based transmissions, e.g. Digital Video Broadcasting – Satellite (DVB-T), orGlobal Navigation Satellite Systems (GNSS) such as the Global Positioning System (GPS), or Galileo.5–7

In this paper the potential of passive radar on moving platforms for moving target indication will be presentedby evaluation of measurement data from a maritime moving platform exploiting the emissions from a DVB-T

Further author information: (Send correspondence to Philipp Wojaczek or Daniel O’Hagan)E-mail: {philipp.wojaczek,daniel.ohagan}@fhr.fraunhofer.de, Telephone: 0049 228 9435-389

transmitter network. The paper is organized as follows: an introduction into passive radar signal processingexploiting DVB-T transmissions is given in Sec. 2. In Sec. 3 a clutter suppression technique and its adaptationfor passive radar on moving platforms is given. In Sec. 4 results from data evaluation from a measurementcampaign are presented. Outlook and conclusions are given in Sec. 5.

2. PASSIVE RADAR SIGNAL PROCESSING USING DVB-T

DVB-T is a variant of Digital Video Broadcasting (DVB), which is used for the transmission of audio and videocontent using terrestrial transmitters.8 The used frequency band is in the UHF band: 474–786 MHz. OneDVB-T channel covers in total a bandwidth of approximately BW = 7.61 MHz in the UHF band, centered atcarrier frequency fC . The DVB-T signal is transmitted as continuous waveform (CW) signal. It constitutesin time-domain of so called symbols, each of constant duration TS . The constant duration enables the radarengineer to process each DVB-T symbol separately as a type of pulse-Doppler radar, where the pulse repetitioninterval (PRI) equals the symbol duration: PRI = TS . In frequency domain the DVB-T signal utilizes theorthogonal frequency division multiplex (OFDM) modulation scheme, which means that each symbol comprisesK separately modulated carriers, where the each carrier is separated by approximately 1.116 kHz. Each carriercarries either deterministic information or non-deterministic information, where the deterministic carriers enablesynchronization and estimation of the transmission channel H(f). The signal received at the passive radarsurveillance channel can be written as a function of time t as

sR(t) =aT sT (t) exp(j2πfDT (αT )t) +

NR∑q=1

∫Φq

aq(α)s(t− τq) exp(j2πfDq(α)t)dα

+

NG∑g=1

ag(α, γ)s(t− τg) exp(j2πfDg(α, γ)t) + e(t) 0 ≤ t ≤ T0

(1)

where T0 is the observation time, sT (t) is the transmitted signal and aT is its corresponding complex amplitudeat the surveillance antenna, NR is the range extent of the observed scene (expressed in range gates), Φq isthe azimuthal angular sector of the illuminated scene, aq(α) is the complex amplitude of the echo receivedwith time delay τq = Rq/c0 from azimuth angle α ∈ Φq, and Rq is the bistatic range of the q-th range gate.fDT = vT

c0fT cosαT is the Doppler frequency for the motion component relative to the transmitter. Finally, the

third contribution of (1) corresponds to contributions of NG moving targets of delay τg and bistatic Dopplershift fDg(α, γ). The component e(t) is white Gaussian noise (WGN).For the digital signal processing, the passive radar receiver synchronizes on the received signal sR(t), and thenreconstructs the transmitted signal sT (t).9 After reconstruction, a copy sT (t) of the transmitted signal is availableat the receiver, which is used as a filter h(t) for the correlation process on a batch-wise basis, that is for eachDVB-T symbol m = [0, . . . ,M − 1] individually: xC(l,m) = sR(l,m) ∗ h(l,m). l = tfS represents the samplesacquired with sampling frequency fS , and xC(l,m) is a so-called slow-time–fast-time matrix resulting from thecorrelation process. The matrix xC(t, TS) can be Fourier transformed into a range-Doppler map z(l, fD), via:

z(l, fD) =∑M−1

m xC(l,m) exp(−j2πυm/M), where fD represents the Doppler frequency in a particular Dopplerbin.

3. PASSIVE RADAR FOR MOVING TARGET INDICATION

Referring to (1), one can easily see, that the clutter returns have a Doppler frequency depending on the velocityof the moving receiver, which can be seen in the range-Doppler map as an interference at Doppler frequenciesfD = vRx

c0fC cosα, usually termed as “clutter ridge”. This is especially a problem for MTI (as well for active

and passive radar), as signal echos of slow moving targets can fall into the clutter ridge and thus being hiddenby the clutter. In order to suppress the clutter, various techniques were created for active radar, e.g. displacedphase center antenna (DPCA).10

DPCA is a simple technique, which requires only two channels in the basic configuration. Provided ideal con-ditions, it allows to completely suppress the clutter. Two antenna elements are displaced mounted along-track,

where the preceding element can be labeled leading antenna (LA) and the rear element can be labeled trail-ing antenna (TA). As the receiver moves, both elements, LA and TA, receive signal echos according to (1).Due to the receiver’s movement, the phase center of the TA will be at the location, where the phase centerof the LA has been a time step before. The spatial overlap of both phase centers – the DPCA condition –is a crucial requirement, as then both LA and TA receive clutter echoes with the same Doppler shift and de-lay. It can be expressed as a time delay TD depending on the antennas’ displacement d and on the receiver’svelocity vRx: TD = d/vRx = K · PRI, K ∈ N. A further crucial requirement is that the transmitted radarpulses are time-invariant. While in active radar this requirement is usually possible to fulfill, as the transmittedwaveform is under control of the radar operator, for a passive radar this is not feasibly, due to the dependenceon the non-cooperative time-varying waveform. To overcome this limitation, the reciprocal (or inverse) filterh(t) = IFFT{S−1

T (f)}, where ST (f) = FFT{sT (t)}, can be used in order to create a time-invariant impulseresponse for the clutter suppression process.11 Applying h(t) on the data from both receiving channels, two

slow-time–fast-time maps x(LA)C (l,m) and x

(TA)C (l,m) for LA and TA can be created. To suppress the clutter,

both maps are subtracted with the respective time delay TD and Fourier transformed to create a range-Dopplermap zD(l, fD):

zD(l, fD) =

M−1∑m=0

(x(TA)C (l,m)− x(LA)

C (l,m− TD)) exp(j2πmfD/M) (2)

4. RESULTS OF PCL-MTI

A measurement campaign with a moving passive radar has been undertaken in order to show and evaluate theperformance of passive radar for moving target indication. As receiving system the Parasol system,12 developedby Fraunhofer FHR, has been used. The passive radar was carried by a small boat (see Fig. 1) which movedalong defined trajectories in the Oslo fjord in Norway (see Fig. 2 with a velocity vRx ≈ 8.5 m/s. Two DVB-Ttransmitters, Tx-1 and Tx-2, transmitting on center frequency fC = 650 MHz were in the vicinity of the trialsite. Tx-1 is marked with a red circle in Fig. 2, while the direction to Tx-2 being further distant to the trial siteis indicated with a black arrow in Fig. 2. The black dashed line in Fig. 2 indicates frequently traveled waterways,convenient for the detection of non-cooperative targets or ships in order to show the potential of passive radarfor moving target indication.Two discone antenna elements were mounted along-track as linear array on the boat, with bore sight to starboard.As the discone antenna elements are omnidirectional in azimuth, radiation absorbing material was mounted onthe array’s left side to achieve a side-looking condition.

The processing described in Sec. 2 and Sec. 3 was applied on the received data. The result before cluttersuppression is shown in Fig. 3. The interference from the direct signal (approx. at Doppler frequency fD ≈−11 Hz) has been suppressed using the ECA-CD filter,13 as the direct signal considerably limits the dynamicrange in the range-Doppler map due its high power level. One can clearly see the clutter returns up to a bistaticrange of 6 km, which overlap the echoes of slow moving targets. The results after clutter suppression are shownin Fig. 4. The improvement is clearly visible, as most clutter echoes are suppressed, so that targets are detectableagainst the background: two slow moving targets (marked with red ellipses in Fig. 4) at bistatic range of ≈ 400 mand ≈ 600 m and Doppler ≈ −17 Hz and ≈ −20 Hz are clearly visible against the background. A third movingtarget (marked with a red ellipse) at bistatic range ≈ 2000 m and Doppler ≈ −7 Hz is as well better detectable.Further results are presented in Figs. 5 and 6. Both Figures present results after clutter suppression, and in bothFigures responses from the same target, but from different illuminations, can be seen: In Fig. 5 one can see thereturn from the target (marked with a black ellipse) illuminated from Tx-1. The target is close to the receiver atDoppler ≈ 21 Hz and bistatic range ≈ 100 m. The second transmitter Tx-2 being further distant to the receiverthan compared to Tx-1 illuminated the same target as well, but due to its greater distance to the receiver, isreceived second. The target response is shown in Fig. 6 at a bistatic distance of ≈ 19.8 km at Doppler ≈ 23 Hz.The echo of the target is marked with a white ellipse. The simultaneous illumination by multiple transmittersand reception is very appealing as it allows the localization of detected targets via ellipsoid intersection.These results show the potential of passive radar on moving platforms for detection and localization of slowmoving targets.

Figure 1. Picture of the moving receiver platform. Thelinear array is mounted on the boat’s stern (recognizabledue to the RAM to achieve a side-looking condition).

Tx-1

Rx

Baseline: 17 km

Direction to Tx-2

Figure 2. Map of the trial site (from OpenStreetMap). Thereceiver’s trajectory is indicated in black. At the time ofthe evaluated data it was moving from North to South onthe left side of the square (indicated with the black circle).The array steering direction was to starboard. The trans-mitter Tx-1 is indicated with the red circle, the baselineis indicated with the blue double-sided arrow. The direc-tion to the second transmitter Tx-2 is highlighted with theblack arrow.

Figure 3. Range-Doppler map of evaluated data from theLA (before clutter suppression). The clutter responses areclearly to be seen and can overlap target echoes.

Figure 4. Range-Doppler map of evaluated data after clut-ter suppression. The overall clutter has been considerablyreduced and allows to detect the three slow moving targets(marked with red ellipses).

5. OUTLOOK AND CONCLUSIONS

This contribution presented passive radar on moving platforms for the purpose of moving target detection usingterrestrial broadcast transmitters as illuminators. The passive radar signal processing was introduced and resultsfrom evaluated measurement data were presented. The results show the feasibility of passive radar for movingtarget detection, even if the targets’ echoes are buried in clutter. This shows the potential and the applicabilityfor the surveillance of coast-line and maritime infrastructure, such as harbors and ports, and for reconnaissanceof large areas. As next steps it is planned to increase the number of receiving elements to have more than twoelements, and to adapt the algorithms in order to be able to do multi-array processing in order to further improvethe clutter suppression and target detection.

6. ACKNOWLEDGMENTS

The authors would like to thank the Norwegian Defence Research Establishment (Forsvarets forskningsinstitutt -FFI) for the support and the participation in conducting the measurement campaign and experiments in Norway.

Figure 5. A moving target (marked with black ellipse) withhigh reflectivity being illuminated by the transmitter Tx-1received first from the network.

Figure 6. The echo from the same moving target (markedwith white ellipse) as in Fig. 5 being illuminated by thetransmitter Tx-2 received second from the network.

REFERENCES

[1] Bournaka, G., Baruzzi, A., Heckenbach, J., and Kuschel, H., “Experimental validation of beamformingtechniques for localization of moving target in passive radar,” in [2015 IEEE Radar Conference (RadarCon) ],1710–1713 (May 2015).

[2] Baruzzi, A., Bournaka, G., Heckenbach, J., and Kuschel, H., “Experimental validation of kalman filter basedtracking in passive radar systems,” in [2015 IEEE Radar Conference (RadarCon) ], 1634–1637 (May 2015).

[3] Atkinson, G. M., Sayin, A., Cherniakov, M., Antoniou, M., Stove, A., and Underwood, C. I., “Passive SARsatellite system (PASSAT): Ground trials,” in [2018 International Conference on Radar (RADAR) ], 1–6(Aug 2018).

[4] Wojaczek, P., Summers, A., Cristallini, D., Walterscheid, I., Lombardo, P., and Colone, F., “Results ofairborne PCL under CCI conditions using DVB-T illuminators of opportunity,” in [2018 InternationalConference on Radar (RADAR) ], 1–6 (Aug 2018).

[5] Antoniou, M., Stove, A. G., Tzagkas, D., Cherniakov, M., and Ma, H., “Marine target localization withpassive GNSS-based multistatic radar: Experimental results,” in [2018 International Conference on Radar(RADAR) ], 1–5 (Aug 2018).

[6] Santi, F., Pieralice, F., and Pastina, D., “Multistatic GNSS-based passive radar for maritime surveillancewith long integration times: Experimental results,” in [2018 IEEE Radar Conference (RadarConf18) ], 1260–1265 (April 2018).

[7] Pisciottano, I., Cristallini, D., Schell, J., and Seidel, V., “Passive ISAR for maritime target imaging: Ex-perimental results,” in [2018 19th International Radar Symposium (IRS) ], 1–10 (June 2018).

[8] ETSI, 650 Route des Lucioles F-06921 Sophia Antipolis Cedex - FRANC, Digital Video Broadcasting (DVB);Framing structure, channel coding and modulation for digital terrestrial television (October 2015).

[9] Poullin, D., “Passive detection using digital broadcasters (DAB, DVB) with COFDM modulation,” IEEProceedings - Radar, Sonar and Navigation 152, 143–152 (June 2005).

[10] Stimson, G. W., Griffiths, H. D., Baker, C. J., and Adamy, D., [Introduction to airborne radar ], SciTechPublishing, Inc. (2014).

[11] Wojaczek, P., Colone, F., Cristallini, D., and Lombardo, P., “Reciprocal-filter-based STAP for passive radaron moving platforms,” IEEE Transactions on Aerospace and Electronic Systems 55, 967–988 (April 2019).

[12] Heckenbach, J., Kuschel, H., Schell, J., and Ummenhofer, M., “Passive radar based control of wind turbinecollision warning for air traffic parasol,” in [2015 16th International Radar Symposium (IRS) ], 36–41 (June2015).

[13] Schwark, C. and Cristallini, D., “Advanced multipath clutter cancellation in ofdm-based passive radarsystems,” in [2016 IEEE Radar Conference (RadarConf) ], 1–4 (May 2016).