HF Surface Wave Radar Operation In Adverse Conditions
HF Surface Wave Radar Operationin Adverse ConditionsAnthony M. Ponsford, Reza M. Dizaji and Richard McKerracherRaytheon Canada Limited, Waterloo, ON N2J 4K6 CanadaE-mail: tony ponsford@raytheon.com, reza dizaji@raytheon.com, rick mckerracher@raytheon.comAbstract—For the past 12 years the Canadian Department ofNational Defence and Raytheon Canada Limited havecollaborated on a cost-shared programme to develop anIntegrated Maritime Surveillance (IMS) system based on HFSurface Wave Radar (HFSWR).The primary objective behind the programme was todemonstrate the capability of HFSWR to continuously detectand track surface targets (ships and icebergs) as well asairborne targets, at all altitudes, to ranges in excess of 200nautical miles, reliably and consistently in real time and in allweathers. A secondary objective was to demonstrate theconcept of IMS, involving the fusing of data from HFSWRradars and other sensors.This paper reviews techniques and methods used in theprocessing of HFSWR data to ensure that performance ismaintained, even under adverse operating conditions.I. INTRODUCTIONThe United Nations’ “Law of the Sea” grants maritimenations sovereign rights over an area of ocean known as theExclusive Economic Zone (EEZ) that extends 200 nauticalmiles from shore. In return, these nations are required toestablish and maintain administration, law enforcement andenvironmental protection over this area. This requires thatship and aircraft activity within their EEZ be monitored.Ships can be monitored intermittently and at great cost byair patrols, sea patrols or possibly satellites, but HFSWR isthe only sensor that offers the capability of inexpensivesurveillance of a large area, with the ability to track targetscontinuously and in all weathers. HFSWR provides trackinformation and can classify contacts in terms of size (i.e.radar cross section), track history, heading and speed.Moreover, by providing current locations and tracks ofspecific contacts, HFSWR can improve the effectiveness ofother reconnaissance assets, such as patrol aircraft, inproviding positive identification.An IMS system uses HFSWR to provide a backgroundlayer of current and past activity, onto which other datafrom complementary sensors and resources are mapped.HFSWR is used to maintain the validity of tracks of targetsthat have been identified by other means, as well as to cueother sensors and assets such as patrol aircraft.Extensive testing of the HF Radar system has beenundertaken over the past three years [1, 2], and it has beenshown that the system performance during daylight hours issatisfactory, but that night time performance can bedegraded by high external interference, range-wrapped0-7803-7871-7/03/ 17.00 2003 IEEEionospheric clutter and external noise. In addition, highclutter levels experienced during periods of high sea statescan adversely affect detection of small targets.This paper introduces some key factors that can improvethe performance of HFSWR, and demonstrates the effectiveness of mitigation techniques. These techniques helpensure optimal performance of an HFSWR system, even inextremely adverse conditions.II. THE HFSWR SYSTEMTwo SWR-503 HFSWRs, developed by RaytheonCanada Limited, have been in operation on Canada’s EastCoast since 1999. Key operational parameters are listed inTable I, below.The system uses a phase-code sequence on transmit,which permits operation at a high Pulse Rate Frequency(PRF), while suppressing range sidelobes and “rangewrapped” ionospheric clutter. The radar uses “mismatched”phase-code sequences to allow the system to suppress strongco-channel interference signals.While the bandwidth gives a range resolution of 7.5 km,over range-sampling produces an accuracy of 0.3 km. Beamwidths are approximately 8 , and target angles of arrivalestimates are determined with a detection error of less than1.0 over the radar coverage area of 60 from boresight.The 1.6 kW (average) transmitter power ensures oceanclutter limitation to greater than 350 km range during theday, but at night the system is typically externally noiselimited from approximately 150 km.TABLE IPARAMETERS OF THE HFSWR AT CAPE RACE,NEWFOUNDLANDFrequencyTransmit antenna:Receive antenna:Transmit power:Waveform:PRF:Pulse bandwidth:Sampling Rate:5933-5 MHz7-element log-periodic monopole.Gain 8 dBiLinear array of 16 monopole doublets.33m spacing.16 kW peak, 1.6kW av.Sequence of phase codes.250 Hz20 kHz100 kHzRadar 2003
Form ApprovedOMB No. 0704-0188Report Documentation PagePublic reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering andmaintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information,including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, ArlingtonVA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if itdoes not display a currently valid OMB control number.1. REPORT DATE2. REPORT TYPE14 APR 2005N/A3. DATES COVERED-4. TITLE AND SUBTITLE5a. CONTRACT NUMBERHF Surface Wave Radar Operation in Adverse Conditions5b. GRANT NUMBER5c. PROGRAM ELEMENT NUMBER6. AUTHOR(S)5d. PROJECT NUMBER5e. TASK NUMBER5f. WORK UNIT NUMBER7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)8. PERFORMING ORGANIZATIONREPORT NUMBERRaytheon Canada Limited, Waterloo, ON N2J 4K6 Canada9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)10. SPONSOR/MONITOR’S ACRONYM(S)11. SPONSOR/MONITOR’S REPORTNUMBER(S)12. DISTRIBUTION/AVAILABILITY STATEMENTApproved for public release, distribution unlimited13. SUPPLEMENTARY NOTESSee also ADM001798, Proceedings of the International Conference on Radar (RADAR 2003) Held inAdelaide, Australia on 3-5 September 2003., The original document contains color images.14. ABSTRACT15. SUBJECT TERMS16. SECURITY CLASSIFICATION OF:a. REPORTb. ABSTRACTc. THIS PAGEunclassifiedunclassifiedunclassified17. LIMITATION OFABSTRACT18. NUMBEROF PAGESUU619a. NAME OFRESPONSIBLE PERSONStandard Form 298 (Rev. 8-98)Prescribed by ANSI Std Z39-18
III. TARGET DETECTIONThe performance of HFSWR is adversely affected by anumber of environmental factors [3]. Digital signal processing techniques are used to mitigate these effects.A target is detected by comparing the power in a givenradar pixel relative to its neighbours. The radar pixel isbounded in azimuth, range and Doppler. A typical HFSWRmay have 10 million pixels per dwell. In an ideal situation, apixel contains a target return or spatially temporally whiteexternal noise. However, other unwanted signals may alsobe present. These unwanted signals fall into two distinctcategories:1) External Interference: where the unwanted signal isindependent of the radar operation, e.g. co-channelinterference and impulsive noise; these signals behaveas additive noise.2) Clutter: where the unwanted signal is a consequence ofthe radar’s operation, e.g. ionospheric clutter, oceanclutter, range wrap clutter and meteor clutter. Thesesignals behave as multiplicative noise.Target detection generally degrades at night. This isbecause ionospheric conditions change with time of day andseason. During daylight hours, ionospheric propagationbetween 3 and 7 MHz (i.e. short-wave) is extremely lossydue to D-layer absorption. At night, the D-layer disappears,leaving the way open for short-wave signals to propagate infrom around the world. This also includes lightening noisethat propagates via the ionosphere and hence increases thebackground noise level.Not all the energy emitted by the radar propagates as asurface wave. Some energy is directed upwards, and mayreflect from the ionosphere, either directly back to the radar,or indirectly by a second reflection of the ocean. This lattercase may be viewed as multipath clutter, and given thegeometry of the problem, these echoes usually appear asrange wrap or “second-time-arounds.”The interaction of the electromagnetic wave with theocean wave results in an ocean clutter spectrum that istypically the limiting factor in surface target detection. Thisclutter spectrum has been extensively modelled and isdescribed in detail [4].IV. CLUTTER MITIGATIONClutter is defined as unwanted echoes. These unwantedechoes are typically characterized as originating from acollection of spatially distributed scatterers, and do not havethe characteristic thumbtack ambiguity function of a pointtarget. For a HFSWR the dominant forms of clutter areocean clutter and ionospheric clutter. Detection in thesesituations can be enhanced by distributing the clutter energyover a larger number of pixels, by either improving therange, Doppler, or azimuth resolution, or by exploiting thecharacteristic of the clutter signal to remove it.The radar returns from the ocean surface have a complexstructure. The sea surface is composed of waves of differentwavelengths and amplitudes travelling in differentdirections. The resultant clutter is dominated by back-scatterfrom components of the sea spectrum which are resonantwith the radar wavelength. Two dominant first-order peaksimpair radar detection at their corresponding Dopplerfrequencies. However, the part of the ocean clutter thataffects the performance of HFSWR in detecting low-speedships is the continuum. The level of the ocean clutter at agiven Doppler is influenced by the power of the wind, andits directionality with regard to the radar look direction,.This directionality of the spectra does not imply that theocean clutter has a strong spatial correlation. In fact, it has avery poor spatial correlation. This can be exploited by usinga sub-space processing technique to suppress the clutter andenhance target detection. A detailed description of thealgorithm can be found in [5].Anne PierceAnne PierceThe doppler plot at 110 kmThedoppler plot at 110 km45605040Power (dBu)Power ( dBu)403020353010025-10-0.6-0.4-0.20Doppler (Hz)0.20.420-0.80.6Fig. 1. Doppler Plot at the Range of the Target – Standard FFT Processing.-0.6-0.4-0.200.2Doppler (Hz)0.40.6Fig. 2. Doppler Plot at the Range of the Target – Sub-Space Processing.5940.8
Anne S PierceRange (km)Range (km)E LayerIonsphericClutterPower (dB)Power (dB)Fig. 3. Target Detection in Ionospheric Clutter.The effectiveness of the algorithm is illustrated using datacollected from a controlled test target - a 40m scallop boat.The trials were conducted during gale-force winds and seasof 3 to 4 metres. Fig. 1 plots the output of the FFTbeamformed Doppler spectrum. It can be observed that astrong clutter continuum prevents the detection of this smalltarget. Fig. 2 presents the data after sub-space processing. Itcan be observed that the ocean clutter has effectively beensuppressed, including the first order, and that the target isnow detectable with a 20 dB signal-to-clutter ratio.Ionospheric clutter, on the other hand, exhibits spatialcorrelation. However, clutter suppression techniques havebeen shown to be effective in improving detection when thetarget’s azimuth is removed from that of the ionosphericclutter. This is illustrated in Fig. 3, which plots the Rangeand Doppler spectrum at the target beam. The data wasobtained from the same trial, but at a time when there werestrong E-layer reflections, resulting in a broad Doppler bandof clutter at 100 km. It can be observed that the subspaceprocessing method has cancelled the clutter to reveal thetarget.The third type of clutter is the result of skywavepropagation that results in second- or higher-order rangewrap and consequently results in the folding of echoesarriving from ranges beyond the maximum unambiguousrange into the radar range. As illustrated in Fig. 4, phasecoding of the transmitted pulse sequence is very effective inremoving these unwanted signals.The final clutter signal to be discussed is meteor clutter.This clutter is the result of radar reflections from the ionisedtrail. Meteor echoes as observed by an HF Radar areusually the result of line-of-sight propagation at a slantrange of 100 to 250 nautical miles. The echo signatureusually lasts for a few seconds, and appears as large peak ata specific range.V. EXTERNAL INTERFERENCE MITIGATIONPower (dB)Power (dB)The HF band is highly congested with frequencyallocations shared between many users. During the day, theoccurrence of the D-layer prevents skywave propagation atthese frequencies, and co-channel interference from localusers in the band can be avoided by careful choice ofRange FoldedClutterFig. 4. Example of Range-Wrap Clutter Suppression using Quadrature Phase Coding. (a) Range Doppler Spectra processed using Frank Quadrature PhaseCode (b) Data taken at approximately the same time using a binary phase code showing range-wrap clutter.595
operating frequency. However, during the night, theionosphere changes such that long-range skywavepropagation is supported, and interference sourcespropagate into the area from around the world.External interference is independent of the radaroperation, and includes both co-channel interference andimpulsive noise. Co-channel interference cancellation isachieved based on mismatched filtering to the radartransmitted codes. The matched filter response containsboth the radar return and the interference, whilst theancillary mismatched data contains only the interference.Subtraction leaves only the radar data. Details of theapproach can be found in [6].The effectiveness of the technique is illustrated in Fig. 5,which plots the beam outputs for the matched filter data(uncancelled beam) and the mismatched filter beam output.Subtraction of one from the other leaves the cleanExternal Interference Cancelled (EIC) beam output, fromwhich the interference has been removed. The fourth plot isa Doppler profile taken at a range index of 40 before andafter cancellation. It can be observed that the externalinterference has been suppressed by up to 20 dB and downto the external night-time noise level.Impulsive noise is the result of local lightning discharges.These result in large spikes that have a short life, and ingeneral only affect a few received pulses.A simple and effective method to remove these spikes isto simply remove those pulses that exceed a given thresholdlevel. This crude method is very effective but does result ina spreading of the Bragg energy that can potentially masksmall targets. An alternative technique uses a predictivefilter to both remove and reconstruct the original signal.The approach is illustrated in Figure 6, which presents aplot of the amplitude of the time series (pulse index) of data.Impulsive noise (in this case a swept tone resulting from anionospheric sounder) can be seen at pulse index 380.The standard method for dealing with this is astraightforward blanking technique. This gives satisfactoryresults in most cases, but can result in a slight smearing ofthe Bragg energy. The alternative is to reconstruct theoriginal signal using a Linear Prediction filter. The resultsfrom the two methods are compared in Fig. 7 using datacollected from a HFSWR system operating at 14.5 MHz.Fig. 5. External Interference Cancellation (EIC) Based on Matched/Mismatched Filtering: Data taken from Cape Race Radar Operating at 3.1 MHz at0350z Mar 13, 2001.Fig. 6. Impulsive Noise Excision: Blanking and Linear Prediction.Fig. 7. Comparison of Impulsive Noise Excision Techniques.596
TABLE IITRACKER LOGICVI. TRACKINGThe next step in the process is to associate consecutivedetections to form tracks. This can be a relatively easyprocess in low-density traffic areas but offers a significantchallenge in dense target situations.It is required that the radar tracks all vessels from firstdetection until they leave the coverage area or exceed theirmaximum detection range. This has to be accomplished witha minimum display of false tracks. Tracking will alsoimprove the positional accuracy of the radar by smoothingthe noise error. For an established track, track accuracy istypically better than 0.25 nm in range, 0.25 degree inazimuth.The Tracker is described in detail in [7]. The tracker is adeferred-decision-based tracker that propagates multiplehypotheses at the report-to-track assignment stage (i.e. is it atrue detection, a false detection, or a missed detection).These multiple track options are maintained over severalupdate periods until a firm decision concerning thelikelihood of a track can be established and competingtracks deleted. The report-to-track assignment is a multidimensional process incorporating target dynamicinformation (range, speed and azimuth) as well as rankinformation (target cross section). False tracks areminimized by using a multiple-stage assignment process, asshown in Table II.Potential Tracks (P):single detectionTentataive Tracks (T):2 or more associated detectionsConfirmed Tracks (C):tracks with at least N associateddetections, where N is user definedto meet false track rateDeleted Tracks (D):Tracks are coasted for a maximumon M consecutive misses prior todeletion.VII. AN EXAMPLE OF MARITIME SURVEILLANCE PICTUREThis paper has shown that signal processing techniquescan effectively remove unwanted interference and clutterfrom the radar data, allowing echoes from point targets to becleanly extracted. Consecutive detections are then processedto produce tracks.An example of typical track activity of the East Coast ofCanada is presented in Figure 8. This data was extensivelyground truthed using Maritime Patrol Aurora CP3 aircraftand a Fisheries aircraft equipped with Airborne SearchRadar. A gale warning was in effect at the time of the trials,with winds Southeast 25-35 kts and seas of 3-4 meters.The radar successfully tracked all reported targets andmaintained a false alarm rate of better than 0.25 perhour.Fig. 8. Typical Radar Display, East Coast of Canada.597
REFERENCESVIII. CONCLUSIONHFSWR has been evaluated as a key sensor in providingcomplete surveillance of surface and air activity within theCanadian EEZ. It has been shown that signal processingtechniques can be applied to the radar data to overcomesevere environmental impacts on radar detectionperformance. The HFSWR is a viable sensor, even thoughits performance is subject to time of day and seasonalvariations, as well as sea state conditions and winddirection.HF Radar has been shown to provide consistent andcontinuous real-time tracking of targets within the EEZ.When associated with detail obtained from other sources, aclear, unambiguous, picture of surface and air activity (at allaltitudes) can be e authors gratefully acknowledge the support of the Department ofNational Defence, Defence Research Development Canada (DRDC) and theCanadian Coast Guard for their continued support of the programme.598A.M. Ponsford, L. Sevgi, H.C. Chan, “An integrated maritimesurveillance system based on high-frequency surface-wave radars. Pt2, Operational status and system performance,” IEEE Antennas andPropagation Magazine, vol. 43, no. 5, pp. 52-63, October 2001.A.M. Ponsford, “Surveillance of the 200 Nautical Mile ExclusiveEconomic Zone (EEZ) Using High Frequency Surface Wave Radar(HFSWR),” Can. J. Rem. Sens. vol. 27, no. 4, Special Issue on ShipDetection in Coastal Waters, pp. 354-360, August 2001.L. Sevgi, A.M. Ponsford, “Propagation and Interference Characteristicof High Frequency Surface Waves,” URSI, XXVIth General Assembly,F7: Wave Propagation and Remote Sensing, August 13-21, 1999.S. Srivastava, J. Walsh, “An analysis of the second order Dopplerreturn from the ocean surface,” IEEE J. Oceanic Engineering, OE-10,pp. 443-445, 1985.R. Dizaji, A.M. Ponsford, “System and Method for SpectralGeneration in Radar: U.S. Patent Application 60/365,163,” March 19,2002.R. Dizaji, A.M. Ponsford, R. McKerracher, “Co-channel InterferenceReduction using Adaptive Array Processing: U.S. Patent Application60/365,152,” March 19, 2002.Z. Ding, K. Hickey, “A multiple hypotheses tracker for a multiplesensor integrated maritime surveillance system,” Proceedings of the2nd International Conference on Information Fusion, San Jose, July6-8, 1999.
Anne Pierce Thedoppler plot at 110 km Fig. 2. Doppler Plot at the Range of the Target – Sub-Space Processing. -0.6 -0.4 -0.2 0 0.2 0.4 0.6-10 0 10 20 30 40 50 60 Doppler (Hz) Power (dBu) The doppler plot at 110 km Anne Pierce Fig. 1. Doppler P
SYNTHETIC APERTURE RADAR (SAR) IMAGING BASICS 1.1 Basic Principles of Radar Imaging / 2 1.2 Radar Resolution / 6 1.3 Radar Equation /10 1.4 Real Aperture Radar /11 1.5 Synthetic Aperture Radar /13 1.6 Radar Image Artifacts and Noise / 16 1.6.1 Range and Azimuth Ambi
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bistatic radar geometry.27 Radar-absorbent material augments fuselage shaping by absorbing radar energy and reducing the strength of the radar echo.28 Future innova-tions may allow stealth aircraft to actively cancel radar echo by retransmitting radar energy and/or by ionizing boundary layer air around the fuselage.29 Counters to Stealth
Wave a and Wave c are constructed of five waves as Elliott originally proposed. As opposed to the five wave impulse move in Elliott’s original version that could form either a Wave 1, Wave 3, Wave 5, Wave A or Wave C the harmonic version can only f
So, the wave 1, wave 3 and wave 5 are parts of impulsive wave in upward direction. [6] Though Elliott waves follow many rules but three basic rules are followed by each wave to interpret Elliott wave. These guidelines are unbreakable. These rules are as follow: Rule 1: Wave 2 is not retracted more than 100% of wave 1.
So, the wave 1, wave 3 and wave 5 are parts of impulsive wave in upward direction. [2] Though Elliott waves follow many rules but three basic rules are followed by each wave to interpret Elliott wave. These guidelines are unbreakable. These rules are as follow: Rule 1: Wave 2 is not retracted more than 100% of wave 1.
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Radar Antennas - 1 PRH 6/18/02 MIT Lincoln Laboratory Introduction to Radar Systems Radar Antennas. Radar Antennas - 2 PRH 6/18/02 MIT Lincoln Laboratory Disclaimer of Endorsement and Liability The video courseware and accompanying viewgraphs presented on this
