Defence

Air Power projectionAir Power projection

FREE to all delegates at Land Warfare Conference2006

Defencetoday

$7.95

Print Post PP424022/00254

DEFENCE CAPABILITIES MAGAZINE

September/October 2006

UAVscan onesize fit all

Land Warfarespecial editionLand Warfarespecial edition

Upgrading ASLAVUpgrading ASLAV

air strike and deterrent force

Since the advent of air warfare,inclement weather has providedthe principal opportunity for groundforce movements, as opticalsensors are unable to penetrate anovercast. Ground mapping radars,using real beam or later Synthetic

Aperture Radar (SAR) technology, provided arobust means of locating and attacking fixedsurface targets, with increasing precision overtime. Slow moving targets however remainedelusive, and it was not until the advent of GroundMoving Target Indicator (GMTI) radars that thissanctuary was removed.An excellent example was observed during the2003 invasion of Iraq, when large sandstormsblown in from the Western desert region broughtmost operations to a standstill, for days. Seeingthis as an opportunity, Saddam's militia unitssought to redeploy south in large road convoys,confident that the sandstorms would provide acloak to conceal them from marauding coalitionstrike aircraft. An orbiting E-8C JSTARS, sweepingthe area with its APY-3 radar in GMTI mode,tracked these convoys. Soon they wereannihilated under a barrage of GPS guided JDAMsand smart submunition dispensing Sensor FusedWeapons. The APY-3 penetrated the sandstormswith ease and provided precision targeting datafor incoming waves of bombers.Over a year later, the US Air Force conducted animportant trial during the Resultant Fury exercisein the Pacific. An E-8 JSTARS tracked a series ofmaritime targets using its APY-3 surveillanceradar in MMTI mode, including barges simulatingamphibious landing craft, and relayed precisionrealtime tracking data via a modified JTIDSnetwork, to modified GBU-31 JDAM inertiallyguided bombs in flight, which subsequentlyannihilated these moving surface targets.Until the advent of precision GMTI radartechnology, both of these strikes would not havebeen possible.

How GMTI evolvedMoving ground targets and slow moving maritimetargets, especially smaller vessels, presentserious challenges for legacy radar technologies.These problems arise as a result of the geometriesand relative speeds, and radar signatures of suchtargets, when viewed from an aircraft. The faintenergy backscattered from such targets is buriedin the enormous reflection from the surface of theearth, termed 'clutter'.The problem of tracking low flying aircraft andcruise missiles was solved during the 1960s, withthe advent of Air Moving Target Indicator (AMTI)and pulse Doppler radar technologies, andsupporting Kalman tracking filter technology.These technologies allowed the radar signal and

data processing software and hardware to siftthrough the jumble of reflected radar signals, andseparate the reflections belonging to aircraft fromthe much stronger clutter reflections from terrain.In pulse Doppler surveillance and air interceptradars this was achieved by discriminatingbetween the Doppler shift in the reflected radarsignals. Doppler shift is an increase or decrease inthe frequency of a reflected radar signal, which isproportional to the relative velocity between theradar and the target. Increasing frequency isproduced by closing targets, decreasing frequencyby receding targets. For relatively fast movingaerial targets, the Doppler shifts of these targetsare quite different from the Doppler shift of theterrain clutter produced by the motion of theaircraft carrying the radar. Therefore it is feasibleto use filtering techniques to separate the targetsfrom the terrain clutter. This is the principleunderpinning all 'look-down shoot-down' fighterradars.

Dr Carlo Kopp

101NCWNETWORKEDOPERATIONS

NCW 101 part 8

tech o

ps

58DefenceToday

Ground andMaritimeMoving TargetIndicator Radar

clockwise from left:The E-8C JSTARS equippedwith the APY-3 GMTI/SAR X-band radar is the most capableground surveillance systemdeployed to date (USAF).

JSTARS imagery of UKcountryside (USAF).

The 'Mother of All Retreats'JSTARS imagery of retreatingIraqi troops in 1991 (USAF).

Difficulties arise however, when the target is slowmoving, since its Doppler shift differs little from theDoppler shift produced by the motion of the radarrelative to the terrain. Unfortunately, movingground vehicles like trucks, 4WDs, tanks, cars andother such targets fall precisely into this category.As powerful as pulse Doppler techniques may bethey are simply unsuitable for this category oftarget.Historically, the problem of tracking opposingground forces became acute during the Cold Warperiod, as the Soviets deployed increasingnumbers of tanks and armoured vehicles inEastern Europe. Given the poor weather prevalentthrough much of the year, and complex forestedand mountainous terrain in many areas of interest,the US were especially concerned about theirability to divine and understand Soviet ground forcemovements. The Red Army juggernaut was not tobe trifled with. While NATO had an overwhelmingadvantage in superior air power, which couldannihilate massed formations of tanks, air powercan only be used with effect where the location ofthe target is known. This problem became ofincreasing concern to US strategic planners duringthe 1960s and 1970s. The result was robustinvestment in research to find a way of trackingground vehicles effectively.The approach first pursued was adaptation ofexisting Airborne Moving Target Indicator (AMTI)radar, at that time being introduced in AEW&Cradars to provide overland capability. All MTI radarsare based on the idea of subtracting radar returnsfrom consecutive pulses sent out by the radar.Signals from moving targets will differ slightly, dueto their motion, more so than the ground clutter theradar sees. When the two returns are subtracted,the targets appear as differences. This technique iseasy to implement in a static ground based orshipboard radar, but harder to do for airborneradars, and techniques were required tocompensate the Doppler shift of the terrain clutter.An MTI mode of this ilk was introduced on theMotorola AN/APS-94 Side Looking Airborne Radarcarried in the OV-1 Mohawk, during the late 1960s.It had limited capability, but was a step in the rightdirection.The next breakthrough was the US Army's SOTAS(Stand Off Target Acquisition System)demonstrator, which installed a rotating AN/APS-94 antenna under a UH-1 Huey helicopter, toprovide continual MTI surveillance through acircular arc. The APS-94 and SOTAS were feasiblesince the Mohawk and UH-1 were very slowmoving vehicles, as a result of which the Dopplershifts in the radar clutter were quite small. For afaster platform, like an aircraft, where Dopplershifts in the clutter were much larger, thistechnique was no longer viable.A completely new approach to this problem wasneeded, and this led to the genesis of the modernGMTI radar. In 1969 the US Air Force entered thegame, by funding the MIT Lincoln Laboratory todevelop the Multi-Lateration Radar Surveillanceand Strike System or MLRS, using a pair of MTIradars. This effort led to the discovery of atechnique termed Displaced Phase Center Antenna(DPCA), which is the basis of all modern GMTI andMMTI radars.DPCA in its simplest form splits a sidelooking radarantenna into two halves. A radar pulse istransmitted, reflects off terrain and moving targets,and travels back to the aircraft. Because theaircraft has forward motion, and the antenna is

split into halves, the half of the antenna nearer tothe tail will be in the physical location occupiedfractions of a second earlier, by the half of theantenna nearer to the nose. Therefore, the motionof the aircraft is compensated for, and the clutterdisappears. This is a very simple but also verypowerful idea, as it allowed clutter to be removedfrom the radar signal by a clever arrangement ofantennas, rather than complex signal and dataprocessing. In such simple DPCA systems, thePulse Repetition Frequency (PRF) of the radar isadjusted to match the forward velocity of theaircraft, and the range at which targets aresearched for, so that pulses transmitted arrive atthe right instant in time to reject the clutter.Further research led to improved signal processing,which reduced the sensitivity of DPCA to aircraftvelocity.Such simple DPCA systems were capable ofdetecting ground vehicles moving at much lowerspeeds than any previous technique could. Theystill had the limitation of poor angular accuracywhen tracking ground targets, critical for targetingapplications.Subsequently, a further discovery was that bysplitting the antenna into three rather than twosegments, it was possible to not only reject theclutter, but also perform a very precise anglemeasurement against a moving ground target. Thiswas the breakthrough which was needed toproduce operationally effective GMTI radars.Armed with this research, the US Air Force andArmy, with DARPA support, launched the PaveMover demonstration program, which wasintended to develop the radar technology needed todefeat massed Soviet armoured attacks in allweather.Hughes and Grumman/Norden were contracted todevelop two DPCA based Pave Mover radardemonstrators, which flew during the early 1980sto support the Assault Breaker smart anti armourmunition demonstrations. The Pave Mover radarswere carried in the weapon bay of an F-111E. Thetarget tracking information produced by theseradars was then relayed over a microwave datalinkto a ground station, which processed the data fordistribution to missile batteries, which would thenfire submunition dispensing ballistic missiles atapproaching armoured columns (referh t tp : / /www.ausa i rpower.ne t /TE-Assau l t -Breaker.html for details).

The Pave Mover trials were a resounding success,and provided the proof of concept for the E-8JSTARS and its massive APY-3 phased array radar.The APY-3 remains the largest DPCA GMTI radarever built, using a 24 foot long phased arrayantenna, mechanically stabilised in roll. The systemis so large, that a Boeing 707-320 airframe isrequired to carry it.The APY-3, like most GMTI radars, operates in thecentimetric X-band, the remainder typicallyoperating in the even shorter Ku-band. The choiceof radar wavelength for these applications is notarbitrary, as these bands were found to be best fordetecting ground targets. Tanks, trucks, 4WDs,artillery pieces and other such targets are of thesize where most of their radar signature isproduced by detail features, so a centimetric bandradar performs best in detecting such targets.Another important performance aspect of GMTIradars is their effective range. The ugly reality isthat to achieve good range in such radars, giventhe often low signatures of the targets involved,considerable power and a large antenna apertureare required (this is not unlike the problem faced bySoviet designers when they built the massive X-band Uspekh maritime targeting radar carried bythe Bear D maritime reconnaissance and targetingaircraft). Building large centimetric band highpower radars is not easy, which is why so few suchsystems exist.With JSTARS in development, other applicationswere sought for this technology. Norden, a keyplayer via prior Pave Mover experience, sought tointroduce this capability into an upgrade for the USNavy's A-6 Intruder via a new Norden AN/APQ-173radar, and into the new phased array WestinghouseAN/APQ-183 radar in the A-12A, intended toreplace the A-6. Both programs were cancelled.The technology was subsequently introduced intothe AN/APG-76, a large Ku band attack radardesigned for an Israeli F-4E upgrade.The large APG-76 was designed with five receiverchannels, and could simultaneously perform highresolution SAR imaging and DPCA GMTI trackingand targeting. This superb but large Ku band radarused a conventional planar antenna, supplementedby three lower auxiliary antennas for three segmentDPCA GMTI modes, which provided it with the mostaccurate GMTI capability in any fighter radar duringthe 1990s.

59DefenceToday

The Grumman/Norden and Hughes PaveMover demonstrators proved the concept ofhighly accurate DPCA GTMI radar. Both werecarried in the weapon bay on an F-111E(Norden).

US Army UH-1 fitted with the SOTAS MTIradar, deployed in Germany during the ColdWar (via http://www.usarmygermany.com).

NCW 101 - GMTI/MMTI radar

tech o

ps

Other manufacturers of multimode X-bandfighter radars soon sought to emulate whatNorden did by providing DPCA GMTIcapabilities in existing fighter radar designs.Invariably they exploited the fact that most ofthese radars had antennas which weresegmented into four quadrants, whichpermitted monopulse angle tracking for air toair engagements.Whether the radar antennas were segmentedin an 'X' or a '+' pattern determined whetherthe highly accurate three segment DPCA orless accurate two segment DPCA techniquecould be used. To date none of themanufacturers have disclosed this, as itwould no doubt have an adverse effect onmarketing if customers knew that only basictwo segment DPCA could be used.Having a 3 segment DPCA GMTI capability isvaluable in radars used on aircraft which areto perform battlefield interdiction role, as thisallows blind all weather attacks with GPSguided weapons like the GBU-31 JDAM orGBU-39/40 Small Diameter Bomb, on movingcolumns of ground vehicles or surfaceshipping. The drawback of GMTI capability onsuch radars is that the useful footprint islimited by the modest power output and smallantenna size on these radars, which results inthem having a very small footprint comparedto specialised Intelligence SurveillanceReconnaissance radars like the APY-3 or theequivalent but smaller European SOSTAR-Xsystem.In the Australian public defence debate,fighter radars with GMTI capability are oftentouted as equivalent somehow to large multi-segment DPCA GMTI radars in the JSTARScategory. This is from a technical andoperational perspective pure nonsense, as thefighter radars not only cannot match theuseful range and footprint of the larger radars,but due to smaller antenna sizes cannot evencome close to competing in angular accuracyand thus achievable precision when used fortargeting or ISR purposes. Never let facts getin the way of a good yarn!

60DefenceToday

The future ofGMTI/MMTIDPCA GMTI radars are tremendously useful,whether used for battlefield surveillance,reconnaissance or targeting, or in maritimeoperations.The latter is especially interesting, sinceconventional pulsed maritime radars are designedprimarily for the detection of full size shipping inblue water operations. That is an environmentwhere the ocean surface clutter is wellunderstood, and targets are both very large andusually highly reflective to radar.The reality of much of contemporary maritimesurveillance is that it takes place in littoral waters,or archipelagic waters, where targets often exploitlandmass clutter to evade radars designed forblue water operations. Another factor is that insuch environments, many targets of interest areoften very small, be they landing craft, barges,fishing boats, launches, yachts, speedboats,dhous and other small traffic. As a result targetsof relatively low radar signature are operating in acomplex maritime clutter environment.Radars designed around DPCA GMTI capabilitiesare well suited for this regime of operations, sincethey can much more effectively reject the clutterenvironment, but also do a better job ofseparating low signature targets from thebackground. This is operationally highly valuable.Another application which has emerged for DPCAGMTI radars is cruise missile defence. Cruisemissiles are very small low flying targets whichhide in clutter, and often have very low radarsignatures head on, but larger signatures fromabeam. Conventional AWACS and AEW&C radarsoften have difficulty tracking cruise missileseffectively, since their designs typically assumeaircraft sized targets with larger radar signatures.X-band DPCA GMTI radars are generallyconsidered more effective against cruise missiles,

OK

129129

Bomber Delivers JDAMs

00130

130U.S. AIR FORCE

(c) 2001, Carlo KopJDAM Moving Target Engagement CONOPS (AMSTE)

Surveillance UAV

Datalink Aimpoint Updates

GMTI Radar Track

JSTARS/MC2A

Overcast

GMTI Radar Track 101NCWNETWORKEDOPERATIONS

The Norden APG-76MMRS was the firstfighter radar to employDPCA GMTI capability,providing exceptionaltracking accuracy.(Northrop-Grumman)

APG-76 GMTI/SAR imagery, convoycrossing bridge and beachhead assault.(Northrop-Grumman)

Whether the attacker is an F-22A dropping eightSDBs or a B-2A dropping 320 SDBs, these systemsare currently limited to fixed aimpoints, even ifthese are loaded into the bombs seconds beforerelease, using the onboard radars for targeting.High precision large DPCA GMTI or MMTI radars areas much the enabling technology for this, as are theradio networks used to communicate with thebombs.The Affordable Moving Surface Target Engagement(AMSTE) trials conducted in 2003, sponsored by theDARPA Special Projects Office (SPO) and the AirForce Research Laboratory Information Directorate,intended to fuse established GPS/inertially guidedbomb, networking, GMTI and SAR radartechnologies, to permit massed blind radarbombing precision attacks on moving surfacetargets.The model developed in the AMSTE program issimple. Munitions such as the JDAM or SDB aremodified to incorporate a datalink receiver, whichpermits the bomb's aimpoint to be continuouslyupdated in flight. Weapons with GPS/inertialguidance have to date been limited to fixed targets,since the target location could not be updated oncethe bomb's cable umbilical to the aircraft wasbroken. Legacy laser and TV guided weapons didnot have this limitation, since the weapon is guidedby laser illumination on the target, or a TV contrastlock on the target.For a GPS/inertial guidance system to be capableof use against a moving target, it must have thecapability to accept continuous position updatesfrom an offboard source, whatever that may be.The modifications first trialled during the AMSTEdrops provide this capability. Understandably thechanges to the internal navigation software of thebomb must be more extensive, since the Kalmantracking filter and autopilot must be able to projectthe future position of the target from a series ofconsecutive network updates, to bias the bomb'sflightpath so it intercepts the target. This wouldinvolve a proportional or lead-pursuit homing

algorithm of the ilk used in air to air missiles.In an operational environment, a GMTI radar likethe APY-3 or MP-RTIP is used to track a formationof surface targets like a convoy, tank squadron onthe move, or wave of amphibious assault craftapproaching a beachhead. Each target isindividually tracked in position and speed.A bomb delivery aircraft is vectored over the target,and its fire control software, via the network,communicates with the targeting GMTI radarsystem's software. The GMTI radar systemsoftware assigns specific bombs to specifictargets, and generates cueing commands to thepilot of the bomber, so he can position optimally todrop the weapons.Once the assigned bombs have separated fromtheir ejectors, the datalink receivers in the bombsgo active and acquire target position updates beingbroadcast over the radio network via the distantGMTI radar system. As the radar continuesgenerating individual tracks for each target, itbroadcasts a stream of position updates to eachbomb individually over the network. The bombseach continuously predict the future position oftheir assigned target and fly to this point, impactingwith a distance error determined by the GPSguidance in the bomb, and the accuracy of thetargeting coordinates generated by the distantGMTI radar system.The advent of wide area differential GPS systemssuch as WAGE or EDGE (referhttp://www.ausairpower.net/TE-GPS-Guided-Weps.html Part V) indicates that future GPS/inertialerrors will be as low as inches in longitude, latitudeand elevation. As a result the dominant error in thebombing equation will be that of the GMTI radarproviding targeting information.This is why in the long term, it is so critical thatappropriate investment is made into DPCAGTMI/MMTI radars, but also why much care mustbe put into selecting such systems, to ensure thatthe basic design can produce very accurate targettrack.

61DefenceToday

since they are able to extract small targets withlesser Doppler shifts from clutter more effectively,but also because the X-band radar signature ofmany cruise missiles is higher than their signaturein the lower radar bands.The longer term outlook is that we will see morespecialised DPCA GMTI/MMTI radars on specialisedISR platforms, and some kind of DPCA GMTI/MMTIcapability in all new fighter radars. How effectivethe latter will be will depend primarily on the designof the radar antenna segmentation, and especiallythe power aperture performance of the radar.The US some years ago launched the MP-RTIP(Multi-Platform Radar Technology InsertionProgram) which is a family of modular X-bandactive phased array (AESA) radars designed for ISRapplications, especially DPCA GMTI/MMTI. The MP-RTIP program aims to design not only individualphased array TR modules, but also larger 'tiles'comprising multiple modules, allowing specificantenna configurations to be built up from standardcomponents, to match specific applications.Three applications have already been announcedfor MP-RTIP modules. The first is an antennaupgrade for the APY-3 in the E-8C JSTARS, toincrease range and sensitivity. The second is anadaptation of the APY-3 upgrade package to theplanned, but currently suspended, E-10A MC2A ISRaircraft, intended as a replacement for the E-8C.The third is a new DPCA GMTI/MMTI radar payloadfor the second generation RQ-4B Global Hawk,intended to replace the U-2's ASARS package instrategic ISR roles.Another application since canvassed by Raytheon isthe planned US Navy P-3C replacement, the P-8AMMA, which would carry a shorter variant of theAPY-3 MP-RTIP radar, to be used in cruise missiledefence, littoral maritime surveillance, and inproviding GMTI ISR support for amphibiousoperations and battlefield interdiction from aircraftcarriers.Networked environments have tremendouspotential for high tempo operations, given theability of a well designed network to distributetargeting data very quickly to large numbers ofplatforms. Current US Air Force thinking is to extendthat network down to munitions like the JDAM andSDB, in flight, and ultimately down to smartsubmunitions where appropriate.The intent, long term, is to gain the capability formassed precision attacks against moving targets,such as an invasion force landing on a beach, or amassed armoured attack. Current US operationalcapabilities permit individual attacks on movingtargets, or massed attacks on fixed targets.

NCW 101 - GMTI/MMTI radar

The P-8A MMA has been proposed as a platformfor carriage of the MP-RTIP radar, for maritimesurveillance, cruise missile defence, and groundsurveillance tasks (Boeing)

The E-10A MC2A isintended to replace theJSTARS, but has beenput on hold due to afunds shortages. It willuse the MP-RTIP AESAGMTI/SAR radar.