USN Shipboard Infrared Search and Track (IRST) Program

Joseph E. Misanin

Program Executive Office, Theater Air DefenseIntegrated Ship Defense Systems

Department of DefenseUnited States of America

ABSTRACT

On May 17, 1987 two EXOCET missiles hit and crippled the frigate USS STARK. Thirty seven Sailorslost their lives due to the inability ofthe ship to defend itself against a seaskimming cruise missile attack.In 1991, as a result ofthis incident, Congress mandated the establishment of a Program Executive Officefor Ship Self Defense. The purpose ofthe legislation was to preclude another incident by placing a highpriority on the Combat System engineering process used to design and field the anti-ship cruise missile(ASCM) defense capability of surface ships. Over 35countries now have seaskimming ASCMs and thistype ofthreat continues to proliferate. The use of IRST is a critical element of ship self defense,providing early and reliable detection of seaskimming cruise missiles. This paper describes thecontribution of IRST in providing self defense and the current status ofthe United States Navy (USN)shipboard IRST development program.

Keywords: ASCM, IRST, PRA, defense, seaskimmer, shipboard, survivability

2. Initiation of Shipboard IRST Development

In 1996, the USN awarded a competitive contract for the design, manufacture and test of a shipboardIRST. The development effort is comprised oftwo phases. The first phase provides an IRSTdemonstrator for early technical and operational evaluation in 1998. The second phase completesdevelopment by using lessons learned from phase one and inserts technological upgrades as appropriateto ensure fielding of an optimally capable system.

The IRST is depicted in Figure 1. It is comprised of a scanning infrared sensor, signal/data processorsand a control console. The scanner is light weight, which allows mounting high on the mast to providemaximum line-of-sight to the horizon with minimized blockage. The below decks electronics andconsole use standard Navy components to improve logistics through commonality. The IRST performsits primary mission of seaskimming ASCM detection and declaration automatically; however, an operatoris required to establish operating parameters associated with doctrine and for maintenance.

2.1 Mission Need

Seaskimming ASCM threats fall into two general categories: supersonic and subsonic. Supersonicmissiles stress the reaction time ofthe combat system. For example, a 3.0 mach missile will travel theapproximately 10 mile distance from the horizon in under 30 seconds. This provides little time for thenormal detect - control - engage process required to defeat a missile attack. The infrared signature ofsupersonic missiles is usually high due to aerodynamic heating, making it easy for IRST systems to detectthem at long ranges. Subsonic missiles can stress the ability of the detection system when designed tominimize radar and IR signatures. Subsonic missiles typically have low infrared signatures due to the

436 SPIE Vol. 3061 • 0277-786X1971$10.00

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lack of aerodynamic heating. One exception is the subsonic missile using solid rocket motors, where amajor contributor to the infrared signature is the rocket plume. The infrared signature of these plumesmakes detection of these missiles as easy as the supersonic threats. The EXOCET missiles that struckthe USS STARK were solid rocket, subsonic missiles having no particularly unique design parametersmaking them easy IRST targets. Both types of missiles, super and subsonic, continue to proliferateworldwide, Figure 2.

The IRST provides continuous surveillance ofthe area near the ship out to and above the horizon. Itsprimary mission is the automatic detection and declaration of seaskimming ASCM missiles at rangessufficient for the combat system to provide a specified probability of ship survivability. Its primary use isas an adjunct to radars and ESM systems, by cueing the fire control sensors to initiate the engagement;however, combat systems with a complete passive engagement capability have added immunity to anyadverse ECM environments. Because the IRST provides a completely passive capability, it is the idealdetection sensor for missions requiring EMCON.

2.2 Contribution of LRST to Ship Survivability

Ship survivability against ASCM attacks is the primary ship defense requirement established by the Chiefof Naval Operations. Each ship class is required to have a basic area and self defense

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Figure 1 - IRST System Description

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Figure 2. Threats

capability that will protect it from an ASCM attack with a specified probability. This measure ofeffectiveness is the Probability of Raid Annihilation (PRA). PRA, in turn, is comprised of five otherprobabilistic elements: coverage, reaction time, firepower, resistance to degradation and availability.These probabilities interact, mostly through timelines, to yield PRA. For a given combat system andASCM scenario, the minimum declaration range required to provide a specific PRA can be derived. Thisdeclaration range is the minimum performance required by any sensor (radar, ESM or IRST) ifthe shipsurvivability requirement is to be met. The elements of PRA are depicted in Figure 3.

2.3 Coverage

Coverage is the probability that the ASCM will be detected and declared to the combat system at theminimum range required to support the desired PRA. The probability of coverage provided by a sensor isaffected by any condition preventing required coverage performance. Blockage by the mast andcoverage overlap with other sensors required for the engagement (such as missile illuminators) areusually the most significant influencing factors for rotating radars. The light weight and small size of theIRST provides the flexibility to allow high mounting on the mast to minimize blockage and maximizeoverlap with radars and weapons.

Coverage required during periods of EMCON can only be provided by passive sensors such as IRST andESM. Though it is hard to foresee a need to "turn-off' shipboard radars, it is important to realize that theship's self defense capability will not be compromised by doing so, if an IRST is used. The IRSTprovides coverage for all seaskimming ASCMs, while ESM systems provide coverage only for RFemitting threats, again providing complementary performance.

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rt7 OF SUVVA3UTY S A TOP LEVELt RW) t\fl1 U\ ) EE\SE RE1EV

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Figure 3: PRA Bubble Chart

IRST, because of its extremely precise and accurate tracking capability, can improve coverageperformance by providing an accurate raid count to the combat system. ASCM raids that are closetogether in bearing or elevation may be interpreted by the radar as a single target. This could lead to toofew defending missiles being fired at the attacking missiles. The resolution of an IRST can be defined byits instantaneous field ofview (IFOV), which is the detector size divided by focal length. For diffractionlimited resolution, the spot size is matched to the detector element size. The "spot size" for a 6" aperturesensor operating at 4 microns is approximately 60 microns. This resolution of 60 microradians is almost300 times better than that of a typical 1 degree fire control radar.

Lastly, using an IRST makes the effectiveness of future stealth enhancements more difficult by requiringboth the RF and JR spectrums to be addressed by ASCM designers. This factor makes the detectioncapability more immune to obsolescence due to advancements in threat design.

When used in conjunction with radar based sensors, the probability of achieving the required coverage ismuch closer to unity than RF-only based systems.

2.4 Reaction Time

Reaction time is the time required by the combat system to initiate and complete the engagement, given athreat declaration from the detection suite. Many combat systems are being designed to be "fullyautomatic" thereby precluding any delay associated with the human decision time. Jn the PRA analysis,the reaction time element can be viewed as the probability that the "fully automatic" reaction time can berealized during an engagement. Given historical precedence and the significant negative consequence ofengaging a friendly target, it is likely that the human decision element will remain an importantcomponent of the reaction time. The problem becomes one of minimizing the operator decision time.This can only be done by providing target track information to the operator or system which can aid inthe discrimination between friendly and hostile intent.

Experience with current radar based detection suites indicate large variability in human decision times.This implies current radar based detection suites do not always provide data sufficient to identify hostiletargets with sufficient confidence to order an engagement. One reliable discriminant is target altitude.

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The target track accuracy of the IRST is one or two orders of magnitude better than a fire control radar.This angle track precision, when combined with the target range data from the radar, eliminates targetaltitude ambiguity, thereby allowing immediate confirmation of the presence of a seaskimmer vice anaircraft.

When IRST is used in conjunction with the radar, the probability of minimizing the human decision timeis increased, leading to higher overall PRA.

2.5 Firepower

Firepower is the probability that the defending missiles will defeat the threat. Sensors such as IRST canaid firepower in several ways. Early declaration, i.e. ranges exceeding the minimum requirement, canincrease battle space to allow more missile firings. Firepower can be improved by providing thedefending missile with precision target position data. Because the accuracy ofthe IRST track data ismeasured in feet, this information can be used by the combat system and defending missile to improvemissile endgame trajectory and hence lethality.

The precision tracking by the IRST can also increase firepower by supporting pointing ofthe fire controlilluminator. The illuminator is a radar emitter which floods the target with coded RF emissions. Theradar returns are used by the defending missile for semi-active terminal guidance. The IRST supportsaccurate pointing of the illuminator during periods when radars may "lose" the track, such as duringfades.

IRST can also provide information sufficient to support a direct engagement, i.e. no radar track required,when IR guided missiles are used. This provides a completely passive engagement capability for theship.

Early declaration and precision target tracking also aids softkill. The IRST can support early declarationranges required to maximize effectiveness of countermeasures. The precision tracking allowsconfirmation of softkill effectiveness in seconds as opposed to the minutes required by radars.

The probability of supporting the required firepower is improved when IRST is used in conjunction withradars.

2.6 Resistance to Degradation

Resistance to degradation is the robustness with which the elements of coverage, reaction time andfirepower can be delivered. For example, coverage performance of a sensor can be degraded by naturalor man-made environmental conditions. Surveillance and fire control radar performance can be degradedby natural factors such as surface based ducting, multipath, fading, and land clutter associated withlittoral operations. Man-made factors include electronic countermeasures which can be on missiles,ships, aircraft, or land based vehicles and installations.

IRST coverage can also be degraded by natural and man-made conditions. IR energy propagation isattenuated by severe weather such as heavy precipitation, fog, humidity and aerosols. This makes itimportant to specify the weather conditions for which coverage performance is specified. IR propagationcan also be affected by heavy refractive conditions causing beam bending near the water surface. Man-made factors include passive and active IR countermeasures which can be on ships, aircraft and landbased vehicles and installations.

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In general, the conditions affecting radars and IRST tend towards mutual exclusivity. A combat systemcombining IRST with radars becomes relatively immune to all performance degrading factors. Thissynergism between IR and RF provides the most significant benefits because it affects all thecornerstones.

2.7 Availabifity

Availability is the probability that the combat system will be available when required. OperationalAvailability is typically defines as UPTIME/(UPTIME+DOWNTIME). For detection sensors,availability is usually a specified requirement and is achieved through a combination of reliability andmaintainability design and the logistics support structure. Typically, an availability above 0.90 is achallenge for most electro-mechanical systems. For the detection capability of the combat system,availability approaching unity can be achieved by using IRST in conjunction with a radar.

2.9 Total PRA

Figure 4 shows how the addition of an IRST to a radar-based combat system detection suite candramatically improve the ability to achieve the required PRA. All the figures are contrived and providedfor exemplary purposes only. As can be seen, designing a single sensor or type of sensor to meet orexceed

. Precision Raid Count

. Increased Declaration Range

. Operates During EMCON

. P31 Performance GrowthS Immediate Seaskimmer

DiscriminationS Allows For Fully Automatic

Reaction Time• Allows Immediate Softkill

AssessmentPrecision Track ImprovesEndgame LethalitySupports Accurate IlluminatorPointingImmune to Conditions ThatDegrade RF

- Surface-Based Ducting- Multipath- Land Clutter- RF Jamming- No Doppler- Future Stealth Enhancements

Availability of Detection SuiteImproves

\ Note: Values Shown Were Selected

\\Rndomly

for Example Purposes Only

Coverage 0.95 0.98 1.00

Reaction Time 0.95 1.00 1.00

Firepower 1.00 1.00 1.00

Resistance to

Degradation

0.90 0.90 0.99

-Availability 0.90 0.90 0.99

Figure 4: PRA Summary Chart

the detection range requirements is a certain strategy for failure, especially in any projected warfightingenvironment that might include jamming or other unfavorable conditions. The key to achieving therequired PRA is to employ sensors which exploit different portions of the EM spectrum. Only in this

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Probability of SupportingShip PRA Requirements

RADAR IRST BOTH

— PRA 0.73 0.79 0.98

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manner can shipboard detection systems become sufficiently robust to achieve the desired result. Astrategy that focuses on the use of RF-only systems is a sure path to quick obsolescence.

3. Conclusions

The methodology employed here has many ramifications for future shipboard combat systems. Futureconflicts will employ seaskimming missiles that are faster and stealthier. Technology that is in theconceptual stages today will be deployed on operational threats. They will be deployed from ships,aircraft and land. Smart and powerful jammers will be employed. Natural propagation anomalies willpersist to create problems for sensors. Most of these factors are aimed at reducing the detectioncapability of the combat system. A detection capability based on using a single (RF) portion of the EMspectrum will have an almost impossible time at achieving PRArequirements in future environments. Itwill also make design of hostile systems easier, thereby accelerating the technological obsolescence ofthe fielded capability. All these factors clearly indicate that shipboard combat systems require detectionsuites combining IR and RF sensors to meet survivability requirements of future conflicts.

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