Navy RadarTrades at the Ship Interface

J. WhiteDPM Consulting

Amy BillupsJohns Hopkins University Applied Physics Laboratory

Abstract:

The next generation radar suite will include a radar that has sensitivity far greater thanshipboard radars in use today. Most likely the radar will be a multi-faced, solid state, phasedarray system with each array consisting of thousands of individual radiating elements powered bytransmit/receive (T/R) modules. It will likely need to be large, and the demands it makes on shipservices will be unprecedented. Itselectrical power demands could be larger than the total ship’s load for present day ships. Array size could significantly impact topside design.

This paper examines top level radar system design choices and illustrates the trends in theship impact of those choices. Radar design options considered are aperture size and shape,coolant operating temperature, number of array faces, and power system architecture. Thesedesign options have an effect onthe radar system’s weight, footprint, power demand, and cooling load. The effects on ship design can be significant. The ship impact analyses consider a radarsystem that includes ship-provided equipment such as electrical power distribution equipment andchill water plants.

An analysis of radar systems, all with the same performance, shows that radar systempower demand can change by as much as four megawatts for a notional surface combatantdepending upon the radar designer’s configuration choices.Radar system weight can vary bymore than 100 metric tons.

There is a cubic relationship between T/R module power and array face area for aconstant sensitivity radar system. This relationship is examined to show that there is often achoice of T/R module power that minimizes radar ship impact. The exact choice of T/R modulepower depends upon the customer’s preferences, the manufacturer’s capabilities and choice of vendors.

This paper does not convey any official U. S. Government position or U. S. Navyendorsement of any particular radar architecture or design approach.

Introduction:

The next generation air and missile defense mission will demand that ships be able todetect and track low radar cross-section objects at great distances. Threat projections for cruisemissiles and other airborne threats suggest that much more capable radar will be required. Radardesigners are already beginning to contemplate shipboard radar systems that are much morecapable than the SPY-1 radar systems currently deployed in Aegis cruisers and destroyers.

The new radar systems will consume much more of the available shipboard resourcesthan their predecessors. Their large array faces will occupy a large portion of the topside surfacearea, and the volume of their power and cooling equipment will require ship designers to provide

Distribution Statement A: Opendistribution.

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more machinery spaces. Radar system demands on electrical power generation and distributionsystemsand upon the ship’s cooling systemswill increase substantially.

The new radar systems will be expensive, as will their effects upon ship size andcomplexity. This paper discusses some aspects of radar design that can affect ship design andways that minimizea radar system’s impact upon the ship.

The Shipboard Radar System

The antenna portion of a shipboard, phased array system is the most visible and often themost complex. But the antenna is backed up by a large assortment of equipment, as shown inFigure 1. This equipment, peripheral to the radar antennas, could have ship impact as great asthat of the antennas themselves.

The radar’s electricalequipment must convert AC electrical power from the ship’s distribution system into high quality 300 VDC input to DC/DC converters in the arrays. The firststep in this process involves a transformer to reduce ship’s voltage. To help reduce harmonic distortion, the three-phase ship’s power input is converted to power with more phases, typically12 or 18. Next, AC/DC converters rectify the power to DC. Large filters then remove whatevervoltage ripple might be present. Additional filters, close to or in the arrays remove any additionalnoise or rippleon the 300 VDC bus. The 300 VDC bus accounts for most of the radar system’s power consumption, but signal processors, heaters, pumps, chill water systems, and controlcircuitry can account for up to 40%.

Figure 1 The Shipboard Radar SystemA shipboard radar system includes power and cooling equipment as well as digital

processing equipment and antenna arrays. (Components in red located within deckhouse)

Typically less than20% of the radar system’s total power demand leaves the radar face as radiated signal. The ship’s cooling system must remove theremaining heat. To facilitate heatremoval, a typical radar face includes channels through which cooling liquid flows. Since theradar arrays are exposed to extreme ambient temperature conditions, radar liquid coolant musthave a low freeze point, and is often a mixture of ethylene glycol and water (EGW) or propyleneglycol and water (PGW). A heat exchanger carries heat from the radar system coolant to theship’s coolant system, which would be theship’s fresh water, sea water, chill water or acombination thereof using temperature control valves.

PrimePowerSource

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Radar System Design Considerations:

Radar Design Consideration 1, Coolant Temperature:

From the ship impact point of view, radar coolant temperature is one of the mostimportant considerations in radar system design because it determines what the ship must supplyto the radar system heat exchanger. If the radar system uses chill water as the heat sink, thencoolant entering the radar can be as low as 10ºC, chill water being provided at about 7°C.However, most radar designers prefer coolant temperatures of approximately 20 ºC to reduce thechance of damaging condensation. On the other hand, seawater temperatures can be as high as37°C so that radar coolant would be as high as 40°C. Moreover, chill water temperature isapproximately constant, but seawater temperature varies with time of year and ship location.

Reliability engineers prefer that electronics operate at the coldest possible temperature,and would therefore choose chill water for radar cooling. Operating the radar at a highertemperature also reduces its output power because the high power amplifiers in the T/R modulesbecome less efficient as temperature increases, and transmit power decreases. The radar system’s noise floor also increases with higher temperatures further degrading performance. Each dB ofradar performance comes at the expense of adding additional radiators and T/R modules to thearray faces or increasing the power per T/R module.

Because of manufacturing differences among the T/R modules and other components, theradar system manufacturer calibrates each array face after its completion. Each T/R module mustinclude a means to adjust both its phase and amplitude. Unfortunately, these adjustments are notstable with temperature, so the calibration is good only for a small range of temperatures. Usingseawater as the heat sink, therefore, requires that the radar be calibrated at the highest expectedtemperature and that the radar’s cooling system maintain that temperature as seawater temperatures vary. Alternatively, the factory might provide calibration data at severaltemperature ranges. Then the system’s software would include provision for adjusting array calibration values as seawater temperature changes. The costs associated with extra arraycalibration and software development trade against costs for additional machinery, powerconsumption, and ship sizewhen chill water is the radar’s heat sink.

If the radar system uses chill water as the heat sink, then the ship design must includeadditional space and weight margin for the necessary machinery. Figure 3 illustrates theelectrical power impact of the chill water plants for radar system that might be suitable for CGX.Table I shows the characteristics of a modern chill water plant. For some advanced radarsystems the Navy might consider, up to five of these plants would be required.

Table IModern Navy Chill Water Plant Characteristics

Capacity 500 Ton @ 97 deg F SeawaterDimensions 13’ 8” x 6’ 2” x 8’ 2”Wet Weight 33,500 lbsMotor HP 650

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Radar Design Consideration Number 2, Number of Array Faces and Array Shape:

A primary decision for the radar designer is the number of radar faces to employ. Single-faced, pedestal-mounted radar systems are adequate for some applications, such as single targetmissile tracking, but a system which must have 360 degree coverage will need three or four faces.Like the Aegis system currently deployed, it will be necessary to track many fast moving targetssimultaneously, which is a challenge for single array radars. The radar designer selects thenumber of faces based upon cost and performance considerations that depend on the radarfunctionality. A study by Trunk, reference (b), shows that for a naval radar which must functionin both the AAW and the BMD environments, the total number of radiating elements remainsabout the same regardless of the number of faces for constant search performance. In the three-faced system, scan losses are greater as the system must scan 60 degrees from broadsidecompared to 45 degrees for the four face system. To make up for the losses, the three-facedsystem has more radiating elements in each face. The face area of the three-faced system must beabout 4/3 that of the four-faced system. That is, the diameter of the three-faced system must beabout 15% greater than that of the four-faced system.

For the likely case where radar face area influences deckhouse size, the four-faced radarsystem would be preferred if deckhouse width is constrained. Two possible face layouts aredepicted in Figure 2 to illustrate this point. Figure 2 assumes the simple case where the radar faceextends across the entire deckhouse and its width determines deckhouse width. In most shipconfigurations, other apertures besides those for the radar would also mount on the deckhouse orsuperstructure faces and increase deckhouse surface area requirements. The sizes of thedeckhouse faces in these configurations would increase equally to accommodate the otherapertures. The analysis in this paper assumes a four-faced configuration.

Array shape is another concern to the radar designer because it determines beam shape.Some designers prefer a round or octagonal shape because it makes the beam cross sectionsymmetrical. But the ship’s structure is mostly orthogonal, so a round array occupies a rectangular space with wasted space at the corners. A rectangular array shape, therefore,occupies less deckhouse surface area than a round one.

Figure 2Number of Array Faces

D D1.15

Four-FacedRadar

Three-FacedRadar

FWD FWD

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Radar Design Option 3, T/R Module Power and Active Array Area:

In reference (a), Frank et al show, by rearranging the discriminating or tracking radarrange equations, that theradar’s sensitivity is directly proportional to T/R module power and tothe cube of the number of elements.

30NPySensitivit

Where P0 is the output power of the T/R module and N is the number of radiating elements in thearray. Since each element requires a specific amount of array area, sensitivity is also proportionalto the active array area cubed, or to the array diameter raised to the sixth power. Thus, a smallincrease in array diameter can cause a large improvement in radar sensitivity. For constantsensitivity, Figure 3 shows the relationship between T/R module power and active array area (asopposed to physical array area, which might be larger because of mounting flanges or transitionpieces).

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Figure 3Relationship between T/R Module Power and Array Area for a Constant Sensitivity

Electrical Power

Most of the electrical power in recent radar design concepts is consumed as 300 VDCpower to the antenna arrays where DC/DC converters make it available for T/R modules. Powerconsumption is much higher during the system’s transmit pulse than when it is receiving. While performing the BMD mission, the radar requires longer transmit pulses than previouslyexperienced with some legacy radar systems, so transmit power draw is higher. But the systemconsumes receive power constantly. Total power to the T/R modules is the time average oftransmitting and receiving power. Figure 4 shows a range of choices the radar designer hastrading active array size against system power demand both with and without ship’s chllwater.Small arrays must have higher power T/R modules in accordance with the cubic relationship. As

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array size increases and T/R module power decreases, receiving power becomes a more importantpart of the total power consumption and total power consumption reaches a minimum.

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Figure 4Electrical Power

Cooling

Only about 20% of the power the radar system consumes radiates from the antenna; theship’s cooling facilities must remove the rest. Since the cooling load percentage is approximatelyconstant regardless of array size, ship impact follows the same trend as electrical power. Figure 5shows the potential impact.

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Figure 5Cooling Load

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Footprint

Deck area required for radar equipment peripheral to the arrays is dominated by electricaland cooling equipment. Therefore it follows the same trend as electrical power. Figure 6 showsthe deck space impact of changing active array area keeping radar performance constant. Thisfigure assumes that sufficient electrical power is available from the ship’s integrated power system and that the ship would need no additional engine capacity for radar power. Were this notthe case, deck footprint chargeable to radar systems would increase dramatically since gasturbines require large intake and exhaust ductwork that must penetrate several decks.

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Figure 6Deck Footprint

Array Weight

Array weight for a constant sensitivity increases with its area, but not linearly. As thenumber of radiating elements increase, the power from each decreases in accordance with thecubic relationship previously discussed. Since a large percentage of the array weight is in powerand cooling components there is an offsetting tendency to decrease weight as array size increases.

To illustrate the effects of active array area changes on array weight, it is helpful to putarray components into two categories and sum the results:

Category One — those components whose weight is proportional to the array area,Category Two–- those whose characteristics change with total power delivered to thearray

In the first category are the radiating elements themselves, the T/R modules and theirassociated circuitry, a portion of the lowest replaceable unit (LRU) structure and infrastructure,

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digital beam forming systems, the antenna radome, the antenna structure that supports electronics,and the structure upon which radiating elements are mounted.

The second category consists of some components within the array including DC/DCconverters, energy storage components, power filters, and a proportionate share of the electronicstructure and infrastructure.

Figure 7 illustrates the relationship between active array area and array weight for aconstant sensitivity. There is a choice of array area that results in a minimum array weight.

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Figure 7Array Weight

Total System Weight

Most of the equipment peripheral to the arrays is for power and cooling, and a decrease inarray area below an inflection point causes an increase in weight. In addition, there are somepieces of equipment such as a processor whose weight depends upon the radar genre and itsmission and is approximately the same no matter what the array area is. Figure 8 shows totalsystem weight as it changes with array area keeping a constant radar performance. Note thatarray area for minimum total system weight is greater than the array area for array weight.

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Figure 8Total System Weight

Fuel

Figure 9 shows that the fuel needed to operate the radar system for five days can weigh asmuch as the radar system itself. (Here the analysis assumes an engine operating with a specificfuel consumption of 0.7 pounds per kilowatt-hour.) Moreover, the life cycle costs of so muchfuel can be a major part of that for the whole ship. Theship’s mission may require that it remainon station for several days at a time without refueling. Figure 10 shows the relationship betweenfuel consumption and array area with a constant sensitivity for a typical ship.

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Figure 9Fuel Weight for Five Days of Radar Operation

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Figure 10Fuel Consumption for a Typical Ship

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Radar Design Consideration Number 4, Power System Architecture and Duty Factor:

Duty factor, the percentage of time the radar is actually transmitting, is one of the majordeterminants of radar power demand. In most cases, the BMD mission uses longer pulse widthand, therefore, larger duty factor than the AAW mission. But, the BMD mission can use oneradar face at a time whereas the AAW mission uses all available faces. Therefore, a goodarrangement for the radar’s power distribution system is one that can deliver all available powerto any single radar face or deliver equal but smaller amounts of power to all faces simultaneously.To accommodate this, the radar designer would insure that the AAW mission can beaccomplished with a duty factor that is one fourth or less of what the BMD mission needs on asingle face for the four-faced system. Systems analyses might show that the radar cannotadequately perform both the BMD and AAW missions with one face sharing time between thetwo missions. In that case, it might be necessary to have the AAW and BMD faces transmittingsimultaneously effectively increasing the total ship duty factor. Such an arrangement wouldincrease transmit power required since it is directly proportional to the duty factor. Ship impactcharacteristics would be increased as well.

Radar Design Consideration Number 5, The Constrained Ship

Next generation shipboard radar systems could be on either a new ship class or back fitonto an existing Naval platform. Stringent constraints can be introduced on the radar designwhen trying to integrate a new advanced radar system on an existing hull form, including: powergeneration capability, cooling capacity, array size restrictions, ship stability and totaldisplacement limits, and available deckhouse and machinery spaces volume.

Introducing constraints from an existing hull form onto a new radar design will have asignificant effect on the radar system. In the constrained ship, the radar system may be driven tonon-optimal radar choices to achieve maximum performance, e.g. in a ship with a constrainedarray size, module power may need to be increased to meet radar performance requirements,negatively impacting power, cooling, and weight.

Figure 12 illustrates an example of the effect of ship constraints on radar sensitivity fordifferent T/R module powers. The two curves represent two different ship platforms each with itsown constraints. In this example, there is an array size constraint in both curves. The secondcurve also has a ship displacement and a ship stability limitation. At lower T/R module powersradar sensitivity is limited by the array size. As T/R module power increases, sensitivityincreases. The weight of the radar system increases with increasing T/R module power duemainly to the additional weight of power, cooling, and power conversion systems. This increasein weight requires a reduction in total number of elements to maintain ship stability anddisplacement. Moreover, the reduction in number of elements results in a decrease in radarsensitivity. Although the maximum aperture size constraint is constant, the additional weight athigher module powers prevents the use of the entire allotted aperture.

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Figure 12Effect on radar sensitivity of array size and ship displacement and stability constraints

for varying T/R module powers

Each set of constraints or each individual hull form requires a trade study. The optimumpoint for any given platform may be significantly different from the next. Figure 13 illustrates theeffect on radar performance of a ship’s primepower limitation. In this case maximum radarperformance can be achieved at low T/R module power levels. As T/R module power increasessubstantially fewer T/R modules can be used to meet the power limitation, thus reducing totalradar sensitivity.

Figure 13Effect on radar sensitivity of prime power constraint vs. T/R Module Power.

Array size constraint

Ship stability/displacement constraint

T/R module power

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Ship limits need to be considered in the all decision points in the radar design. To achieveoptimum radar performance in the constrained ship several design iterations between the radarand ship designers may be required. Trade studies need to be conducted to achieve optimum radarperformance for each specific set of constraints. Therefore, ship and radar design must becooperative.

Conclusions:

There is a great deal radar designers can do to reduce the ship impact of large, advancedradar systems. Paramount is the need to use a largerarray size consistent with the ship’s deckhouse dimensions and radar costs. For ship impact, four-face radar systems are probablybetter than three-faced systems. Radars designers should consider systems that are not dependenton the ship’s chill water system. Radar electrical power architecture should account for both the BMD mission and the AAW mission. In total, these efforts to reduce ship impact can be a hugeimprovement to the ship designs.

Next generation radar systems will have a significant impact on many facets of shipdesign. To achieve optimum performance and cost, radar and ship design must be integrated.Independent designs will not provide the best radar/ship system design. Cooperation betweenradar designers and ship designers is difficult, but it is essential to achieve the best total systemfor the Navy.

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References:

(a) J. Frank et al,“Impact of T/R Module Power Level on the Attributes ofActive Phased Array Radars,”Proceedings of the 50th Tri-Service Radar Symposium, Monterey,CA, June 2004

(b)G.V. Trunk, “Optimal Number of Phased Array Faces and Signal Processors for Horizon and Volume Surveillance -Revisited,” 2003 Tri-Service Radar Symposium, Boulder, Colorado23-27 June, 2003

Acknowledgement:

This work was done at JHU/APL under task BKM14 of contract N00024-03-D-6606 with the USNavy.