Distribution Statement A: distribution. Open Navy Radar Trades at the Ship Interface J. White DPM Consulting Amy Billups Johns Hopkins University Applied Physics Laboratory Abstract: The next generation radar suite will include a radar that has sensitivity far greater than shipboard radars in use today. Most likely the radar will be a multi-faced, solid state, phased array system with each array consisting of thousands of individual radiating elements powered by transmit/receive (T/R) modules. It will likely need to be large, and the demands it makes on ship services 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 the ship impact of those choices. Radar design options considered are aperture size and shape, coolant operating temperature, number of array faces, and power system architecture. These design 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 radar system that includes ship-provided equipment such as electrical power distribution equipment and chill water plants. An analysis of radar systems, all with the same performance, shows that radar system power demand can change by as much as four megawatts for a notional surface combatant depending upon the radar designer s configuration choices.radar system weight can vary by more than 100 metric tons. There is a cubic relationship between T/R module power and array face area for a constant sensitivity radar system. This relationship is examined to show that there is often a choice of T/R module power that minimizes radar ship impact. The exact choice of T/R module power 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. Navy endorsement of any particular radar architecture or design approach. Introduction: The next generation air and missile defense mission will demand that ships be able to detect and track low radar cross-section objects at great distances. Threat projections for cruise missiles and other airborne threats suggest that much more capable radar will be required. Radar designers are already beginning to contemplate shipboard radar systems that are much more capable 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 resources than their predecessors. Their large array faces will occupy a large portion of the topside surface area, and the volume of their power and cooling equipment will require ship designers to provide
more machinery spaces. Radar system demands on electrical power generation and distribution systemsand upon the ship s cooling systemswill increase substantially. The new radar systems will be expensive, as will their effects upon ship size and complexity. This paper discusses some aspects of radar design that can affect ship design and ways 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 the most complex. But the antenna is backed up by a large assortment of equipment, as shown in Figure 1. This equipment, peripheral to the radar antennas, could have ship impact as great as that 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 first step in this proces 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, typically 12 or 18. Next, AC/DC converters rectify the power to DC. Large filters then remove whatever voltage ripple might be present. Additional filters, close to or in the arrays remove any additional noise 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 control circuitry can account for up to 40%. Prime Power Source Electrical Distribution Electrical Conversion Electrical Conditionin Primary Heat Exchanger Chill Water Plant Monitoring and Control Cooling Water Distribution Cooling System Signal and Data Processing Heat Exchanger Beam Steering Control Array Figure 1 The Shipboard Radar System A 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 heat removal, a typical radar face includes channels through which cooling liquid flows. Since the radar arrays are exposed to extreme ambient temperature conditions, radar liquid coolant must have a low freeze point, and is often a mixture of ethylene glycol and water (EGW) or propylene glycol and water (PGW). A heat exchanger carries heat from the radar system coolant to the ship s coolant system, which would be theship s fresh water, sea water, chill water or a combination thereof using temperature control valves. 2
Radar System Design Considerations: Radar Design Consideration 1, Coolant Temperature: From the ship impact point of view, radar coolant temperature is one of the most important considerations in radar system design because it determines what the ship must supply to the radar system heat exchanger. If the radar system uses chill water as the heat sink, then coolant 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 the chance of damaging condensation. On the other hand, seawater temperatures can be as high as 37 C so that radar coolant would be as high as 40 C. Moreover, chill water temperature is approximately 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 higher temperature also reduces its output power because the high power amplifiers in the T/R modules become les eficient as temperature increases, and transmit power decreases. The radar system s noise floor also increases with higher temperatures further degrading performance. Each db of radar performance comes at the expense of adding additional radiators and T/R modules to the array faces or increasing the power per T/R module. Because of manufacturing differences among the T/R modules and other components, the radar system manufacturer calibrates each array face after its completion. Each T/R module must include a means to adjust both its phase and amplitude. Unfortunately, these adjustments are not stable with temperature, so the calibration is good only for a small range of temperatures. Using seawater as the heat sink, therefore, requires that the radar be calibrated at the highest expected temperature and that the radar s cooling system maintain that temperature as seawater temperatures vary. Alternatively, the factory might provide calibration data at several temperature ranges. Then the system s software would include provision for adjusting aray calibration values as seawater temperature changes. The costs associated with extra array calibration and software development trade against costs for additional machinery, power consumption, and ship sizewhen chil water is the radar s heat sink. If the radar system uses chill water as the heat sink, then the ship design must include additional space and weight margin for the necessary machinery. Figure 3 illustrates the electrical 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 radar systems the Navy might consider, up to five of these plants would be required. Table I Modern Navy Chill Water Plant Characteristics Capacity 500 Ton @ 97 deg F Seawater Dimensions 13 8 x 6 2 x 8 2 Wet Weight 33,500 lbs Motor HP 650 3
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. Singlefaced, pedestal-mounted radar systems are adequate for some applications, such as single target missile 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 targets simultaneously, which is a challenge for single array radars. The radar designer selects the number of faces based upon cost and performance considerations that depend on the radar functionality. A study by Trunk, reference (b), shows that for a naval radar which must function in both the AAW and the BMD environments, the total number of radiating elements remains about the same regardless of the number of faces for constant search performance. In the threefaced system, scan losses are greater as the system must scan 60 degrees from broadside compared to 45 degrees for the four face system. To make up for the losses, the three-faced system has more radiating elements in each face. The face area of the three-faced system must be about 4/3 that of the four-faced system. That is, the diameter of the three-faced system must be about 15% greater than that of the four-faced system. For the likely case where radar face area influences deckhouse size, the four-faced radar system would be preferred if deckhouse width is constrained. Two possible face layouts are depicted in Figure 2 to illustrate this point. Figure 2 assumes the simple case where the radar face extends across the entire deckhouse and its width determines deckhouse width. In most ship configurations, other apertures besides those for the radar would also mount on the deckhouse or superstructure faces and increase deckhouse surface area requirements. The sizes of the deckhouse faces in these configurations would increase equally to accommodate the other apertures. 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 section symmetrical. But the ship s structure is mostly orthogonal, so a round aray occupies a rectangular space with wasted space at the corners. A rectangular array shape, therefore, occupies less deckhouse surface area than a round one. Four- Three- Faced Faced Radar Radar FWD FWD D 1.1 5 D Figure 2 Number of Array Faces 4
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 radar range equations, that theradar sensitivity is directly proportional to T/R module power and to the cube of the number of elements. Sensitivity P 0 N 3 Where P 0 is the output power of the T/R module and N is the number of radiating elements in the array. Since each element requires a specific amount of array area, sensitivity is also proportional to the active array area cubed, or to the array diameter raised to the sixth power. Thus, a small increase in array diameter can cause a large improvement in radar sensitivity. For constant sensitivity, Figure 3 shows the relationship between T/R module power and active array area (as opposed to physical array area, which might be larger because of mounting flanges or transition pieces). Active Array Area, ft 2 900 800 700 600 500 400 300 200 100 0 T/R Module Power Figure 3 Relationship 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 VDC power to the antenna arrays where DC/DC converters make it available for T/R modules. Power consumption 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 previously experienced with some legacy radar systems, so transmit power draw is higher. But the system consumes receive power constantly. Total power to the T/R modules is the time average of transmitting and receiving power. Figure 4 shows a range of choices the radar designer has trading active array size against system power demand both with and without ship s chlwater. Small arrays must have higher power T/R modules in accordance with the cubic relationship. As 5
array size increases and T/R module power decreases, receiving power becomes a more important part of the total power consumption and total power consumption reaches a minimum. 8.0 7.0 6.0 With Chill Water Without Chill Water 5.0 Electrical Power from Power Generatlor, 4.0 from Generator, 3.0 MW 2.0 1.0 Cooling 0.0 200 400 600 800 Active Array Area, ft 2 Figure 4 Electrical Power Only about 20% of the power the radar system consumes radiates from the antenna; the ship s cooling facilities must remove the rest. Since the cooling load percentage is approximately constant regardless of array size, ship impact follows the same trend as electrical power. Figure 5 shows the potential impact. 1400 1200 1000 Tons of Refrigeration 800 600 400 200 0 200 400 600 800 Active Array Area, ft 2 Figure 5 Cooling Load 6
Footprint Deck area required for radar equipment peripheral to the arrays is dominated by electrical and cooling equipment. Therefore it follows the same trend as electrical power. Figure 6 shows the deck space impact of changing active array area keeping radar performance constant. This figure 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 not the case, deck footprint chargeable to radar systems would increase dramatically since gas turbines require large intake and exhaust ductwork that must penetrate several decks. 4000 3500 3000 2500 Without Chill Water Plant With Chill Water Plant Deck Footprint, ft 2 2000 1500 1000 500 0 200 400 600 800 Active Array Area, ft 2 Figure 6 Deck Footprint Array Weight Array weight for a constant sensitivity increases with its area, but not linearly. As the number of radiating elements increase, the power from each decreases in accordance with the cubic relationship previously discussed. Since a large percentage of the array weight is in power and 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 put array 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 the array In the first category are the radiating elements themselves, the T/R modules and their associated circuitry, a portion of the lowest replaceable unit (LRU) structure and infrastructure, 7
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/DC converters, energy storage components, power filters, and a proportionate share of the electronic structure and infrastructure. Figure 7 illustrates the relationship between active array area and array weight for a constant sensitivity. There is a choice of array area that results in a minimum array weight. 270 250 230 Array Weight, mt 210 190 170 150 200 300 400 500 600 700 800 Active Array Area, ft 2 Figure 7 Array Weight Total System Weight Most of the equipment peripheral to the arrays is for power and cooling, and a decrease in array area below an inflection point causes an increase in weight. In addition, there are some pieces of equipment such as a processor whose weight depends upon the radar genre and its mission and is approximately the same no matter what the array area is. Figure 8 shows total system weight as it changes with array area keeping a constant radar performance. Note that array area for minimum total system weight is greater than the array area for array weight. 8
360 340 320 300 Radar System Including Chill Water Plants Weight, mt 280 260 240 220 200 200 400 600 800 Active Array Area, ft 2 Figure 8 Total System Weight Fuel Figure 9 shows that the fuel needed to operate the radar system for five days can weigh as much as the radar system itself. (Here the analysis assumes an engine operating with a specific fuel consumption of 0.7 pounds per kilowatt-hour.) Moreover, the life cycle costs of so much fuel can be a major part of that for the whole ship. Theship s mission may require that it remain on station for several days at a time without refueling. Figure 10 shows the relationship between fuel consumption and array area with a constant sensitivity for a typical ship. 9
300 250 Without Chill Water With Chill Water Fuel Weight, mt 200 150 100 50 0 200 300 400 500 600 700 800 Active Array Area, ft2 Figure 9 Fuel Weight for Five Days of Radar Operation Fuel Consumption 200 300 400 500 600 700 800 Active Array Area, ft 2 Figure 10 Fuel Consumption for a Typical Ship 10
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 major determinants of radar power demand. In most cases, the BMD mission uses longer pulse width and, therefore, larger duty factor than the AAW mission. But, the BMD mission can use one radar face at a time whereas the AAW mission uses all available faces. Therefore, a good arangement for the radar s power distribution system is one that can deliver all available power to 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 be accomplished with a duty factor that is one fourth or less of what the BMD mission needs on a single face for the four-faced system. Systems analyses might show that the radar cannot adequately perform both the BMD and AAW missions with one face sharing time between the two missions. In that case, it might be necessary to have the AAW and BMD faces transmitting simultaneously effectively increasing the total ship duty factor. Such an arrangement would increase transmit power required since it is directly proportional to the duty factor. Ship impact characteristics 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 fit onto an existing Naval platform. Stringent constraints can be introduced on the radar design when trying to integrate a new advanced radar system on an existing hull form, including: power generation capability, cooling capacity, array size restrictions, ship stability and total displacement limits, and available deckhouse and machinery spaces volume. Introducing constraints from an existing hull form onto a new radar design will have a significant effect on the radar system. In the constrained ship, the radar system may be driven to non-optimal radar choices to achieve maximum performance, e.g. in a ship with a constrained array 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 for different T/R module powers. The two curves represent two different ship platforms each with its own constraints. In this example, there is an array size constraint in both curves. The second curve also has a ship displacement and a ship stability limitation. At lower T/R module powers radar sensitivity is limited by the array size. As T/R module power increases, sensitivity increases. The weight of the radar system increases with increasing T/R module power due mainly to the additional weight of power, cooling, and power conversion systems. This increase in weight requires a reduction in total number of elements to maintain ship stability and displacement. Moreover, the reduction in number of elements results in a decrease in radar sensitivity. Although the maximum aperture size constraint is constant, the additional weight at higher module powers prevents the use of the entire allotted aperture. 11
Array size constraint Radar Sensitivity Ship displacement/stability constraint Array size constraint Ship stability/displacement constraint T/R module power Figure 12 Effect 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 optimum point for any given platform may be significantly different from the next. Figure 13 illustrates the efect on radar performance of a ship s primepower limitation. In this case maximum radar performance can be achieved at low T/R module power levels. As T/R module power increases substantially fewer T/R modules can be used to meet the power limitation, thus reducing total radar sensitivity. Radar Sensitivity T/R module power Figure 13 Effect on radar sensitivity of prime power constraint vs. T/R Module Power. 12
Ship limits need to be considered in the all decision points in the radar design. To achieve optimum radar performance in the constrained ship several design iterations between the radar and ship designers may be required. Trade studies need to be conducted to achieve optimum radar performance for each specific set of constraints. Therefore, ship and radar design must be cooperative. Conclusions: There is a great deal radar designers can do to reduce the ship impact of large, advanced radar systems. Paramount is the need to use a largeraray size consistent with the ship s deckhouse dimensions and radar costs. For ship impact, four-face radar systems are probably better than three-faced systems. Radars designers should consider systems that are not dependent on the ship s chil 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 huge improvement to the ship designs. Next generation radar systems will have a significant impact on many facets of ship design. 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 between radar designers and ship designers is difficult, but it is essential to achieve the best total system for the Navy. 13
References: (a) J. Frank et al, Impact of T/R Module Power Level on the Attributes of Active Phased Array Radars, Proceedings of the 50th Tri-Service Radar Symposium, Monterey, CA, June 2004 (b)g.v. Trunk, Optimal Number of Phased Aray Faces and Signal Procesors for Horizon and Volume Surveillance -Revisited, 2003 Tri-Service Radar Symposium, Boulder, Colorado 23-27 June, 2003 Acknowledgement: This work was done at JHU/APL under task BKM14 of contract N00024-03-D-6606 with the US Navy. 14