Airborne active element array radars come of age.
and Enabling Techniques
Fire control and other airborne radars have experienced major increases in capability and sophistication in the last two decades, boosted by the introduction of powerful digital signal processors; but their uncertain reliability reduces system availability and increases life cycle costs. Their low reliability is due to the use of tube-type transmitters and power supplies. Their lifetime also is limited by wear of moving antenna parts in a high dynamic load environment. Basic physical limits on mechanically-scanned antennas also prevent the instantaneous scanning or dwells, increasingly needed as threat levels escalate. AESA antennas are a proven solution to the reliability problem, promise even higher levels of performance, and presently are demonstrating excellent progress toward competitive costs.
X-band AESA Matures
The benefits of AESA radar antennas are many, but the primary areas are system power efficiencies; high reliability and graceful degradation; inertialess beam agility; and the ability to form dynamic beam pattern shapes. Power efficiency is higher primarily because both transmit and receive amplification of RF signals is done directly behind the radiating surface, not incurring several dB of waveguide or manifold losses. The inertialess agility of electronically scanning allows more flexible and efficient coverage of required scan volumes, and using techniques such as selective target dwells increases detection and tracking capability against low radar cross section targets. AESA arrays also may be configured to provide specialized beam shapes to support mission specific radar modes and/or to reject jamming signals.
AESA antennas consist of a grid of transmit and receive (T/R) modules, spaced regularly on half wave-length centers. The modules contain solid-state and passive microwave circuitry, and exhibit mean time between failure (MTBF) at or above 100,000 hours. These two factors result in AESA antennas having inherent high reliability and very long times between system-critical failures (MTBCF) due to the distributed architecture of these high MTBF building blocks. The overall MTBCF of an AESA array, including its power supplies and beam steering digital units, easily can exceed 1000 hours. This compares well with modern conventional radars, which after many years of product maturity are now showing MTBF growth above 100 hours. This AESA advantage promises not only a major increase in flight availability of airborne radars, but also significant reduction in life cycle maintenance costs.
The Evolution of X-band AESA
The evolution of X-band AESA radar has been paced by several key technologies, but the emergence of MICs has been the latest and most important. This evolution is keyed to developments illustrated in Figure 1. This figure shows a series of AESA radar antennas developed by Texas Instruments under the sponsorship of the USAF Wright Avionics Laboratories during the past 25 years. As summarized below, this sequence of advanced developments was pivotal first in showing feasibility, and later in demonstrating the reliability, performance and cost attributes of AESA.
The introduction of silicon digital ICs made it feasible to control an array of tightly packed, complex electronic modules. TI first developed the X-band modular electronics for radar application (MERA) AESA antenna in the early 1960s. MERA's first generation T/R modules used GaAs components to select the transmit or receive path circuitry, and to control switched line phase shift circuits used to scan the pencil beam pattern. Both transmitter and receiver amplifiers used silicon transistors. The overall power efficiency of the MERA system was low, due to limitations of silicon devices at X-band; but it did fully demonstrate the basic feasibility of applying AESA technology to airborne radar application.
As a successor to MERA, TI developed the reliable advanced solid-state radar (RASSR) antenna in the early 1970s. This program demonstrated T/R module reliability, as well as the inherent reliability of AESA architecture. It also clearly showed AESA's ability to form and scan variable shaped radiation patterns. RASSR antenna radiation pattern performance was equal to contemporary airborne radar requirements; but it was equipped with low efficiency silicon active devices, multipliers and frequency converters between its X-band in-space signal and S-band amplifiers.
Building on the success of RASSR, beginning in 1983, the solid-state phased-array (SSPA) was the first USAF/TI X-band AESA array to realize the performance efficiency of direct power and receive amplification at X-band using GaAs devices. Individual GaAs FETs were used in hybrid curcuit amplifiers for both transmit and receive functions. The SSPA program also baselined the cost of X-band AESA technology. More than 2000 GaAs-based T/R modules were mass produced to build the array. The SSPA was sized to approximate the radiating area of existing air-to-air fire control radars. Two way radiation pattern performance comparable to modern flat plate radar antennas was measured throughout the eight month SSPA test period and over an additional 18 months of use in the ultra reliable radar (URR) program. Design breakthroughs in thermal technology were made to control junction temperatures to a level required for high reliability operation. Over almost 6,000,000 module operating hours, the T/R modules demonstrated an MTBF of over 70,000 hours and the SSPA antenna easily met the 1500 hours design reliability goal.
A typical functional schematic diagram for an AESA-equipped radar is shown in Figure 2, where the system is configured into three major divisions. The AESA group conditions the signal, steers and shapes the beams, and radiates and receives RF energy. It normally consists of the T/R module array and its supporting structure, manifolds to connect DC and RF to all modules, thermal control equipment, low noise power supplies for the array and digital functions for beam steering.
The receiver-exciter (Rx/Ex) group controls the operation and timing of the radar, sets the excitation waveform, and converts the received signal to digital form for processing. It also executes overall control of the radar system function and monitors system performance. The processor group derives target information from this digital stream and formats the desired synthetic data for operator display. This separated processor arrangement also can support sharing computers between multiple sensors, for example, EO or EW, for efficiency or data fusion before display. While the Rx/Ex and processor groups are important technical areas, this article concentrates on the discussion of the AESA group.
Overall, the most obvious difference between AESA systems and conventional radars is the absence of mechanical means to scan the radiating beam. Also, the high power microwave signal is generated in the T/R modules, located just behind the radiating aperture surface. Similarly, each T/R module also contains the first stages of noise amplification, as shown in Figure 3, to set the noise figure and dynamic range of the system.
Passive electronically-scanned array radars are similar to AESAs in that they feature inertialess beam scanning agility by means of modules that phase shift only; elimination of the amplifier function in the modules results in the term passive. They are akin to conventional radars in that they require the use of a high power transmitter. Passive ESA antennas provide less flexibility in radiation pattern shape. Transmitter characteristics also limit the operational bandwidth and represent the principle failure mechanism of this type of radar. The increased high power RF losses incurred between the transmitter and the radiating aperture, and subsequent losses back through the antenna to the receiver must be compensated for by increasing the generated power; this can amount to an additional 4 to 6 dB of additional power in an already limited environment. Further, the increased weight of a transmitter three to four times as powerful is a significant disadvantage of this type of system.
Advancements in microwave semiconductor technology gives active ESA antenna performance an increasing advantage on a pound for pound basis as range performance demands increase.
The feasibility of development of AESA radar for the 1990s is keyed to three issues, performance, reliability and system cost; with cost considered to be the highest remaining risk.
Overall AESA radar performance is primarily dependent on the capability of the microwave circuits within the T/R module, which provide transmit power and amplification; phase shifting to steer and control the transmit and receive radiation patterns; and low noise receive amplifiers to set the system noise figure.
Over the last few years, several generations of MMIC circuits have been developed to support each of these functions. These MMICs not only satisfy the demands of high performance, but also are highly producible.
MMIC circuit performance now makes possible X-band AESA radars sized for tactical aircraft. The availability of power semiconductor amplifiers at X-band has increased dramatically over the past 20 years, as shown in Figure 4. In the late 1970s, silicon power transistors demonstrated up to 1 W peak power at X-band; and efficiencies improved from 15 to 20 percent. More recently, GaAs power amplifiers are demonstrating 3 to 5 W per device with junction efficiencies in the 30 to 40 percent range, and MMICs with parallel output stages have achieved powers approaching 10 W.
The much smaller size and repeatable performance of MMICs (vs. earlier hybrids) very strongly impacts overall module size, and manufacturing complexity and cost. To illustrate, Figure 5 compares an all-hybrid SSPA module made in 1986 with a later, all-MMIC module with equivalent performance parameters. The all-MMIC module contains fewer components (59 vs. 348), sub-assemblies (2 vs. 5) and interconnections (200 vs. 561).
MMICs for low noise and power amplifiers, plus peripheral circuit functions such as phases shifters, limiters and switches, are becoming available today. Figure 6 shows an aggregation of such available components, including control functions, attenuators, switches, phase shifters and vector modulators; receiver functions with low noise amplifiers operating at frequencies from 100 MHz through Ku-band, and featuring variable gain dual gate FETs; narrowband transmitters, both driver and power stages that operate at X- and Ku-bands; and wideband amplifiers capable of three to four octave bandwidths.
Many programs, funded by both the government and industry, are pushing the evolution of MMIC technology. The DoD MIMIC program, for example, is developing not only MMIC circuits but also the processes by which such circuits may be developed rapidly, repeatably, and at low production cost. Other programs sponsored by the Air Force, Navy and Army Laboratory Commands are expanding the performance, reliability and cost envelope of such components.
Another key component in implementing AESA arrays is compact, highly reliable power supplies to develop the low voltage, high current, low noise power required by the solid-state T/R modules. Power densities have increased from a few tenths of one watt per cubic inch to a level of 10 to 30 W per cubic inch. As summarized in Table 1, similar improvements in MTBF and volume power density also have been realized.
Table : TABLE 1 COMPARISON OF POWER SUPPLY TECHNOLOGIES
Linear, Switching, Switching, Discrete Discrete High Density Efficiency 20%-50% 60%-90% 60%-90% Power Density (w/inch) 0.5-1 1-3 10-30 Weight Density (W/lb) 10-20 20-40 100-400 Reliability, MTBF/hrs 15,000-30,000 10,000-20,000 15,000-30,000
(200 W, airborne uninhabited, MIL-HNBK 217 predition)
AESA Reliability Attributes
The inherently parallel structure of AESA architecture greatly reduces the sensitivity of the system to single point or serial failures. The random failures of individual T/R modules make only undetectably minor changes to the radiation pattern as the element is removed from the phase/amplitude distribution of the desired beam excitation function. These independent losses reduce accuracy in a very minor way but do not represent loss of total desired radar function. For example, the transmit function in AESA radar is distributed over many T/R modules (hundreds of thousands) rather than a single, high power tube. The reduction in radar power caused by the loss of a power amplifier in a phased-array or one TWT transmitter in a conventional or passive ESA radar is proportional to (N-1)/N in both cases. However, the result is significantly different for N=2000 (99.95 percent remaining power) for a typical X-band AESA than for either of the tube radars where N=1 (0.0 percent remaining power).
On receive, the indication of T/R module failure is in the very fine details of the radiation pattern, such as the depth of the null at the center of a monopulse beam or the low level sidelobes at wide separations from the main beam. The fine detail of the sidelobe increase for a particular set of module failures will differ as the beam is scanned from place to place. However, the average impact of this type of degradation can be expressed as a particular set of failed modules in RMS sidelobes.
Figure 7 quantifies this RMS sidelobe degradation as a function of the cumulative, independent loss of modules. This degradation can be expressed in terms of percentage of module failure or equivalently as the expected time to loss of a certain percentage of modules. An additional abscissa is included in Figure 7 to illustrate the anticipated timeline for RMS sidelobe increase in AESA of 1200 T/R modules, each with an MTBF of 150,000 hours. Typically, AESA loss of function is defined as the loss of 5 percent of the radiating elements. Figure 8 illustrates that the functional loss is not catastrophic. This graceful degradation allows significant flexibility in maintenance scheduling procedures, since degradation over the course of many two to three hour sorties will not be noticeable for any practical purpose.
One of the most extensive X-band AESA reliability data bases available was developed on the SSPA program. The SSPA array now has logged 3000 total operating hours, which is equal to 5.8 million module hours. At the time of delivery to the US Air Force, in May 1988, the total modules failure count was 58 out of the total array of 1980; but the measured radiation pattern RMS sidelobe increase was less than 1 dB. The array then was operated for another 15 months (1300 additional array hours) at Westinghouse on the URR program without refurbishment. To this point, the hybrid SSPA modules have demonstrated an MTBF of 70,000 hours.
Other AESA system failures, such as loss of a cluster of elements by failure of a modular power supply, a manifold or connector will have more impact than loss of a single T/R module. In the configuration of Figure 2, for example, the array transmit input connector easily may be seen to be a single point failure item. These critical points can, however, be minimized in number and implemented with highly reliable components. Or these elements can be connected in parallel to assure high MTBCF for the system.
A method for improving reliability through power supply redundancy is illustrated in Figure 8. Cases l through IV show different methods for distributing redundant circuits and subsystems.
Case I shows five (assumed infinitely reliable) subarrays, each with a dedicated power supply. For purposes of discussion, we will assume that the system has a design margin that would allow 10 percent reduction in power supply capability as the minimum acceptable limit. In the configuration of Case I, loss of a single supply results in a 20 percent power reduction, which is unacceptable. For this case then, both the system MTBF and the system MTBCF are determined to be 2000 hours, the expected time to loss of one power supply.
Each 10,000 hour power supply unit of Case I can be constructed of five identical subassemblies, each having a reliability of 50,000 hours MTBF. The loss of one subassembly then defines the unit failure rate (MTBF = 10,000 hours).
Case II illustrates the impact of parallel connecting the power supplies to a single bus feeding the subarrays. The 25 subassemblies are grouped in five power supply units, each interconnected with the array. The serial MTBF is 2000 hours, the time to loss of a single subassembly. In this configuration, failure of the system is tied to loss of greater than 10 percent of the power supply capacity or the loss of any three of the 25 available subassemblies. This can be expected to happen at an MTBCF of 6257 hours.
Cases III and IV show two ways of extending this parallel architecture to include redundancy in the system. Case III adds redundancy to the configuration of Case I by adding an additional 50,000 hours subassembly to each unit. Table II shows the serial MTBF decreasing due to the increased number of subassemblies. The MTBCF associated with loss of two subassemblies in any box slightly is increased to 6418 hours. Case IV shows the impact of parallel-connected, redundant units that increases the MTBCF to 15,209 hours.
Table : TABLE II REDUNDANCY MULTIPLIES POWER SUPPLY RELIABILITY
# Subassemblies Serial MBTF Failure MTBCF Case I 25 2000 1 unit 2000 Case II 25 2000 3 S/A 6257 Casel III 30 1667 1 unit 6418 Case IV 30 1667 8 S/A 15,209
The addition of redundant power supply modules does not increase material system cost or complexity, yet it strongly complements the very high reliability and fail-soft character of the T/R module array.
Cost and Affordability
Affordability of the AESA radar is keyed to the production cost of modules. Current AESA cost models indicate that T/R module costs constitute 40 to 50 percent of the radar cost. This, in turn, depends both on manufacturing technology progress and on production volume requirements that allow learning. The latter need will be met in the radar production environment required to support fighter aircraft systems. Typically, current aircraft production rates may lie between 6F-15 and 16 F-16 per month. Each X-band AESA radar requires 2000 T/R modules, resulting in a T/R module production need of 12,000 to 32,000 per month. By contrast, production rates of hybrid modules for the one-of-a-kind SSPA and RASSR arrays have peaked at 300 modules per month.
Both the DoD MIMIC program and other manufacturing technology programs are developing much of the factory technologies required to meet the rate and cost needs for T/R modules. For example, a current USAF mantech program is aimed at investigating the technologies required to produce 1000 T/R modules per day. These and other developments are leading toward fully automated, work cell oriented microwave production factory units with production capacities meeting the needs of modern fighter aircraft.
In the early 1980s, TI set a production cost goal for X-band T/R modules of $500 (1985 dollars) each by the mid-1990s. Since that time, both GaAs MMIC production capability and design/architecture innovations have been developed that baselined both the performance and cost trends of this technology. As a practical test of the feasibility of these goals, actual module costs were measured on SSPA. Two years later, during the Dem-Val phase of the USAF advanced tactical fighter radar development (executed by a Westinghouse-TI joint venture) the manufacture of several thousand first generation MMIC modules confirmed steady, adequate progress toward this goal.
In the mid-1980s, the availability of microwave integrated circuits and the development of modular array architectures completed a twenty year evolution of X-band active element phased-arrays suitable for application in tactical aircraft and other defense systems. Such systems offer not only previously unavailable functional capability but also higher reliabilities, true fail-soft characteristics, and substantially higher radar loop gain for a given prime power level. The parallel availability of very dense, high speed computers also allows such systems to take full advantage of the agile, multipleshaped beam performance of AESA. Complementary technologies, such as efficient, compact power supplies, may be configured to attain a reliability matching well with the characteristically high MTBF of the AESA array.
The final hurdle to be overcome in order to realize the full benefits of AESA technology is affordability. The evolution of MMIC technology supports its achievement by significant reduction in parts count and labor content of T/R modules. The results of recent AESA programs, and projections based on conservative semiconductor-like learning curves, support a measure of confidence that production cost goals below $400 per module will be achieved for radar developments in the mid 1990s.
Robert M. Lockerd received his BA and BSEE at Rice University and his MSEE at Yale University in 1961 and 1962, respectively. He joined Texas Instruments in 1964, and has held various management positions. In January 1987, he was elected VP, Defense Systems and Electronics Group. Currently, he is responsible for general management of the division including development, testing and production of advanced avionics systems for next generation defense aircraft. Lockerd is a member of Tau Beta Pi and a senior member of IEEE. He holds several patents.
Jerry Crain received his BSEE degree from Wichita State University in 1964, and his MSEE and PhD from the University of Colorado in 1966 and 1970, respectively. He joined Texas Instruments in 1970, and is currently manager of the advanced programs department in the Advanced Development Division. Prior to joining TI, he worked at Standard Telecommunications Laboratories Ltd. Crain was a section chairman of the IEEE.
PHOTO : Fig. 1 25 years of active ESA advanced development; (a) modular electronics for radar
PHOTO : application (1964); (b) reliable advanced solid-state radar (1970); and solid-state phase - array radar (1983).
PHOTO : Fig. 2 A schematic diagram of functional AESA radar.
PHOTO : Fig. 3 Transmit/receive (T/R) module functions and interfaces.
PHOTO : Fig. 4 Evolution of X-band transmit semiconductor technology.
PHOTO : Fig. 5 Hybrid circuit and monolithic microwave integrated (MMIC) X-band module comparison.
PHOTO : Fig. 6 Examples of MMIC building blocks for T/R modules; (a) Ku-band 2 W power amplifier.
PHOTO : Fig. 6b L-band vector module.
PHOTO : Fig. 6c Low noise Ku-band power amplifier.
PHOTO : Fig. 6d Five-stage Ku-band driven amplifier.
PHOTO : Fig. 6e X-band, dual gate LNA.
PHOTO : Fig. 7 Graceful degradation of array RMS sidelobes vs. time.
PHOTO : Fig. 8 Power supplies interconnected for reliability.
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|Title Annotation:||modern design allows replacement of moving antennas with multiple-element arrays|
|Author:||Lockerd, R.M.; Crain, G.E.|
|Date:||Jan 1, 1990|
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