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Upgrading the Nation's Largest Space Surveillance Radar
Radar installation gets a boost from SwRI-developed high-power transmitter
by J. Mark Major
Southwest Research Institute is leading an engineering development effort to upgrade the reliability and performance of the U.S. Space Command's largest surveillance radar.
The world's first large phased-array radar, the AN/FPS-85 was constructed in the 1960s at Eglin Air Force Base, Florida. Other large radars have been introduced since then, but the Grand Old Lady of the South, as the radar installation is known at Eglin, remains the nation's primary space surveillance radar because of its unsurpassed power and coverage.
The AN/FPS-85 is a valued asset to the U.S. Air Force, but one with an aging technology base that must be supported into the future. For example, the on-site maintenance crew repairs an average of 17 radar transmitter units per day at an expense of $2 million annually, a figure that will rise as the vacuum tube market diminishes. Recognizing that maintenance costs could be reduced by reliability improvements, the Air Force contracted with SwRI in 1992 to study ways of improving the installation's transmitter array system.
SwRI engineers determined that reliability, supportability, and reliability gains in the transmitter array system could be realized through modern design approaches that would replace high-power vacuum tubes with RF power transistor and integrated electronic technology. As the project progressed, new transmitter designs were developed, prototyped, and tested by modifying government-furnished radar transmitter units. The basic concept has been successfully demonstrated, and a large four-year production effort to modify the full transmitter array system is planned. The Air Force has endorsed the upgrade plan and is prepared to carry out the modification program with SwRI as the principle engineering consultant.
Benefits of the Phased Array
A phased array refers to an antenna configuration composed of a large number of elements that together form a radio beam. The phase of the radio signal at each antenna element is independently controlled, usually by computer. By actively manipulating the relative phasing of the antenna elements, a radio beam can be electronically steered. The beam-steering concept is bidirectional in that it works equally well for both radio transmitters and receivers.
The fundamental advantage of electronic control is speed; consequently, phased arrays are very useful in radar applications where it is desirable to interlace complex search and track functions for multiple objects. With a phased array, it is even possible to produce multiple, truly simultaneous beams. Mechanically-steered antennas are simpler in composition but usually cannot be maneuvered with the speed, accuracy, and reliability of a phased array.
The AN/FPS-85 radar is dedicated to space surveillance, which includes searching for, detecting, tracking, and reporting objects in space. Its mission grows more vital as the number of satellites orbiting Earth increases. Information from the Space Command is used, for example, to help NASA plan shuttle launches.
The AN/FPS-85 was designed and constructed by the Bendix Corporation, Communications Division. When first installed in 1961 it was effectively a large-scale scientific experiment, because it was the first phased-array radar of such size and power. Destroyed by fire in 1965, the facility was brought back on line in 1969 and has operated since then on a 24-hour-a-day, seven-day-a-week basis.
More than 7,000 spaceborne objects in the AN/FPS-85 computer database are regularly tracked as they enter the radar's coverage area. Time, elevation, azimuth, range, and range rate data are relayed in real time to the NORAD facility at Cheyenne Mountain, Colorado. An average of 20,000 observations are made each day. Many are of uncorrelated targets, otherwise known as space junk.
The AN/FPS-85 is well-suited for space surveillance. Its location and 45-degree angle to the horizon allow it to achieve excellent coverage of space. Other large radars designed to detect ballistic missiles are positioned at boresight only slightly above the horizon. They support the space surveillance mission but cannot equal the coverage of the AN/FPS-85 because of their limited elevation range. The AN/FPS-85 can track 90 percent of the Space Command's object catalog, but is usually tasked for 75 percent. It can penetrate deep space to the range of geosynchronous orbiting satellites - up to 37,500 kilometers - and can track 200 near earth targets simultaneously.
An incident from the early 1970s concerning astronaut Ed White illustrates the exceptional resolution of the AN/FPS-85 radar. White lost a glove during a space walk and was not able to retrieve it as it floated off into space. Using the AN/FPS-85, site operations staff were able to locate and track the glove.
Transmitter Array System
The transmitter array system contains 5,184 dipole antenna elements mounted in a 72 by 72 matrix orthogonal to a 132-square-foot ground plane. The system produces a combined peak power of 30 MW at 442 MHz. Each element is driven by a dedicated radar transmitter unit rated for 10 kW peak power, 250-Ás pulse width, and 0.5 percent duty factor. The transmitter units dissipate about 120 W and are forced-air cooled.
The radar beam is electronically steered by the phasing of 72 row and 72 column signals with a dynamic control resolution of three degrees achieved by switched delay line networks. The row (alpha) signals control the beam elevation component and are supplied to the transmitter at 85.3 MHz and 100 mW. The column (beta) signals control the beam azimuth component and are supplied to the transmitters at 14.7 MHz and 100 mW.
Radar carrier frequency is controlled by the frequency of the master oscillator (MO) signal, which is common to all 5,184 transmitter units and supplied to them at 542 MHz and 300 W. Linear frequency modulation (pulse chirping) of up to 1 MHz is often applied to the MO signal for radar signal processing advantages.
The radar transmitter units require power supply inputs of 5.5 kVDC, 3.5 kVDC, 0.5 kVDC, 1 kV pulse-modulated, 200 V pulse-modulated, and 117 VAC at 60 Hz. The high-voltage power supplies are impressive because of their size and capability. For instance, although each radar transmitter unit draws only a few milliamps of current, the combined requirement for the array is nearly 100 amps.
A current monitoring station for the transmitter array system provides calibrated readings of the peak power and phase of all 5,184 radar transmitter units. Measurements made at the monitoring station are synchronized to a 60-Ás calibration pulse, which is generated once per second. The calibration pulse is transmitted at boresight (zero phase shift), but does not affect normal radar operation, with the exception of throughput.
Upgrade Design Features
The Institute conducted a comprehensive study to identify transmitter array system components, assemblies, and subassemblies that could be upgraded to improve the reliability and maintainability of the AN/FPS-85 system. The study included analyses of system failure records and a solicitation of industry for applicable modern technology. Preliminary designs for the most promising upgrades were prepared and analyzed to estimate implementation cost and schedule, as well as return on investment. A life-cost model was developed for the transmitter array system and used to assess potential upgrade alternatives. The following system features were recommended as worthwhile and feasible upgrades:
Reduced MO Signal Power
The existing radar transmitter units use a high-level (300-W) MO signal input; however, redesigning the radar transmitter unit for a 100-mW MO signal input provides substantial system benefits. Chief among these is that the MO distribution group, including 180 17-kW RF amplifiers and their associated power supplies, is eliminated, saving significant prime power and maintenance dollars.
Solid-State RF Amplifiers
The six vacuum tubes in each of the existing radar transmitter units can be reduced to one or none by redesigning the transmitter using solid-state amplifiers. By using the reduced MO signal power feature, a 437-447 MHz amplifier requirement of 60 dB is required (10 mW in, 10 kW out). A two-fold efficiency improvement is attained with modern amplifier technology, which allows the radar duty factor and maximum pulse width to be extended by the same factor. The SwRI team is considering two design approaches--a fully solid-state amplifier, and a hybrid configuration consisting of a solid-state driver amplifier and a tube final-stage amplifier.
Both amplifier configurations were pursued for prototype development because of intangible factors that could affect the final decision. For example, there is a reliance on sole sources for critical components with both design approaches, and the associated risk with each might change as the production effort nears. Three hybrid prototypes and one fully solid-state prototype were constructed.
Remote Power and Phase Adjustment Capability
The power output of the existing radar transmitter units cannot be adjusted in the system and the phase must be manually calibrated, requiring a two-man operation and the use of an intercom system. SwRI invented a method for sending digital commands to the radar transmitter units through the central monitoring system using existing cabling; thus, the possibility of remote power and phase adjustment became feasible. An integrated microcontroller within each radar transmitter unit is required to accept and respond to power and phase adjustment commands. The microcontroller also provides protection benefits because it can shut down the transmitter in the event of potentially damaging conditions, such as lack of cooling air, loss of voltage, or a faulty antenna feed.
The central monitoring system tests all 5,184 radar transmitter units each day, but the data are not recorded or used, other than to identify failed units. SwRI engineers designed a PC-based host system and interfaced it to the calibration and monitoring group for storage of radar transmitter unit power and phase data, so transmitter performance trends can be analyzed. The host system also serves as the digital data command interface for automated power and phase adjustment. Finally, a substantial upgrade of the coaxial relay switching system was achieved by using a modern relay-card switch matrix in the monitoring system.
Upgrade Design Technology
SwRI integrated a variety of electronic technologies in the development of upgrades for the AN/FPS-85 transmitter array system. The system involves high-voltage and high-power electronics, power grid tubes, RF power transistors, analog/digital electronics, RF signal processing electronics, electronic packaging, and PC-based instrumentation. In most cases, circuits were engineered at SwRI using standard electronic components, but in the case of the RF amplifiers, commercial industry was solicited for custom-developed products built to Institute specifications because it was judged that the high base component costs could be reduced by a competitive field.
The RF signal processor implements the advantages of the reduced MO signal power feature. It was designed at SwRI using discrete RF hybrid and integrated circuit component technologies packaged in standard, hermetic metal containers. Included are RF signal processing components such as power dividers, mixers, filters, fixed attenuators, current-controlled variable attenuators, RF amplifiers, and voltage-variable phase shifters. Device connections are made on a printed circuit board with microstrip circuit traces designed for a 50-W transmission line impedance. The circuit produces a 437-447 MHz RF output signal based on the alpha, beta, and MO input signals. Frequency, phase, and pulse modulations applied to the input signals are directly reflected in the output signal. Power and phase control signals from the microcontroller allow the RF output signal power to be adjusted over the range of 0-17 dBm with 0.1-dB resolution, and the phase to be adjusted ▒180░ with 1░ resolution.
The microcontroller was designed at SwRI using a combination of standard analog and digital integrated circuits mounted on a custom multilayer printed circuit board. Its primary function is to provide power and phase control signals to the RF signal processor in response to digital commands received from the host system. In addition, the microcontroller continuously monitors operating conditions via temperature and voltage sensors, and is programmed to shut the transmitter down if potentially damaging conditions are detected. A bidirectional communications port allows problems with the microcontroller to be diagnosed by a maintenance technician.
Designed at SwRI using primarily commercial off-the-shelf (COTS) electronic equipment, with significant custom packaging and cabling, the host system consists of a personal computer system on a cart and a half-height equipment rack enclosure. The computer system is standard, with the exception of added cards for a general purpose interface bus and a multichannel digital-to-analog converter. A COTS switch matrix, analog-to-digital converter, and multiple-output power supply are included in the equipment rack. Rack-mounted assemblies were designed for measurement, calibrator, and modulator units. The measurement unit accepts the 442-MHz system reference and selected radar signals and produces output signals proportional to the power and phase of the input signals. The calibrator unit provides a relay-switchable signal path and a 442-MHz test signal for automated calibration of the measurement unit with a single external measurement instrument used as the calibration standard. The modulator unit provides a digital command transmitter for the power and phase adjustment feature by using an 830-MHz oscillator pulse modulated with 40-bit serial data commands at 38.4 kbaud.
SwRI is now engaged in transmitter unit prototype design development. Three prototype units, using the hybrid amplifier configuration, were tested at the AN/FPS-85 radar site in February 1994. A fourth prototype, using a fully solid-state amplifier configuration, is scheduled for delivery and test in November 1994. An extended data collection effort on the first three prototypes has been in progress since the site tests were completed. Design improvements have been identified, and the prototypes are being reworked so they can be retested with the fourth prototype.
A final decision on production is scheduled for March 1995. If the Air Force proceeds with production, SwRI will play an important role as engineering consultant to ensure that the selected prototype design is properly implemented.
Ron Patton and Terry Tislow of the AN/FPS-85 radar site staff are thanked for their valuable input to this article. SwRI project team members Monty Grimes, Kevin Honeyager, Jeff Lucas, and Tom Warnagiris are recognized for their key technical contributions.This article was published in the September 1994 Technology Today®. For more information about space surveillance radar, contact Thomas C. Untermeyer, Automation and Data Systems Division, Southwest Research Institute, P.O. Drawer 28510, San Antonio, Texas 78228-0510, Phone (210) 522-5040, Fax (210) 522-3396.