Sensors make-or-break ballistic missile defense
Development of the eyes and ears of future missile shields is progressing slowly, despite political opposition, and overhype of potential anti-missile capabilities.
by J.R. Wilson
Development of the eyes and ears of future missile shields is progressing slowly, despite political opposition, and overhype of potential anti-missile capabilities. Space-based sensors, airborne weapons, and ground-based counter-missiles are taking shape to help defend the U.S. homeland and deployed military forces abroad.
Military defenders can kill a missile in flight in one of three ways — explode a warhead near it, hit it directly with a kinetic warhead, or heat it with a laser until it ruptures. Each of these approaches has its share of advantages and disadvantages, as well as a host of technological problems yet to be resolved. Yet all share common requirements. Anti-missile systems must have:
- adequate sensor technology to detect and acquire the target;
- the ability to judge the target's speed and trajectory; and
- the ability to direct the kill mechanism to the incoming missile.
Some of those sensors may be common to the wide assortment of anti-missile programs currently in development; others are unique to missile defense. Even a common base technology, such as radar, can be highly diverse depending on whether it is ground-air- or space-based, or which specific approach is employed, such as various forms of millimeter wave (MMW) radar.
The U.S. FPQ-14 radar at Kaena Point, Hawaii, is a collateral sensor to the Space Surveillance Network.
Each defense system also requires several different layers of sensors that work together to form a complete picture of the target as it moves through the atmosphere — or the edge of space — and to discriminate actual threats from collateral debris and deliberate attempts to fool the sensors, such as dummy warheads.
Air Force Lt. Gen. Ronald Kadish, director of the Ballistic Missile Defense Organization (BMDO) in Arlington, Va., gave a description of sensors for the National Missile Defense (NMD) program in testimony he offered last September before the House Government Reform subcommittee on national security, veterans affairs, and international relations. In his testimony, Kadish covers ground also applicable, at least in part, to other programs, such as the Space-Based Laser (SBL), Airborne Laser (ABL), Theater High Altitude Area Defense (THAAD), and others.
"We are designing a system that allows each element to gather and share data throughout the engagement in order to enhance discrimination and improve kill probability," Kadish told Congress. "We have designed a system of systems that uses more than the kill vehicle to discriminate among countermeasures. Major advances in focal plane array technology and computer processing allow us to deploy extremely sensitive 'eyes' in space and on the ground.
Future space-based defense may consist of Brilliant Pebbles, a kinetic-energy weapon with about 4,600 small interceptors, each capable of homing in on and destroying incoming hostile warheads.
"Space-based infrared (IR) sensors would detect and project a tracking path and monitor such things as booster burnout, which might help identify the type of missile. Information from Defense Support Program (DSP) satellites — and later Space Based Infrared System (SBIRS) 'High' satellites — will be handed over to the ground-based radars. Early Warning Radars (EWRs) would acquire and classify the target complex," Kadish said. "The discrimination capability of EWRs would be refined over the length of time that it viewed the target cluster, helping to distinguish and do initial characterization of objects," he testified.
"The cluster is then tracked and information handed over to the X-band radar [XBR] or the in-flight EKV [exoatmospheric kill vehicle]," Kadish continued. "The XBR would discriminate using a variety of techniques to determine, in some cases very precisely, the number, characteristics, and movements of objects in the cluster. By way of illustrating a portion of its capability, the XBR will be powerful enough to distinguish a golf ball 2,400 miles away — or the distance between Washington, D.C., and Seattle."
NMD's planned Expanded Capability 1 (C1) architecture includes upgrades to five existing ballistic missile early warning radars and an advanced X-band radar in Shemya, Alaska. These new ground-based sensor elements are direct descendants of the 1960s era Ballistic Missile Early Warning System (BMEWS). Space-based elements began with the DSP effort in the early 1970s, leading to planned deployment of SBIRS-High and SBIRS-Low during this decade. The information processing, sensor fusion, and battle management aspects continue to evolve and include new fast microprocessors and data-transfer capabilities as the various missile defense programs progress.
"The elements that are the system's 'eyes', 'nerve network', and 'brain' continue to perform at or above expectations," Kadish told Congress.
The role of radar
X-band radar is a forward-deployed ground-based, multi-function radar providing high-resolution target tracking data to the interceptor in flight through an In-Flight Interceptor Communications System. High frequency and advanced radar signal processing technology improve target resolution as the X-band radar attempts to separate closely spaced warheads from debris, decoys, and other countermeasures. The interceptor will use that data to maneuver closely enough to the target missile for the on-board kill vehicle sensor to make a final discrimination of warheads from decoys and debris. Sensors on the kill vehicle provide final, precise course corrections to take it to the target.
The Integrated Flight Test-5 last summer launched from Meck Island in the Kwajalein Island Atoll to evaluate sensors and missile technology for the National Missile Defense program.
The Upgraded Early Warning Radars (UEWRs) are large, fixed, phased-array surveillance radars that will detect and track ballistic missiles during their midcourse phase and cue the more accurate X-band radar. Later, the SBIRS-Low satellites will take over most of that effort.
The Air Force's SBIRS-High constellation of four geosynchronous Earth orbit (GEO) satellites and two highly elliptical orbit satellites will replace the existing DSP satellites, starting in 2002. They will acquire and track ballistic missiles throughout their trajectories, giving NMD an "over-the-horizon" capability to enable missile-defense personnel to launch interceptors long before the ground-based X-band radar could detect a threat. Lockheed Martin Missiles & Space in Sunnyvale, Calif., is the prime contractor on SBIRS-High, leading a team that includes Aerojet in Rancho Cordova, Calif., and Lockheed Martin Federal Systems in Owego, N.Y. (satellite control, mission data processing and telemetry and tracking and operations), Northrop Grumman Corp. in Los Angeles (primary IR sensor payload) and Honeywell Inc. in Clearwater, Fla. (on-board data processing).
The SBIRS-Low constellation of 20 low Earth orbit (LEO) satellites will have two sensors — one for acquisition, the other for tracking. These satellite sensors will operate in a variety of wavebands, including short-wave infrared, medium-wave infrared, long-wave infrared and visible light. These wavebands enable the sensors to acquire and track targets of different temperatures.
TRW Space & Electronics Group in Redondo Beach, Calif., teams with the Raytheon Surveillance & Reconnaissance Systems Division in El Segundo, Calif., in competition with a team from Spectrum Astro in Gilbert, Ariz., and the Northrop Grumman Electronic Sensors and Systems Sector in Baltimore to design SBIRS-Low. Downselect for full-scale development is scheduled for late 2002, with deployment in 2006.
Raytheon's Ground Based Radar-Prototype, illuminated at night on Kwajalein Atoll in the Marshall Islands.
"Up to now, the technological immaturity of our sensors did not allow us to discriminate, or pick out the countermeasures within a target cluster," Kadish told the Year 2000 Multinational BMD Conference last year. "During the past decade, we've made significant advances in our sensor and discrimination technologies, including in the areas of new high-resolution radars — digital radars with sophisticated electronic counter-countermeasures and infrared seekers. Steady improvements in computer processing power, which has been doubling every 18 months for the last 30 years, have helped us to develop an interceptor that flies out quickly, processes the sensor data faster and with greater accuracy and destroys the warhead."
As improvements are worked into one system, they are studied for possible application to other sensors.
"We try to leverage the work we do to cover as many of our programs as practical, to save money and share information," says Walter Dyer, assistant chief scientist at BMDO. "We've been working on improving the accuracy and speed of our sensors and keeping the costs affordable. We try to fuse the data from all available sensors, each of which has unique capabilities and features."
"The space-based element has the worldwide surveillance capability and multi-aspect capability, with several satellites seeing the target from different angles and fusing the results," Dyer continues. "The ground-based radar has the attributes of ranging and multifrequency; the interceptor is closing on the target and has the advantage of signal-to-noise as it gets closer and sorts out closely spaced objects."
The exoatmospheric NMD interceptor will use electro-optical longwave infrared (LWIR) sensors. These are strap-down sensors, not gimbals, using focal plane arrays. There also is a visible-light sensor, which uses a silicon focal plane. The endoatmospheric interceptors will use gimbaled radar seekers (C-band and millimeter wave) — and, in the case of the U.S. Navy, with an IR adjunct — that are outgrowths of existing missiles — the Navy Standard missile and the Army Patriot. The THAAD interceptor uses a gimbaled passive medium wavelength IR (MWIR) sensor.
"The formats of the focal planes have improved in terms of processors and readouts, as well as sensitivities, where we've gone from platinum silicide MWIR focal planes to indium and antimonide (InSB), which was an SDIO/BMDO-developed technology," Dyer says. "It has much higher sensitivity, measured by quantum efficiency. Basically, it can see dimmer targets.
"We're also doing some interesting things in the area of emerging technologies," Dyer continues. "We have a program called the discriminating interceptor technology program, where we are developing multispectral IR focal plane arrays. Those operate in two or more wavelength bands simultaneously, which allows us to observe two or more wavelengths as targets move or rotate, presenting different thermal characteristics. If we don't observe those simultaneously, we'll get erroneous results because each band will present different information. We're working on three- and four-color for even better discrimination in the future."
The future Airborne Laser system is to provide an early defense against theater ballistic missiles by destroying them in the boost phase.
Some BMD-unique applications also have required research into very long wavelengths, says Dr. Meimei Tidrow, senior development scientist at BMDO: "In regular IR fields, midwave is 3 to 5 microns, long is 8 to 12 — anything larger than 12 microns is very long. Most of the DOD effort stops at around 10 microns because anything beyond that is very challenging, but for BMD, we need that capability."
BMDO also is developing lightweight, ultracompact laser radars, such one called angle-angle-range, which measures those three parameters and is small enough to integrate onto an interceptor for improved homing accuracy.
"We've also developed the world's first range-resolved Doppler imaging radar. This is the current planned future advanced upgrade to our exoatmospheric interceptors. It gives us four-dimensional imaging with the Doppler image and significantly improved discrimination," Dyer says. "Right now these are in technology development and we don't have a set schedule for integrating them, but I would imagine it will be by the end of this decade."
For endoatmospheric sensor applications, BMDO leaders turn to their atmospheric interceptor technology program, which is developing solid-state millimeter wave radar for interceptors. These are dual-mode seekers that use millimeter wave radar and infrared sensors to improve homing navigation all-weather capability. This approach uses strap-down seekers to reduce the complexity and weight of gimbals," Dyer says. "We're also developing fusion processors and algorithms to fuse the data from these multiple color radars and sensor data. It doesn't do you much good to have these data if you can't fuse them."
BMDO experts also are involved in multiband radar research by contributing efforts to develop efficient transmit/receive modules that put out higher power with less waste heat than do today's modules. Those are expected to emerge from the labs during the next 10 years.
To every extent possible, managers of all of these systems are looking to leverage commercial products to cut costs and to stay up with fast-moving technologies, such as microprocessors. Still, some specific requirements — such as multispectral radars — offer unique challenges.
Another new sensor system with applications for more than one defense system is the Airborne Laser (ABL) program, which is a weapons-grade laser aboard a Boeing 747 jetliner. The ABL's Track Illuminator Laser (TILL) involves a pulsed solid-state laser based on ytterbium-doped yttrium aluminum garnet (Yb:YAG), which lases at 1.03 microns. Most solid-state lasers used in this type of application have been neodymium-doped yttrium aluminum garnet (Nd:YAG), operating at 1.06 microns. But developers say the Yb:YAG approach will require less power and weight than Nd:YAG, making the laser more efficient.
"Originally, we were using a hydrogen gas cell to shift the wavelength of neodymium, which was our original baseline, because we need two colors of illuminators for our specific system," says Paul Shattuck, ABL program manager at Lockheed Martin Space Systems in Sunnyvale, Calif. "That raised weight issues as well as safety concerns about having a pressurized hydrogen vessel aboard an aircraft. We decided to do a source selection and update our baseline to the ytterbium YAG in mid-1998 when we determined that technology had reached a sufficient maturity level."
The track illuminator subsystem includes the pumpheads, resonator optics, laser diodes, cooling units, and some electronics, all in a modular line replaceable unit (LRU) that technicians could swap out in the field if necessary. Additional elements include a rack of electronics containing the power supplies, processors, thermal control system, and all the electronics and software necessary to run the device. One cooling system handles the track illuminator and the beacon illuminator on the ABL.
On 30 March, experts from Raytheon Electronic Systems conducted a "first light" test of the TILL at their High Energy Laser Center in El Segundo, Calif. They demonstrated that the design has sufficient power to meet the signal needs of the system. The TILL is scheduled to be delivered in November to Lockheed Martin's Beam Control/Fire Control Integration and Test Facility in Sunnyvale, Calif. Systems integrators will blend it with the remainder of the beam transfer optics early next year, followed by an end-to-end test of the Beam Control/Fire Control system in the first half of 2002. Each subsystem will undergo flight tests separately, beginning early next year, culminating in a total-system flight test in 2003. BMDO officials plan to make the first ABL attempted missile shootdown by the end of 2003.
Shattuck says he and his fellow ABL engineers at Lockheed Martin are directing considerable effort toward maintaining a baseline of commercial and military off-the-shelf (COTS and MOTS) components. Most of the power supplies are MOTS. Lockheed Martin engineers are developing the majority of the processing, in terms of determining the pulse repetition frequency and sensor control. They are using PowerPC (G4)-based hardware from Mercury Computer Systems Inc. of Chelmsford, Mass., for the high-end processing, linked with fiber optics through a high-speed data transfer card from Systran Corp. in Dayton, Ohio. The Lockheed Martin-developed software is in the C and C++ languages. The algorithms that do the tracking are from Boeing SVS of Albuquerque, N.M.
"We do have some Ada in a control processor (a Raytheon 960, the militarized version of the Compaq Alpha microprocessor), but it's not part of our real time system," Shattuck says. "It's our top level communications interface with the battle management segment."
The track illuminator performs two functions; it tracks the range to target, and enables the ABL to lock onto the nose of the missile and stabilize its aim point. In order to do that, the illuminator pulses about 5,000 times a second. A low-light sensor, which is an electron bombarded charged coupled device, receives the information. That is a 128-by-128-pixel focal plane array, which Shattuck says is eight times more sensitive than other low light level sensors available and has high bandwidth — 5-to-10 MHz compared to 60-to-120 Hz for most cameras.
"We need to close high-speed loops," he says. "So we pulse the illuminator, get the return in the sensor, and use that information to stabilize steering mirrors to control line-of-sight to the target. That compensates for the standard motion and jitter caused by the aircraft."
The beacon illuminator signals the wavefront control. A laser beam moving through the atmosphere is subject to distortion. TILL uses a deformable mirror to pre-distort the beam so it corrects itself in transit.
Shattuck describes how the ABL sensors work together: "We have IR sensors that look at the plume of the target. We have a coarse sensor with a wider field of view and a separate aperture. We rotate the nose turret of the 747 until we acquire a target in the acquisition sensor, then we center it and hand off to our fine IR tracker, which has better granularity and a substantially smaller field of view. It also uses the same telescope to gather light from the target plume as we use to send out the laser. That is used to determine where to point the track illuminator.
A member of Team ABL -- The U.S. Air Force, Boeing, Lockheed Martin, and TRW -- works on a scaled laser beam control system that demonstrates the performance necessary for the Air Force's Airborne Laser.
"All of these are directly applicable to the Space-Based Laser and other directed-energy systems," Shattuck continues. "There also are variants of the low-light-level sensor that are being considered for programs other than directed energy."
ABL also includes several IR search-and-track systems, such as those on U.S. Navy Northrop Grumman F-14 carrier-based jet fighters, and a modified LANTIRN (Low-Altitude Navigation & Targeting Infrared for Night), similar to that on the U.S. Air Force F-15 jet fighter. These sensors are on top of the ABL aircraft, and provide a 360-degree field of view for the surveillance segment. That, Shattuck says, means ABL could operate autonomously.
There is no on-board radar working with ABL's electro-optical sensors. However, it can take cues from off-board platforms, such as SBIRS-High, and will be one of the first users tied into the unified communications network that will link not only to the Airborne Warning and Control System (AWACS) and Joint Surveillance Target Attack Radar System (Joint STARS) aircraft, but also with all of the existing and future command and control nodes, such as unmanned aerial vehicles. All information from those sources will pass to ABL in real-time.
It is this combination of sensors that Kadish says he believes will make the multi-layered U.S. missile defense programs successful, no matter what efforts potential adversaries may take to counter it.
"I am confident that countermeasures initially deployed by states of concern will not be sophisticated enough to fool all of the discrimination capabilities employed by the planned NMD system," he told Congress. "Each of the elements contributes uniquely to the discrimination mission using various measures and extrapolating additional information derived from physical principles [e.g., launch trajectories]. The system is redundant and synergistic, so the total capability is greater than the sum of the parts. This synergy among the elements should be expected to improve as the system evolves by upgrading software and hardware, increasing the number of existing elements and augmenting the system using additional platforms in other geographic environments."
Even without countermeasures to fight, the importance of advanced sensors to all of the anti-missile programs is crucial, Kadish told a military appreciation dinner in Alaska on 2 March 2001: "There's one inflexible rule about missile defense — the later you detect and intercept an enemy missile, the closer it will be when you destroy it and the smaller the area you can defend. Conversely, the earlier you can detect, decide and act, the farther away it will be when you destroy it and the greater the area you can defend. In this business, farther is better; it gives you enough time to gain a chance for a second or third shot if you miss."