Ultra-Wideband / Impulse Radar
An impulse radar is one whose waveform is a single-cycle sinewave. Its most distinctive characteristic is its very wide relative-bandwidth. Much has been said and claimed for such a radar, as compared to conventional narrowband radar; but there is very little written about what it is, what it can and cannot do, and what is required to achieve such radars in practice. All three major subsystems for an impulse radar (transmitters, receivers, and antennas) are presently far from adequate for practical applications (other than for probing underground). A number of problem areas include electromagnetic compatibility, target scattering, the radar is discussed and the differences noted. Several potential applications include target-to-clutter enhancement, target recognition, resolution of low-altitude multipath, and target scattering enhancement.
Conventional radars employ a long pulsewidth and typically have an instantaneous bandwidth on the order of 100 MHz. Conventional radars have difficulty detecting target such as low-flying cruise missiles because of the images or reflections that cause the signal return from the missile to fade in and out when the direct path and multipath signals reinforce or cancel each other. A radar that employs a train of short pulses on the order of 200 picoseconds having an instantaneous bandwidth on the order of 5 GHz is capable of separating the direct path return from the multipath return, thus allowing detection of low flying sea skimmer missiles.
The wide bandwidth signal may enhance the signal return from a target having a low radar cross-section because of resonant scattering characteristics of the target over a wide frequency band. Thus, such a short pulse radar may also have potential application as an adjunct radar to existing ship air defense radars.
An impulse radar emits a wideband signal at a low power level. The signal is difficult to detect and intercept, and thus provides for a low probability of intercept radar. A multiplicity of the receivers are used to detect the multiplicity of transmitted frequencies. The output signals from each receiver are coherently combined by the processor. Since the arriving waves are coherent across the channels while the noise is not, a strong detected signal results. In each receiver channel, the operational signal-to-noise ratio is generally set substantially below the detection threshold for a single receiver. In effect, this array of coherently combined receivers constitutes a matched filter for the transmitted waveform. Efficient detection of the waveform requires an a priori knowledge of the transmitter architecture. Various waveform parameters, such as frequencies and phases, may be rapidly changed to make unintended detection difficult.
Impulse radars employing trains of short pulses are currently being investigated in research laboratories. These existing experimental radars typically take the approach of switching the RF transmit signal on and off in picoseconds to generate a train of extremely short pulses. Such radars require the impulse generator to have a peak power on the order of several megawatts due to the fact that it has a low duty factor in that the pulse width of the impulse generator is extremely short compared to the required interpulse period. On receive, an extremely wideband receiver is required to detect the entire bandwidth of the impulse. Hence, it is difficult to provide satisfactory gain in the receiver amplifier.
It has been established that stealth technology is more susceptible to low frequency radars, so it appeared that this technology might be able to detect low cross-section stealth targets. A report prepared by MITRE Corporation for the Air Force(Report M90-l8, "Ultra Wideband Radar Applicability to Air Defense - Red Team Assessment," dated March l990) addressed long-range airborne surveillance. The conclusions of the report were based on a strawman model for an Ultra Wideband Impulse Radar. "The use of short pulses on the order of l nanosecond in duration is not feasible for long-range(approximately 200 nmi) surveillance, owing to the extremely high peak powers that are required by this approach. ... Although the use of short pulses reduces the ground clutter level by several orders of magnitude over systems with megahertz bandwidths, an additional reduction of clutter by a factor of about 10,000 is required in order to detect cruise missile sized objects... A pervasive problem for an Ultra Wideband broadcasting (transmitting) system is the fact that it shares the spectrum with a large number of critical military and civilian services (such as UHF, VHF, voice communications, cellular telephone, TV and FM broadcasting). It is susceptible to interference from these services and could potentially interfere with them. The problem has been managed in the laboratory for short-range radar and communication systems; however, along-range wide area surveillance radar requiring perhaps 10,000,000 times as much effective radiated power makes the electromagnetic interference problem effectively insurmountable."
Congressional interest in the potential use of Ultra Wideband technology resulted from a presentation made to Representative Norman Dicks in 1989. The presentation was given to secure Government funding to continue internal research and development work on light-activated high-power microwave switch technology, which had been ongoing for several years. One of the primary uses of the switch technology is thegeneration of extremely narrow pulses of high energy content. The wide frequency bandwidth of a narrow pulse relates to high radar range resolution. Another characteristic of the narrowpulse is its low frequency content. The Balanced Technology Initiative [BTI] Office directed DARPA to convene a Panel of experts to review Ultra Wideband technology and applications and to identify and prioritize Ultra Wideband research to be pursued. DARPA contracted with Battelle, Columbus Division to form the Ultra Wideband Radar Review Panel.
The OSD/DARPA Ultra Wideband Radar Review Panel Report was released on July l3, 1990. The Panel recommended that the Department of Defense fund analyses of point designs using impulse and non-impulse approaches for four radar applications that appear tohave important military uses: (a) A short-range radar for detecting moving targets behind walls or foliage.(b) A short-range airborne imaging radar for detecting military targets under canopy or in wooded terrain.(c) A medium-range (20 km) air defense radar for detection and non-cooperative identification of airborne targets, including but not limited to helicopters in the treeline.(d) A medium-range (20 km) radar for detection of sea skimming missiles in fleet defense applications. The Panel stated that it had found interesting work under way and recommended additional efforts, but that it did not believe Impulse Radar offered a major new military capability nor did it present the threat of a serious technological surprise.
Some proponents of UWB Radar claim that Impulse Radar had a low probability of intercept, that Impulse Radar could defeat radar absorbing materials, that Maxwell's equations did not apply to Impulse type waveforms without profound reformulation, and that Impulse Radar effects could not be understood using conventional spectral analysistechniques. They assert that the Soviets recognized the special attributes of nonsinusoidal waves and had developed a significant technical program to exploit these properties. This position was supported by a few presenters to the Panel. The Ultra Wideband Panel Chairman assigned Panel members the task of examining these issues for substance. They were asked to organize their conclusions and report to the full Panel. The Panel found no merit to these claims.
Ultra-wideband radio and micro-power impulse radar are two applications of a similar technology. Both are by-products of recent advances in high-speed digital circuitry. Both transmit, receive and measure the timing between short (nominally nanosecond) impulses. Both are also short-range in nature and both are highly affected by objects in the propagation path. It may be practical to fuse these similar concepts into a single device using common electronics and waveforms for position determination, sensor calibration, sensing and communicating.
Remote Casualty Location Assessment Device (RCLAD) is a developmental casualty location tool focused toward the search and rescue mission. Using various technologies, including Micro Impulse Radar or Ultra-Wide Band Radar, it locates casualties buried in rubble by remotely sensing heart beat and breathing. It is intended to be low cost, portable and able to detect minute motion at close range though 10 feet of rubble. This technology was developed because existing heat sensor technology had a very limited range and could not medically assess a casualty, and passive listening devices require all quiet conditions - meaning the stop of all local rescue work.
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