AMTA Paper Archive
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Methods for enhancing the utility and performance of coherent background subtraction
Coherent background subtraction is an established method of reducing additive range clutter in radar cross section measurements. In some measurement situations, it is neither practical nor convenient to directly make a coherent measurement of the range background. The Environmental Research Institute of Michigan has devel oped two methods of synthesizing background measure ments for the coherent subtraction of additive clutter in these cases. The first method synthesizes a background for measurements of pylon-supported targets by remov ing unterminated pylon returns using software gating. The second method improves background subtraction by compensating for phase drift between target and back ground measurements. In this paper, these methods of improving the performance and utility of background subtraction will be described and demonstrated on mea sured data.
Frequency dependent scattering effects on Fourier domain imaging of ultra-wideband data
Forming radar images from large fractional bandwidth data can often lead to unusual artifacts or resolutions degraded from "expected" theoretical point-target values. The frequency dependencies of typical scatter ing mechanisms, such as diffractions, surface waves and speculars, can be significant over processing apertures when data are collected using large fractional bandwidth measurement systems. For example, it is well known that resonant scatterers exhibit blurring in the downrange direction of an image. Other scattering mechanisms have linear or quadratic amplitude dependencies which can also alter the impulse response from that of an ideal point scatterer. This paper will first provide a brief description of the frequency dependencies of various scattering mechanisms. The paper will then describe the corresponding effects seen in the impulse response, primarily in the range profile domain. Impulse response plots will be compared for data with large and small fractional bandwidths. Lastly, the effects of frequency dependent scattering on the impulse response will be shown using images generated from data collected in indoor compact ranges.
Influence of noise and calibration errors on HRR and ISAR
Several approaches are known for the identification of noncooperative air-borne targets with radar. Assuming that the tar get can be tracked during a certain flight path, observations from different aspect angles will be obtained. High-resolution radar (HRR) systems use these observations to create one-dimensional range profiles. With Inverse Synthetic Aperture Radar (ISAR) the data from all observed aspect angles are combined to obtain two-dimensional images. In recent years, techniques for resolution enhancement have been developed for both techniques. The choice for one of the two approaches should depend on the applicability of the target representation for identification. ISAR is the most suitable for reproduction on a display and identification by human observers. In case of identification by a machine, for example an algorithm on a computer, the choice is not straight forward. In this paper an overview of the influence of several errors on the performance of HRR and ISAR will be given. The error sources that will be evaluated are: • uncertainty of the absolute distance of the target; • errors in the mutual alignment of observations; • additive noise. The errors are generated numerically and applied to data from simulations and low-noise measurements. The influence of the bandwidth and angular span on the quality of the target reconstruction will be regarded as well as the performance of some high-resolution techniques. Finally, conclusions are drawn concerning the applicability of ISAR and HRR.
Bistatic coherent measurement system (BICOMS), A
The U.S. Air Force 46 Test Group, Radar Target Scattering Division (RATSCAT), at Holloman AFB, NM, in conjunction with the US Army, Navy and Georgia Tech Research Institute (GTRI), has developed a concept for a bistatic coherent radar measurement system (BICOMS). It will be used to measure both the monostatic and bistatic RCS of targets, as well as create two-dimensional images of monostatic and bistatic signature data. It will consist of two mobile radar units, each of which is capable of simultaineously collecting coherent monostatic and bistatic RCS data. This paper will cover the systetn design specificatiovs, layout and design of equipment, and discuss the operating parameters for the radar (power, antenna sizes, sensitivities, timing, etc.).
2095P pulsed microwave measurement system for the Naval Surface Warfare Center, Crane Division
Modem pulsed phased array radar systems bring new challenges to antenna measurement. These antennas generally consist of hundreds of Transmit-Receive (TR) modules controlled via a beam steering computer to fonn the antenna beam. Attempting to operate these modules with a CW wavefonn will not only quickly damage the mod ules but will not properly characterize the antenna. The Navel Surface Warfare Center, Crane Division, recog nized the need to add pulsed capability when specifying their latest antenna measurement system. Scientific Atlanta met these requirements by integrating their newly introduced Model l 795P Pulsed Microwave Receiver into their proven 2095 Microwave Measurement System to make the Model 2095P Pulsed Microwave Measurement System.
Calibrated real-time RCS measurements using the DDRE modular radar system (MRS)
The Modular Radar System has been developed at the Danish Defence Research Establishment (DDRE) in cooperation with the Danish company CRIMP. The unique system is capable of performing nearly all types of calibrated radar measurements. The modularized highly flexible system is presented along with a number of measurement. RCS of very small targets at short ranges 400'- l 000', medium range measurements of Navy targets, aircraft and chaff at ranges from 1-10 nautical miles. The real time high resolution range profiles are used for positive identification of "hot spots" on Navy vessels leading to very efficient RCS reductions.
3-D high resolution radar imaging using the MUSIC algorithm
Superresolution techniques based on the Multiple Signal Classification (MUSIC) have recently been applied to two-dimensional (2-D) Inverse Synthetic Aperture Radar (ISAR) imaging with demonstrated results. These techniques exhibit much higher spa tial resolution than other approaches using a 2-D Fourier transform. This paper a MUSIC based superresolution algorithm for 3-D radar imaging, which is especially useful for measurements with both small frequency and aspect angle (in azimuth and elevation) spans. This algorithm models the measured 3-D data set as a sum of point source emissions plus noise. Once the positions in the 3-D space of such scattering centers are obtained using the MU SIC algorithm, the weights (or RCS) of the scattering centers are obtained through a pseudo-inverse matrix inversion computed by means of a Singular Value De composition (SYD).
Convenient, multi-platform, boresight mounting scheme for compact range, A
Accurate mechanical-to-electrical axis alignment (boresighting), gain, and pattern testing of radar antennae requires specialized tooling/fixturing. This requirement is often taken for granted and seldom discussed in the EE community. Particularly in a production environment, where rapid change of test configurations to accommodate multiple radar platforms are required, a convenient mounting scheme is mandatory. This paper describes and illustrates a method implemented at the Warner Robins Air Logistics Center to satisfy this demand. Drawings and/or photos of a three-point Universal Adapter fixture and several UUT Specific radar mounting fixtures are discussed. The paper discusses tolerances, materials, manufacturing processes, alignment, and antenna boresight methodologies.
Fiber optic link phase thermal noise performance in a coherent bistatic instrumentation radar
Instrumentation grade, coherent, bistatic, radar cross section (RCS) measurement systems require a reliable low-noise method to link the reference, local oscillator (LO) and intermediate frequency (IF) coherent signals between the transmit and receive subsystems. One approach to this is the use of a fiber optic link (FOL). Phase noise measurements have been performed on a distributed feedback (DFB) type laser transmitter-photodiode receiver link with a delay of up to 2.26 kilometers, operating at 5 GHz, using a standard HP 3048A phase noise test measurement setup. System level tests have been performed, incorporating a FOL into a coherent bistatic instrumentation radar system local oscillator path, and performing image processing on an emulated target A first level analysis was conducted regarding the effects of the thermal noise on the radar perfonnance.
Development and measurement of a frequency selective surface highway stripe
A frequency selective surface has been developed for use as a part of an automatic highway system. The FSS is attached as a stripe along the edge or center of the lane, and is designed to a strong retro-reflective echo for the design frequency, polarization, and elevation angle of the forward-looking radar installed on an automobile. The stripe provides directional information for automated steering, as well as other coded information such as lane number, and exit advance warning. This paper reports on initial development and testing of a prototype FSS highway stripe. The stripe was designed for an operating frequency of 10.5 GHz, and was built and tested using a prototype autonomous vehicle. Both FSS stripe performance, and performance of the vehicle will be reported.
RCS range characterization using an orbiting sphere
Proper characterization of metal walled chambers or other non-anechoic facilities is normally difficult and time consuming. A novel technique for rapid charac terization is described that is available to high PRF, pulsed, chirp radar systems. A sphere is tethered to a crosspiece mounted on the axis of a motor using a fine cord. The system can be mounted on the ceiling or affixed to a variable height pole. adjusting the motor speed and length of the cord, a stable orbit is achieved having a fixed radius and height above the suspension point. Chirp data can be processed into range-time-intensity (RTI) plots that provide clear evidence of multipath and beam taper. By changing the orbit parameters it is possible to characterize a large volume and remedy problems in a very short period of time.
Ultra-wideband transient antenna measurement techniques
In the past few years there have been new application of transient, ultra-wide band microwaves include cooperating aircraft identification and ground penetration Radar's, high power microwave weapons and others. These applications typically require the use of ultra-wideband antennas with characteristics suitable to radiate transient pulses. This paper describes the capabilities of the USAF Phillips Laboratory's new Transient Antenna Range. The antenna range can measure the radiated characteristics of sources/antennas wave forms with risetimes in the 75 ps regime, and with greater than 50 ns pulse width. The antenna range incorporates a hardware suite controlled by a powerful software data acquisition system that runs on a PC. Automatic data reduction can yield values of wave form peak electric field, risetime and waveform spectrum at a single point, or across an azimuthal scan. This paper will also describe a unique wave form splicing technique used in the data processing algorithms of the Transient Antenna Range. This splicing technique allows test personnel to record the (typically) very fast early time history of the radiated waveform with an SCD-5000 scan converter (operating at a maximum bandwidth, 5 ns of record available), and the long time history of the waveform with a DSA-602 transient digitizer.
Antenna/RCS range evaluation using a spherical synthetic aperture radar
We describe an imaging technique which allows the isolation of sources of unwanted radiation on an antenna/RCS range. The necessary data may be collected by using a roll-over azimuth mount to scan a probe over a spherical measurement surface.
Inflatable antenna technology with preliminary shuttle experiment results and potential applications
This paper describes the status of NASA’s Inflatable Antenna Experiment (IAE) and a brief discussion on potential future applications. The space experiment of a 14-meter diameter reflector antenna was flown and deployed successfully aboard the Space Shuttle, STS-11, launched May 19, 1996. Since the flight data is still being processed and reduced, only preliminary results can be presented at this time. The development of the IAE will be discussed along with the results of ground test measurements which were conducted to determine the overall mechanical and projected electrical performance characteristics of this inflatable concept. Large, space-deployable antennas are needed for numerous applications which include mobile communications, Earth remote sensing, and space-based radar systems. Due to the traditionally high cost to develop and launch such large antennas, new technology must be developed which is cheaper, faster, and better. Inflatable antenna technology provides the opportunity to accomplish these objectives.
Measurement system considerations for characterizing antennas with integrated sources
Antennas with integrated RF or microwave sources are becoming more prevalent as the wireless explosion continues to evolve into specific programs and products. These types of antenna modules span several different business areas such as communication satellites, radars, and collision warning systems and cellular or wireless systems. In order to evaluate a device’s true performance parameters, it is desirable to test the device in its actual operating environment. There are a number of different tradeoffs that must be considered when configuring an antenna measurement system to test antennas with integrated sources or transceiver based products. This paper will discuss the tradeoffs available in the antenna measurement system design for a test range that can measure antennas with integrated sources. Several antenna test ranges will be presented and the advantages and disadvantages of each configuration will be discussed.
A Beam-steered array for ground penetrating radar
This paper reports on a ground penetrating radar (GPR) antenna with an electronically steered beam, currently being developed at South Dakota Tech. The increased power and directivity that result from beam-steered operation have potential utility in deep/lossy GPR environments. The antenna is a transmitting array of up to eight bow-tie dipoles, each driven by a narrow pulse generator connected directly to the dipole. The beam is steered in real time by controlling the timing of the individual element transmitters using digitally-programmed pulse delay units. Reception is through a conventional GPR receiver using a single bow-tie antenna. Modeling the air-ground interface as a lossy half-space, numerical results indicate that, under certain conditions, time-domain beam-forming is possible in such an environment. Antenna patterns and standard antenna measurement parameters, such as beamwidth and directivity, are presented in support of this finding.
Performance analysis of the image-based near field-to-far field transformation
At last year’s conference we presented the discrete implementation of an image-based near field to far field transform (IB-NFFFT) for predicting far field radar cross-section (RCS) from spherically-scanned near field measurements, along with some preliminary transform results using numerically-simulated data. This paper quantifies this expected performance in terms of the RCS prediction error (RMS dB difference) using numerically-simulated data for two ten wavelength-long canonical bodies, a thin wire and a conesphere. It will be shown that for the highly-resonant wire target, the NFFFT’s algorithm performance is limited by the multiple interactions resulting from the travelling wave reflections between the end of the wire, except at near broadside aspect angles. Conversely, very good performance is obtained for the conesphere at nearly all aspect angles, except very close to nose and tail-on. We will also shown that the IB-NFFFT algorithm performance is robust with respect to clutter and scan angle coverage.
Performance comparison of the analog and digital ramps in a linear-FM chirp RCS measurement radar
The designer of a linear-FM homodyne RCS measurement system must consider the nonlinearity present in the chirp waveform. Two basic methods employed in obtaining the chirp waveform are to apply either a digital ramp or an analog ramp to a YIG oscillator source. Nonlinearity can occur as the result of the characteristics of the YIG oscillator and the applied ramp waveform. The point spread functions useful in characterizing the performance of both the digital and analog ramp excited YIG oscillator systems are given. Both range resolution and dynamic range of the measurement system are dependent on the target range and can be adversely effected by the nonlinearity. Theory shows that the point spread function of a digital ramp is suitable for short range RCS measurements. However the analog ramp system has improved performance at extended range. By using the analog ramp, we have been able to improve performance of RCS measurements over the digital ramp. Experimental data from both the digital and analog ramp systems are provided.
Experimental radar cross section reduction of an array of aluminum cubes using a genetic algorithm
We use a genetic algorithm to optimize a linear array of aluminum cubes in order to reduce the relative Radar Cross Section at a specific angle. A genetic algorithm models biological reproduction and natural selection on a computer in order to optimize the output of some function or experiment. We also examine the initial parameter tradeoffs required to achieve an optimal design with a minimal number of unique measurements. We actually discovered two very different designs, each providing more than 44 dB reduction at five degrees off broadside.
ISAR imaging using UWB noise radar
It is possible to build a very inexpensive radar which transmits wide band radio noise. On receive, the signal is cross correlated with a delayed version of the transmitted signal. In this paper we will discuss the design and operation of a UWB noise radar which was installed in the OSU compact RCS measurement range. Scattering measurements were made for a number of targets over 360 degrees of aspect angle. Calibration was performed, and then the data converted to ISAR images. Example ISAR images will be shown.
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