The Event Horizon Telescope (EHT) is a very-long-baseline interferometry (VLBI) experiment that aims to observe supermassive black holes with an angular resolution that is comparable to the event horizon scale. The South Pole occupies an important position in the array, greatly increasing its north-south extent and therefore its resolution. <p> </p>The South Pole Telescope (SPT) is a 10-meter diameter, millimeter-wavelength telescope equipped for bolometric observations of the cosmic microwave background. To enable VLBI observations with the SPT we have constructed a coherent signal chain suitable for the South Pole environment. The dual-frequency receiver incorporates state-of-the-art SIS mixers and is installed in the SPT receiver cabin. The VLBI signal chain also includes a recording system and reference frequency generator tied to a hydrogen maser. Here we describe the SPT VLBI system design in detail and present both the lab measurements and on-sky results.
We present a prototype bistatic compact radar range operating at 160 GHz and capable of collecting fullypolarimetric radar cross-section and electromagnetic scattering measurements in a true far-field facility. The bistatic ISAR system incorporates two 90-inch focal length, 27-inch-diameter diamond-turned mirrors fed by 160 GHz transmit and receive horns to establish the compact range. The prototype radar range with its modest sized quiet zone serves as a precursor to a fully developed compact radar range incorporating a larger quiet zone capable of collecting X-band bistatic RCS data and 3D imagery using 1/16th scale objects. The millimeter-wave transmitter provides 20 GHz of swept bandwidth in the single linear (Horizontal/Vertical) polarization while the millimeter-wave receiver, that is sensitive to linear Horizontal and Vertical polarization, possesses a 7 dB noise figure. We present the design of the compact radar range and report on test results collected to validate the system’s performance.
In recent years, UHF synthetic aperture radar has become a growing area of interest among the radar community. Due to their relatively long wavelengths, UHF systems provide advantages that may not be attainable by microwave and millimeter-wave radar systems. These advantages include excellent target detection statistics in high clutter environments, wide-area surveillance, and long stand-off ranges. UHF systems also have proven synergistic properties with higher frequency radar systems in applications such as topographical mapping. However, the ability to study the characteristics of these lower frequency radar systems in a controlled and systematic environment is difficult. In this work, a physical scale modeling process is utilized to generate three-dimensional UHF imagery that may be used to study scattering phenomenology at these wavelengths. Dimensionally and dielectrically scaled targets and scenes are measured in a 6 - 18 GHz microwave compact range to model the backscatter of the full-size target at UHF wavelengths. The microwave compact radar range and transceiver hardware utilized to model UHF radar signature data are briefly described. A description of the image processor used to generate three-dimensional UHF imagery from wide-band/wide-angle data collections is described as well. Finally, imagery of radar signature data collected from a M1A1 Abrams main battle tank model is examined. The high resolution imagery resulting from the wide-band/wide-angle collection will show that sub-wavelength features of ground targets are resolvable at these wavelengths.
Radar detection and identification of ground targets in diverse environments is a subject of continuing interest. It has long been known that different radar bands have advantages for different environmental conditions. For example, it has been shown that detection of targets under foliage is more easily accomplished using longer wavelength radars since there is less attenuation at these frequencies. However, higher frequency radars offer greater resolution that is crucial in target identification. Because each radar band has its own unique strengths and weakness, one current approach is the use of dual-band radar platforms. With two radar bands working simultaneously, the strengths of each radar band can be used to compliment the other. ERADS has constructed two full polarimetric compact radar ranges to acquire X-Band and UHF ISAR imagery data using 1/35th scale models. The new compact ranges allow data to be taken that can simulate a multi-frequency radar platform with frequencies low enough to detect obscured targets and high enough to provide useful resolution to aid in target identification once they have been detected. Since both compact ranges use the same scale factor, this allows measurement of the same target at the two spectral regions simply by moving the target model from one compact range to the other. Data can thus be taken whose differences in scattering are due only to the difference in radar frequency, eliminating variations due to differences in target models as well as the surrounding ground clutter. Detailed descriptions of the new compact ranges will be presented along with results from sample data sets.
VV and HH-polarized radar signatures of several ground targets were acquired in the VHF/UHF band (171-342 MHz) by using 1/35th scale models and an indoor radar range operating from 6 to 12 GHz. Data were processed into medianized radar cross sections as well as focused, ISAR imagery. Measurement validation was confirmed by comparing the radar cross section of a test object with a method of moments radar cross section prediction code. The signatures of several vehicles from three vehicle classes (tanks, trunks, and TELs) were measured and a signature cross-correlation study was performed. The VHF/UHF band is currently being exploited for its foliage penetration ability, however, the coarse image resolution which results from the relatively long radar wavelengths suggests a more challenging target recognition problem. One of the study's goals was to determine the amount of unique signature content in VHF/UHF ISAR imagery of military ground vehicles. Open-field signatures are compared with each other as well as with simplified shapes of similar size. Signatures were also acquired on one vehicle in a variety of configurations to determine the impact of monitor target variations on the signature content at these frequencies.
A far-field radar range has been constructed at the University of Massachusetts Lowell Submillimeter-Wave Technology Laboratory to investigate electromagnetic scattering and imagery of threat military targets located in forested terrain. The radar system, operating at X-band, uses 1/35<sup>th</sup> scale targets and scenes to acquire VHF/UHF signature data. The trees and ground planes included in the measurement scenes have been dielectrically scaled in order to properly model the target/clutter interaction. The signature libraries acquired by the system could be used to help develop automatic target recognition algorithms. The difficulty in target recognition in forested areas is due to the fact that trees can have a signature larger than that of the target. The rather long wavelengths required to penetrate the foliage canopy also complicate target recognition by limiting image resolution. The measurement system and imaging algorithm will be presented as well as a validation of the measurements obtained by comparing measured signatures with analytical predictions. Preliminary linear co-polarization (HH,VV) and cross-polarization (HV,VH) data will be presented on an M1 tank in both forested and open-field scenarios.
The monostatic VV and HH-polarized radar signatures of several targets and trees have been measured at foliage penetration frequencies (VHF/UHF) by using 1/35th scale models and an indoor radar range operating at X-band. An array of high-fidelity scale model ground vehicles and test objects as well as scaled ground terrain and trees have been fabricated for the study. Radar measurement accuracy has been confirmed by comparing the signature of a test object with a method of moments radar cross section prediction code. In addition to acquiring signatures of targets located on a smooth, dielectric ground plane, data have also been acquired with targets located in simulated wooded terrain that included scaled tree trunks and tree branches. In order to assure the correct backscattering behavior, all dielectric properties of live tree wood and moist soil were scaled properly to match the complex dielectric constant of the full-scale materials. The impact of the surrounding tree clutter on the VHF/UHF radar signatures of ground vehicles was accessed. Data were processed into high-resolution, polar-formatted ISAR imagery and signature comparisons are made between targets in open-field and forested scenarios.