Radio Imaging System Research Articles

Solar Observations with Single-Dish INAF Radio Telescopes: Continuum Imaging in the 18 – 26 GHz Range

We present a new solar radio imaging system implemented through the upgrade of the large single-dish telescopes of the Italian National Institute for Astrophysics (INAF), not originally conceived for solar observations.During the development and early science phase of the project (2018 – 2020), we obtained about 170 maps of the entire solar disk in the 18 – 26 GHz band, filling the observational gap in the field of solar imaging at these frequencies. These solar images have typical resolutions in the 0.7 – 2 arcmin range and a brightness temperature sensitivity <10 K. Accurate calibration adopting the Supernova Remnant Cas A as a flux reference provided typical errors <3% for the estimation of the quiet-Sun level components and active regions flux measurements.As the first early scientific result of the project, we present a catalog of radio continuum solar imaging observations with Medicina 32-m and SRT 64-m radio telescopes, including the multi-wavelength identification of active regions, their brightness and spectral characterization. The interpretation of the observed emission as thermal bremsstrahlung components combined with gyro-magnetic variable emission paves the way for the use of our system for long-term monitoring of the Sun. We also discuss useful outcomes both for solar physics (e.g., study of the chromospheric network dynamics) and space weather applications (e.g., flare precursors studies).

Simulation of subnanosecond radio pulse electro-optical repeater

Objectives . The study aimed to construct and analyze a computational model of an electro-optical repeater of radio pulses, capable of reconstructing a pulsed radio image with subnanosecond resolution in a single reception under conditions of additive noise. Methods . Numerical methods of network analysis were used, which are the basis of specialized computer-aided design systems, numerical methods of statistical radio engineering. Results . A radiophotonic radio pulse repeater scheme was constructed, which is implemented based on the principle of fractional multiplexing with delayed feedback. Software simulation of infrared repeaters in the Simulink environment (Trial Version Soft) was developed, which allows for analyzing and investigating the efficiency of the optical reconstruction method of radio pulses using a fractional multiplexing with delayed feedback. It is shown that repeaters schemes, implemented on the principles of fractional multiplexing with delayed feedback, are able to effectively solve the scientific and practical problems of multiple probing of objects with ultrashort pulses (USP) for obtaining a radio image of a target with reliable reproducibility. In the course of numerical simulations, it was found that the two- and four-cascade schemes of delay lines with feedback do not provide reliable reproducibility in the case of reconstruction of an ultrashort pulse with a complex time profile. At the same time, the scheme with a cascade of 8 delay lines showed good results, providing a correlation reproducibility of more than 0.9. In this case, the scheme of an electro-optical repeater with a cascade of 16 delay lines did not make a significant contribution to increasing the accuracy of the USP reconstruction if compared to the scheme with a cascade of 8 lines; therefore, the latter can be determined as an optimal solution. An electro-optical method was proposed for solving the radio engineering problem of stroboscopic registration and reconstruction of subnanosecond radio pulses, which represent the signature of the radio image of dynamic objects for active radio imaging systems. Conclusions . It was found that an electro-optical repeater with 8 delay lines is able to recover a complex pulse reflected from a target in 30 iterations with a correlation coefficient greater than 0.9 between the reference and reconstructed pulses at a signal-to-noise ratio of at least 9 dB.

Array radio imaging based on heterodyne detection with application of the continuous-wave radar technique

A new approach to the creation of millimeter-wave radio imaging systems is proposed. This approach is based on the use of an array receiver consisting of a densely packed (pixel size of 4 mm) array of planar mixers located in the focal plane of a quasi-optical objective, with application of the frequency-modulated continuous-wave radar technique. It has been demonstrated that the implementation of the heterodyne type of reception makes it possible to increase the distance range of the array radio imaging system up to ~ 100 m while maintaining the angular resolution at the previous level. Keywords: millimeter-wave imaging, frequency-modulated continuous-wave (FMCW) radar, focal-plane array (FPA) receiver

Матричное радиовидение на основе гетеродинного приема с применением радиолокации непрерывным излучением

A new approach to the creation of millimeter-wave radio imaging systems is proposed. This approach is based on the use of an array receiver consisting of a densely packed (pixel size - 4 mm) array of planar mixers located in the focal plane of a quasi-optical objective, with application of the frequency-modulated continuous-wave radar technique. It has been demonstrated that the implementation of the heterodyne type of reception makes it possible to increase the distance range of the array radio imaging system up to ~ 100 m while maintaining the angular resolution at the previous level.

Space-Time Processing of Object Images in Passive Radio Imaging Systems

This paper describes the concept and methods of space-time processing of radio images of small objects in a multiposition radiometer system, which make it possible to increase the performance of this system. The spatial processing is carried out in each overlook cycle of scanning radiometers in order to determine the spatial coordinates of objects and mutual orientation of coordinate systems. The time processing is carried out in a sequence of overlook cycles of radiometers for determining the trajectory of the objects that change mutual positions.

A millimeter-band planar lens based on an inhomogeneous medium with forced refraction

A millimeter-band planar lens designed on the basis of an inhomogeneous medium with forced refraction, which is formed by a periodic grating of metal strips placed on the surfaces of a dielectric plate, is considered. The possibility of application of this lens as the basis for the design of a multibeam receiving antenna for a radio imaging system is analyzed. The structure of an antenna consisting of a planar lens and a multichannel feed is proposed. The results of the design of a planar lens based on an inhomogeneous medium with forced refraction are presented. A multichannel feed in the form of an array of E-plane horns is analyzed and the results of simulation of this feed with the help of an approximate technique and numerical solution of an electromagnetic problem are given. The design of the multibeam antenna and its experimental characteristics are presented. The possibility of computer correction of radiation patterns of the multibeam antenna is considered.

Passive millimeter wave imaging system with tilt scanning

A model of a passive 3-mm range radioimaging system with tilt scanning was designed and built to detect potentially dangerous items hidden under clothing. The basic elements and characteristics of the system are described. Obtained radio images are shown.

Methods for generating three-dimensional surface images in airborne radio imaging systems

Methods are proposed to generate three-dimensional surface images with improved angular resolution for low-altitude airborne multichannel radars based on the optimal reconstruction of reflection fields and narrow-band Doppler filtering.

Planar detectors for multielement systems of millimeter-wave imaging

We discuss the concept of developing a millimeter-wave multielement matrix receiver on the basis of planar antennas with directly coupled low-barrier Schottky diodes. Three main problems are considered which involve choosing and developing a design of the planar antenna coupled with the low-barrier Schottky diode, optimizing the parameters of the low-barrier Schottky diodes for obtaining maximum sensitivity, and ensuring compact arrangement with weak mutual influence of the planar detectors in a two-dimensional array. We propose a design of the slot antenna with an active resistance of about 800 Ω at a resonant frequency of 94 GHz. The detection characteristics of diodes with the differential Schottky-barrier resistance in the range Rj = 0.4–1000 kΩ for a zero bias are studied experimentally. The mutual influence of the neighboring antennas is examined for developing the multielement radio-imaging system. The conditions of weak cross influence of the closely located planar detectors are determined.

Radio-imaging in small animals.

As the resolution of radio-imaging systems improves, the prospect of in-vivo imaging of small animals becomes more attractive. Purpose-built positron emission tomography (PET) scanners capable of imaging individual tissues within the rat brain are now in routine experimental use and in-vivo tracer and saturation-kinetic studies are now possible. The study of small animals in this way does have intrinsic problems and constraints associated with it. For example, the animal must be completely immobilized, stable ligand within the radiolabelled preparation may be limiting and anatomical definition may be poor. In spite of this, consistent, semi-quantitative data can be produced and in-vivo radio-imaging can provide a genuine and unique complement to more conventional techniques. Animal numbers can be significantly reduced and the quality of data improved due to reduced inter-animal variation, and longitudinal studies, to monitor disease progression, are feasible. As the resolution of imaging systems improves still further, such studies could be extended to mouse, in addition to rat, models of disease.

Radio-imaging for quantitative autoradiography in biology

We present here an overview of new in vitro and ex vivo radio-imaging systems developed to overcome the limitations of films and emulsions currently used in histological autoradiography experiments. The shortcomings of films for quantitative studies are first introduced. Principles and performances of each family of imagers are discussed and illustrated in various biological contexts. Finally, perspectives of development including nonradioactive labeling techniques are briefly presented.