Synthetic Aperture Radiometry Research Articles

Review of Terahertz Antenna Technology for Science Missions in Space

The utilization of terahertz (THz) band in the electromagnetic spectrum has great potential value both in scientific research and practical applications, therefore reasonable design of antenna system for radiating and receiving THz wave is especially critical. This article reviews the THz antenna technology toward atmospheric remote sensing, radio astronomy, and deep space exploration over the past three decades. Some typical scientific and engineering applications in space are discussed, which include the specifications, design issues, and the performance of various space THz antenna systems. Horn array antennas (synthetic aperture radiometry), reflector antennas fed by quasi-optical networks and traditional microwave networks have been widely applied in the designs for science missions. Finally, some promising THz antennas for future missions are introduced, and viewpoints on research challenges and future directions are given.

A Novel Imaging Method Using Fractional Fourier Transform for Near-Field Synthetic Aperture Radiometer Systems

In the domain of synthetic aperture radiometry, a Fourier transform (FT) relationship can be established between the brightness temperature of the target in the far-field region and the visibility function output by the system. However, when the target is in the near-field range of the imaging system, the existing far-field imaging methods cannot be used directly for the inversion of the near-field visibility function, because the target radiation signal cannot simply be regarded as a plane wave signal. In this letter, we propose a novel imaging method for synthetic aperture radiometer systems on the basis of the characteristics of the near-field target radiation signal. We introduce the near-field error term to reformulate the relationship between visibility function and brightness temperature in near field. Based on the reestablished signal model, we present an image reconstruction algorithm via fractional Fourier transformation, named near-field fractional FT (NF-FRFT), to estimate the near-field brightness temperature. Compared with the conventional near-field imaging method, the proposed NF-FRFT method can achieve better image reconstruction quality without extra hardware consumption. Furthermore, another advantage of this approach is that no additional array layout design is required. The validity of this method is demonstrated by a series of simulations and experiments.

A 1.5-GHz 6.144T Correlations/s 64 $\times $ 64 Cross-Correlator With 128 Integrated ADCs for Real-Time Synthetic Aperture Imaging

A 65-nm CMOS, 18-mm2, 1.5-GHz 64 $\times $ 64 cross-correlator with 128 on-chip analog-to-digital converters (ADCs) enables real-time synthetic aperture radiometric imaging. This brief enables high-resolution synthetic aperture radiometry by greatly improving the integration and energy efficiency of the mixed-signal baseband cross-correlator. The design supports analog-in and digital correlation out, removing approximately 5 W of power that would otherwise be needed for I/O between the ADCs and the digital correlation core. The prototype 6.144T correlation/s 1.5 Gsamples/s 64 $\times $ 64 correlator is designed for satellite-based radiometric imaging of water in the atmosphere. Massive parallelism together with an optimized correlation scheme leads to a measured energy consumption of only 0.35 pJ/correlation/cycle. A correlation efficiency greater than 90% is achieved for input signal levels greater than −30 dBm.

Common Mathematical Framework for Real and Synthetic Aperture by Interferometry Radiometers

This work focuses on the relationship between real and synthetic aperture radiometers giving a general and common mathematical framework for both of them. It will be demonstrated that a real aperture radiometer array can be understood as a synthetic aperture one, with a high level of redundancy. Therefore, all the recent results from synthetic aperture radiometry can be translated to real aperture radiometer arrays. This fact can be used to create beamforming arrays in a new way. To do this, the beamforming array equation will be written in terms of cross-correlations between all pair of antenna signals (visibility samples). By properly combining the visibility samples, a new type of digital beamforming radiometer can be implemented, which will benefit from the high calibration accuracy achieved in synthetic aperture radiometers (e.g., The Y-shaped MIRAS radiometer used in the Soil Moisture and Ocean Salinity mission of the European Space Agency) and the flexibility to electronically steer the beam.

Technical Development and Application Prospect of Synthetic Aperture Radiometer

Interferometric synthetic aperture radiometry is a relative new technique in the area of microwave earth observation to measure the brightness temperature distribution of the earth. It can enhance the spatial resolution of the passive microwave remote sensing effectively. Steady progresses of this technology have been achieved in both one dimensional and two dimensional cases since 1990's. In this paper, the technique development of the interferometric synthetic aperture radiometry in the past ten years was reviewed. The imaging principle of the aperture synthesis was introduced. Both of the two most common means of the aperture synthesis, one dimensional and two dimensional, were described. Another aperture synthesis means based on antenna array rotating was also described as the latest progress. Then the existing and proposed instruments were reviewed, including ESTAR, 2D-STAR, GeoSTAR from the US, MIRAS of the ESA mission SMOS, HUT-2D developed by Helsinki University of Technology, and the C-band and X-band systems developed by Center for Space Science and Applied Research, Chinese Academy of Sciences, etc. Finally, the application prospect of the interferometric synthetic aperture technology was discussed, especially in the area of earth observation, space exploration, reconnaissance and some security imaging application.

Foreword to the Special Issue on the 9th Specialist Meeting on Microwave Radiometry and Remote Sensing Applications (MicroRad '06)

The 38 papers in this special issue were originally presented at the 9th Specialist Meeting on Microwave Radiometry and Remote Sensing Applications (MicroRad '06). These papers are organized into topical areas and applications, which are in the general order of the MicroRad technical sessions: Radiometer Calibration and RFI Mitigation (4); Synthetic Aperture Radiometry (3); Land and Vegetation (6); Ocean Salinity (5); Ocean Wind (4); Atmosphere (3); Temperature and Humidity Sounding (8); and Precipitation (5).

Research Activity on Synthetic Aperture Radiometry in CSSAR/CAS

Interferometric synthetic aperture radiometry is a relative new technique in the area of microwave earth observation to measure the brightness temperature distribution of the earth. It can enhance the spatial resolution of the passive microwave remote sensing effectively. Steady progress of this technology have been achieved in both one dimensional and two dimensional cases since 1990’s. The typical instruments are ESTAR and MIRAS, developed by NASA (and umass) and ESA respectively. Relative research has also been conducted in China, mainly by National Microwave Remote Sensing Laboratory (NMRS Lab), Center for Space Science and Applied Research (CSSAR), Chinese Academy of Sciences (CAS), since the middle of 1990’s. A C-band and an X-band instrument has been developed. In this paper, research activities on synthetic aperture radiometry in CSSAR/CAS will be reviewed and summarized, including the development of the instruments. Finally, further plans in synthetic aperture radiometry in CSSAR/CAS will also be prospected.

About off-axis radiometric polarimetric measurements

Polarization changes for off-axis rays, while a minor effect for narrow-beam antennas, become a significant issue for wide-beam antennas required by synthetic aperture radiometry. This note provides the angle-dependent relationship between upwelling fields and collected signals; results are illustrated by the case of the Surface Moisture and Ocean Salinity (SMOS) mission.

Synthetic aperture radiometry evaluated by a two-channel demonstration model

The Technical University of Denmark (TUD) Synthetic Aperture Radiometer (SARad) is a two-channel demonstration model that can simulate a two-dimensional (2D) thinned array radiometer having an unfilled aperture populated with several small antenna elements. Aperture synthesis obtained by interferometric measurements using the antenna elements in pairs, followed by an image reconstruction based on an inverse Fourier transform, results in an imaging instrument without the need of mechanical scan. The thinned aperture and the nonscanning feature make the technique attractive for spaceborne radiometer systems, especially at low frequencies. The TUD SARad demonstration model consists of a two-channel K/sub u/-band correlation radiometer with two horn antennas and an antenna mounting structure enabling the horns to be mounted in relevant positions within a certain aperture. A total aperture synthesis is obtained by sequentially placing the two antenna elements in all required pairs of positions and measuring the corresponding samples of the visibility function. The system has been used to demonstrate 2D synthetic aperture imaging of complex targets in outdoor ground experiments, a special feature of the system is that it uses a focused antenna system, thus enabling a short distance to the target. Set still utilizing image reconstruction algorithms identical to those used in a normal far-field situation. The aperture synthesis theory is discussed, with special emphasis on focused systems; the radiometer system is described; and images suitable for demonstration of resolution and other imaging properties are presented and discussed.

Threshold and timing errors of 1 bit/2 level digital correlators in Earth observation synthetic aperture radiometry

Analytical expressions for the errors generated in 1 bit/2-level digital correlators (1B/2L) are derived: threshold errors in comparators and timing (skew and jitter) errors in samplers. These expressions are used to specify the parameters of the comparators and samplers in high performance 1B/2L digital correlators that are required in Earth observation synthetic aperture radiometry.