Synthetic aperture radar interferometry is an imaging technique for measuring the topography of a surface, its changes over time, and other changes in the detailed characteristic of the surface. By exploiting the phase of the coherent radar signal, interferometry has transformed radar remote sensing from a largely interpretive science to a quantitative tool, with applications in cartography, geodesy, land cover characterization, and natural hazards. This paper reviews the techniques of interferometry, systems and limitations, and applications in a rapidly growing area of science and engineering.

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Synthetic aperture radar interferometry

When surrounded by a transparent emission region, black holes are expected to reveal a dark shadow caused by gravitational light bending and photon capture at the event horizon. To image and study this phenomenon, we have assembled the Event Horizon Telescope, a global very long baseline interferometry array observing at a wavelength of 1.3 mm. This allows us to reconstruct event-horizon-scale images of the supermassive black hole candidate in the center of the giant elliptical galaxy M87. We have resolved the central compact radio source as an asymmetric bright emission ring with a diameter of 42 +/- 3 mu as, which is circular and encompasses a central depression in brightness with a flux ratio greater than or similar to 10: 1. The emission ring is recovered using different calibration and imaging schemes, with its diameter and width remaining stable over four different observations carried out in different days. Overall, the observed image is consistent with expectations for the shadow of a Kerr black hole as predicted by general relativity. The asymmetry in brightness in the ring can be explained in terms of relativistic beaming of the emission from a plasma rotating close to the speed of light around a black hole. We compare our images to an extensive library of ray-traced general-relativistic magnetohydrodynamic simulations of black holes and derive a central mass of M = (6.5 +/- 0.7) x 10(9) M-circle dot. Our radio-wave observations thus provide powerful evidence for the presence of supermassive black holes in centers of galaxies and as the central engines of active galactic nuclei. They also present a new tool to explore gravity in its most extreme limit and on a mass scale that was so far not accessible.

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First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole

Over the past 5 years, there has been a significant interest in employing terahertz (THz) technology, spectroscopy and imaging for security applications. There are three prime motivations for this interest: (a) THz radiation can detect concealed weapons since many non-metallic, non-polar materials are transparent to THz radiation; (b) target compounds such as explosives and illicit drugs have characteristic THz spectra that can be used to identify these compounds and (c) THz radiation poses no health risk for scanning of people. In this paper, stand-off interferometric imaging and sensing for the detection of explosives, weapons and drugs is emphasized. Future prospects of THz technology are discussed.

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THz imaging and sensing for security applications—explosives, weapons and drugs

Cosmology at low frequencies: The 21 cm transition and the high-redshift Universe

The Murchison Widefield Array (MWA) is one of three Square Kilometre Array Precursor telescopes and is located at the Murchison Radio-astronomy Observatory in the Murchison Shire of the mid-west of Western Australia, a location chosen for its extremely low levels of radio frequency interference. The MWA operates at low radio frequencies, 80-300 MHz, with a processed bandwidth of 30.72 MHz for both linear polarisations, and consists of 128 aperture arrays (known as tiles) distributed over a ∼3-km diameter area. Novel hybrid hardware/software correlation and a real-time imaging and calibration systems comprise the MWA signal processing backend. In this paper, the as-built MWA is described both at a system and sub-system level, the expected performance of the array is presented, and the science goals of the instrument are summarised. © 2013 Astronomical Society of Australia.

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The Murchison widefield array: The square kilometre array precursor at low radio frequencies

Preface to the Second Edition Preface to the First Edition Introduction and Historical Review Introductory Theory of Interferometry and Synthesis Imaging Analysis of the Interferometer Response Geometric Relationships and Polarimetry Antennas and Arrays Response of the Receiving System Design of the Analog Receiving System Digital Signal Processing Very-Long-Baseline Interferometry Calibration and Fourier Transformation of Visibility Data Deconvolution, Adaptive Calibration, and Applications Interferometer Techniques for Astrometry and Geodesy Propagation Effects Van Cittert-Zernike Theorem, Spatial Coherence, and Scattering Radio Interference Related Techniques Principal Symbols Author Index Subject Index

Interferometry and Synthesis in Radio Astronomy

Interferometry and Synthesis in Radio Astronomy, 2nd Edition

Preface -- Introduction and Historical Review -- Introductory Theory of Interferometry and Synthesis Imaging -- Analysis of the Interferometer Response -- Geometric Relationships and Polarimetry -- Antennas and Arrays -- Response of the Receiving System -- Design of the Analog Receiving System -- Digital Signal Processing -- Very-Long-Baseline Interferometry -- Calibration and Fourier Transformation of Visibility Data -- Deconvolution, Adaptive Calibration and Applications -- Interferometer Techniques for Astrometry and Geodesy -- Propagation Effects -- Van Cittert-Zernike Theorem, Spatial Coherence and Scattering -- Radio Interference -- Related Techniques -- Principal Symbols -- Index.

The development of radio technology in World War II opened a completely new window on the universe. When astronomers turned radio antennas to the heavens, they began to find a previously unknown universe of solar and planetary radio bursts, quasars, pulsars, radio galaxies, giant molecular clouds and cosmic masers. Not only do the radio waves reveal a new world of astronomical phenomena but also-because they are much longer than light waves-they are not as severely distorted by atmospheric turbulence or small imperfections in the telescope. About 25 years ago radio astronomers became aware that they could synthesize a resolution equivalent to that of a large aperture by combining data from smaller radio antennas that are widely separated. The effective aperture size would be about equal to the largest separation between the antennas. The technique is called synthesis imaging and is based on the principles of interferometry. Radio astronomers in the U.S. are now building a synthesis radio telescope called the Very-Long-Baseline Array, or VLBA. With 10 antennas sited across the country from the Virgin Islands to Hawaii, it will synthesize a radio antenna 8,000 kilometers across, nearly the diameter of the earth. The VLBA'S angular resolution will be less thanmore » a thousandth of an arc-second-about three orders of magnitude better than that of the largest conventional ground-based optical telescopes. Astronomers eagerly await the completion early in the next decade of the VLBA, which is expected, among other things, to give an unprecedentedly clear view into the cores of quasars and galactic nuclei and to reveal details of the processe-thought to be powered by black holes-that drive them.« less

The Very-Long-Baseline Array

This chapter is concerned with the van Cittert–Zernike theorem, including an examination of the assumptions involved in its derivation, the requirement of spatial incoherence of a source, and the interferometer response to a coherent source. Some optical terminology is used, for example, mutual coherence, which includes complex visibility. There is also a brief discussion of some aspects of scattering by irregularities in the propagation medium. Much of the development of the theory of coherence and similar concepts of electromagnetic radiation is to be found in the literature of optics. The terminology is sometimes different from that which has evolved in radio interferometry, but many of the physical situations are similar or identical. However, in spite of the similarity, the literature shows that in the early development of radio astronomy, the optical experience was hardly ever mentioned, an exception being the reference by Bracewell (1958) to Zernike (1938) for the concept of the complex degree of coherence. The van Cittert–Zernike theorem contains a simple formalism that includes the basic principles of correlation in electromagnetic fields.

A. Richard Thompson

Papers

Interferometry and Synthesis in Radio Astronomy

Interferometry and Synthesis in Radio Astronomy, 2nd Edition

Interferometry and Synthesis in Radio Astronomy

The Very-Long-Baseline Array

Van Cittert–Zernike Theorem, Spatial Coherence, and Scattering