Synthetic aperture radar

A short introduction to the background and theory of synthetic aperture radar (SAR) imaging is given. Some of the key issues in SAR design are discussed and possible future developments involving SAR operation with phased arrays are suggested     

Thermal behavior of millimeter wavelength radio telescopes

The passive and/or active thermal control of significant structural components, for instance the backstructure and the feed legs, of the IRAM 15-m MM and 30-m MM wavelengths telescopes is discussed, and their thermal behavior is illustrated. The design of the thermal protection was supported by dynamic time-dependent model calculations, which are explained and compared with in situ recorded temperatures of the telescope components     

Scanning the special issue on radio telescopes

The present issue (vol.82, no.2, 1994) is the third in the Proceedings of the IEEE to be devoted to radio and radar astronomy. The first appeared in January 1958, a significant date because it was 25 years after Karl G. Jansky's first report of his discovery of Galactic radio noise. The second was 15 years later in September 1973, and was devoted to reporting how the science had progressed. The 15-year timespan had marked a period of rapid advance that had yielded a wealth of discoveries. Now, 21 years later, the field is still dynamic and continues to produce an abundance of findings, due in part to the large number of new observatories. In the present issue, the focus is on the design and performance of the antennas and the instrumentation used in radio astronomy. The issue concentrates on already existing and operational telescopes. Perhaps yet a future issue is merited, one devoted to those new telescopes that are still on the drawing boards     

Synthetic aperture radar interferometry

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     

Synthetic aperture radar equation

The synthetic aperture radar equation for extended diffuse clutter depends on the area of such clutter processed by the radar. It is shown here that approximating the illuminated area by the product of the range and azimuth lineal resolutions can give results which are optimistic or pessimistic, depending on the amount of range and azimuth weighting used to suppress sidelobes.     

Synthetic aperture radiometer systems

Aperture synthesis is an emerging technology for passive microwave remote sensing from space. It is an interferometric technique similar to earth rotation synthesis employed in radio astronomy in which pairs of small antennas and signal processing are used to obtain the resolution of a single large antenna. The technique has the potential to overcome the barriers that antenna size has plated on passive microwave remote sensing from space. The technique has been demonstrated successfully for remote sensing at L-band with the aircraft prototype ESTAR. New aircraft instruments are under development and proposals have been submitted for instruments to demonstrate this technology in space     

Command preprocessor for radio telescopes and microwave antennas

The LQG (linear-quadratic-Gaussian) controllers, designed for the NASA Deep Space Network antennas, have small tracking errors and are resistant to wind disturbances. However, they induce limit cycling during antenna-slewing operations and target acquisition. The cycling is caused by the violation of the antenna's rate and acceleration limits. This problem can be avoided by introduction of a command that does not exceed the limits. The command preprocessor, presented, generates a command that is equal to the original command if the latter does not exceed the limits, and that varies with the maximum (or minimum) allowable rate and acceleration if the limits are met or exceeded. The preprocessor is comparatively simple, since it requires knowledge of the command at only the current and the previous instants of time, while the other known preprocessor requires knowledge of the terminal state and the acquisition time. Analysis and field tests of the preprocessor are presented. The dynamics of the preprocessor itself, and of the antenna with the preprocessor, are illustrated with the antenna's measured responses to typical commands     

Phase retrieval in the radio holography of reflector antennas and radio telescopes

Methods of phase retrieval from simulated intensity information have been tested for use in the radio holography of reflector antennas. In numerical simulations the Misell algorithm has been used successfully to retrieve the aperture phase distribution from two numerically simulated power polar diagrams, one in focus and the second defocused. The technique uses no auxilliary reference antenna. However, it does need a high signal to noise ratio, typically 50 dB if a60 times 60array is to be measured to a precision such that the gain is within 1 percent of ideal. It should be most useful where no direct phase measurements are possible and ground-based or satellite transmitters can be used as sources. The use of astronomical maser sources (22 GHz) can give information on large scale deformations.     

Millimeter wave radio telescopes: Gain and pointing characteristics

The basic gain and pointing characteristics of millimeter-wave radio telescopes are described in this paper. Formal definitions of relevant telescope parameters are given. Performance limits set by conventional technology are discussed and compared with measurements of existing telescopes. Methods are also suggested for minimizing pointing errors.     

Calculated thermal behavior of ventilated high precision radio telescopes

Radio telescopes that operate at millimeter and sub-millimeter wavelengths need a reflector-surface precision of a few tens of microns and a pointing accuracy of a few arcseconds. When built in a conventional way from steel and aluminum, as in the case of larger-diameter telescopes, thermal control must be applied to reduce temperature-induced deformations, in particular of the reflector backup structure. We illustrate that it is possible to make model calculations - for instance, during the design phase - that simulate the thermal behavior and the operation of a telescope when servo-loop-controlled ventilation or climatization (air-conditioned ventilation) of the backup structure is applied. We explain the technique of model calculations, and present as an example the calculated thermal behavior of a ventilated 64-m-diameter telescope and of the climatized 30-m IRAM telescope. It is explained that the thermal control of a telescope mount is less demanding if frequent pointing corrections can be made.     

A subspace-tracking approach to interference nulling for phasedarray-based radio telescopes

Several next-generation radio telescopes, now in the planning stages, are based on phased-array technology. One reason for this is to make use of adaptive nulling techniques to combat radio frequency interference, which is a growing problem for radio astronomy. This paper presents a low-complexity approach to interference nulling which is suitable for use in such systems. The approach uses subspace-tracking to identify interference, followed by spatial projections to place deep nulls in the directions of interferers. This technique overcomes two limitations of power-minimization algorithms (e.g., "minimum variance"), namely power inversion and pattern rumble, which create serious problems for radio astronomy. Furthermore, this technique imposes a lower computational burden and provides side information which is useful in later stages of data processing. Performance results from a phased array demonstrator system and a simulation are presented     

大型射电望远镜电磁环境频谱监测

射电天文的发展需要更宽的观测频率,但是随着通讯的不断发展,射频无线电干扰(Radio Frequency Interference,RFI)对射电观测的影响日益加重,射电望远镜需要选择电磁环境优良的站址并进行保护.文中描述了目前国内在建和预研阶段的望远镜的电磁环境的测试过程,阐述了数据分析和处理方法.通过对我国射电望远镜台址选择和保护进行的电磁环境测试,优选出适合建立大型射电望远镜的台址.     

Synthetic aperture radar calibration using reference reflectors

A simple expression for the terrain backscatter coefficient is derived in terms of the integrated power of an adjacent known radar reflector in a synthetic aperture radar (SAR) image. It is shown that this technique for SAR image calibration is independent of the radar system focus or partial coherence and thereby possesses an important advantage over the usual technique, which relies on an estimate of the peak of the reflector impulse response. Results from airborne SAR overflights of corner reflectors and active radar calibrators are used to demonstrate the validity and consistency of the method and to show that the method is robust under defocus caused by an incorrect FM rate or inadequate motion compensation of data collected during turbulence. It is also shown that the fading errors associated with the integral method are comparable to or slightly worse than those associated with the peak estimation method. However, this small disadvantage is outweighed by the fact that the integral method is independent of actual resolution     

Synthetic aperture processing for full-deramp radar altimeters

It is shown how the technique of aperture synthesis may be used to provide high spatial resolution for satellite-borne geophysical radar altimeters. Unfocused synthetic aperture processing is appropriate, since this provides adequate resolution and allows simple processing. It is also shown how this is compatible with the full-deramp pulse compression technique     

Optimization of aperture illumination for radio wave power transmission

A basic design of an aperture illumination of antennas used for radio wave power transmission is presented. In the power transmission using microwave such as solar power satellite, not only the transmission efficiency, but also the transmitting power capacity allowed by the necessary electromagnetic environment is a very important factor for realization. In this paper the circular antenna aperture illumination is optimized in order to give the maximum transmission efficiency with the constraint on the radiation levels at the edge of the receiving aperture and at the radiation peaks of the subsequent sidelobes. This constrained optimization problem is solved by successive use of the sequential unconstrained minimization techniques (SUMT method). The maximum transmission efficiency and receivable average power density are obtained for various constrained levels of the spilled radiation, and are compared with the values in the unconstrained case.     

Synthetic aperture radar imaging of moving ocean waves

A theory for the radar imaging of ocean waves is presented under the assumptions that a swell propagates through an ensemble of Bragg scatterers and that the integration time of the synthetic aperture radar (SAR) is small compared to the angular velocity of the swell. Results are prsented which show image development and distortions caused by the radial velocities and accelerations of the swell. Neglecting small wave bunching and tilts due to the longer underlying waves, and considering only one-dimensional geometries, the mechanism of wave motions are considered and their efforts on the production of the usual intensity Pattern representing the wave image are studied. The analysis shows that in certain situations a processed image can appear which has twice the spatial period of the actual long wave on the ocean, which can confuse the interpretation of ocean wave analysis.     

Synthetic aperture radiometry evaluated by a two-channeldemonstration 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_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     

Synthetic aperture radar autofocus based on a bilinear model

Autofocus algorithms are used to restore images in nonideal synthetic aperture radar imaging systems. In this paper, we propose a bilinear parametric model for the unknown image and the nuisance phase parameters and derive an efficient maximum-likelihood autofocus (MLA) algorithm. In the special case of a simple image model and a narrow range of look angles, MLA coincides with the successful multichannel autofocus (MCA). MLA can be interpreted as a generalization of MCA to a larger class of models with a larger range of look angles. We analyze its advantages over previous extensions of MCA in terms of identifiability conditions and noise sensitivity. As a byproduct, we also propose numerical approximations to the difficult constant modulus quadratic program that lies at the core of these algorithms. We demonstrate the superior performance of our proposed methods using computer simulations in both the correct and mismatched system models. MLA performs better than other methods, both in terms of the mean squared error and visual quality of the restored image.