The spatial resolution of a conventional imaging laser radar system is constrained by the diffraction limit of the telescope’s aperture. We investigate a technique known as synthetic-aperture imaging laser radar (SAIL), which employs aperture synthesis with coherent laser radar to overcome the diffraction limit and achieve fine-resolution, long-range, two-dimensional imaging with modest aperture diameters. We detail our laboratory-scale SAIL testbed, digital signal-processing techniques, and image results. In particular, we report what we believe to be the first optical synthetic-aperture image of a fixed, diffusely scattering target with a moving aperture. A number of fine-resolution, well-focused SAIL images are shown, including both retroreflecting and diffuse scattering targets, with a comparison of resolution between real-aperture imaging and synthetic-aperture imaging. A general digital signal-processing solution to the laser waveform instability problem is described and demonstrated, involving both new algorithms and hardware elements. These algorithms are primarily data driven, without a priori knowledge of waveform and sensor position, representing a crucial step in developing a robust imaging system.

Synthetic-aperture imaging laser radar: laboratory demonstration and signal processing

The theoretical angular resolution of an optical imaging instrument such as a telescope is given by the ratio of the imaging wavelength lambda over the aperture diameter D of the instrument. For real-world instrument, optical aberrations often prevent this so-called diffraction-limit resolution lambda/D from being achieved. These aberrations may arise both from the instrument itself and from the propagation medium of the light. The aberrations can be compensated either during the image acquisition by real-time techniques or a posteriori, i.e., by post-processing. Most of these techniques require the measurement of the aberrations, also called wave-front, by a wave-front sensor (WFS). The focal-plane family of sensors was born from the very natural idea that an image of a given object contains information not only about the object, but also about the wave-front. A focal-plane sensor thus requires little or no optics other than the imaging sensor; it is also the only way to be sensitive to all aberrations down to the focal plane. The first practical method for wave-front sensing from focal-plane data was proposed by Gerchberg and Saxton (1972). This so-called “phase-retrieval” method has two major limitations. Firstly, it only works with a point source. Secondly, there is generally a sign ambiguity in the recovered phase, i.e., the solution is not unique, as will be detailed below. Gonsalves (1982) showed that by using a second image with an additional known phase variation with respect to the first image (such as defocus), it is possible to estimate the unknown phase even when the object is extended and unknown. The presence of this second image additionally removes the above-mentioned sign ambiguity of the solution. This technique is referred to as “phase diversity”. This contribution attempts to provide a survey of the phase diversity technique, with an emphasis on its wave-front sensing capabilities. Section 1 gives an introduction to the image formation for the considered instruments (i.e. those working with spatially incoherent light, such as telescopes), reviews the sources of image degradation, and states the inverse (estimation) problem to be solved in phase diversity. Section 2 reviews the domains of application of phase diversity. Then, Sections 3 and 4 review the wave-front estimation methods associated with this technique and their properties, while Section 5 examines the possible object estimation (i.e., image restoration) methods. Section 6 gives some background on the various minimization algorithms that have been used for phase diversity. Section 7 illustrates the use of phase diversity on experimental data for wave-front sensing. Finally, Sections 8 and 9 highlight two fields of phase diversity wave-front sensing that have witnessed noteworthy advances: Section 8 reviews the methods used to estimate the large-amplitude aberrations that one faces when imaging through turbulence, and proposes a novel approach for this difficult problem. And Section 9 reviews the developments of phase diversity for a recent application: the phasing (also called cophasing) of multi-aperture telescopes.

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Phase Diversity: A Technique for Wave-Front Sensing and for Diffraction-Limited Imaging

Telescopes and imaging interferometers with sparsely filled apertures can be lighter weight and less expensive than conventional filled-aperture telescopes. However, their greatly reduced MTF’s cause significant blurring and loss of contrast in the collected imagery. Image reconstruction algorithms can correct the blurring completely when the signal-to-noise (SNR) is high, but only partially when the SNR is low. This paper compares both linear (Wiener) and nonlinear (iterative maximum likelihood) algorithms for image reconstruction under a variety of circumstances. These include high and low SNR, Gaussian noise and Poisson-noise dominated, and a variety of aperture configurations and degrees of sparsity. The quality metric employed to compare algorithms is image utility as quantified by the National Imagery Interpretability Rating Scale (NIIRS). On balance, a linear reconstruction algorithm with a power-law power-spectrum estimate performed best.

Comparison of reconstruction algorithms for images from sparse-aperture systems

Fourier telescopy (FT) is an active imaging technique that is a good candidate for high resolution imaging systems that can be used to obtain satellite images out to geosynchronous target ranges. Fourier telescopy uses multiple beams that illuminate the target with a fringe pattern that sweeps across it due to frequency differences between beams. In this way the target spatial frequency components are encoded in the temporal signal that is reflected from the target. The FT receiver can then be composed of a large area "light bucket" collector, since only the integrated temporal signal is necessary to reconstruct the target image. The GEO Light Imaging National Testbed (GLINT) system was previously designed to obtain satellite images at geosynchronous ranges by using this technique. Laboratory experiments by several groups have demonstrated the validity of this technique to produce images from simulated targets. In this paper we expand upon these previous experiments to present results from both a FT laboratory and field experimental setup that simulated realistic photon noise, speckle noise, and atmospheric turbulence that will be encountered in an actual FT imaging system. To obtain the scaling for the FT experiment, we have used the GLINT system design parameters for our experimental setup. We will also discuss the phase closure process used to eliminate the random phase differences between the beams from the target spatial frequency measurements and the basic reconstruction algorithm used to produce the target image. Results will also be given that demonstrate the phase closure variance is reduced by averaging a small number of high SNR measurements together, as compared to averaging a larger number of low SNR measurements. Target reconstruction improvements obtained by "unbiasing" the average of the individual low SNR phase closure measurements will also be discussed.

Laboratory and field experimental demonstration of a Fourier telescopy imaging system

Detection of barycenter of planar target based on laser reflective tomography

Object identification in deep space is a surveillance mission crucial to our national defense. Satellite health/status monitoring is another important space surveillance task with both military and civilian applications. Deep space satellites provide challenging targets for ground-based optical sensors due to the extreme range imposed by geo-stationary and geo-synchronous orbits. The Air Force Research Laboratory, in partnership with Trex Enterprises and our other contractor partners, will build a new ground-based sensor to address these deficiencies. The Geo Light Imaging National Testbed (GLINT) is based on an active imaging concept known as Fourier telescopy. In this technique, the target satellite is illuminated by two or more laser sources. The corresponding fields interfere at the satellite to form interference fringes. These fringes may be made to move across the target by the introduction of a frequency shift between the laser beams. The resulting time-varying laser backscatter contains information about a Fourier component of the target reflectivity and may be collected with a large solar heliostat array. This large unphased receiver provides sufficient signal-to-noise ratio for each Fourier component using relatively low power laser sources. A third laser source allows the application of phase closure in the image reconstruction software. Phase closure removes virtually all low frequency phase distortion and guarantees that the phases of all fringes are relatively fixed. Therefore, the Fourier phase associated with each component can be recovered accurately. This paper briefly reviews the history of Fourier telescopy, the proposed design of the GLINT system, and the future of this research area.

Geo Light Imaging National Testbed (GLINT): past, present, and future

The GEO Light Imaging National Testbed (GLINT) system will image objects in geo-synchronous and semi-synchronous orbits using a synthetic aperture technique known as Fourier Telescopy. The testbed will be located in the vicinity of Socorro, New Mexico, and will form one of the most powerful imaging systems on Earth in terms of resolution, with an angular resolution of about 10 nano-radians, or 2 milli-arc seconds. Various parts of the system have strong similarities to astronomical instruments, and these similarities can be exploited to perform long-baseline interferometry, long- baseline intensity interferometry, gamma-ray observation, stellar spectrometry, and remote sensing with unprecedented sensitivities and state-of-the-art resolution.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

GLINT: program overview and potential science objectives

The HI-CLASS is a high power, wideband, coherent laser radar (ladar) for long range detection, tracking, and imaging located at the Maui Space Surveillance Site. HI-CLASS will be used to provide high precision metrics as well as information for images of space objects and remote sensing with the same system. The four phases of the HI-CLASS hardware development program were completed in Fall 1997. During this development contract, hardware and software were developed for two different modes of operation; a ladar mode for active imaging of satellites, and a lidar model for remote sensing atmospheric measurements. Throughout the contract, data were collected which provided a demonstration of the system capabilities which validated technology and designs required for fielding operational systems. The HI- CLASS follow-on demonstration program is currently being performed under an Air Force contract. The follow-on demonstrations will provide the groundwork to an upgrade program currently under consideration by the Air Force. HI- CLASS provides high accuracy tracking in position and velocity simultaneously, and by ultimately providing size, shape and orientation information, it will help assess adversary capabilities. HI-CLASS has the potential to address operational areas of need for increased capability for information about space objects. The follow-on contract effort and the HI-CLASS upgrade effort will provide a demonstration of these potential applications of the HI- CLASS system.

Applications of the HI-CLASS (high-performance CO2 laser radar surveillance sensor) laser system for active imaging of space objects

The HI-CLASS is a high power, wideband, coherent laser radar for long range detection, tracking, and imaging located at the Maui Space Surveillance Site (MSSS). HI-CLASS will be used to provide high precision metrics as well as information for images of space objects and remote sensing with the same system. The HI-CLASS system is currently in the final of four phases. During Phase 1, breadboard hardware was built which led to a fully integrated laser radar system at MSSS. During Phase 2, an improved system oscillator and receiver-processor were built and integrated which brought the system capability to 12 Joules at 30 Hz. The Phase 3 system has the addition of a power amplifier to the transmitter which brought the system capability to 30 Joules Hz. The HI-CLASS system will validate technology and designs required for fielding operational systems since it has the potential to address operational areas of need for increased capability for information about space objects. HI-CLASS will provide high accuracy tracking in position and velocity simultaneously, and by ultimately providing size, shape and orientation information which will help assess adversary capabilities.

Active imaging of space objects using the HI-CLASS (high-performance CO 2 ladar surveillance sensor) laser system

The Air Force Phillips Laboratory is in the process of demonstrating an advanced space surveillance capability with a heterodyne laser radar system to be used, among other applications, for range-resolved imaging of orbiting satellites. In this paper, we present our first satellite feature reconstruction from field results using reflective tomographic techniques.

Stanley R. Czyzak

Papers

Geo Light Imaging National Testbed (GLINT): past, present, and future

GLINT: program overview and potential science objectives

Applications of the HI-CLASS (high-performance CO2 laser radar surveillance sensor) laser system for active imaging of space objects

Active imaging of space objects using the HI-CLASS (high-performance CO 2 ladar surveillance sensor) laser system

Satellite feature reconstruction using reflective tomography: field results