Cross-correlation registration algorithm enhancements for multipixel inverse synthetic aperture LiDAR.

The accurate registration and realignment of complex signal volumes is critical for cross-range aperture gain in 3D LiDAR aperture synthesis. For targets at long range, only a limited number of diffraction-limited pixels will be projected on the target, resulting in low cross-range support. In addition, the signal-to-noise ratio (SNR) is typically low. This research describes an enhanced cross-correlation registration algorithm for 3D inverse synthetic aperture LiDAR data volumes that improves performance for low cross-range support, low SNRs, and relatively large aperture shifts. The registration performance is improved through statistical removal of the cross-correlation noise pedestal and compensation for the reduced signal overlap caused by larger shifts. The registration performance is characterized as a function of SNR, signal shift (target rotation rate), and target pixel support. The algorithm's improvements allow for registration convergence at 1-5 dB lower SNR than the baseline cross-correlation algorithm. In addition, the algorithm enhancements allow for registration convergence at 10%-20% greater shifts.

Interferometric synthetic aperture ladar using code division multiple access apertures.

This paper describes a multi-static interferometric synthetic aperture ladar (IFSAL) for high-resolution, high-precision 3D imaging. Code division multiple access apertures with periodic, pseudorandom noise waveforms are used to create aperture diversity and overcome the ambiguity associated with the aperture separation requirements for interferometric synthetic aperture ladar. The basic theory for mapping relative aperture phase to a high-precision elevation profile is derived for a multi-static IFSAL system and subsequent processing steps are presented. An analytic model and computer simulation are developed to demonstrate the 3D imaging capability of an IFSAL system.

Wave optics simulations of synthetic aperture ladar performance through turbulence.

This paper utilizes a combination of theory and simulations to examine synthetic aperture imaging across a wide range of turbulence conditions. Extensive wave optics simulations are used to validate existing theory and to investigate the use of a common measurement technique. It demonstrates the applicability of earlier synthetic aperture laser radar (ladar) (SAL) research across a wide range of turbulence conditions, and examines the metric approaches and limitations for the imaging conditions normally seen in practical SAL systems. To examine the full impact of turbulence on SAL, the derivations, simulations, and analyses include three different resolution metrics as well as a commonly used contrast metric: the integrated sidelobe ratio. This paper demonstrates the integrated effects of turbulence on SAL imaging. Finally, suggestions are given for measuring the true resolving power of operational SAL systems.

Demonstrated resolution enhancement capability of a stripmap holographic aperture ladar system.

Holographic aperture ladar (HAL) is a variant of synthetic aperture ladar (SAL). The two processes are related in that they both seek to increase cross-range (i.e., the direction of the receiver translation) image resolution through the synthesis of a large effective aperture. This is in turn achieved via the translation of a receiver aperture and the subsequent coherent phasing and correlation of multiple received signals. However, while SAL imaging incorporates a translating point detector, HAL takes advantage of a two-dimensional translating sensor array. For the research presented in this article, a side-looking stripmap HAL geometry was used to sequentially image a set of Ronchi ruling targets. Prior to this, theoretical calculations were performed to determine the baseline, single subaperture resolution of our experimental, laboratory-based system. Theoretical calculations were also performed to determine the ideal modulation transfer function (MTF) and expected cross-range HAL image sharpening ratio corresponding to the geometry of our apparatus. To verify our expectations, we first sequentially captured an oversampled collection of pupil plane field segments for each Ronchi ruling. A HAL processing algorithm incorporating a high-precision speckle field registration process was then employed to phase-correct and reposition the field segments. Relative interframe piston phase errors were also removed prior to final synthetic image formation. By then taking the Fourier transform of the synthetic image intensity and examining the fundamental spatial frequency content, we were able to produce experimental modulation transfer function curves, which we then compared with our theoretical expectations. Our results show that we are able to achieve nearly diffraction-limited results for image sharpening ratios as high as 6.43.

Saturated semiconductor optical amplifier phase modulation for long range laser radar applications.

We investigate the use of a semiconductor optical amplifier operated in the saturation regime as a phase modulator for long range laser radar applications. The nature of the phase and amplitude modulation resulting from a high peak power Gaussian pulse, and the impact this has on the ideal pulse response of a laser radar system, is explored. We also present results of a proof-of-concept laboratory demonstration using phase-modulated pulses to interrogate a stationary target.

Experimental verification of sparse frequency linearly frequency modulated ladar signals modeling.

We present the results of an experiment designed to verify the results of a previously published theoretical model that predicts the range resolution and peak-to-side lobe ratio of sparse frequency linearly frequency modulated (SF-LFM) ladar signals. We use two ultra stable diode lasers which are frequency locked and can be current tuned in order to adjust the difference frequency between the two lasers. The results of the experiment verify the previously developed model proving that SF-LFM ladar signals have the ability to increase the range resolution of a ladar system without the need for larger bandwidth modulators. Finally we simulate a target at a range of approximately 150 meters through the use of a fiber optic delay line, and demonstrate the ability of SF-LFM ladar signals to detect a target at range.

Experimental demonstration of a stripmap holographic aperture ladar system.

By synthesizing large effective apertures through the translation of a smaller imaging sensor and the subsequent proper phasing and correlation of detected signals in postprocessing, holographic aperture ladar (HAL) systems seek to increase the resolution of remotely imaged targets. The stripmap HAL process was demonstrated in the laboratory, for the first time to our knowledge. Our results show that the stripmap HAL transformation can precisely account for off-axis transmitter induced phase migrations. This in turn allows multiple pupil plane field segments, sequentially collected across a synthetic aperture, to be coherently mosaiced together. As a direct consequence, we have been able to confirm the capability of the HAL method to potentially provide substantial increases in longitudinal cross-range resolution. The measurement and sampling of complex pupil plane field segments, as well as target related issues arising from short laboratory ranges, have also been addressed.

Periodic, pseudonoise waveforms for multifunction coherent ladar.

We report the use of periodic, pseudonoise waveforms in a multifunction coherent ladar system. We exploit the Doppler sensitivity of these waveforms, as well as agile processing, to enable diverse ladar functions, including high range resolution imaging, macro-Doppler imaging, synthetic aperture ladar, and range-resolved micro-Doppler imaging. We present analytic expressions and simulations demonstrating the utility of pseudonoise waveforms for each of the ladar modes. We also discuss a laboratory pseudonoise ladar system that was developed to demonstrate range compression and range-resolved micro-Doppler imaging, as well as the phase recovery common to each of the coherent modes.

Monte Carlo simulation of multiple photon scattering in sugar maple tree canopies.

Detecting objects hidden beneath forest canopies is a difficult task for optical remote sensing systems. Rather than relying upon the existence of gaps between leaves, as other researchers have done, our ultimate goal is to use light scattered by leaves to image through dense foliage. Herein we describe the development of a Monte Carlo model for simulating the scattering of light as it propagates through the leaves of an extended tree canopy. We measured several parameters, including the gap fraction and maximum leaf-area density, of a nearby sugar maple tree grove and applied them to our model. We report the results of our simulation in both the ground and the receiver planes for an assumed illumination angle of 80 degrees. To validate our model, we then illuminated the sugar maple tree grove at 80 degrees and collected data both on the canopy floor and at our monostatic receiver aperture. Experimental results were found to correlate well with our simulated expectations.

Sparse frequency LFM ladar signals.

Through modeling we explored the possibility of utilizing a sparse frequency linear frequency modulation (LFM) signal for laser radar (ladar) applications. We propose a potential transmit and receive experiment utilizing the superposition of two LFM laser sources with a known difference frequency to provide the necessary segmented bandwidth. Finally we analyzed the signal performance of the proposed system showing that the range resolution of the signal can be improved by two to three times while utilizing the same modulator bandwidth as that of a continuous LFM signal.

Holographic aperture ladar.

Holographic aperture ladar is a variant of synthetic aperture ladar that seeks to increase cross-range scene resolution by synthesizing a large effective aperture through the motion of a smaller receiver and through the subsequent proper phasing and correlation of the detected signals in postprocessing. Unlike in conventional synthetic aperture ladar, however, holographic aperture ladar makes use of a two-dimensional translating sensor array, not simply a translating point detector. Also unlike in conventional synthetic aperture ladar, holographic aperture images will be formed in the two orthogonal cross-range dimensions parallel and perpendicular to the sensor platform's direction of motion. The central focus is on the development of the stripmap and spotlight holographic aperture transformations. These transformations will allow sequentially collected pupil plane field segments to be coherently stitched together in order to synthesize complex pupil plane fields with larger spatial extent. The challenge in this process is in accounting for the practical fact that both the receiver aperture and the transmitter will be in motion in real-world airborne applications. However, we demonstrate that, owing to the synchronous motion of the transmitter and receiver, resolution enhancements of more than two (stripmap case) or three (spotlight case) times the ratio of the synthetic aperture to the real receiver aperture diameter can be realized. We also demonstrate that in practical applications the holographic aperture ladar image formation process is relatively insensitive to scene depth if a good estimate of nominal scene range is available.

Bidirectional scattering distribution functions of maple and cottonwood leaves.

We present our investigations into the optical scattering properties of both sugar maple (Acer saccarum) and eastern cottonwood (Populus deltoides) leaves in the near-IR wavelength regime. The bidirectional scattering distribution function (BSDF) describes the fractions of light reflected by and transmitted through a leaf for a given set of illumination and observation angles. Experiments were performed to measure the BSDF of each species at a discrete set of illumination and observation angles. We then modeled the BSDFs in such a way that other researchers may interpolate their values for scattering in any direction under illumination at any angle.

Optical sparse aperture imaging.

The resolution of a conventional diffraction-limited imaging system is proportional to its pupil diameter. A primary goal of sparse aperture imaging is to enhance resolution while minimizing the total light collection area; the latter being desirable, in part, because of the cost of large, monolithic apertures. Performance metrics are defined and used to evaluate several sparse aperture arrays constructed from multiple, identical, circular subapertures. Subaperture piston and/or tilt effects on image quality are also considered. We selected arrays with compact nonredundant autocorrelations first described by Golay. We vary both the number of subapertures and their relative spacings to arrive at an optimized array. We report the results of an experiment in which we synthesized an image from multiple subaperture pupil fields by masking a large lens with a Golay array. For this experiment we imaged a slant edge feature of an ISO12233 resolution target in order to measure the modulation transfer function. We note the contrast reduction inherent in images formed through sparse aperture arrays and demonstrate the use of a Wiener-Helstrom filter to restore contrast in our experimental images. Finally, we describe a method to synthesize images from multiple subaperture focal plane intensity images using a phase retrieval algorithm to obtain estimates of subaperture pupil fields. Experimental results from synthesizing an image of a point object from multiple subaperture images are presented, and weaknesses of the phase retrieval method for this application are discussed.