The Firepond Long Range Imaging CO2 Laser Radar

In the current scenario of high-resolution inverse synthetic aperture radar (ISAR) imaging, the non-cooperative targets may have strong maneuverability, which tends to cause time-variant Doppler modulation and imaging plane in the echoed data. Furthermore, it is still a challenge to realize ISAR imaging of maneuvering targets from sparse aperture (SA) data. In this paper, we focus on the problem of 3D geometry and motion estimations of maneuvering targets for interferometric ISAR (InISAR) with SA. For a target of uniformly accelerated rotation, the rotational modulation in echo is formulated as chirp sensing code under a chirp-Fourier dictionary to represent the maneuverability. In particular, a joint multi-channel imaging approach is developed to incorporate the multi-channel data and treat the multi-channel ISAR image formation as a joint-sparsity constraint optimization. Then, a modified orthogonal matching pursuit (OMP) algorithm is employed to solve the optimization problem to produce high-resolution range-Doppler (RD) images and chirp parameter estimation. The 3D target geometry and the motion estimations are followed by using the acquired RD images and chirp parameters. Herein, a joint estimation approach of 3D geometry and rotation motion is presented to realize outlier removing and error reduction. In comparison with independent single-channel processing, the proposed joint multi-channel imaging approach performs better in 2D imaging, 3D imaging, and motion estimation. Finally, experiments using both simulated and measured data are performed to confirm the effectiveness of the proposed algorithm.

A simple analytical model of a laser radar's subtended irradiance probability-density-function has been developed for both direct detection and coherent detection laser radar. The vacuum speckle irradiance statistics are developed following Goodman's M parameter treatment for direct detection ladar and also by setting the M-parameter equal to one (negative-exponential power statistics) for coherent laser radar. A turbulence M parameter is then computed using the round-trip aperture averaging analyses of Gudimetla and Holmes based on the Rytov-variance parameter computation over an atmospheric path of interest. The vacuum M parameter and the turbulence M parameter are then combined to form an effective M parameter. This effective M parameter is used in an analytically simple gamma distribution probability-density-function for the laser radar's subtended irradiance. We will show excellent agreement with the more analytically complicated two-parameter K-distribution from the literature. We will also indicate how one may include the turbulence scintillation in addition to the fundamental vacuum speckle, with increasing levels of turbulence to determine ladar performance.

One potential approach to monitoring atmospheric pollutants is the use of differential absorption lidar (DIAL). In this technique two laser pulses are transmitted: one laser pulse on a resonant absorption of the species being monitored and the other off resonance. The CO2 TEA laser has been successfully used to obtain path-averaged concentration measurements using a retroreflector or a topographical target1. The usefulness of this technique would be greatly increased by using a distributed scatterer such as the naturally occurring atmospheric aerosols. A CO2 TEA laser can provide sufficient peak power to perform this task; however data collection is limited by the low PRF's (1-100Hz) available. A possible alternative is a low pressure Q-switched CO2 laser in conjunction with a heterodyne receiver such as is currently being used for existing CO2 laser radar systems2. Heterodyne detection lowers the required transmitter power while the high PRF (20-50kHz) of a Q-switched laser reduces the data collection time. Such a system designed for DIAL operation could provide range-resolved blanket area coverage with high up-date rates (~1 per min.).

In this paper, a method for Inverse Synthetic Aperture Radar (ISAR) image formation based on the use of the Complex Empirical Mode Decomposition (CEMD) is proposed. The CEMD [1] which based on the Empirical Mode Decomposition (EMD) is used in conjunction with a Time-Frequency Representation (TFR) to estimate a 3-D time-range-Doppler Cubic image, which we can use to effectively extract a sequence of ISAR 2-D range-Doppler images. The potential of the proposed method to construct ISAR image is illustrated by simulations results performed on synthetic data and compared to 2-D Fourier Transform and TFR methods. The simulation results indicate that this method can provide ISAR images with a good resolution. These results demonstrate the potential application of the proposed method for ISAR image formation.

Coherent Laser Radar is a powerful remote sensing tool, which can be applied to range-finding, target discrimination, vibrometric monitoring, air pollution monitoring, aircraft wake-vortex and clear-air turbulence analysis. A high power, highly efficient, near diffraction limited and highly reliable pulsed coherent laser source is a key sub-system required in a coherent Lidar sensor. When humans are involved, eye safe laser emission is also typically required. Therefore a highly efficient fiber laser system based on a coherent Master-Oscillator followed by a chain of Erbium (EDFA) and Erbium co-doped with Ytterbium fiber amplifiers (EYDFA) is ideally suited for this application suite. In this paper, we are presenting an all polarization-maintaining fiber architecture and experimental results on such a high peak power fiber laser system allowing for versatile modulation strategies at a wavelength of 1563nm commensurate with a clear atmospheric transmission window and eye-safe operation. The system is constituted by three amplification stages, all based on Polarization-Maintaining fiber. With 660ns and 20Kpps, over 500W peak power pulses have been experimentally demonstrated with near diffraction limited performance with this all PM fiber system.

A methodology for the estimation of ladar cross sections from high-resolution image data of geometrically complex targets is presented. Coherent CO(2) laser radar was used to generate high-resolution amplitude imagery of a UC-8 Buffalo test aircraft at a range of 1.3 km at nine different aspect angles. The average target ladar cross section was synthesized from these data and calculated to be sigma(T) = 15.4 dBsm, which is similar to the expected microwave radar cross sections. The aspect angle dependence of the cross section shows pronounced peaks at nose on and broadside, which are also in agreement with radar results. Strong variations in both the mean amplitude and the statistical distributions of amplitude with the aspect angle have also been observed. The relative mix of diffuse and specular returns causes significant deviations from a simple Lambertian or Swerling II target, especially at broadside where large normal surfaces are present.

In this paper basic principle of range-Doppler imaging in inverse synthetic aperture radar (ISAR) is analyzed. Simulating experiments of ISAR imaging on far-distant rotating multi-point targets are finished. The data films of ISAR in rectangular and polar coordinate formats are generated by microcomputer. Two-dimensional image with high- resolution are obtained by optical processing technology. Possible motion errors are discussed in the paper. Compensating methods and experimental results for some motion errors are given. Finally, feasibility of optoelectronic hybrid real-time processing for ISAR data is investigated.

In inverse synthetic aperture radar (ISAR) imaging, translation compensation should be done before range-Doppler imaging process, and range alignment is the first step for translation compensation. In order to remove the limitation of integer range bin and align the echoes precisely for ISAR range alignment, combining with the advantage of array signal processing at fractional unit delay compensation, we propose a novel range alignment method based on linear transmitting/receiving (T/R) array. Firstly the ISAR imaging system is modeled as a linear T/R array. Then based on the snapshot imaging model of linear T/R array, range alignment is accomplished by wave path difference compensation which is transformed into the phase difference compensation in frequency domain between adjacent array elements. The phase difference compensation consists of integer range bin alignment and decimal time delay compensation which is implemented by the phase rotation's estimation and compensation in frequency domain. Finally, the results of simulation data and real radar data are provided to demonstrate the effectiveness of the proposed method.

: Previously, a novel formulation for the generation of three-dimensional (3D) inverse synthetic aperture radar (ISAR) images based on recent developments in high resolution spectral estimation theory was presented. Because this technique requires only snapshots of data, where a is defined as a block of Nyquist sampled frequency-time pulses of data, we refer to it as snapshot imaging. Concomitant with these results, recent advances in interferometric SAR imaging have demonstrated the use of two-dimensional (2D) range-Doppler image phase information to extract out-of-plane height information to obtain 3D images of ground scenes. In this case, a unique sampling grid is generated that allows overlaying of nearly identical 2D range-Doppler images and uses phase differences between these images to estimate the out-of-plane height information, from which a 3D image is developed. In this report, we develop a framework connecting these two techniques, particularly applicable to forming 3D images of target types typically dominated by smaller numbers (<20) of scattering centers, and characterized by deterministic exoatmospheric motion having torque-free Euler dynamic spin and precession. Applications of the development are illustrated using simulation data: specifically, the wideband, monostatic field is simulated using a simple point scatter model of a generic cone-like target, and the phase-enhanced 3D image is generated for differing cases. Extensions of the technique to bistatics are also discussed.

Diode-pumped alkali lasers (DPALs) have drawn much attention since they were proposed in 2001. The narrow-linewidth DPAL can be potentially applied in the fields of coherent communication, laser radar, and atomic spectroscopy. In this study, we propose a novel protocol to narrow the width of one kind of DPAL, diode-pumped rubidium vapor laser (DPRVL), by use of an injection locking technique. A kinetic model is first set up for an injection-locked DPRVL with the end-pumped configuration. The laser tunable duration is also analyzed for a continuous wave (CW) injection-locked DPRVL system. Then, the influences of the pump power, power of a master laser, and reflectance of an output coupler on the output performance are theoretically analyzed. The study should be useful for design of a narrow-linewidth DPAL with the relatively high output.

In conventional imaging laser radar, the resolution of target is constrained by the diffraction-limited, which includes the beamwidth of the laser in the target plane and the telescope's aperture. Synthetic aperture imaging Ladar (SAIL) is an imaging technique which employs aperture synthesis with coherent laser radar, the resolution is determined by the total frequency spread of the source and is independent of range, so can achieve fine resolution in long range. Ray tracing is utilized here to obtain two-dimensional scattering properties from three-dimensional geometric model of actual target, and range-doppler algorithm is used for synthetic aperture process in laser image simulation. The results show that the SAIL can support better resolution.

A low average power, pulsed, solid-state, 1.06-micron coherent laser radar (CLR) for range and velocity measurements of atmospheric and hard targets has been developed. The system has been operating at a field test site near Boulder, CO since September, 1988. Measurements have been taken on moving targets such as atmospheric aerosol particles, belt sanders, spinning disks, and various stationary targets. The field measurements have shown that this system exhibits excellent velocity measurement performance. A fast-tuning CW Nd:YAG oscillator has also been developed which has a frequency tuning range of greater than 30 GHz (which spans a target radial velocity range of over 16 km/s) and a tuning speed greater than 30 GHz/ms.

The relative merits of coherent and noncoherent laser radars are reviewed. An information theoretic approach is used to investigate the tradeoffs between degrees of freedom (resolution) and signal-to-noise ratio in maximizing information in radar images. Similar tradeoffs between carrier-to-noise ratio and degrees of freedom are obtained for maximizing the signal-to-noise ratio in detection and estimation problems. It is shown that under some conditions noncoherent laser radars provide better performance than coherent radars, while under other conditions, the reverse is true.

The maneuverability of targets cannot be neglected in many circumstances for Inverse Synthetic Aperture Radar (ISAR) imaging procedure. For these circumstances, the Range Instantaneous Doppler (RID) algorithm is required to form the well-focused ISAR images. Simultaneously, as the inevitable means of obtaining the target’s cross-range size, the cross-range scaling approach for the RID images also needs to be considered consequently. Since there are few cross-range scaling methods focused on the RID images, we propose a novel method in this paper based on the phase coefficients estimation and scatterers pairing process to solve the problem. Without the special requirements for the scatterers or complicated optimization process, the proposed approach is widely adaptive and efficient. Otherwise, besides the novel estimation method for the rotation parameters, we also propose the novel technique for calculating the cross-range scaling factor of the RID images, which is quite different from that of the Range Doppler (RD) images. Finally, by using the recommended RID imaging algorithm and the proposed cross-range scaling approach, the results of ISAR imaging and cross-range scaling experiments on the simulation and real-data experiment are represented, which demonstrate the effectiveness of the proposed approach in this paper.

In advanced multifunctional radar, inverse synthetic aperture radar (ISAR) imaging of sparse apertures for maneuvering targets is a challenge problem. In general, the Doppler modulation of rotation motion can be modeled as linear frequency for uniformly accelerated rotation targets, which is spatial-variant in two-dimension (2-D). The signal diversity inherently reflects the maneuverability and provides a rationale of rotation motion estimation. In this paper, we focus on the problem of sparse apertures ISAR imaging and scaling for maneuvering targets. The maneuvering signal model is formulated as chirp code and represented using a chirp-Fourier basis. Then sparse representation is applied to realize range-Doppler (RD) imaging from the sparse apertures, where the superposition of chirp parameters is acquired using the modified discrete chirp Fourier transform (MDCFT). After preprocessing, such as sample selection, rotation center determination, and noise reduction, the chirp parameters are used to estimate the parameters of rotation motion using the weighted least square (WLS) method. Finally, a high-resolution scaled-ISAR image is achieved by rescaling the acquired RD image using the estimated rotation velocity. Experiments are performed to confirm the effectiveness of the proposal.