A portable differential image motion sensor (DIMS) has been developed and field demonstrated to measure the
atmospheric coherence diameter, or Fried parameter, r0, both at daytime and at night. The hardware design was
developed using system requirements and performance analysis. A graphical user interface (GUI) and software were
developed to automatically measure r0 from collected imagery data. The DIMS system uses a short wave infrared
(SWIR) camera, IR telescope with custom environmental enclosure, a rack-mount computer accessed remotely through a
laptop, and an equatorial mount and tripod for accurate pointing at a selected star. The system is two-man portable. The
sensor continuously measures r0 from star imagery during clear weather at any time of day or night, with an update rate
of 10 minutes. A continuously nutating optical wedge moves the star image in a circle allowing automatic background
subtraction. Data output is provided at the SWIR 1.6 μm wavelength and scaled to 0.55 μm and pointing at zenith. Valid
r0 measurements range is from 1 cm to 20 cm (in the visible waveband). The r0 measurements over time were performed
at daytime at sea level in San Diego. The largest values of r0 were observed near and after the sunset. This approach
provides a straight-forward path to sea-based seeing measurements with an addition of a stabilized platform.
A concept of a Hybrid Wavefront-based Stochastic Parallel Gradient Decent (WSPGD) Adaptive Optics
(AO) system for correcting the combined effects of Beacon Anisoplanatism and Thermal Blooming is
introduced. This system integrates a conventional phase conjugate (PC) AO system with a WSPGD AO
system. It uses on-axis wavefront measurements of a laser return from an extended beacon to generate
initial deformable mirror (DM) commands. Since high frequency phase components are removed from the
wavefront of a laser return by a low-pass filter effect of an extended beacon, the system also uses off-axis
wavefront measurements to provide feedback for a multi-dithering beam control algorithm in order to
generate additional DM commands that account for those missing high frequency phase components.
Performance of the Hybrid WSPGD AO system was evaluated in simulation using a wave optics code.
Numerical analysis was performed for two tactical scenarios that included ranges of L = 2 km and L = 20
km, ratio of aperture diameter to Fried parameter, D/r0, of up to 15, ratio of beam spot size at the target to
isoplanatic angle, θB/θ0, of up to 40, and general distortion number characterizing the strength of Thermal
Blooming, Nd = 50, 75, and 100. A line-of-sight in the corrected beam was stabilized using a target-plane
tracker. The simulation results reveal that the Hybrid WSPGD AO system can efficiently correct the effects
of Beacon Anisoplanatism and Thermal Blooming, providing improved compensation of Thermal
Blooming in the presence of strong turbulence. Simulation results also indicate that the Hybrid WSPGD
AO system outperforms a conventional PC AO system, increasing the Strehl ratio by up to 300% in less
than 50 iterations. A follow-on laboratory demonstration performed under a separate program confirmed
our theoretical predictions.
For a non-cooperative target, a laser beacon is created by illuminating the target with a beacon beam. When a beacon
beam propagates though deep turbulence, turbulence spreads the beam. A conventional phase conjugate adaptive optics
(AO) system is not efficient in the presence of Beacon Anisoplanatism when the beacon beam spot size at the target
includes many isoplanatic patch sizes. We introduce a concept of the wavefront-based stochastic parallel gradient decent
(WSPGD) AO system, which uses an off-axis wavefront sensor to provide feedback for the beam control algorithm. This
concept is based on the finding that the phase aberrations of laser return from the target contain information about beam
spot size at the target, and that correction of a limited number of low-order Zernike modes increases on-axis intensity
and power in the bucket at the target. We evaluated the WSPGD AO system performance in simulation for two tactical
engagement scenarios in the presence of strong turbulence. We found that that the WSPGD AO system can efficiently
compensate the effects of strong turbulence including Beacon Anisoplanatism, even when the beam spot size at the
target includes up to 20 isoplanatic patch sizes and the isoplanatic angle is by a factor of 2.6 less than the diffraction
limit. The Strehl ratio gain for this scenario is 1.6 - 2.5, and the maximum Strehl ratio is achieved after 15-20 iterations.
A laboratory demonstration performed under a separate program confirmed our theoretical predictions.
Turbulence inner scale affects scintillation in laser projection and laser communication systems especially in strong
scintillation regime. Analytical and numerical models are used for performance analysis and design of these systems.
Turbulence inner scale is critically important to anchor theoretical predictions to an experiment. However, the inner scale
is usually not measured in the experiments along extended atmospheric paths. Commercial scintillometer commonly
operates over the range of a few hundreds meters and requires an optical transmitter and receiver at different ends of the
propagation path. We introduced a concept for turbulence inner scale sensor, which is based on phase related
phenomenon and can operate along arbitrary atmospheric paths including the strong scintillation regime both during
daytime and nighttime. We evaluated the feasibility of this approach. We developed an analytical model for a tilt-corrected
point spread function (PSF) of a distant source that enables turbulence inner scale sensor determination from
optical measurements, evaluated the PSF sensitivity to the inner scale variations for ground-to-ground and space-to-ground
engagement scenarios, designed and built a sensor breadboard prototype Finally, for the first time we performed
turbulence inner scale measurements along space-to-ground propagation paths by imaging stars. We found that the
turbulence inner scale on space-to-ground paths is in the range from 1 cm to 3 cm, whereas it is in the range from 0.2 cm
to 1.2 cm near the ground. Thus, initial inner scale measurements by imaging stars revealed that turbulence inner scale
on extended elevated paths exceeds that value near the ground.
A laboratory demonstration of two novel tactical beam control methods for correcting the effects of strong turbulence
including Beacon Anisoplanatism, and the combined effects of Beacon Anisoplanatism and Thermal Blooming,
respectively, were performed in SAIC's Tactical Beam Control Test-Bed. Both systems were tested with ratio of
aperture diameter to Fried parameter, D/r0, of up to 7, and ratio of beam spot size at the target to isoplanatic angle, θB/θo,
of up to 10. The first method was implemented in a Wavefront-based Stochastic Parallel Gradient Decent (WSPGD)
adaptive optics (AO) system, which uses an off-axis wavefront sensor (WFS) to provide feedback for a multi-dithering
beam control algorithm. The second method was implemented in a Hybrid WSPGD AO system, which incorporates the
WSPGD AO system with a conventional Phase Conjugate (PC) AO system. The Hybrid system uses an on-axis WFS to
generate initial deformable mirror commands and an off-axis WFS to generate additional commands that account for the
high frequency phase components removed from the wavefront of a laser return by Beacon Anisoplanatism. We
developed a low speed PC-based WSPGD controller, implemented designs of the WSPGD and Hybrid WSPGD AO
systems in SAIC's Test-Bed, and tested both AO systems in static and dynamic turbulence over a wide range of
turbulence conditions. A target-plane tracker was used to stabilize the line-of-sight in the AO corrected beam. Test
results show that the WSPGD AO system efficiently compensates the effects of Beacon Anisoplanatism for both static
and dynamic turbulence, providing a mean performance gain of 1.8 averaged over multiple turbulent realizations. We
also found in testing that the Hybrid WSPGD system efficiently compensates for Beacon Anisoplanatism in the presence
of Thermal Blooming - providing improved compensation for both Thermal Blooming and turbulence. In the presence
of strong Beacon Anisoplanatism with θB/θo of up to 10, the maximum performance gain is 4.9 and the mean
performance gain for multiple turbulence realizations is 2.1.
We investigated the spatial structure of atmospheric turbulence at Maui Space Surveillance Site (MSSS) using a 3.6 m telescope and a spatial filtering receiver. This receiver simultaneously records four star images on one camera frame. The star images are formed through pupil masks representing aperture diameters of 0.1 m, 0.5m, 1.5 m, and 3.6 m. We determined the camera orientation for each data set by moving the telescope at a given angle in azimuth and elevation. We calculated the horizontal and vertical components of the image centroid and evaluated the statistics of the horizontal and vertical wavefront tilt as a function of the aperture diameter and seeing conditions. We found several evidences of anisotropy of turbulence at MSSS. On four nights we observed that the variance of on-axis horizontal tilt exceeded the variance of the vertical tilt by a factor of 1.3-3.3. We believe that this is due to anisotropy of large-scale turbulence, where the horizontal scale of the turbulent inhomogeneities exceeds their vertical scale. The estimates of the horizontal and vertical turbulence outer scale confirmed this conclusion. In addition, in several data sets the horizontal image spot diameter of the long-exposure star image exceeded the vertical image spot diameter. We also found that large apertures are more likely to have higher anisotropy coefficient values than small apertures. This is because the contribution of small-scale isotropic turbulence to the image centroid reduces with increasing telescope diameter. In the case of isotropic turbulence, the power spectral densities (PSDs) of wavefront tilt are consistent with theoretical models. The telescope vibration modes were observed at 20 Hz. In the case of anisotropic turbulence, the PSDs of the horizontal tilt component have lower slope in the high frequency range, and difference between PSDs for large and small apertures is reduced. The anisotropy of turbulence and atmospheric tilt may affect the design and performance analysis of both active and passive optical systems.
The tracking algorithm is presented that reduces the influence of the camera motion on the tracking performance. The algorithm uses a change detector. The target motion is described by parameterized optical flow. The flow parameters are estimated using Kalman filtering. The algorithm allows us to estimate the target motion without any bias associated with the camera motion. The effects of thermal blooming on high-energy laser beacon for air-to-ground directed energy system are evaluated. The laser fluence at the target and power in the bucket are evaluated for various tactical engagement scenarios and different atmospheric conditions. The critical laser power that can be efficiently transmitted through the atmosphere is evaluated. Two techniques for mitigating the effects of thermal blooming including a method based on pointing of a high energy beam "downwind" to correct for the thermal blooming tilt and focusing a high energy beam beyond the target range are evaluated. We found that the power in the bucket at the target at the optical axis of a high energy beam for tactical directed energy system increases about one order of magnitude due to correction of the thermal blooming tilt.
Laser communication has an enormous potential to provide a secure, jam-resistant, low detection probability and high-bandwidth means of communmication to support multimedia, imagery, video, mapping and other command and control functions in battlefield environments. In this paper, we discuss the development of a complete numerical model for free-space laser - PamCom model. We examine results obtained in the preliminary study that validate the feasibility of a selected approach. We review the estimates for atmospheric transmission for various lasercom links at 1.55 μm, analysze spatial-temporal statistics of the scintillation mititigation techniques. In addition, we review results for the link performance analysis including the SNR and BER calculations and examine predictions for the irradiance probability density function (PDF) from various models. Finally, we discuss the composition and top level architecture of the Pam Com model, which will be usable for a wide variety of scenarios, involving terminals on aricrafts, satellites, and the ground, and will be designed for use in DoD and commercial lasercom system design, test, and evaluation.
We investigated an edge response of an extended object in a turbulent atmosphere using imagery data acquired with a double-waveband passive imaging system operating in the visible IR wavebands and an actively illuminated optical sensor. We made two findings. We found that the edge response of an extended object is independent of an exposure time, and an atmospheric tilt does not contribute to the image blur of an extended object. In addition, we found that turbulence-induced image blur for an extended object reduces, not increases, with the imager diameter. Therefore, one can reduce the turbulence-induced image blur for an extended object reduces, not increases, with the imager diameter. Therefore, one can reduce the turbulence-induced blur by increasing aperture diameter of an imaging lens. Both findings contradict the predictions of the conventional imaging theory, suggesting that the conventional theory is not applicable to extended anisoplanatic objects. We provided physical interpretation for the results obtained. In addition, we discussed the mitigation techniques that allow us to reduce both turbulence-induced image blur and edge waviness in optical images.