Mikhail S. Belen'kii

Experimental study of spatial structure of turbulence at Maui Space Surveillance Site (MSSS)

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.

Full-aperture tilt measurement technique with a laser guide star

A technique for measuring a full aperture tilt (FAT) with a laser guide star (LGS) is proposed. It is shown that information about a FAT is lost in a conventional LGS scheme because of the reciprocity of propagation paths. As a consequence neither the conventional LGS scheme nor its modifications with the receiver coaxial with the transmitter can be used to sense the FAT. A bistatic scheme that permits us to overcome the above difficulty is considered. This scheme permits us to single out the tilt component corresponding to the transmitting beam which is highly correlated with the FAT for a natural star. The tilt component corresponding to the reflected wave can be averaged out by averaging a LGS image motion over its angular extent. Such an averaging, however, does not affect the tilt component corresponding to the transmitting beam. This tilt conservation effect occurs due to the fact that a random motion of the transmitting beam causes a displacement of the LGS as a whole. The accuracy of measuring a FAT with a LGS is determined and the requirements for the measurement scheme are discussed.

Tilt angular correlation and tilt sensing techniques with a laser guide star

The two techniques for sensing full aperture tilt with a laser guide star (LGS) are described. The first technique exploits a full aperture beam transmitting through the main optical train. The two auxiliary telescopes, which are separated from the transmitter in transverse directions, are used to measure a laser beacon image motion. The contribution of the down propagation path to the tilt that is measured with an auxiliary telescope is eliminated by averaging LGS image over a laser beacon angular extent. Such averaging requires FOV of the receiver which greatly exceeds the tilt angular correlation scale. A second method exploits a small aperture beam transmitted from behind a portion of a primary mirror of the main telescope. A laser beacon image motion is measured simultaneously with the main and auxiliary telescopes. A full aperture tilt is determined by subtracting the tilt measured with the main telescope from that measured with an auxiliary one. This method does not require transmitting laser irradiance through the main optical train, and it might be used for the mesospheric sodium layer. A scheme for measuring uncontrolled motion of the main telescope is also considered.

Fundamental limitation in adaptive optics: how to eliminate it? A full-aperture tilt measurement technique with a laser guide star

A theory for a new laser guide star technique is developed. This technique, for the first time, permits the sensing of a full-aperture tilt of the atmospheric wave front distortions using a laser guide star, eliminating one of the fundamental limitations of adaptive optics. By using a detailed analysis of the laser guide star image jitter for a conventional scheme and for its possible modifications, it is shown that, because of reciprocity of propagation paths, a conventional laser beacon is unable to sense a full aperture tilt and can be only used to measure the higher-order wavefront distortions. The full aperture tilt can be measured with a divergent beam if its effective size at a laser beacon altitude coincides with the radius of a receiving telescope. Estimates of the modified laser guide star image jitter rms, signal-to-noise ratio, and the mean square deviation between the instantaneous tilts for the modified laser beacon and a natural star are obtained, and it is shown that the proposed technique is practical.

Experimental validation of the differential image motion lidar concept

We have experimentally validated the concept of a differential image motion (DIM) lidar for measuring vertical profiles of the refractive-index structure characteristic C(n)(2) by building a hard-target analog of the DIM lidar and testing it against a conventional scintillometer on a 300-m horizontal path throughout a range of turbulent conditions. The test results supported the concept and confirmed that structure characteristic C(n)(2) can be accurately measured with this method.

Residual turbulent scintillation effect and impact of turbulence on the Fourier telescopy system

The e ffects o f a tmospheric turbulence o n the g round—to-space p ropagation p ath a nd P oisson sh ot n oise o n a n a ctive laser-based imaging system for high-resolution imaging of Geosynchronous (GEO) satellites are investigated using a wave-optics simulation code. The phase and scintillation statistics in tilt corrected and uncorrected beams are examined at the top ofthe atmosphere and at the satellite. The effects of intensity and phase variations in the illuminating beams on the fringe visibility and spatial frequency of the interference pattern formed by the illuminating beams at the satellite are investigated. The Fourier phase variance caused by Poisson shot noise and turbulence on the uplink path is evaluated. We found that tilt correction reduces the scintillation in the laser beam at the satellite. In the Fourier telescopy system the scintillation variance at the edge of the beam is reduced by a factor of up to 3 for a tilt corrected beam. Long-range propagation in free space reduces scintillation in the illuminating beam. The scintillation variance in the Fourier telescopy system on the optical axis at the satellite is reduced by 26% to 36 %, as compared to that at the top of the atmosphere. The latter is due to diffraction of the laser beam in free space and enhancement of the spatial coherence of the beam described by the Van Cittert-Zernike theorem. The intensity spatial correlation scale in the scintillation pattern exceeds the satellite dimensions. This leads to a so-called residual turbulent scintillation effect, when the scintillation in the illuminating beam modulates the total reflected energy flux. As a result, an arbitrarily large receiver on the ground cannot average the received signal variations. This degrades the Fourier telescopy system performance. Also intensity and phase variations in the illuminating beams degrade the interference pattern formed at the satellite. The turbulence effect on the fringe visibility is stronger at high spatial frequencies. Intensity variations in the illuminating beams degrade the fringe visibility the most. Poisson shot noise and scintillation on the uplink path strongly impacts the Fourier phase of the object. In the turbulent atmosphere the Fourier phase variance increases by a factor of 1 .5-3, as compared to that in free space. The increase of the phase variance is caused by a non-linear interaction between the two statistically independent noise sources. For the nominal signal level and number of averaged pulses the Fourier phase variance is less than 0.1, or ()L I20)2 . This suggests that the Fourier telescopy method is feasible.

Free-space laser communication model

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.

New remote sensing technique for lidar monitoring of atmospheric turbulence

A new remote sensing technique is proposed for determining the turbulent parameters of the atmosphere using a single-ended lidar system. This technique is based on the enhanced backscattering effect and is insensitive to the scattering volume averaging effect on the intensity fluctuations of the reflected wave and the sounding beam. The corresponding measurements are independent of the turbulent scintillation spectrum and that permits the use of high power pulsed lasers with a relatively low repetition rate for determining the refractive index structure characteristic Cn2, its vertical profile Cn2(h) and inner scale of turbulence lo in the atmosphere. A theory of the method is developed, and the conditions are obtained for observing the backscattering amplification effect in the atmosphere with a laser beam scattered by aerosol. The signal-to-noise ratio and the sensitivity of the measured quantities to the inner scale of turbulence lo variations are estimated. A planned demonstration of this technique in the boundary layer of the atmosphere with an eyesafe lidar which has been developed at Georgia Tech is discussed.

Tracking through laser-induced clutter for air-to-ground directed energy system

The agility and speed with which directed energy can be retargeted and delivered to the target makes a laser weapon highly desirable in tactical battlefield environments. A directed energy system can effectively damage and possibly destroy relatively soft targets on the ground. In order to accurately point a high-energy beam at the target, the directed energy system must be able to acquire and track targets of interest in highly cluttered environments, under different weather, smoke, and camouflage conditions and in the presence of turbulence and thermal blooming. To meet these requirements, we proposed a concept of a multi spectral tracker, which integrates three sensors: SAR radar, a passive MWIR optical tracker, and a range-gated laser illuminated tracker. In this paper we evaluated the feasibility of the integrated optical tracker and arrived to the following conclusions: a) the contrast enhancement by mapping the original pixel distribution to the desired one enhances the target identification capability, b) a reduction of the divergence of the illuminating beam reduces rms pointing error of a laser tracker, c) a clutter removal algorithm based on active contours is capable of capturing targets in highly cluttered environments, d) the daytime rms pointing error caused by anisoplanatism of the track point to the aim point is comparable to the diffraction-limited beam spot size, f) the peak intensity shift from the optical axis caused by thermal blooming at 5 km range for the air-to-ground engagement scenario is on the order of 8 μrad, and it is 10 μrad at 10 km range, and e) the thermal blooming reduces the peak average power in a 2 cm bucket at 5 km range by a factor of 8, and it reduces the peak average power in the bucket at 10 km range by a factor of 22.

Design of an optical remote-sensing system for measuring refractive turbulence in the Antarctic boundary layer

We have investigated the feasibility of building an innovative optical remote sensing instrument to monitor the vertical profile of the refractive index structure characteristic Cn2. There is currently no active optical remote sensing instrument which is capable of doing this. Calculations have been performed for a system designed specifically to resolve a site survey question at the South Pole, where recent balloon soundings suggest that excellent astronomical seeing conditions could be obtained by mounting telescopes above a thin layer of atmospheric refractive turbulence near the surface. The new sensor considered here is essentially an imaging lidar which measures range- dependent laser beam wander, from which the vertical profile of Cn2 can be derived. Calculations based on atmospheric characteristics and preliminary design parameters have been carried out for a practical system based on commercially available components. Design parameters include the choice of operating wavelength, elevation angle, transmitter and receiver diameters, and image scale. The calculations indicate that it is feasible to develop an optical remote sensor for monitoring vertical profiles of Cn2 at the South Pole.