Atmospheric propagation issues relevant to optical communications

Atmospheric propagation issues relevant to space-to-ground optical communications for near-earth applications are studied. Propagation effects, current optical communication activities, potential applications, and communication techniques are surveyed. It is concluded that a direct-detection space-to-ground link using redundant receiver sites and temporal encoding is likely to be employed to transmit earth-sensing satellite data to the ground some time in the future. Low-level, long-term studies of link availability, fading statistics, and turbulence climatology are recommended to support this type of application

Effects of turbulence on the geodynamic laser ranging system

The Geodynamic Laser Ranging System (GLRS) is one of several instruments being developed by the National Aeronautics and Space Administration (NASA) for implementation as part of the Earth Observing System in the mid-1990s (Cohen et al., 1987; Bruno et al., 1988). It consists of a laser transmitter and receiver in space and an array of retroreflectors on the ground. The transmitter produces short (100 ps) pulses of light at two harmonics (0.532 and 0.355 microns) of the Nd:YAG laser. These propagate to a retroreflector on the ground and return. The receiver collects the reflected light and measures the round-trip transit time. Ranging from several angles accurately determines the position of the retroreflector, and changes in position caused by geophysical processes can be monitored

Airborne lidar detection and characterization of internal waves in a shallow fjord

A dual-polarization lidar and photography are used to sense internal waves in West Sound, Orcas Island, Washington, from a small aircraft. The airborne lidar detected a thin plankton layer at the bottom of the upper layer of the water, and this signal provides the depth of the upper layer, amplitude of the internal waves, and the propagation speed. The lidar is most effective when the polarization filter on the receiver is orthogonal to the transmitted light, but this does not depend significantly on whether the transmitted light is linearly or circularly polarized. The depolarization is greater with circular polarization, and our results are consistent with a single parameter Mueller scattering matrix. Photographs of the surface manifestation of the internal waves clearly show the propagation direction and width of the phase fronts of the internal waves, even though the contrast is low (2%). Combined with the lidar profile, the total energy of the internal wave packet was estimated to be 9 MJ

Ocean Color Inferred from Radiometers on Low-Flying Aircraft

The color of sunlight reflected from the ocean to orbiting visible radiometers hasprovided a great deal of information about the global ocean, after suitable corrections aremade for atmospheric effects. Similar ocean-color measurements can be made from a lowflyingaircraft to get higher spatial resolution and to obtain measurements under clouds.A different set of corrections is required in this case, and we describe algorithms to correctfor clouds and sea-surface effects. An example is presented and errors in the correctionsdiscussed

Lidar TS measurements on Northeast Atlantic mackerel (Scomber scombrus)

A green linearly polarized laser and a digital video camera were used to find the average reflectivity and lidar target strength of live mackerel. The light reflected from the fish was compared to that reflected from a target having a known total reflectivity of 20 %. The target was 50 % depolarizing, resulting in 10 % reflectivity in the copolarized plane and 10 % in the cross-polarized plane. Using a lidar having two receivers with different polarization, one might be able to distinguish between the reflected laser light from mackerel and other fish (e.g. herring) as they depolarize the light differently. Mackerel was found to reflect 8.6 % of the light in the plane copolarized with the laser and 6.1 % in the cross-polarized plane, giving a total average reflectivity of 14.7 %. A similar experiment on sardines gave a co-polarized return of 9.7 % and a cross-polarized return of 3.1 %. The large difference in depolarization between the two (41 % for mackerel and 24 % for sardines, respectively) can be used for species identification. The average reflectivity was combined with the size of the fish to find the lidar target strength of mackerel at 532 [nm]. When the video camera and laser were co-polarized, the target strength was found to be between 1.06x10-4 and 3.61x10-4 [m2sr-1]. The cross-polarized setup resulted in a target strength between 8.92x10-5 and 2.21x10-4 [m2sr-1]. Multiplying this by fish density, we get the lidar volume backscatter. Adding a second receiver gives the lidar great new species identification capabilities

Subsurface Ocean Signals from an Orbiting Polarization Lidar

Abstract: Detection of subsurface returns from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite were demonstrated. Despite the coarse range resolution of this aerosol lidar, evidence of subsurface scattering was observed as a delay and broadening of the cross-polarized signal relative to the co-polarized signal in the three near-surface range bins. These two effects contributed to an increased depolarization at the nominal depth of 25 m. These features were all correlated with near-surface chlorophyll concentrations. An increase in the depolarization was also seen at a depth of 50 m under certain conditions, suggesting that chlorophyll concentration at that depth could be estimated if an appropriate retrieval technique can be developed. At greater depths, the signal is dominated by the temporal response of the detectors, which was approximated by an analytical expression. The depolarization caused by aerosols in the atmosphere was calculated and eliminated as a possible artifact