There is increasing interest in free space optical communications as an alternative to fibre optics and radio frequency communications, particularly in 'last mile' applications and applications with weight and power restrictions e.g. communications with unmanned aerial vehicles. The potential advantages of free space optical communications include: high bandwidth; no licensing issues; smaller, lighter payloads; low probability of intercept; and immunity from interference/jamming. However, propagation through the atmosphere is subject to atmospheric scintillation noise affecting the signal-to-noise ratio (SNR), effectively reducing the range and bandwidth of the communication link. This scintillation is experienced even over relatively short propagation paths and is caused by small temperature variations in the atmosphere, resulting in index of refraction changes. In this paper we present a technique to correct for atmospheric scintillation noise in free space optical communications and laser remote sensing. It uses common mode rejection to remove co-channel noise, where each channel is transmitted on separate, but closely spaced, wavelengths. The signal-to-noise ratio is significantly increased, thereby increasing the range and/or bandwidth of the link. To date, tests have been conducted with analogue audio and video transmissions. This has been successful, with improvements of up to 12dB in SNR having been demonstrated. This has been limited by the current implementation, which is only at prototype stage -- the ultimate achievable improvement in SNR is anticipated to be significantly higher.
A method for reducing noise in near-IR laser communications has been proposed that relies upon the dual wavelength output of the He-Xe laser having a high level of noise coherence. However, in transmissions through the atmospheric boundary layer, an additional and significant noise component is added by atmospheric scintillation. These scintillations are mainly limited to frequencies of less than 1 kHz and are correlated in the two laser channels to a degree determined by the channel wavelength separation, the transmission range and the severity of the turbulence regime. To analyze the propagation of waves in random media one normally considers the statistics of the field. In the case of small angle forward scattering, which is the case of interest in laser propagation, field moments higher than the fourth are so difficult to solve that no solutions are known outside of the asymptotic weak and strong approximations. An alternative approach is to conduct numerical experiments in which one generates a realization of the random medium (with the desired statistics) and then calculates the wave field. We have numerically modeled the spatial irradiance intensity as a function of range from a point source under turbulence regimes typical of daytime conditions near the Earth’s surface. Simulations were performed for two closely separated channels in the near-IR (1556.5 and 1558.1 nm). We present the results of these simulations together with the implications for the mitigation of atmospheric scintillation noise by common mode rejection.
A great need exists amongst X-band direct broadcast regional users for near real-time, high spatial resolution cloud detection and cloud property retrieval to support regional interdisciplinary applications. As part of the International MODIS and AIRS Processing Package (IMAPP), the objective treatment of spatial and spectral information, including principal component and residual techniques, is provided by the AIRS single field of view clear and cloud detection and cloud property retrieval algorithm. This algorithm, known as Minimum Local Emissivity Variance (MLEV), is used to retrieve both cloud height and cloud spectral emissivity. The ECMWF model analysis is used to demonstrate that high quality clear radiances can improve the yield and quality of cloud spectral emissivity and height, quantities that are precursors to retrieving cloud micro-physical properties and cloudy sounding profiles. In this paper we describe in detail the procedure employed to achieve this goal. The use of cloud spectral emissivity and height in retrieving cloud micro-physical properties is discussed together with their utility in identifying cloud contaminated soundings in the IMAPP AIRS only single field of view retrieval.
Atmospheric correction of ocean colour is routinely achieved by fitting radiometric observations at near near-infrared wavelengths to radiances predicted for a range of aerosol types. The best-fitting candidate aerosol model can then be used to compute radiances in the visible part of the spectrum, enabling an atmospheric correction to
be applied there. The Navy Aerosol Model (NAM) is a multi-component aerosol model which may be suitable for this purpose. The components of NAM are closely tied to the physical processes which generate them and this allows for some expectation on the spatial homogeneity of the component optical depths. Presented is an atmospheric correction scheme based upon NAM and implemented for SeaWiFS. Some conclusions are drawn about the efficacy of extrapolating to visible wavelengths
those estimates of aerosol type and amount made at near-infrared wavelengths.
Passive remote sensing of ocean color relies on the sunlight backscattered from the ocean, the water-leaving radiance, to convey information on the concentrations of optically active marine constituents. Observations of ocean color from space also contain light scattered by the overlying atmosphere. The determination and removal of the atmospheric contribution to the satellite detected radiance, in order to accurately determine the water-leaving radiance, is known as atmospheric correction. The atmospheric correction must be applied carefully since the ocean color community requires water-leaving radiance estimates to better than 5 percent from beneath an atmosphere that can, in some spectral regions of interest, account for 90 percent of the total satellite signal. Aerosols play an important role in determining the atmospheric scattered radiance to the extent that the principal difficulty that remains in atmospheric correction is accounting for the variability in space and time of aerosol optical properties. We are developing an atmospheric correction algorithm based on a tri-modal maritime aerosol model. Results are presented within the context of present and future ocean color sensors.