The Hyperspectral Imager for the Coastal Ocean (HICO) is the only spaceborne
hyperspectral sensor designed for characterization of the coastal maritime environment. It
was taken from a set of written requirements to a complete hardware package ready for
spacecraft-level testing in 16 months. It had to meet NASA's safety requirements for the
ISS. A means of directing the sensor's line of sight to off-nadir directions was essential.
Construction of HICO was made possible by extensive use of commercial off-the-shelf
(COTS) components, with minor modifications for spaceflight/vacuum conditions where
necessary. Efficient engineering combined these components into a complete system that
met all requirements.
The Naval Research Laboratory and the Boeing Company have teamed to fly the NRL ocean Portable Hyperspectral Imager for Low Light Spectroscopy (ocean PHILLS) on board the International Space Station (ISS). This joint program is named the Hyperspectral Sensor for Global Environmental Imaging and Analysis (HyGEIA). Hyperspectral images spanning the wavelength range 400 to 1000 nm will be collected at a ground sample distance of 25 m, with 10 nm spectral binning, and 200 to 1 signal to noise over the visible wavelengths for a 5% albedo scene. These images will be used to characterize the coastal ocean and littoral zone, crops, and forest areas. The PHILLS will also image over the same wavelength range at 130 m GSD to produce similar environmental products over a larger ground area. This paper will describe the modification of PHILLS required for use on the ISS, the modeled on orbit performance, and the planned on orbit configuration.
Production of science or Naval products from hyperspectral data requires the careful calibration of the sensor and the validation of the algorithms to demonstrate that they produce the correct products at the required accuracy. Thus a key part of the Navy’s Hyperspectral Remote Sensing Technology Program is the maintenance of accurate calibration for the Naval EarthMap Observer (NEMO) spacecraft’s Coastal Ocean Imaging Spectrometer (COIS) during the lifetime of the spacecraft. On-Orbit COIS is calibrated in three ways: Moon imaging, using on-board calibration lamps, and imaging of well characterized ocean and land scenes. The primary standard for COIS on-orbit calibration will be monthly imaging of the moon. The approach is similar to that used in NASA's SeaWiFS and MODIS programs, with the added complication that COIS images the Moon surface at much higher resolution than the NASA 1 km resolution sensors. On-board calibration lamps will not provide absolute calibration, but will be used to provide a stability check as frequently as once per orbit. Imaging the known reflectance land and open ocean sites will provide a data set for validating the calibration and atmospheric correction against measured surface reflectances. As an additional check COIS will be cross-calibrated with two well calibrated aircraft sensors, NRL’s Ocean PHILLS and NASA's AVIRIS, which will under-fly COIS and image the same ocean and land scenes.
This paper reports progress on the development of a fast modulating retroreflector for a free space optical data link. A previous publication reported sustaining video over a 17 meter link using a multiple quantum well shutter with a diameter of 0.5 cm at a rate on the order of 0.5 Mbps, limited by the demonstration electronics. This work describes improvements in the device performance, which is on the order of 4 Mbps to 6 Mbps with a Bit Error Rates of 10-6 over a robust optical link. This device lends itself to an array configuration for long range applications and will clearly support T1 rates of 1.54 Mbps, and higher.
KEYWORDS: Modulators, Retroreflectors, Quantum wells, Modulation, Free space optics, Video, Free space optical communications, Camera shutters, Free space, Data communications
In this paper, we describe a demonstration using a Multiple Quantum Well modulator combined with an optical retroreflector which supported a high speed free space optical data link. Video images were transmitted over an 859 nanometer link at a rate of 460 kilo bits per second, where rate of modulation was limited by demonstration hardware, not the modulator. Reflection architectures for the modulator were used although transmission architectures have also been investigated but are not discussed in this paper. The modulator was a GaAs/Al0.3Ga0.7As quantum well which was designed and fabricated for use as a shutter at the Naval Research Laboratory. We believe these are the first results reported demonstrating a high speed free space optical data link using multiple quantum well shutters combined with retroreflectors for viable free space optical communications.
Chemical vapor deposited (CVD) silicon carbide mirrors were exposed to bombardment by 8 km/s (5.2 eV) oxygen atoms that simulated exposure in low earth orbit for periods up to 7.5 years. The reflectances of four mirrors were measured before and after exposure at 584, 736, 1048, 1216, and 1610 angstroms and at eleven angles of incidence ranging from 5 degree(s) to 80 degree(s). The oxygen exposure reduced the normal incidence reflectances by factors of 1.5 to 4.5 in the VUV but had no effect on the visual appearance. The optical constants and thicknesses of the surface layers present on the SiC substrates were determined from reflectance measurements. This analysis indicated that before exposure the surface layers were composed of SiOx (where x approximately equals 1.5) with thicknesses of 8 - 18 angstroms. After exposure the thicknesses had increased to 35 - 45 angstrom. There were no systematic differences in the reflectances after simulated space exposures of 1.5, 4.5, and 7.5 years. This implied that most of the growth in thickness of the SiOx layers occurred early in the exposure and stabilized at thicknesses of 35 - 45 angstroms. The optical results were consistent with x-ray photoelectron spectroscopy of the four mirrors after oxygen exposure.
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