The primary payload on a small-satellite, the Air Force Research Laboratory's MightySat II.1, is a spatially modulated Fourier Transform Hyperspectral Imager (FTHSI) designed for terrain classification. The heart of this instrument is a solid block Sagnac interferometer with 85cm-1 spectral resolution over the 475nm to 1050nm bands and 30m spatial resolution. Coupled with this hyperspectral imager is a Quad-C40 card, used for on-orbit processing. The satellite was launched on 19 July 2000 into a 575km, 97.8 degree inclination, sun-synchronous orbit. The hyperspectral imager collected its first data set on 1 August 2000, and has been in continuous operation since that time. To the best of our knowledge, the MightySat II.1 sensor is the first true hyperspectral imager to be successfully operated in space. The paper will describe the satellite and instrument, pre-launch calibration results, on-orbit performance, and the calibration process used to characterize the sensor. We will also present data on the projected lifetime of the sensor along with samples of the types of data being collected.
A Fourier Transform hyperspectral imager was integrated onto a standard clinical fundus camera, a Zeiss FF3, for the purposes of spectrally characterizing normal anatomical and pathological features in the human ocular fundus. To develop this instrument an existing FDA approved retinal camera was selected to avoid the difficulties of obtaining new FDA approval. Because of this, several unusual design constraints were imposed on the optical configuration. Techniques to calibrate the sensor and to define where the hyperspectral pushbroom stripe was located on the retina were developed, including the manufacturing of an artificial eye with calibration features suitable for a spectral imager. In this implementation the Fourier transform hyperspectral imager can collect over a hundred 86 cm-1 spectrally resolved bands with 12 micro meter/pixel spatial resolution within the 1050 nm to 450 nm band. This equates to 2 nm to 8 nm spectral resolution depending on the wavelength. For retinal observations the band of interest tends to lie between 475 nm and 790 nm. The instrument has been in use over the last year successfully collecting hyperspectral images of the optic disc, retinal vessels, choroidal vessels, retinal backgrounds, and macula diabetic macular edema, and lesions of age-related macular degeneration.
Previous papers have described the concept behind the MightySat II.1 program, the satellite's Fourier Transform imaging spectrometer's optical design, the design for the spectral imaging payload, and its initial qualification testing. This paper discusses the on board data processing designed to reduce the amount of downloaded data by an order of magnitude and provide a demonstration of a smart spaceborne spectral imaging sensor. Two custom components, a spectral imager interface 6U VME card that moves data at over 30 MByte/sec, and four TI C-40 processors mounted to a second 6U VME and daughter card, are used to adapt the sensor to the spacecraft and provide the necessary high speed processing. A system architecture that offers both on board real time image processing and high-speed post data collection analysis of the spectral data has been developed. In addition to the on board processing of the raw data into a usable spectral data volume, one feature extraction technique has been incorporated. This algorithm operates on the basic interferometric data. The algorithm is integrated within the data compression process to search for uploadable feature descriptions.
The clear waters of Lake Superior constitute the heart of one of the most significant fresh water ecosystems in the world. Lake Superior is the world's largest lake by surface area (82,100 km2) holding approximately 10% of the earth's freshwater (12,230 km3) that is not locked into glaciers or ice caps. Although Superior is arguably the most significant fresh water ecosystem on earth, questions relating to the lake and its watershed remain unanswered, including the effects of human habitation, exploitation, and economic potential of the region. There is a great diversity of scientific disciplines with a common interest in remote sensing of the Lake Superior ecosystem which have the need for data at all spatial, spectral, and temporal scales-from scales supplied by satellites, ships or aircraft at low spatial, spectral or temporal resolution, to a requirement for synoptic high resolution spatial (approximately 1 meter)/spectral (1 - 10 nm) data. During May and August of 1998, two week-long data collection campaigns were performed using the Kestrel airborne visible hyperspectral imager to acquire hyperspectral data of a broad taxonomy of ecologically significant targets, including forests, urban areas, lakeshore zones and rivers, mining industry tailing basins, and the Lake itself. We will describe the Kestrel airborne hyperspectral sensor, the collection and data reduction methodology, and flight imagery from both campaigns.
Previous papers have described the concept behind the MightySat II.1 program, the satellite' Fourier Transform imaging spectrometer's optical design, and the design for the hyperspectral imaging payload. Initial qualification testing of the payload has been completed. All component level qualification tests have been finished. The solid block optics, interferometer, camera and telescope where all successfully tested and a payload Critical Deign Review was passed. Early optical testing of the monolithic interferometer has shown that it has the designed spectral resolution of less than 100 cm-1. Bench testing of a custom VME data interface board that operates the sensor in a variety of spatial and spectral resolution modes can transfer data satisfactorily at data rates up to 24.3 Mbytes/sec over a VSB bus to spacecraft solid state memory. Problems in manufacturing the hardened C-40 processors has caused a change to an unhardened version of the C-40 using tantalum foil for protection. This still allows all hyperspectral 'smart' imaging spectrometer demonstrations including a 10:1 data compression technique. The payload is scheduled to be delivered in April 1999 for integration on to the spacecraft bus.
UV-VIS-NIR ratiometric reflectance data was obtained for several commonly utilized remote sensing calibration standards used in Fourier Transform Hyperspectral Imaging. We found that single layer reflectance depends on the degree of translucency and hence on the particular choice of background material, from which multiple layer reflectance and extracted absorption and scattering curves logically follow. These data are given as a function of incident wavelength for each calibration standard. Because optical properties are determined by the combination of scattering and absorption, we deconvolved their effects on each material's spectrum.
The Kestrel Corporation visible-near IR band (525 to 1016 nm) airborne Fourier Transform Hyperspectral Imager was modified to include measurement of the polarization characteristics of several ground cover classes. The polarization contrast of typical terrestrial background and target objects was characterized. First, the t statistic was used as an index of class separation to determine whether polarized images were more useful for discriminating several cover classes than unpolarized images. Second, the information present in polarized images which is not present in unpolarized images was identified and described. This was done by regressing polarized and unpolarized images, generating images of predicted values for the polarized images using the regression coefficients, generating images of residuals by subtracting the actual values from the predicted values, and analyzing the statistical separation of cover classes in the residual images. A single polarized image was not more useful for identifying the cover classes than an unpolarized image. A residual image derived from a single polarized image and an unpolarized image provided a mean maximum statistical separation of t equals 18.3 for all cover class combinations. The sum of two orthogonal polarized images provided slightly greater separation, with a mean maximum separation of t equals 23.7.
The MightySat II.1 satellite carries as one of its primary payloads a Fourier transform hyperspectral imager, the first such sensor to be flown in space. Over the last year the sensor has passed its preliminary design and an engineering model of the sensor has been constructed. The model has started to be qualified. To date the sensor has met its weight, volume and power design goals. An unusually high random vibration qualification level has forced the redesign of two mirror mounting techniques. Custom, space qualified, VME electronic camera interface and control cards to handel 20 Mbytes/sec of imagery data has been designed, fabricated, and coupled to a set of four C-40 processors to provide 160 MIPS of onboard processing. Mission operations are now being developed that will demonstrate a 30 m GSD by using the on orbit three axis maneuvering capability of the satellite. The payload is on schedule for a delivery in early 1999 for integration on the bus.
Kestrel Corporation has designed and is now building a dual- band infrared Fourier transform ultraspectral imager for aircraft deployment. Designed for installation in a Cessna 206, this instrument will have a 15 degree FOV, with an IFOV of 1.0 mrad. The target spectral resolution is better than 1.5 cm-1 over 2000 to 3000 cm-1 and 0.4 cm-1 over 850 to 1250 cm(superscript -1$. using 512 spectral channels. The device will use a variety of spectral enhancement techniques to achieve this unprecedented spectral resolution. Computer simulations of the optical systems demonstrates sub-wavenumber resolutions and signal to noise ratios of over 900.
Kestrel Corporation is designing and building the first Fourier transform hyperspectral imager to be operated from a spacecraft. Performance enhancements offered by the Fourier transform approach have shown it to be one of the more promising spaceborne hyperspectral concepts. Simulations of the payload's performance have indicate that the instrument is capable of separating a wide range of subtle spectral differences. The concept design for the payload has been completed and hardware is in fabrication for an engineering model.
During the past year, Kestrel Corporation has designed and built a low cost Fourier transform visible hyperspectral imager (FTVHSI) for deployment in a light aircraft (Cessna TU-206). The instrument is an imaging spectrometer employing a Sagnac (triangle) interferometer, that operates over a range of 450 - 1050 nm with 256 spectral channels, and a 13 degree FOV with an 0.8 mrad pixel IFOV (450 spatial channels). To aid in the calibration of the instrument, calibration and downwelling signals are recorded with every frame. Installed with the optical instrument are attitude sensors and a scene camera. This auxiliary data allows us to place a hyperspectral slice to within less than 5 m of its true position (using selective availability 'on' and differential GPS). We have performed extensive testing and calibration studies, including data collection conducted synchronously with ground measurements at locations including a White Sands radiometric calibration site. This paper reports some of the calibration studies and their results.
During the past year, Kestrel Corporation has designed and built a low cost Fourier transform hyperspectral imager for deployment in a light aircraft. The instrument is a pushbroom imaging spectrometer employing a Sagnac interferometer. The instrument operates over a range of 350- 1050 nm with 256 spectral channels, and a 13 degree FOV with an 0.8 mrad IFOV. Installed with the optical instrument are attitude sensors, a scene camera, a downwelling sensor and in-flight calibration equipment. This paper will focus on the description of both the optical system and the support equipment used in this revolutionary instrument.
A new hyperspectral imager has recently been developed by Kestrel Corporation for use in light aircraft platforms. The instrument provides 256 spectral channels with 87 cm-1 spectral bandwidth over the 450 nm to 1000 nm portion of the spectrum. Operated as a pushbroom imager, the FTVHSI has been shown to have a IFOV of 0.75 mrad, and a FOV of 0.23 rad. The sensor includes an internal spectral/radiometric calibration source, a self contained spectrally resolved downwelling sensor, and complete line of sight and GPS positioning information. The instrument is now operating from a Cessna TU-206 single engine aircraft.