Bruce Rafert

SMIFTS: a cryogenically cooled, spatially modulated imaging infrared interferometer spectrometer

We describe a novel cryogenically cooled, spatially modulated, imaging, Fourier transform interferometer spectrometer for spectral measurements in the 1 - 5 micrometers range. Using spatial modulation and a detector array to sample the interferogram, the instrument employs no moving parts to obtain spectra. It is extremely robust and potentially more reliable than other interferometers in addition to taking advantage of the multiplexing afforded by array detectors. The instrument technology possesses a unique combination of characteristics which forms a niche for spectral measurement not widely known but of great potential value. These characteristics include broad wavelength range, wide field of view if desired, simultaneous measurement of all spectral channels, compactness, no moving parts, and moderate resolution. We present a small amount of test data derived from the instrument.

Design of an airborne Fourier transform visible hyperspectral imaging system for light aircraft environmental remote sensing

Kestrel Corporation and the Florida Institute of Technology have designed, and are now manufacturing, a Fourier transform visible hyperspectral imager system for use in a single engine light aircraft. The system is composed of a Sagnac-based interferometer optical subsystem, a data management system, and an aircraft attitude and current position sybsystem. The system is designed to have better than 5 nm spectral resolution at 450 nm, operates over the 440 nm to 1150 nm spectral band and has a 2D spatial resolution of 0.8 mrad. An internal calibration source is recorded with every frame of data to retain radiometric accuracy. The entire system fits into a Cessna 206 and uses a conventional downward looking view port located in the baggage compartment. During operation, data are collected at a rate of 15 Mbytes per second and stored direct to a disk array. Data storage has been sized to accommodate 56 minutes of observations. Designed for environmental mapping, this Fourier transform imager has uses in emergency response and military operations.

MightySat II.1 Fourier-transform hyperspectral imager payload performance

Using a new microsat called MightySat II as a platform, Kestrel Corporation is designing and building the first Fourier transform hyperspectral imager (FTHSI) to be operated from a spacecraft. This payload will also be the first to fly on the Phillips Laboratory MightySat II spacecraft series, a new, innovative approach, to affordable space testing of high risk, high payoff technologies. Performance enhancements offered by the Fourier transform approach have shown it to be one of the more promising spaceborne hyperspectral concepts. Simulations of the payloads performance have shown that the instrument is capable of separating a wide range of subtle spectral differences. Variations in the return from the Georges Bank and shoals are discernible and various types of coastal grasses (sea oats and spartina) can be isolated against a sand background.

Hyperspectral imaging Fourier transform spectrometers for astronomical and remote sensing observations

The Florida Institue of Technology and the Phillips Laboratory have developed several advanced visible (0.4-0.8 micrometers ) imaging fourier transform spectrometer (IFTS) brassboards, which simultaneously acquire one spatial and one spectral dimension of the hyperspectral image cube. The initial versions of these instruments may be scanned across a scene or configured with a scan mirror to pick up the second spatial dimension of the image cube. The current visible hyperspectral imagers possess a combination of features, including (1) low to moderate spectral resolution for hundreds/thousands of spectral channels, (2) robust design, with no moving parts, (3) detector limited free spectral range, (4) detector-limited spatial and spectral resolution, and (5) field widened operation. The utility of this type of instrument reaches its logical conclusion however, with an instrument designed to acquire all three dimensions of the hyperspectral image cube (both spatial and one spectral) simultaneously. In this paper we present the (1) detailed optical system designs for the brassboard instruments, (2) the current data acquisition system, (3) data reduction and analysis techniques unique to hyperspectral sensor systems which operate with photometric accuracy, and (4) several methods to modify the basic instrument design to allow simultaneous acquistion of the entire hyperspectral image cube. The hyperspectral sensor systems which are being developed and whose utility is being pioneered by Florida Tech and the Phillips Laboratory are applicable to numerous DoD and civil applications including (1) space object identification, (2) radiometrically correct satellite image and spectral signature database observations, (3) simultaneous spactial/spectral observations of booster plumes for strategic and surrogate tactical missile signature identification, and (4) spatial/spectral visible and IR astronomical observations with photometric accuracy.

LWIR and MWIR ultraspectral Fourier transform imager

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

Onboard spectral imager data processor

. 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.

THRIFTI: tomographic hyperspectral remote imaging Fourier transform interferometer

Previous papers have described the concept behind the MightySat II.1 program, the satellites Fourier Transform imaging spectrometers 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.

Measured performance of an airborne Fourier-transform hyperspectral imager

Hyperspectral imaging spectrometers (HSIs) utilizing Sagnac interferometer are throughput-limited if a slit is employed in their designs. This paper describes the Tomographic Hyperspectral Remote Imaging Fourier Transform Interferometer (THRIFTI) optical design. THRIFTI is capable of producing spectral autocorrelation fringe modulation over an image plane defined by a two-dimensional CCD array without the throughput disadvantage encountered by the Sagnac-based imaging spectrometers that incorporate a slit. This approach is utilized to recapture the full spatial-spectral characteristics of an image hypercube via tomography or linear deconvolution. In addition to its large throughput, THRIFTI is robust and simple to construct. The optical design of THRIFTI is discussed and the first experimental results are presented.

Engineering model for the MightySat II.1 hyperspectral imager

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.

Hyperspectral observations of space objects

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.