Leonard John Otten

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 hyperspectral imager: summary of on-orbit performance

The primary payload on a small-satellite, the Air Force Research Laboratorys 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.

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.

Measurement and data-processing approach for estimating the spatial statistics of turbulence-induced index of refraction fluctuations in the upper atmosphere

We present a method of data reduction and analysis that has been developed for a novel experiment to measure the spatial statistics of atmospheric turbulence in the tropopause. We took measurements of temperature at 15 points on a hexagonal grid for altitudes from 12,000 to 18,000 m while suspended from a balloon performing a controlled descent. From the temperature data we estimate the index of refraction and study the spatial statistics of the turbulence-induced index of refraction fluctuations. We present and evaluate the performance of a processing approach to estimate the parameters of isotropic models for the spatial power spectrum of the turbulence. In addition to examining the parameters of the von Kármán spectrum, we have allowed the so-called power law to be a parameter in the estimation algorithm. A maximum-likelihood-based approach is used to estimate the turbulence parameters from the measurements. Simulation results presented here show that, in the presence of the anticipated levels of measurement noise, this approach allows turbulence parameters to be estimated with good accuracy, with the exception of the inner scale.

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

Comparison between a Shack-Hartmann and a distorted grating wavefront sensor

. 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.

Onboard spectral imager data processor

A comparison of the wavefronts recorded simultaneously by a Shack-Hartmann and a Distorted Grating Wavefront Sensor (DGWFS) has been completed. The DGWFS is a phase diversity/wavefront curvature type of sensor using a grating to generate the multiple image planes. Data were collected under simulated propagation conditions using the Advanced Concept Laboratory at Lincoln Laboratory. The sensors were arranged such that both recorded a time varying sample of the wavefront at exactly the same time. Dynamic and static comparisons of the two sensors under conditions that ranged from a benign path, D/r0 = 2, to a propagation condition with significant scintillation, D/r0 =50, were completed. The data show that the two techniques measure static, low amplitude, on the order of a few waves, aberrations with little difference. Under conditions where there are significant aberrations, the wavefronts measured by the two sensors show notable differences with the DGWFS exhibiting a smoothed, low passed, rendition of the wavefront. As the aberrations increase to produce scintillated condition the differences become more pronounced.

High-bandwidth atmospheric-turbulence data collection platform

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

In applications of adaptive optics, understanding the statistical nature of the upper atmospheric turbulence is critical. Today there is a lack of detailed knowledge of upper atmospheric turbulence in the region of the tropopause. Tropopause measurements have been made with simplifications in experimental designs and the potential for experimentally induced artifacts. Recent measurements by various groups indicate the presence of non-Kolmogorov behavior, asymmetric turbulence structure and finite outer scale sizes. Analysis of current balloon borne data collection techniques suggests that signal processing and the balloons own wake may have influenced the sensor. To address some of these concerns, a new measurement platform has been designed that carries 15 constant current probes that are simultaneously sampled at 16 bits at over 1500 samples per second during a controlled descent. All of the signals are then telemetered to the ground without on board processing. The data are then merged with atmospheric data and payload orientation to product key turbulence parameters, including turbulence strength, the inner and outer scales, the temperature structure function, power spectral density of the turbulence, and turbulence isotropic behavior.

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.