A low-cost thermal IR hyperspectral imager for Earth observation

A novel niche for Earth observations exploiting new technologies in focused, short-duration missions is opening with the growth of the market for small satellites. To demonstrate the ways in which a university's scientific and instrument development programs can innovate under this new model, we are building a low-cost thermal IR spectral sensor. The sensor is a low-mass, power-efficient thermal hyperspectral imager (THI) with electronics contained in a pressure vessel to enable the use of commercial-off-the-shelf (COTS) electronics (see Figure 1). Since the early 1990s, we have been developing spatial Fourier transform thermal hyperspectral imagers for Earth-based and airborne remote sensing as well as laboratory thermal IR microscopy.^1–4 The space-qualified THI is an implementation of this approach. It will allow us to assess our data quality, map out improvements to the design, and quantify cost savings possible through a COTS-based approach.

Figure 1. Schematic of the thermal hyperspectral imager (THI) showing the calibration system, interferometer cube, and location of the camera and electronics within a pressure vessel.

Current space-based thermal spectral sensors for science of Earth's surface are multispectral, collecting measurements of IR radiation in discrete bands. True hyperspectral data has been collected for atmospheric science by, for example, NASA's Atmospheric Infrared Sounder instrument, which collects data in 2378 channels between 3.7 and 15.4(μ)m,⁵ but only at low spatial resolution. Earth surface science can greatly benefit from hyperspectral imaging—gathering reflectance or emittance data in tens or hundreds of narrow, contiguous wavebands—especially to derive laboratory-quality spectra from space.

Although NASA's Hyperion instrument acquired hyperspectral data in the 0.4–2.5(μ)m region,⁶ there is no equivalent to Hyperion in the thermal IR wavelengths (8–14(μ)m). This is in part because traditional thermal IR technology requires cooled optics that have large cost, mass, and power penalties. Our innovative design overcomes the traditional barriers to hyperspectral thermal IR data acquisition, with a signal-to-noise ratio of up to 1000 and a compact, low-power, low-mass configuration using uncooled microbolometer detectors.^1–3

Our sensor is based on a Sagnac interferometer (see Figures 2 and 3) and uses an FLIR^® Photon uncooled 320×256 microbolometer array similar to the focal planes used by NASA's recent LCROSS lunar-impact experiment.⁷ The sensor will collect calibrated hyperspectral radiance data at thermal IR wavelengths in 230m pixels with 20-wavenumber spectral resolution from a hypothetical 400km Earth orbit. THI is designed to be compatible in terms of mass, volume, power, and software-hardware interfaces with small-satellite-platform concepts. The sensor and its components, including the pressure vessel and calibration system, will be no more than 15kg in mass and will draw no more than 20W for 20 minutes with a peak limit of 80W.

Figure 2. Image of a laboratory version of the THI interferometer using an uncooled detector array and optics.¹

Figure 3. Schematic of the Sagnac interferometer and camera system. The camera is focused at infinity and views the scene through the interferometer.³

A spatial Fourier transform hyperspectral imager uses an interferometer to record spectral information as an interferogram using a 2D focal plane array. Each row in a stationary interferometer introduces a unique optical path difference (OPD) between the transmitted and reflected beams measured from each ground target. If the instrument is scanned across the scene at an appropriate rate, light from the ground target can be sampled at each OPD. After data collection, the intensity at the detector array as a function of OPD can be reconstructed into an interferogram for each scene element, which can be used to obtain the spectral radiance as a function of wavelength.^1,2

The target's molecular structure determines the spectral features obtained. The primary scientific objective of THI is to record spectral radiance emitted from targets on Earth's surface as a function of wavelength (see Figure 4) to characterize their temperature and spectral emissivity.² This data is of interest in geologic analysis and resource mapping, determining water body temperatures, and estimating abundance of atmospheric trace gases.

Figure 4. Left: Spectral radiance of an apatite (calcium phosphate) grain obtained by THI compared with the radiance of a blackbody at the same temperature. Right: Emissivity spectrum of the same grain derived from the spectral radiance, compared with a reference spectrum for apatite (gray curve). Although in this case, the THI spectrum is of coarser spectral resolution, the important absorption features are clearly resolved.

The relatively lower overall costs of these small-satellite missions bring several benefits to Earth-observation science and engineering communities. For example, low mission cost makes the use of unconventional sensors an acceptable risk, which in turn enables further decreases in costs from the use of COTS parts. The relatively short mission duration and greater frequency of these flights will allow a greater number of younger scientists and engineers to gain experience with similar projects. And the low-cost missions provide an experimental platform for testing new sensor technologies that may transition to larger, more long-lived platforms.

THI represents a novel approach to Earth remote sensing using an innovative thermal IR sensor design that will allow collection of hyperspectral thermal IR data from orbit for the first time. Its low cost, low mass, and low power requirements make it an ideal example of the way a university's instrument development program can exploit the experimental platform provided by the emerging small-satellite market. We are currently building a prototype of the THI instrument that will be used for airborne testing and data collection within the state of Hawaii during 2011.

Funding is provided by the NASA Experimental Project to Stimulate Competitive Research program.