The Multispectral Thermal Imager (MTI) was designed as an imaging radiometer with absolute calibration requirements established by Department of Energy (DOE) mission goals. Particular emphasis was given to water surface temperature retrieval using two mid wave and three long wave infrared spectral bands, the fundamental requirement was a surface temperature determination of 1K at the 68% confidence level. For the ten solar reflective bands a one-sigma radiometric performance goal of 3% was established. In order to address these technical challenges a calibration facility was constructed containing newly designed sources that were calibrated at NIST. Additionally, the design of the payload and its onboard calibration system supported post launch maintenance and update of the ground calibration. The on-orbit calibration philosophy also included vicarious techniques using ocean buoys, playas and other instrumented sites; these became increasingly important subsequent to an electrical failure which disabled the onboard calibration system. This paper offers various relevant lessons learned in the eight-year process of reducing to practice the calibration capability required by the scientific mission. The discussion presented will include observations pertinent to operational and procedural issues as well as hardware experiences; the validity of some of the initial assumptions will also be explored.
The mission of the Multispectral Thermal Imager (MTI) satellite is to demonstrate the efficacy of highly accurate multispectral imaging for passive characterization of urban and industrial areas, as well as sites of environmental interest. The satellite makes top-of-atmosphere radiance measurements that are subsequently processed into estimates of surface properties such as vegetation health, temperatures, material composition and others. The MTI satellite also provides simultaneous data for atmospheric characterization at high spatial resolution. To utilize these data the MTI science program has several coordinated components, including modeling, comprehensive ground-truth measurements, image acquisition planning, data processing and data interpretation and analysis. Algorithms have been developed to retrieve a multitude of physical quantities and these algorithms are integrated in a processing pipeline architecture that emphasizes automation, flexibility and programmability. In addition, the MTI science team has produced detailed site, system and atmospheric models to aid in system design and data analysis. This paper provides an overview of the MTI research objectives, data products and ground data processing.
One element of a multi-year calibration program between the National Institute of Standards and Technology (NIST) and the National Aeronautical and Space Administration (NASA) Earth Observing System (EOS) Project Science Office has been the development and deployment of a portable transfer radiometer for verifying the thermal-infrared scales being used for flight-instrument pre-launch calibration. This instrument, the Thermal-infrared Transfer Radiometer (TXR), has been built and the first deployment test was completed successfully, as has been reported previously.1 The 5 µm channel, based on a photovoltaic Indium Antimonide (InSb) detector, so far has demonstrated a pre-deployment to post-deployment uncorrected repeatability of better than 30 mK to 60 mK, which is sufficient to enable intercomparisons at useful uncertainty levels for the EOS program. However, the 10 µm channel, based on a photovoltaic Mercury Cadmium Telluride (MCT) detector, shows uncorrected repeatability levels of about 0.5 K, the response changes being induced by cryocycling. This paper describes the technique that has been developed for correcting these changes. A portable black body check-source travels with the TXR that is used to verify the repeatability during the deployment trip. The check-source, in combination with the stability of the 5 µm channel, is used to restore a higher accuracy scale to the 10 µm channel than would otherwise be possible. This application is analogous to the use of an on-orbit calibration source to check for and correct for launch-induced or degradation-induced flight instrument detector response changes.
The Multispectral Thermal Imager (MTI) is a satellite-based imaging system that provides images in fifteen spectral bands covering large portions of the spectrum from 0.45 through 10.7 microns. This article describes the current MTI radiometric image calibration, and will provide contrast with pre-launch plans discussed in an earlier article. The MTI system is intended to provide data with state-of-the-art radiometric calibration. The on-orbit calibration relies on the pre-launch ground calibration and is maintained by vicarious calibration campaigns. System drifts before and between the vicarious calibration campaigns are monitored by several on-board sources that serve as transfer sources in the calibration of external images. The steps used to transfer calibrations to image products, additional radiometric data quality estimates performed as part of this transfer, and the data products associated with this transfer will all be examined.
The Multi-spectral Thermal Imager (MTI) will be a satellite- based imaging system that will provide images in fifteen spectral bands covering large portions of the spectrum from 0.45 through 10.7 microns. An important goal of the mission is to provide data with state-of-the-art radiometric calibration. The on-orbit calibration will rely on the pre-launch ground calibration and will be maintained by vicarious calibration campaigns. System drifts before and between the vicarious calibration campaigns will be monitored by several on-board sources that serve as transfer sources in the calibration of external images. These sources can be divided into two groups: a set of sources at an internal aperture, primarily intended to monitor short term drifts in the detectors and associated electronics; and two sources at the external aperture, intended to monitor longer term drifts in the optical train before the internal aperture. The steps needed to transfer calibrations to image products, additional radiometric data quality estimates performed as part of this transfer, and the data products associated with this transfer will all be examined.
Los Alamos National Laboratories has completed the design, manufacture and calibration of a vacuum-compatible, tungsten lamp, integrating sphere. The light source has been calibrated at the National Institute of Standards and Technology and is intended for use as a calibration standard for remote sensing instrumentation. Calibration 2(sigma) uncertainty varied with wavelength from 1.21% at 400 nm and 0.73% at 900 nm, to 3.95% at 2400 nm. The inner radius of the Spectralon-coated sphere is 21.2 cm with a 7.4 cm square exit aperture. A small satellite sphere is attached to the main sphere and its output coupled through a stepper motor driven aperture. The variable aperture allows a constant radiance without effecting the color temperature output from the main sphere. The sphere's output is transmitted into a vacuum test environment through a fused silica window that is an integral part of the outer housing of the vacuum shell assembly. The atmosphere within this outer housing is composed of 240 degree(s)K nitrogen gas, provided by a custom LN2 vaporizer unit. Use of the nitrogen gas maintains the internal temperature of the sphere at a nominal 300 degree(s)K +/- 10 degree(s). The calibrated spectral range of the source is 0.4 micrometers through 2.4 micrometers . Three, color temperature matched, 20 W bulbs together with a 10 W bulb are within the main integrating sphere. Two 20 W bulbs, also color temperature matched, reside in the satellite integrating sphere. A silicon and a germanium broadband detector are situated within the inner surface of the main sphere. Their purpose is for the measurement of the internal broadband irradiance. A fiber-optic-coupled spectrometer measures the internal color temperature that is maintained by current control on the lamps. Each lamp is independently operated allowing for radiances with common color temperatures ranging from near 0.026 W/cm2/sr to about 0.1 W/cm2/sr at a wavelength of 0.9 micrometers (the location of the peak spectral radiance).
A radiometric calibration station (RCS) is being assembled at the Los Alamos National Laboratory (LANL) which will allow for calibration of sensors with detector arrays having spectral capability from about 0.4-15 micrometers. The configuration of the LANL RCS is shown. Two blackbody sources have been designed to cover the spectral range from about 3-15 micrometers, operating at temperatures ranging from about 180-350 K within a vacuum environment. The sources are designed to present a uniform spectral radiance over a large area to the sensor unit under test. THe thermal uniformity requirement of the blackbody cavities has been one of the key factors of the design, requiring less than 50 mK variation over the entire blackbody surface to attain effective emissivity values of about 0.999. Once the two units are built and verified to the level of about 100 mK at LANL, they will be sent to the National Institute of Standards and Technology (NIST), where at least a factor of two improvements will be calibrated into the blackbody control system. The physical size of these assemblies will require modifications of the existing NIST Low Background Infrared (LBIR) Facility. LANL has constructed a bolt-on addition to the LBIR facility that will allow calibration of our large aperture sources. Methodology for attaining the two blackbody sources at calibration levels of performance equivalent to present state of the art will be explained in the paper.