Bruce Guenther

Multiyear On-Orbit Calibration and Performance of Terra MODIS Reflective Solar Bands

Terra Moderate Resolution Imaging Spectroradiometer (MODIS) has made continuous global observations for more than six years since its launch in December 1999. MODIS has 36 spectral bands: 20 reflective solar bands (RSBs) with wavelengths from 0.41-2.2 mum and 16 thermal emissive bands with wavelengths from 3.7-14.4 mum. It is a cross-track scanning radiometer that collects data at three nadir spatial resolutions: 0.25 km (2 bands), 0.5 km (5 bands), and 1 km (29 bands). An onboard solar diffuser (SD) and an SD stability monitor (SDSM) are used biweekly for RSB on-orbit radiometric calibration. Another onboard calibrator (OBC), a spectroradiometric calibration assembly, is used periodically to evaluate and monitor RSB spatial and spectral performance. In addition to measurements made using OBCs, lunar observations at nearly identical phase angles are used to track RSB calibration stability. This paper provides an overview of MODIS RSB on-orbit calibration algorithms and operational activities. It discusses sensor characteristics that could impact RSB calibration accuracy and data product quality, including degradation of the SD bidirectional reflectance factor (BRF), degradation of the scan mirror reflectance in the visible spectral region, and changes in operational configuration. The Terra MODIS OBCs have performed well in monitoring SD degradation and tracking changes in RSB response. Band 8 (0.41 mum) has experienced the largest response decrease with an approximate annual rate of 4.5% (mirror side 1). Band 9 (0.44 mum) has an annual response decrease of about 2.3% (mirror side 1). For most RSB bands with wavelengths greater than 0.5 mum, the annual response changes are generally less than 1.0%. Results from the SDSM on-orbit observations show that the SD BRF also has a similar wavelength-dependent degradation, with the largest degradation appearing at the shortest wavelengths. Among the 330 RSB detectors, there are no inoperable detectors, and only a few noisy detectors have appeared postlaunch

MODIS Reflective Solar Bands On-Orbit Lunar Calibration

The moderate resolution imaging spectroradiometer (MODIS) protoflight model on-board the Terra spacecraft and the MODIS flight model 1 on-board the Aqua spacecraft were launched on December 18, 1999 and May 4, 2002, respectively. They view the moon through the space view (SV) port approximately once a month to monitor the long-term radiometric stability of their reflective solar bands (RSBs). The lunar irradiance observed by MODIS depends on the viewing geometry. Algorithms were developed to select lunar views such that these geometric effects are minimized. In each MODIS lunar observation, the moon can be viewed in multiple scans. The lunar irradiance of a MODIS RSB can be derived from the response of all detectors of a spectral band in one scan which fully covers the moon, from that of one detector in multiple scans or from the response of all detectors in multiple scans. Based on lunar observations, a set of coefficients is defined and derived to trend MODIS system response degradation at the angle of incidence (AOI) of its SV port. It is shown that the degradation is both wavelength and mirror side dependent. Since launch, Terra and Aqua MODIS band 8 (412 nm) mirror side one have degraded 36% and 17%, respectively, at the AOI of the SV. A comparison between the lunar coefficients and those derived from the MODIS on-board solar diffuser (SD) calibrations shows that the response change of the MODIS RSB is both AOI and time dependent. Time-dependent response versus scan angle (RVS) lookup tables derived from lunar views, SD calibration, and Earth-view observations have been used to maintain the quality of the L1B data for both the Terra and Aqua MODIS RSB. The corrections provided by the RVS in the Terra and Aqua MODIS data from the 412-nm band are as large as 14% and 6.2%, respectively.

Prelaunch algorithm and data format for the Level 1 calibration products for the EOS-AM1 Moderate Resolution Imaging Spectroradiometer (MODIS)

The Moderate Resolution Imaging Spectroradiometer (MODIS) radiometric calibration product (Level 1B) is described for the thermal emissive and reflective solar bands. A band-integrated radiance is produced for all measurements. A reflectance factor product is also produced for the reflected solar band measurements. Specific sensor design characteristics are identified to assist in understanding how the calibration algorithm software product is designed. The product file format is summarized, and the location for the current file format is provided.

Multiyear On-Orbit Calibration and Performance of Terra MODIS Thermal Emissive Bands

Since its launch in December 1999, Terra MODIS has been making continuous Earth observations for more than seven years. It has produced a broad range of land, ocean, and atmospheric science data products for improvements in studies of global climate and environmental change. Among its 36 spectral bands, there are 20 reflective solar bands and 16 thermal emissive bands (TEBs). MODIS TEBs cover the mid-wave infrared and long-wave infrared spectral regions with wavelengths from 3.7 to 14.4 . They are calibrated on-orbit using an onboard blackbody (BB) with its temperature measured by a set of thermistors on a scan-by-scan basis. This paper will provide a brief overview of MODIS TEB calibration and characterization methodologies and illustrate onboard BB functions and TEB performance over more than seven years of on-orbit operation and calibration. Discussions will be focused on TEB detector short-term stability and noise characterization and changes in long-term response (or system gain). Results show that Terra MODIS BB operation has been extremely stable since launch. When operated at its nominal controlled temperature of 290 K, the BB temperature variation is typically less than 0.30 mK on a scan-by-scan basis, and there has been no time-dependent temperature drift. In addition to excellent short-term stability, most TEB detectors continue to meet or exceed their specified noise characterization requirements, thus enabling calibration accuracy and science data product quality to be maintained. Excluding the noisy detectors identified prelaunch and those that occurred postlaunch, the changes in TEB responses have been less than 0.7% on an annual basis. The optical leak corrections applied to bands 32-36 have been effective and stable over the entire mission.

MODIS reflective solar bands calibration algorithm and on-orbit performance

The MODerate Resolution Imaging Spectroradiometer (MODIS)is one of the key instruments for the NASA s Earth Observing System (EOS).The MODIS ProtoFlight Model (PFM)was launched on-board the EOS Terra spacecraft on December 18,1999 and has been providing the science community and public users global data sets for the study of the land,oceans,and atmosphere for more than two and a half years.This coverage is further enhanced by the data sets from the MODIS Flight Model (FM-1)that was launched on-board the EOS Aqua spacecraft on May 4,2002.MODIS has 36 spectral bands with wavelengths ranging from 0.41 to 14.5 μm and nadir spatial resolutions of 250m (2 bands), 500m (5 bands),and 1km (29 bands).The sensor s 20 reflective solar bands (RSB)from 0.41 to 2.1 μm are calibrated on-orbit by a solar diffuser (SD)and a solar diffuser stability monitor (SDSM)system.The other 16 thermal emissive bands (TEB)with wavelengths above 3.7 μm are calibrated by a blackbody. This paper describes the RSB on-orbit calibration approach using the SD/SDSM system,its implementation in the Level 1B algorithm,and the RSB on-orbit characterization and performance for both Terra and Aqua MODIS. The TEB calibration algorithm and performance are presented in a separate paper in these proceedings.

Degradation of MODIS optics and its reflective solar bands calibration

The MODerate Resolution Imaging Spectroradiometer (MODIS) has 36 spectral bands with wavelength ranging from 0.41(mu) to 14.5(mu) and spatial resolution of 0.25 km (2 bands), 0.5 km (5 bands), and 1.0 km (29 bands) at Nadir. Its ProtoFlight Model (PFM) on the NASA EOS Terra spacecraft has been providing global coverage of the Land, Ocean, and Atmosphere for the science community since the instrument opened its Nadir door on 24 February 2000. The MODIS optical system includes a 2-sided paddle wheel scan mirror, a fold mirror, and a primary mirror. The sensors 20 reflective solar bands (RSB) from 0.41(mu) to 2.1(mu) are calibrated on- orbit by a solar diffuser (SD) and a solar diffuser stability monitor (SDSM). In addition to the SD, degradation of the MODIS optics in the reflective solar bands has been observed, including variations in degradation between the two sides of the MODIS scan mirror. During MODIS first year of on-orbit operation, the overall degradation at the shortest wavelength of 0.41(mu) is about 2.5% for the SD, and in excess of 8% for the MODIS system. In this paper, we present our degradation analysis results and discuss their impact on the RSB on-orbit calibration.

Modeling the Detector Radiometric Gains of the Suomi NPP VIIRS Reflective Solar Bands

Right after the opening of the nadir door of the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi National Polar-orbiting Partnership satellite, the detector gains of the near-infrared bands had decreased much faster than expected, indicating large degradation of the VIIRS optical system. To help determine the root cause and to access the potential outcome of the degradation, we developed a mathematical model based on a physical hypothesis that the observed degradation was due to the sensor Rotating Telescope Assembly (RTA) mirror surface contamination. To date, the detector gains have been consistent with a physical model of a thin contaminant layer of material on each of the four RTA reflective mirrors. The contaminated material, after exposure to solar radiation, reduces the mirror reflectance over the reflective solar band (RSB) wavelength region. We describe the mathematical model and apply the model to predict the RSB detector gains at the end of seven-year mission operation. The model also projects that the signal-to-noise ratios of the RSB will all be larger than the design requirements with a margin of at least 25% at the end of seven years of mission operation. In addition, the detector relative spectral response (RSR) is modulated by the wavelength-dependent optical throughput degradation. We compute the modulated RSR and its impacts on sensor radiometric calibration and the computed top-of-the-atmosphere spectral reflectance at the Sensor Data Record level.

Satellite-Earth remote sensor scatter effects on Earth scene radiometric accuracy

A general modelling formalism is used to extend near-field point spread function (PSF) measurements over a wide-field off-axis angular range, covering a dynamic range of 5 to 6 orders of magnitude, using bidirectional reflectance distribution function (BRDF) measurements of the scan mirror and other key optical elements. Line spread function (LSF) model results were compared with measured near-field LSF measurements for the Moderate Resolution Imaging Spectroradiometer (MODIS) of the Earth Observing System (EOS), demonstrating excellent agreement between model and measurements. When realistic effects of sensor scattered light are taken into account, significant radiometric bias errors are produced near high-contrast structured scenes (e.g. bright clouds over dark oceans and land; broken snow and ice scenes). Image restoration using an asymptotically exact PSF is shown to produce results significantly different from those produced by traditional near-field 5 × 5, 7 × 7, ..., PSF kernel matrix inversion techniques. The results reported have implications for future remote-sensor specifications and testing, in-flight and surface-based calibration comparisons, and the assessment of radiometric bias errors in the presence of moderate- to high-contrast Earth scenes.

MODIS thermal emissive bands calibration algorithm and on-orbit performance

The MODerate Resolution Imaging Spectroradiometer (MODIS)is one of the key instruments for the NASA s Earth Observing System (EOS).MODIS ProtoFlight Model (PFM)was launched on-board the EOS Terra spacecraft on December 18,1999 and the MODIS Flight Model (FM-1)was launched on-board the EOS Aqua spacecraft on May 4, 2002.MODIS has 36 spectral bands with wavelengths ranging from 0.41 to 14.5 μm and nadir spatial resolutions of 250m (2 bands),500m (5 bands),and 1km (29 bands). The sensor s 20 reflective solar bands (RSB)from 0.41 to 2.1 μm are calibrated on-orbit by a solar diffuser (SD)and a solar diffuser stability monitor (SDSM).The other 16 thermal emissive bands (TEB)with wavelengths above 3.7 μm are calibrated by a blackbody.This paper follows the discussions on the RSB calibration and instrument performance presented in a separate paper (Xiong et.al.)in these proceedings, and focuses on the 16 thermal emissive bands (TEB).

Trending results of MODIS optics on-orbit degradation

The MODIS Protoflight Model (PFM), on-board the NASA EOS Terra spacecraft, has been in operation for more than two years. Its 20 reflective solar bands (RSB) from 0.412μ to 2.13μ are calibrated on-orbit by a solar diffuser (SD) with its degradation tracked by a solar diffuser stability monitor (SDSM). The results derived from the SD/SDSM calibration data have shown that SD degradation is wavelength dependent. After nearly 2.5 years, the SD has degraded about 7.0% at 0.412μ, 4.0% at 0.466μ, 2.1% at 0.530μ, and the degradation is smaller at other longer wavelengths. The MODIS optical system includes a rotating scan mirror and other fixed aft optics. Overall system response in the visible spectral range has also shown wavelength dependent degradation over time. This degradation varies with the angle of incidence (AOI) to the scan mirror and the degradation rate is different between two sides of the scan mirror. During the first 20 months of instrument on-orbit operation, the system degradation (mirror side 1) at SD calibration AOI (50.2β) is about 11% at 0.412μ (MODIS Band 8), 6.5% at 0.443μ (Band 9), 5.0% at 0.469μ (Band 3), and 4.0% at 0.488μ (Band 10). Again the degradation is smaller for other bands with longer wavelengths. At other smaller AOIs, our results show that the degradation rate is higher. Since Oct./Nov. 2001, the system response degradation has essentially stopped. In this paper, we present MODIS RSB degradation analyses and the associated trending results including degradation at different AOIs to the scan mirror. We also address their impact on and application to the RSB on-orbit calibration.