NASA's Earth Observing System (EOS) Terra spacecraft was launched in December 1999 and the Aqua spacecraft in May 2002. The Moderate Resolution Imaging Spectroradiometer (MODIS) is one of the key instruments for NASA's EOS missions, currently operated on both the Terra and Aqua spacecrafts. Together they have made continuous global observations for more than 8 years and led to many applications and studies for the Earth's system of land, oceans, and atmosphere. Compared to its heritage sensors, the MODIS was designed with more stringent requirements on the sensor's calibration accuracy and data product quality. Because of this it is equipped with a set of on-board calibrators (OBCs), including a solar diffuser (SD) and a solar diffuser stability monitor (SDSM) for the reflective solar bands (RSB) calibration and a blackbody (BB) for the thermal emissive bands (TEB) calibration. In addition to the sensor's intrinsic design characteristics, the quality of MODIS data products depends on the quality of its on-orbit calibration and characterization and on its on-orbit performance. The primary objective of this paper is to provide an overview of MODIS on-orbit radiometric calibration approaches and a summary of the calibration uncertainties for both RSB and TEB (Terra and Aqua). This paper provides an update to our previous reports with considerations based on each sensor's characteristics identified pre-launch, measured and validated on-orbit. It also serves as a useful reference for the users of MODIS data products.
The Moderate Resolution Imaging Spectroradiometer (MODIS) reflective solar bands (RSB) cover wavelengths from 0.41 to 2.2μm. They are calibrated on-orbit by a solar diffuser (SD) panel, made of space-grade Spectralon. During each SD calibration a solar diffuser stability monitor (SDSM) is operated concurrently to track the changes of the SD bidirectional reflectance factor (BRF). The SDSM views alternately the sunlight (Sun View) through a fixed transmission
screen and the sunlight diffusely reflected from the SD panel (SD view). A design error in the SDSM system, not discovered until post-launch, has caused significant ripples in the SDSM Sun view responses. Consequently an alternative normalization approach has been developed to remove the ripples in the SDSM Sun view responses and their impacts on the SD degradation analysis. This approach has been successfully used in the SDSM measurements on-orbit. In order to reduce the direct solar exposure onto the SD panel, the MODIS instrument was designed with a SD door that is normally commanded to an "open" position during SD/SDSM observations and to a "closed" position when the calibration is completed. For Terra MODIS launched in December 1999, an SD door related anomaly occurred in May 2003 that led to a decision to set the SD door permanently at the open position. This operational configuration has resulted in extra time of direct solar illumination on the SD plate and therefore a much faster SD degradation rate. In this paper we provide a brief description of the MODIS RSB calibration algorithm and the on-board SD and SDSM system used for the calibration. We examine the Terra MODIS SD degradation rate and its spectral dependency. The results from five years of SDSM observations are summarized in this paper and used to evaluate the SD on-orbit performance and its impact on the MODIS RSB calibration uncertainty. Prior to the SD door anomaly, the SD annual degradation rate was approximately 3% at 0.41μm, 2% at 0.47μm, and 1% at 0.53μm. After the SD door anomaly, the SD annual degradation rate has increased to 10% at 0.41μm, 7% at 0.47μm, and 4.5% at 0.53μm.
Terra MODIS, also referred to as the MODIS Protoflight Model (PFM), was launched on-board the NASA's EOS Terra spacecraft on December 18, 1999. It has been in operation for more than four years and continuously providing the science community quality data sets for studies of the Earth's land, oceans, and atmosphere. It has also served as the primary source of information for the MODIS Land Rapid Response System for observing and reporting on natural disasters, and providing active fire information around the Earth. The MODIS instrument has 36 spectral bands with wavelengths ranging from 0.41mm to 14.5mm: 20 bands with wavelengths below 2.2mm are the reflective solar bands (RSB) and the other 16 bands are the thermal emissive bands (TEB). The RSB are calibrated on-orbit using a solar diffuser (SD) with the degradation of its bi-directional reflectance factor (BRF) tracked by an on-board solar diffuser stability monitor (SDSM). The calibration coefficients are updated via Look-Up Tables (LUTs) for the Level 1B code that converts the sensor's Earth view response from digital counts to calibrated reflectance and radiance. In this paper we review the MODIS RSB on-orbit calibration algorithm and the methodology of computing and updating the calibration coefficients determined from the SD and SDSM data sets. We present examples of the sensor's long-term and short-term stability trending of key RSB calibration parameters using over four years of on-orbit calibration data sets. Special considerations due to changes in instrument configuration and sensor response are also discussed.
A key instrument for the NASA Earth Observing System (EOS) mission, the Moderate Resolution Imaging Spectroradiometer (MODIS), is currently operating on-board the Terra and Aqua spacecrafts. This paper discusses the calibration uncertainty analysis for the MODIS Reflective Solar Bands. Each MODIS, either on the Terra or on the Aqua spacecraft, has 20 reflective solar bands, making observations at three different nadir spatial resolutions: 250m (B1-2), 500m (B3-7), and 1000m (B8-19, and B26). The 250m, 500m, and 100m bands have 40, 20, and 10 detectors per band, respectively. The reflective solar bands spectral wavelengths are between 0.41 and 2.3 μm. On-orbit, a solar diffuser is used for the reflective solar bands calibration. For the high gain ocean color bands (B8-16), a retractable attenuation pinhole screen is placed in front of the solar diffuser during each calibration. For the reflective solar bands, the specified uncertainty at the typical scene is 2% in reflectance and 5% in radiance. The uncertainty analysis to be presented in this paper will include the approaches and estimated results for Terra MODIS. Aqua MODIS L1B uncertainty is not reported but is extremely similar to Terra. Emphasis will be on the solar diffuser bi-directional reflectance factor characterization at pre-launch since it is a major contributor to the reflective solar bands uncertainty. Other factors include the Earth view response-versus-scan angle, solar diffuser degradation and attenuation screen effect. For the Terra MODIS instrument, the estimated uncertainties based on the instrument characterization and performance will be compared with the specifications.
The MODerate Resolution Imaging Spectroradiometer (MODIS) uses an on-board solar diffuser (SD) panel made of Spectralon for the radiometric calibration of its 20 reflective solar bands (RSB). The spectral wavelengths of the RSB range from 0.41 to 2.1 micrometers. The on-orbit calibration coefficients are determined from the sensor s responses to the diffusely reflected solar illumination from the SD. This method requires an accurate pre-launch characterization of solar diffuser s bi-directional reflectance factors (BRF) that should cover the sensor s spectral range and illumination/viewing angles and accurate on-orbit monitoring of SD degradation over time. The MODIS SD panel s bi-directional reflectance factors were characterized prior to the sensor s final system integration (pre-launch by the instrument vendor using reference samples traceable to the NIST reflectance standards at a number of wavelengths and carefully selected combinations of the illumination/viewing angles. The measured BRF values were fitted into smooth surfaces and then interpolated for each of the MODIS reflective solar bands. In this paper, we describe an approach designed for the MODIS on-orbit characterization and validation of its SD BRF using multiple SD solar observations at several spacecraft yaw angels. This approach has been successfully applied to both the Terra and Aqua MODIS. This paper presents the algorithm used to derive the SD s relative BRF from observations during spacecraft yaws and compares the on-orbit results with corresponding pre-launch values.
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.
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.
The MODIS instrument's solar diffuser is used in its radiometric calibration for the reflective solar bands (VIS, NIR, and SWIR) ranging from 0.41 to 2.1 micron. The sun illuminates the solar diffuser either directly or through an attenuation screen. The attenuation screen consists of a regular array of pin holes. The attenuated illumination pattern on the solar diffuser is not uniform, but consists of a multitude of pin-hole images of the sun. This non-uniform illumination produces small, but noticeable radiometric effects. A description of the computer model used to simulate the effects of the attenuation screen is given and the predictions of the model are compared with actual, on-orbit, calibration measurements.
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 sensor's 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.
The development of large area xenon drift chambers as imaging systems for the advanced Gamma-Ray Astronomy Telescope Experiment (AGATE), sensitive in the energy range 20 MeV - 100 GeV, is presented here. AGATE is visualized as the successor to the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory, and will add to the wide range of important results currently being obtained by EGRET. Experiments were carried out with a laboratory prototype consisting of a stack of sixteen 1/2m X 1/2m active area drift chambers using both xenon and argon gas mixtures. The spatial resolution of the drift chamber stack was measured with a multi-wire readout plane using atmospheric muons traversing the active volume. A spatial resolution of about 0.23 mm was measured with drift chambers using xenon- methane gas mixtures. The experiments with the argon-isobutane gas mixtures yielded a spatial resolution of about 0.14 mm.