The Moderate Resolution Imaging Spectroradiometer (MODIS) is one of the key instruments for NASA's Earth Observing System (EOS) missions. Its high quality land, ocean, and atmosphere data products heavily rely on accurate on-orbit calibration and characterization. MODIS reflective solar bands (RSB) on-orbit calibration is reflectance based and is provided by an on-board Solar Diffuser (SD). The Bi-directional Reflectance Factor (BRF) of the SD was characterized pre-launch. The earthshine (ES) contamination to the SD signal was estimated to be approximately 0.5%. On-orbit observations and geometric optical modeling results show that this contamination strongly depends on the wavelength, calibration viewing geometry, and the Earth surface type. In this paper, the contribution from the earthshine on MODIS RSB calibration is illustrated and characterized systematically. A simple approach is proposed to reduce its impact on the RSB calibration. This approach has been applied to both Terra and Aqua MODIS RSB calibration and proved to be effective in reducing the earthshine contaminations.
Launched in May 2002, the Aqua MODIS has successfully operated on-orbit for more than five years and continuously
produced many high quality data products that have significantly contributed to studies of the Earth's climate and
environmental changes. The MODIS collects data in 36 spectral bands ranging from the visible (VIS) to the long-wave
infrared (LWIR) spectral region and at three (nadir) spatial resolutions: 250m (2 bands), 500m (5 bands), and 1km (29
bands). Bands 1-19 and 26 are the reflective solar bands (RSB) with wavelengths from 0.41 to 2.2μm and bands 20-25
and 27-36 are the thermal emissive bands (TEB) with wavelengths from 3.7 to 14.4μm. The MODIS on-board
calibrators, noticeably improved over those of its heritage sensors, include a solar diffuser (SD), a solar diffuser stability
monitor (SDSM), a blackbody (BB), a spectro-radiometric calibration assembly (SRCA), and a space view (SV) port.
This paper provides an overview of Aqua MODIS on-orbit operation and calibration activities with emphasis on the
performance of its on-board calibrators. Results discussed in this paper include TEB and RSB detector noise
characterization, short-term stability and long-term response change. The sensor's overall spectral (RSB) and spatial
(RSB and TEB) parameters are also presented in this paper.
MODIS has 20 reflective solar bands (RSB) that are calibrated on-orbit using a solar diffuser (SD) and a solar diffuser
stability monitor (SDSM). The MODIS SD bi-directional reflectance factor (BRF) was characterized pre-launch. Its on-orbit
degradation is regularly monitored by the SDSM at wavelengths ranging from 0.41 to 0.94μm. During each
SD/SDSM calibration event, the SDSM views alternately the sunlight directly through a fixed attenuation screen and the
sunlight diffusely reflected from the SD panel. The time series of SDSM measurements (ratios of the SD view response
to the Sun view response) is used to determine the SD BRF degradation at SDSM wavelengths. Since launch Terra
MODIS has operated for more than seven years and Aqua for over five years. The SD panel on each MODIS instrument
has experienced noticeable degradation with the largest changes observed in the VIS spectral region. This paper provides
a brief description of MODIS RSB calibration methodology and SD/SDSM operational activities, and illustrates the SD
on-orbit degradation results for both Terra and Aqua MODIS. It also discusses the impact on the SD degradation due to
sensor operational activities and SD solar exposure time. Aqua MODIS has been operated under nearly the same
condition for more than five years. Its SD annual degradation rate is estimated to be 2.7% at 0.41μm, 1.7% at 0.47μm,
and less than 1.0% at wavelengths above 0.53μm. Terra MODIS, on the other hand, has experienced two different SD
solar exposure conditions due to an SD door (SDD) operation related anomaly that occurred in May 2003 that had led to
a decision to keep the SDD permanently at its "open" position. Prior to this event, Terra MODIS SD degradation rates
were very similar to Aqua MODIS. Since then its SD has experienced much faster degradation rates due to more
frequent solar exposure.
KEYWORDS: MODIS, Calibration, Sensors, Signal to noise ratio, Mirrors, Diffusers, Reflectivity, Space operations, Long wavelength infrared, Black bodies
MODIS is currently onboard NASA's EOS Terra and Aqua spacecraft launched in December 1999 and May 2002, respectively. Together, Terra and Aqua MODIS have generated over 10 years of global observations for the study of changes in the Earth's land, oceans, and atmosphere. Each sensor produces more than 40 science data products using measurements from 36 spectral bands with wavelengths ranging from 0.41 to 14.4μm. MODIS on-orbit radiometric calibration is performed using a solar diffuser (SD) and a solar diffuser stability monitor (SDSM) for the reflective solar bands (RSB) and a blackbody (BB) for the thermal emissive bands (TEB). In addition, regularly scheduled lunar observations are used to track RSB radiometric calibration stability. This paper discusses Aqua MODIS radiometric calibration performance using four-years of on-orbit calibration data. Results include detector noise characterization (SNR for the RSB and NEdT for the TEB), short- and long-term radiometric stability, and optics (scan mirror and solar diffuser) degradation.
The MODerate Resolution Imaging Spectroraiometer (MODIS) reflective solar bands (RSB) are calibrated on-orbit
using solar illuminations reflected from its onboard solar diffuser (SD) plate. The specified calibration uncertainty
requirements for MODIS RSB are ±2% in reflectance and ±5% in radiance at their typical top of atmosphere (TOA)
radiances. The onboard SD bi-directional reflectance factor (BRF) was characterized pre-launch by the instrument
vendor using reference samples traceable to NIST reflectance standard. The SD on-orbit degradation is monitored
using a solar diffuser stability monitor (SDSM). One of contributors to the RSB calibration uncertainty is the
earthshine (ES) illumination on the SD plate during SD calibration. This effect was estimated pre-launch by the
instrument vendor to be of 0.5% for all RSB bands. Analyses of on-orbit observations show that some of the SD
calibration data sets have indeed been contaminated due to extra ES illumination and the degree of ES impact on the
SD calibration is spectrally dependent and varies with geo-location and atmospheric conditions (ground surface type
and cloudiness). This paper illustrates the observed ES impacts on the MODIS RSB calibration quality and compare
them with the effects derived from an ES model based on the viewing geometry of MODIS SD aperture door and
likelihood atmospheric conditions. It also describes an approach developed to minimize the ES impact on MODIS
RSB calibration.
MODIS is a major instrument for NASA's EOS missions, currently operating aboard the EOS Terra and Aqua spacecraft
launched in December 1999 and May 2002, respectively. It was designed to extend heritage sensor measurements and
data records and to enable new research studies of the Earth's land, oceans, and atmosphere. MODIS has 36 spectral
bands (0.41 - 14.4μm) located on four focal plane assemblies (FPA). It makes measurements at three nadir spatial
resolutions: 0.25km, 0.5km, and 1km. Because of instrument design complexity and more stringent calibration
requirements, extensive calibration and characterization activities were conducted pre-launch by the sensor vendor for
both Terra and Aqua MODIS. For the 20 reflective solar bands (RSB) with wavelengths below 2.2μm, these activities
include detector noise characterization, radiometric response at different instrument temperatures and at different scan
angles, and relative spectral response. On-orbit RSB calibration is performed using a solar diffuser (SD) and a solar
diffuser stability monitor (SDSM). In addition, regular lunar observations are made to track RSB radiometric stability.
This paper provides a summary of Terra and Aqua MODIS RSB pre-launch and on-orbit calibration and characterization
activities, methodologies, data analysis results, and lessons learned. It focuses on major issues that could impact MODIS
RSB calibration and data product quality. Results presented in this paper include RSB detector noise characterization,
response versus scan angle and instrument temperature, SD bi-directional reflectance factors characterization, and on orbit
calibration stability. Similar discussions on MODIS thermal emissive bands (TEB) are presented in a separate paper in these proceedings (Xiong et. al).
The Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the NASA EOS Aqua spacecraft has been in operation for more than three years since its launch in May 2002. MODIS is a multi-spectral cross-track scanning radiometer that has 20 reflective solar bands (RSB) from 0.41 to 2.2μm and 16 thermal emissive bands (TEB) from 3.7 to 14.4μm. It makes continuous observations that can be applied to a wide range of studies of the Earth's environment and climate. The sensor's in-flight calibration and characterization activities (radiometric, spatial, and spectral) are performed using a set of onboard calibrators (OBCs) that include a solar diffuser (SD), a solar diffuser stability monitor (SDSM), a blackbody, and a spectro-radiometric calibration assembly (SRCA). In this paper we present on-orbit performance of the Aqua MODIS onboard calibrators using its calibration data sets. We illustrate use of the SD for RSB calibrations and current trending of SD degradation. For the TEB calibration, we discuss BB temperature noise characterization, its short- and long-term stability, and the use of BB warm-up and cool-down cycles to track key TEB calibration parameters. The results of Aqua MODIS spatial and spectral characterization are also addressed. In general the overall Aqua MODIS on-orbit performance has been stable and satisfactory when compared to its design parameters, performance specifications, and pre-launch determined characteristics. In a number of areas, Aqua MODIS is performing better than its predecessor - Terra MODIS.
Twenty of the 36 MODIS spectral bands are reflective solar bands (RSB). They are calibrated on-orbit by an onboard solar diffuser (SD). For the high-gain ocean color bands (8-16), an attenuating solar diffuser screen (SDS) is used in front of the SD panel to avoid detector saturation caused by direct solar illumination of the SD. The use of the SDS, a metal plate with uniformly distributed pinholes, introduces an additional factor to the radiometric calibration uncertainty. Since a system level characterization of the SDS transmission versus SD viewing geometry was not performed pre-launch, the vignetting function (VF) for both Terra and Aqua MODIS had to be characterized on-orbit. The VF can be derived either from SD observations made with and without the SDS in place during specially planned spacecraft yaw maneuvers or by using routine SD calibration pairs (with and without the SDS) accumulated over a long period in order to cover all possible viewing geometries. In this paper we present details of the methods used to characterize the MODIS SDS VFs and examine the results derived from both spacecraft yaw maneuvers and long-term SD calibration pairs. The VF results obtained for Terra and Aqua MODIS are discussed and compared. In addition, an estimate of the calibration uncertainties introduced by the SDS is provided.
The Moderate Resolution Imaging Spectroradiometer (MODIS) on the
Earth Observing System (EOS) Aqua platform uses biweekly solar diffuser measurements for the radiometric calibration of the ocean
color bands. The solar angle relative to the spacecraft changes
throughout the year. This document describes correlations in the solar diffuser measurements of the ocean color bands to the sun yaw angle. The functional form of the correlations depends on the position of the respective band and detector on the focal plane. The proposed corrections often exceed 0.5% (peak-to-peak). The most likely source of the correlations is the radiometric characterization of the solar diffuser screen. These results show the importance of a complete prelaunch characterization for spaceborne sensors regarding the radiometric calibration.
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.
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