J. Sun

On-Orbit Calibration and Performance of Aqua MODIS Reflective Solar Bands

Aqua MODIS has successfully operated on-orbit for more than six years since its launch in May 2002, continuously making global observations and improving studies of changes in the Earths climate and environment. Twenty of the 36 MODIS spectral bands, covering wavelengths from 0.41 to 2.2 ?m, are the reflective solar bands (RSBs). They are calibrated on-orbit using an onboard solar diffuser (SD) and an SD stability monitor. In addition, regularly scheduled lunar observations are made to track the RSB calibration stability. This paper presents Aqua MODIS RSB on-orbit calibration and characterization activities, methodologies, and performance. Included in this paper are characterizations of detector signal-to-noise ratio, short-term stability, and long-term response change. Spectral-wavelength-dependent degradation of the SD bidirectional reflectance factor and scan mirror reflectance, which also varies with the angle of incidence, is examined. On-orbit results show that Aqua MODIS onboard calibrators have performed well, enabling accurate calibration coefficients to be derived and updated for the Level 1B production and assuring high-quality science data products to be continuously generated and distributed. Since launch, the short-term response, on a scan-by-scan basis, has remained extremely stable for most RSB detectors. With the exception of band 6, there have been no new RSB noisy or inoperable detectors. Like its predecessor, i.e., Terra MODIS, launched in December 1999, the Aqua MODIS visible spectral bands have experienced relatively large changes, with an annual response decrease (mirror side 1) of 3.6% for band 8 at 0.412 ?m, 2.3% for band 9 at 0.443 ?m, 1.6% for band 3 at 0.469 ?m, and 1.2% for band 10 at 0.488 ?m. For other RSB bands with wavelengths greater than 0.5 ?m, the annual response changes are typically less than 0.5%. In general, Aqua MODIS optics degradation is smaller than Terra MODIS, and the mirror-side differences are much smaller. Overall, Aqua MODIS RSB on-orbit performance is better than that of Terra MODIS.

Cross calibration of SeaWiFS and MODIS using on-orbit observations of the Moon.

Observations of the Moon provide a primary technique for the on-orbit cross calibration of Earth remote sensing instruments. Monthly lunar observations are major components of the on-orbit calibration strategies of SeaWiFS and MODIS. SeaWiFS has collected more than 132 low phase angle and 59 high phase angle lunar observations over 12 years, Terra MODIS has collected more than 82 scheduled and 297 unscheduled lunar observations over nine years, and Aqua MODIS has collected more than 61 scheduled and 171 unscheduled lunar observations over seven years. The NASA Ocean Biology Processing Group Calibration and Validation Team and the NASA MODIS Characterization Support Team use the USGS RObotic Lunar Observatory (ROLO) photometric model of the Moon to compare these time series of lunar observations over time and varying observing geometries. The cross-calibration results show that Terra MODIS and Aqua MODIS agree, band to band, at the 1%-3% level, while SeaWiFS and either MODIS instrument agree at the 3%-8% level. The combined uncertainties of these comparisons are 1.3% for Terra and Aqua MODIS, 1.4% for SeaWiFS and Terra MODIS, and 1.3% for SeaWiFS and Aqua MODIS. Any residual phase dependence in the ROLO model, based on these observations, is less than 1.7% over the phase angle range of -80° to -6° and +5° to +82°. The lunar cross calibration of SeaWiFS, Terra MODIS, and Aqua MODIS is consistent with the vicarious calibration of ocean color products for these instruments, with the vicarious gains mitigating the calibration biases for the ocean color bands.

Terra and Aqua MODIS calibration algorithms and uncertainty analysis

NASAs 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 NASAs 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 Earths system of land, oceans, and atmosphere. Compared to its heritage sensors, the MODIS was designed with more stringent requirements on the sensors 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 sensors 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 sensors characteristics identified pre-launch, measured and validated on-orbit. It also serves as a useful reference for the users of MODIS data products.

Performance of Terra MODIS solar diffuser and solar diffuser stability monitor

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 Band 2 Electronic Crosstalk: Cause, Impact, and Mitigation

The MODerate-resolution Imaging Spectroradiometer (MODIS) is one of the primary instruments in the Earth Observing System (EOS). The first MODIS instrument was launched in December, 1999 on-board the NASA EOS Terra spacecraft. MODIS has 36 bands, covering a wavelength range from 0.4 μm to 14.4 μm. MODIS collects data at three spatial resolutions: 0.25 km (2 bands), 0.5 km (5 bands), and 1 km (29 bands). In the Earth scene images of Terra MODIS band 2 (0.85μm), two sets of regularly distributed anomalous pixels are observed in each scan, of which one is brighter and the other is darker than surrounding pixels. MODIS band 2 is a 0.25 km resolution band, having 40 detectors and 4 subframes for each detector. The brighter dots correspond to the subframe 1 pixels of detector 30 and the darker dots correspond to the same subframe of detector 29. In this manuscript, it is demonstrated that the anomaly is due to electronic crosstalk. The sending bands and detectors for the crosstalk are identified using lunar images and are confirmed using the Spectroradiometric Calibration Assembly (SRCA) observations. A linear algorithm is developed to describe the crosstalk, and crosstalk coefficients are derived using lunar observations. With the derived coefficients, the dotted features in Earth view images of Terra band 2 can be significantly reduced.

Use of the Moon for Calibration and Characterization of MODIS, SeaWiFS, and VIRS

The need for long term global data sets to enable quantitative studies of changes in the Earth’s climate leads directly to a requirement for a radiometric calibration source that is stable over several decades (and possibly several centuries) to levels of better than one part in a thousand, that can be readily used on-orbit and that is suitable for inter-comparison of a wide variety of imagng radiometers. For the visible through shortwave-infrared portion of the electromagnetic spectrum the moon meets this requirement. It’s surface is stable over thousands of years, the sun’s illumination is well characterized, and, if care is taken to account for changes in phase and libration, it can be used for radiometric calibration, calibration transfer and intercomparison of sensors.

Using the moon for MODIS on-orbit spatial characterization

The moon is a very stable reference source that has been used for the space-borne sensors’ radiometric calibration and/or radiometric stability monitoring. In this paper, we present an approach that uses the sensors’ on-orbit lunar observations for their spatial characterization and apply the method to the MODIS instruments that are currently operating on board NASA EOS Terra and Aqua satellites. Both MODIS instruments perform monthly lunar observations. The spatial characterization results derived from the lunar observations using this algorithm are compared with those obtained from the MODIS Spectro-Radiometric Calibration Assembly (SRCA), which is an on-board calibrator capable of performing spatial characterizations for all MODIS spectral bands. The new approach can be applied to other remote sensing instruments.

Analysis of MODIS solar diffuser screen vignetting function

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.

An Overview of Inter-comparison Methodologies for Terra and Aqua MODIS Calibration

With increasing efforts on data fusion and long-term climate data records (CDR) using observations made by multiple sensors on the same or different platforms, sensor cross-calibration has become increasingly important. It is known that the uncertainty of climate models or science data records depends not only on the calibration quality of individual sensors but also on their calibration consistency. This paper provides an overview of inter-comparison methodologies applied by the MODIS Characterization Support Team (MCST) at NASA/GSFC for the studies of Terra and Aqua MODIS on-orbit calibration consistency. Improved over heritage sensors, MODIS was built with a set of on-board calibrators (OBC) that include a blackbody (BB), a space view (SV) port, a solar diffuser (SD), and a solar diffuser stability monitor (SDSM). The BB is primarily used for the thermal emissive bands (TEB) calibration and the SD/SDSM system for the reflective solar bands (RSB) calibration. Detector responses to the SV provide measurements for the instrument background. Although instrument design requirements and calibration approaches are nearly identical for both Terra and Aqua MODIS and they all went through extensive and similar pre-launch calibration and characterization activities, their on-orbit calibration consistency still has to be carefully examined and validated as many science products are generated from observations made by both instruments. Methodologies discussed in this paper include inter-comparison studies using the Moon, a third sensor, and ground targets. Our results show that Terra and Aqua reflective solar bands and thermal emissive bands have been calibrated consistently to within their combined uncertainty requirements. For the 11mm and 12mm bands used for surface temperature measurements, the calibration differences between Terra and Aqua MODIS are less than ±0.15K at scene temperatures from 240-280K and less than ±0.50K at cold scene temperatures from 190 to 230K (before corrections). For most reflective solar bands, their reflectance calibration differences are typically less than ±2%.

Aqua MODIS first year on-orbit calibration and performance

The MODerate Resolution Imaging Spectroradiometer (MODIS) Flight Model 1 (FM1) was launched on May 4, 2002 onboard the NASA Earth Observing System (EOS) Aqua spacecraft. It has provided more than a year of global data for studies of the Earth’s land, oceans, and atmosphere in support of the science community and public users. To assure the quality of the data and science products, extensive efforts have been made to collect and analyze data on the instrument’s on-orbit performance using its on-board calibrators (OBCs). MODIS has 36 spectral bands: 20 reflective solar bands (RSBs) with wavelengths from 0.41 micrometer to 2.2 micrometers and 16 thermal emissive bands (TEBs) from 3.7 to 14.2 micrometers. For radiometry, the RSBs are calibrated by a solar diffuser (SD) and a solar diffuser stability monitor (SDSM) system and the TEBs by a blackbody (BB). An on-board Spectroradiometric Calibration Assembly (SRCA) is used for the instrument’s spectral (RSBs only) and spatial (all 36 bands) characterization. Using the first year’s calibration data sets, this paper presents Aqua MODIS on-orbit performance in three areas: radiometric, spatial, and spectral. Comparisons with the sensor’s specifications and with the performance of its predecessor, Terra MODIS (launched in December 1999), are discussed. Excluding a few problems identified pre-launch, such as non-functional detectors in the 1.6 micrometers band and the out of specification performance for the band to band registration (BBR), the on-orbit observations and analyses show that Aqua MODIS has been performing according to its design characteristics.