X. Xie

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.

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.

Results and Lessons from MODIS Reflective Solar Bands Calibration: Pre-launch to On-orbit

MODIS is a major instrument for NASAs 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 Earths 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).

On-orbit performance of Aqua MODIS onboard calibrators

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 Earths environment and climate. The sensors 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.

Residual correlations in the solar diffuser measurements of the MODIS Aqua ocean color bands to the sun yaw angle

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.

Summary of terra and aqua MODIS On-orbit calibration and characterization results

Since launch the NASA EOS Terra MODIS has been in operation for more than seven years and the Aqua MODIS for nearly five years. Each MODIS has 20 reflective solar bands and 16 thermal emissive bands. It makes observations at three nadir spatial resolutions: 0.25 km, 0.5 km, and 1 km and is calibrated on- orbit by a set of on-board calibrators (OBC) that include a solar diffuser, a solar diffuser stability monitor, a blackbody, and a spectro-radiometric calibration assembly. This paper provides an overview of Terra and Aqua MODIS on-orbit calibration and characterization activities and results.

Four years of Aqua MODIS on-orbit radiometric calibration

MODIS is currently onboard NASAs 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 Earths 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.

An Overview of Aqua MODIS after Five-year On-orbit Operation and Calibration

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 Earths 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 sensors overall spectral (RSB) and spatial (RSB and TEB) parameters are also presented in this paper.