The Moderate Resolution Imaging Spectroradiometer (MODIS) is one of the key instruments for NASA’s Earth Observing System (EOS), currently operating on both the Terra and Aqua satellites. The MODIS is a major advance over the previous generation of sensors in terms of its spectral, spatial, and temporal resolutions. It has 36 spectral bands: 20 reflective solar bands (RSB) with center wavelengths from 0.41 to 2.1 µm and 16 thermal emissive bands (TEB) with center wavelengths from 3.7 to 14.4 µm, making observations at three spatial resolutions: 250 m (bands 1–2), 500 m (bands 3–7), and 1km (bands 8–36). MODIS is a cross-track scanning radiometer with a wide field-of-view, providing a complete global coverage of the Earth in less than 2 days. Both Terra and Aqua MODIS went through extensive pre-launch calibration and characterization at various levels. In orbit, the calibration and characterization tasks are performed using its on-board calibrators (OBCs) that include a solar diffuser (SD) and a solar diffuser stability monitor (SDSM), a v-grooved flat panel blackbody (BB), and a spectro-radiometric calibration assembly (SRCA). In this paper, we present au overview of MODIS calibration and characterization activities, methodologies, and lessons learned from pre-launch characterization and in-orbit operation. Key issues discussed in this paper include in-orbit efforts of monitoring the noise characteristics of the detectors, tracking the solar diffuser and optics degradations, and updating the sensor’s response versus scan angle. The experiences and lessons learned through MODIS have played and will continue to play major roles in the design and characterization of future sensors.

An overview of MODIS radiometric calibration and characterization

Intercalibration of satellite instruments is critical for detection and quantification of changes in the Earth's environment, weather forecasting, understanding climate processes, and monitoring climate and land cover change. These applications use data from many satellites; for the data to be interoperable, the instruments must be cross-calibrated. To meet the stringent needs of such applications, instruments must provide reliable, accurate, and consistent measurements over time. Robust techniques are required to ensure that observations from different instruments can be normalized to a common scale that the community agrees on. The long-term reliability of this process needs to be sustained in accordance with established reference standards and best practices. Furthermore, establishing physical meaning to the information through robust Systeme International d'unites traceable calibration and validation (Cal/Val) is essential to fully understand the parameters under observation. The processes of calibration, correction, stability monitoring, and quality assurance need to be underpinned and evidenced by comparison with “peer instruments” and, ideally, highly calibrated in-orbit reference instruments. Intercalibration between instruments is a central pillar of the Cal/Val strategies of many national and international satellite remote sensing organizations. Intercalibration techniques as outlined in this paper not only provide a practical means of identifying and correcting relative biases in radiometric calibration between instruments but also enable potential data gaps between measurement records in a critical time series to be bridged. Use of a robust set of internationally agreed upon and coordinated intercalibration techniques will lead to significant improvement in the consistency between satellite instruments and facilitate accurate monitoring of the Earth's climate at uncertainty levels needed to detect and attribute the mechanisms of change. This paper summarizes the state-of-the-art of postlaunch radiometric calibration of remote sensing satellite instruments through intercalibration.

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Overview of Intercalibration of Satellite Instruments

Monitoring on-orbit calibration stability of the Terra MODIS and Landsat 7 ETM+ sensors using pseudo-invariant test sites

For the most part, satellite observations of climate are not presently sufficiently accurate to establish a climate record that is indisputable and hence capable of determining whether and at what rate the climate is changing. Furthermore, they are insufficient for establishing a baseline for testing long-term trend predictions of climate models. Satellite observations do provide a clear picture of the relatively large signals associated with interannual climate variations such as El Nino-Southern Oscillation (ENSO), and they have also been used to diagnose gross inadequacies of climate models, such as their cloud generation schemes. However, satellite contributions to measuring long-term change have been limited and, at times, controversial, as in the case of differing atmospheric temperature trends derived from the U.S. National Oceanic and Atmospheric Administration's (NOAA) microwave radiometers.

http://www.star.nesdis.noaa.gov/star/documents/ASIC3-071218-webversfinal.pdf

Achieving satellite instrument calibration for climate change

The MODerate-resolution Imaging Spectroradiometer (MODIS) is the keystone instrument for the NASA’s Earth Observing System (EOS). Currently two nearly identical MODIS instruments are operating on-board the EOS Terra spacecraft (launched in December 1999) and the EOS Aqua spacecraft (launched in May 2002), providing global coverage of the Earth’s land, oceans, and atmosphere with both morning and afternoon observations. This paper reviews the EOS MODIS development history, its design concepts, system implementation and calibration, current status and the follow-on Visible/Infrared Imaging Radiometer Suite (VIIRS) under development for the National Polar Orbiting Environmental Satellite System (NPOESS).

Development, characterization, and performance of the EOS MODIS sensors

The MODIS Protoflight Model (PFM) on-board the Terra spacecraft has been in operation for more than two and half years since its launch on December 18, 1999. In addition to the on-board calibrators (OBCs), the observations of the moon have been planned monthly with carefully chosen viewing conditions. The data from these observations is used to support the instrument's on-orbit calibration and characterization. In this paper, we describe the use of lunar observations for monitoring the MODIS reflective solar bands (RSB) radiometric stability and discuss related applications. For Terra MODIS, the lunar views have also been used to derive correction parameters for the optical leak among the photoconductive (PC) detectors (bands 31-36), to characterize the electronic crosstalk under different focal plane operational configurations, and to track on-orbit band-to-band registration (BBR). The same strategies are being applied to the Aqua MODIS (Flight Model 1 - FM1) launched on May 4, 2002. The lunar observation results from both instruments are compared.

MODIS on-orbit characterization using the Moon

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.

Using the moon for MODIS on-orbit spatial characterization

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. The science data acquisition started on February 24, 2000. Since then it has been providing the science community and public users unprecedented amount of data sets for the global monitoring of the Earth’s land, oceans, and atmosphere. MODIS has 36 spectral bands with wavelengths ranging from 0.41 micrometer to 14.5 micrometers. Its 16 thermal emissive bands (TEB) range from 3.7 to 14.2 micrometers and have a total of 160 individual detectors (10 detectors per band). The thermal emissive bands are calibrated on-orbit by an on-board calibrator blackbody (OBC BB) on a scan by scan basis. The detectors responses to the BB source track their operational stability and therefore their noise characteristics as well. In this paper, we provide a brief review of the MODIS TEB on-orbit calibration algorithm with a focus on detector stability using over three years of on-orbit calibration data sets. The on-orbit changes in detectors responses from one operational configuration to another, the changes within the same operational condition, and the impact of these changes on the calibration and on the Earth scene observations are carefully examined. Except for a few detectors that were identified from pre-launch or became noisy on-orbit, the overall performance of MODIS TEB detectors is very satisfactory according to the design specifications.

S. Xiong

Papers

MODIS on-orbit characterization using the Moon

Using the moon for MODIS on-orbit spatial characterization

Detector stability of the Terra MODIS thermal emissive bands