Amit Angal

Time-Dependent Response Versus Scan Angle for MODIS Reflective Solar Bands

The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments currently operate onboard the National Aeronautics and Space Administration (NASAs) Terra and Aqua spacecraft, launched on December 18, 1999 and May 4, 2002, respectively. MODIS has 36 spectral bands, among which 20 are reflective solar bands (RSBs) covering a spectral range from 0.412 to 2.13 μm. The RSBs are calibrated on orbit using a solar diffuser (SD) and an SD stability monitor and with additional measurements from lunar observations via a space view (SV) port. Selected pseudo-invariant desert sites are also used to track the RSB on-orbit gain change, particularly for short-wavelength bands. MODIS views the Earth surface, SV, and the onboard calibrators using a two-sided scan mirror. The response versus scan angle (RVS) of the scan mirror was characterized prior to launch, and its changes are tracked using observations made at different angles of incidence from onboard SD, lunar, and Earth view (EV) measurements. These observations show that the optical properties of the scan mirror have experienced large wavelength-dependent degradation in both the visible and near infrared spectral regions. Algorithms have been developed to track the on-orbit RVS change using the calibrators and the selected desert sites. These algorithms have been applied to both Terra and Aqua MODIS Level 1B (L1B) to improve the EV data accuracy since L1B Collection 4, refined in Collection 5, and further improved in the latest Collection 6 (C6). In C6, two approaches have been used to derive the time-dependent RVS for MODIS RSB. The first approach relies on data collected from sensor onboard calibrators and mirror side ratios from EV observations. The second approach uses onboard calibrators and EV response trending from selected desert sites. This approach is mainly used for the bands with much larger changes in their time-dependent RVS, such as the Terra MODIS bands 1-4, 8, and 9 and the Aqua MODIS bands 8 and 9. In this paper, the algorithms of these approaches are described, their performance is demonstrated, and their impact on L1B products is discussed. In general, the shorter wavelength bands have experienced a larger on-orbit RVS change, which, in general, are mirror side and detector dependent. The on-orbit RVS change due to the degradation of band 8 can be as large as 35% for Terra MODIS and 20% for Aqua MODIS. Vital to maintaining the accuracy of the MODIS L1B products is an accurate characterization of the on-orbit RVS change. The derived time-independent RVS, implemented in C6, makes an important improvement to the quality of the MODIS L1B products.

Applications of Spectral Band Adjustment Factors (SBAF) for Cross-Calibration

To monitor land surface processes over a wide range of temporal and spatial scales, it is critical to have coordinated observations of the Earths surface acquired from multiple spaceborne imaging sensors. However, an integrated global observation framework requires an understanding of how land surface processes are seen differently by various sensors. This is particularly true for sensors acquiring data in spectral bands whose relative spectral responses (RSRs) are not similar and thus may produce different results while observing the same target. The intrinsic offsets between two sensors caused by RSR mismatches can be compensated by using a spectral band adjustment factor (SBAF), which takes into account the spectral profile of the target and the RSR of the two sensors. The motivation of this work comes from the need to compensate the spectral response differences of multispectral sensors in order to provide a more accurate cross-calibration between the sensors. In this paper, radiometric cross-calibration of the Landsat 7 Enhanced Thematic Mapper Plus (ETM+) and the Terra Moderate Resolution Imaging Spectroradiometer (MODIS) sensors was performed using near-simultaneous observations over the Libya 4 pseudoinvariant calibration site in the visible and near-infrared spectral range. The RSR differences of the analogous ETM+ and MODIS spectral bands provide the opportunity to explore, understand, quantify, and compensate for the measurement differences between these two sensors. The cross-calibration was initially performed by comparing the top-of-atmosphere (TOA) reflectances between the two sensors over their lifetimes. The average percent differences in the long-term trends ranged from -5% to +6%. The RSR compensated ETM+ TOA reflectance (ETM+*) measurements were then found to agree with MODIS TOA reflectance to within 5% for all bands when Earth Observing-1 Hyperion hyperspectral data were used to produce the SBAFs. These differences were later reduced to within 1% for all bands (except band 2) by using Environmental Satellite Scanning Imaging Absorption Spectrometer for Atmospheric Cartography hyperspectral data to produce the SBAFs.

Characterization of Terra and Aqua MODIS VIS, NIR, and SWIR Spectral Bands' Calibration Stability

The Moderate Resolution Imaging Spectroradiometer (MODIS) has successfully operated onboard the Terra spacecraft for more than 12 years and the Aqua spacecraft for more than ten years. It has 20 reflective solar bands covering the visible (VIS), near infrared (NIR), and short-wave infrared (SWIR) spectral regions. They are calibrated on orbit using regularly scheduled solar diffuser measurements and lunar observations. In recent years, observations over selected ground targets are also used to monitor detector responses at different angles of incidence. This paper provides a brief description of MODIS on-orbit calibration and characterization methodologies and examines the calibration stability of the VIS, NIR, and SWIR spectral bands over the entire missions of both instruments. Results obtained from four different vicarious approaches (deserts, Dome Concordia, deep convective cloud, and simultaneous nadir overpass) show that Terra MODIS VIS and NIR spectral bands have a wavelength-dependent drift in reflectance with a drop up to 8% in the shortest wavelength region. All four approaches have a relative agreement to within 2.0% with an uncertainty of less than 1.5% for most bands. It is anticipated that the improvements made in the MODIS Collection 6, with additional corrections based on the desert reflectance trending results, will significantly reduce, if not completely remove, some of the trending drifts identified in the Collection-5 data product.

Terra and Aqua moderate-resolution imaging spectroradiometer collection 6 level 1B algorithm

Abstract The moderate-resolution imaging spectroradiometer (MODIS) was launched on the Terra spacecraft on Dec.18, 1999 and on Aquaon May 4, 2002. The data acquired by these instruments have contributed to the long-term climate data record for more than a decade and represent a key component of NASA’s Earth observing system. Each MODIS instrument observes nearly the whole Earth each day, enabling the scientific characterization of the land, ocean, and atmosphere. The MODIS Level 1B (L1B) algorithms input uncalibrated geo-located observations and convert instrument response into calibrated reflectance and radiance, which are used to generate science data products. The instrument characterization needed to run the L1B code is currently implemented using time-dependent lookup tables. The MODIS characterization support team, working closely with the MODIS Science Team, has improved the product quality with each data reprocessing. We provide an overview of the new L1B algorithm release, designated collection 6. Recent improvements made as a consequence of on-orbit calibration, on-orbit analyses, and operational considerations are described. Instrument performance and the expected impact of L1B changes on the collection 6 L1B products are discussed.

Using the Sonoran and Libyan Desert test sites to monitor the temporal stability of reflective solar bands for Landsat 7 enhanced thematic mapper plus and Terra moderate resolution imaging spectroradiometer sensors

Remote sensing imagery is effective for monitoring environmental and climatic changes because of the extent of the global coverage and long time scale of the observations. Radiometric calibration of remote sensing sensors is essential for quantitative & qualitative science and applications. Pseudo-invariant ground targets have been extensively used to monitor the long-term radiometric calibration stability of remote sensing sensors. This paper focuses on the use of the Sonoran Desert site to monitor the radiometric stability of the Landsat 7 (L7) Enhanced Thematic Mapper Plus (ETM+) and Terra Moderate Resolution Imaging Spectroradiometer (MODIS) sensors. The results are compared with the widely used Libya 4 Desert site in an attempt to evaluate the suitability of the Sonoran Desert site for sensor inter-comparison and calibration stability monitoring. Since the overpass times of ETM+ and MODIS differ by about 30 minutes, the impacts due to different view geometries or test site Bi-directional Reflectance Distribution Function (BRDF) are also presented. In general, the long-term drifts in the visible bands are relatively large compared to the drift in the near-infrared bands of both sensors. The lifetime Top-of-Atmosphere (TOA) reflectance trends from both sensors over 10 years are extremely stable, changing by no more than 0.1% per year (except ETM+ Band 1 and MODIS Band 3) over the two sites used for the study. The use of a semi-empirical BRDF model can reduce the impacts due to view geometries, thus enabling a better estimate of sensor temporal drifts.

Absolute Calibration of Optical Satellite Sensors Using Libya 4 Pseudo Invariant Calibration Site

The objective of this paper is to report the improvements in an empirical absolute calibration model developed at South Dakota State University using Libya 4 (+28.55°, +23.39°) pseudo invariant calibration site (PICS). The approach was based on use of the Terra MODIS as the radiometer to develop an absolute calibration model for the spectral channels covered by this instrument from visible to shortwave infrared. Earth Observing One (EO-1) Hyperion, with a spectral resolution of 10 nm, was used to extend the model to cover visible and near-infrared regions. A simple Bidirectional Reflectance Distribution function (BRDF) model was generated using Terra Moderate Resolution Imaging Spectroradiometer (MODIS) observations over Libya 4 and the resulting model was validated with nadir data acquired from satellite sensors such as Aqua MODIS and Landsat 7 (L7) Enhanced Thematic Mapper (ETM+). The improvements in the absolute calibration model to account for the BRDF due to off-nadir measurements and annual variations in the atmosphere are summarized. BRDF models due to off-nadir viewing angles have been derived using the measurements from EO-1 Hyperion. In addition to L7 ETM+, measurements from other sensors such as Aqua MODIS, UK-2 Disaster Monitoring Constellation (DMC), ENVISAT Medium Resolution Imaging Spectrometer (MERIS) and Operational Land Imager (OLI) onboard Landsat 8 (L8), which was launched in February 2013, were employed to validate the model. These satellite sensors differ in terms of the width of their spectral bandpasses, overpass time, off-nadir-viewing capabilities, spatial resolution and temporal revisit time, etc. The results demonstrate that the proposed empirical calibration model has accuracy of the order of 3% with an uncertainty of about 2% for the sensors used in the study.

MODIS reflective solar bands calibration improvements in Collection 6

Since launch, Terra and Aqua MODIS have performed more than 12 and 10 years of scientific measurements of the Earth’s surface. MODIS has 36 spectral bands, among which 20 are Reflective Solar Bands (RSB), covering a spectral range from 0.41 μm to 2.1 μm. MODIS was developed with stringent requirements for calibration and uncertainty and is equipped with a set of on-board calibrators (OBC) that facilitate a constant monitoring and update of its on-orbit calibration coefficients. The RSB are calibrated on-orbit using a Solar Diffuser (SD) and a Solar Diffuser Stability Monitor (SDSM), with help from the lunar observations via a Space View (SV) port and an onboard Spectroradiometric Calibration Assembly (SRCA). The algorithms to accurately characterize the sensor’s gain change and the on-orbit change in the response versus scan-angle (RVS) have been applied to improve the quality of the Earth-view measurements. Various improvements to the calibration algorithms have been incorporated since launch and the following paper will discuss the calibration algorithms and enhancements developed for MODIS Collection 6 (C6) processing. In addition, to supplement the measurements from the on-board calibrators, pseudo-invariant desert targets are also used to track the on-orbit response change for selective RSB. Discussions of the on-orbit calibration uncertainty and the Level 1B (L1B) Uncertainty index (UI) product are also included. A comprehensive assessment of the impact on the L1B product in comparison to Collection 5 (C5) is also discussed. Significant improvements are observed in the case of VIS bands wherein the long-term bias observed in C5 products is eliminated to provide a more accurate radiometric product.

Monitoring MODIS calibration stability of visible and near-IR bands from observed top-of-atmosphere BRDF-normalized reflectances over Libyan Desert and Antarctic surfaces

MODIS is one of the major instruments for the National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) missions. It is on-board both the EOS Terra and Aqua spacecrafts, launched in December 1999 and May 2002, respectively. Each MODIS provides spectral observations in multiple angular views of reflectance at the top of atmosphere (TOA) around the globe. This study focuses on one visible (0.65μm) band and one near-IR (0.84μm) band to examine the variations of observed TOA reflectances due to the impact of the bi-directional reflectance distribution function (BRDF). Two highly uniform ground sites in the Libyan Desert and Antarctica are selected. The variation of reflectance as a function of view zenith angle at a fixed solar zenith angle is studied based on reflectances obtained from multiple granules over passing each site. The variation of reflectance as a function of solar zenith angle at a fixed view zenith angle is studied based on reflectances collected from 16-day repeatable orbits, which have the same view geometry relative to each site. Results show that variations of the near nadir reflectances at the desert site are close to the Lambertian pattern while those at the Dome C. site are strongly anisotropic. Comparison of a simple Lambertian and a BRDF correction is made in terms of their effectiveness in removing reflectance trending variations. The BRDF correction is based on a semi-empirical model consisting of two kernel-driven components. Results show that both corrections are able to significantly remove trending variations at the desert site, which produce a remarkably low variability of less than 2% relative to the linear fit. The normalized reflectance trends show that from year 2000 to 2007, the visible and near-IR bands dropped only 1.7 and 1.2% in total, respectively.

Multitemporal Cross-Calibration of the Terra MODIS and Landsat 7 ETM+ Reflective Solar Bands

In recent years, there has been a significant increase in the use of remotely sensed data to address global issues. With the open data policy, the data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Enhanced Thematic Mapper Plus (ETM+) sensors have become a critical component of numerous applications. These two sensors have been operational for more than a decade, providing a rich archive of multispectral imagery for analysis of mutitemporal remote sensing data. This paper focuses on evaluating the radiometric calibration agreement between MODIS and ETM+ using the near-simultaneous and cloud-free image pairs over an African pseudo-invariant calibration site, Libya 4. To account for the combined uncertainties in the top-of-atmosphere (TOA) reflectance due to surface and atmospheric bidirectional reflectance distribution function (BRDF), a semiempirical BRDF model was adopted to normalize the TOA reflectance to the same illumination and viewing geometry. In addition, the spectra from the Earth Observing-1 (EO-1) Hyperion were used to compute spectral corrections between the corresponding MODIS and ETM+ spectral bands. As EO-1 Hyperion scenes were not available for all MODIS and ETM+ data pairs, MODerate resolution atmospheric TRANsmission (MODTRAN) 5.0 simulations were also used to adjust for differences due to the presence or lack of absorption features in some of the bands. A MODIS split-window algorithm provides the atmospheric water vapor column abundance during the overpasses for the MODTRAN simulations. Additionally, the column atmospheric water vapor content during the overpass was retrieved using the MODIS precipitable water vapor product. After performing these adjustments, the radiometric cross-calibration of the two sensors was consistent to within 7%. Some drifts in the response of the bands are evident, with MODIS band 3 being the largest of about 6% over 10 years, a change that will be corrected in Collection 6 MODIS processing.

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.