Since its launch in May, 2002, Aqua MODIS has successfully operated for more than 20 years and has continuously generated a wide range of data products that have enabled and supported the remote sensing community and users worldwide for their studies of the Earth’s system by monitoring changes in its key environmental parameters. Although Aqua MODIS, designed with a lifetime requirement of 6 years, is currently operated in its extended mission phase, it continues to make high quality global observations of the Earth’s surface via its 36 spectral bands that cover wavelengths from visible to long-wave infrared. To date, all instrument on-board calibrators (OBC) remain capable of performing their design functions, providing various calibration data sets to help monitor on-orbit changes in sensor responses and performance characteristics. In addition to the OBC, regularly scheduled lunar observations and select Earth-view targets are used extensively to support sensor on-orbit calibration, especially for the calibration of the visible channels (or bands). We provide an overview of Aqua MODIS on-orbit calibration activities and methodologies for both reflective solar bands and thermal emissive bands, illustrate its on-orbit performance over the past 20 years using examples derived from OBC measurements, lunar observations, and Earth-view response trends, and describe various calibration improvements made over its entire mission. We focus on key issues identified since launch, such as solar diffuser degradation, electronic crosstalk, and on-orbit changes in sensor response versus scan-angle, along with approaches and strategies developed to mitigate their impact on sensor calibration quality. Also discussed in this paper are some of the key calibration enhancements incorporated recently in the Collection 6.1 and the upcoming Collection 7 Level-1B algorithms.
Aqua MODIS has successfully operated for more than 20 years and continuously generated a wide range of data products to enable and support the remote sensing community and users worldwide for their studies of the Earth’s system. Although it is currently operated in its extended mission phase, Aqua MODIS continues to make high quality global observations of the Earth’s surface and its on-board calibrators (OBC) remain capable of performing their design functions, providing essential calibration data sets to help monitor on-orbit changes in sensor responses. In this paper, we provide an overview of Aqua MODIS on-orbit calibration methodologies for both reflective solar bands and thermal emissive bands, illustrate its on-orbit performance over 20 years using examples derived from OBC measurements, lunar observations, and Earth view response trends, and describe various calibration improvements made over its entire mission. We focus on several issues identified since launch, such as solar diffuser degradation, electronic crosstalk, and on-orbit changes in sensor response versus scan-angle, and discuss the approaches developed to mitigate their impact on sensor calibration quality.
The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on-board the Aqua and Terra spacecraft have provided valuable Earth data to the science community for the last 20 and 22 years, respectively. The Spectro-Radiometric Calibration Assembly (SRCA) is an on-board calibrator (OBC) that can characterize the radiometric, spatial, and spectral properties of the MODIS reflective solar bands (RSBs). In radiometric mode, the SRCA is able to monitor the gain trends of the RSBs on a detector level. Nominal radiometric mode measurements are collected during a 10-minute period during spacecraft night using a combination of SRCA halogen lamps. These measurements are intended for deriving on-orbit gains, however due to several 10-watt lamp failures on-orbit, the frequency of SRCA calibrations has been reduced and no longer used in the official L1B LUT algorithm. Once per calendar year the SRCA is operated in radiometric mode over several consecutive orbits using a backup lamp, monitoring the gain changes of the RSBs under unique conditions. This paper will provide insight into the 1W short-term stability of the MODIS RSBs using these calibrations over both the Aqua and Terra missions, along with the long-term trends of these multi-orbit gain observations.
KEYWORDS: MODIS, Calibration, Sensors, Reflectivity, Short wave infrared radiation, Long wavelength infrared, Detection and tracking algorithms, Polarization, Mirrors, Algorithm development
Moderate resolution imaging spectroradiometer (MODIS) continues to be an important instrument for NASA’s earth observing system. Terra and Aqua MODIS have produced more than 22 and 20 years of global datasets that have significantly helped scientists better understand the Earth’s systems, respectively. The MODIS level-1B (L1B) algorithms use the uncalibrated, geolocated Earth scene observations as input and convert the instrument response into calibrated reflectance and radiance, which are used to generate the downstream science products. The sustained calibration and characterization activities undertaken by the MODIS characterization support team have resulted in several upgrades to the L1B algorithms to maintain accurate calibration in the data products. We present an overview of the L1B algorithm designated as Collection 7. Various algorithm enhancements both in the reflective bands and thermal bands characterization are currently under science testing and evaluation. Once applied in data processing (projected in early 2023), they are expected to manifest in improved science products, both in terms of radiometric accuracy and long-term stability.
Calibration of Terra and Aqua MODIS reflective solar bands (RSB) has evolved significantly since the launch of the first MODIS instrument on the Terra satellite more than 21 years ago. In NASA’s current Collection 6 and 6.1 Level 1B products (C6/C6.1 L1B), the RSB calibration algorithm continues to rely primarily on the onboard solar diffuser to calibrate the instrument gain. Lunar observations are used to track on-orbit changes in the response versus scan angle (RVS), and data from pseudo-invariant desert sites are used to apply adjustments to the gain and RVS calibration for select bands. The resulting reflectance products have in general shown a very stable performance. In recent years, some performance degradation has been noted for a few bands and algorithm changes have been tested to further improve the calibration accuracy for the upcoming Collection 7 (C7) L1B reprocess. In this paper, we present the MODIS RSB calibration improvements that will be included in C7. Major improvements include: applying polarization correction to the desert data before using it to generate RVS for Terra bands 8, 9, 3, and 10; using ocean scene data and an interband calibration approach to correct for long-term drift of Terra bands 11 and 12; applying an updated crosstalk correction to Terra SWIR bands over the entire mission; and using data from deep convective clouds in Terra SWIR band calibration, including the addition of time-dependent RVS for bands 5 and 26. All other minor calibration changes are also covered. Overall, the reflectance differences at nadir between C6.1 and C7 are within a few percent, though the differences increase in some cases at large scan angles. The Terra visible (3, 8-12) and SWIR bands (5-7, 26) have the most significant improvements. For all other Terra bands and all Aqua bands, the C7-C6.1 differences are mostly within 1%.
To provide the best science data quality, an accurate characterization of the response versus scan angle (RVS) function is critical for the MODIS reflective solar bands (RSB) on-orbit calibration. In every MODIS operational scan, the Earth’s surface, referred to here as Earth view (EV), the space view (SV) port, and the onboard calibrators are viewed via a twosided scan mirror. The RVS is defined as the sensor’s relative response as a function the angle of incidence (AOI) to the scan mirror. Many different approaches have been developed to derive the time-dependent RVS and its look-up table (LUT) applied to MODIS Level 1B (L1B) products since calibration Collection 4. For most MODIS RSB, the on-board calibrators can reasonably track the RVS change with time. In practice, their RVS is derived using data from on-board calibrators and the EV mirror side ratio (for mirror side 2). For Terra bands 1-4, 8-10 and Aqua bands 1-4, 8-9, an enhancement has been employed in Collections 6 and 6.1 (C6/C6.1) by using Earth scene response trending from pseudoinvariant desert sites in addition to the onboard calibrators. The current C6/C6/1 RVS algorithm is focused on fitting the EV data at each AOI over time and then deriving the relative change at different AOI. The EV response trending is currently fitted with multiple segments over time. Alternatively, the EV responses can be fit first as a function of AOI before fitting temporally in order to reduce the dependence on the stability of the desert site. These pre-treatment methods on the EV data provide improvement in the derived calibration coefficients. However, evidence of insufficient calibration is still observed in the MODIS L1B reflectance data, especially in the form of differences between the mirror sides. In this paper, we review the current methodologies that utilize the EV response trends from the pseudo-invariant Libyan desert targets to supplement the gain derived from the onboard calibrators. An improvement is then proposed and investigated such that a sliding window average (SWA) is used to pre-process the raw EV data. The SWA parameters are carefully selected using trade-off studies to accurately track the Earth scene response trending in multiple cases to overcome the reflectance differences between two mirror sides. Calibration results show improvements for both Aqua and Terra MODIS RSB L1B data products. This new adjustment has been included in the recently delivered Collection 7 LUT that will be evident in the L1B products expected to be released in late 2021.
Aqua MODIS has successfully operated for more than 18 years since its launch in May 2002 and has generated numerous science products in support of studies of the Earth’s system and its changes via a set of geophysical and environmental parameters. On-orbit calibration and characterization activities have played a vital role in maintaining the quality of MODIS data products. In addition to data collected from sensor on-board calibrators (OBC), near-monthly lunar observations and select earth view targets have been used to monitor and characterize on-orbit changes in sensor responses and to derive and update the calibration look-up tables. In this paper, we provide an overview of Aqua MODIS on-orbit operations, calibration activities and approaches, and algorithm improvements and also illustrate the sensor on-orbit performance using examples derived from various calibration sources and targets. We will focus on issues identified from instrument operations and calibrations, such as solar diffuser degradation, electronic crosstalk, variations in the cold focal plane temperatures, and changes in response versus scan-angle. Also discussed in this paper are remaining challenges and future improvements.
The Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Terra and Aqua platforms is a multi-spectral, whiskbroom scanning radiometer with 36 spectral bands covering wavelengths from 0:4 - 14:2 μm. Among these bands, the 20 reflective solar bands (RSB, 0:4 - 2:1 μm) use an on-board solar difuser (SD) and SD stability monitor (SDSM) to track the detector gain changes on orbit. In addition to this, lunar and Earth-view (EV) observations are used in order to characterize the scan-mirror response versus scan angle (RVS), which improves the Earth-view retrieval over the full width of each scan. For the short-wave infrared bands (SWIR, 1:2 - 2:1 μm), the prelaunch RVS has been used for both Aqua and Terra MODIS throughout each mission. While these bands are not expected to have a significant change in the RVS on-orbit, issues such as electronic crosstalk and an out-of-band optical leak have prevented the use of the Moon for deriving the on-orbit RVS for validation. However, recent improvements to the MODIS lunar calibration for the SWIR bands allow us to mitigate the electronic crosstalk impact and derive the RVS for the SWIR bands. In this work, we will assess the impact of this newly derived RVS on the EV data compared to MODIS Collection 6.1. By comparing to EV data obtained from pseudo-invariant calibration sites at different angles-of-incidence on the scan mirror, we can assess the effectiveness of the newly derived RVS from the Moon. We find that while the change in the RVS on-orbit for the SWIR bands is relatively small, applying a correction to the RVS based on the Moon and SD data can provide an improvement for some bands.
Near-identical MODIS instruments launched on-board the Terra and Aqua spacecraft in 1999 and 2002, respectively. Each MODIS instrument has 36 spectral bands covering 0.41 to 14.2 μm mounted among four focal plane assemblies, along with a series of on-board calibrators (OBCs) used to characterize the instrument performance on-orbit. One such OBC is the Spectro-radiometric Calibration Assembly (SRCA), which is a multi-function calibrator, able to provide calibration sources to measure spatial, spectral, or radiometric properties of the MODIS bands depending on its configuration. The MODIS instrument performance, including measurements of the signal cross-contamination (crosstalk) between bands, was measured on-orbit during early-mission characterization for both instruments. This crosstalk test used the SRCA in its spatial mode while utilizing the thin slit, which is normally used for spectral calibrations. A similar crosstalk test was recently performed for Terra MODIS. Since the Terra safe mode event in 2016, the PV LWIR bands specifically (6.7-9.7 μm) have shown increased influence from crosstalk. The process involved in preparing and performing this crosstalk test is included in this work, as well as the findings from the recent and previous SRCA-based crosstalk characterizations.
The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on-board the Aqua and Terra spacecraft have provided valuable science data for the last 18 and 20 years, respectively. Each instrument is equipped with 36 spectral bands, 20 of which are reflective solar bands (RSBs). These bands cover a wavelength range of 0.4 - 2.2 μm and are calibrated on-orbit using several on-board calibrators (OBCs), such as a solar diffuser (SD) and a solar diffuser stability monitor (SDSM), along with regularly-scheduled lunar observations through the space view (SV) port. The gain (1/m1) and response-verses-scan angle (RVS) are updated on a near-monthly basis and act as the primary look-up-tables (LUTs) for the RSB calibration. A set of separate uncertainty LUTs for each of the RSBs are also delivered regularly and incorporated into the Level 1B (L1B) product to generate a pixel-level Uncertainty Index (UI). In addition to the gain, RVS and uncertainty, there are several other LUTs associated with the reflective bands that are either updated less frequently or remain static. The accuracy of both the forward-predicted and historical RSB LUTs, which are derived by the MODIS Characterization Support Team (MCST), is important in maintaining the quality and accuracy of the L1B and science products. To ensure a timely and accurate LUT update, MCST has established a comprehensive set of procedures. This paper provides an overview of the calibration process, along with the current LUT delivery process for the RSBs in Collection 6 (C6) and Collection 6.1 (C6.1). Improvements to be implemented in future collections are also discussed.
The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on-board the Terra spacecraft has provided valuable Earth data to the science community for the last 20 years. Equipped with several on-board calibrators (OBCs), MODIS has continued to operate nominally since its launch in December 1999. The Spectro- Radiometric Calibration Assembly (SRCA) is one such OBC that is able to provide on-orbit measurements of the MODIS reflective solar bands (RSBs) in radiometric, spatial and spectral modes. While the SRCA is operating in spectral mode, it is able to monitor the center wavelength (CW), bandwidth (BW) and in-band relative spectral response (RSR) of most RSBs. Prelaunch measurements of the CWs, BWs and RSRs of the RSBs were performed at the system level using the Spectral measurement Assembly (SpMA). Using both the prelaunch measurements and the measurements obtained on-orbit using the SRCA, the changes in the spectral response of the MODIS reflective bands can be monitored throughout the mission. This paper will provide a brief description of the spectral calibration approach and report on-orbit changes in these spectral performance parameters and their uncertainties over the last 20 years. It will also address changes to the SRCA operation on-orbit and their impact on measured spectral calibration results. Despite two decades in orbit, the spectral responses for most Terra MODIS re ective bands continue to remain within their design specifications.
The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on-board the NASA’s Earth Observing System Terra satellite has continued successful Earth-sensing operations for over 20 years. To aid in its mission in providing calibrated science data to the worldwide user community, the MODIS instrument is equipped with several on-board calibrators designed to measure changes in the instrument response over time. One such calibrator is the Spectro- Radiometric Calibration Assembly (SRCA), which can provide a source signal for radiometric, spectral, or spatial characterization. When commanded into its spatial calibration mode, the SRCA is able to produce light across the MODIS band spectral range (0.412μm to 14.2μm) at a variety of signal levels thanks to several internal halogen lamps, an IR glow bar, and a neutral density filter. This signal, used in combination with commanded sub-sample measurements of the MODIS detectors, provides a basis for determining changes in the spatial performance of the MODIS spectral bands. This work summarizes the spatial calibration process using the SRCA and presents 20 years of Terra MODIS spatial performance characterized through co-registration between MODIS bands, detectors, and focal plane assemblies. Results from pre-launch testing using the Integration and Alignment Collimator and the SRCA are incorporated in the history of the Terra MODIS mission-long spatial performance. We also note modifications to the spatial characterization methodology brought on by changes to the SRCA’s operational configuration and changes to the MODIS spectral band performance, particularly after the recovery from the safe-mode event in February 2016. Results are compared against the MODIS design specifications.
KEYWORDS: Equipment, Crosstalk, Short wave infrared radiation, Calibration, MODIS, Sensors, Long wavelength infrared, Staring arrays, Signal to noise ratio, Algorithm development
Since its launch in December 1999, Terra Moderate Resolution Imaging Spectroradiometer (MODIS) has successfully operated for more than 20 years, with its observations generating a broad range of science data products that have greatly enabled the remote sensing community and users worldwide in their studies of many key geophysical parameters of the Earth’s systems. MODIS collects data in 36 spectral bands, covering wavelengths from 0.41 to 14.4 μm, which are calibrated by a set of onboard calibrators (OBCs). Also contributing to the sensor’s mission-long on-orbit calibration and characterization are near-monthly scheduled lunar observations and multiple time series of the sensor’s responses over select ground targets at a variety of scan angles. To a large extent, the quality of MODIS data products relies strongly on the dedicated efforts to operate and calibrate the instrument, to derive and update calibration parameters, and to develop and implement new calibration strategies and algorithms in response to on-orbit changes of the sensor’s characteristics and its OBC functions. We provide an overview of the Terra MODIS on-orbit operation and calibration activities over the last 20 years, including changes made to extend and preserve the instrument and OBC functions and their operation strategies. It also illustrates the sensor’s on-orbit performance with results derived from its OBC, lunar observations, and select ground targets and discusses major changes in sensor characteristics and corrections applied to the L1B algorithms as well as calibration lookup table updates. To date, the Terra MODIS instrument and its OBCs continue to operate and function normally. Except for those identified prelaunch, most spectral bands and detectors continue to meet their specified calibration requirements. Also discussed in our paper are lessons learned from Terra MODIS operation and calibration, as well as future efforts to further extend and maintain the quality of its long-term data records.
For nearly 20 years, Terra MODIS observations have generated a broad range of data products, enabling the remote sensing community and users worldwide for their studies of many key geophysical parameters of the Earth’s system. MODIS collects data in 36 spectral bands, covering wavelengths from 0.41 to 14.4 μm, that are calibrated by a set of on-board calibrators (OBC). Also contributed to sensor on-orbit calibration and characterization are near monthly-scheduled lunar observations and long-term trends of sensor responses over select ground targets. The quality of MODIS data products relies strongly on the dedicated efforts to the operate instrument, derive and update calibration parameters, and improve calibration strategies and algorithms in order to address on-orbit changes of sensor characteristics and its OBC functions. This paper provides an overview of Terra MODIS on-orbit operation and calibration activities over the last 20 years, including changes made to extend and preserve instrument and OBC functions and their implementation strategies. It illustrates sensor on-orbit performance using data from its OBC, lunar observations, and select ground targets and discusses major changes in sensor characteristics and corrections applied to the L1B algorithms or updates of calibration look-up tables (LUTs). Also described in this paper are lessons learned from Terra MODIS and future efforts to further extend its long-term data records.
KEYWORDS: Signal to noise ratio, Calibration, MODIS, Sensors, Diffusers, Reflectivity, Signal detection, Interference (communication), Space operations, Sun
The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on-board the Terra and Aqua space- craft are equipped with several on-board calibrators (OBCs) and continue to operate normally since launch. One such calibrator is the solar diffuser (SD), which allows for the calibration of the 20 reflective solar bands (RSBs) with wavelengths ranging from 0.41 to 2.3 μm. In order to accurately characterize the RSBs on-orbit, the changes associated with the SD bi-directional reflectance factor (BRF) are tracked using a solar diffuser stability monitor (SDSM). The SDSM consists of nine detectors located within a spherical integration source (SIS) and covers wavelengths from 0.41 to 0.94 μm. During each calibration event, the SDSM alternately views sunlight through an attenuation screen and the sunlight reflected from the SD in order to accurately characterize the degradation of the SD at those nine wavelengths. This paper provides a brief overview of the SD/SDSM calibration and operation, with more emphasis on the recent performance of the SD degradation and the SDSM detectors. A methodology to compute the signal-to-noise ratio (SNR) for each of the SDSM detectors is formulated and the noise performance is tracked over the mission lifetime. The importance of the detector noise to the RSB calibration uncertainty and to other instruments, such as the VIIRS SDSM, is also discussed.
The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on board the Terra and Aqua space- craft are equipped with several on-board calibrators (OBCs) and continue to operate normally since launch. One such calibrator is the Spectro-Radiometric Calibration Assembly (SRCA), whose regular calibrations provide accurate measurements in radiometric, spatial and spectral modes. The SRCA is able to monitor and measure the center wavelength (CW) shift, the bandwidth (BW) shift and a major portion of the relative spectral response (RSR) for each of the MODIS reflective solar bands (RSBs) while operating in spectral mode. However, there are several factors that influence the uncertainties when calculating these results. This paper provides a brief overview of the SRCA in spectral mode, along with how the CWs, BWs and RSRs of the MODIS RSBs are calculated. The operational factors that contribute to the spectral uncertainty are also discussed, including the variation of the half-included angle (β) and the grating motor offset angle (θoff ) of the monochromator. A comparison between the theoretical and on-board CW uncertainty is also provided.
We initiated a multi-technique campaign to understand the physics and properties of the massive binary system MWC 314. Our observations included optical high-resolution spectroscopy and Johnson photometry, nearinfrared spectrophotometry, and K′−band long-baseline interferometry with the CHARA Array. Our results place strong constraints on the spectroscopic orbit, along with reasonable observations of the phase-locked photometric variability. Our interferometry, with input from the spectrophotometry, provides information on the geometry of the system that appears to consist of a primary star filling its Roche Lobe and loosing mass both onto a hidden companion and through the outer Lagrangian point, feeding a circumbinary disk. While the multi-faceted observing program is allowing us to place some constraints on the system, there is also a possibility that the outflow seen by CHARA is actually a jet and not a circumbinary disk.
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