Xiaoxiong J. Xiong

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.

On-orbit noise characterization of MODIS reflective solar bands

Abstract. The Moderate Resolution Imaging Spectroradiometer (MODIS), launched on the Terra and Aqua spacecrafts, was designed to collect complementary and comprehensive measurements of the Earth’s properties on a global scale. The 20 reflective solar bands (RSBs), covering a wavelength range from 0.41 to 2.1  μm, are calibrated on-orbit using regularly scheduled solar diffuser (SD) observations. Although primarily used for on-orbit gain derivation, the SD observations also facilitate the characterization of the detector signal-to-noise ratio (SNR). In addition to the calibration requirement of 2% for the reflectance factors and 5% for the radiances, the required SNRs are also specified for all RSB at their typical scene radiances. A methodology to characterize the on-orbit SNR for the MODIS RSB is presented. Overall performance shows that a majority of the RSB continue to meet the specification, therefore performing well. A temporal decrease in the SNR, observed in the short-wavelength bands, is attributed primarily to the decrease in their detector responses. With the exception of the inoperable and noisy detectors in band 6 identified prelaunch, the detectors of Aqua MODIS RSB perform better than Terra MODIS. The approach formulated for on-orbit SNR characterization can also be used by other sensors that use on-board SDs for their on-orbit calibration (e.g., Suomi National Polar-Orbiting Partnership [SNPP]-Visible Infrared Imaging Radiometer Suite).

Status of Terra MODIS Operation, Calibration, and Performance

Since launch in December 1999, Terra MODIS has successfully operated for nearly 15 years, making continuous observations. Data products derived from MODIS observations have significantly contributed to a wide range of studies of key geophysical parameters of the earth’s eco-system of land, ocean, and atmosphere, and their changes over time. The quality of MODIS data products relies on the dedicated effort to monitor and sustain instrument health and operation, to calibrate and update sensor parameters and properties, and to improve calibration algorithms. MODIS observations are made in 36 spectral bands, covering wavelengths from visible to long-wave infrared. The reflective solar bands (1-19 and 26) are primarily calibrated by a solar diffuser (SD) panel and regularly scheduled lunar observations. The thermal emissive bands (20-25 and 27- 36) calibration is referenced to an on-board blackbody (BB) source. On-orbit changes in the sensor spectral and spatial characteristics are monitored by a spectroradiometric calibration assembly (SRCA). This paper provides an overview of Terra MODIS on-orbit operation and calibration activities and implementation strategies. It presents and summarizes sensor on-orbit performance using nearly 15 years of data from its telemetry, on-board calibrators, and lunar observations. Also discussed in this paper are changes in sensor characteristics, corrections applied to maintain MODIS level 1B (L1B) data quality, and efforts for future improvements.

Updates of MODIS on-orbit calibration uncertainty assessments

The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments have successfully operated for more than 17 and 15 years, respectively, on-board the NASA’s Earth Observing System (EOS) Terra and Aqua spacecraft. MODIS level 1B (L1B) data products include top of the atmosphere (TOA) reflectance factors for the reflective solar bands (RSB) and radiances for both the RSB and the thermal emissive bands (TEB), and their associated uncertainty indices (UI) at a pixel-by-pixel level. This paper provides a brief review of MODIS L1B calibration algorithms, including improvements made in recent years. It presents an update of sensor calibration uncertainty assessments with a focus on several new contributors resulting from changes in sensor characteristics and on-orbit calibration approaches and the impact due to these changes on the L1B data quality. Also discussed in this paper are potential changes that could be made to continue improving the quality of MODIS L1B uncertainty product.

MODIS solar diffuser degradation at short-wave infrared band wavelengths.

The MODIS instruments aboard the Terra and Aqua spacecraft have 20 reflective solar bands (RSB) with wavelengths spanning 412 nm to 2130 nm. The primary on-board calibration source for the RSB is a sunlit solar diffuser (SD), with its degradation tracked by a SD stability monitor (SDSM). The SDSM measurements show that the decrease in SD reflectance over time has a strong wavelength dependence, with longer wavelengths showing less degradation. The SDSM has 9 detectors to track the SD degradation at wavelengths from 412 nm to 936 nm, but is not designed to track the degradation at short-wave infrared (SWIR) wavelengths. In recent years, the SDSM has measured non-negligible degradation in the SD reflectance at 936 nm for both Terra (>;2%) and Aqua (>0.5%) MODIS. In addition, comparison of SD calibration results to earth view targets suggests that smaller but non-negligible SD degradation also exists at the SWIR band wavelengths. In this paper, we review the current status of the MODIS SD degradation as measured by the SDSM. We present efforts to extend the SD degradation measurements to the SWIR band wavelengths (1240 nm to 2130 nm) by fitting a wavelength-dependent model to the SDSM results from the visible and NIR wavelength detectors. The predicted degradation results are used to correct the MODIS SWIR band degradation, and comparisons are made with trends from pseudo-invariant earth targets. Results are presented for both Terra and Aqua MODIS.

Simulation of the long term radiometric responses of the Terra MODIS and EO-1 ALI using Hyperion spectral responses over Railroad Valley Playa in Nevada (RVPN)

The Earth Observing-1 (EO-1) Hyperion instrument provides 220 spectral bands with wavelengths between 400 and 2500 nm at 30 m spatial resolution, which covers a 7.5 km by 100 km area on the ground. The EO-1 spacecraft has another multispectral sensor called the Advanced Land Imager (ALI), which has 10 spectral bands with wavelengths between 400 and 2350 nm at 30 m spatial resolution. The Moderate Resolution Imaging Spectroradiometer (MODIS) sensor onboard the Terra spacecraft was launched in Dec., 1999, and flies approximately 30 minutes behind EO-1. Nearsimultaneous observations from Terra MODIS, EO-1 ALI and Hyperion over a well characterized Railroad Valley Playa in Nevada (RVPN) target are chosen for this study. A uniform region of interest (ROI) within the playa within latitudes and longitudes of 38.48 and -115.71 to 38.53 and -115.66 was chosen for this analysis. A representation of the ground spectra during every near-simultaneous acquisition of MODIS and ALI is obtained using EO-1 Hyperion data. Using the EO-1 Hyperion derived top-of-atmosphere (TOA) reflectance profile along with the ALI and MODIS relative spectral responses (RSR), simulated reflectance for the matching band pairs is calculated. The Hyperion simulated TOA reflectance results are compared to the measured TOA reflectance trends of ALI and MODIS. The long-term measured versus simulated reflectance results are used to examine the relationships and calibration differences between the ALI and MODIS sensors.

Assessment of polarization correction impact on the calibration of Terra MODIS reflective solar bands

The Moderate-Resolution Imaging Spectroradiometer (MODIS), launched in 1999 on Terra and 2002 on Aqua spacecraft respectively, is a scanning radiometer that covers a wavelength range from 0.4 μm to 14.4 μm and scans the Earth over an angular range from -55° to +55°. After a few years in the Terra mission, it became extremely challenging to characterize the changes in the sensor gain and response versus scan angle (RVS) at short wavelengths due to significant degradation and increased polarization sensitivity. To better characterize the system-level degradation, the MODIS Characterization Support Team (MCST) developed an enhanced approach in Collection-6 (C6) L1B algorithm by supplementing the on-board calibration data with the Earth-scene response trends at various scan angles obtained from the pseudo-invariant desert sites. However, the trends at short wavelengths experienced significant impact due to the increased polarization sensitivity, especially at the end of scan. In this study, a polarization correction algorithm developed by MCST is applied to the Terra MODIS RSB response trends obtained from the desert sites. The trends after polarization correction are used to derive the gain and RVS based on the existing MODIS C6 calibration algorithm. Impact of the polarization correction is examined for gain, RVS and their fitting uncertainties over the entire mission. The results of this study provide useful information on how to further improve accuracy and stability of the calibrated L1B product.

Lunar calibration improvements for the short-wave infrared bands in Aqua and Terra MODIS

The Moderate Resolution Imaging Spectroradiometer (MODIS) is one of the key sensors among a suite of remote sensing instruments on board the Terra and Aqua spacecrafts. Since the beginning of each mission, regularly scheduled lunar observations have been used in order to track the on-orbit gain changes of the reflective solar bands. However, for the short-wave infrared bands, 5-7 and 26, the measured signal is contaminated by both electronic crosstalk and an out-of-band response due to transmission through the MODIS filters at undesired wavelengths. These contaminating signals cause significant oscillations in the derived gain from lunar observations for these bands, which limits their use in determining the scan mirror response versus scan angle at these wavelengths. In this paper, we show a strategy for correcting the electronic crosstalk contamination using lunar observations, where the magnitude and the source of the contaminating signal is clear. For Aqua MODIS, we find that the magnitude of the electronic crosstalk contamination is small, and the lunar calibration remains relatively unaffected. For Terra MODIS, the contamination is more significant, and the electronic crosstalk correction shows a significant reduction in the oscillations of the lunar calibration results.

Cross comparison of the Collection 6 and Collection 6.1 Terra and Aqua MODIS Bands 1 and 2 using AVHRR N15 and N19

The Moderate Resolution Imaging Spectroradiometer (MODIS) is a key scientific instrument that was launched into Earth orbit by NASA in 1999 on board the Terra (EOS AM) satellite and in 2002 on board the Aqua (EOS PM) satellite. Terra and Aqua MODIS collect the entire Earth’s images every 1 to 2 days in 36 spectral bands. MODIS band 1 (0.620- 0.670 μm) and band 2 (0.841-0.876 μm) have nadir spatial resolution of 250 m and their measurements are crucial to derive key land surface products. This study evaluates the performance of the Collection 6 (C6, and C6.1) L1B of both Terra and Aqua MODIS bands 1 and 2 using Simultaneous Nadir Overpass (SNO) data to compare with AVHRR/3 sensors. We examine the relative stability between Terra and Aqua MODIS in reference to NOAA N15 and N19 the Advanced Very High Resolution Radiometer (AVHRR/3). The comparisons for MODIS to AVHRR/3 are over a fifteenyear period from 2002 to 2017. Results from this study provide a quantitative assessment of Terra and Aqua MODIS band 1 and band 2 calibration stability and the relative differences through the NOAA N15 and N19 AVHRR/3 sensors.

Assessment of MODIS reflective solar bands calibration stability

Terra and Aqua MODIS instruments have continued to operate normally since their launch in December 1999 and May 2002. MODIS reflective solar bands (RSB) with wavelengths ranging from 0.41 to 2.3 μm are calibrated on-orbit by an on-board solar diffuser (SD) and a solar diffuser stability monitor (SDSM). In addition, a spectroradiometric calibration assembly (SRCA) is used regularly to characterize and track on-orbit changes in RSB spectral, spatial, and radiometric performance. On a near-monthly basis, lunar observations are scheduled and performed to support sensor on-orbit calibration and characterization, such as radiometric calibration stability monitoring for the RSB. This paper provides a brief review of MODIS design requirements related to its RSB calibration performance and an assessment of MODIS RSB on-orbit calibration stability on both the short- and long-term timescales. Examples from different approaches are presented to demonstrate on-orbit performance of MODIS RSB calibration stability.