Sriharsha Madhavan

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.

Terra MODIS Band 27 Electronic Crosstalk Effect and Its Removal

The Moderate Resolution Imaging Spectroradiometer (MODIS) is one of the primary instruments in the National Aeronautics and Space Administration Earth observing system. The first MODIS instrument was launched in December, 1999 onboard the Terra spacecraft. MODIS has 36 bands, covering a wavelength range 0.4-14.4 μm. MODIS band 27 (6.72 μm) is a water vapor band, which is designed to be insensitive to Earth surface features. In recent Earth view images of Terra band 27, surface feature contamination is clearly seen with pronounced striping. In this paper, it is shown that these band-27 issues are caused by electronic crosstalk from bands 28-30. An algorithm using a linear approximation is developed to correct the crosstalk effect. The crosstalk coefficients are derived from Terra MODIS lunar observations. They show that the crosstalk is strongly detector-dependent and the crosstalk pattern has changed in a noticeable fashion since launch. The crosstalk contributions were positive to the instrument response of band 27 early in the mission but became negative and much larger in magnitude at later stages of the mission for most detectors of the band. The algorithms are applied to both the black body (BB) calibration and the MODIS L1B calibrated products. With the crosstalk effect significantly removed, the calibration coefficients of Terra MODIS band 27 derived from the BB show that the detector differences become smaller. With the algorithms applied to MODIS L1B products, the Earth surface features are significantly removed, thereby restoring the radiometric balance of the band and substantially reducing the striping features in the image.

Long-term drift induced by the electronic crosstalk in Terra MODIS Band 29

Terra MODerate Resolution Imaging Spectroradiometer (MODIS) is one of the key sensors in the NASAs Earth Observing System, which has successfully completed 15u2009years of on-orbit operation. Terra MODIS continues to collect valuable information of the Earths energy radiation from visible to thermal infrared wavelengths. The instrument has been well characterized over its lifetime using onboard calibrators whose calibration references are traceable to the National Institute of Standards and Technology standards. In this paper, we focus on the electronic crosstalk effect of Terra MODIS band 29, a thermal emissive band (TEB) whose center wavelength is 8.55u2009µm. Previous works have established the mechanism to describe the effect of the electronic crosstalk in the TEB channels of Terra MODIS. This work utilizes the established methodology to apply to band 29. The electronic crosstalk is identified and characterized using the regularly scheduled lunar observations. The moon being a near-pulse-like source allowed easy detection of extraneous signals around the actual Moon surface. First, the crosstalk-transmitting bands are identified along with their amplitudes. The crosstalk effect then is characterized using a moving average mechanism that allows a high fidelity of the magnitude to be corrected. The lunar-based analysis unambiguously shows that the crosstalk contamination is becoming more severe in recent years and should be corrected in order to maintain calibration quality for the affected spectral bands. Finally, two radiometrically well-characterized sites, Pacific Ocean and Libya 1 desert, are used to assess the impact of crosstalk effect. It is shown that the crosstalk contamination induces a long-term upward drift of 1.5u2009K in band 29 brightness temperature of MODIS Collection 6 L1B, which could significantly impact the science products. The crosstalk effect also induces strong detector-to-detector differences, which result in severe stripping in the Earth view images. With crosstalk correction applied, both the long-term drift and detector differences are significantly reduced.

Statistical analysis of the electronic crosstalk correction in Terra MODIS Band 27

The first MODerate-resolution Imaging Spectroradiometer (MODIS), also known as the Proto-Flight model (PFM), is on-board the Terra spacecraft and has completed 14 years of on orbit flight as of December 18, 2013. MODIS remotely senses the Earth in 36 spectral bands, with a wavelength range from 0.4 μm to 14.4 μm. The 36 bands can be subdivided into two groups based on their spectral responsivity as Reflective Solar Bands (RSBs) and Thermal Emissive Bands (TEBs). Band 27 centered at 6.77 μm is a TEB used to study the global water vapor distribution. It was found recently that this band has been severely affected by electronic crosstalk. The electronic crosstalk magnitude, its on-orbit change and calibration impact have been well characterized in our previous studies through the use of regularly scheduled lunar observations. Further, the crosstalk correction was implemented in Earth view (EV) images and quantified the improvements of the same. However, improvements remained desirable on several fronts. Firstly, the effectiveness of the correction needed to be analyzed spatially and radiometrically over a number of scenes. Also, the temporal aspect of the correction had to be investigated in a rigorous manner. In order to address these issues, a one-orbit analysis was performed on the Level 1A (L1A) scene granules over a ten year period from 2003 through 2012. Results have been quantified statistically and show a significant reduction of image striping, as well as removal of leaked signal features from the neighboring bands. Statistical analysis was performed by analyzing histograms of the one-orbit granules at a scene and detector level before and after correction. The comprehensive analysis and results reported in this paper will be very helpful to the scientific community in understanding the impacts of crosstalk correction on various scenes and could potentially be applied for future improvements of band 27 calibration and, therefore, its retrieval for the Level 2 (L2) geophysical parameters.

Long-term band-to-band calibration stability of MODIS thermal emissive bands

The Terra and Aqua MODIS instruments have operated continuously for over 12 and 10 years respectively and are key contributors to the NASA Earth Observing System mission. The calibration for the 16 thermal emissive bands (TEB) is maintained on-orbit through scan-by-scan observations of a temperature controlled blackbody and deep space. Recently a potential calibration issue with Terra Band 29 (8.55 μm) was identified resulting in a possible long-term drift in Band 29 detector response. The long-term performance of Band 31 (11 μm) is considered stable and is used as a reference to track the relative stability of other TEB. Multiple observations of different Earth targets with a range of scene temperatures as a function of time are analyzed to assess MODIS TEB band-to-band calibration stability for Band 29.

Monitoring and assessment of the temperature fluctuation of Aqua MODIS cold focal plane assembly

The MODIS instruments on-board the Terra and Aqua spacecrafts have 16 thermal emissive bands (TEB), located on two cold focal plane assemblies (CFPA). The CFPAs are cryogenically cooled by a passive radiative cooler, with their temperatures further controlled at a nominal value of 83K. For Aqua MODIS, the cooler margin has gradually decreased since launch, which deteriorates the CFPA temperature stability. Since 2006, Aqua CFPA temperature fluctuates with the instrument temperature in both seasonal and orbital oscillation patterns. The magnitude of the fluctuation steadily increases on yearly basis. The gains of TEB detectors change with the CFPA temperature in a nearly linear way, as is demonstrated by both pre-launch calibration and on-orbit monitoring. As of mid 2012, the magnitude of the CFPA temperature fluctuation reaches 0.65K, causing band-dependent detector gain fluctuation of up to 6%. In this paper, the CFPA temperature and its related telemetries are monitored over both a short-term and long-term basis. The impact of the fluctuation to TEB radiometric calibration is assessed, too. Because the calibration is normally performed on a scanby- scan basis based on the observation of an onboard blackbody (BB), the detector gain change can be retrieved in nearly real time. Therefore, the impact is insignificant in general. However, for bands 33, 35 and 36, their detectors saturate when observing BB at BB temperature above certain saturation limits during quarterly held BB warmupcooldown (WUCD) activities. Since there is no valid scan-by-scan calibration during these periods, a special treatment has to be applied to calibrate these bands to reflect the detector gain fluctuation.

MODIS calibration algorithm improvements developed for Collection 6 Level-1B

The Moderate Resolution Imaging Spectroradiometer (MODIS) has been operating on both the Terra and Aqua spacecraft for over 10.5 and 8 years, respectively. Over 40 science products are generated routinely from MODIS Earth images and used extensively by the global science community for a wide variety of land, ocean, and atmosphere applications. Over the mission lifetime, several versions of the MODIS data set have been in use as the calibration and data processing algorithms evolved. Currently Version 5 MODIS data is the baseline Level-1B calibrated science product. The MODIS Characterization Support Team (MCST), with input from the MODIS Science Team, developed and delivered a number of improvements and enhancements to the calibration algorithms, Level-1B processing code and Look-up Tables for the Version 6 Level-1B MODIS data. Version 6 implements a number of changes in the calibration methodology for both the Reflective Solar Bands (RSB) and Thermal Emissive Bands (TEB). This paper describes the improvements introduced in Collection 6 to the RSB and TEB calibration and detector Quality Assurance (QA) handling.

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.

Analog and digital saturation in the MODIS reflective solar bands

The MODIS instrument on the Terra and Aqua spacecrafts is a 12 bit sensor with an analog-to-digital (A/D) range of 0 to 4095 DN. Each sensor system is limited by a range at the low and high ends of the dynamic scale. At the low end, quantization noise is the limiting factor whereas at the high end the maximum value is limited by the capability of the amplifier, 4095 in the case of MODIS. However, in both Terra and Aqua MODIS certain detectors in the Reflective Solar Bands (RSB) tend to pre-saturate at a value lower than 4095. This paper serves as a comprehensive report on the algorithms developed to characterize the pre-saturation limit in the RSB. The paper also provides the digital and pre-saturation (analog saturation) limits for the RSB that are currently being used in the Level 1B (L1B) products. The digital and analog saturation limits are well characterized using the Level 1A (L1A) raw Earth-View (EV) data and through the on-board Electronic Calibration (E-CAL). Also, in this paper an analysis is done to study the sensors dynamic range due to the long term changes in the instrument response. In summary, the algorithms and results reported in this paper are important as the radiometric accuracy / uncertainty for instruments such as MODIS, VIIRS (NPP) tends to be coupled to pre-saturation.

On-orbit Radiometric Stability Assessment of MODIS Thermal Emissive Bands with Lunar Observation

The MODIS thermal emissive bands (TEB) are radiometrically calibrated on-orbit on a scan-by-scan basis, with reference to an aboard blackbody operated at 290 K for Terra MODIS and at 285 K for Aqua MODIS. The quality of the calibration can be evaluated with independent thermal sources at other temperatures. As a spectrally, spatially and radiometrically stable source, the Moon has become more important to the on-orbit calibration of space-borne spectral sensors that have regular lunar observation capability. MODIS is scheduled to observe the Moon on a nearly monthly basis at approximately the same lunar phase angle through its space view port. In this paper, the long-term stability of MODIS TEB radiometric calibration is assessed through the multi-year trends of the brightness temperatures (BT) of the lunar surface retrieved from the scheduled lunar observation. The highest lunar surface temperature is approximately 390 K, higher than the saturation temperatures of most TEB. For the non-saturated bands, the trending is based on the BT of the hottest area of the Moon. For the partially saturated bands, the trending is based on the BT difference of the unsaturated matching pixels between the band and a non-saturated reference band, given the fact that all MODIS bands are spatially registered. Overall, the trends have been stable throughout MODIS lifetime. The results also prove that the Moon can be used as a source to monitor the stability of the thermal bands.