Summary of Terra and Aqua MODIS Long-Term Performance

Since launch in December 1999, the MODIS ProtoFlight Model (PFM) onboard the Terra spacecraft has successfully operated for more than 11 years. Its Flight Model (FM) onboard the Aqua spacecraft, launched in May 2002, has also successfully operated for over 9 years. MODIS observations are made in 36 spectral bands at three nadir spatial resolutions and are calibrated and characterized regularly by a set of on-board calibrators (OBC). Nearly 40 science products, supporting a variety of land, ocean, and atmospheric applications, are continuously derived from the calibrated reflectances and radiances of each MODIS instrument and widely distributed to the world-wide user community. Following an overview of MODIS instrument operation and calibration activities, this paper provides a summary of both Terra and Aqua MODIS long-term performance. Special considerations that are critical to maintaining MODIS data quality and beneficial for future missions are also discussed

Comparison of Two Methodologies for Calibrating Satellite Instruments in the Visible and Near Infrared

Traditionally, satellite instruments that measure Earth-reflected solar radiation in the visible and near infrared wavelength regions have been calibrated for radiance response in a two-step method. In the first step, the spectral response of the instrument is determined using a nearly monochromatic light source, such a lamp-illuminated monochromator. Such sources only provide a relative spectral response (RSR) for the instrument, since they do not act as calibrated sources of light nor do they typically fill the field-of-view of the instrument. In the second step, the instrument views a calibrated source of broadband light, such as lamp-illuminated integrating sphere. In the traditional method, the RSR and the sphere spectral radiance are combined and, with the instrument's response, determine the absolute spectral radiance responsivity of the instrument. More recently, an absolute calibration system using widely tunable monochromatic laser systems has been developed, Using these sources, the absolute spectral responsivity (ASR) of an instrument can be determined on a wavelength-hy-wavelength basis. From these monochromatic ASRs. the responses of the instrument bands to broadband radiance sources can be calculated directly, eliminating the need for calibrated broadband light sources such as integrating spheres. Here we describe the laser-based calibration and the traditional broad-band source-based calibration of the NPP VIIRS sensor, and compare the derived calibration coefficients for the instrument. Finally, we evaluate the impact of the new calibration approach on the on-orbit performance of the sensor

Initial Stability Assessment of S-NPP VIIRS Reflective Solar Band Calibration Using Invariant Desert and Deep Convective Cloud Targets

The latest CERES FM-5 instrument launched onboard the S-NPP spacecraft will use the VIIRS visible radiances from the NASA Land Product Evaluation and Analysis Tool Elements (PEATE) product for retrieving the cloud properties associated with its TOA flux measurement. In order for CERES to provide climate quality TOA flux datasets, the retrieved cloud properties must be consistent throughout the record, which is dependent on the calibration stability of the VIIRS imager. This paper assesses the NASA calibration stability of the VIIRS reflective solar bands using the Libya-4 desert and deep convective clouds (DCC). The invariant targets are first evaluated for temporal natural variability. It is found for visible (VIS) bands that DCC targets have half of the variability of Libya-4. For the shortwave infrared (SWIR) bands, the desert has less variability. The brief VIIRS record and target variability inhibits high confidence in identifying any trends that are less than 0.6yr for most VIS bands, and 2.5yr for SWIR bands. None of the observed invariant target reflective solar band trends exceeded these trend thresholds. Initial assessment results show that the VIIRS data have been consistently calibrated and that the VIIRS instrument stability is similar to or better than the MODIS instrument

MODIS On-orbit Calibration and Lessons Learned

The Moderate Resolution Imaging Spectroradiometer (MODIS) is a key instrument for NASA’s Earth Observing System (EOS) Terra and Aqua missions. Since launch, Terra and Aqua MODIS have successfully operated for more than 12 and 10 years, respectively, and generated an unprecedented amount of data products for the science and user community over a wide range of applications. MODIS was developed with improved design and stringent calibration requirements over its heritage sensors in order to extend and enhance their long-term data records. Its follow-on instrument, the Visible/Infrared Imager Radiometer Suite (VIIRS), was launched on-board the Suomi National Polar-orbiting Partnership (NPP) spacecraft October 28, 2011. MODIS collects data in 36 spectral bands, covering wavelengths from 0.41 to 14.5mm, and at 250m, 500m, and 1km spatial resolutions (nadir). MODIS on-orbit calibration is provided by a set of onboard calibrators (OBC), including a solar diffuser (SD), a solar diffuser stability monitor (SDSM), a blackbody (BB), and a spectroradiometric calibration assembly (SRCA). In addition to the onboard calibrators, regular lunar observations are made by both Terra and Aqua MODIS to track their calibration stability in the reflective solar region. This tutorial session provides an overview of MODIS on-orbit calibration and characterization methodologies. It discusses challenging issues and lessons learned from sensor design, operation, calibration, and inter-comparisons. Examples of instrument on-orbit performance are illustrated with a focus on the improvements made based on various lessons learned. It is expected that MODIS experience and lessons will continue to provide valuable information for future earth observing missions/sensors

S – NPP VIIRS On-orbit Uncertainty Estimate for Emissive Bands

The SNPP VIIRS instrument includes seven thermal emissive bands covering a range of 3.7 to 12 μm. The top of the atmosphere radiances and brightness temperatures generated by the instrument are used by the science community involved in environmental and climate research to create various science products. It is important to characterize the uncertainty of the VIIRS thermal bands measurements to ensure that the science accuracy requirements are being met. This work will focus on estimating the various error contributors and propagating the uncertainty inherent in the measurement to the product level using a standard formulation. Individual error sources include spectral and temperature uncertainties, uncertainty in the relative reflectance of the scan mirror, uncertainty in the detector response and radiance model parameters. Results of the error propagation are investigated using on-orbit data, covering various scene types as well as the retrieval of the on-board blackbody radiance. Estimates of the uncertainty are also made using prelaunch data from sensor level thermal vacuum testing, referenced to both the internal blackbody as well as a well characterized external blackbody. The prelaunch and postlaunch uncertainties are then both compared to the sensor specification to determine if the instrument is operating within the expected accuracy

Assessment of MODIS RSB Detector Uniformity Using Deep Convective Clouds

For satellite sensor, the striping observed in images is typically associated with the relative multiple detector gain difference derived from the calibration. A method using deep convective cloud (DCC) measurements to assess the difference among detectors after calibration is proposed and demonstrated for select reflective solar bands (RSBs) of the Moderate Resolution Imaging Spectroradiometer (MODIS). Each detector of MODIS RSB is calibrated independently using a solar diffuser (SD). Although the SD is expected to accurately characterize detector response, the uncertainties associated with the SD degradation and characterization result in inadequacies in the estimation of each detector's gain. This work takes advantage of the DCC technique to assess detector uniformity and scan mirror side difference for RSB. The detector differences for Terra MODIS Collection 6 are less than 1% for bands 1, 3-5, and 18 and up to 2% for bands 6, 19, and 26. The largest difference is up to 4% for band 7. Most Aqua bands have detector differences less than 0.5% except bands 19 and 26 with up to 1.5%. Normally, large differences occur for edge detectors. The long-term trending shows seasonal oscillations in detector differences for some bands, which are correlated with the instrument temperature. The detector uniformities were evaluated for both unaggregated and aggregated detectors for MODIS band 1 and bands 3-7, and their consistencies are verified. The assessment results were validated by applying a direct correction to reflectance images. These assessments can lead to improvements to the calibration algorithm and therefore a reduction in striping observed in the calibrated imagery

Benefits of an Explicit Calibration Procedure for VIIRS Reflective Solar Bands

We have developed a new calibration method that is easier to understand than the historical attenuator method used in VIIRS pre-launch reflective band calibration by separating the determination of the attenuator\u27s transmittance from the fitting of a detector\u27s calibration coefficients. Other than straightforward to understand, the new method brings benefits that are not available using the historical method. For example, the non-linear fitting is eliminated for a quadratic calibration equation, and instead of trusting some standard routines and hidden assumptions in the historical method which is often used as a black box, every step in the new method is explicit and easy to visualize. In addition, the influence of individual data point can be examined, making the new method a more useful tool. This presentation demonstrates some of the benefits of the new attenuator method using the VIIRS J1 pre-launch test data. Some sensitivity tests are also discussed

Initial Stability Assessment of S-NPP VIIRS Reflective Solar Band Calibration Using Invariant Desert and Deep Convective Cloud Targets

The latest CERES FM-5 instrument launched onboard the S-NPP spacecraft will use the VIIRS visible radiances from the NASA Land Product Evaluation and Analysis Tool Elements (PEATE) product for retrieving the cloud properties associated with its TOA flux measurement. In order for CERES to provide climate quality TOA flux datasets, the retrieved cloud properties must be consistent throughout the record, which is dependent on the calibration stability of the VIIRS imager. This paper assesses the NASA calibration stability of the VIIRS reflective solar bands using the Libya-4 desert and deep convective clouds (DCC). The invariant targets are first evaluated for temporal natural variability. It is found for visible (VIS) bands that DCC targets have half of the variability of Libya-4. For the shortwave infrared (SWIR) bands, the desert has less variability. The brief VIIRS record and target variability inhibits high confidence in identifying any trends that are less than ±0.6%/yr for most VIS bands, and ±2.5%/yr for SWIR bands. None of the observed invariant target reflective solar band trends exceeded these trend thresholds. Initial assessment results show that the VIIRS data have been consistently calibrated and that the VIIRS instrument stability is similar to or better than the MODIS instrument

A Study of Out-of-band Uncertainties for On-orbit Ocean Color Measurements Based on Laser Calibration of Flight Radiometers

Laser-based laboratory calibrations in the facility for Spectral Irradiance and Radiance responsivity Calibrations using Uniform Sources (SIRCUS) of the National Institute of Standards and Technology (NIST) have achieved detector calibration with uncertainties less than 0.1 % in the silicon spectral region. Because of the high power of lasers, there is also a drastic increase in dynamic range covering more than 5 orders of magnitude. The low measurement uncertainty and high dynamic range allow accurate assessment of out-of-band performance for flight instruments such as the Suomi National Polar-orbiting Partnership (NPP) Visible and Infrared Imager Radiometer Suite (VIIRS) sensor which was calibrated in 2010 for Absolute Spectral Responsivity (ASR) with full aperture illumination using a tunable laser and a large integrating sphere with an approximately 0.5% uncertainty in radiance responsivity. With the calibration data, it is now possible to fully evaluate the effect of out-of-band (OOB) contribution on at-sensor water-leaving radiance and derive strategy to alleviate variations from OOB scattering and reduce data product uncertainties. In this work, we present a sensitivity analysis of the Top-of-Atmosphere (TOA) measurement for varying chlorophyll concentrations and column water vapor based on measured detector Relative Spectral Responsivity (RSR). A conventional band-averaged radiance approach is taken and histograms are presented for different bands of radiometers to illustrate the data uncertainty from these variations as well as the global seasonal differences. We show how conventional lamp calibration at ground and on board solar calibration resulted in large deviation from OOB scattering because of mismatch with ocean color spectra. We will also present similar analysis for the detectors of the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS)

Guidelines for Radiometric Calibration of Electro-Optical Instruments for Remote Sensing

Sensor calibration increases the probability of mission success by quantifying the sensor’s response to known radiometric input, characterizing the interactions between the sensor\u27s components, and allowing systematic errors to be discovered and resolved before launch. This poster provides guidelines for conducting a successful EO sensor calibration campaign. It is intended for use by managers, technical oversight personnel, scientists, and engineers as a useful reference in planning and carrying out a sensor calibration. This content of this poster is based on a publication titled Guidelines for Radiometric Calibration of Electro-Optical Instruments for Remote Sensing. The publication is based on the authors\u27 many years of combined experience planning, reviewing, preparing, conducting, analyzing, implementing, and reporting on a variety of calibration efforts. Authors involved with this publication include, in alphabetical order: Daniel Bancroft, Jim Butler, Changyong Cao, Raju Datla, Scott Hansen, Dennis Helder, Raghu Kacker, Harri Latvakoski, Martin Mlynczak, Tom Murdock, James Peterson, David Pollock, Ray Russell, Deron Scott, John Seamons, Tom Stone, Joe Tansock, Alan Thurgood, Richard Williams, Xiaoxiong (Jack) Xiong, and Howard Yoon