CLARREO: Advances in High Accuracy Spectrally Resolved Radiance Measurements for Climate Forecast Testing and Global Climate Benchmarks: James Anderson – Harvard University; Henry Revercomb – University of Wisconsin, SSEC; John Dykema, Jonathan Gero – Harvard University; David Tobin, Fred Best – University of Wisconsin, SSEC

ABSTRACT: Developments of high accuracy subsystems that establish SI traceable spectrally resolved radiance measurements from Earth orbit are presented in the context of climate forecast testing using an array of mathematical methods that optimize the information content obtained from the CLARREO satellite mission. A strategy is presented for CLARREO that considers the NRC decadal study objectives for climate, the sampling issues and orbit selection, the instrument design, and the integrated spacecraft design.

AIRS Pre–Launch Spectral Calibration: Spectral Resolving Power Measurement vs. Prediction: Charles Dionne, Ken Overoye – BAE Systems

ABSTRACT: The Atmospheric Infrared Sounder (AIRS) pre–launch spectral calibration included a measurement of the spectral resolving power across the infrared band of 3.8 to 15.4 microns. The measurements became increasingly larger and deviated from the model prediction as the wavelengths became longer. A discrepancy approaching 15% occurred at the wavelength 15 microns. We show that we can now account for the discrepancy between measurement and prediction by generalizing the model to accommodate partial coherence for the slit illumination. The model used at AIRS pre–launch calibration assumed spatial incoherence for the slit illumination which lead to the discrepancy. The AIRS pre–launch measurements will be shown compared to the improved model and the incoherent model as a reference. There is a good comparison across the infrared band with the generalized model. A remaining residual discrepancy of about 1.5% increasing to 2.5% for wavelengths greater than 14 microns may be attributable to a chopper drive amplitude error. A chopper was required for the longer wavelengths since the detectors were photoconductive.

The On–Orbit Calibration of SeaWiFS: Functional Fits to the Lunar Time Series: Gene Eplee – Science Applications International Corporation; Gerhard Meister – Futuretech Corporation; Fred Patt – Science Applications International Corporation; Chuck McClain – NASA Goddard Space Flight Center

ABSTRACT: The NASA Ocean Biology Processing Group's Calibration and Validation Team uses SeaWiFS on-orbit lunar calibration data to monitor the radiometric response of the instrument over time and to generate corrections to the ocean color data products for these calibration drifts. With over ten years of lunar measurements (121 monthly observations) available for this analysis, the Cal/Val Team has undertaken an investigation of how the choice of mathematical functions to fit the time series affects the fidelity of the radiometric correction applied to the ocean data. Since band 8 (865 nm) shows the greatest change in response over time, the analysis has concentrated on that band. The initial goal of the SeaWiFS on-orbit calibration effort has been to use a single function to fit the mission-long lunar time series. To date, that goal had been met with a pair of simultaneous decaying exponentials with short-period and long-period time constants. As late mission observations are added to the time series (beyond seven years into the mission), the long-term radiometric change over time is approaching a linear function of time. For the simultaneous exponential function fit or a short-period exponential and a linear function fit to the lunar time series, the long-term trend is staring to bias the fit to the time series for the first three years of the mission. The best fit for the early mission is provided by simultaneous exponential functions with periods of 200 and 2500 days. The best fit for the late mission is provided by a exponential with a 400-day time constant and a linear function. Assessment of application of these two sets of radiometric corrections to the ocean data shows that the early-mission corrections under-correct the late-mission ocean data, while the late-mission corrections under-correct the early-mission ocean data. The optimum approach for the mission-long ocean data set is put epochs into the radiometric correction time series. Examination of the band 8 lunar time series shows a radiometric discontinuity of ~0.5% approximately 1045 days into the mission, which was confirmed in the daily solar calibration time series at the level of ~0.8%. This discontinuity has been traced to a spacecraft GPS receiver anomaly on July 19, 2001 which resulted in a one-orbit excursion in the spacecraft attitude control system. A temporary excursion of SeaWiFS into the velocity vector of the spacecraft may have affected the instrument optics, resulting in a change in the radiometric degradation rate of the instrument. The time of this discontinuity is the optimum point for the epoch between the early-mission and late-mission radiometric corrections to the ocean data. This work shows the increased fidelity of the two-epoch radiometric correction compared to the single-epoch correction, further demonstrating the necessity of a mission-long calibration validation program for the generation of a climate-quality ocean color data set.

Status of CrIS Instrument Pre–launch Calibration: Denise Hagan, Chunming Wang, Paul Lee – Northrop Grumman Space Technology; Denis Tremblay – Science Data Processing Inc.; Gene Kratz – Raytheon Information Systems

ABSTRACT: The Cross-Track Infrared Sounder (CrIS) is manifest on the National Polar-orbiting Operational Environmental Satellite System (NPOESS) for NOAA and the U.S. Air Force. CrIS provides cross-track measurements of top-of-atmosphere (TOA) radiances to permit the calculation of vertical profiles of temperature and moisture in the Earth’s atmosphere. The CrIS is a Fourier-transform Michelson interferometer covering the spectral range of 3.9 to 15.4 µm (650 to 2550 cm-1) developed by ITT under contract to Northrop Grumman Space Technology. Thermal vacuum (TVAC) tests of the CrIS Flight Module (F1) sensor were carried out by ITT in the spring and summer of 2008 to establish the radiometric, spectral and spatial properties of CrIS. This paper describes status of the evaluation of the TVAC measurement suite including radiometric noise, linearity and gas cell spectral response, and current efforts to achieve an accurate characterization transfer of the External Calibration Target to the instrument Internal Calibration Target.

Statistical Analysis of Contrast Ratios in Earth Observed Scenes for VIIRS: Stephen Mills – Northrop Grumman Space Technology

ABSTRACT: Statistical analysis is conducted of several orbits of Earth observed scenes simulated to represent imagery from the Visible Infrared Imaging Radiometer Suite (VIIRS) developed for the NPOESS Preparatory Project (NPP) and NPOESS in support of the tri-agency Integrated Project Office for NASA, NOAA and the DoD. The statistical analysis takes contrast ratios of local regions. The statistics of local contrast ratios are important in assessing the impact of scattering and crosstalk and other spatial effects. Various types of contrast ratios are considered; standard deviation over the mean, maximum over the minimum, maximum over the mean and mean over the minimum. Each of these are different types of contrasts that are relevant to different issues in retrieving environmental data records as products.

Apparent 3rd Order Effects in VIIRS MWIR (3 to 5 microns) and the Relation to Calibration Errors Using a Blackbody: Stephen Mills – Northrop Grumman Space Technology

ABSTRACT: Modeling will be presented for several types of errors. MWIR bands have a peculiar problem with fitting radiometric response because of the steep Planck Blackbody function at low brightness temperatures (230 K to 260 K). Methods for preventing spurious third-order effects include piece-wise fitting, weighted fitting, and fitting over a limited range. Each of these methods for suppressing 3rd order effects will be considered and compared.

MODIS On–board Blackbody Performance: Jack Xiong – NASA Goddard Space Flight Center; Jennifer Dodd – Science Systems and Applications, Inc.

ABSTRACT: Currently, there are two MODIS instruments operated on-orbit: one on-board the Terra spacecraft launched in December 1999 and the other on-board the Aqua spacecraft launched in May 2002. MODIS is a scanning radiometer that has 16 thermal emissive bands (TEBs) in the MWIR and LWIR regions. The remaining spectral bands are in the VIS/NIR and SWIR regions. The TEBs have a total of 160 detectors (10 detectors per band), which are calibrated on-orbit using an on-board blackbody (BB). MODIS TEB calibration is performed via a quadratic algorithm with its linear calibration coefficients updated on a scan-by-scan basis using each detector’s response to the BB. The offset and nonlinear terms of the quadratic calibration equation are stored in a look-up table (LUT). The LUT parameters are derived from pre-launch calibration and updated on-orbit from BB observations, as needed. Typically, the BB is set at a fixed temperature. Periodically, a warm-up and cool-down activity is performed, which enables the BB temperature to be varied from instrument ambient up to 315K. The temperature of the BB is measured each scan using 12 thermistors, which were fully characterized pre-launch with reference to the NIST temperature scale. This paper describes MODIS on-board BB operational activities and performance. The TEB detector response (short-term stability and long-term changes) and noise characterization results derived from BB observations and their impact on the TEB calibration uncertainty are also presented.

MODIS Solar Diffuser On-orbit Performance: Jack Xiong – NASA Goddard Space Flight Center; Honda Chen – Science Systems and Applications, Inc.

ABSTRACT: MODIS is a key instrument for the NASA Earth Observing System (EOS), currently operated on both the Terra and Aqua missions. Each MODIS instrument has 20 reflective solar bands (RSBs) and 16 thermal emissive bands (TEBs). MODIS RSB on-orbit calibration is reflectance based using an on-board solar diffuser (SD). The SD bi-directional reflectance factors (BRFs) were characterized pre-launch using reference diffuser samples, which are traceable to NIST reflectance standards. The SD BRF on-orbit degradation (or change) is tracked by another on-board device, called the solar diffuser stability monitor (SDSM). The SDSM is operated during each scheduled SD calibration event, making alternate observations of direct sunlight and the diffusely reflected sunlight from the SD. The time series of the ratios of SDSM’s SD view to its Sun view provide SD degradation information. This paper presents and compares the Terra and Aqua MODIS SD on-orbit performance. Results show that the SD on-orbit degradation depends on the amount of solar exposure of the SD plate. In addition, it is strongly wavelength-dependent, with a larger degradation rate at shorter wavelengths. For Terra MODIS, an SD door anomaly occurred in May 2003 that led to a decision to fix the door permanently at an “open” position. Since then, the SD degradation rate has significantly increased due to more frequent solar exposure. As expected, the SD on-orbit performance directly impacts the RSB calibration performance. The lessons learned from MODIS on-orbit calibration will provide useful insights into the development and operation of future SD calibration systems.

Measuring and Correcting for the Non-linearity of Microwave Radiometers: Giovanni De Amici, Phil Moffa – Northrop Grumman Space Technology

ABSTRACT: Microwave (and sometimes even IR) instruments used for Earth science observations often rely for calibration on the observation of two well-known loads at widely different brightness temperatures, usually ambient (about 300 K) and cold space (3 K at microwave frequencies), where the temperature characteristics of the scene of interest are somewhere in between these extremes. Linearity of the instrument's transfer function between the calibration points is sometimes assumed. Other times non-linearity is represented by an assumed quadratic dependency on temperature using a limited set of data collected during pre-launch testing. We investigate novel approaches to modeling the non-linear behavior of microwave receivers and ways to correct the instrument output data to enable well-characterized science data products.