Wednesday, 17 January 2007
Simulation and test of the VIIRS sensor data record (SDR) algorithm for NPOESS
217D (Henry B. Gonzalez Convention Center)
The Visible/Infrared Imager Radiometer Suite (VIIRS) built by Raytheon Santa Barbara Remote Sensing (SBRS) will be one of the primary earth-observing remote-sensing instruments on the National Polar-Orbiting Operational Environmental Satellite System (NPOESS). It will also be installed on the NPOESS Preparatory Project (NPP). These satellite systems fly in near-circular, sun-synchronous low-earth orbits at altitudes of approximately 830 km. VIIRS has 15 bands designed to measure reflectance with wavelengths between 412 nm and 2250 nm, and an additional 7 bands measuring primarily emissive radiance between 3700nm and 11450 nm. The calibration source for the reflective bands is a solar diffuser (SD) that is illuminated once per orbit as the satellite passes from the dark side to the light side of the earth near the poles. Sunlight enters VIIRS through an opening in the front of the instrument. An attenuation screen covers the opening, but other than this there are no other optical elements between the SD and the sun. The bi-directional reflectance distribution function (BRDF) of the SD and the transmittance of the attenuation screen is measured pre-flight, and so with knowledge of the angles of incidence, the radiance of the sun can be computed and is used as a reference to produce calibrated reflectances and radiances. Emissive bands are calibrated using an on-board blackbody (BB) that has also been carefully characterized. The BB temperature is carefully controlled using heater elements and thermistors in the BB. The calibration algorithm, using knowledge of BB temperature and emissivity, predicts radiances and compares it with counts to determine gain adjustments. Because of emissive background variations caused by the half-angle scanning mirror, additional corrections must be made for this scan-angle dependent modulation. Knowledge of spacecraft ephemeris, alignment errors and instrument scan rate are used to accurately geolocate the sensor data. The combined calibrated radiances with geolocation are referred to as Sensor Data Records (SDR) in the NPOESS/NPP program. Using environmental and radiative transfer models (RTM) within the Integrated Weather Products Test Bed (IWPTB)*, simulated earth view radiances are generated, and these are input into model of the VIIRS sensor in the IWPTB, which produces simulated raw counts. The raw counts are processed through the calibration algorithm and the resultant radiances, reflectances and brightness temperatures compared against the known truth radiances from the RTM to determine the residual calibration error. By varying parameters in the sensor model, the sensitivity of sensor performance can be determined.
*Note: IWPTB is referred to as Environmental Products Verification and Remote Sensing Testbed (EVEREST) when not used in conjunction with NPOESS/NPP. EVEREST is a proprietary software tool of NGST.