11th Conference on Satellite Meteorology and Oceanography

Thursday, 18 October 2001
VIIRS (Visible Infrared Imager Radiometer Suite): A Next-Generation Operational Environmental Sensor for NPOESS
Carol Welsch, U.S. Air Force and NPOESS/Integrated Program Office, Silver Spring, MD; and H. Swenson and J. M. Haas
The Department of Commerce (DOC), Department of Defense (DoD), and the National Aeronautics and Space Administration (NASA), are working together in a newly formed Integrated Program Office (IPO). The IPO is to develop and fly a new National Polar-Orbiting Operational Environmental Satellite System (NPOESS). NPOESS replaces two systems currently flying-the DOC Polar-orbiting Operational Environmental Satellite (POES) and the DoD Defense Meteorological Satellite Program (DMSP)-beginning about 2009. In this paper, we will review just one of the instruments for NPOESS, the Visible Infrared Imager Radiometer Suite (VIIRS) currently being developed by Raytheon Santa Barbara.

VIIRS replaces DOC's third generation visible and infrared sensor, the Advanced Very High Resolution Radiometer (AVHRR/3), and DoD's Operational Line Scanner (OLS), flown on POES and DMSP respectively. VIIRS represents a dramatic improvement over the heritage sensors. VIIRS 22 spectral bands may be characterized as either fine resolution or moderate resolution. Fine bands will have horizontal sampling intervals (HSIs) of about 400 to 800 meters across the swath. Moderate bands will have HSIs of twice this size. The resolution of OLS averages 0.65 km in the Hi Resolution Diode (HRD) channels, and 3.25 km in the Photomultiplier Tube (PMT) channel.

Raytheon's basic design philosophy balanced performance and cost. Judicious use of flight heritage components lowered risk and development costs. The resulting compact, efficient design was made possible principally by the availability of high performance, photovoltaic LWIR detector arrays.

The VIIRS afocal, rotating telescope design is modeled upon the successful SeaWiFS instrument. The VIIRS optical train consists of the fore optics (an off-axis afocal three-mirror anastigmat, or TMA); the aft optics (an all reflective four-mirror anastigmat imager), and the back end optics, which include microlenses for the cooled focal planes.

A three stage Cryoradiator will cool the four focal plane arrays (FPAs). The cryoradiator will provide cooling to 80 K with a heat load of almost 300 mW. VIIRS will fly an on-board calibration subsystem, which has a rich MODIS heritage.

The VIIRS will scan the earth with a cross-track field of regard of 56 degrees at a nominal altitude of 833 km. This produces a ground swath width of 3029 km. The down track swath is 11.87 km, comprised of either 16 or 32 detectors, depending whether the band is of moderate or fine resolution. VIIRS has a maximum data rate of 10.8 Megabits per second (Mbps), and a scene dependent average of 6.7 Mbps.

The Day Night Band (DNB) provides a nearly constant contrast visible image as the VIIRS scans across the earth's terminator from bright daylight into twilight into night. The DNB must accommodate a dynamic range of 45,000,000:1. The DNB provides a nearly constant ground sample distance of 742 km.

VIIRS will have a 160 Kg mass and a stowed dimension of 65x129x138 cm (nadir, velocity, and antisolar axis respectively). Instrument average power will be 134 W, with a peak power of 177 W. Fault tolerant design enables long mission life (up to 8 years storage and 7 years operation).

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