Raytheon's Next-Generation Sensor for Weather/Climate Forecasting VIIRS
Raytheon's Visible/Infrared Imager Radiometer Suite
VIIRS is the next-generation imaging spectroradiometer for the National Oceanic and Atmospheric Administration's (NOAA) Joint Polar Satellite System (JPSS) that emerged from cancellation of the National Polar-orbiting Operational Environmental Satellite System (NPOESS). The first VIIRS flight unit was delivered in March 2010 and was launched onboard the NPOESS Preparatory Project (NPP) satellite on Oct. 28, 2011. The second and third flight units are being built now.
Capabilities and Applications
VIIRS' characteristics are listed in Figure 1. The sensor provides highly accurate measurements of light radiated by the Earth at visible through infrared wavelengths. Incorporating a flexible design architecture, VIIRS can be adapted to future mission needs for the next 20 to 30 years. It replaces and improves upon three different sensors operating today:
- The MODerate-resolution Imaging Spectroradiometer (MODIS), the Raytheon-built keystone of NASA's Earth Observing System, in flight since 1999.
- The Advanced Very High Resolution Radiometer (AVHRR), operating onboard the NOAA Polar Operational Environmental satellites and the European Meteorological Operational (MetOp) satellites since 1978.
- The Operational Line Scanner (OLS), operating onboard Defense Meteorological Satellite Program satellites since 1976.
VIIRS instruments are scheduled to fly in two 833-km polar sun synchronous orbits: one that passes over all locations on Earth at approximately 1:30 a.m. and 1:30 p.m. local time each day, and another that passes over at 5:30 a.m. and 5:30 p.m. Each VIIRS instrument covers the entire Earth twice a day with data at visible and infrared wavelengths (0.4–12 µm). It provides well calibrated moderate (~km) spatial resolution measurements of light upwelling from Earth in support of a large number of high-priority applications, ranging from weather prediction for civilian and military needs to climate change monitoring, land usage, public health alerts and predictions of electrical power usage.
VIIRS offers significant improvements over the systems it replaces by providing:
- Twenty-two spectral bands with four times better spectral coverage than AVHRR, thereby enabling new agricultural, climate, disaster monitoring, public health and weather data products.
- Three times better spatial resolution than AVHRR and MODIS at end-of–scan, enabling sharper imagery over a much larger area.
- A fully calibrated day/night band that improves nighttime weather forecasting and military applications compared with OLS.
VIIRS has benefited from substantial U.S. research and development investment in MODIS and other NASA Earth observing systems that led to a wide range of new environmental data products for operational use, including size of aerosol particles suspended in the atmosphere and biological productivity in the ocean.
System Description and Operation
A key to the VIIRS architecture is a rotating telescope assembly that provides the flexibility needed to meet a diverse set of requirements for multispectral imaging spectroradiometry and low light level day/night imaging. Advantages of the rotating telescope design relative to scan mirror based systems like AVHRR and MODIS include:
- Better control of stray light.
- Smaller range in angle of incidence of light on the fore optics to reduce image distortion.
- Immunity to image rotation seen in AVHRR as the scan moves out from nadir.
- Better protection from contamination and degradation over time because all of the optical elements are placed deep inside the instrument housing.
Design, fabrication and testing of the rotating telescope assembly responded successfully to challenging stray light and instrument background requirements to produce a well understood high-performance subsystem. The result is an imager that is ready to provide high fidelity data for the international science, weather and other environmental data product communities with much better spatial resolution at end-of-scan than AVHRR and MODIS.
The rotating telescope assembly is followed by a fixed telescope along with other back-end optics that image the scene and separate light into three focal planes with filters that define each spectral band. A cryoradiator radiates heat from the infrared detector arrays to deep space to maintain a stable detector operating temperature as low as 78 Kelvin. The focal plane interface electronics carry signals from the detector arrays to the externally mounted Electronics Module. The EM synchronizes the rotating telescope assembly with a rotating flat mirror, making it possible to image the scene onto the detector arrays without image rotation. The EM also provides onboard processing of detector samples to enable a nearly constant pixel size across the entire scan, data compression, processing of operational and housekeeping data, and formatting of these data into the consultative committee on space data systems (CCSDS) format. The EM also communicates via a 1394a data bus with the spacecraft, to provide VIIRS operational data and telemetry, and to receive commands, spacecraft telemetry and software uploads. A fault tolerant design enables long mission life.
VIIRS has an on-board calibration subsystem consisting of a carefully stabilized blackbody source to provide a reference signal for emissive infrared bands and a diffuser to provide a reference for bands dominated by reflected sunlight. VIIRS includes a monitor to detect changes in the optical characteristics of the solar diffuser over time. The VIIRS calibration subsystem has a rich MODIS heritage — a key to maintaining continuity with data from the MODIS instruments onboard the Terra and Aqua satellites.
Scientists and weather forecasters have been preparing to use VIIRS data for years. A group at the Naval Research Laboratory in Monterey, Calif., has created a tool called NextSat that simulates VIIRS data and the near real time creation of environmental data products by combining measurements from MODIS, AVHRR, OLS and the U.S. geosynchronous orbit environmental monitoring system with new processing and display methods. Figure 2 shows an example of NextSat output that illustrates the intricate detail expected from VIIRS data and the usefulness of this wealth of information for distinguishing artificial phenomena such as contrails from naturally occurring clouds.
VIIRS will continue to improve upon the record of environmental data used by scientists to measure climate change. It is the primary instrument for 22 of 38 environmental data records to be collected by the JPSS for weather forecasting, sea surface temperature, ocean color, land use, biomass fires, aerosols and cloud top properties. The enhanced imagery provided by VIIRS will expand the environmental data record and improve weather forecasting and climate monitoring for generations.