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Ocean Color Instrument
PACE's primary sensor, the Ocean Color Instrument (OCI), is a highly advanced optical spectrometer that will be used to measure properties of light over portions of the electromagnetic spectrum. It will enable continuous measurement of light at finer wavelength resolution than previous NASA satellite sensors, extending key system ocean color data records for climate studies.

The color of the ocean is determined by the interaction of sunlight with substances or particles present in seawater such as chlorophyll, a green pigment found in most phytoplankton species. By monitoring global phytoplankton distribution and abundance with unprecedented detail, the OCI will help us to better understand the complex systems that drive ocean ecology.

The OCI design is based on a long heritage of NASA technology development and flight programs. Its functionality – rotating telescope (mechanism and timing), charged couple device (CCD) detector, optics – benefits from previous technology development efforts such as Ocean Radiometer for Carbon Assessment (ORCA).

The OCI's operational concept, cross-track rotating telescope / half-angle mirror, system timing, and data processing infrastructure have been successfully used on previous and existing flight missions such as the Coastal Zone Color Scanner (1978 to 1986), Sea-Viewing Wide Field-of-View Sensor or SeaWiFS (1997 to 2010), Suomi National Polar-orbiting Partnership (Visible Infrared Imaging Radiometer Suite or VIIRS), Aqua and Terra (Moderate Resolution Imaging Spectroradiometer or MODIS instrument). In addition, the OCI's avionics (communications, positioning) will use a significantly smaller electronics system developed by the iMUSTANG effort.

Instrument Overview
With all of its features, the OCI will be the first ocean color instrument to include all of the following:
  • 2-day global coverage at 1-km (0.6-mi) resolution
  • Hyperspectral radiometry from the ultraviolet (350 nm) to near-infrared (885 nm)
  • Shortwave infrared bands (940, 1038, 1250, 1378, 1615, 2130, 2260 nm)
  • Single science detector to inhibit image striping
  • Signal-to-noise ratios that rival or exceed previous instruments
  • Total calibration of instrument artifacts < 0.5% at top-of-atmosphere
  • Fore / aft tilt to avoid sun glint
  • Semi-monthly lunar calibration and on-board solar diffusor mechanisms

The OCI is being built at Goddard Space Flight Center with an expected height of about 1 m (3.3 ft) and mass of 205 kg (452 lb), requiring 260 Watts of power. It will consist of a telescope rotating mechanism; thermal radiators; solar calibration assembly; tilt mechanism; Blue/Red Focal Plane Array spanning from ultraviolet to near-infrared wavelengths; various electronics boxes; and a shortwave infrared focal plane array. View OCI Webinar »

How the OCI Will Work
It will measure the spectral radiance exiting the top of the atmosphere. Of the total amount of radiance seen by the OCI, only a small portion is actually coming from the ocean; by far the dominant portion comes from the atmosphere, and this "noise" effectively hides the desired signal. To retrieve the portion of the signal exiting the water, scientists and programmers apply atmospheric correction algorithms that remove the radiance contribution from the atmosphere; what remains is the small portion passing through the ocean surface—the component of interest for ocean color measurements. That radiance is then converted to spectral remote-sensing reflectances, which are essentially the ratio of the light coming from the ocean normalized to the light from the sun entering the ocean. Once these reflectances are known, then bio-optical algorithms are used to produce estimates of geophysical and optical properties, such as the near-surface concentration of the phytoplankton pigment chlorophyll-a and spectral marine inherent optical properties (IOPs). Read the full article »