Ocean Color Instrument

View images of the OCI construction

PACE's primary sensor, the Ocean Color Instrument (OCI), is a highly advanced optical spectrometer that measures properties of light over portions of the electromagnetic spectrum. It enables 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 was built at Goddard Space Flight Center (GSFC). It consists of a cross-track rotating telescope, thermal radiators, along with half-angle mirror and solar calibration mechanisms. The OCI's tilt will help avoid sun glint and single science detector design will inhibit image striping. Its signal-to-noise ratios will rival or exceed previous ocean color instruments.

The OCI features:

Cross track, 360° continuous rotating telescope
Two slit grating hyperspectral spectrographs (ultraviolet to visible & visible to near-infrared, NIR)
Fiber-coupled multiband filter spectrograph (NIR-to shortwave-infrared)

OCI overview (Meister et al., 2024) »
OCI relative spectral response (Kitchen-McKinley et al., 2023) »
OCI spectral accuracy on-orbit (Eplee et al., 2024) »
OCI lunar measurements (Lee et al., 2024) »
OCI solar diffusers (Knuble et al., 2024) »
OCI SWIR bands (Gliese et al., 2023) »

OCI Heritage

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 or CZCS (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) uses a significantly smaller electronics system developed by the iMUSTANG effort.

Visible infrared imaging radiometer suite

OCI Overview

Mass

Power

Volume

Field of Regard

Coverage

Along-track tilt

Ultraviolet-to-Near-infrared Spectral Range

SWIR Bands

Calibration

OCI is a hyperspectral imaging radiometer whose continuous coverage extends from 315 nm in the ultraviolet (UV) to 895 nm in the near infrared spectrum at 2.5 nm resolution (with bandwidths of 5 nm). At selected wavelength ranges across the chlorophyll-a fluorescence and oxygen A and B bands, the spectral steps are 1.25 nm (with bandwidth remaining at 5 nm). The prelaunch characterization of OCI focused on wavelengths above 340 nm, therefore the radiometric quality of spectral bands from 315 nm to 340 nm will be evaluated on-orbit. OCI also includes 7 discrete bands from 940 nm to 2260 nm in the shortwave infrared (SWIR) spectrum. Like SeaWiFS, OCI will perform a tilt maneuver every orbit at approximately the sub-solar point to avoid Sun glint reflected off the ocean, looking ~20° north (fore) in the northern hemisphere and ~20° south (aft) in the southern hemisphere.

The OCI telescope scans from west to east at a rotation rate of 5.77 Hz, acquiring Earth view data at a 1.2 km × 1.2 km ground sample footprint at the center of the scan (higher at the scan edges) and a ground swath width of ~2700 km. The OCI fore optics design follows that of SeaWiFS, with a rotating telescope, a half angle mirror, and a depolarizer that is transmissive (rather than reflective as with CZCS and SeaWiFS). Dichroics – material which splits visible light into distinct beams – direct the light to three different detection units: 1) a blue spectrograph (315-605 nm) with wavelength separation via grating and light detection using a CCD; 2) a red spectrograph (600-895 nm) using the same approach; and 3) SWIR detection assembly with wavelength separation using dichroics and bandpass filters, and with light detection via semiconductor devices that convert light into an electrical current, known as "photodiodes." The photodiodes use indium gallium arsenide (InGaAs) and mercury cadmium telluride (HgCdTe) as detector materials. The SWIR detection assembly guides light to individual detection units via fiber optic cables.

There are 3 on-board solar diffusers on OCI: 2 bright diffusers for radiometric gain trending, and 1 dim diffuser for linearity trending. Additionally, the Solar Pulse Calibration Assembly (SPCA) is used to monitor on-orbit changes of hysteresis in the SWIR bands. OCI will measure the lunar irradiance twice a month (for long-term radiometric gain trending), and use Fraunhofer lines and atmospheric absorption lines for spectral trending of the blue and red spectrographs.

ORCA is a prototype of an advanced ocean biology/biogeochemistry satellite sensor. The concept development began in 2000 with the eventual fabrication of a functional bench top instrument completed in 2013. The prototype development was supported by NASA/GSFC with the actual fabrication and testing conducted under two consecutive grants (three years each) funded through the NASA Earth Science Technology Office (ESTO) Instrument Incubator Program (IIP).