Instrument Design: Ocean
Ongoing studies continue to refine GEO-CAPE measurement and instrument requirements.
Specific instruments for GEO-CAPE have not yet been selected. At this preliminary stage,
several concepts are being studied to ensure that a range of potential instruments can meet GEO-CAPE requirements.
Coastal oceans are more dynamic in time and more heterogeneous in space than the open ocean,
therefore more demanding in terms of measurement frequency (temporal) and resolution (spatial).
The objective of differentiating the various phytoplankton functional groups also demands more spectral bands.
While existing polar-orbiting multi-band instruments provide near-daily observations of the surface ocean at ~1-km resolution,
the key science questions of the GEO-CAPE mission can only be answered through enhanced measurements
of the spectral radiance with sufficient temporal frequency and spatial resolution.
This measurement requirement [PDF] further puts strict requirements on the instrument design.
With spectral bands in the UV and shortwave infrared (SWIR) wavelengths, as well as high temporal measurements,
GEO-CAPE will significantly improve ocean color data quality in the coastal ocean region.
A broad spectral range and high spectral resolution in the UV-VIS will permit atmospherically-corrected
retrievals of spectral remote sensing reflectances from the UV to NIR. The contribution of water-leaving
radiances to the top-of-the atmosphere (TOA) is typically <10%.
This requires atmospheric correction to account for molecular scattering (Rayleigh),
gaseous absorption (ozone, water vapor, oxygen, nitrogen dioxide), and aerosol scattering and
absorption to the TOA radiances. The spectral range and resolution requirements have been
established to provide appropriate atmospheric correction to the TOA radiances.
Coastal data products will require spectral remote sensing reflectances from 350-760nm with a high signal-to-noise ratio.
An optimal spatial resolution to resolve coastal ocean geophysical features (and hence in-water constituents)
would be < 200 to 100 m (ground sample distance; GSD) for turbid waters within 10km of the shore.
Since spatial resolution represents one of the principal drivers of instrument size and mass, a compromise
must be made between resolving in-water constituents within the near shore and developing a geostationary
satellite sensor that is both reasonable in size and mass and technologically feasible.
Preliminary specifications have been drafted for a Strawman
Coastal Ecosystem Mission (CEM) Sensor [PDF],
and a broad range of instrument concepts has been identified, including 2D frame capture, very large 1D iFOV, multi-slit,
and traditional Offner concepts. The results of a preliminary (January 2010)
Coastal Ecosystem Dynamics Imager [PDF]
instrument design study show that at least one example of such a CEM sensor that could meet all GEO-CAPE requirements is feasible.
Instrument Design: Atmosphere
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