While the Point observational conventions document is undergoing final review,
I want to initiate a discussion on a complementary topic - Swath observational
conventions. This model addresses satellite observational measurements and
potentially airborne measurements.
The Swath conceptual model is essentially a grid in spacecraft coordinates. One
dimension of this grid ("along_track") follows the path of the satellite.
Normally there are one or two additional dimensions: "cross_track" and/or
"vertical". The "cross_track" dimension is perpendicular to the satellite path,
as the instrument typically makes "side views" of the surface rather than just
at the nadir. The "vertical" dimension is present when a vertical profiler
instrument is used. CF:FeatureType will need to account for each possible
combination of these 2-D and 3-D swaths.
Typically, time is explicitly stored and associated only with the along-track
dimension. Spatial resolution generally will differ in the along_track and
cross_track directions.
Orbits are not mapped to files in any consistent way: a file might correspond
to a complete orbit, a half-orbit, or some other value. However, it is common
to explicitly consider yet another dimension: "satellite_node", with values
"ascending" (crosses equator going northward) and "descending" (crosses equator
going southward).
Common satellites are in sun-synchronous polar orbits such that the ascending
node remains at a near constant Local Solar Time (LST), while the descending
node remains at a near constant LST shifted by 12 hours. For example, the
ascending node may be at 6am LST and the descending node at 6pm LST. Often
gridded data products are produced from these swaths, with separate grids
corresponding to the AM and PM cases. A new CF time representation for "LST" is
required to indicate that the global data are all at a time such as 6am LST.
Unrelated to the swath geometry, some measurements use spectral band as an
independent variable, as they sample at multiple "channels". This capability
requires a new standard name for "spectral_band" or "spectral_channel" with
values that may be numeric, a numeric range, or string.
Swath data include many new dependent variables that correspond to engineering
parameters of the retrieval rather than geophysical parameters (point spread
function is a common example). If these names are standardized at all, they
should be indicated as being of the engineering type.
In the case of an airborne (rather than satellite) measurement, there is more
commonality with the "trajectory" representation from the Point observation
model. Hence, the focus here is on spacecraft measurements.
Finally, on an unrelated note, I have semantically mapped the entire CF
Standard Name list to an ontological representation. But that is the subject of
a separate communication.
-Rob
------------------------------------
Rob Raskin
Group Supervisor, Science Data Engineering and Archiving
Instrument Software and Science Data Systems Section
Jet Propulsion Laboratory
Pasadena, CA 91109
(818) 354-4228
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