Aleksander and other interested parties.
I am working with Randy Horne on the GOES-R program and it seems that
some good progress has been made to
tackle the thorny issue of how to represent what is the fundamental
product of remote sensing observations, i.e, the
measurement of top of atmosphere radiance incident at the sensor. We
have struggled with the same issue concerning
radiance - that is, does the difference between radiance measured as a
function of wavelength or frequency represent
physically distinct quantities representing unique standard_names or are
these fundamentally equivalent quantities
represented by a simple units conversion and therefore can be
represented by a common standard_name. The remote
sensing community is used to dealing with the conversion between these
quantities as different programs have chosen
to adopt different units. For my part I am used to radiances as
function of wavelength for the solar reflectance bands and
as a function of wavenumber for the emissive bands. For those reading
this who may no be aware, the conversion between
these quantities is a function of the wavelength-squared (or
wavenumber-squared). What is new over the
last few weeks is the general acceptance that wavelength, or wavenumber
(or some other physical quantity) can be adopted
as a coordinate variable. Therefore the radiance conversion in question
is a function of the coordinate variable itself. I think
was is needed to close this issue is to determine if this type of
functionality constitutes a unique quantity and therefore a
unique standard name. I think we are pretty close to achieving this.
Also whatever is chosen for the standard_name, the corresponding units
often will differ from the actual units used for the
product (i.e., mW vs W, cm vs m, etc) so the specific choice of units is
somewhat arbitrary - though for the sake clarity
should adopt values that are if not universal at least wide-spread
throughout the community. The quantity itself will always
be described by the product specific units attribute for all applications.
With that introduction, you have proposed three unique standard_names
corresponding to radiance in units of wavelength, wavenumber,
and frequency. I am most interested in consensus rather than the choice
of 1 versus 3 unique standard names, mostly because I believe
what is written in the units field is what really matters to the
applications. If 3 names are adopted, then applications that are
ignorant about the data will probably need to look for one of those
three standard_names in the file - and may still need to convert from
one set of units to
another anyway depending on the application. If one name is adopted, we
will need to think about if there are any implications for the
coordinate variable being in one set of units versus another (i.e.,
hyperspectral data cubes in the infrared often adopt frequency). We would
want the convention to be general enough that it is not tied to one
coordinate variable versus another. (I may just need to do more research
on CF on this point.) Again depending on the application, a conversion
between actual units and needed units may be required.
In terms of the standard_names options you have proposed, I personally
prefer:
toa_outgoing_spectral_radiance_per_unit_wavelength
units: mW m-2 sr-1 um-1
toa_outgoing_spectral_radiance_per_unit_wavenumber
units: mW m-2 sr-1 (cm-1)-1
toa_outgoing_spectral_radiance_per_unit_frequency
units: W m−2 sr−1 Hz−1
though I wonder why you choose mW for two and W for the other. (This is
a secondary issue - not really of central importance.)
So for those who have gotten this far, in this response, I hope we can
reach consensus on the debate of 1 versus 3 standard names for
the different flavors of toa_spectral_radiance. I think it depends on
the central issue that if the quantity is a function of the units of the
coordinate variable, then does a change in the coordinate variable units
imply a unique physical quantity and therefore a unique
standard_name.
thanks for your attention,
Ted Kennelly
Aleksandar Jelenak - NOAA Affiliate wrote:
Dear all,
I proposed last November a standard name:
toa_outgoing_spectral_radiance
Canonical units: mW m-2 sr-1 (cm-1)-1
Definition:
"toa" means top of atmosphere; "outgoing" means toward outer space;
"spectral" means per unit wavenumber or as a function of wavenumber.
Radiance is the radiant power per unit area in a particular direction
per unit of solid angle.
***
Spectral radiance can also be expressed as a function of either
wavelength or frequency so the problem here is that my proposal would
prevent using these two alternative forms of an invaluable piece of
standard name real estate for satellite remote sensing data. One way
to solve it is to allow more than one canonical units per standard
name where is justified, or have more than one standard name for the
same physical quantity.
If having more than one canonical units is deemed too much of a
radical change, how about these standard names:
toa_outgoing_spectral_radiance_per_unit_wavelength
units: mW m-2 sr-1 um-1
toa_outgoing_spectral_radiance_per_unit_ wavenumber
units: mW m-2 sr-1 (cm-1)-1
toa_outgoing_spectral_radiance_per_unit_frequency
units: W m−2 sr−1 Hz−1
or (I could not find any example to copy from):
toa_outgoing_spectral_radiance_function_of_wavelength
units: mW m-2 sr-1 um-1
toa_outgoing_spectral_radiance_function_of_ wavenumber
units: mW m-2 sr-1 (cm-1)-1
toa_outgoing_spectral_radiance_function_of_frequency
units: W m−2 sr−1 Hz−1
Any suggestion how to proceed will be welcome.
-Aleksandar
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