Hi David,
Thanks for the question. Constant coordinate variables are hopefully not a big deal -- they can be easily specified as scalar coordinate variables, as noted in the example here:
http://cfconventions.org/Data/cf-convetions/cf-conventions-1.7/build/cf-conventions.html#scalar-coordinate-variables.
That's an example for analysis time and pressure level, but it's OK to use coordinate variables for any critical reference variable. (Chapter 4: "Coordinate types other than latitude, longitude, vertical, and time are allowed." Incidentally, the text in reference [1] was proposed as a replacement for that sentence in
http://kitt.llnl.gov/trac/ticket/100, which has been accepted but not implemented.)
I had to search the archives to fully understand the motivation, here's what I found (ooh, FAQ question!):
(1) To locate the data in an axis other than space or time. [1]
(2) To provide a consistent way to specify the value of a certain other parameter, or even multiple parameters; if the parameter is unvarying, it can be specified as a scalar. [2]
So your declaration that both the radiation wavelength and scatter angle were essential led to my suggestion. Adopting it standardizes the method for citing the needed information (consistent with other standard names, and across users of this standard name), thereby maximizing interoperability.
By all means reply further if this seems problematic, I'm at the edge of my experience but others can jump in.
John
[1] On Apr 11, 2013, at 12:45, Jonathan Gregory <j.m.gregory at reading.ac.uk> wrote:
> The commonest use of coordinate variables is to locate the data in space and
> time, but coordinates may be provided for any other continuous geophysical
> quantity (e.g. density, temperature, radiation wavelength, zenith angle of
> radiance, sea surface wave frequency) or discrete category (see Section 4.5,
> "Discrete axis", e.g. area type, model level number, ensemble member number)
> on which the data variable depends.
[2] On Dec 24, 2010, at 13:26, Jonathan Gregory <j.m.gregory at reading.ac.uk> wrote:
> We quite often recommend, for instance in connection with particular standard names,
> that the value of a certain other parameter could be specified, e.g. a
> radiation_wavelength for radiative quantities. Scalar coord vars are a neat
> way to do this. They are something between multivalued coord vars and
> attributes in terms of function: easier than coord vars, and more powerful
> than attributes because they can themselves have attributes.
[3]
On Jul 23, 2014, at 17:39, Moroni, David F (398M) <David.F.Moroni at jpl.nasa.gov> wrote:
> Hi John (G),
>
> John (N) and myself are working together on this effort.
>
> Thanks for your inputs here.
>
> In general, I agree with the first part of your revision of the description, but I don't suggest including a reference to the coordinate values for the other standard names as you've suggested, namely "radiation_wavelength" and "scattering_angle", simply because this type of measurement assumes: 1) constant wavelength and 2) constant scattering angle. I simply don't follow your reasoning for why such coordinate values would be needed given the nature of these values being constant. Can you provide some rationale as to why we would want to list these as coordinate values?
>
> Cheers,
> David
>
> ==================================================
> David Moroni
> Ocean Wind and Scatterometry Data Engineer
> Physical Oceanography Distributed Active Archive Center
> Jet Propulsion Laboratory
> 4800 Oak Grove Dr
> M/S 158-242
> Pasadena, CA 91109
> Phone: 818.354.2038
> Fax: 818.353.2718
> ==================================================
>
> From: John Graybeal <john.graybeal at marinexplore.com>
> Date: Wednesday, July 23, 2014 3:52 PM
> To: "Niedfeldt, John C (398M-Affiliate)" <John.C.Niedfeldt at jpl.nasa.gov>
> Cc: CF Metadata List <cf-metadata at cgd.ucar.edu>, David F Moroni <David.F.Moroni at jpl.nasa.gov>
> Subject: Re: [CF-metadata] normalized_radar_backscatter_coefficient
>
>> Hi John (N),
>>
>> Thanks for the detailed explanation. I am convinced the new term is distinct. I did not expect to redefine the old term unless one was clearly a refinement of the other, which is not the case.
>>
>> I still am concerned about the description I think you are proposing for this term ("normalized_radar_backscatter_coefficient is also called the normalized radar cross section, which are commonly used in the microwave remote sensing community.") Based on your inputs, here is a second attempt:
>>
>>> The fraction of incident power at a given wavelength that reaches a receiver, after reflection by a surface at a given reflection angle. (In microwave remote sensing this is also known as the 'normalized radar cross section' or 'sigma naught'.) Coordinate values for radiation wavelength and reflection angle should be given the standard names radiation_wavelength and scattering_angle.
>>
>> This description reflects: always 1 wavelength; always 1 backward scattering angle; and that all lost power is included in the coefficient, not simply the surface reflectance/absorption.
>>
>> If I correctly got your 3 points, the term "attenuated" applies, as it is used elsewhere in CF: "'The attenuated backwards scattering function includes the effects of two-way attenuation by the medium between a radar source and receiver." Since I can't imagine needing an unattenuated backscatter coefficient, the extra word seems unneeded for this name.
>>
>> John (G)
>>
>> On Jul 23, 2014, at 10:37, Niedfeldt, John C (398M-Affiliate) <John.C.Niedfeldt at jpl.nasa.gov> wrote:
>>
>>> Dear John et al,
>>> Here are three major distinctions between the surface_backwards_scattering_coefficient_of_radar_wave (old) and normalized_radar_backscatter_coefficient (new):
>>> ? The (old) definition has the default definition as being an integral over all wavelengths, but the radar backscatter coefficient is always measured using one wavelength (new) and must always be specified.
>>> ? The part about scattering radiation having no loss in energy in the (old) definition is not clear, but in practice and theory energy is always lost once the initial wave is transmitted (indeed, it is in part the loss due to the ground that we are measuring)(new).
>>> ? The backscatter in the (old) definition refers to summing all backwards scattering angles, where in remote sensing we look at just one backscatter angle (new).
>>> And yes, if the old variable pertains to the normalized radar cross section--which I believe it does not--then the transmitted wavelength and backscatter angle (elevation angle) should be required as they are essential to understanding the product and being able to correlate and verify data.
>>>
>>> So yes, we could change the old definition to meet the new needs, but it would require a change in base assumptions that would render any current data using that standard name as invalid. For these reasons and more, I believe we should make a new standard name. The definition I have provided is accurate and once approved additional attributes and values can be made required to suit all needs for those dealing with the normalized radar backscatter coefficient.
>>>
>>> Sincerely,
>>> John
>>>
>>> --
>>> John Niedfeldt
>>> Data Engineering
>>> PO.DAAC, JPL
>>>
>>> From: John Graybeal <john.graybeal at marinexplore.com>
>>> Date: Tuesday, July 22, 2014 at 3:53 PM
>>> To: JPL <John.C.Niedfeldt at jpl.nasa.gov>
>>> Cc: CF Metadata List <cf-metadata at cgd.ucar.edu>, "Moroni, David F (398M)" <David.F.Moroni at jpl.nasa.gov>
>>> Subject: Re: [CF-metadata] normalized_radar_backscatter_coefficient
>>>
>>> John, I think we (I, anyway) were waiting for a little more clarification as to what was needed. Sorry for that delay. I like the name itself, makes sense to me.
>>>
>>> Unless I am mistaken, from your email I infer that the meaning of this is a narrow case of surface_backwards_scattering_coefficient_of_radar_wave. That description is:
>>>> The scattering/absorption/attenuation coefficient is assumed to be an integral over all wavelengths, unless a coordinate of radiation_wavelength is included to specify the wavelength. Scattering of radiation is its deflection from its incident path without loss of energy. Backwards scattering refers to the sum of scattering into all backward angles i.e. scattering_angle exceeding pi/2 radians. Ascattering_angle should not be specified with this quantity.
>>>
>>> I can't tell from the description if this item is different, so the description could use a little bit more meat to tease that out. Looking at your thread, I see this:
>>>
>>>> we are calculating sigma_naught which is the fraction of incident power that is reflected by the surface. It is also very important in scatterometry to record the angle of incidence as the sigma_naught changes based on the incidence angle in addition to various other parameters which are essential to being able to correlate data from various scatterometers.
>>>
>>> So does it work for the description could say something like the following? This is still similar to the other standard name, so if there are specific things that make the distinction clear that would be important to add. ("This differs from surface_backwards_...")
>>>
>>>> The fraction of incident power that is reflected by the surface. (In microwave remote sensing this is also known as the 'normalized radar cross section' or 'sigma naught', when produced from one angle of incidence and from one wavelength.) Scattering of radiation is its deflection from its incident path without loss of energy. Backwards scattering refers to the sum of scattering into all backward angles i.e. scattering_angle exceeding pi/2 radians. A scattering_angle should not be specified with this quantity. Coordinates of radiation_wavelength and angle_of_incidence are used to specify those baseline parameters.
>>>
>>> I'm not sure about the last part -- if they are always needed these variables should be required.
>>>
>>> John
>>>
>>>
>>>
>>> On Jul 22, 2014, at 14:49, Niedfeldt, John C (398M-Affiliate) <John.C.Niedfeldt at jpl.nasa.gov> wrote:
>>>
>>>> Dear all,
>>>> Hello again! About two months back I sent in a request, which is referenced below, in which I requested that we add the ?normalized_radar_backscatter_coefficient? in CF. Having heard nothing to the contrary, and seeing as no other standards name match our needs, we at PO.DAAC will be moving forward in implementing this new standard name.
>>>>
>>>> As our newly reprocessed netCDF datasets shall soon serve as an online replacement for existing datasets already in use by hundreds of interdisciplinary scatterometry data users, we hope to likewise hear back from you soon as to whether there is consensus on our proposed standard name. If there is anything further we can do to build community consensus on our proposed standard name, please let me know.
>>>>
>>>> Sincerely,
>>>> John
>>>>
>>>> --
>>>> John Niedfeldt
>>>> Data Engineering
>>>> PO.DAAC, JPL
>>>>
>>>> From: Lauret Olivier <olauret at cls.fr>
>>>> Date: Friday, May 30, 2014 at 8:11 AM
>>>> To: "cf-metadata at cgd.ucar.edu" <cf-metadata at cgd.ucar.edu>
>>>> Cc: JPL <John.C.Niedfeldt at jpl.nasa.gov>
>>>> Subject: TR: normalized_radar_backscatter_coefficient
>>>>
>>>> Hi all,
>>>>
>>>> Just forwarding you some discussion we have with J. Niedfeldt about some standard name for sigma naught variable. I thought the available ?surface_backwards_scattering_coefficient_of_radar_wave? could be used considering some changes in the definition [in short I wish we could mix the description of sigma naught from radar altimetry with the one from scatterometers]. But it seems that the quantities are different enough to introduce a new standard name (see the message below).
>>>>
>>>> Can we introduce ?normalized_radar_backscatter_coefficient? in CF?
>>>>
>>>> Thanks,
>>>>
>>>> Olivier
>>>>
>>>> De : Niedfeldt, John C (398M-Affiliate) [mailto:John.C.Niedfeldt at jpl.nasa.gov]
>>>> Envoy? : jeudi 29 mai 2014 21:26
>>>> ? : Lauret Olivier
>>>> Objet : normalized_radar_backscatter_coefficient
>>>>
>>>> Dear Mr. Olivier,
>>>> After discussion with Dr. David Long of BYU and reviewing the current definition with him it was determined that we do in fact need a new variable. In microwave remote sensing the normalized radar cross section, sigma naught, is always produced from one angle of incidence and from one wavelength. I understand the desire to consolidate the number of standard names and to not have duplication, but adding this standard name would reduce confusion and error for many I believe. It is also general enough that we can add attributes to it in the future to allow further specification for various endeavors. If you have any more questions feel free to contact me and thank you again for your assistance. We changed the standard_name to be more descriptive.
>>>>
>>>> standard_name:
>>>> normalized_radar_backscatter_coefficient
>>>>
>>>> Definition:
>>>> normalized_radar_backscatter_coefficient is also called the normalized radar cross section, which are commonly used in the microwave remote sensing community.
>>>>
>>>> Canonical Units:
>>>> 1
>>>>
>>>>
>>>> Sincerely,
>>>> John Niedfeldt
>>>>
>>>>
>>>> Cliquez ici si ce message est ind?sirable (pourriel).
>>>>
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>>>
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>>
Received on Wed Jul 23 2014 - 22:32:59 BST
