Dear John (G.) and Aleksandar,
Thank you both for your comments. I think this discussion has raised a number of issues that need to be resolved.
John G. writes:
>
> > "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. "
>
> I think we have all agreed these words do not apply
>
Yes, indeed! I copied and pasted this from the definition of an existing backscattering name, but as you rightly say, it has already been stated that it doesn't apply to this name, so I will take them out again. Thanks for pointing this out.
It would be helpful if we could take this opportunity to clarify the meaning of the terms 'backscattering' versus 'backward_scattering' in the existing names, as the two look very similar and the current definitions don't make any distinction between them.
We have four existing backwards_scattering names:
surface_backwards_scattering_coefficient_of_radar_wave
volume_attenuated_backwards_scattering_function_in_air
volume_attenuated_backwards_scattering_function_in_air_assuming_no_aerosol_or_cloud
volume_backwards_scattering_coefficient_of_radiative_flux_in_sea_water
all of which have definitions stating "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."
There are two existing backscattering names:
backscattering_ratio
histogram_of_backscattering_ratio_over_height_above_reference_ellipsoid
whose definitions also say "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." The ratio definitions then go on to say " "Backscattering ratio" is the ratio of the quantity with standard name volume_attenuated_backwards_scattering_function_in_air to the quantity with standard name volume_attenuated_backwards_scattering_function_in_air_assuming_no_aerosol_or_cloud."
In the definitions of the existing names it would appear that 'backscattering' is simply being used as short-hand for a ratio of two 'backwards_scattering' quantities and, looking back in the archives to 2009 when those names were proposed, I think that was all that was intended at the time (see
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2009/053016.html). However, in the current proposal it seems that we are now making a different, more specific, definition of 'backscattering' in which the particular scattering angle becomes important. I think we can cope with this by first making the two existing backscattering names into aliases as follows:
backscattering_ratio becomes backwards_scattering_ratio;
histogram_of_backscattering_ratio_over_height_above_reference_ellipsoid becomes
histogram_of_backwards_scattering_ratio_over_height_above_reference_ellipsoid.
Their definitions would be also need to be slightly amended by replacing 'backscattering' with 'backwards_scattering'. I think it would be useful to make this change in any case, regardless of what we decide for the current proposal.
This would then leave us free to define 'backscatter' as a different quantity, distinct from 'backwards_scattering'. It could be argued that, even with a clear definition, 'backscatter' sounds too similar to 'backwards_scattering' and that we should adopt one of the alternative terms (sigma_naught or normalized_radar_cross_section) for the proposed name. However, neither of those seems particularly self explanatory, so I would vote to stick with the name as currently proposed: surface_normalized_radar_backscatter_coefficient.
It is clear from the discussion that the incident and scattering angles are all-important to interpreting the quantity correctly. Aleksandar writes:
>
> I am not sure that incident angles can always be a single value. That
> depends on the scatterometer, I think. Also, another angle is needed to
> specify the position of the instrument to the scattering surface and the
> standard name to use for those values is platform_azimuth_angle.
>
I think a diagram would be useful here to help ensure that we are all talking about the same thing. I found this one:
http://earth.esa.int/ers/eeo4.10075/scatt_design.html. In this particular example there are three radar beams on a single instrument - is this why you say that incident angles can't always be a single value? in this case there could be three separate scalar coordinate variables, with different variable names but all with the standard_name of angle_of_incidence, and each could contain the angle of one of the beams. Would that work? We haven't done it before, but I think the definition of the name could even contain a link to a diagram like the one here to help make the meaning clear. We can also say that platform_azimuth_angle should be included as a scalar coordinate variable.
Aleksander writes:
>
> In this case only the signal scattered right back
> toward the instrument is measured. This is the situation where the
> scattering angle is 180 degrees.
>
So if I understand correctly, the scatterometer is transmitting towards the surface at (say) three different angles of incidence and then measuring the signal scattered directly back along each of the paths. Is that right? In that case we should define the term 'backscatter' to mean, strictly, scattering at an angle of 180 degrees relative to the angle of incidence. Or we could generalise a bit and say that it is scattering at a specified angle exceeding pi/2 radians and require that the exact value be provided in a scalar coordinate variable (with a new standard name of scattering_angle). Which approach would be most useful?
I'd welcome comments on any of the above.
Best wishes,
Alison
------
Alison Pamment Tel: +44 1235 778065
NCAS/British Atmospheric Data Centre Email: alison.pamment at stfc.ac.uk
STFC Rutherford Appleton Laboratory
R25, 2.22
Harwell Oxford, Didcot, OX11 0QX, U.K.
> -----Original Message-----
> From: John Graybeal [mailto:john.graybeal at marinexplore.com]
> Sent: 25 July 2014 19:12
> To: CF Metadata List
> Subject: Re: [CF-metadata] normalized_radar_backscatter_coefficient
>
> Alison, in your suggested rephrasing, some things appear that you seemed
> to previously reject:
>
> > "It is the fraction of incident power that is reflected by the surface."
>
> As I understood it, in this case it is the fraction of incident power that is
> reflected by the surface AND not scattered by intervening deflections from
> its incident path, PLUS the fraction of incident power that is deflected back
> to the receiver along the measured angle (but not by the surface).
>
> > "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. "
>
> I think we have all agreed these words do not apply, and you ask:
>
> > However, if it is just plain wrong then perhaps we should change it to read
> simpy "scattering of radiation is its deflection from its incident path" in the
> definitions of all scattering names. Do others agree?
>
> My word intuition says "backscattering" is, like our "backwards_scattering",
> the deflection into all angles < 180 degrees. If this community uses the term
> to mean reflection along a particular path, this seems to me strictly a
> reflection -- though I expect some refer to it as something like
> backwards_scattering_defined_by_angle, i.e., a narrowing of the term.
>
> Backscattering seems a specific case of scattering. Surface backscattering
> seems a specific case of backscattering, and has the two variants
> with/without attenuation. My impression is that some of the existing terms
> count on the distinctions.
>
> Aleksandar, you said
>
> > Also, another angle is needed to specify the position of the instrument to
> the scattering surface and the standard name to use for those values is
> platform_azimuth_angle.
>
>
> Is this strictly true in every case? Is it always one position (not separate
> sending/receiving platforms)? Assuming no, and no: Would it work to say
> "The standard_name platform_azimuth_angle may be used to specify the
> orientation of the sending and/or receiving instrument relative to the
> scattering surface." ? Makes it less mandatory and more general.
>
> John
>
>
>
> On Jul 25, 2014, at 05:15, alison.pamment at stfc.ac.uk wrote:
>
> > Dear John N, John G, David and Jonathan,
> >
> > It is clear from the discussion so far that this is a surface quantity, so in fact
> we should include that in the name:
> >
> > surface_normalized_radar_backscatter_coefficient
> >
> > canonical units: 1
> >
> > I agree that scalar coordinate variables would be needed for the
> radiation_wavelength and angle_of_incidence. A similar approach is taken
> for many existing standard names. Unless the values of those parameters
> are always the same for all scatterometer measurements, then they need to
> be included with the data. If they are truly fixed values then they need to
> be specified in the definition, but from the discussion so far I think that is
> not the case.
> >
> > It sounds as though it is not appropriate to say "Scattering of radiation is its
> deflection from its incident path without loss of energy", as in the existing
> definition for surface_backwards_scattering_coefficient_of_radar_wave,
> since John N says this is factually incorrect. I'm not sure of the origin of the
> sentence, but suspect that it may have crept into the definition of scattering
> standard names to differentiate from absorption/attenuation/quantities.
> However, if it is just plain wrong then perhaps we should change it to read
> simpy "scattering of radiation is its deflection from its incident path" in the
> definitions of all scattering names. Do others agree?
> >
> > We need to keep the wording of the definition as consistent as possible
> with other similar names, so I suggest some rephrasing of what has already
> been proposed:
> >
> > "The quantity called surface_normalized_radar_backscatter_coefficient is
> also commonly called the "normalized radar cross section" or "sigma
> naught" in the microwave remote sensing community. It is the fraction of
> incident power that is reflected by the surface. The
> surface_normalized_radar_backscatter_coefficient is measured at a single
> wavelength and at a single angle of incidence, which should be specified
> using scalar coordinate variables with standard names, respectively, of
> radiation_wavelength and angle_of_incidence. Scattering of radiation is its
> deflection from its incident path. 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. The
> surface_normalized_radar_backscatter_coefficient differs from the quantity
> surface_backwards_scattering_coefficient_of_radar_wave in that the latter
> does not require a radiation_wavelength and angle_o
> f_
> > incidence to be specified."
> >
> > Does that sound OK?
> >
> > Best wishes,
> > Alison
> >
> > ------
> > Alison Pamment Tel: +44 1235 778065
> > NCAS/British Atmospheric Data Centre Email:
> alison.pamment at stfc.ac.uk
> > STFC Rutherford Appleton Laboratory
> > R25, 2.22
> > Harwell Oxford, Didcot, OX11 0QX, U.K.
> >
> >
> >> -----Original Message-----
> >> From: Jonathan Gregory [mailto:j.m.gregory at reading.ac.uk]
> >> Sent: 24 July 2014 15:43
> >> To: cf-metadata at cgd.ucar.edu
> >> Subject: Re: [CF-metadata] normalized_radar_backscatter_coefficient
> >>
> >> Dear David and John
> >>
> >>> 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.
> >>
> >> I agree that if these parameters are essential for interpretation of the
> data,
> >> and if they don't have very commonly applied standard values (which
> could
> >> be
> >> included in the definition as defaults), then they should be specified as
> >> size-one or scalar coordinate variables.
> >>
> >> Best wishes
> >>
> >> Jonathan
> >> _______________________________________________
> >> CF-metadata mailing list
> >> CF-metadata at cgd.ucar.edu
> >> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
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