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Morning Jim,
Thank you for your prompt and constructive comments.
In simple terms, the sub-satellite point is a reference longitude and latitude
point where the data is measured from. It may not be the actual longitude and
latitude of the satellite. Internally at EUMETSAT, we call this the
rectification point. I think its needed but will check if an update can be made
to clarify this.
The discussions we (internally, with our Met services, with our international
partners and users) had before the submission of the proposal suggested we make
the standard names satellite specific as it reduces ambiguity. For example the
platform_altitude for aircraft is different to a satellite. From Mike Grant's
feedback, we will look if the word platform should be replaced by satellite.
A side comments is I'm not a meteorologist or a scientist but a software
engineer, and whilst compiling these proposals, I endured many discussion on
what 'things' mean so being specific removes ambiguity. Nevertheless, I will
take your comments onboard check with the relevant forces here and see if
updates can be made.
Regards,
Pete.
------------------------------
Message: 3
Date: Tue, 19 Feb 2013 13:35:55 -0500
From: Jim Biard <jim.biard@xxxxxxxx>
To: cf-satellite@xxxxxxxxxxxxxxxx
Subject: Re: [cf-satellite] EUMETSAT proposed CF convention updates to
the Standard Names and Units for Satellite Data.
Message-ID: <30695AF9-D6D6-43D8-B08F-2BD412550726@xxxxxxxx>
Content-Type: text/plain; charset="windows-1252"
A few comments/questions regarding the platform latitude, longitude, and
altitude:
Sub-satellite point means the intersection of a line from the center of the
Earth to the satellite with the surface of the Earth.
Is referring to the sub-satellite point necessary? At first glance, I think it
isn't needed.
I don't see a need to make any of these names satellite-specific.
Use the word height instead of altitude in the definition for platform_altitude
to avoid recursion (the altitude is the altitude).
Latitude, longitude, and altitude should be considered in relation to the
coordinate system defined for the file. I think it would be best to remove the
reference to mean sea level from the definition of the altitude. It is the
height above the coordinate system vertical datum, which might not be sea
level. (In particular, if geometric heights on an ellipsoid are used, there
can be significant differences. And, in fact, WGS84 and the WGS84 ellipsoid
are often used in satellite work.)
And while I'm at it, would it also be useful to include platform_x, platform_y,
and platform_z in the set? These would be useful in the case where the
coordinates are in Earth-centered Cartesian coordinates (ECF, ECI), as well as
cases where a projected coordinate system was used. The projected coordinate
case would clearly be for a subset of an orbit, but could still be completely
valid. This would also be useful for other moving platform cases (aircraft,
ships, etc).
Grace and peace,
Jim
Jim Biard
Research Scholar
Cooperative Institute for Climate and Satellites
Remote Sensing and Applications Division
National Climatic Data Center
151 Patton Ave, Asheville, NC 28801-5001
jim.biard@xxxxxxxx
828-271-4900
On Feb 19, 2013, at 12:47 PM, Mike Grant <mggr@xxxxxxxxx> wrote:
> On 19/02/13 15:00, Mike Grant wrote:
>> To make replies easier and as some of us (normally me!) may be too lazy
>
> And now for a reply (comments inline)..
>
> First, did you coordinate with Aleksandar Jelenak's proposed new names? They
> should be in the list archive for this list 19/Sept/2012 "New Standard Names
> for Satellite Data" and for the main CF-metadata list on 7/Oct/2012 "New
> standard names for satellite obs data" and summarised at
> http://wiki.esipfed.org/index.php?title=Standard_Names_For_Satellite_Observations,
> before it got derailed by string-based time variables debate.
>
> I think the platform ones fit well with your proposals (so perhaps you
> already coordinated?) - perhaps by reincluding or referencing them in your
> submission, you could rescue those parts from the time-string swamp.
>
> Either way, it may be wise to split these proposals into multiple parts to
> ensure that the whole set aren't hung up on a single argument. Perhaps split
> into 3 parts by platform, sensor and toa names?
>
>> azimuth_angle
>> * degrees
>> * Azimuth angle is the angle measured towards the east, from north,
>> along the astronomical horizon to the intersection of the great circle
>> passing through the point and the astronomical zenith with the
>> astronomical horizon.
>>
>> platform_latitude
>> * degrees_north
>> * Latitude of the satellite measured at the sub-satellite point
>>
>> platform_longitude
>> * degrees_east
>> * Longitude of the satellite measured at the sub-satellite point
>>
>> platform_altitude
>> * m
>> * Altitude of the satellite above mean sea level
>
> If these are specifically about satellites, it may be worth renaming them to
> satellite_XXX. I think it would be more useful to keep them as platforms, in
> which case perhaps change the word "satellite" to "platform" in the
> descriptions.
>
> Either way, it would be nice to more clearly define what's meant by the
> "sub-satellite point" for non-specialists (CF is used by all sorts of people)
> or, better, rephrase it in generic "platform" language. Is this the position
> on the surface/geoid/ellipsoid (intersection of the line between the platform
> location and the centre of the Earth with the geoid/ellipsoid)?
>
> It might be worth stealing parts of the definition of the general term
> "altitude" as it's nicely written:
> "Altitude is the (geometric) height above the geoid, which is the reference
> geopotential surface. The geoid is similar to mean sea level."
>
>> sensor_band_spectral_width
>> * cm-1
>> * Bandwidth of the satellite?s spectral channel
>
> This also fits nicely with the sensor_band_ proposals Aleksandar made on this
> list, but they didn't seem to go onto the CF list. Might be worth rescuing
> too, unless Aleksander is reading and had another reason not to submit?
>
> Again, it would be good to replace "satellite" with "sensor". Unless it's
> intentionally vague, perhaps define more clearly what's meant by the
> bandwidth (e.g. full width half max?)
>
>> toa_spectral_irradiance
>> * mW m-2 (cm-1)-1
>> * "toa" means top of atmosphere; irradiance which is relevant for any
>> sensor measuring in the UV-VIS and NIR. This parameter is reported by
>> integrating over the whole sphere.
>
> Might be worth stealing wording from other standard definitions incorporating
> irradiance, e.g.
>
> omnidirectional_spectral_spherical_irradiance_in_sea_water
> - "spectral" means per unit wavelength or as a function of wavelength;
> spectral quantities are sometimes called "monochromatic". Radiation
> wavelength has standard name radiation_wavelength. Omnidirectional spherical
> irradiance is the radiation incident on unit area of a spherical (or "4-pi")
> collector. It is sometimes called "scalar irradiance". Radiation incident on
> a 2-pi collector has standard names of "spherical irradiance" which specify
> up/downwelling.
>
> In fact, perhaps the name should be
> toa_omnidirectional_spectral_spherical_irradiance ?
>
>> toa_spectral_reflectance
>> * 1 (dimensionless)
>> * Ratio of radiance to irradiance I/I0, reflection from a thick layer
>> where the layer, here the atmosphere, is part of the reflection's property.
>
> Stealing a few bits from toa_bidirectional_reflectance, perhaps..
>
> Reflectance is the ratio of the energy of the reflected to the incident
> radiation. "spectral" means per unit wavelength or as a function of
> wavelength; spectral quantities are sometimes called "monochromatic". A
> coordinate variable of radiation_wavelength or radiation_frequency can be
> used to specify the wavelength or frequency, respectively, of the radiation.
> "toa" means top of atmosphere. toa_spectral_reflectance is the ratio of
> radiance to irradiance I/I0, reflection from a thick layer where the layer,
> here the atmosphere, is part of the reflection's property.
>
>> toa_outgoing_inband_radiance
>> * mW m-2 sr-1
>> * "toa" means top of atmosphere; "outgoing" means emitted toward outer
>> space; the radiance is integrated over a discrete band.
>
> I think this one is quite new to CF (integrating over a band), so you're
> probably setting a standard here. It may be worth saying that the band can
> be specified by (e.g.) a coordinate variable or other ancillary data.
>
>> toa_reflectance
>> * Percent
>> * Ratio of the energy of reflected to incident light at the top of
>> atmosphere.
>
> Steal most of the text from toa_bidirectional_reflectance?
>
> Should this be unitless, like toa_bidirectional_reflectance and
> toa_spectral_reflectance?
>
> Ok, that was longer than I thought - hope it was helpful, but feel free to
> take what you wish and reject what you don't. I think the splitting idea may
> be worth it to avoid getting tied up in arguments though.
>
> Cheers,
>
> Mike.
>
>
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