Dear Martin: Note that the CF “geostationary projection” has been design to accommodate both GOES-R and Meteosat (and Himawari, etc.), The ways the different imagers do their scans is different causing the same (x.y) angular coordinates to resolve to a different earth location. A projection parameter “sweep angle” is included to allow the same projection to work for both imager designs.
A GOES-R writeup on this projection is located at: https://www.goes-r.gov/users/docs/PUG-L1b-vol3.pdf, paragraph 5.1.2 ABI Fixed Grid. v/r randy > On Apr 20, 2018, at 10:06 AM, Daniel Lee <[email protected]> wrote: > > Hi Martin, > > Yes, you need both coordinates in order to find the position viewed on the > Earth's surface - it's not a simple distance measurement. > > I'm a bit confused about your use of azimuth vs. polar - do you mean > meridional and zonal? > > I'd be cautious about bringing in too much information about how > geostationary satellites function into the grid mapping. Not all satellites > use the same scanning principals - technology changes and there have already > been a number of different geostationary satellites deployed with different > scanning characteristics, so that the geostationary constellation currently > contains satellites with different scanning methods. This will surely > continue to change in the future. > > At the end of the day, describing geostationary satellite data with the > method NOAA is currently using is well-established and has worked well for > decades in several data formats, so I don't think we'll benefit much from > changing anything except for maybe the name of the variables. The methodology > should clearly remain the same. > > Cheers, > Daniel > >> -----Original Message----- >> From: CF-metadata [mailto:[email protected]] On Behalf >> Of Martin Juckes - UKRI STFC >> Sent: 20 April 2018 15:41 >> To: Randy Horne <[email protected]> >> Cc: [email protected] >> Subject: Re: [CF-metadata] Fix Geostationary projection, including proposal >> for two new standard names >> >> Hello Randy, >> >> >> thanks, that clears up a lot of my confusion. >> >> >> Since the coordinates are N/S and E/W aligned, at least at the origin, it may >> be better to include this in the names. "x" and "y" are generally used for >> coordinates which have an arbitrary orientation relative to the Earth's axis >> which then needs to be specified in additional attribute values. >> >> >> Your answer does not completely define the angles for me. If we consider a >> point (A) which is, for example, at 45N at the same longitude, then it >> angular >> distance in the N/S direction is uniquely defined, but if we take another >> point >> (B) 45degrees to the east, then we have two angles and their values will >> depend on the definition of the coordinate system. >> >> >> I've found some documentation on geostationary satellites which suggests >> that the viewing angles are related to the gimbal system, with an outer >> "sweep" axis and an inner "fixed-angle" axis (this is from >> proj4.org/projections/geos.html). Relating this back to the mathematical >> terminology of spherical coordinates that I'm familiar with, I believe the >> angle >> of rotation around the sweep axis is the azimuthal angle and the rotation >> around the fixed axis is the polar angle. I.e. we have a spherical coordinate >> system relative to the sweep axis. >> >> >> The proj4.org document also states that the GEOS series have the sweep axis >> aligned E/W, which would imply that projection_y_angular is an azimuthal >> angle and projection_x_angular is a polar angle. With this information (and >> the height of the satellite) I would be able to calculate the two angles for >> point B. I'll get a different answer if projection_y_angular is the polar >> angle >> and projection_x_angular is azimuthal, so it is important to know which is >> which. Perhaps projection_polar_angle, projection_azimuthal_angle would >> be better? >> >> >> The calculation would indeed be complicated, as has already been >> emphasized below, but I think it is worth going back to the fundamentals >> here, and stating the underlying assumptions behind the coordinate system. >> E.g. the fact that it is defined relative to a fixed point above the Earth's >> surface which corresponds to an ideal satellite position. >> >> >> regards, >> >> Martin >> >> >> >> ________________________________ >> From: Randy Horne <[email protected]> >> Sent: 20 April 2018 13:56 >> To: Juckes, Martin (STFC,RAL,RALSP) >> Cc: [email protected] >> Subject: Re: [CF-metadata] Fix Geostationary projection, including proposal >> for two new standard names >> >> Hi Martin: >> >> RE: I agree with Jim that a little more basic information is needed about >> what >> the angles are. I may be misinterpreting the discussion, but I had imagined >> that the angles as components of a spherical coordinate system centred on >> the satellite, with the nadir at (0,0) ... is that correct? >> >> The projection_x_angular_coordinate and projection_y_angular coordinates >> are the angular distances from the satellite’s nadir in the E/W an N/S >> direction, respectively, from the ideal location of the imaging instrument in >> geostationary orbit. >> >> >> v/r >> >> randy >> >> >> >>> On Apr 20, 2018, at 4:06 AM, Martin Juckes - UKRI STFC >> <[email protected]> wrote: >>> >>> I agree with Jim that a little more basic information is needed about what >> the angles are. I may be misinterpreting the discussion, but I had imagined >> that the angles as components of a spherical coordinate system centred on >> the satellite, with the nadir at (0,0) ... is that correct? >> >> _____________________________________ >> >> Randy C Horne ([email protected]) Principal Engineer, Excalibur >> Laboratories Inc. >> voice & fax: (321) 952.5100 >> cell: (321) 693.1074 >> url: http://www.excaliburlabs.com >> >> >> >> >> _______________________________________________ >> CF-metadata mailing list >> [email protected] >> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > Any email message from EUMETSAT is sent in good faith but shall neither be > binding nor construed as constituting a commitment by EUMETSAT, except where > provided for in a written agreement or contract or if explicitly stated in > the email. Please note that any views or opinions presented in this email are > solely those of the sender and do not necessarily represent those of > EUMETSAT. 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