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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