Hi all, thanks @erget for trying to clarify this issue! It's causing some confusion in the community, as it's a tricky thing to understand. I'm fully in line with your last recap.
As it's already been mentioned here, imho one thing that each description should really clarify (including the one from PROJ), is that this gimbal scanning geometry we are describing for the geostationary projection is independent of the actual scanning geometry of the actual instrument in orbit. It rather describes how the image is aquired by a fictitious, simplified, two-axis gimbal imaging instrument in an idealized geostationary position. Elaborating on this, for future newbies like me delving into this issue: Higher-lever geostationary imagery (e.g. SEVIRI L1.5, FCI L1c, and GOES-R L1b) is rectified to a specified regular azimuth/elevation grid. With this, each image pixel has a pair of azimuth and elevation angles, that define how this idealized gimbal needs to be rotated to reach that specific pointing towards Earth. The sweep-angle axis describes which convention is used to define these azimuth/elevation rotations (either the "standing" gimbal as in the image above, or a turned-over version). The origin of the reference frame used to define these angles (i.e. the center of the gimbal and the intersection of the two rotation axes), is fixed and placed on an ideal satellite position. With this, each pixel of the rectified image is always at the same location on Earth, independently of the instrument scanning pattern, and the wobbling and drifting of the satellite in the non-perfect and perturbed geostationary orbit. This ideal satellite position is specified by the other variables of the geostationary projection. -- You are receiving this because you commented. Reply to this email directly or view it on GitHub: https://github.com/cf-convention/cf-conventions/issues/258#issuecomment-613407787 This list forwards relevant notifications from Github. It is distinct from [email protected], although if you do nothing, a subscription to the UCAR list will result in a subscription to this list. To unsubscribe from this list only, send a message to [email protected].
