**Title:** Add standard names for angular coordinates
**Moderator:** TBD
**Moderator Status Review [last updated: YY/MM/DD]:** Pending
**Requirement Summary:** As discussed in #230 new standard names are needed for 
encoding angular coordinates.
**Technical Proposal Summary:** Add 2 new standard names to be adopted in 
conjunction with the changes discussed in #230 
**Benefits:** Users of angular coordinates, particularly geostationary imagery
**Status Quo:** This is discussed in #230. The `standard_name`s currently in 
use have canonical names that do not fulfil the needs of geostationary imagery.
**Detailed Proposal:** Add the following `standard_name`s:

| Name | Canonical units | Definition |
| ---- | --------------- | -----------|
| projection_x_angular_coordinate | radian | "x" indicates a vector component 
along the grid x-axis, when this is not true longitude, positive with 
increasing x. Angular projection coordinates are angular distances in the x- 
and y-directions on a plane onto which the surface of the Earth has been 
projected according to a map projection. The relationship between the angular 
projection coordinates and latitude and longitude is described by the 
grid_mapping. |
| projection_y_angular_coordinate | radian | "y" indicates a vector component 
along the grid y-axis, when this is not true latitude, positive with increasing 
y. Angular projection coordinates are angular distances in the x- and 
y-directions on a plane onto which the surface of the Earth has been projected 
according to a map projection. The relationship between the angular projection 
coordinates and latitude and longitude is described by the grid_mapping. |

A known use case is in the [geostationary 
projection](http://cfconventions.org/cf-conventions/cf-conventions.html#_geostationary_projection).

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