Adding aperture rotation (#308)

Its possible that an aperture may be rotated along the boresight
axis in a way that is no longer orthogonal to the reference plane.
This is most common for inversion, but technically any rotation is
possible. This field is added here.

extensions/spatial.sigmf-ext.md

@@ -119,6 +119,7 @@ The `spatial` extension adds the following fields to `captures` segment objects:
 |----|--------|----|-----|-----------|
 |`aperture_azimuth`|false|double|degrees|Azimuth of the aperture boresight in degrees east of true north.|
 |`aperture_bearing`|false|[bearing](spatial.sigmf-ext.md#01-the-bearing-object)|N/A|Bearing of aperture boresight in this segment.|
+|`aperture_rotation`|false|double|degrees|Right-handed rotation of the aperture in degrees (other rotational degrees of freedom are captured in `aperture_bearing`).|
 |`emitter_bearing`|false|[bearing](spatial.sigmf-ext.md#01-the-bearing-object)|N/A|Bearing of signals in this segment.|
 |`element_geometry`|false|array|N/A|An array containing `cartesian_point` objects that specify the relative physical geometry of the antenna elements.|
 |`phase_offset`|false|double|degrees|Phase offset of the data in this capture relative to a common phase reference plane.|
@@ -133,6 +134,11 @@ The azimuth is specified in degrees east of true north and elevation (if
 provided) is in degrees above the horizon. The `aperture_azimuth` field is also
 available for simpler specification when this is more appropriate.
 
+If the antenna is rotated boresight axis, this can also be specified wit
+with the `aperture_rotation` field. This rotation does not affect the antenna
+boresight but can be used to indicate the antenna is not perpindicular (e.g.:
+mounted upside down if this value is 180 degrees).
+
 The `emitter_bearing` field within a `captures` segment is used to specify the
 ground truth bearing of all signals contained within a multichannel dataset,
 relative to the `aperture_bearing`. This is useful for reference data which is