如下图所示，posangle=315度

关于角度

4.4.1.2.5 Position Angles
Position angle definitions can be a great source of confusion. To reduce
ambiguity, we will attach a Reference attribute to position angles that may have
the values “North”, “X” (default), or “Y”. If the reference is “North”, position angles
will be counted from north through east; this should only be used with coordinate
systems where “North” has meaning. If the reference is “X” (the default), the
position angle will be counted from the positive first coordinate axis through the
positive second coordinate axis. If the reference is “Y”, the position angle will be
counted from the positive second coordinate axis through the positive first
coordinate axis. These definitions are independent of the handedness of the
coordinate system. For spherical systems, the longitude is considered the first
axis in this context, and latitude the second. Note that in most (though not all)
cases “North” is equivalent to “Y”.

关于长短轴

4.5.1.3 Ellipse
The Ellipse (2-dimensional) is similar to the Circle but has, in addition, a minor
radius and a position angle. Position angles are measured following the definition
in Section 4.4.1.2.5 and refer to the first axis. The definition of an ellipse in a
Cartesian coordinate system is unambiguous, but this is not the case for
spherical coordinates. In a spherical coordinate system the ellipse shall be
defined as the intersection of an elliptical cone with the unit sphere, where the
axes and position angle describe the geometry of the cone.

参考：http://www.starlink.ac.uk/docs/sun183.htx/node22.html#KEY:BOX

ELLIPSE- A 2-dimensional ellipse. The argument listshould contain 5 values; the first pair give the user coordinates ofthe centre of the ellipse, the second pair give the half-lengths of thetwo axes of the ellipse (in user coordinates), and the fifth value givesthe angle (in degrees) from the first axis of the user coordinate systemto the first axis of the ellipse (positive rotation is from the first tothe second user axis). This keyword can only be used in 2-dimensional ARDdescriptions.

它的ARD Description是这样的

#   ARD description generated by ARDGEN
COFRAME(SKY,SYSTEM=FK5,EQUINOX=2000.0,EPOCH=2000.0)
ELLIPSE( 16:28:34.6, -24:13:41, 0:34:57, 0:09:35, -118.6131 )

角度由STC-S到ARD：90-alfa，alfa是STC-S对应的角度

它的ARD Description是这样的

#   ARD description generated by ARDGEN
COFRAME(SKY,SYSTEM=FK5,EQUINOX=2000.0,EPOCH=2000.0)
ELLIPSE( 16:26:40.4, -24:12:01, 0:33:33, 0:10:52, -62.57536 )

角度由STC-S到ARD：270-alfa，alfa是STC-S对应的角度