XMM Angles

XMM orientation

This is the orientation of XMM in the x, y and z axis:

The XMM solar panels lie along the y axis.
The XMM pointing direction is along the -X axis.
The XMM RGS dispersion direction lies along the z axis.

Roll angle, solar aspect angle and position angle

For the satellite to receive enough power from the solar panels, the Z axis (which is orthogonal to the Y axis, ie to the solar panels) MUST lie within a 20 degree cone around the Sun.

As can be seen from the above diagram, this condition also restricts the X and Y axis movements to + or -20 degrees.

The solar aspect angle is defined as the angle between the pointing direction (the -X axis) and the vector to the Sun.

Given the condition that the Z axis MUST lie within a 20 degree cone around the Sun, this restricts the allowed solar aspect angle range to lie between 70 and 110 degrees.

The roll angle is defined in the User Handbook as the rotation angle between the plane defined by the major rotation axis of the spacecraft (the spacecraft X-axis) and the spacecraft Z-axis, to that defined by the spacecraft X-axis and the spacecraft-Sun direction. The roll angle increases in a clockwise direction with respect to the positive X-axis.

Given the condition that the Z axis MUST lie within a 20 degree cone around the Sun, this restricts the allowed roll angle range to + or -20 degrees

Please note that in the diagram below the roll angle has been exaggerated to approximately 70 degrees for clarity.

The position angle is defined in the User Handbook as the rotation between the spacecraft X-Z plane and the plane defined by the spacecraft X-axis and celestial North. Again, the position angle increases in a clockwise direction with respect to the positive X-axis. Note that the satellite's -Z-axis, which is also the RGS dispersion axis, is North-aligned under a position angle of 0 degrees, i.e. the declination axis.

Last updated: Thu Apr 8th 1999