Tracking Mount

Description

The Tracking Mount is a dynamic component that enables ground station antennas and sensors to track objects in the sky. It receives access information containing azimuth and elevation angles to a target and rotates accordingly, orienting any child objects (such as antennas or optical systems) toward the tracked object. This component is essential for ground-based communication systems, radar installations, and optical tracking facilities that require continuous pointing toward moving spacecraft or space objects.

Example Use Cases

Antenna Pointing: Orient a ground station antenna dish to maintain communication link with an orbiting spacecraft.
Optical Tracking: Direct a telescope or laser ranging system toward a satellite for observation or ranging measurements.
Radar Tracking: Point a tracking radar toward a space object for orbit determination or conjunction assessment.

Module Implementation

The Tracking Mount is a Physical Object that updates its orientation based on the azimuth and elevation angles provided by an Access Message.

Coordinate Convention

The tracking mount uses a local topocentric coordinate system where:

The default orientation points straight up (zenith), corresponding to 90° elevation
Azimuth is measured clockwise from north in the horizontal plane
Elevation is measured upward from the horizon toward zenith

Orientation Computation

The mount orientation is computed through two sequential rotations:

Azimuth Rotation

The first rotation is about the local vertical (up) axis to align with the target azimuth:

[A] = [R_{3} (- α)]

where $α$ is the azimuth angle and $[R_{3}]$ represents a rotation about the third (up) axis.

Elevation Rotation

The second rotation tilts from zenith toward the horizon to achieve the commanded elevation:

[E] = [R_{2} (90° - ϵ)]

where $ϵ$ is the elevation angle. The term $(90° - ϵ)$ converts from elevation (measured from horizon) to the tilt angle from zenith.

Combined Orientation

The final orientation is the composition of both rotations:

[BN] = [E] [A]

This direction cosine matrix is applied to the mount’s transform, which propagates to all child objects.

Access Gating

The mount only updates its orientation when the target is accessible. If the access message indicates the target is not in view (below the horizon or otherwise inaccessible), the mount retains its previous orientation.

Assumptions/Limitations

The mount assumes the default orientation is pointing at zenith (straight up).
Child objects inherit the mount orientation; their local offsets are preserved.
No gimbal limits or slew rate constraints are modeled; the mount achieves commanded orientation instantaneously.
The mount does not update orientation when the target is inaccessible.
Mechanical dynamics such as backlash, friction, or servo response