Dock Alignment Pointing Software

SWITCH TO API

Description

The Dock Alignment Pointing Software computes an inertial attitude reference that aligns the spacecraft’s docking adapter with a target adapter for mating. The reference orients the chaser’s port axis anti-parallel to the target’s port axis while achieving a commanded clocking (roll) angle about the mating axis. The module accounts for the body-frame mounting of the docking adapter so that the output commands the spacecraft body attitude, not the port frame directly.

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Example Use Cases

• Docking Approach: Generate attitude commands during final approach to align the spacecraft for capture.
• Berthing Operations: Maintain port alignment while an external manipulator positions the spacecraft.
• Clocking Control: Achieve a specific roll orientation for mechanical or electrical interface alignment.

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Module Implementation

Reference Frame Construction

The module constructs a desired port frame ${\mathcal{L}}_{des}$ in inertial coordinates from the target adapter’s orientation. The desired chaser port Up axis is the negation of the target port Up axis to achieve anti-parallel mating:

$${\hat{\mathbf{u}}}_{des}=-{\hat{\mathbf{u}}}_{target}$$

The clocking (Right) axis is derived from the target’s clock vector, rotated by the commanded clocking angle $\varphi$ about the mating axis using the Rodrigues rotation formula:

$${\hat{\mathbf{c}}}_{des}={\hat{\mathbf{c}}}_{target}\cos \varphi +({\hat{\mathbf{u}}}_{des}\times {\hat{\mathbf{c}}}_{target})\sin \varphi +{\hat{\mathbf{u}}}_{des}({\hat{\mathbf{u}}}_{des}\cdot {\hat{\mathbf{c}}}_{target})(1-\cos \varphi )$$

The Forward axis completes the right-handed frame:

$${\hat{\mathbf{f}}}_{des}={\hat{\mathbf{u}}}_{des}\times {\hat{\mathbf{c}}}_{des}$$

Body Frame Correction

The chaser adapter is mounted at a fixed orientation in the spacecraft body frame. To command the body attitude rather than the port attitude, the module removes this mount offset:

$$[R_{B/N}]=[R_{L/B}{]}^T[R_{L/N}{]}_{des}$$

where $[R_{L/B}]$ is the DCM from body to port frame (constructed from the adapter’s body-frame axes) and $[R_{L/N}{]}_{des}$ is the desired port-to-inertial DCM. The resulting body-to-inertial DCM is converted to Modified Rodrigues Parameters ${\mathbf{\sigma }}_{R/N}$.

Target State Propagation

When the target adapter telemetry is stale, the module propagates the target’s port axes forward using the published angular velocity. The propagation time is limited by MaxPoseLookAhead to bound extrapolation error:

$$\hat{\mathbf{u}}(t)=R(\mathbf{\omega },\Delta t)\hat{\mathbf{u}}(t_0)$$

where $R(\mathbf{\omega },\Delta t)$ is the Rodrigues rotation by angle $|\mathbf{\omega }|\Delta t$ about axis $\hat{\mathbf{\omega }}$.

Reference Rate Computation

The angular velocity and acceleration of the reference frame are computed by finite-differencing the MRP using the kinematic B-matrix relationship:

$${\mathbf{\omega }}_{R/N}=\frac{4}{(1+{\sigma }^2{)}^2}[B(\mathbf{\sigma }){]}^T\dot{\mathbf{\sigma }}$$

The module averages the B-matrices at the current and previous time steps to improve numerical accuracy. When analytic angular velocity from the target adapter is available, it is used directly instead of the finite-difference estimate.

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Assumptions/Limitations

• Requires docking adapter telemetry from both the chaser and target spacecraft.
• The first few rate and acceleration outputs are zeroed to avoid numerical transients from finite-differencing.
• Target state propagation assumes constant angular velocity; accelerating targets may exhibit tracking lag.
• If the target or chaser axes degenerate (near-zero length), the previous reference is held.