GNC & ADCS Assessment Skill

Read CONVENTIONS.md at the repo root before proceeding.

This skill performs sizing and performance estimation for the Guidance, Navigation, and Control (GNC) subsystem — also called Attitude Determination and Control System (ADCS).

Before You Begin

Ask the user (if not already known):

1. What are the pointing requirements? (accuracy, stability, and knowledge — in degrees or arcseconds)
2. What is the orbit? (affects disturbance torques — LEO has drag/magnetic, GEO has solar radiation pressure)
3. Is 3-axis stabilization required, or is spin stabilization acceptable?
4. Are there slew requirements? (e.g., agile imaging needs fast repointing)
5. What design phase?

Applicable Phases

• Primary: Phase A (requirements flowdown, actuator trade), Phase B (detailed sizing)
• Supporting: Phase C (pointing budget refinement), Phase D (calibration planning)

Attitude Mode Definitions

Define these modes for the mission (tailor as needed):

• Detumble: Post-separation rate damping (typically B-dot control using magnetorquers)
• Sun-Safe / Safe Mode: Sun-pointing for power, minimal actuator use
• Nadir-Pointing: Earth/body-facing for payload or comms
• Inertial-Pointing: Fixed celestial target (e.g., star observation)
• Slew: Transition between targets
• Thrust: Attitude hold during propulsive maneuvers

Core Analysis Workflows

1. Pointing Error Budget

• Components: Accuracy (knowledge error), Stability (jitter), Control Error.
• Statistical method: Root Sum Square (RSS) for independent sources.
• Output: Table of error sources, values (arcsec or deg), and contribution.
• Key drivers: Star tracker accuracy, reaction wheel vibration, structural flexibility.

2. Disturbance Torque Estimation

• Solar Radiation Pressure: $T_{SRP} = F_{SRP} \cdot A \cdot c_{ps}$ (force × area × CG-to-CP offset)
• Gravity Gradient: $T_{GG} = \frac{3\mu}{2R^3}|I_z - I_y|\sin(2\theta)$ (uses MOI from structural-assessment)
• Magnetic: Relevant for LEO — residual dipole interacting with Earth's field
• Aerodynamic drag: LEO only — $T_{drag} = F_{drag} \cdot c_{pa}$

3. Actuator Sizing

• Reaction Wheels: Momentum storage $H = T_{dist} \cdot t_{accumulation}$ over one orbit or desaturation interval.
• Magnetorquers: Dipole moment ($Am^2$) to desaturate wheels — only usable near magnetic-field body.
• Thrusters: For large slews, orbit maneuver attitude hold, or when magnetic desaturation isn't available.
• Slew rate: $T = I \cdot \alpha$ — check that wheels can provide the required angular acceleration.

4. Sensor Suite

• Sun sensors: Coarse (~1°) for safe mode, fine (~0.1°) for power-positive pointing
• Star trackers: High accuracy (~arcsec), but blinded by Sun/Moon exclusion zones
• IMUs: Rate sensing, needed for slew control and during eclipses
• Magnetometers: LEO only — for B-dot detumble and magnetic field model
• GPS receivers: LEO orbit determination (not attitude, typically)

Output Format

1. Pointing Budget (pointing_budget.md): Error breakdown against requirements.
2. ADCS Sizing Report (adcs_report.md): Actuator and sensor selection with rationale, disturbance torque estimates.
3. Mode Transition Table: Which modes use which actuators/sensors.

Interface

• Reads from: /requirements/, /analysis/structural-assessment/ (MOI, CG-CP offset), /analysis/mission-analysis-specialist/ (orbit parameters, eclipse)
• Writes to: /analysis/gnc-assessment/
• Consumed by: systems-engineering-assessment (mass/power), flight-software-architect (processing needs), power-assessment (actuator power)