Constellation Design Skill

Read CONVENTIONS.md at the repo root before proceeding.

This skill designs and evaluates satellite constellations — multiple spacecraft working together to provide coverage, capacity, or capability that a single satellite cannot. Constellation design is fundamentally different from single-satellite design: the system-level architecture (number of planes, phasing, altitude) drives individual satellite requirements, not the other way around.

Before You Begin

Ask the user (if not already known):

1. Mission objective? (Communications, Earth observation, weather, navigation, IoT, SSA, science — each has different coverage metrics)
2. Coverage requirement? (Continuous global, regional, specific latitude bands, revisit time, max gap duration)
3. Number of satellites (range)? (Budget-driven constraint — can be 3 or 30,000)
4. Inter-satellite links? (Yes = mesh network, reduced ground stations. No = simpler satellites, more ground infrastructure)
5. What design phase?

Applicable Phases

• Primary: Phase A (constellation architecture trade), Phase B (orbit and phasing design)
• Supporting: Phase C (deployment sequencing), Phase D (on-orbit constellation management)

Core Concepts

1. Walker Constellation Notation

The standard notation for symmetric constellations: T/P/F

• T: Total number of satellites
• P: Number of orbital planes (equally spaced in RAAN)
• F: Phasing parameter (relative spacing between planes, 0 ≤ F < P)
• Example: GPS = 24/6/1 at 55° inclination, 20,200 km altitude
• Example: Iridium = 66/6/1 at 86.4° inclination, 780 km altitude

2. Coverage Geometry

• Half-cone angle (footprint): $\rho = \arccos(R_E / (R_E + h))$ — the angular radius of visibility from one satellite.
• Minimum elevation angle ($\epsilon_{min}$): Typical 5-15°. Higher elevation = smaller footprint but better link quality.
• Effective footprint: $\theta = 90° - \epsilon_{min} - \arcsin((R_E \cdot \cos\epsilon_{min}) / (R_E + h))$
• Street-of-coverage width: For a single plane, the ground swath covered by N satellites evenly spaced.

3. Coverage Metrics

| Metric | Definition | Typical Target | |:---|:---|:---| | Continuous coverage | 100% of target area has ≥1 satellite visible at all times | GPS, comms constellations | | Revisit time | Max time between consecutive passes over a point | EO: 1-24 hours | | Max gap | Longest period with no coverage at any point | Comms: 0 (continuous) | | Number of folds | Minimum simultaneous visible satellites at any point | Navigation: ≥4 for 3D fix | | Contact duration | Time a satellite is visible per pass | LEO: 5-15 min per pass |

4. Key Trade Parameters

| Parameter | Lower Value | Higher Value | |:---|:---|:---| | Altitude | Lower drag, shorter life, smaller footprint, more sats needed | Higher footprint, fewer sats, radiation, higher launch ΔV | | Inclination | Covers equatorial region well, misses poles | Near-polar covers all latitudes, ground track complexity | | N satellites | Cheaper, less coverage, longer revisit | Better coverage, higher cost, more complex management | | N planes | Simpler deployment (all in one plane) | Better longitudinal distribution, needs multiple launches or RAAN drift |

Analysis Workflows

1. Coverage Analysis

• Analytical (Phase A): Use the Walker formula to estimate single-fold or multi-fold coverage for a given T/P/F/i combination.
• Grid-based: Discretize the Earth's surface and compute visibility from each satellite at time steps.
• Higher fidelity: Recommend STK, GMAT, or Orekit propagation for detailed gap analysis.
• Latitude dependency: Coverage is always better near the inclination-matching latitude. Polar coverage requires i > 80°.

2. Inter-Satellite Links (ISL)

• Intra-plane links: Between adjacent satellites in the same plane. Geometry is relatively stable.
• Cross-plane links: Between satellites in adjacent planes. Range and angle vary — most complex for seam planes (where ascending/descending nodes meet).
• RF ISL: Proven (TDRS heritage), lower data rate (1-10 Gbps typical).
• Optical ISL: Higher data rate (10-100+ Gbps), narrower beam, needs precision pointing (Starlink V2 uses optical ISL).
• Latency benefit: ISL routing can be faster than fiber for long distances — signal travels at c in vacuum vs. ~0.67c in fiber.

3. Deployment Strategy

• Shared launch: Multiple satellites per launch to the same plane. Then phase within the plane using differential drag or low-thrust maneuvering.
• RAAN spreading: Either launch to different RAANs directly (expensive) or use J2 precession at different altitudes to naturally drift planes apart.
• Build-up: Deploy in phases — initial operating capability (IOC) with partial constellation, full operating capability (FOC) with all satellites.

4. Constellation Management

• Station-keeping: Drag makeup (LEO), orbit maintenance, RAAN/argument-of-perigee corrections.
• Spare strategy: On-orbit spares (parked at different altitude), ground spares, or rapid launch capability.
• Collision avoidance: Maneuver authority, conjunction assessment cadence, coordination with other operators.
• End-of-life: Deorbit within 5 years of mission end (current guidelines, moving toward 0 years). $\Delta V_{deorbit}$ budget.

5. Spectrum & Regulatory

• ITU coordination: Frequency filing, interference analysis with existing systems.
• Orbital debris compliance: FCC/ITU 25-year rule (legacy), accelerated timelines for large constellations.
• Licensing: National licensing (FCC for US, Ofcom for UK, etc.) before deployment.

Output Format

1. Constellation Design Report (constellation_report.md): Walker notation, orbit parameters, coverage analysis, ISL architecture.
2. Coverage Map / Statistics: Revisit time, max gap, number of folds by latitude.
3. Deployment Plan: Launch manifest, phasing strategy, IOC/FOC timeline.
4. Per-Satellite Requirements: Derived requirements for each satellite (mass, power, propulsion for station-keeping and deorbit).
5. 🟢 / 🟡 / 🔴 status: Coverage compliance, regulatory readiness, collision risk.

Interface

• Reads from: /requirements/, /analysis/mission-analysis-specialist/ (orbital mechanics, ΔV), /analysis/communications-assessment/ (link budgets for ISL and ground), /analysis/cost-modeling/ (per-satellite and total constellation cost)
• Writes to: /analysis/constellation-design/
• Consumed by: systems-engineering-assessment (per-satellite requirements flowdown), cost-modeling (constellation economics — per-sat × quantity), trade-study-manager (constellation architecture as a trade option)