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docs/session_summaries/2026-01-24-unified-orbital-elements-phases-1-3.md

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+# Session Summary - 2026-01-24
+
+## Overview
+Implemented Phases 1-3 of unified orbital elements configuration system.
+
+## Changes Made
+
+### Phase 1: Create Orbital Mechanics Module
+- **Created** `src/orbital_mechanics.h` - Defines `OrbitalElements` struct with Keplerian elements
+- **Created** `src/orbital_mechanics.cpp` - Implements `orbital_elements_to_cartesian()` conversion function
+- Supports all orbit types: circular (e=0), elliptical (0<e<1), parabolic (e=1), hyperbolic (e>1)
+- Planar orbits only (inclination=0), 3D orientation deferred to future work
+
+### Phase 2: Update Data Structures
+- **Modified** `CelestialBody` struct (simulation.h):
+  - Added `OrbitalElements orbit` field
+  - Renamed `position` → `global_position`
+  - Renamed `velocity` → `global_velocity`
+  - Reorganized field order for clarity
+
+- **Modified** `Spacecraft` struct (spacecraft.h):
+  - Added `OrbitalElements orbit` field
+  - Renamed `position` → `global_position`
+  - Renamed `velocity` → `global_velocity`
+
+- **Renamed** `OrbitalElements` → `OrbitalMetrics` in simulation.h (output/analysis use case)
+  - Updated all references in `simulation.cpp`, `config_loader.cpp`, `renderer.cpp`, `ui_renderer.cpp`
+
+### Phase 3: Update Config Parser
+- **Modified** `parse_toml_body()` (config_loader.cpp):
+  - Replaced old `position` field requirement with `orbit` table parsing
+  - Removed old `eccentricity` and `semi_major_axis` top-level fields
+  - Parses all orbital element fields with defaults
+  - Supports `altitude` convenience field for LEO/GEO orbits
+  - Required: `semi_major_axis` or `altitude` in orbit table
+  - Optional: `eccentricity` (default 0.0), `true_anomaly` (default 0.0), 3D orientation elements (default 0.0)
+  - Position/velocity initialized to zero (calculated from orbital elements later)
+
+- **Modified** `parse_toml_spacecraft()` (config_loader.cpp):
+  - Replaced `position` + `velocity` fields with `orbit` table parsing
+  - Uses same orbital element parsing logic as bodies
+  - Position/velocity initialized to zero (calculated from orbital elements later)
+
+- **Added** validation for orbital elements:
+  - `semi_major_axis` must not be zero
+  - `eccentricity` must be >= 0
+  - For elliptical orbits (e < 1): `semi_major_axis > 0`
+  - Validates both bodies and spacecraft
+
+- **Modified** `load_spacecraft_from_toml()`:
+  - Removed manual position/velocity calculation from parent
+  - Now initializes both bodies and spacecraft in `initialize_orbital_objects()`
+
+## Commits
+1. **dc1f9f4** - Phase 1: Create orbital mechanics module
+2. **121f1b9** - Phase 2: Update data structures with global_position/global_velocity
+3. **ec1d115** - Phase 3: Update config parser to use orbital elements
+
+## Net Line Count
+- **+172 lines added** (orbital_mechanics module, updated structs, config parsing, validation)
+- **-93 lines deleted** (removed manual velocity calculations, old config format support)
+
+## Remaining Issues
+1. **Phase 4 incomplete**: Need to update initialization to use `orbital_elements_to_cartesian()` function
+   - Current state: `add_body_to_simulation()` calls old `calc_orbital_velocity()` (removed)
+   - Old `initialize_bodies()` function still references removed function
+   - Need to replace with `initialize_orbital_objects()` that calls `orbital_elements_to_cartesian()`
+
+2. **Function name typo**: File has `orbital_mechanics.h` but code uses `calc_orbital_velocity` (should be `orbital` not `orbi`)
+
+3. **Config format**: All existing test configs need to be updated to new `orbit` table format (13 config files)
+
+4. **Renderer references**: Need to verify all `render_orbit()` calls are correct after struct changes
+
+## Next Steps (Phase 4)
+1. Remove old `calc_orbital_velocity()` function from simulation.cpp
+2. Implement `initialize_orbital_objects()` using `orbital_elements_to_cartesian()`:
+   - Bodies: Convert orbital elements to local position/velocity, compute global coordinates
+   - Spacecraft: Handle altitude convenience, add parent radius, convert to local/global coordinates
+   - Calculate SOI radii for bodies
+3. Update `load_system_config()` to call `initialize_orbital_objects()` instead of `initialize_bodies()`
+4. Update all 13 test configs to new format with `orbit` tables
+5. Verify renderer displays orbits correctly
+6. Run full test suite to ensure all changes work correctly
+
+## Notes
+- All orbital calculations use planar orbits (inclination=0), 3D orientation deferred
+- `OrbitalElements` struct in separate module to enable clean separation of concerns
+- Config parser now validates orbital elements at load time (better error messages)
+- Altitude convenience simplifies LEO/GEO orbit specification (e.g., altitude=400km instead of semi_major_axis calculation)