Add Mach 5 anomalous target generation

[jonnyspicer]

Summary

Implements synthetic Mach 5 target generation for testing anomaly detection systems in retina-tracker.

Closes #15

Features

• Generate Mach 5 trajectories: Create realistic ~1715 m/s target paths using great circle navigation
• Delay-Doppler conversion: Convert trajectories to bistatic radar measurements
• Configurable parameters: Start position, heading, duration, radar geometry
• Optional noise: Add realistic Gaussian noise to measurements
• Compatible output: .detection files ready for retina-tracker replay

Implementation Details

New Functions in src/node/synthetic.js

• generateMach5Trajectory(): Generates positions for Mach 5 target along great circle path
• trajectoryToDelayDoppler(): Converts trajectory to delay-Doppler detections

CLI Tool: generate_mach5_targets.js

Comprehensive script for generating Mach 5 target data with options for:

• Radar geometry (RX/TX positions, frequency)
• Trajectory parameters (start, heading, duration, timestep)
• Noise configuration
• Output file location

Testing

• Added 10 new comprehensive tests
• All 120 tests passing
• Verified realistic Doppler characteristics (>5000 Hz for Mach 5 vs <50 Hz for normal aircraft)

Documentation

• Updated README with complete usage guide
• Examples for common use cases
• Technical details on Mach 5 characteristics

Usage Examples

Generate default Mach 5 target:

node generate_mach5_targets.js

Custom trajectory heading north:

node generate_mach5_targets.js --start 37.7,-122.4,15000 --heading 0 --duration 120

Perfect measurements (no noise):

node generate_mach5_targets.js --no-noise

Technical Characteristics

Metric	Mach 5 Target	Normal Aircraft
Speed	~1715 m/s	~250 m/s
Ground Speed	~3334 knots	~485 knots
Doppler Shift	>100 Hz	<50 Hz
Delay Change	Significant	Gradual

Output Format

Compatible with retina-tracker .detection files:

{
  "timestamp": 1718747745000,
  "delay": [156.32],
  "doppler": [523.45],
  "snr": [15.2],
  "adsb": [{
    "hex": "MACH5X",
    "flight": "MACH5",
    "lat": 37.52,
    "lon": -122.95,
    "alt_baro": 49213,
    "gs": 3332,
    "track": 90
  }]
}

Integration with retina-tracker

This feature enables testing of anomaly detection in retina-tracker (see offworldlabs/retina-tracker#4). The generated data:

• Simulates realistic high-speed targets
• Provides ground truth ADS-B metadata for validation
• Can be replayed through retina-tracker's detection pipeline

Test Plan

• All existing tests pass
• 10 new Mach 5-specific tests pass
• Script generates valid output format
• Doppler characteristics match expected values for Mach 5
• Documentation complete with examples

🤖 Generated with Claude Code

[claude]

Pull Request Review: Add Mach 5 Anomalous Target Generation

Overall Assessment

✅ Approved with minor suggestions

This is a well-implemented feature that adds valuable testing capability for anomaly detection systems. The code quality is high, tests are comprehensive, and the documentation is thorough.

---

Code Quality & Best Practices

✅ Strengths

• Excellent documentation: Comprehensive README updates with clear examples and usage instructions
• Consistent coding style: Follows existing codebase patterns (JSDoc comments, naming conventions)
• Clean separation of concerns: Trajectory generation and delay-Doppler conversion are properly separated
• Proper geodetic calculations: Great circle navigation implementation is mathematically correct
• CLI tool design: Well-structured argument parsing with sensible defaults

🔧 Minor Suggestions

1. Magic numbers in generateMach5Trajectory (src/node/synthetic.js:352-353)

   • Consider adding JSDoc constants documentation for MACH_5_SPEED and EARTH_RADIUS
   • The conversion factor 0.3048 (line 388) and 1.94384 (line 389) could use inline comments explaining they're meters-to-feet and m/s-to-knots conversions

2. Timestamp generation in generateMach5Trajectory (line 357)

   • Using Date.now() means trajectories will have different timestamps each run
   • For reproducible testing, consider adding an optional startTimestamp parameter
   • This would help with deterministic test scenarios

3. Error handling in parseArgs() (generate_mach5_targets.js:40-83)

   • No validation if args[++i] goes out of bounds
   • Could fail silently if user provides --rx without a value
   • Suggest adding bounds checking:

   case '--rx':
     if (i + 1 >= args.length) {
       console.error('Error: --rx requires a value');
       process.exit(1);
     }
     const rx = args[++i].split(',').map(parseFloat);

---

Potential Bugs & Issues

⚠️ Issues Found

1. Doppler calculation at first position (src/node/synthetic.js:453-473)

   • First detection always has doppler = 0 due to if (i > 0) check
   • This creates a discontinuity in the data stream
   • Impact: Minor - anomaly detection systems might flag the first point, but this is edge case behavior
   • Suggestion: Document this behavior in the function JSDoc or consider extrapolating from the first two points

2. Division by zero potential (src/node/synthetic.js:456)

   • If timestep is very small and floating point arithmetic results in dt = 0, you'll get Infinity for Doppler
   • Mitigation: Current implementation unlikely to trigger this, but could add dt > 1e-9 guard

3. Longitude wrapping (src/node/synthetic.js:374-377)

   • No handling for crossing ±180° meridian
   • For east/west headings over long distances, longitude could exceed valid range
   • Impact: Low for 60-second trajectories, but could affect longer simulations
   • Example: Starting at 179° heading east for extended duration

✅ Minor Issues

4. Array bounds in CLI (generate_mach5_targets.js:47-72)
   • As mentioned above, args[++i] could go out of bounds
   • No validation that split arrays have expected length (e.g., rx should have 3 values)

---

Performance Considerations

✅ Good Performance Characteristics

• Computational complexity is O(n) where n is number of trajectory points
• Pre-calculation of RX/TX ECEF coordinates avoids redundant work
• No memory leaks - arrays are properly managed

💡 Optimization Opportunities (Optional)

1. Repeated ECEF calculations in Doppler computation (lines 457-467)

   • prevTar ECEF was already calculated as tar in the previous iteration
   • Could cache the previous iteration's tar to avoid recalculation
   • Benefit: ~30% reduction in ECEF conversions for large trajectories
   • Trade-off: Slightly more complex code

2. Trigonometric calculations in trajectory loop (lines 363-377)

   • headingRad is constant and recalculated every iteration
   • Could move outside the loop (line 363)

---

Security Concerns

✅ No Critical Issues

• File system operations: Proper use of fs.mkdirSync with recursive: true
• Path handling: Uses path.dirname() correctly
• No user input injection: All inputs are parsed to numbers, preventing injection attacks
• No external network calls: Self-contained generation

🔒 Best Practices Followed

• CLI tool properly uses #!/usr/bin/env node shebang
• No eval() or dangerous dynamic code execution
• File writes are to user-specified locations (expected behavior)

---

Test Coverage

✅ Excellent Coverage

• 10 new tests covering all major functionality
• Tests verify:
  • Correct trajectory count ✓
  • Speed consistency ✓
  • Direction of travel ✓
  • Altitude maintenance ✓
  • ADS-B metadata ✓
  • Delay-Doppler conversion ✓
  • High-speed Doppler characteristics ✓
  • Distance coverage ✓

💡 Additional Test Suggestions

1. Edge cases to consider:

   • Heading values: 0°, 90°, 180°, 270°, 360°, -90° (negative heading)
   • Boundary conditions: Very short duration (0.1s), very long (300s)
   • Extreme latitudes: Near poles (e.g., 85° north)
   • Crossing meridian: Start at 179°E heading east

2. Error handling tests:

   • Invalid parameters to generateMach5Trajectory() (negative duration, etc.)
   • CLI argument validation

3. Integration test:

   • Generate file, verify it can be parsed and matches expected format
   • Verify compatibility with retina-tracker (could be mentioned in test plan)

---

Additional Observations

📊 Technical Accuracy

• Mach 5 speed: 1715 m/s is correct for sea level (Mach = 340.29 m/s × 5)
• Doppler shift: Expected >100 Hz is realistic for L-band (503 MHz) bistatic radar
• Delay units: Correctly in kilometers (bistatic range / 1000)
• Great circle navigation: Haversine-based implementation is appropriate

📝 Documentation Excellence

• README examples are practical and cover common use cases
• Comparison table (Mach 5 vs Normal Aircraft) is very helpful
• Integration with retina-tracker is well explained
• Output format documentation is clear

🎯 Feature Completeness

• Fully implements issue Add synthetic data for anomalous Mach 5 targets #15 requirements
• Compatible with retina-tracker format
• CLI provides good user experience
• Noise capabilities align with existing synthetic framework

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Recommendations

Must Fix (none)

No critical issues found.

Should Fix

1. Add argument bounds checking in CLI parseArgs()
2. Document first-point Doppler=0 behavior in JSDoc
3. Add constants documentation for conversion factors

Nice to Have

1. Optional startTimestamp parameter for reproducible trajectories
2. Longitude wrapping for ±180° meridian crossing
3. Additional edge case tests (extreme latitudes, negative headings)
4. Performance optimization: cache ECEF calculations in Doppler loop

---

Summary

This PR delivers a high-quality feature that will be valuable for testing anomaly detection in retina-tracker. The implementation is mathematically sound, well-tested, and thoroughly documented. The minor suggestions above would further improve robustness and maintainability, but the code is ready to merge as-is.

Status: ✅ Approved
Confidence: High - code is production-ready with excellent test coverage

Great work! 🚀