This addresses Issue #18 by correcting the CPR global decoding algorithm
implementation that was causing aircraft to appear tens of kilometers
east of their actual positions.
Root Cause:
- CPR longitude zone calculation always used NL-1 regardless of frame type
- Specification requires: even frames use NL zones, odd frames use NL-1 zones
- This caused systematic eastward displacement for all aircraft positions
Changes:
- Implement frame-consistent zone calculations per RTCA DO-260B specification
- Even frame (i=0): ni = max(1, NL - 0) = max(1, NL)
- Odd frame (i=1): ni = max(1, NL - 1)
- Add comprehensive documentation with authoritative source references
- Ensure latitude and longitude use same frame choice for consistency
Testing:
- Aircraft south of Oslo now appears correctly (was near Skarlandsvatnet)
- Aircraft north of Oslo should now appear correctly (was east of Lillestrøm)
- Systematic eastward bias eliminated
References:
- RTCA DO-260B / EUROCAE ED-102A: ADS-B Performance Standards
- ICAO Annex 10, Volume IV: Surveillance Systems
- "Decoding ADS-B position" by Edward Lester
- PyModeS reference implementation
Fixes#18🤖 Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>
