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+# GPS-Denied Onboard Navigation System
+
+## What This Is
+
+Real-time GPS-independent position estimation system for a fixed-wing UAV operating in GPS-denied/spoofed environments (flat terrain, Ukraine). Runs onboard a Jetson Orin Nano Super (8GB shared, 67 TOPS). Fuses visual odometry (cuVSLAM), satellite image matching (TensorRT FP16), and IMU via an ESKF to output MAVLink GPS_INPUT to an ArduPilot flight controller at 5-10Hz, while also streaming position and confidence over SSE to a ground station.
+
+## Core Value
+
+The flight controller must receive valid MAVLink GPS_INPUT at 5-10Hz with position accuracy ≤50m for 80% of frames — without this, the UAV cannot navigate in GPS-denied airspace.
+
+## Requirements
+
+### Validated
+
+- ✓ FastAPI service scaffold with SSE streaming — existing
+- ✓ FlightProcessor orchestrator with NORMAL/LOST/RECOVERY state machine — existing
+- ✓ CoordinateTransformer (GPS↔ENU, pixel→camera→body→NED→WGS84) — existing
+- ✓ SatelliteDataManager (tile fetch, diskcache, GeoHash lookup) — existing
+- ✓ ImageInputPipeline (frame queue, batch validation, storage) — existing
+- ✓ SQLAlchemy async DB layer (flights, waypoints, frames, results) — existing
+- ✓ Pydantic schema contracts for all inter-component data — existing
+- ✓ ABC interfaces for all core components (VO, GPR, metric, graph) — existing
+
+### Active
+
+- [ ] ESKF implementation (15-state error-state Kalman filter: IMU prediction + VO update + satellite update + covariance propagation)
+- [ ] MAVLink GPS_INPUT output (pymavlink, UART/UDP, 5-10Hz loop, ESKF state→GPS_INPUT field mapping)
+- [ ] Real VO implementation (cuVSLAM on Jetson / OpenCV ORB stub on dev for CI)
+- [ ] Real TensorRT inference (SuperPoint+LightGlue for VO, XFeat for satellite matching — FP16 on Jetson)
+- [ ] Satellite feature matching pipeline (tile selection by ESKF uncertainty, RANSAC homography, WGS84 extraction)
+- [ ] GlobalPlaceRecognition implementation (AnyLoc/DINOv2 candidate retrieval, FAISS index, tile scoring)
+- [ ] FactorGraph implementation (pose graph with VO edges + satellite anchor nodes, optimization loop)
+- [ ] FailureRecoveryCoordinator (tracking loss detection, re-init protocol, operator re-localization hint)
+- [ ] End-to-end pipeline wiring (processor.process_frame → VO → ESKF → satellite → GPS_INPUT)
+- [ ] Docker SITL test harness (ArduPilot SITL, camera replay, tile server mock, CI integration)
+- [ ] Confidence scoring and GPS_INPUT fix_type mapping (HIGH/MEDIUM/LOW → fix_type 3/2/0)
+- [ ] Object GPS localization endpoint (POST /objects/locate with gimbal angle projection)
+
+### Out of Scope
+
+- TensorRT engine building tooling — engines are pre-built offline, system only loads them
+- Google Maps tile download tooling — tiles pre-cached before flight, not streamed live
+- Full ArduPilot integration testing on hardware — Jetson hardware validation is post-v1
+- Mobile/web ground station UI — SSE stream is consumed by external systems
+- Multi-UAV coordination — single UAV instance only
+
+## Context
+
+**Hardware target:** Jetson Orin Nano Super (8GB LPDDR5 shared, JetPack 6.2.2, CUDA 12.6, TRT 10.3.0). All development happens on x86 Linux; cuVSLAM and TRT are Jetson-only — dev machine uses OpenCV ORB stub and MockInferenceEngine.
+
+**Camera:** ADTI 20L V1 (5456×3632, APS-C, 16mm lens, nadir fixed, 0.7fps). AI detection camera: Viewpro A40 Pro (separate).
+
+**Flight controller:** ArduPilot via MAVLink UART. System sends GPS_INPUT; receives IMU (200Hz) and GLOBAL_POSITION_INT (1Hz) from FC.
+
+**Key latency budget:** <400ms end-to-end per frame (camera @ 0.7fps = 1430ms window).
+
+**Existing scaffold:** ~2800 lines of Python code exist as a well-structured scaffold. All modules are present with ABC interfaces and schemas, but critical algorithmic kernels (ESKF, real VO, TRT inference, MAVLink) are missing or mocked.
+
+**Test data:** 60 UAV frames (AD000001-AD000060.jpg), coordinates.csv with ground-truth GPS, expected_results/position_accuracy.csv. 43 documented test scenarios across 7 categories.
+
+## Constraints
+
+- **Performance**: <400ms/frame end-to-end, <8GB RAM+VRAM — non-negotiable for real-time flight
+- **Hardware**: cuVSLAM v15.0.0 (aarch64-only wheel) — stub interface required for CI
+- **Platform**: JetPack 6.2.2, Python 3.10+, TensorRT 10.3.0, CUDA 12.6
+- **Navigation accuracy**: 80% frames ≤50m, 60% frames ≤20m, max drift 100m between satellite corrections
+- **Resilience**: Handle sharp turns (disconnected VO segments), 3+ consecutive satellite match failures
+
+## Key Decisions
+
+| Decision | Rationale | Outcome |
+|----------|-----------|---------|
+| ESKF over EKF/UKF | 15-state error-state formulation avoids quaternion singularities, standard for INS | — Pending |
+| XFeat over LiteSAM for satellite matching | LiteSAM may exceed 400ms budget on Jetson; XFeat is faster | — Pending (benchmark required) |
+| OpenCV ORB stub for dev/CI | cuVSLAM is aarch64-only; CI must run on x86 | — Pending |
+| AnyLoc/DINOv2 for GPR | Validated on UAV-VisLoc benchmark (17.86m RMSE) | — Pending |
+| diskcache + GeoHash for tiles | O(1) tile lookup, no DB overhead, LRU eviction | ✓ Good |
+| AsyncSQLAlchemy + aiosqlite | Non-blocking DB for async FastAPI service | ✓ Good |
+
+## Evolution
+
+This document evolves at phase transitions and milestone boundaries.
+
+**After each phase transition** (via `/gsd:transition`):
+1. Requirements invalidated? → Move to Out of Scope with reason
+2. Requirements validated? → Move to Validated with phase reference
+3. New requirements emerged? → Add to Active
+4. Decisions to log? → Add to Key Decisions
+5. "What This Is" still accurate? → Update if drifted
+
+**After each milestone** (via `/gsd:complete-milestone`):
+1. Full review of all sections
+2. Core Value check — still the right priority?
+3. Audit Out of Scope — reasons still valid?
+4. Update Context with current state
+
+---
+*Last updated: 2026-04-01 after initialization*