# Solution Draft

## Assessment Findings


| Old Component Solution                                                               | Weak Point (functional/security/performance)                                                                                                                                                                            | New Solution                                                                                                                                                                                                                                                   |
| ------------------------------------------------------------------------------------ | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| ESKF described as "16-state vector, ~10MB" with no mathematical specification        | **Functional**: No state vector, no process model (F,Q), no measurement models (H for VO, H for satellite), no noise parameters, no scale observability analysis. Impossible to implement or validate accuracy claims.  | **Define complete ESKF specification**: 15-state error vector, IMU-driven prediction, dual measurement models (VO relative pose, satellite absolute position), initial Q/R values, scale constraint via altitude + satellite corrections.                      |
| GPS_INPUT at 5-10Hz via pymavlink — no field mapping                                 | **Functional**: GPS_INPUT requires 15+ fields (velocity, accuracy, hdop, fix_type, GPS time). No specification of how ESKF state maps to these fields. ArduPilot requires minimum 5Hz.                                  | **Define GPS_INPUT population spec**: velocity from ESKF, accuracy from covariance, fix_type from confidence tier, GPS time from system clock conversion, synthesized hdop/vdop.                                                                               |
| Confidence scoring "unchanged from draft03" — not in draft05                         | **Functional**: Draft05 is supposed to be self-contained. Confidence scoring determines GPS_INPUT accuracy fields and fix_type — directly affects how ArduPilot EKF weights the position data.                          | **Define confidence scoring inline**: 3 tiers (satellite-anchored, VO-tracked, IMU-only) mapping to fix_type + accuracy values.                                                                                                                                |
| Coordinate transformations not defined                                               | **Functional**: No pixel→camera→body→NED→WGS84 chain. Camera is not autostabilized, so body attitude matters. Satellite match → WGS84 conversion undefined. Object localization impossible without these transforms.    | **Define coordinate transformation chain**: camera intrinsics K, camera-to-body extrinsic T_cam_body, body-to-NED from ESKF attitude, NED origin at mission start point.                                                                                       |
| Disconnected route segments — "satellite re-localization" mentioned but no algorithm | **Functional**: AC requires handling as "core to the system." Multiple disconnected segments expected. No tracking-loss detection, no re-localization trigger, no ESKF re-initialization, no cuVSLAM restart procedure. | **Define re-localization pipeline**: detect cuVSLAM tracking loss → IMU-only ESKF prediction → trigger satellite match on every frame → on match success: ESKF position reset + cuVSLAM restart → on 3 consecutive failures: operator re-localization request. |
| No startup handoff from GPS to GPS-denied                                            | **Functional**: System reads GLOBAL_POSITION_INT at startup but no protocol for when GPS is lost/spoofed vs system start. No validation of initial position.                                                            | **Define handoff protocol**: system runs continuously, FC receives both real GPS and GPS_INPUT. GPS-denied system always provides its estimate; FC selects best source. Initial position validated against first satellite match.                              |
| No mid-flight reboot recovery                                                        | **Functional**: AC requires: "re-initialize from flight controller's current IMU-extrapolated position." No procedure defined. Recovery time estimation missing.                                                        | **Define reboot recovery sequence**: read FC position → init ESKF with high uncertainty → load TRT engines → start cuVSLAM → immediate satellite match. Estimated recovery: ~35-70s. Document as known limitation.                                             |
| 3-consecutive-failure re-localization request undefined                              | **Functional**: AC requires ground station re-localization request. No message format, no operator workflow, no system behavior while waiting.                                                                          | **Define re-localization protocol**: detect 3 failures → send custom MAVLink message with last known position + uncertainty → operator provides approximate coordinates → system uses as ESKF measurement with high covariance.                                |
| Object localization — "trigonometric calculation" with no details                    | **Functional**: No math, no API, no Viewpro gimbal integration, no accuracy propagation. Other onboard systems cannot use this component as specified.                                                                  | **Define object localization**: pixel→ray using Viewpro intrinsics + gimbal angles → body frame → NED → ray-ground intersection → WGS84. FastAPI endpoint: POST /objects/locate. Accuracy propagated from UAV position + gimbal uncertainty.                   |
| Satellite matching — GSD normalization and tile selection unspecified                | **Functional**: Camera GSD ~15.9 cm/px at 600m vs satellite ~0.3 m/px at zoom 19. The "pre-resize" step is mentioned but not specified. Tile selection radius based on ESKF uncertainty not defined.                    | **Define GSD handling**: downsample camera frame to match satellite GSD. Define tile selection: ESKF position ± 3σ_horizontal → select tiles covering that area. Assemble tile mosaic for matching.                                                            |
| Satellite tile storage requirements not calculated                                   | **Functional**: "±2km" preload mentioned but no storage estimate. At zoom 19: a 200km path with ±2km buffer requires ~~130K tiles (~~2.5GB).                                                                            | **Calculate tile storage**: specify zoom level (18 preferred — 0.6m/px, 4× fewer tiles), estimate storage per mission profile, define maximum mission area by storage limit.                                                                                   |
| FastAPI endpoints not in solution draft                                              | **Functional**: Endpoints only in security_analysis.md. No request/response schemas. No SSE event format. No object localization endpoint.                                                                              | **Consolidate API spec in solution**: define all endpoints, SSE event schema, object localization endpoint. Reference security_analysis.md for auth.                                                                                                           |
| cuVSLAM configuration missing (calibration, IMU params, mode)                        | **Functional**: No camera calibration procedure, no IMU noise parameters, no T_imu_rig extrinsic, no mode selection (Mono vs Inertial).                                                                                 | **Define cuVSLAM configuration**: use Inertial mode, specify required calibration data (camera intrinsics, distortion, IMU noise params from datasheet, T_imu_rig from physical measurement), define calibration procedure.                                    |
| tech_stack.md inconsistent with draft05                                              | **Functional**: tech_stack.md says 3fps (should be 0.7fps), LiteSAM at 480px (should be 1280px), missing EfficientLoFTR.                                                                                                | **Flag for update**: tech_stack.md must be synchronized with draft05 corrections. Not addressed in this draft — separate task.                                                                                                                                 |


## Overall Maturity Assessment


| Category                      | Maturity (1-5) | Assessment                                                                                                                              |
| ----------------------------- | -------------- | --------------------------------------------------------------------------------------------------------------------------------------- |
| Hardware & Platform Selection | 3.5            | UAV airframe, cameras, Jetson, batteries — well-researched with specs, weight budget, endurance calculations. Ready for procurement.    |
| Core Algorithm Selection      | 3.0            | cuVSLAM, LiteSAM/XFeat, ESKF — components selected with comparison tables, fallback chains, decision trees. Day-one benchmarks defined. |
| AI Inference Runtime          | 3.5            | TRT Engine migration thoroughly analyzed. Conversion workflows, memory savings, performance estimates. Code wrapper provided.           |
| Sensor Fusion (ESKF)          | 1.5            | Mentioned but not specified. No implementable detail. Blockerfor coding.                                                                |
| System Integration            | 1.5            | GPS_INPUT, coordinate transforms, inter-component data flow — all under-specified.                                                      |
| Edge Cases & Resilience       | 1.0            | Disconnected segments, reboot recovery, re-localization — acknowledged but no algorithms.                                               |
| Operational Readiness         | 0.5            | No pre-flight procedures, no in-flight monitoring, no failure response.                                                                 |
| Security                      | 3.0            | Comprehensive threat model, OP-TEE analysis, LUKS, secure boot. Well-researched.                                                        |
| **Overall TRL**               | **~2.5**       | **Technology concept formulated + some component validation. Not implementation-ready.**                                                |


The solution is at approximately **TRL 3** (proof of concept) for hardware/algorithm selection and **TRL 1-2** (basic concept) for system integration, ESKF, and operational procedures.

## Product Solution Description

A real-time GPS-denied visual navigation system for fixed-wing UAVs, running on a Jetson Orin Nano Super (8GB). All AI model inference uses native TensorRT Engine files. The system replaces the GPS module by sending MAVLink GPS_INPUT messages via pymavlink over UART at 5-10Hz.

Position is determined by fusing: (1) CUDA-accelerated visual odometry (cuVSLAM in Inertial mode) from ADTI 20L V1 at 0.7 fps sustained, (2) absolute position corrections from satellite image matching (LiteSAM or XFeat — TRT Engine FP16) using keyframes from the same ADTI image stream, and (3) IMU data from the flight controller via ESKF. Viewpro A40 Pro is reserved for AI object detection only.

The ESKF is the central state estimator with 15-state error vector. It fuses:

- **IMU prediction** at 5-10Hz (high-frequency pose propagation)
- **cuVSLAM VO measurement** at 0.7Hz (relative pose correction)
- **Satellite matching measurement** at ~0.07-0.14Hz (absolute position correction)

GPS_INPUT messages carry position, velocity, and accuracy derived from the ESKF state and covariance.

**Hard constraint**: ADTI 20L V1 shoots at 0.7 fps sustained (1430ms interval). Full VO+ESKF pipeline within 400ms per frame. Satellite matching async on keyframes (every 5-10 camera frames). GPS_INPUT at 5-10Hz (ESKF IMU prediction fills gaps between camera frames).

```
┌─────────────────────────────────────────────────────────────────────┐
│                    OFFLINE (Before Flight)                           │
│  1. Satellite Tiles → Download & Validate → Pre-resize → Store      │
│     (Google Maps)     (≥0.5m/px, <2yr)     (matcher res)  (GeoHash)│
│  2. TRT Engine Build (one-time per model version):                  │
│     PyTorch model → reparameterize → ONNX export → trtexec --fp16  │
│     Output: litesam.engine, xfeat.engine                            │
│  3. Camera + IMU calibration (one-time per hardware unit)           │
│  4. Copy tiles + engines + calibration to Jetson storage            │
└─────────────────────────────────────────────────────────────────────┘
                              │
                              ▼
┌─────────────────────────────────────────────────────────────────────┐
│                    ONLINE (During Flight)                            │
│                                                                     │
│  STARTUP:                                                           │
│  1. pymavlink → read GLOBAL_POSITION_INT → init ESKF state         │
│  2. Load TRT engines + allocate GPU buffers                         │
│  3. Load camera calibration + IMU calibration                       │
│  4. Start cuVSLAM (Inertial mode) with ADTI 20L V1                 │
│  5. Preload satellite tiles ±2km into RAM                           │
│  6. First satellite match → validate initial position               │
│  7. Begin GPS_INPUT output loop at 5-10Hz                           │
│                                                                     │
│  EVERY CAMERA FRAME (0.7fps from ADTI 20L V1):                     │
│  ┌──────────────────────────────────────┐                           │
│  │ ADTI 20L V1 → Downsample (CUDA)     │                           │
│  │             → cuVSLAM VO+IMU (~9ms)  │ ← CUDA Stream A          │
│  │             → ESKF VO measurement    │                           │
│  └──────────────────────────────────────┘                           │
│                                                                     │
│  5-10Hz CONTINUOUS (IMU-driven between camera frames):              │
│  ┌──────────────────────────────────────┐                           │
│  │ IMU data → ESKF prediction           │                           │
│  │ ESKF state → GPS_INPUT fields        │                           │
│  │ GPS_INPUT → Flight Controller (UART) │                           │
│  └──────────────────────────────────────┘                           │
│                                                                     │
│  KEYFRAMES (every 5-10 camera frames, async):                       │
│  ┌──────────────────────────────────────┐                           │
│  │ Camera frame → GSD downsample        │                           │
│  │ Select satellite tile (ESKF pos±3σ)  │                           │
│  │ TRT inference (Stream B): LiteSAM/   │                           │
│  │   XFeat → correspondences            │                           │
│  │ RANSAC → homography → WGS84 position │                           │
│  │ ESKF satellite measurement update    │──→ Position correction    │
│  └──────────────────────────────────────┘                           │
│                                                                     │
│  TRACKING LOSS (cuVSLAM fails — sharp turn / featureless):         │
│  ┌──────────────────────────────────────┐                           │
│  │ ESKF → IMU-only prediction (growing  │                           │
│  │   uncertainty)                        │                           │
│  │ Satellite match on EVERY frame       │                           │
│  │ On match success → ESKF reset +      │                           │
│  │   cuVSLAM restart                    │                           │
│  │ 3 consecutive failures → operator    │                           │
│  │   re-localization request            │                           │
│  └──────────────────────────────────────┘                           │
│                                                                     │
│  TELEMETRY (1Hz):                                                   │
│  ┌──────────────────────────────────────┐                           │
│  │ NAMED_VALUE_FLOAT: confidence, drift │──→ Ground Station         │
│  └──────────────────────────────────────┘                           │
└─────────────────────────────────────────────────────────────────────┘
```

## Architecture

### Component: ESKF Sensor Fusion (NEW — previously unspecified)

**Error-State Kalman Filter** fusing IMU, visual odometry, and satellite matching.

**Nominal state vector** (propagated by IMU):


| State      | Symbol | Size | Description                                      |
| ---------- | ------ | ---- | ------------------------------------------------ |
| Position   | p      | 3    | NED position relative to mission origin (meters) |
| Velocity   | v      | 3    | NED velocity (m/s)                               |
| Attitude   | q      | 4    | Unit quaternion (body-to-NED rotation)           |
| Accel bias | b_a    | 3    | Accelerometer bias (m/s²)                        |
| Gyro bias  | b_g    | 3    | Gyroscope bias (rad/s)                           |


**Error-state vector** (estimated by ESKF): δx = [δp, δv, δθ, δb_a, δb_g]ᵀ ∈ ℝ¹⁵
where δθ ∈ so(3) is the 3D rotation error.

**Prediction step** (IMU at 5-10Hz from flight controller):

- Input: accelerometer a_m, gyroscope ω_m, dt
- Propagate nominal state: p += v·dt, v += (R(q)·(a_m - b_a) - g)·dt, q ⊗= Exp(ω_m - b_g)·dt
- Propagate error covariance: P = F·P·Fᵀ + Q
- F is the 15×15 error-state transition matrix (standard ESKF formulation)
- Q: process noise diagonal, initial values from IMU datasheet noise densities

**VO measurement update** (0.7Hz from cuVSLAM):

- cuVSLAM outputs relative pose: ΔR, Δt (camera frame)
- Transform to NED: Δp_ned = R_body_ned · T_cam_body · Δt
- Innovation: z = Δp_ned_measured - Δp_ned_predicted
- Observation matrix H_vo maps error state to relative position change
- R_vo: measurement noise, initial ~0.1-0.5m (from cuVSLAM precision at 600m+ altitude)
- Kalman update: K = P·Hᵀ·(H·P·Hᵀ + R)⁻¹, δx = K·z, P = (I - K·H)·P

**Satellite measurement update** (0.07-0.14Hz, async):

- Satellite matching outputs absolute position: lat_sat, lon_sat in WGS84
- Convert to NED relative to mission origin
- Innovation: z = p_satellite - p_predicted
- H_sat = [I₃, 0, 0, 0, 0] (directly observes position)
- R_sat: measurement noise, from matching confidence (~5-20m based on RANSAC inlier ratio)
- Provides absolute position correction — bounds drift accumulation

**Scale observability**:

- Monocular cuVSLAM has scale ambiguity during constant-velocity flight
- Scale is constrained by: (1) satellite matching absolute positions (primary), (2) known flight altitude from barometer + predefined mission altitude, (3) IMU accelerometer during maneuvers
- During long straight segments without satellite correction, scale drift is possible. Satellite corrections every ~7-14s re-anchor scale.

**Tuning approach**: Start with IMU datasheet noise values for Q. Start with conservative R values (high measurement noise). Tune on flight test data by comparing ESKF output to known GPS ground truth.


| Solution                   | Tools           | Advantages                                                    | Limitations                            | Performance   | Fit         |
| -------------------------- | --------------- | ------------------------------------------------------------- | -------------------------------------- | ------------- | ----------- |
| Custom ESKF (Python/NumPy) | NumPy, SciPy    | Full control, minimal dependencies, well-understood algorithm | Implementation effort, tuning required | <1ms per step | ✅ Selected  |
| FilterPy ESKF              | FilterPy v1.4.5 | Reference implementation, less code                           | Less flexible for multi-rate fusion    | <1ms per step | ⚠️ Fallback |


### Component: Coordinate System & Transformations (NEW — previously undefined)

**Reference frames**:

- **Camera frame (C)**: origin at camera optical center, Z forward, X right, Y down (OpenCV convention)
- **Body frame (B)**: origin at UAV CG, X forward (nose), Y right (starboard), Z down
- **NED frame (N)**: North-East-Down, origin at mission start point
- **WGS84**: latitude, longitude, altitude (output format)

**Transformation chain**:

1. **Pixel → Camera ray**: p_cam = K⁻¹ · [u, v, 1]ᵀ where K = camera intrinsic matrix (ADTI 20L V1: fx, fy from 16mm lens + APS-C sensor)
2. **Camera → Body**: p_body = T_cam_body · p_cam where T_cam_body is the fixed mounting rotation (camera points nadir: 90° pitch rotation from body X-forward to camera Z-down)
3. **Body → NED**: p_ned = R_body_ned(q) · p_body where q is the ESKF quaternion attitude estimate
4. **NED → WGS84**: lat = lat_origin + p_north / R_earth, lon = lon_origin + p_east / (R_earth · cos(lat_origin)) where (lat_origin, lon_origin) is the mission start GPS position

**Camera intrinsic matrix K** (ADTI 20L V1 + 16mm lens):

- Sensor: 23.2 × 15.4 mm, Resolution: 5456 × 3632
- fx = fy = focal_mm × width_px / sensor_width_mm = 16 × 5456 / 23.2 = 3763 pixels
- cx = 2728, cy = 1816 (sensor center)
- Distortion: Brown model (k1, k2, p1, p2 from calibration)

**T_cam_body** (camera mount):

- Navigation camera is fixed, pointing nadir (downward), not autostabilized
- R_cam_body = R_x(180°) · R_z(0°) (camera Z-axis aligned with body -Z, camera X with body X)
- Translation: offset from CG to camera mount (measured during assembly, typically <0.3m)

**Satellite match → WGS84**:

- Feature correspondences between camera frame and geo-referenced satellite tile
- Homography H maps camera pixels to satellite tile pixels
- Satellite tile pixel → WGS84 via tile's known georeference (zoom level + tile x,y → lat,lon)
- Camera center projects to satellite pixel (cx_sat, cy_sat) via H
- Convert (cx_sat, cy_sat) to WGS84 using tile georeference

### Component: GPS_INPUT Message Population (NEW — previously undefined)


| GPS_INPUT Field         | Source                                         | Computation                                                                                             |
| ----------------------- | ---------------------------------------------- | ------------------------------------------------------------------------------------------------------- |
| lat, lon                | ESKF position (NED)                            | NED → WGS84 conversion using mission origin                                                             |
| alt                     | ESKF position (Down) + mission origin altitude | alt = alt_origin - p_down                                                                               |
| vn, ve, vd              | ESKF velocity state                            | Direct from ESKF v[0], v[1], v[2]                                                                       |
| fix_type                | Confidence tier                                | 3 (3D fix) when satellite-anchored (last match <30s). 2 (2D) when VO-only. 0 (no fix) when IMU-only >5s |
| hdop                    | ESKF horizontal covariance                     | hdop = sqrt(P[0,0] + P[1,1]) / 5.0 (approximate CEP→HDOP mapping)                                       |
| vdop                    | ESKF vertical covariance                       | vdop = sqrt(P[2,2]) / 5.0                                                                               |
| horiz_accuracy          | ESKF horizontal covariance                     | horiz_accuracy = sqrt(P[0,0] + P[1,1]) meters                                                           |
| vert_accuracy           | ESKF vertical covariance                       | vert_accuracy = sqrt(P[2,2]) meters                                                                     |
| speed_accuracy          | ESKF velocity covariance                       | speed_accuracy = sqrt(P[3,3] + P[4,4]) m/s                                                              |
| time_week, time_week_ms | System time                                    | Convert Unix time to GPS epoch (GPS epoch = 1980-01-06, subtract leap seconds)                          |
| satellites_visible      | Constant                                       | 10 (synthetic — prevents satellite-count failsafes in ArduPilot)                                        |
| gps_id                  | Constant                                       | 0                                                                                                       |
| ignore_flags            | Constant                                       | 0 (provide all fields)                                                                                  |


**Confidence tiers** mapping to GPS_INPUT:


| Tier   | Condition                                         | fix_type   | horiz_accuracy                  | Rationale                              |
| ------ | ------------------------------------------------- | ---------- | ------------------------------- | -------------------------------------- |
| HIGH   | Satellite match <30s ago, ESKF covariance < 400m² | 3 (3D fix) | From ESKF P (typically 5-20m)   | Absolute position anchor recent        |
| MEDIUM | cuVSLAM tracking OK, no recent satellite match    | 3 (3D fix) | From ESKF P (typically 20-50m)  | Relative tracking valid, drift growing |
| LOW    | cuVSLAM lost, IMU-only                            | 2 (2D fix) | From ESKF P (50-200m+, growing) | Only IMU dead reckoning, rapid drift   |
| FAILED | 3+ consecutive total failures                     | 0 (no fix) | 999.0                           | System cannot determine position       |


### Component: Disconnected Route Segment Handling (NEW — previously undefined)

**Trigger**: cuVSLAM reports tracking_lost OR tracking confidence drops below threshold

**Algorithm**:

```
STATE: TRACKING_NORMAL
  cuVSLAM provides relative pose
  ESKF VO measurement updates at 0.7Hz
  Satellite matching on keyframes (every 5-10 frames)

STATE: TRACKING_LOST (enter when cuVSLAM reports loss)
  1. ESKF continues with IMU-only prediction (no VO updates)
     → uncertainty grows rapidly (~1-5 m/s drift with consumer IMU)
  2. Switch satellite matching to EVERY frame (not just keyframes)
     → maximize chances of getting absolute correction
  3. For each camera frame:
     a. Attempt satellite match using ESKF predicted position ± 3σ for tile selection
     b. If match succeeds (RANSAC inlier ratio > 30%):
        → ESKF measurement update with satellite position
        → Restart cuVSLAM with current frame as new origin
        → Transition to TRACKING_NORMAL
        → Reset failure counter
     c. If match fails:
        → Increment failure_counter
        → Continue IMU-only ESKF prediction
  4. If failure_counter >= 3:
     → Send re-localization request to ground station
     → GPS_INPUT fix_type = 0 (no fix), horiz_accuracy = 999.0
     → Continue attempting satellite matching on each frame
  5. If operator sends re-localization hint (approximate lat,lon):
     → Use as ESKF measurement with high covariance (~500m)
     → Attempt satellite match in that area
     → On success: transition to TRACKING_NORMAL

STATE: SEGMENT_DISCONNECT
  After re-localization following tracking loss:
  → New cuVSLAM track is independent of previous track
  → ESKF maintains global NED position continuity via satellite anchor
  → No need to "connect" segments at the cuVSLAM level
  → ESKF already handles this: satellite corrections keep global position consistent
```

### Component: Satellite Image Matching Pipeline (UPDATED — added GSD + tile selection details)

**GSD normalization**:

- Camera GSD at 600m: ~15.9 cm/pixel (ADTI 20L V1 + 16mm)
- Satellite tile GSD at zoom 18: ~0.6 m/pixel
- Scale ratio: ~3.8:1
- Downsample camera image to satellite GSD before matching: resize from 5456×3632 to ~1440×960 (matching zoom 18 GSD)
- This is close to LiteSAM's 1280px input — use 1280px with minor GSD mismatch acceptable for matching

**Tile selection**:

- Input: ESKF position estimate (lat, lon) + horizontal covariance σ_h
- Search radius: max(3·σ_h, 500m) — at least 500m to handle initial uncertainty
- Compute geohash for center position → load tiles covering the search area
- Assemble tile mosaic if needed (typically 2×2 to 4×4 tiles for adequate coverage)
- If ESKF uncertainty > 2km: tile selection unreliable, fall back to wider search or request operator input

**Tile storage calculation** (zoom 18 — 0.6 m/pixel):

- Each 256×256 tile covers ~153m × 153m
- Flight path 200km with ±2km buffer: area ≈ 200km × 4km = 800 km²
- Tiles needed: 800,000,000 / (153 × 153) ≈ 34,200 tiles
- Storage: ~10-15KB per JPEG tile → ~340-510 MB
- With zoom 19 overlap tiles for higher precision: ×4 = ~1.4-2.0 GB
- Recommended: zoom 18 primary + zoom 19 for ±500m along flight path → ~500-800 MB total


| Solution                               | Tools                     | Advantages                                                               | Limitations                                            | Performance (est. Orin Nano Super TRT FP16) | Params | Fit                             |
| -------------------------------------- | ------------------------- | ------------------------------------------------------------------------ | ------------------------------------------------------ | ------------------------------------------- | ------ | ------------------------------- |
| LiteSAM (opt) TRT Engine FP16 @ 1280px | trtexec + tensorrt Python | Best satellite-aerial accuracy (RMSE@30=17.86m UAV-VisLoc), 6.31M params | MinGRU TRT export needs verification (LOW-MEDIUM risk) | Est. ~165-330ms                             | 6.31M  | ✅ Primary                       |
| EfficientLoFTR TRT Engine FP16         | trtexec + tensorrt Python | Proven TRT path (Coarse_LoFTR_TRT). Semi-dense. CVPR 2024.               | 2.4x more params than LiteSAM.                         | Est. ~200-400ms                             | 15.05M | ✅ Fallback if LiteSAM TRT fails |
| XFeat TRT Engine FP16                  | trtexec + tensorrt Python | Fastest. Proven TRT implementation.                                      | General-purpose, not designed for cross-view gap.      | Est. ~50-100ms                              | <5M    | ✅ Speed fallback                |


### Component: cuVSLAM Configuration (NEW — previously undefined)

**Mode**: Inertial (mono camera + IMU)

**Camera configuration** (ADTI 20L V1 + 16mm lens):

- Model: Brown distortion
- fx = fy = 3763 px (16mm on 23.2mm sensor at 5456px width)
- cx = 2728 px, cy = 1816 px
- Distortion coefficients: from calibration (k1, k2, p1, p2)
- Border: 50px (ignore lens edge distortion)

**IMU configuration** (Pixhawk 6x IMU — ICM-42688-P):

- Gyroscope noise density: 3.0 × 10⁻³ °/s/√Hz
- Gyroscope random walk: 5.0 × 10⁻⁵ °/s²/√Hz
- Accelerometer noise density: 70 µg/√Hz
- Accelerometer random walk: ~2.0 × 10⁻³ m/s³/√Hz
- IMU frequency: 200 Hz (from flight controller via MAVLink)
- T_imu_rig: measured transformation from Pixhawk IMU to camera center (translation + rotation)

**cuVSLAM settings**:

- OdometryMode: INERTIAL
- MulticameraMode: PRECISION (favor accuracy over speed — we have 1430ms budget)
- Input resolution: downsample to 1280×852 (or 720p) for processing speed
- async_bundle_adjustment: True

**Initialization**:

- cuVSLAM initializes automatically when it receives the first camera frame + IMU data
- First few frames used for feature initialization and scale estimation
- First satellite match validates and corrects the initial position

**Calibration procedure** (one-time per hardware unit):

1. Camera intrinsics: checkerboard calibration with OpenCV (or use manufacturer data if available)
2. Camera-IMU extrinsic (T_imu_rig): Kalibr tool with checkerboard + IMU data
3. IMU noise parameters: Allan variance analysis or use datasheet values
4. Store calibration files on Jetson storage

### Component: AI Model Inference Runtime (UNCHANGED)

Native TRT Engine — optimal performance and memory on fixed NVIDIA hardware. See draft05 for full comparison table and conversion workflow.

### Component: Visual Odometry (UNCHANGED)

cuVSLAM in Inertial mode, fed by ADTI 20L V1 at 0.7 fps sustained. See draft05 for feasibility analysis at 0.7fps.

### Component: Flight Controller Integration (UPDATED — added GPS_INPUT field spec)

pymavlink over UART at 5-10Hz. GPS_INPUT field population defined above.

ArduPilot configuration:

- GPS1_TYPE = 14 (MAVLink)
- GPS_RATE = 5 (minimum, matching our 5-10Hz output)
- EK3_SRC1_POSXY = 1 (GPS), EK3_SRC1_VELXY = 1 (GPS) — EKF uses GPS_INPUT as position/velocity source

### Component: Object Localization (NEW — previously undefined)

**Input**: pixel coordinates (u, v) in Viewpro A40 Pro image, current gimbal angles (pan_deg, tilt_deg), zoom factor, UAV position from GPS-denied system, UAV altitude

**Process**:

1. Pixel → camera ray: ray_cam = K_viewpro⁻¹(zoom) · [u, v, 1]ᵀ
2. Camera → gimbal frame: ray_gimbal = R_gimbal(pan, tilt) · ray_cam
3. Gimbal → body: ray_body = T_gimbal_body · ray_gimbal
4. Body → NED: ray_ned = R_body_ned(q) · ray_body
5. Ray-ground intersection: assuming flat terrain at UAV altitude h: t = -h / ray_ned[2], p_ground_ned = p_uav_ned + t · ray_ned
6. NED → WGS84: convert to lat, lon

**Output**: { lat, lon, accuracy_m, confidence }

- accuracy_m propagated from: UAV position accuracy (from ESKF) + gimbal angle uncertainty + altitude uncertainty

**API endpoint**: POST /objects/locate

- Request: { pixel_x, pixel_y, gimbal_pan_deg, gimbal_tilt_deg, zoom_factor }
- Response: { lat, lon, alt, accuracy_m, confidence, uav_position: {lat, lon, alt}, timestamp }

### Component: Startup, Handoff & Failsafe (UPDATED — added handoff + reboot + re-localization)

**GPS-denied handoff protocol**:

- GPS-denied system runs continuously from companion computer boot
- Reads initial position from FC (GLOBAL_POSITION_INT) — this may be real GPS or last known
- First satellite match validates the initial position
- FC receives both real GPS (if available) and GPS_INPUT; FC EKF selects best source based on accuracy
- No explicit "switch" — the GPS-denied system is a secondary GPS source

**Startup sequence** (expanded from draft05):

1. Boot Jetson → start GPS-Denied service (systemd)
2. Connect to flight controller via pymavlink on UART
3. Wait for heartbeat
4. Initialize PyCUDA context
5. Load TRT engines: litesam.engine + xfeat.engine (~1-3s each)
6. Allocate GPU I/O buffers
7. Create CUDA streams: Stream A (cuVSLAM), Stream B (satellite matching)
8. Load camera calibration + IMU calibration files
9. Read GLOBAL_POSITION_INT → set mission origin (NED reference point) → init ESKF
10. Start cuVSLAM (Inertial mode) with ADTI 20L V1 camera stream
11. Preload satellite tiles within ±2km into RAM
12. Trigger first satellite match → validate initial position
13. Begin GPS_INPUT output loop at 5-10Hz
14. System ready

**Mid-flight reboot recovery**:

1. Jetson boots (~30-60s)
2. GPS-Denied service starts, connects to FC
3. Read GLOBAL_POSITION_INT (FC's current IMU-extrapolated position)
4. Init ESKF with this position + HIGH uncertainty covariance (σ = 200m)
5. Load TRT engines (~2-6s total)
6. Start cuVSLAM (fresh, no prior map)
7. Immediate satellite matching on first camera frame
8. On satellite match success: ESKF corrected, uncertainty drops
9. Estimated total recovery: ~35-70s
10. During recovery: FC uses IMU-only dead reckoning (at 70 km/h: ~700-1400m uncontrolled drift)
11. **Known limitation**: recovery time is dominated by Jetson boot time

**3-consecutive-failure re-localization**:

- Trigger: VO lost + satellite match failed × 3 consecutive camera frames
- Action: send re-localization request via MAVLink STATUSTEXT or custom message
- Message content: "RELOC_REQ: last_lat={lat} last_lon={lon} uncertainty={σ}m"
- Operator response: MAVLink COMMAND_LONG with approximate lat/lon
- System: use operator position as ESKF measurement with R = diag(500², 500², 100²) meters²
- System continues satellite matching with updated search area
- While waiting: GPS_INPUT fix_type=0, IMU-only ESKF prediction continues

### Component: Ground Station Telemetry (UPDATED — added re-localization)

MAVLink messages to ground station:


| Message                       | Rate     | Content                                             |
| ----------------------------- | -------- | --------------------------------------------------- |
| NAMED_VALUE_FLOAT "gps_conf"  | 1Hz      | Confidence score (0.0-1.0)                          |
| NAMED_VALUE_FLOAT "gps_drift" | 1Hz      | Estimated drift from last satellite anchor (meters) |
| NAMED_VALUE_FLOAT "gps_hacc"  | 1Hz      | Horizontal accuracy (meters, from ESKF)             |
| STATUSTEXT                    | On event | "RELOC_REQ: ..." for re-localization request        |
| STATUSTEXT                    | On event | Tracking loss / recovery notifications              |


### Component: Thermal Management (UNCHANGED)

Same adaptive pipeline from draft05. Active cooling required at 25W. Throttling at 80°C SoC junction.

### Component: API & Inter-System Communication (NEW — consolidated)

FastAPI (Uvicorn) running locally on Jetson for inter-process communication with other onboard systems.


| Endpoint              | Method    | Purpose                                | Auth |
| --------------------- | --------- | -------------------------------------- | ---- |
| /sessions             | POST      | Start GPS-denied session               | JWT  |
| /sessions/{id}/stream | GET (SSE) | Real-time position + confidence stream | JWT  |
| /sessions/{id}/anchor | POST      | Operator re-localization hint          | JWT  |
| /sessions/{id}        | DELETE    | End session                            | JWT  |
| /objects/locate       | POST      | Object GPS from pixel coordinates      | JWT  |
| /health               | GET       | System health + memory + thermal       | None |


**SSE event schema** (1Hz):

```json
{
  "type": "position",
  "timestamp": "2026-03-17T12:00:00.000Z",
  "lat": 48.123456,
  "lon": 37.654321,
  "alt": 600.0,
  "accuracy_h": 15.2,
  "accuracy_v": 8.1,
  "confidence": "HIGH",
  "drift_from_anchor": 12.5,
  "vo_status": "tracking",
  "last_satellite_match_age_s": 8.3
}
```

## UAV Platform

Unchanged from draft05. See draft05 for: airframe configuration (3.5m S-2 composite, 12.5kg AUW), flight performance (3.4h endurance at 50 km/h), camera specifications (ADTI 20L V1 + 16mm, Viewpro A40 Pro), ground coverage calculations.

## Speed Optimization Techniques

Unchanged from draft05. Key points: cuVSLAM ~9ms/frame, native TRT Engine (no ONNX RT), dual CUDA streams, 5-10Hz GPS_INPUT from ESKF IMU prediction.

## Processing Time Budget

Unchanged from draft05. VO frame: ~17-22ms. Satellite matching: ≤210ms async. Well within 1430ms frame interval.

## Memory Budget (Jetson Orin Nano Super, 8GB shared)


| Component                 | Memory         | Notes                                       |
| ------------------------- | -------------- | ------------------------------------------- |
| OS + runtime              | ~1.5GB         | JetPack 6.2 + Python                        |
| cuVSLAM                   | ~200-500MB     | CUDA library + map                          |
| LiteSAM TRT engine        | ~50-80MB       | If LiteSAM fails: EfficientLoFTR ~100-150MB |
| XFeat TRT engine          | ~30-50MB       |                                             |
| Preloaded satellite tiles | ~200MB         | ±2km of flight plan                         |
| pymavlink + MAVLink       | ~20MB          |                                             |
| FastAPI (local IPC)       | ~50MB          |                                             |
| ESKF + buffers            | ~10MB          |                                             |
| **Total**                 | **~2.1-2.9GB** | **26-36% of 8GB**                           |


## Key Risks and Mitigations


| Risk                                                    | Likelihood | Impact                                                | Mitigation                                                                                                                                                                |
| ------------------------------------------------------- | ---------- | ----------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| LiteSAM MinGRU ops unsupported in TRT 10.3              | LOW-MEDIUM | LiteSAM TRT export fails                              | Day-one verification. Fallback: EfficientLoFTR TRT → XFeat TRT.                                                                                                           |
| cuVSLAM fails on low-texture terrain at 0.7fps          | HIGH       | Frequent tracking loss                                | Satellite matching corrections bound drift. Re-localization pipeline handles tracking loss. IMU bridges short gaps.                                                       |
| Google Maps satellite quality in conflict zone          | HIGH       | Satellite matching fails, outdated imagery            | Pre-flight tile validation. Consider alternative providers (Bing, Mapbox). Robust to seasonal appearance changes via feature-based matching.                              |
| ESKF scale drift during long constant-velocity segments | MEDIUM     | Position error exceeds 100m between satellite anchors | Satellite corrections every 7-14s re-anchor. Altitude constraint from barometer. Monitor drift rate — if >50m between corrections, increase satellite matching frequency. |
| Monocular scale ambiguity                               | MEDIUM     | Metric scale lost during constant-velocity flight     | Satellite absolute corrections provide scale. Known altitude constrains vertical scale. IMU acceleration during turns provides observability.                             |
| AUW exceeds AT4125 recommended range                    | MEDIUM     | Reduced endurance, motor thermal stress               | 12.5 kg vs 8-10 kg recommended. Monitor motor temps. Weight optimization.                                                                                                 |
| ADTI mechanical shutter lifespan                        | MEDIUM     | Replacement needed periodically                       | ~8,800 actuations/flight at 0.7fps. Estimated 11-57 flights before replacement. Budget as consumable.                                                                     |
| Mid-flight companion computer failure                   | LOW        | ~35-70s position gap                                  | Reboot recovery procedure defined. FC uses IMU dead reckoning during gap. Known limitation.                                                                               |
| Thermal throttling on Jetson                            | MEDIUM     | Satellite matching latency increases                  | Active cooling required. Monitor SoC temp. Throttling at 80°C. Our workload ~8-15W typical — well under 25W TDP.                                                          |
| Engine incompatibility after JetPack update             | MEDIUM     | Must rebuild engines                                  | Include engine rebuild in update procedure.                                                                                                                               |
| TRT engine build OOM on 8GB                             | LOW        | Cannot build on target                                | Models small (6.31M, <5M). Reduce --memPoolSize if needed.                                                                                                                |


## Testing Strategy

### Integration / Functional Tests

- **ESKF correctness**: Feed recorded IMU + synthetic VO/satellite data → verify output matches reference ESKF implementation
- **GPS_INPUT field validation**: Send GPS_INPUT to SITL ArduPilot → verify EKF accepts and uses the data correctly
- **Coordinate transform chain**: Known GPS → NED → pixel → back to GPS — verify round-trip error <0.1m
- **Disconnected segment handling**: Simulate tracking loss → verify satellite re-localization triggers → verify cuVSLAM restarts → verify ESKF position continuity
- **3-consecutive-failure**: Simulate VO + satellite failures → verify re-localization request sent → verify operator hint accepted
- **Object localization**: Known object at known GPS → verify computed GPS matches within camera accuracy
- **Mid-flight reboot**: Kill GPS-denied process → restart → verify recovery within expected time → verify position accuracy after recovery
- **TRT engine load test**: Verify engines load successfully on Jetson
- **TRT inference correctness**: Compare TRT output vs PyTorch reference (max L1 error < 0.01)
- **CUDA Stream pipelining**: Verify Stream B satellite matching does not block Stream A VO
- **ADTI sustained capture rate**: Verify 0.7fps sustained >30 min without buffer overflow
- **Confidence tier transitions**: Verify fix_type and accuracy change correctly across HIGH → MEDIUM → LOW → FAILED transitions

### Non-Functional Tests

- **End-to-end accuracy** (primary validation): Fly with real GPS recording → run GPS-denied system in parallel → compare estimated vs real positions → verify 80% within 50m, 60% within 20m
- **VO drift rate**: Measure cuVSLAM drift over 1km straight segment without satellite correction
- **Satellite matching accuracy**: Compare satellite-matched position vs real GPS at known locations
- **Processing time**: Verify end-to-end per-frame <400ms
- **Memory usage**: Monitor over 30-min session → verify <8GB, no leaks
- **Thermal**: Sustained 30-min run → verify no throttling
- **GPS_INPUT rate**: Verify consistent 5-10Hz delivery to FC
- **Tile storage**: Validate calculated storage matches actual for test mission area
- **MinGRU TRT compatibility** (day-one blocker): Clone LiteSAM → ONNX export → polygraphy → trtexec
- **Flight endurance**: Ground-test full system power draw against 267W estimate

## References

- ArduPilot GPS_RATE parameter: [https://github.com/ArduPilot/ardupilot/pull/15980](https://github.com/ArduPilot/ardupilot/pull/15980)
- MAVLink GPS_INPUT message: [https://ardupilot.org/mavproxy/docs/modules/GPSInput.html](https://ardupilot.org/mavproxy/docs/modules/GPSInput.html)
- pymavlink GPS_INPUT example: [https://webperso.ensta.fr/lebars/Share/GPS_INPUT_pymavlink.py](https://webperso.ensta.fr/lebars/Share/GPS_INPUT_pymavlink.py)
- ESKF reference (fixed-wing UAV): [https://github.com/ludvigls/ESKF](https://github.com/ludvigls/ESKF)
- ROS ESKF multi-sensor: [https://github.com/EliaTarasov/ESKF](https://github.com/EliaTarasov/ESKF)
- Range-VIO scale observability: [https://arxiv.org/abs/2103.15215](https://arxiv.org/abs/2103.15215)
- NaviLoc trajectory-level localization: [https://www.mdpi.com/2504-446X/10/2/97](https://www.mdpi.com/2504-446X/10/2/97)
- SatLoc-Fusion hierarchical framework: [https://www.scilit.com/publications/e5cafaf875a49297a62b298a89d5572f](https://www.scilit.com/publications/e5cafaf875a49297a62b298a89d5572f)
- Auterion GPS-denied workflow: [https://docs.auterion.com/vehicle-operation/auterion-mission-control/useful-resources/operations/gps-denied-workflow](https://docs.auterion.com/vehicle-operation/auterion-mission-control/useful-resources/operations/gps-denied-workflow)
- PX4 GNSS-denied flight: [https://docs.px4.io/main/en/advanced_config/gnss_degraded_or_denied_flight.html](https://docs.px4.io/main/en/advanced_config/gnss_degraded_or_denied_flight.html)
- ArduPilot GPS_INPUT advanced usage: [https://discuss.ardupilot.org/t/advanced-usage-of-gps-type-mav-14/99406](https://discuss.ardupilot.org/t/advanced-usage-of-gps-type-mav-14/99406)
- Google Maps Ukraine imagery: [https://newsukraine.rbc.ua/news/google-maps-has-surprise-for-satellite-imagery-1727182380.html](https://newsukraine.rbc.ua/news/google-maps-has-surprise-for-satellite-imagery-1727182380.html)
- Jetson Orin Nano Super thermal: [https://edgeaistack.app/blog/jetson-orin-nano-power-consumption/](https://edgeaistack.app/blog/jetson-orin-nano-power-consumption/)
- GSD matching research: [https://www.kjrs.org/journal/view.html?pn=related&uid=756&vmd=Full](https://www.kjrs.org/journal/view.html?pn=related&uid=756&vmd=Full)
- VO+satellite matching pipeline: [https://polen.itu.edu.tr/items/1fe1e872-7cea-44d8-a8de-339e4587bee6](https://polen.itu.edu.tr/items/1fe1e872-7cea-44d8-a8de-339e4587bee6)
- PyCuVSLAM docs: [https://wiki.seeedstudio.com/pycuvslam_recomputer_robotics/](https://wiki.seeedstudio.com/pycuvslam_recomputer_robotics/)
- Pixhawk 6x IMU (ICM-42688-P) datasheet: [https://invensense.tdk.com/products/motion-tracking/6-axis/icm-42688-p/](https://invensense.tdk.com/products/motion-tracking/6-axis/icm-42688-p/)
- All references from solution_draft05.md

## Related Artifacts

- AC Assessment: `_docs/00_research/gps_denied_nav/00_ac_assessment.md`
- Completeness assessment research: `_docs/00_research/solution_completeness_assessment/`
- Previous research: `_docs/00_research/trt_engine_migration/`
- Tech stack evaluation: `_docs/01_solution/tech_stack.md` (needs sync with draft05 corrections)
- Security analysis: `_docs/01_solution/security_analysis.md`
- Previous draft: `_docs/01_solution/solution_draft05.md`

