@mytec: iter3.10 start, baseline rc ready

RFCP-3.10-LinkBudget-Fresnel-Interference.md

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+# RFCP — Iteration 3.10: Link Budget, Fresnel Zone & Interference Modeling
+
+## Overview
+Add three interconnected RF analysis features: link budget calculator panel, Fresnel zone visualization on terrain profiles, and basic interference (C/I) modeling for multi-site scenarios. These build on existing infrastructure — propagation models, terrain profiles, and multi-site coverage.
+
+## Priority Order
+1. Link Budget Calculator (simplest, standalone UI)
+2. Fresnel Zone Visualization (extends terrain profile)  
+3. Interference Modeling (extends coverage engine)
+
+---
+
+## Feature 1: Link Budget Calculator
+
+### Description
+A panel/dialog that shows the complete RF link budget as a table — from transmitter to receiver. Uses existing propagation model values but presents them in the standard telecom link budget format.
+
+### Implementation
+
+**New component:** `frontend/src/components/panels/LinkBudgetPanel.tsx`
+
+The panel should display a table with rows for each element in the link chain. It should use the currently selected site's parameters and a configurable receiver point (either clicked on map or manually entered coordinates).
+
+**Link Budget Table Structure:**
+```
+TRANSMITTER
+  Tx Power (dBm)              [from site config, e.g. 43 dBm]
+  Tx Antenna Gain (dBi)       [from site config, e.g. 18 dBi]  
+  Tx Cable/Connector Loss (dB) [new field, default 2 dB]
+  EIRP (dBm)                  = Tx Power + Gain - Cable Loss
+
+PATH
+  Distance (km)               [calculated from Tx to Rx point]
+  Free Space Path Loss (dB)   [existing formula: 20log(d) + 20log(f) + 32.45]
+  Terrain Diffraction Loss (dB) [from terrain_los model if available]
+  Vegetation Loss (dB)         [from vegetation model if available]
+  Atmospheric Loss (dB)        [from atmospheric model if available]
+  Total Path Loss (dB)         = sum of all path losses
+
+RECEIVER  
+  Rx Antenna Gain (dBi)        [configurable, default 0 dBi for handset]
+  Rx Cable Loss (dB)           [configurable, default 0 dB]
+  Rx Sensitivity (dBm)         [configurable, default -100 dBm]
+
+RESULT
+  Received Power (dBm)         = EIRP - Total Path Loss + Rx Gain - Rx Cable
+  Link Margin (dB)             = Received Power - Rx Sensitivity
+  Status                       = "OK" if margin > 0, "FAIL" if < 0
+```
+
+**Backend addition:** Add a new endpoint or extend existing coverage API.
+
+**File:** `backend/app/api/routes/coverage.py` (or new `link_budget.py`)
+
+```python
+@router.post("/api/link-budget")
+async def calculate_link_budget(request: dict):
+    """Calculate point-to-point link budget.
+    
+    Body: {
+        "site_id": "...",           # or tx_lat/tx_lon/tx_params
+        "tx_lat": 48.46,
+        "tx_lon": 35.04,
+        "tx_power_dbm": 43,
+        "tx_gain_dbi": 18,
+        "tx_cable_loss_db": 2,
+        "tx_height_m": 30,
+        "rx_lat": 48.50,
+        "rx_lon": 35.10,
+        "rx_gain_dbi": 0,
+        "rx_cable_loss_db": 0,
+        "rx_sensitivity_dbm": -100,
+        "rx_height_m": 1.5,
+        "frequency_mhz": 1800
+    }
+    """
+    from app.services.terrain_service import terrain_service
+    
+    # Calculate distance
+    distance_m = terrain_service.haversine_distance(
+        request["tx_lat"], request["tx_lon"],
+        request["rx_lat"], request["rx_lon"]
+    )
+    distance_km = distance_m / 1000
+    
+    # Get elevations
+    tx_elev = await terrain_service.get_elevation(request["tx_lat"], request["tx_lon"])
+    rx_elev = await terrain_service.get_elevation(request["rx_lat"], request["rx_lon"])
+    
+    # EIRP
+    eirp_dbm = request["tx_power_dbm"] + request["tx_gain_dbi"] - request["tx_cable_loss_db"]
+    
+    # Free space path loss
+    freq = request["frequency_mhz"]
+    fspl_db = 20 * math.log10(distance_km) + 20 * math.log10(freq) + 32.45 if distance_km > 0 else 0
+    
+    # Terrain profile for LOS check
+    profile = await terrain_service.get_elevation_profile(
+        request["tx_lat"], request["tx_lon"],
+        request["rx_lat"], request["rx_lon"],
+        num_points=100
+    )
+    
+    # Simple LOS check - does terrain block line of sight?
+    tx_total_height = tx_elev + request.get("tx_height_m", 30)
+    rx_total_height = rx_elev + request.get("rx_height_m", 1.5)
+    
+    terrain_loss_db = 0
+    los_clear = True
+    for i, point in enumerate(profile):
+        if i == 0 or i == len(profile) - 1:
+            continue
+        # Linear interpolation of LOS line at this point
+        fraction = i / (len(profile) - 1)
+        los_height = tx_total_height + fraction * (rx_total_height - tx_total_height)
+        if point["elevation"] > los_height:
+            los_clear = False
+            # Simple knife-edge diffraction estimate
+            terrain_loss_db += 6  # ~6dB per obstruction (simplified)
+    
+    total_path_loss = fspl_db + terrain_loss_db
+    
+    # Received power
+    rx_power_dbm = eirp_dbm - total_path_loss + request["rx_gain_dbi"] - request["rx_cable_loss_db"]
+    
+    # Link margin
+    margin_db = rx_power_dbm - request["rx_sensitivity_dbm"]
+    
+    return {
+        "distance_km": round(distance_km, 2),
+        "distance_m": round(distance_m, 1),
+        "tx_elevation_m": round(tx_elev, 1),
+        "rx_elevation_m": round(rx_elev, 1),
+        "eirp_dbm": round(eirp_dbm, 1),
+        "fspl_db": round(fspl_db, 1),
+        "terrain_loss_db": round(terrain_loss_db, 1),
+        "total_path_loss_db": round(total_path_loss, 1),
+        "los_clear": los_clear,
+        "rx_power_dbm": round(rx_power_dbm, 1),
+        "margin_db": round(margin_db, 1),
+        "status": "OK" if margin_db >= 0 else "FAIL",
+        "profile": profile,
+    }
+```
+
+### UI Requirements
+- New panel accessible from sidebar or toolbar button (calculator icon)
+- Click on map to set Rx point (with crosshair cursor)
+- Auto-populates Tx params from selected site
+- Shows result table with color coding (green margin = OK, red = FAIL)
+- Optionally draws line on map from Tx to Rx
+
+---
+
+## Feature 2: Fresnel Zone Visualization
+
+### Description
+Draw Fresnel zone ellipse overlay on the Terrain Profile chart, showing where terrain intrudes into the first Fresnel zone. This is critical for understanding if a radio link will actually work — even if terrain doesn't block direct LOS, Fresnel zone obstruction causes significant signal loss.
+
+### Implementation
+
+**Modify:** The existing Terrain Profile component/chart
+
+**Fresnel Zone Radius Formula:**
+```python
+import math
+
+def fresnel_radius(n: int, frequency_mhz: float, d1_m: float, d2_m: float) -> float:
+    """Calculate nth Fresnel zone radius at a point along the path.
+    
+    Args:
+        n: Fresnel zone number (1 = first zone, most important)
+        frequency_mhz: Frequency in MHz
+        d1_m: Distance from transmitter to this point (meters)
+        d2_m: Distance from this point to receiver (meters)
+    
+    Returns:
+        Radius of nth Fresnel zone in meters
+    """
+    wavelength = 300.0 / frequency_mhz  # meters
+    d_total = d1_m + d2_m
+    if d_total == 0:
+        return 0
+    radius = math.sqrt((n * wavelength * d1_m * d2_m) / d_total)
+    return radius
+```
+
+**Backend endpoint:** `backend/app/api/routes/coverage.py`
+
+```python
+@router.post("/api/fresnel-profile")
+async def fresnel_profile(request: dict):
+    """Calculate terrain profile with Fresnel zone boundaries.
+    
+    Body: {
+        "tx_lat": 48.46, "tx_lon": 35.04, "tx_height_m": 30,
+        "rx_lat": 48.50, "rx_lon": 35.10, "rx_height_m": 1.5,
+        "frequency_mhz": 1800,
+        "num_points": 100
+    }
+    """
+    from app.services.terrain_service import terrain_service
+    
+    tx_lat, tx_lon = request["tx_lat"], request["tx_lon"]
+    rx_lat, rx_lon = request["rx_lat"], request["rx_lon"]
+    tx_height = request.get("tx_height_m", 30)
+    rx_height = request.get("rx_height_m", 1.5)
+    freq = request.get("frequency_mhz", 1800)
+    num_points = request.get("num_points", 100)
+    
+    # Get terrain profile
+    profile = await terrain_service.get_elevation_profile(
+        tx_lat, tx_lon, rx_lat, rx_lon, num_points
+    )
+    
+    total_distance = profile[-1]["distance"] if profile else 0
+    
+    # Get endpoint elevations
+    tx_elev = profile[0]["elevation"] if profile else 0
+    rx_elev = profile[-1]["elevation"] if profile else 0
+    tx_total = tx_elev + tx_height
+    rx_total = rx_elev + rx_height
+    
+    wavelength = 300.0 / freq  # meters
+    
+    # Calculate Fresnel zone at each profile point
+    fresnel_data = []
+    los_blocked = False
+    fresnel_blocked = False
+    worst_clearance = float('inf')
+    
+    for i, point in enumerate(profile):
+        d1 = point["distance"]  # distance from tx
+        d2 = total_distance - d1  # distance to rx
+        
+        # LOS height at this point (linear interpolation)
+        if total_distance > 0:
+            fraction = d1 / total_distance
+        else:
+            fraction = 0
+        los_height = tx_total + fraction * (rx_total - tx_total)
+        
+        # First Fresnel zone radius
+        if d1 > 0 and d2 > 0 and total_distance > 0:
+            f1_radius = math.sqrt((1 * wavelength * d1 * d2) / total_distance)
+        else:
+            f1_radius = 0
+        
+        # Fresnel zone boundaries (height above sea level)
+        fresnel_top = los_height + f1_radius
+        fresnel_bottom = los_height - f1_radius
+        
+        # Clearance: how much space between terrain and Fresnel bottom
+        clearance = fresnel_bottom - point["elevation"]
+        
+        if clearance < worst_clearance:
+            worst_clearance = clearance
+        
+        if point["elevation"] > los_height:
+            los_blocked = True
+        if point["elevation"] > fresnel_bottom:
+            fresnel_blocked = True
+        
+        fresnel_data.append({
+            "distance": point["distance"],
+            "lat": point["lat"],
+            "lon": point["lon"],
+            "terrain_elevation": point["elevation"],
+            "los_height": round(los_height, 1),
+            "fresnel_top": round(fresnel_top, 1),
+            "fresnel_bottom": round(fresnel_bottom, 1),
+            "f1_radius": round(f1_radius, 1),
+            "clearance": round(clearance, 1),
+        })
+    
+    return {
+        "profile": fresnel_data,
+        "total_distance_m": round(total_distance, 1),
+        "tx_elevation": round(tx_elev, 1),
+        "rx_elevation": round(rx_elev, 1),
+        "frequency_mhz": freq,
+        "wavelength_m": round(wavelength, 4),
+        "los_clear": not los_blocked,
+        "fresnel_clear": not fresnel_blocked,
+        "worst_clearance_m": round(worst_clearance, 1),
+        "recommendation": (
+            "Clear — excellent link" if not fresnel_blocked
+            else "Fresnel zone partially blocked — expect 3-6 dB additional loss"
+            if not los_blocked
+            else "LOS blocked — significant diffraction loss expected"
+        ),
+    }
+```
+
+### Frontend Visualization
+On the existing Terrain Profile chart:
+- Draw the LOS line (straight line from Tx to Rx) — this may already exist
+- Draw first Fresnel zone as a **semi-transparent elliptical area** around the LOS line
+  - Upper boundary = `fresnel_top` series  
+  - Lower boundary = `fresnel_bottom` series
+  - Color: light blue with ~20% opacity
+- Where terrain intersects Fresnel zone, highlight in red/orange
+- Show clearance info in the profile tooltip
+- Add a summary badge: "LOS Clear ✓" / "Fresnel 60% Clear ⚠" / "LOS Blocked ✗"
+
+---
+
+## Feature 3: Interference Modeling (C/I Ratio)
+
+### Description
+Add carrier-to-interference ratio calculation to the coverage engine. For each grid point, calculate the C/I ratio: the signal from the serving cell vs the sum of signals from all other cells on the same frequency. Display as a separate heatmap layer.
+
+### Implementation
+
+**Backend changes:**
+
+**File:** `backend/app/services/coverage_service.py` (or gpu_service.py)
+
+Add C/I calculation after existing coverage computation:
+
+```python
+def calculate_interference(self, sites: list, coverage_results: dict) -> np.ndarray:
+    """Calculate C/I ratio for each grid point.
+    
+    For each point:
+    - C = signal strength from strongest (serving) cell
+    - I = sum of signal strengths from all other co-frequency cells
+    - C/I = C - 10*log10(sum of linear interference powers)
+    
+    Returns array of C/I values in dB.
+    """
+    # Get all RSRP grids (already calculated)
+    # For each point, find:
+    #   1. Best server (strongest signal) = C
+    #   2. Sum of all others on same frequency = I
+    #   3. C/I = C(dBm) - I(dBm)
+    
+    # Group sites by frequency
+    freq_groups = {}
+    for site in sites:
+        freq = site.get("frequency_mhz", 1800)
+        if freq not in freq_groups:
+            freq_groups[freq] = []
+        freq_groups[freq].append(site)
+    
+    # Only calculate interference for frequency groups with 2+ sites
+    # For single-site frequencies, C/I = infinity (no interference)
+    
+    # The RSRP values are already in dBm, need to convert to linear for summing
+    # P_linear = 10^(P_dBm / 10)
+    # I_total_linear = sum(P_linear for all interferers)
+    # I_total_dBm = 10 * log10(I_total_linear)
+    # C/I = C_dBm - I_total_dBm
+    pass
+```
+
+**Key algorithm (for GPU pipeline in gpu_service.py):**
+```python
+# After computing RSRP for all sites at all grid points:
+# rsrp_grid shape: (num_sites, num_points) in dBm
+
+# Convert to linear (mW)
+rsrp_linear = 10 ** (rsrp_grid / 10.0)  # CuPy array
+
+# For each point, best server
+best_server_idx = cp.argmax(rsrp_grid, axis=0)
+best_rsrp_linear = cp.take_along_axis(rsrp_linear, best_server_idx[cp.newaxis, :], axis=0)[0]
+
+# Total power from all sites
+total_power = cp.sum(rsrp_linear, axis=0)
+
+# Interference = total - serving
+interference_linear = total_power - best_rsrp_linear
+
+# C/I ratio in dB
+# Avoid log10(0) with small epsilon
+epsilon = 1e-30
+ci_ratio_db = 10 * cp.log10(best_rsrp_linear / (interference_linear + epsilon))
+
+# Clip to reasonable range
+ci_ratio_db = cp.clip(ci_ratio_db, -20, 50)
+```
+
+### Frontend Visualization
+- Add a toggle in the coverage controls: "Show: Signal (RSRP) | Interference (C/I)"
+- C/I heatmap uses different color scale:
+  - Dark red: < 0 dB (interference dominant — no service)
+  - Orange: 0-10 dB (marginal)  
+  - Yellow: 10-20 dB (acceptable)
+  - Green: 20-30 dB (good)
+  - Blue: > 30 dB (excellent, minimal interference)
+- The C/I map only makes sense with 2+ sites on same frequency
+- Show warning if all sites are on different frequencies (no co-channel interference)
+
+### API Response Extension
+Add `ci_ratio` field to coverage calculation response alongside existing `rsrp` values.
+
+---
+
+## Testing Checklist
+
+### Link Budget
+- [ ] Panel opens from toolbar/sidebar
+- [ ] Click on map sets Rx point
+- [ ] Tx parameters auto-populate from selected site
+- [ ] Link budget table shows all rows correctly
+- [ ] Margin calculation is correct (manual verification)
+- [ ] Color coding: green for positive margin, red for negative
+- [ ] Line drawn on map from Tx to Rx
+
+### Fresnel Zone  
+- [ ] Terrain profile shows Fresnel zone overlay
+- [ ] Fresnel ellipse is widest at midpoint (correct shape)
+- [ ] Red highlighting where terrain enters Fresnel zone
+- [ ] Summary shows LOS/Fresnel status
+- [ ] Works at different frequencies (zone size changes with frequency)
+- [ ] Clearance values are reasonable (first Fresnel zone at 1800 MHz, 10km = ~22m radius at midpoint)
+
+### Interference
+- [ ] C/I toggle appears when 2+ sites exist
+- [ ] C/I heatmap renders with correct color scale
+- [ ] Single-site scenario shows "no interference" or infinite C/I
+- [ ] Two sites on same frequency show interference zones between them
+- [ ] C/I values are reasonable (> 20 dB near serving cell, < 10 dB at cell edge)
+
+## Build & Deploy
+
+```bash
+cd D:\root\rfcp
+
+# Backend — just restart uvicorn (Python, no build)
+cd backend
+python -m uvicorn app.main:app --host 0.0.0.0 --port 8000 --reload
+
+# Frontend — rebuild if UI components changed
+cd frontend  
+npm run build
+
+# Full installer rebuild if needed
+# (use existing build script)
+```
+
+## Commit Message
+
+```
+feat(rf): add link budget, Fresnel zone, and interference modeling
+
+- Add /api/link-budget endpoint with full path analysis
+- Add /api/fresnel-profile endpoint with zone clearance calculation
+- Add C/I ratio computation to GPU coverage pipeline
+- Add LinkBudgetPanel frontend component
+- Add Fresnel zone overlay to terrain profile chart
+- Add C/I heatmap toggle alongside RSRP display
+- Group interference by frequency for co-channel analysis
+```
+
+## Success Criteria
+
+1. Link budget shows correct margin for known test case (Dnipro, 10km, 1800MHz)
+2. Fresnel zone visually shows ellipse on terrain profile
+3. Two co-frequency sites show interference pattern between them
+4. All three features work with existing terrain data (no new downloads needed)
+5. GPU pipeline performance not significantly degraded by C/I calculation

RFCP-3.9.1-Terra-Integration.md

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+# RFCP — Iteration 3.9.1: Terra Tile Server Integration
+
+## Overview
+Connect terrain_service.py to our SRTM tile server (terra.eliah.one) as primary download source, add terrain status API endpoint, and create a bulk pre-download utility. The `data/terrain/` directory already exists.
+
+## Context
+- terra.eliah.one is live and serving tiles via Caddy file_server
+- SRTM3 (90m): 187 tiles, 515 MB — full Ukraine coverage (N44-N51, E018-E041)
+- SRTM1 (30m): 160 tiles, 3.9 GB — same coverage area
+- terrain_service.py already has bilinear interpolation (3.9.0)
+- Backend runs on Windows with RTX 4060, tiles stored locally in `data/terrain/`
+- Server is download source, NOT used during realtime calculations
+
+## Changes Required
+
+### 1. Update SRTM_SOURCES in terrain_service.py
+
+**File:** `backend/app/services/terrain_service.py`
+
+Replace current SRTM_SOURCES (lines 22-25):
+```python
+SRTM_SOURCES = [
+    "https://elevation-tiles-prod.s3.amazonaws.com/skadi/{lat_dir}/{tile_name}.hgt.gz",
+    "https://s3.amazonaws.com/elevation-tiles-prod/skadi/{lat_dir}/{tile_name}.hgt.gz",
+]
+```
+
+With prioritized source list:
+```python
+SRTM_SOURCES = [
+    # Our tile server — SRTM1 (30m) preferred, uncompressed
+    {
+        "url": "https://terra.eliah.one/srtm1/{tile_name}.hgt",
+        "compressed": False,
+        "resolution": "srtm1",
+    },
+    # Our tile server — SRTM3 (90m) fallback
+    {
+        "url": "https://terra.eliah.one/srtm3/{tile_name}.hgt",
+        "compressed": False,
+        "resolution": "srtm3",
+    },
+    # Public AWS mirror — SRTM1, gzip compressed
+    {
+        "url": "https://elevation-tiles-prod.s3.amazonaws.com/skadi/{lat_dir}/{tile_name}.hgt.gz",
+        "compressed": True,
+        "resolution": "srtm1",
+    },
+]
+```
+
+Update `download_tile()` to handle the new source format:
+```python
+async def download_tile(self, tile_name: str) -> bool:
+    """Download SRTM tile from configured sources, preferring highest resolution."""
+    tile_path = self.get_tile_path(tile_name)
+
+    if tile_path.exists():
+        return True
+
+    lat_dir = tile_name[:3]  # e.g., "N48"
+
+    async with httpx.AsyncClient(timeout=60.0, follow_redirects=True) as client:
+        for source in self.SRTM_SOURCES:
+            url = source["url"].format(lat_dir=lat_dir, tile_name=tile_name)
+            try:
+                response = await client.get(url)
+
+                if response.status_code == 200:
+                    data = response.content
+
+                    # Skip empty responses
+                    if len(data) < 1000:
+                        continue
+
+                    if source["compressed"]:
+                        if url.endswith('.gz'):
+                            data = gzip.decompress(data)
+                        elif url.endswith('.zip'):
+                            with zipfile.ZipFile(io.BytesIO(data)) as zf:
+                                for name in zf.namelist():
+                                    if name.endswith('.hgt'):
+                                        data = zf.read(name)
+                                        break
+
+                    # Validate tile size
+                    if len(data) not in (3601 * 3601 * 2, 1201 * 1201 * 2):
+                        print(f"[Terrain] Invalid tile size {len(data)} from {url}")
+                        continue
+
+                    tile_path.write_bytes(data)
+                    res = source["resolution"]
+                    size_mb = len(data) / 1048576
+                    print(f"[Terrain] Downloaded {tile_name} ({res}, {size_mb:.1f} MB)")
+                    return True
+
+            except Exception as e:
+                print(f"[Terrain] Failed from {url}: {e}")
+                continue
+
+    print(f"[Terrain] Could not download {tile_name} from any source")
+    return False
+```
+
+### 2. Add Terrain Status API Endpoint
+
+**File:** `backend/app/api/routes.py` (or wherever API routes are defined)
+
+Add a new endpoint:
+```python
+@router.get("/api/terrain/status")
+async def terrain_status():
+    """Return terrain data availability info."""
+    from app.services.terrain_service import terrain_service
+
+    cached_tiles = terrain_service.get_cached_tiles()
+    cache_size = terrain_service.get_cache_size_mb()
+
+    # Categorize by resolution
+    srtm1_tiles = [t for t in cached_tiles
+                   if (terrain_service.terrain_path / f"{t}.hgt").stat().st_size == 3601 * 3601 * 2]
+    srtm3_tiles = [t for t in cached_tiles if t not in srtm1_tiles]
+
+    return {
+        "total_tiles": len(cached_tiles),
+        "srtm1": {
+            "count": len(srtm1_tiles),
+            "resolution_m": 30,
+            "tiles": sorted(srtm1_tiles),
+        },
+        "srtm3": {
+            "count": len(srtm3_tiles),
+            "resolution_m": 90,
+            "tiles": sorted(srtm3_tiles),
+        },
+        "cache_size_mb": round(cache_size, 1),
+        "memory_cached": len(terrain_service._tile_cache),
+        "terra_server": "https://terra.eliah.one",
+    }
+```
+
+### 3. Add Bulk Pre-Download Endpoint
+
+**File:** Same routes file
+
+```python
+@router.post("/api/terrain/download")
+async def terrain_download(request: dict):
+    """Pre-download tiles for a region.
+
+    Body: {"center_lat": 48.46, "center_lon": 35.04, "radius_km": 50}
+    Or:   {"tiles": ["N48E034", "N48E035", "N47E034", "N47E035"]}
+    """
+    from app.services.terrain_service import terrain_service
+
+    if "tiles" in request:
+        tile_list = request["tiles"]
+    else:
+        center_lat = request.get("center_lat", 48.46)
+        center_lon = request.get("center_lon", 35.04)
+        radius_km = request.get("radius_km", 50)
+        tile_list = terrain_service.get_required_tiles(center_lat, center_lon, radius_km)
+
+    missing = [t for t in tile_list if not terrain_service.get_tile_path(t).exists()]
+
+    if not missing:
+        return {"status": "ok", "message": "All tiles already cached", "count": len(tile_list)}
+
+    # Download missing tiles
+    downloaded = []
+    failed = []
+    for tile_name in missing:
+        success = await terrain_service.download_tile(tile_name)
+        if success:
+            downloaded.append(tile_name)
+        else:
+            failed.append(tile_name)
+
+    return {
+        "status": "ok",
+        "required": len(tile_list),
+        "already_cached": len(tile_list) - len(missing),
+        "downloaded": downloaded,
+        "failed": failed,
+    }
+```
+
+### 4. Add Tile Index Endpoint
+
+**File:** Same routes file
+
+```python
+@router.get("/api/terrain/index")
+async def terrain_index():
+    """Fetch tile index from terra server."""
+    import httpx
+    try:
+        async with httpx.AsyncClient(timeout=10.0) as client:
+            resp = await client.get("https://terra.eliah.one/api/index")
+            if resp.status_code == 200:
+                return resp.json()
+    except Exception:
+        pass
+    return {"error": "Could not reach terra.eliah.one", "offline": True}
+```
+
+## Testing Checklist
+
+- [ ] `GET /api/terrain/status` returns tile counts and sizes
+- [ ] `POST /api/terrain/download {"center_lat": 48.46, "center_lon": 35.04, "radius_km": 10}` downloads missing tiles from terra.eliah.one
+- [ ] Tiles downloaded from terra are valid HGT format (2,884,802 or 25,934,402 bytes)
+- [ ] SRTM1 is preferred over SRTM3 when downloading
+- [ ] Existing tiles are not re-downloaded
+- [ ] Coverage calculation works with terrain data (test with Dnipro coordinates)
+- [ ] `GET /api/terrain/index` returns terra server tile list
+
+## Build & Deploy
+
+```bash
+cd D:\root\rfcp\backend
+# No build needed — Python backend, just restart
+# Kill existing uvicorn and restart:
+python -m uvicorn app.main:app --host 0.0.0.0 --port 8000 --reload
+```
+
+## Commit Message
+
+```
+feat(terrain): integrate terra.eliah.one tile server
+
+- Add terra.eliah.one as primary SRTM source (SRTM1 30m preferred)
+- Keep AWS S3 as fallback source
+- Add /api/terrain/status endpoint (tile inventory)
+- Add /api/terrain/download endpoint (bulk pre-download)
+- Add /api/terrain/index endpoint (terra server index)
+- Validate tile size before saving
+- Add follow_redirects=True to httpx client
+```
+
+## Success Criteria
+
+1. terrain_service downloads from terra.eliah.one first
+2. /api/terrain/status shows correct tile counts by resolution
+3. /api/terrain/download fetches tiles for any Ukrainian coordinate
+4. Offline mode works — no downloads attempted if tiles exist locally
+5. Coverage calculation uses real elevation data instead of flat terrain

UMTC-Wiki-MEGA-TASK.md

@@ -1,352 +0,0 @@
-# UMTC Wiki v2.0 — MEGA TASK: Integration & Polish
-
-Read UMTC-Wiki-v2.0-REFACTOR.md and UMTC-Wiki-v2.0-ROADMAP.md for full context.
-
-This is a comprehensive task covering all remaining fixes and integration work.
-Take your time, think hard, work through each section systematically.
-Report after completing each major section.
-
----
-
-## SECTION A: Fix Critical Tauri 404 Bug
-
-The sidebar loads the full content tree correctly but clicking ANY article shows 404.
-
-### Debug steps:
-1. In `frontend/src/lib/api.ts` — find where getPage is called with a slug
-   Add `console.log('[WIKI] getPage called with slug:', slug)` 
-
-2. In `frontend/src/lib/utils/backend.ts` — in the tauriGetPage function
-   Add `console.log('[WIKI] Tauri invoke get_page with:', slug)`
-
-3. In `desktop/src-tauri/src/commands/content.rs` — in the get_page handler
-   Add `eprintln!("[WIKI] get_page received slug: {}", slug)`
-   Add `eprintln!("[WIKI] trying path: {:?}", resolved_path)`
-
-4. Check the Sidebar.svelte component — what href/slug does it generate when user clicks?
-   The web version uses `/api/pages/{slug}` — in desktop mode it should invoke with just the slug part.
-
-5. Common mismatches to check:
-   - Leading slash: sidebar sends `/lte/bbu` but Rust expects `lte/bbu`
-   - File extension: Rust looks for `lte/bbu.md` but file is `lte/bbu/index.md`
-   - URL encoding: Ukrainian characters in slugs
-   - The SvelteKit catch-all route `[...slug]` may pass the slug differently
-
-6. Fix the mismatch. Test navigation to at least 10 different pages including:
-   - Root sections (lte/, ran/, mikrotik/)
-   - Nested pages (lte/bbu, ran/srsenb-config)
-   - Glossary terms (glossary/prb)
-   - Deep nesting if any
-
----
-
-## SECTION B: Fix Web Deployment
-
-The web version must keep working. Test and fix:
-
-1. Check that `backend/content.py` imports work:
-   - `from wiki_frontmatter import ArticleFrontmatter` 
-   - `from wikilinks import WikiLinksExtension`
-   - `from backlinks import BacklinksIndex`
-   - `from admonitions import AdmonitionsExtension`
-   
-   If any import fails, fix the module.
-
-2. Add the admonitions extension to the markdown pipeline in content.py
-   (wikilinks was already integrated, verify admonitions too)
-
-3. Make sure the backlinks API endpoint in main.py works:
-   - GET /api/pages/{slug:path}/backlinks
-   - Should return { "slug": "...", "backlinks": [...], "count": N }
-
-4. Add grade/status/category to the page API response:
-   - GET /api/pages/{slug} should now include grade, status, category fields
-
-5. Create a simple test script `scripts/test_web.py`:
-   ```python
-   # Test that backend starts and key endpoints work
-   import requests
-   BASE = "http://localhost:8000"
-   
-   # Test navigation
-   r = requests.get(f"{BASE}/api/navigation")
-   assert r.status_code == 200
-   nav = r.json()
-   print(f"Navigation: {len(nav)} sections")
-   
-   # Test page load
-   r = requests.get(f"{BASE}/api/pages/index")
-   assert r.status_code == 200
-   print(f"Home page: {r.json().get('title', 'OK')}")
-   
-   # Test search
-   r = requests.get(f"{BASE}/api/search?q=LTE")
-   assert r.status_code == 200
-   print(f"Search 'LTE': {len(r.json())} results")
-   
-   # Test backlinks
-   r = requests.get(f"{BASE}/api/pages/glossary/enb/backlinks")
-   print(f"Backlinks for eNB: {r.status_code}")
-   
-   print("\nAll tests passed!")
-   ```
-
----
-
-## SECTION C: Frontend Wiki Components — Full Integration
-
-### C.1: Article Grade Badge on Pages
-
-In the wiki page view (`frontend/src/routes/[...slug]/+page.svelte` or equivalent):
-- Import ArticleGrade component
-- Display the grade badge next to the page title
-- The grade comes from the page API response (field: `grade`)
-- If no grade, don't show badge
-- Style: small badge inline with title, not a separate block
-
-### C.2: Breadcrumbs Component
-
-Create/update `frontend/src/lib/components/wiki/Breadcrumbs.svelte`:
-```svelte
-<!-- Example: Головна > LTE > BBU Setup -->
-<nav class="breadcrumbs">
-  <a href="/">Головна</a>
-  <span class="separator">/</span>
-  <a href="/lte">LTE</a>
-  <span class="separator">/</span>
-  <span class="current">BBU Setup</span>
-</nav>
-```
-- Generate from current page slug
-- Each segment is a link except the last
-- Use titles from navigation tree if available, otherwise humanize slug
-- Works in both web and desktop mode
-- Integrate into the page layout — show above article title
-
-### C.3: Admonition CSS
-
-Add styles for admonition boxes to the global CSS or a component:
-```css
-.admonition {
-  border-left: 4px solid;
-  border-radius: 4px;
-  padding: 12px 16px;
-  margin: 16px 0;
-}
-.admonition-note { border-color: #3b82f6; background: rgba(59,130,246,0.1); }
-.admonition-warning { border-color: #f59e0b; background: rgba(245,158,11,0.1); }
-.admonition-tip { border-color: #10b981; background: rgba(16,185,129,0.1); }
-.admonition-danger { border-color: #ef4444; background: rgba(239,68,68,0.1); }
-
-/* Dark mode */
-:global(.dark) .admonition-note { background: rgba(59,130,246,0.15); }
-:global(.dark) .admonition-warning { background: rgba(245,158,11,0.15); }
-:global(.dark) .admonition-tip { background: rgba(16,185,129,0.15); }
-:global(.dark) .admonition-danger { background: rgba(239,68,68,0.15); }
-
-.admonition-title {
-  font-weight: 600;
-  margin-bottom: 4px;
-}
-.admonition-icon {
-  margin-right: 8px;
-}
-```
-
-### C.4: Wiki-Link CSS
-
-Add styles for wiki-links:
-```css
-.wiki-link {
-  color: #3b82f6;
-  text-decoration: none;
-  border-bottom: 1px dotted #3b82f6;
-}
-.wiki-link:hover {
-  border-bottom-style: solid;
-}
-.red-link {
-  color: #ef4444;
-  border-bottom-color: #ef4444;
-}
-.red-link:hover::after {
-  content: " (сторінку не знайдено)";
-  font-size: 0.75em;
-  color: #9ca3af;
-}
-```
-
-### C.5: Backlinks Panel Integration
-
-In the page view, after the article content:
-- Show BacklinksPanel component
-- Pass current page slug
-- Works in both web (API) and desktop (Tauri IPC)
-- Only show if there are backlinks (don't show empty panel)
-
-### C.6: Table of Contents (sidebar)
-
-If the page has headings, generate a table of contents:
-- Extract h2/h3 from rendered HTML or use TOC data from backend
-- Show as a floating sidebar on wide screens (>1200px)
-- Collapsible on smaller screens
-- Highlight current section on scroll (intersection observer)
-- Works in both modes
-
----
-
-## SECTION D: Search Integration for Desktop
-
-1. Test Tantivy search in Tauri:
-   - The search command should be wired to the Search component
-   - Type in search bar → results appear
-   - Cyrillic text should work (test: "мережа", "антена", "LTE")
-   
-2. If search doesn't work, debug:
-   - Is the search index built on startup? Check Rust logs
-   - Are content files found? Check content path resolution
-   - Is the query reaching the search command?
-
-3. Search results should show:
-   - Page title
-   - Brief excerpt (first 150 chars of content)
-   - Click navigates to page
-
-4. Keyboard shortcut: Ctrl+K should focus the search bar (already exists in web, verify in desktop)
-
----
-
-## SECTION E: Content Quality Pass
-
-### E.1: Content Audit Script
-
-Create `scripts/analyze_content.py`:
-- Scan all .md files in content/
-- For each file report: has_frontmatter, word_count, has_code_blocks, grade, broken_wiki_links
-- Summary: total articles, by grade, articles needing work
-- Print actionable output
-
-### E.2: Add More Glossary Terms (20 more)
-
-Create glossary entries with proper frontmatter (grade: B, category: glossary):
-
-**Radio/RF terms:**
-- SGW (Serving Gateway)
-- PGW (PDN Gateway)  
-- HSS (Home Subscriber Server)
-- RSRP (Reference Signal Received Power)
-- RSRQ (Reference Signal Received Quality)
-- SINR (Signal to Interference plus Noise Ratio)
-- EARFCN (E-UTRA Absolute Radio Frequency Channel Number)
-- OFDM (Orthogonal Frequency Division Multiplexing)
-- MIMO (Multiple Input Multiple Output)
-- QoS (Quality of Service)
-
-**Infrastructure terms:**
-- WireGuard
-- MikroTik
-- Mesh Network
-- VLAN (Virtual LAN)
-- BGP (Border Gateway Protocol)
-- mTLS (Mutual TLS)
-- Caddy (Web Server)
-
-**Protocol terms:**
-- S1AP (S1 Application Protocol)
-- GTP (GPRS Tunnelling Protocol)
-- SCTP (Stream Control Transmission Protocol)
-
-Each glossary term should:
-- Have title in English with Ukrainian description
-- Use [[wiki-links]] to cross-reference other terms
-- Include: what it is, why it matters for UMTC, key parameters
-- Be 100-300 words
-
-### E.3: Upgrade 5 Key Articles to Grade B
-
-Pick the 5 most important articles and upgrade them:
-- Add proper frontmatter with all fields
-- Add :::note and :::warning admonitions where useful
-- Add [[wiki-links]] to glossary terms
-- Add "Див. також" (See also) section with related articles
-- Verify technical accuracy
-- Set grade: B
-
-Good candidates: 
-- Main LTE overview
-- srsENB configuration  
-- WireGuard setup
-- Open5GS overview
-- MikroTik basics
-
----
-
-## SECTION F: Desktop App Polish
-
-### F.1: Window Title
-
-Show current page title in the window title bar:
-`UMTC Wiki — {Page Title}`
-
-### F.2: Keyboard Navigation
-
-- Arrow keys in sidebar to navigate
-- Enter to open selected item
-- Backspace to go back
-- Ctrl+K for search (verify)
-
-### F.3: Error Handling
-
-- If page not found, show a friendly Ukrainian message instead of generic 404
-- If content directory is missing, show setup instructions
-- If search index fails to build, log error but don't crash
-
-### F.4: About Dialog
-
-Add a simple about/info accessible from a gear icon or Help menu:
-- UMTC Wiki v2.0
-- Built with Tauri + SvelteKit + Rust
-- Content articles count
-- "Офлайн документація для УМТЗ"
-
----
-
-## SECTION G: Production Builds
-
-### G.1: Web Docker Build Test
-
-Update docker-compose.yml if needed to include new backend modules.
-Make sure Dockerfile copies:
-- backend/wiki_frontmatter.py
-- backend/wikilinks.py  
-- backend/backlinks.py
-- backend/admonitions.py
-- All content/ files
-
-### G.2: Tauri Production Build
-
-Run `npx tauri build` and fix any remaining compilation errors.
-Report the output binary size and location.
-
----
-
-## Order of Operations
-
-Do these in order — each section builds on the previous:
-
-1. **SECTION A** — Fix 404 bug (CRITICAL, everything depends on this)
-2. **SECTION B** — Verify web backend
-3. **SECTION C** — Frontend components
-4. **SECTION D** — Search
-5. **SECTION E** — Content
-6. **SECTION F** — Desktop polish
-7. **SECTION G** — Production builds
-
-Report after each section with:
-- What was done
-- What files were changed
-- Any issues found
-- Ready for next section?
-
-Think hard about edge cases. Don't break existing functionality.
-Good luck! 🚀

backend/app/api/routes/terrain.py

@@ -180,3 +180,93 @@ async def get_terrain_file(region: str):
     if os.path.exists(terrain_path):
         return FileResponse(terrain_path)
     raise HTTPException(status_code=404, detail=f"Region '{region}' not found")
+
+
+@router.get("/status")
+async def terrain_status():
+    """Return terrain data availability info."""
+    cached_tiles = terrain_service.get_cached_tiles()
+    cache_size = terrain_service.get_cache_size_mb()
+
+    # Categorize by resolution based on file size
+    srtm1_tiles = []
+    srtm3_tiles = []
+    for t in cached_tiles:
+        tile_path = terrain_service.terrain_path / f"{t}.hgt"
+        try:
+            if tile_path.stat().st_size == 3601 * 3601 * 2:
+                srtm1_tiles.append(t)
+            else:
+                srtm3_tiles.append(t)
+        except Exception:
+            pass
+
+    return {
+        "total_tiles": len(cached_tiles),
+        "srtm1": {
+            "count": len(srtm1_tiles),
+            "resolution_m": 30,
+            "tiles": sorted(srtm1_tiles),
+        },
+        "srtm3": {
+            "count": len(srtm3_tiles),
+            "resolution_m": 90,
+            "tiles": sorted(srtm3_tiles),
+        },
+        "cache_size_mb": round(cache_size, 1),
+        "memory_cached": len(terrain_service._tile_cache),
+        "terra_server": "https://terra.eliah.one",
+    }
+
+
+@router.post("/download")
+async def terrain_download(request: dict):
+    """Pre-download tiles for a region.
+
+    Body: {"center_lat": 48.46, "center_lon": 35.04, "radius_km": 50}
+    Or:   {"tiles": ["N48E034", "N48E035", "N47E034", "N47E035"]}
+    """
+    if "tiles" in request:
+        tile_list = request["tiles"]
+    else:
+        center_lat = request.get("center_lat", 48.46)
+        center_lon = request.get("center_lon", 35.04)
+        radius_km = request.get("radius_km", 50)
+        tile_list = terrain_service.get_required_tiles(center_lat, center_lon, radius_km)
+
+    missing = [t for t in tile_list if not terrain_service.get_tile_path(t).exists()]
+
+    if not missing:
+        return {"status": "ok", "message": "All tiles already cached", "count": len(tile_list)}
+
+    # Download missing tiles
+    downloaded = []
+    failed = []
+    for tile_name in missing:
+        success = await terrain_service.download_tile(tile_name)
+        if success:
+            downloaded.append(tile_name)
+        else:
+            failed.append(tile_name)
+
+    return {
+        "status": "ok",
+        "required": len(tile_list),
+        "already_cached": len(tile_list) - len(missing),
+        "downloaded": downloaded,
+        "failed": failed,
+    }
+
+
+@router.get("/index")
+async def terrain_index():
+    """Fetch tile index from terra server."""
+    import httpx
+    try:
+        async with httpx.AsyncClient(timeout=10.0) as client:
+            resp = await client.get("https://terra.eliah.one/api/index")
+            if resp.status_code == 200:
+                return resp.json()
+    except Exception:
+        pass
+    return {"error": "Could not reach terra.eliah.one", "offline": True}

backend/app/services/coverage_service.py

@@ -526,19 +526,33 @@ class CoverageService:
             progress_fn("Loading terrain", 0.25)
             await asyncio.sleep(0)
         t_terrain = time.time()
+
+        # Check for missing tiles before attempting download
+        radius_km = settings.radius / 1000.0
+        missing_tiles = self.terrain.get_missing_tiles(site.lat, site.lon, radius_km)
+        if missing_tiles:
+            _clog(f"⚠ Missing terrain tiles: {missing_tiles} - will attempt download")
+
         tile_names = await self.terrain.ensure_tiles_for_bbox(
             min_lat, min_lon, max_lat, max_lon
         )
         for tn in tile_names:
             self.terrain._load_tile(tn)
 
+        # Check what actually loaded
+        loaded_tiles = [tn for tn in tile_names if tn in self.terrain._tile_cache]
+        failed_tiles = [tn for tn in tile_names if tn not in self.terrain._tile_cache]
+        if failed_tiles:
+            _clog(f"⚠ TERRAIN WARNING: Failed to load tiles {failed_tiles}. "
+                  "Coverage accuracy reduced - using flat terrain for affected areas.")
+
         site_elevation = self.terrain.get_elevation_sync(site.lat, site.lon)
 
         point_elevations = {}
         for lat, lon in grid:
             point_elevations[(lat, lon)] = self.terrain.get_elevation_sync(lat, lon)
         terrain_time = time.time() - t_terrain
-        _clog(f"Tiles: {len(tile_names)}, site elev: {site_elevation:.0f}m, "
+        _clog(f"Tiles: {len(loaded_tiles)}/{len(tile_names)} loaded, site elev: {site_elevation:.0f}m, "
               f"pre-computed {len(grid)} elevations")
         _clog(f"━━━ PHASE 2 done: {terrain_time:.1f}s ━━━")
 

backend/app/services/gpu_service.py

@@ -277,10 +277,11 @@ class GPUService:
         lons: np.ndarray,
         terrain_cache: dict,
     ) -> np.ndarray:
-        """Look up elevations from cached terrain tiles.
+        """Look up elevations from cached terrain tiles with bilinear interpolation.
 
         Vectorized implementation: processes per-tile (1-4 tiles) instead of
-        per-point (thousands of points). Inner operations are all NumPy vectorized.
+        per-point (thousands of points). Uses bilinear interpolation for
+        sub-meter accuracy (vs 15m error with nearest-neighbor at 30m resolution).
 
         Args:
             lats, lons: Flattened arrays of coordinates
@@ -313,16 +314,39 @@ class GPUService:
             tile_lons = lons[mask]
 
             size = tile.shape[0]
-            # Vectorized row/col calculation
-            rows = ((1 - (tile_lats - lat_int)) * (size - 1)).astype(int)
-            cols = ((tile_lons - lon_int) * (size - 1)).astype(int)
-            rows = np.clip(rows, 0, size - 1)
-            cols = np.clip(cols, 0, size - 1)
 
-            # Vectorized lookup - single operation for ALL points in tile
-            tile_elevs = tile[rows, cols].astype(np.float64)
-            tile_elevs[tile_elevs == -32768] = 0.0
-            elevations[mask] = tile_elevs
+            # Vectorized bilinear interpolation
+            lat_frac = tile_lats - lat_int
+            lon_frac = tile_lons - lon_int
+
+            row_exact = (1.0 - lat_frac) * (size - 1)
+            col_exact = lon_frac * (size - 1)
+
+            r0 = np.clip(row_exact.astype(int), 0, size - 2)
+            c0 = np.clip(col_exact.astype(int), 0, size - 2)
+            r1 = r0 + 1
+            c1 = c0 + 1
+
+            dr = row_exact - r0
+            dc = col_exact - c0
+
+            # Get four corner values for all points at once
+            z00 = tile[r0, c0].astype(np.float64)
+            z01 = tile[r0, c1].astype(np.float64)
+            z10 = tile[r1, c0].astype(np.float64)
+            z11 = tile[r1, c1].astype(np.float64)
+
+            # Bilinear interpolation (vectorized)
+            result = (z00 * (1 - dr) * (1 - dc) +
+                      z01 * (1 - dr) * dc +
+                      z10 * dr * (1 - dc) +
+                      z11 * dr * dc)
+
+            # Handle void values (-32768) - set to 0
+            void_mask = (z00 == -32768) | (z01 == -32768) | (z10 == -32768) | (z11 == -32768)
+            result[void_mask] = 0.0
+
+            elevations[mask] = result
 
         return elevations
 

backend/app/services/terrain_service.py

@@ -20,8 +20,24 @@ class TerrainService:
     """
 
     SRTM_SOURCES = [
-        "https://elevation-tiles-prod.s3.amazonaws.com/skadi/{lat_dir}/{tile_name}.hgt.gz",
-        "https://s3.amazonaws.com/elevation-tiles-prod/skadi/{lat_dir}/{tile_name}.hgt.gz",
+        # Our tile server — SRTM1 (30m) preferred, uncompressed
+        {
+            "url": "https://terra.eliah.one/srtm1/{tile_name}.hgt",
+            "compressed": False,
+            "resolution": "srtm1",
+        },
+        # Our tile server — SRTM3 (90m) fallback
+        {
+            "url": "https://terra.eliah.one/srtm3/{tile_name}.hgt",
+            "compressed": False,
+            "resolution": "srtm3",
+        },
+        # Public AWS mirror — SRTM1, gzip compressed
+        {
+            "url": "https://elevation-tiles-prod.s3.amazonaws.com/skadi/{lat_dir}/{tile_name}.hgt.gz",
+            "compressed": True,
+            "resolution": "srtm1",
+        },
     ]
 
     def __init__(self):
@@ -48,7 +64,7 @@ class TerrainService:
         return self.terrain_path / f"{tile_name}.hgt"
 
     async def download_tile(self, tile_name: str) -> bool:
-        """Download SRTM tile if not cached locally"""
+        """Download SRTM tile from configured sources, preferring highest resolution."""
         tile_path = self.get_tile_path(tile_name)
 
         if tile_path.exists():
@@ -56,33 +72,45 @@ class TerrainService:
 
         lat_dir = tile_name[:3]  # e.g., "N48"
 
-        async with httpx.AsyncClient(timeout=60.0) as client:
-            for source_url in self.SRTM_SOURCES:
-                url = source_url.format(lat_dir=lat_dir, tile_name=tile_name)
+        async with httpx.AsyncClient(timeout=60.0, follow_redirects=True) as client:
+            for source in self.SRTM_SOURCES:
+                url = source["url"].format(lat_dir=lat_dir, tile_name=tile_name)
                 try:
                     response = await client.get(url)
 
                     if response.status_code == 200:
                         data = response.content
 
-                        if url.endswith('.gz'):
-                            data = gzip.decompress(data)
-                        elif url.endswith('.zip'):
-                            with zipfile.ZipFile(io.BytesIO(data)) as zf:
-                                for name in zf.namelist():
-                                    if name.endswith('.hgt'):
-                                        data = zf.read(name)
-                                        break
+                        # Skip empty responses
+                        if len(data) < 1000:
+                            continue
+
+                        if source["compressed"]:
+                            if url.endswith('.gz'):
+                                data = gzip.decompress(data)
+                            elif url.endswith('.zip'):
+                                with zipfile.ZipFile(io.BytesIO(data)) as zf:
+                                    for name in zf.namelist():
+                                        if name.endswith('.hgt'):
+                                            data = zf.read(name)
+                                            break
+
+                        # Validate tile size (SRTM1: 25,934,402 bytes, SRTM3: 2,884,802 bytes)
+                        if len(data) not in (3601 * 3601 * 2, 1201 * 1201 * 2):
+                            print(f"[Terrain] Invalid tile size {len(data)} from {url}")
+                            continue
 
                         tile_path.write_bytes(data)
-                        print(f"[Terrain] Downloaded {tile_name} ({len(data)} bytes)")
+                        res = source["resolution"]
+                        size_mb = len(data) / 1048576
+                        print(f"[Terrain] Downloaded {tile_name} ({res}, {size_mb:.1f} MB)")
                         return True
 
                 except Exception as e:
                     print(f"[Terrain] Failed from {url}: {e}")
                     continue
 
-        print(f"[Terrain] Could not download {tile_name}")
+        print(f"[Terrain] Could not download {tile_name} from any source")
         return False
 
     def _load_tile(self, tile_name: str) -> Optional[np.ndarray]:
@@ -149,56 +177,179 @@ class TerrainService:
 
         return self._load_tile(tile_name)
 
+    def _bilinear_sample(self, tile: np.ndarray, lat: float, lon: float) -> float:
+        """Sample elevation with bilinear interpolation for sub-meter accuracy.
+
+        SRTM1 at 30m means nearest-neighbor can have 15m positional error.
+        Bilinear interpolation reduces this to sub-meter accuracy.
+        """
+        size = tile.shape[0]
+
+        # Tile southwest corner
+        lat_int = int(lat) if lat >= 0 else int(lat) - 1
+        lon_int = int(lon) if lon >= 0 else int(lon) - 1
+
+        # Fractional position within tile (0.0 to 1.0)
+        lat_frac = lat - lat_int  # 0 = south edge, 1 = north edge
+        lon_frac = lon - lon_int  # 0 = west edge, 1 = east edge
+
+        # Convert to row/col (note: rows go north to south!)
+        row_exact = (1.0 - lat_frac) * (size - 1)  # 0 = north, size-1 = south
+        col_exact = lon_frac * (size - 1)          # 0 = west, size-1 = east
+
+        # Four surrounding grid points
+        r0 = int(row_exact)
+        c0 = int(col_exact)
+        r1 = min(r0 + 1, size - 1)
+        c1 = min(c0 + 1, size - 1)
+
+        # Fractional position between grid points
+        dr = row_exact - r0
+        dc = col_exact - c0
+
+        # Get four corner values
+        z00 = tile[r0, c0]
+        z01 = tile[r0, c1]
+        z10 = tile[r1, c0]
+        z11 = tile[r1, c1]
+
+        # Handle void (-32768) values - fall back to nearest valid
+        void_val = -32768
+        corners = [(z00, r0, c0), (z01, r0, c1), (z10, r1, c0), (z11, r1, c1)]
+        if z00 == void_val or z01 == void_val or z10 == void_val or z11 == void_val:
+            valid = [(z, r, c) for z, r, c in corners if z != void_val]
+            if not valid:
+                return 0.0
+            # Return nearest valid value
+            return float(valid[0][0])
+
+        # Bilinear interpolation
+        elevation = (z00 * (1 - dr) * (1 - dc) +
+                     z01 * (1 - dr) * dc +
+                     z10 * dr * (1 - dc) +
+                     z11 * dr * dc)
+
+        return float(elevation)
+
     async def get_elevation(self, lat: float, lon: float) -> float:
-        """Get elevation at specific coordinate (meters above sea level)"""
+        """Get elevation at specific coordinate with bilinear interpolation."""
         tile_name = self.get_tile_name(lat, lon)
         tile = await self.load_tile(tile_name)
 
         if tile is None:
             return 0.0
 
-        size = tile.shape[0]
-
-        # Calculate position within tile
-        lat_int = int(lat) if lat >= 0 else int(lat) - 1
-        lon_int = int(lon) if lon >= 0 else int(lon) - 1
-
-        lat_frac = lat - lat_int
-        lon_frac = lon - lon_int
-
-        # Row 0 = north edge, last row = south edge
-        row = int((1 - lat_frac) * (size - 1))
-        col = int(lon_frac * (size - 1))
-
-        row = max(0, min(row, size - 1))
-        col = max(0, min(col, size - 1))
-
-        elevation = tile[row, col]
-
-        # -32768 = void/no data
-        if elevation == -32768:
-            return 0.0
-
-        return float(elevation)
+        return self._bilinear_sample(tile, lat, lon)
 
     def get_elevation_sync(self, lat: float, lon: float) -> float:
-        """Sync elevation lookup from memory cache. Returns 0.0 if tile not loaded."""
+        """Sync elevation lookup with bilinear interpolation. Returns 0.0 if tile not loaded."""
         tile_name = self.get_tile_name(lat, lon)
         tile = self._tile_cache.get(tile_name)
         if tile is None:
             return 0.0
 
-        size = tile.shape[0]
-        lat_int = int(lat) if lat >= 0 else int(lat) - 1
-        lon_int = int(lon) if lon >= 0 else int(lon) - 1
+        return self._bilinear_sample(tile, lat, lon)
 
-        row = int((1 - (lat - lat_int)) * (size - 1))
-        col = int((lon - lon_int) * (size - 1))
-        row = max(0, min(row, size - 1))
-        col = max(0, min(col, size - 1))
+    def get_elevations_batch(self, lats: np.ndarray, lons: np.ndarray) -> np.ndarray:
+        """Vectorized elevation lookup with bilinear interpolation.
 
-        elevation = tile[row, col]
-        return 0.0 if elevation == -32768 else float(elevation)
+        Handles points spanning multiple tiles efficiently.
+        Groups points by tile, processes each tile with full NumPy vectorization.
+        Tiles must be pre-loaded into memory cache.
+
+        Args:
+            lats: Array of latitudes
+            lons: Array of longitudes
+
+        Returns:
+            Array of elevations (0.0 for missing tiles or void data)
+        """
+        elevations = np.zeros(len(lats), dtype=np.float32)
+
+        # Compute tile indices for each point
+        lat_ints = np.floor(lats).astype(int)
+        lon_ints = np.floor(lons).astype(int)
+
+        # Group by tile using unique key
+        unique_tiles = set(zip(lat_ints, lon_ints))
+
+        for lat_int, lon_int in unique_tiles:
+            # Get tile name
+            lat_letter = 'N' if lat_int >= 0 else 'S'
+            lon_letter = 'E' if lon_int >= 0 else 'W'
+            tile_name = f"{lat_letter}{abs(lat_int):02d}{lon_letter}{abs(lon_int):03d}"
+
+            tile = self._tile_cache.get(tile_name)
+            if tile is None:
+                continue
+
+            # Mask for points in this tile
+            mask = (lat_ints == lat_int) & (lon_ints == lon_int)
+            tile_lats = lats[mask]
+            tile_lons = lons[mask]
+
+            size = tile.shape[0]
+
+            # Vectorized bilinear interpolation for all points in this tile
+            lat_frac = tile_lats - lat_int
+            lon_frac = tile_lons - lon_int
+
+            row_exact = (1.0 - lat_frac) * (size - 1)
+            col_exact = lon_frac * (size - 1)
+
+            r0 = np.clip(row_exact.astype(int), 0, size - 2)
+            c0 = np.clip(col_exact.astype(int), 0, size - 2)
+            r1 = r0 + 1
+            c1 = c0 + 1
+
+            dr = row_exact - r0
+            dc = col_exact - c0
+
+            # Get four corner values for all points at once
+            z00 = tile[r0, c0].astype(np.float32)
+            z01 = tile[r0, c1].astype(np.float32)
+            z10 = tile[r1, c0].astype(np.float32)
+            z11 = tile[r1, c1].astype(np.float32)
+
+            # Bilinear interpolation (vectorized)
+            result = (z00 * (1 - dr) * (1 - dc) +
+                      z01 * (1 - dr) * dc +
+                      z10 * dr * (1 - dc) +
+                      z11 * dr * dc)
+
+            # Handle void values (-32768) - set to 0
+            void_mask = (z00 == -32768) | (z01 == -32768) | (z10 == -32768) | (z11 == -32768)
+            result[void_mask] = 0.0
+
+            elevations[mask] = result
+
+        return elevations
+
+    def get_required_tiles(self, center_lat: float, center_lon: float, radius_km: float) -> list:
+        """Determine which tiles are needed for a coverage calculation."""
+        # Convert radius to degrees (approximate)
+        lat_delta = radius_km / 111.0  # ~111 km per degree latitude
+        lon_delta = radius_km / (111.0 * np.cos(np.radians(center_lat)))
+
+        min_lat = center_lat - lat_delta
+        max_lat = center_lat + lat_delta
+        min_lon = center_lon - lon_delta
+        max_lon = center_lon + lon_delta
+
+        tiles = []
+        for lat in range(int(np.floor(min_lat)), int(np.floor(max_lat)) + 1):
+            for lon in range(int(np.floor(min_lon)), int(np.floor(max_lon)) + 1):
+                lat_letter = 'N' if lat >= 0 else 'S'
+                lon_letter = 'E' if lon >= 0 else 'W'
+                tile_name = f"{lat_letter}{abs(lat):02d}{lon_letter}{abs(lon):03d}"
+                tiles.append(tile_name)
+
+        return tiles
+
+    def get_missing_tiles(self, center_lat: float, center_lon: float, radius_km: float) -> list:
+        """Check which needed tiles are not available locally."""
+        required = self.get_required_tiles(center_lat, center_lon, radius_km)
+        return [t for t in required if not self.get_tile_path(t).exists()]
 
     async def get_elevation_profile(
         self,

docs/devlog/gpu_supp/RFCP-3.9.0-SRTM-Terrain-Integration.md

@@ -0,0 +1,436 @@
+# RFCP 3.9.0 — SRTM1 Real Terrain Data Integration
+
+## Context
+
+RFCP currently downloads terrain tiles from an elevation API at runtime.
+This works but has limitations:
+- Requires internet connection
+- Unknown data source quality
+- No offline capability (critical for tactical/field use)
+- No control over resolution or caching
+
+Goal: Replace with SRTM1 (30m resolution) HGT files, offline-first architecture.
+
+## SRTM1 Data Format
+
+HGT files are dead simple:
+- 1°×1° tiles, named by southwest corner: `N48E033.hgt`
+- 3601×3601 grid of signed 16-bit integers (big-endian)
+- Each value = elevation in meters
+- File size: exactly 25,934,402 bytes (3601 × 3601 × 2)
+- Row order: north to south (first row = northernmost)
+- Column order: west to east
+- Adjacent tiles overlap by 1 pixel on shared edges
+- Void/no-data value: -32768
+
+Compressed (.hgt.zip): ~10-15 MB per tile typically.
+
+## Architecture
+
+### Tile Storage Layout
+
+```
+{app_data}/terrain/
+├── srtm1/                    # 30m resolution tiles
+│   ├── N48E033.hgt           # Uncompressed for fast access
+│   ├── N48E034.hgt
+│   ├── N48E035.hgt
+│   └── ...
+├── tile_index.json           # Metadata: available tiles, checksums, dates
+└── downloads/                # Temporary download staging
+```
+
+On Windows, `{app_data}` = the application's data directory.
+For PyInstaller exe: `data/terrain/` relative to exe location.
+The path must be configurable (environment variable or config file).
+
+### Tile Manager (new file: `terrain_manager.py`)
+
+```python
+class SRTMTileManager:
+    """Manages SRTM1 HGT tile storage, loading, and caching."""
+    
+    def __init__(self, terrain_dir: str):
+        self.terrain_dir = Path(terrain_dir)
+        self.srtm1_dir = self.terrain_dir / "srtm1"
+        self.srtm1_dir.mkdir(parents=True, exist_ok=True)
+        
+        # In-memory cache: tile_name -> numpy array
+        self._tile_cache: Dict[str, np.ndarray] = {}
+        self._max_cache_tiles = 16  # ~16 tiles = ~400 MB RAM
+    
+    def get_tile_name(self, lat: float, lon: float) -> str:
+        """Convert lat/lon to SRTM tile name."""
+        # Floor to get southwest corner
+        lat_int = int(lat) if lat >= 0 else int(lat) - 1
+        lon_int = int(lon) if lon >= 0 else int(lon) - 1
+        
+        lat_prefix = "N" if lat_int >= 0 else "S"
+        lon_prefix = "E" if lon_int >= 0 else "W"
+        
+        return f"{lat_prefix}{abs(lat_int):02d}{lon_prefix}{abs(lon_int):03d}"
+    
+    def get_required_tiles(self, center_lat, center_lon, radius_km) -> List[str]:
+        """Determine which tiles are needed for a coverage calculation."""
+        # Calculate bounding box from center + radius
+        # Return list of tile names
+    
+    def has_tile(self, tile_name: str) -> bool:
+        """Check if tile exists locally."""
+        return (self.srtm1_dir / f"{tile_name}.hgt").exists()
+    
+    def load_tile(self, tile_name: str) -> Optional[np.ndarray]:
+        """Load tile from disk into memory. Returns 3601x3601 int16 array."""
+        if tile_name in self._tile_cache:
+            return self._tile_cache[tile_name]
+        
+        hgt_path = self.srtm1_dir / f"{tile_name}.hgt"
+        if not hgt_path.exists():
+            return None
+        
+        # Read raw HGT: big-endian signed 16-bit
+        data = np.fromfile(str(hgt_path), dtype='>i2')
+        tile = data.reshape((3601, 3601))
+        
+        # Replace void values
+        tile = tile.astype(np.float32)
+        tile[tile == -32768] = np.nan
+        
+        # Cache management (LRU-style: evict oldest if full)
+        if len(self._tile_cache) >= self._max_cache_tiles:
+            oldest_key = next(iter(self._tile_cache))
+            del self._tile_cache[oldest_key]
+        
+        self._tile_cache[tile_name] = tile
+        return tile
+    
+    def get_elevation(self, lat: float, lon: float) -> Optional[float]:
+        """Get elevation at a single point with bilinear interpolation."""
+        tile_name = self.get_tile_name(lat, lon)
+        tile = self.load_tile(tile_name)
+        if tile is None:
+            return None
+        return self._bilinear_sample(tile, lat, lon)
+    
+    def get_elevations_batch(self, lats: np.ndarray, lons: np.ndarray) -> np.ndarray:
+        """Get elevations for array of points. Vectorized."""
+        # Group points by tile
+        # Load needed tiles
+        # Vectorized bilinear interpolation per tile
+        # Return array of elevations
+    
+    async def download_tile(self, tile_name: str) -> bool:
+        """Download a single tile from remote source (if online)."""
+        # Try multiple sources in order:
+        # 1. Own server (future: UMTC sync endpoint)
+        # 2. srtm.fasma.org (no auth required)  
+        # 3. viewfinderpanoramas.org (no auth, void-filled)
+        # Returns True if successful
+    
+    def get_missing_tiles(self, center_lat, center_lon, radius_km) -> List[str]:
+        """Check which needed tiles are not available locally."""
+        required = self.get_required_tiles(center_lat, center_lon, radius_km)
+        return [t for t in required if not self.has_tile(t)]
+```
+
+### Bilinear Interpolation (CRITICAL for accuracy)
+
+Current system uses nearest-neighbor (pick closest grid cell).
+SRTM1 at 30m means nearest-neighbor can have 15m positional error.
+Bilinear interpolation reduces this to sub-meter accuracy.
+
+```python
+def _bilinear_sample(self, tile: np.ndarray, lat: float, lon: float) -> float:
+    """Sample elevation with bilinear interpolation."""
+    # Tile southwest corner
+    lat_int = int(lat) if lat >= 0 else int(lat) - 1
+    lon_int = int(lon) if lon >= 0 else int(lon) - 1
+    
+    # Fractional position within tile (0.0 to 1.0)
+    lat_frac = lat - lat_int  # 0 = south edge, 1 = north edge
+    lon_frac = lon - lon_int  # 0 = west edge, 1 = east edge
+    
+    # Convert to row/col (note: rows go north to south!)
+    row_exact = (1.0 - lat_frac) * 3600.0  # 0 = north, 3600 = south
+    col_exact = lon_frac * 3600.0           # 0 = west, 3600 = east
+    
+    # Four surrounding grid points
+    r0 = int(row_exact)
+    c0 = int(col_exact)
+    r1 = min(r0 + 1, 3600)
+    c1 = min(c0 + 1, 3600)
+    
+    # Fractional position between grid points
+    dr = row_exact - r0
+    dc = col_exact - c0
+    
+    # Bilinear interpolation
+    z00 = tile[r0, c0]
+    z01 = tile[r0, c1]
+    z10 = tile[r1, c0]
+    z11 = tile[r1, c1]
+    
+    # Handle NaN (void) values
+    if np.isnan(z00) or np.isnan(z01) or np.isnan(z10) or np.isnan(z11):
+        # Fall back to nearest non-NaN
+        valid = [(z00, 0, 0), (z01, 0, 1), (z10, 1, 0), (z11, 1, 1)]
+        valid = [(z, r, c) for z, r, c in valid if not np.isnan(z)]
+        return valid[0][0] if valid else 0.0
+    
+    elevation = (z00 * (1 - dr) * (1 - dc) +
+                 z01 * (1 - dr) * dc +
+                 z10 * dr * (1 - dc) +
+                 z11 * dr * dc)
+    
+    return float(elevation)
+```
+
+### Vectorized Batch Elevation (for GPU pipeline)
+
+This replaces the current `_batch_elevation_lookup` in gpu_service.py.
+Must handle multi-tile seamlessly.
+
+```python
+def get_elevations_batch(self, lats: np.ndarray, lons: np.ndarray) -> np.ndarray:
+    """Vectorized elevation lookup with bilinear interpolation.
+    
+    Handles points spanning multiple tiles efficiently.
+    Groups points by tile, processes each tile with full NumPy vectorization.
+    """
+    elevations = np.zeros(len(lats), dtype=np.float32)
+    
+    # Compute tile indices for each point
+    lat_ints = np.where(lats >= 0, np.floor(lats).astype(int), 
+                        np.floor(lats).astype(int))
+    lon_ints = np.where(lons >= 0, np.floor(lons).astype(int),
+                        np.floor(lons).astype(int))
+    
+    # Group by tile
+    tile_keys = lat_ints * 1000 + lon_ints  # unique key per tile
+    unique_keys = np.unique(tile_keys)
+    
+    for key in unique_keys:
+        mask = tile_keys == key
+        lat_int = int(key // 1000)
+        lon_int = int(key % 1000)
+        if lon_int > 500:  # handle negative longitudes
+            lon_int -= 1000
+        
+        tile_name = self._make_tile_name(lat_int, lon_int)
+        tile = self.load_tile(tile_name)
+        
+        if tile is None:
+            elevations[mask] = 0.0  # no data
+            continue
+        
+        # Vectorized bilinear for all points in this tile
+        tile_lats = lats[mask]
+        tile_lons = lons[mask]
+        
+        lat_frac = tile_lats - lat_int
+        lon_frac = tile_lons - lon_int
+        
+        row_exact = (1.0 - lat_frac) * 3600.0
+        col_exact = lon_frac * 3600.0
+        
+        r0 = np.clip(row_exact.astype(int), 0, 3599)
+        c0 = np.clip(col_exact.astype(int), 0, 3599)
+        r1 = np.clip(r0 + 1, 0, 3600)
+        c1 = np.clip(c0 + 1, 0, 3600)
+        
+        dr = row_exact - r0
+        dc = col_exact - c0
+        
+        z00 = tile[r0, c0]
+        z01 = tile[r0, c1]
+        z10 = tile[r1, c0]
+        z11 = tile[r1, c1]
+        
+        result = (z00 * (1 - dr) * (1 - dc) +
+                  z01 * (1 - dr) * dc +
+                  z10 * dr * (1 - dc) +
+                  z11 * dr * dc)
+        
+        # Handle NaN voids
+        nan_mask = np.isnan(result)
+        if nan_mask.any():
+            result[nan_mask] = 0.0
+        
+        elevations[mask] = result
+    
+    return elevations
+```
+
+## Integration Points
+
+### 1. Replace terrain_service.py elevation lookup
+
+Current terrain service downloads elevation data from an API.
+Replace with SRTMTileManager calls:
+
+```python
+# OLD: 
+elevation = await self.terrain_service.get_elevation(lat, lon)
+
+# NEW:
+elevation = self.tile_manager.get_elevation(lat, lon)
+# Or for batch (GPU pipeline Phase 2.6):
+elevations = self.tile_manager.get_elevations_batch(lats_array, lons_array)
+```
+
+### 2. Replace _batch_elevation_lookup in gpu_service.py
+
+The vectorized elevation lookup in gpu_service.py currently loads tiles
+and does nearest-neighbor sampling. Replace with tile_manager.get_elevations_batch()
+which does bilinear interpolation.
+
+### 3. Coverage service pre-check
+
+Before starting calculation, check if all needed tiles are available:
+
+```python
+missing = self.tile_manager.get_missing_tiles(site_lat, site_lon, radius_km)
+if missing:
+    if has_internet:
+        # Try to download missing tiles
+        for tile_name in missing:
+            await self.tile_manager.download_tile(tile_name)
+    else:
+        # Return warning to frontend
+        return {"warning": f"Missing terrain tiles: {missing}. Using flat terrain."}
+```
+
+### 4. Frontend notification
+
+When tiles are missing, show a warning banner:
+"⚠ Terrain data not available for this area. Coverage accuracy reduced."
+
+When tiles are being downloaded:
+"⬇ Downloading terrain data... (N48E033.hgt, 12.5 MB)"
+
+### 5. Terrain Profile Viewer
+
+The terrain profile viewer should use the same tile_manager
+for consistent elevation data. With bilinear interpolation,
+profiles will be much smoother and more accurate.
+
+## Download Sources (Priority Order)
+
+For auto-download when online:
+
+1. **srtm.fasma.org** (no auth, direct HGT.zip download)
+   URL: `https://srtm.fasma.org/N48E033.SRTMGL1.hgt.zip`
+   - Free, no registration
+   - SRTM1 (30m) data
+   - May be slow or unreliable
+   
+2. **viewfinderpanoramas.org** (no auth, void-filled data)
+   URL: `http://viewfinderpanoramas.org/dem1/{region}/{tile}.hgt.zip`
+   - Free, no registration  
+   - Void areas filled from topographic maps
+   - Better quality in mountainous areas
+   - File naming might differ by region
+
+3. **Future: UMTC sync server**
+   URL: `https://rfcp.{your-domain}/api/terrain/tiles/{tile_name}.hgt`
+   - Self-hosted on your infrastructure
+   - Accessible via WireGuard mesh
+   - Can pre-populate with full Ukraine dataset
+
+## Offline Bundle Strategy
+
+For installer / field deployment:
+
+### Option A: Region packs
+Pre-package tiles by operational area:
+- `terrain-dnipro.zip` — 4 tiles around Dnipro area (~100 MB)
+- `terrain-ukraine-east.zip` — ~50 tiles, eastern Ukraine (~1.2 GB)
+- `terrain-ukraine-full.zip` — ~171 tiles, all Ukraine (~4.3 GB)
+
+### Option B: On-demand with cache
+Ship empty, download tiles as needed on first calculation.
+Cache permanently. Works well for development/testing.
+
+### Option C: Live USB bundle  
+For tactical deployment, include full Ukraine terrain data
+on the live USB alongside the application. 4.3 GB is acceptable
+for a USB drive.
+
+Recommend: **Option B for now** (development), **Option C for deployment**.
+
+## File Changes
+
+### New Files
+- `backend/app/services/terrain_manager.py` — SRTMTileManager class
+
+### Modified Files  
+- `backend/app/services/terrain_service.py` — Replace API calls with tile_manager
+- `backend/app/services/gpu_service.py` — Replace _batch_elevation_lookup
+- `backend/app/services/coverage_service.py` — Add missing tile pre-check
+- `backend/app/main.py` — Initialize tile_manager on startup
+
+### Config
+- Add `TERRAIN_DIR` environment variable / config option
+- Default: `./data/terrain` relative to backend exe
+
+## Testing
+
+```powershell
+# Build and test
+cd D:\root\rfcp\backend
+pyinstaller ..\installer\rfcp-server-gpu.spec --noconfirm
+.\dist\rfcp-server\rfcp-server.exe
+```
+
+### Test 1: First run (no tiles cached)
+- Start app, trigger calculation
+- Should attempt to download required tile(s)
+- If online: downloads, caches, calculates
+- If offline: warning, flat terrain fallback
+
+### Test 2: Cached tiles
+- Run same calculation again
+- Tile loaded from disk cache, no download
+- Should be fast (tile load from disk < 100ms)
+
+### Test 3: Accuracy comparison  
+- Compare elevation at known points (e.g., Dnipro city center)
+- Cross-reference with Google Earth elevation
+- Expected accuracy: ±5m horizontal, ±16m vertical (SRTM spec)
+
+### Test 4: Multi-tile calculation
+- Set radius to 50km+ to span multiple tiles
+- Verify seamless stitching at tile boundaries
+- No elevation jumps or artifacts at edges
+
+### Test 5: Terrain profile
+- Draw terrain profile across tile boundary
+- Should be smooth, no discontinuity
+- Compare with Google Earth profile for same path
+
+### Test 6: Performance
+- Tile load time from disk: <100ms
+- Batch elevation lookup (6000 points): <50ms
+- Should not regress overall calculation time
+- Memory: ~25 MB per loaded tile, max 16 tiles = 400 MB
+
+## What NOT to Change
+
+- Don't modify GPU pipeline architecture (Phase 2.5/2.6/2.7)
+- Don't change propagation model math
+- Don't change API endpoints or response format
+- Don't change frontend map or heatmap rendering
+- Don't change OSM building/vegetation fetching
+- Don't change PyInstaller build process (just add data dir)
+
+## Success Criteria
+
+- [ ] SRTM1 tiles load correctly (3601×3601, 30m resolution)
+- [ ] Bilinear interpolation working (smoother than nearest-neighbor)
+- [ ] Offline mode works with pre-cached tiles
+- [ ] Auto-download works when online
+- [ ] Missing tile warning shown to user
+- [ ] Multi-tile seamless stitching
+- [ ] Terrain profile accuracy matches Google Earth within 20m
+- [ ] No performance regression (calculation time same or faster)
+- [ ] Tile cache directory configurable