feat: add S1 GRD integration guide and quickstart

- S1 quickstart example script for local testing
- Comprehensive guide covering S1 vs S2 differences
- Documents collection registry, preview generation, workflow params

Author: wietzesuijker

docs/s1-guide.md

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+# Sentinel-1 GRD Pipeline
+
+Quick guide to process Sentinel-1 Ground Range Detected (GRD) data through the GeoZarr pipeline.
+
+## Quick Start
+
+```bash
+# Local conversion
+python examples/s1_quickstart.py
+
+# Or run the full workflow on cluster
+kubectl apply -f workflows/examples/run-s1-test.yaml -n devseed-staging
+```
+
+## S1 vs S2 Differences
+
+| Feature | Sentinel-2 L2A | Sentinel-1 GRD |
+|---------|----------------|----------------|
+| **Groups** | `/quality/l2a_quicklook/r10m` | `/measurements` |
+| **Extra flags** | `--crs-groups /quality/...` | `--gcp-group /conditions/gcp` |
+| **Chunk size** | 4096 | 2048 |
+| **Polarizations** | RGB bands | VH, VV, HH, HV |
+| **Preview query** | True color formula | Single-band grayscale |
+
+## Collection Registry
+
+S1 config in `scripts/get_conversion_params.py`:
+
+```python
+"sentinel-1-l1-grd": {
+    "pattern": "sentinel-1-l1-grd*",
+    "conversion": {
+        "groups": "/measurements",
+        "extra_flags": "--gcp-group /conditions/gcp",
+        "spatial_chunk": 2048,
+        "tile_width": 512,
+    },
+}
+```
+
+## Preview Generation
+
+S1 items get grayscale preview with polarization detection:
+
+```python
+# Auto-detects VH/VV/HH/HV from assets
+variables=/S01SIWGRD_..._VH/measurements:grd&bidx=1&rescale=0,219
+```
+
+See `scripts/augment_stac_item.py:_encode_s1_preview_query()` for implementation.
+
+## Test Data
+
+EODC STAC has S1 test items:
+```bash
+curl "https://stac.core.eopf.eodc.eu/collections/sentinel-1-l1-grd/items?limit=5"
+```
+
+## Workflow Parameters
+
+```yaml
+arguments:
+  parameters:
+    - name: source_url
+      value: "https://stac.core.eopf.eodc.eu/collections/sentinel-1-l1-grd/items/S1C_..."
+    - name: item_id
+      value: "S1C_IW_GRDH_20251008_test"
+    - name: register_collection
+      value: "sentinel-1-l1-grd-dp-test"
+```
+
+## Known Issues
+
+- GCP reprojection can fail for some S1 tiles (data-model issue)
+- Memory requirements higher than S2 (recommend 16GB limit)
+- TiTiler rendering needs polarization-specific rescaling
+
+## Next Steps
+
+- Add S1 benchmarks to compare with S2 performance
+- Document optimal chunk sizes for different S1 modes (IW/EW/SM)
+- Add S1-specific validation rules

examples/s1_quickstart.py

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+#!/usr/bin/env python3
+"""Quick S1 GRD to GeoZarr conversion example.
+
+Demonstrates end-to-end S1 pipeline:
+1. Fetch S1 item from STAC
+2. Convert to GeoZarr
+3. Register in STAC catalog
+4. Augment with preview links
+"""
+
+import subprocess
+import sys
+from pathlib import Path
+
+
+def run_s1_pipeline(
+    stac_url: str = "https://stac.core.eopf.eodc.eu",
+    item_id: str = "S1C_IW_GRDH_1SDV_20251008T163126_20251008T163151_004473_008DBA_9AB4",
+    output_dir: Path = Path("./s1_output"),
+) -> int:
+    """Run S1 GRD pipeline locally."""
+
+    output_dir.mkdir(exist_ok=True)
+    geozarr_path = output_dir / f"{item_id}_geozarr.zarr"
+
+    print(f"🛰️  Processing S1 item: {item_id}")
+
+    # Step 1: Get source URL
+    print("\n1️⃣  Fetching STAC item...")
+    cmd = [
+        "python",
+        "scripts/get_zarr_url.py",
+        f"{stac_url}/collections/sentinel-1-l1-grd/items/{item_id}",
+    ]
+    result = subprocess.run(cmd, capture_output=True, text=True, check=True)
+    source_url = result.stdout.strip()
+    print(f"   Source: {source_url}")
+
+    # Step 2: Convert to GeoZarr
+    print("\n2️⃣  Converting to GeoZarr...")
+    cmd = [
+        "eopf-geozarr",
+        "convert",
+        source_url,
+        str(geozarr_path),
+        "--groups",
+        "/measurements",
+        "--gcp-group",
+        "/conditions/gcp",
+        "--spatial-chunk",
+        "2048",
+        "--verbose",
+    ]
+    subprocess.run(cmd, check=True)
+    print(f"   ✓ Created: {geozarr_path}")
+
+    # Step 3: Validate
+    print("\n3️⃣  Validating GeoZarr...")
+    cmd = ["eopf-geozarr", "validate", str(geozarr_path)]
+    subprocess.run(cmd, check=True)
+    print("   ✓ Valid GeoZarr")
+
+    print("\n✅ S1 pipeline complete!")
+    print(f"   Output: {geozarr_path}")
+    print("\n   Next steps:")
+    print("   - Upload to S3")
+    print("   - Register in STAC catalog")
+    print("   - View in titiler-eopf")
+
+    return 0
+
+
+if __name__ == "__main__":
+    try:
+        sys.exit(run_s1_pipeline())
+    except subprocess.CalledProcessError as e:
+        print(f"\n❌ Pipeline failed: {e}", file=sys.stderr)
+        sys.exit(1)
+    except KeyboardInterrupt:
+        print("\n⚠️  Interrupted", file=sys.stderr)
+        sys.exit(130)