Supporting new ESA ZARR format

[mgovorcin]

Summary

ESA is developing the EOPF-CPM zarr format
as the future operational delivery format for Sentinel-1 products, replacing the current SAFE/zip.
This issue tracks the work needed to support zarr in s1-reader.

An initial implementation is available at:
mgovorcin/s1-reader → branch eopf-zarr-reader

Current status

compass geocoding works end-to-end from zarr — geocoded amplitude matches SAFE output visually.
However, several fields differ from SAFE due to limitations in the early release format (see below).

Known differences vs SAFE (EOPF-CPM early release, 2024-04)

Field	Issue
abs_orbit_number	Hex burst group counter ≠ SAFE abs orbit — must read from STAC root attrs
first/last_valid_sample/line	Not provided — full burst extent used as fallback
iw2_mid_range	swst_value is raw PRI offset, not slant range time — use GCP slant_range_time
GCP data	Only first GCP populated, rest NaN — burst border degenerates to a point
FM rate / Doppler	Only 1 polynomial entry per burst vs 3+ in SAFE — duplicate with 1s offset as workaround
Doppler LUT2D az times	Must be orbit-relative seconds (ISCE3 convention) — not stored directly
Ascending node time	Not in per-burst attrs — available in STAC root sat:anx_datetime
RFI information	Not available

What needs to be resolved upstream (in EOPF-CPM format)

• Correct absolute orbit number per burst
• firstValidSample / lastValidSample arrays per burst line
• Absolute slant range time (not raw SWST)
• All GCP entries populated (not just the first)
• Multiple FM rate / Doppler polynomial entries per burst

References

• EOPF-CPM sample data: https://stac.browser.user.eopf.eodc.eu/collections/sentinel-1-l1-slc
• Initial zarr reader PR: [mgovorcin/s1-reader#eopf-zarr-reader]{https://github.com/mgovorcin/s1-reader/blob/eopf-zarr-reader/src/s1reader/s1_reader_zarr.py}
• Field comparison script: scripts/compare_zarr_vs_safe.py compare_safe_zarr_fields.py

Testing

I managed to run COMPASS on this sample data
test_zarr_vs_safe.py
https://stac.browser.user.eopf.eodc.eu/collections/sentinel-1-l1-slc/items/S1A_IW_SLC__1SDV_20240205T051225_20240205T051253_052419_0656D8_454B?.asset=asset-product

and got this results
Zarr

Same product S1A_IW_SLC__1SDV_20240205T051225_20240205T051253_052419_0656D8_454B.zip but from SAFE