Metadata Specification for Polarimetric Radar

1. Introduction

Polarimetric radar systems acquire electromagnetic returns in multiple polarization states (HH, HV, VH, VV). The richness of the data supports advanced analysis such as target classification, soil moisture estimation, and forest structure mapping. To make this data useful across platforms, agencies, and research groups, a welldefined metadata specification is essential. Metadata describes the context, acquisition parameters, processing history, and quality of the dataset, ensuring that users can interpret the measurements correctly.

2. Why Metadata Matters

• Interoperability: Standardized metadata enables seamless exchange between archives, software packages and cloud services.
• Reproducibility: Detailed acquisition and processing records let researchers repeat or extend analyses.
• Data Discovery: Searchable fields (e.g., sensor, orbit, incidence angle) make large collections searchable.
• Quality Assessment: Flags and uncertainty values help users gauge the reliability of derived products.

3. Core Elements of a Polarimetric Radar Metadata Record

The following categories are common to most international standards (ISO 19115, CF Conventions, CEOS, and OGC). They can be grouped into highlevel sections for clarity.

3.1 Identification

3.2 Spatial and Temporal Coverage

3.3 Sensor and Platform

3.4 Acquisition Parameters

• Pulse Repetition Frequency (PRF)
• Transmit Power
• Swath Width
• Ground Range and Slant Range definitions
• Calibration constants (antenna gain, system noise figure).

3.5 Processing Information

Metadata must describe every processing step applied after raw data capture.

3.6 Quality Assurance

• Noise Equivalent Sigma Zero (NESZ) values.
• Calibration accuracy (percentage or dB).
• Data gaps or missing swaths.
• Flag fields for saturation, layover, or shadow.

3.7 Access and Distribution

• File format (e.g., GeoTIFF, ENVI, NetCDF, SAF).
• Compression method.
• Access URL or DOI.
• Use constraints and licensing.

4. Relevant International Standards

Many agencies adopt a combination of the following specifications.

• ISO 19115/19139 General geographic metadata schema; includes gml encoding.
• CF Conventions NetCDF attribute conventions useful for multiband polarimetric data.
• CEOS SAR Metadata Convention Established by the Committee on Earth Observation Satellites; defines mandatory fields for SAR missions.
• OGC SensorML Describes sensor characteristics and processing chains in an XML format.
• NASA ASF Metadata Format Used for data distributed by the Alaska Satellite Facility, with explicit polarization tags.

5. Example Metadata Record (JSON)

{  "id": "urn:uuid:3f8d6e9b-4c2a-4b58-9a12-5c2d3e6f7a90",  "title": "Sentinel1A QuadPol LBand SAR Interferogram",  "abstract": "Fully calibrated quadpolarimetric interferometric product over the Amazon basin, acquired on 20250812.",  "keywords": ["polarimetric SAR", "Lband", "interferometry", "forest"],  "spatial": {    "footprint": "POLYGON((-68.5 -5.2,-67.8 -5.2,-67.8 -4.1,-68.5 -4.1,-68.5 -5.2))",    "bbox": [-68.5, -5.2, -67.8, -4.1]  },  "temporal": {    "start": "2025-08-12T02:15:00Z",    "end":   "2025-08-12T02:20:00Z"  },  "sensor": {    "platform": "Sentinel1A",    "instrument": "CSAR",    "frequencyBand": "L",    "polarizationMode": "HH HV VH VV",    "incidenceAngle": {      "mean": 28.4,      "min": 27.9,      "max": 28.9    },    "resolution": {"range": 5.0, "azimuth": 5.0}  },  "processing": {    "calibration": {"algorithm":"SARCal v2.3","gain":"1.0"},    "geocoding": {"dem":"SRTM 30m","projection":"EPSG:4326"},    "speckleFilter": {"type":"Lee","window":7},    "decomposition": {"method":"Cloude-Pottier","output":["alpha","entropy","anisotropy"]},    "productType":"Covariance matrix (C4)"  },  "quality": {    "NESZ_dB": -25.3,    "calibrationError_percent": 2.1,    "missingLines": 0  },  "distribution": {    "format":"NetCDF-4",    "compressed":true,    "url":"https://data.example.org/s1a/20250812/quadpol.nc",    "license":"CC-BY-4.0"  }}            

The JSON example follows a logical grouping that mirrors the sections above. It can be easily translated to XML or ISO 19139 XML when required.

6. Best Practices for Implementers

1. Use controlled vocabularies. Adopt terms from the NASA/ESA keyword lists to avoid ambiguity.
2. Validate against schema. Run XML/JSON/NetCDF files through the appropriate validator (e.g., ISO 19139 XSD) before publishing.
3. Document processing software versions. Include the exact version numbers of calibration and decomposition tools.
4. Provide both raw and processed metadata. Users may need to reconstruct intermediate steps.
5. Include provenance links. Reference the original Level0 product identifier.
6. Regularly review and update. Standards evolve; schedule periodic compliance checks.

7. Conclusion

A comprehensive metadata specification is the backbone of reliable polarimetric radar data ecosystems. By capturing acquisition geometry, sensor specifics, processing lineage, and quality indicators in a standardized format, data providers enable crossmission analysis, reproducible science, and efficient data discovery. Aligning with international standards such as ISO 19115, CEOS, and CF ensures that these rich datasets can be shared globally and integrated into emerging Earth observation platforms.