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Data groups:
- SLC complex measurements by swath and burst
- azimuth / time, slant_range as dimensions, polarisation as variables
- azimuth / slant_range coordinates as distances instead of times for
easier interpretation? (slant_range == two-ways-time * speed-of-light,
azimuth as linear distance from ascending node?) - rejected for now
- keep time coordinates as UTC, TAI, UT1 and elapsed time from ascending node (NOT PRESENT??)
- calibration information (azimuth / slant_range dimensions on a reduced grid)
- ground control points (azimuth / slant_range dimensions on one more reduced grid)
- de-ramping parameters
- kinematic description:
- orbit / state vectors
- attitude / quaternions
- antenna pattern
- Doppler centroid / Doppler rate
- incidence angle & Co.
Not loaded:
- noise
Attributes:
- mission, acquisition, processing, etc
Conformance in order of precedence:
- STAC metadata for attributes with SAT and SAR extensions
- CF conventions for the coordinates (with special attentions to time)
High level requirements:
- keep all naming as close to the original al possible (with XML camel case to Python snake case?)
- support opening a swath when other swaths are missing (especially the tifs)
# User experience
```python
>>> import xarray as xr
>>> ds = xr.open_dataset("tests/data/S1B_IW_SLC__1SDV_20210401T052622_20210401T052650_026269_032297_EFA4.SAFE/manifest.safe")
>>> ds = xr.open_dataset("tests/data/S1B_IW_SLC__1SDV_20210401T052622_20210401T052650_026269_032297_EFA4.SAFE")
>>> ds
<xarray.Dataset>
...
>>> ds_iw1_gpc = xr.open_dataset("tests/data/S1B_IW_SLC__1SDV_20210401T052622_20210401T052650_026269_032297_EFA4.SAFE", group="IW1/VV/gcp")
```
Structure:
- root / SAFE
- swaths "IW1" "IW2" "S3" etc / duplicated in VH-VV annotation XML
- bursts "R022_N433_E0120" etc
- polarizations are data variables
- gcp "gcp"
- calibration "calibration"
- orbit "orbit" / duplicated in annotation XML for different polarizations
- attitude "attitude" / same as above
- antenna pattern "antenna"
- zero-Doppler "doppler"
examples: `group="IW2/orbit"`, `group="IW2/N433_E0120`, `group="S3/gcp"` etc
Dimensions, coordinates and variables
We may either keep names and unit of measures as close as possible to the original
or going for easier-to-use choices.
In all cases we add the XML tag name in the long_name so it is clear the provenance of the
information: e.g. for `slant_range_time` -> `"long_name": "two way delay (slantRangeTime)"`
- `azimuth_time` as CF time in UTC (warn: may fail on leap seconds)
- `slant_range_time` as CF time interval in `"s"`
# Accuracy considerations
- `azimuth_time` can be expressed as `np.datetime64[ns]` because
spatial resolution at LEO speed is 10km/s * 1ns ~= 0.001cm
- `slant_range_time` cannot be expressed as `np.timedelta64[ns]` because
spatial resolution at the speed of light is 300_000km/s * 1ns / 2 ~= 15cm,
that it is not enough for interferometric applications.
`slant_range_time` needs a spatial resolution of 0.001cm at a 1_000km distance
so around 1e-9 that is well within 1e-15 resolution of IEEE-754 float64.
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