fix: Fix issue with improper geolocation of IMP products. (#51)

* Fix issue with improper geolocation of IMP products.

Fix antenna gain with IMS mode.

* style: extract code into auxiliary functions

* style: extract code into auxiliary functions

* style: added missing type hints

---------

Co-authored-by: Achaad <achaad@achaad.eu>

Author: priit111

asar_xarray/asar.py

@@ -25,7 +25,7 @@ def get_metadata(gdal_dataset: gdal.Dataset) -> Dict[str, Any]:
     :param gdal_dataset: Dataset with metadata
     :return: Dictionary with metadata attributes
     """
-    attributes: dict[str, Any] = dict()
+    attributes: dict[str, Any] = {}
     process_general_metadata(gdal_dataset, attributes)
     process_records_metadata(gdal_dataset, attributes)
     process_derived_subdatasets_metadata(gdal_dataset, attributes)
@@ -35,35 +35,36 @@ def get_metadata(gdal_dataset: gdal.Dataset) -> Dict[str, Any]:
     return attributes
 
 
-def open_asar_dataset(filepath: str | os.PathLike[Any] | ReadBuffer[Any] | AbstractDataStore) -> xr.Dataset:
+def open_asar_dataset(filename_or_obj: str | os.PathLike[Any] | ReadBuffer[
+        Any] | bytes | memoryview | AbstractDataStore) -> xr.Dataset:
     """
     Open an ASAR dataset and converts it into an xarray Dataset.
 
     This function reads the metadata and pixel data from the given ASAR dataset file,
     processes the metadata into attributes, and constructs an xarray Dataset.
 
-    :param filepath: The path to the ASAR dataset file. It can be a string,
+    :param filename_or_obj: The path to the ASAR dataset file. It can be a string,
                      a PathLike object, a ReadBuffer, or an AbstractDataStore.
     :return: An xarray Dataset containing the pixel data and metadata attributes.
     :raises NotImplementedError: If the filepath is not a string.
     """
-    if not isinstance(filepath, str):
-        raise NotImplementedError(f'Filepath type {type(filepath)} is not supported')
+    if not isinstance(filename_or_obj, str):
+        raise NotImplementedError(f'Filepath type {type(filename_or_obj)} is not supported')
 
-    logger.debug('Opening ASAR dataset {}', filepath)
+    logger.debug('Opening ASAR dataset {}', filename_or_obj)
 
     # Open the dataset using a custom reader
-    gdal_dataset: gdal.Dataset = reader.get_gdal_dataset(filepath)
+    gdal_dataset: gdal.Dataset = reader.get_gdal_dataset(filename_or_obj)
 
     # Extract metadata attributes
     metadata = get_metadata(gdal_dataset)
 
     # Duplicate, read directly from file, as gdal does not parse some necessary metadata
 
-    metadata["direct_parse"] = envisat_direct.parse_direct(filepath, metadata)
+    metadata["direct_parse"] = envisat_direct.parse_direct(filename_or_obj, metadata)
 
     # Create an xarray Dataset with pixel data and metadata attributes
-    dataset: xr.Dataset = create_dataset(metadata, filepath)
+    dataset: xr.Dataset = create_dataset(metadata, filename_or_obj)
 
     return dataset
 
@@ -89,10 +90,16 @@ def create_dataset(metadata: dict[str, Any], filepath: str) -> xr.Dataset:
     range_sampling_rate = metadata["records"]["main_processing_params"]["range_samp_rate"]
     image_slant_range_time = metadata["direct_parse"]["slant_time_first"] * 1e-9
 
-    if metadata["sph_descriptor"] == "Image Mode SLC Image":
+    range_pixel_spacing = metadata["records"]["main_processing_params"]["range_spacing"]
+
+    product_str = metadata["sph_descriptor"]
+
+    if product_str == "Image Mode SLC Image":
         product_type = "SLC"
+    elif product_str == "Image Mode Precision Image":
+        product_type = "GRD"
     else:
-        raise RuntimeError("Only Image mode SLC files(IMS) supported for now")
+        raise RuntimeError("Only Image mode files supported(IMS & IMP) supported for now")
 
     attrs = {
         "family_name": "Envisat",
@@ -106,8 +113,7 @@ def create_dataset(metadata: dict[str, Any], filepath: str) -> xr.Dataset:
         "product_type": product_type,
         "start_time": product_first_line_utc_time,
         "stop_time": product_last_line_utc_time,
-        "range_pixel_spacing" : metadata["range_spacing"],
-        "azimuth_pixel_spacing" : metadata["azimuth_spacing"],
+        "range_pixel_spacing": metadata["range_spacing"],
         "radar_frequency": metadata["records"]["main_processing_params"]["radar_freq"] / 1e9,
         "ascending_node_time": "",
         "azimuth_pixel_spacing": metadata["records"]["main_processing_params"]["azimuth_spacing"],
@@ -116,15 +122,14 @@ def create_dataset(metadata: dict[str, Any], filepath: str) -> xr.Dataset:
         "azimuth_time_interval": azimuth_time_interval,
         "image_slant_range_time": image_slant_range_time,
         "range_sampling_rate": range_sampling_rate,
-        "incidence_angle_mid_swath": metadata["direct_parse"]["incidence_angle_center"] * 2 * math.pi / 360 ,
+        "incidence_angle_mid_swath": metadata["direct_parse"]["incidence_angle_center"] * 2 * math.pi / 360,
         "metadata": metadata
     }
 
     azimuth_time = compute_azimuth_time(
         product_first_line_utc_time, product_last_line_utc_time, number_of_lines
     )
 
-
     swap_dims = {"line": "azimuth_time", "pixel": "slant_range_time"}
 
     coords: dict[str, Any] = {
@@ -140,12 +145,34 @@ def create_dataset(metadata: dict[str, Any], filepath: str) -> xr.Dataset:
         ),
     }
 
-    slant_range_time = np.linspace(
-        image_slant_range_time,
-        image_slant_range_time + (number_of_samples - 1) / range_sampling_rate,
-        number_of_samples,
-    )
-    coords["slant_range_time"] = ("pixel", slant_range_time)
+    if product_type == "SLC":
+        slant_range_time = np.linspace(
+            image_slant_range_time,
+            image_slant_range_time + (number_of_samples - 1) / range_sampling_rate,
+            number_of_samples,
+        )
+        coords["slant_range_time"] = ("pixel", slant_range_time)
+    else:
+
+        # generate slant range times from ground ranges for ground range products
+        # this is easier due to the Envisat only having GRSR metadata unlike Sentinel1
+        # however xarray/Sarsen handle GRD conversion differently for S1
+        ground_range = np.linspace(
+            0,
+            range_pixel_spacing * (number_of_samples - 1),
+            number_of_samples,
+        )
+
+        # numpy polyval expects the polynomial top be highest ranked first
+        coeffs = list(reversed(metadata["direct_parse"]["grsr_coeffs"]))
+
+        slant_ranges = np.polyval(coeffs, ground_range)
+        slant_ranges *= 2
+
+        c = 299792458
+        slant_range_times = slant_ranges / c
+
+        coords["slant_range_time"] = ("pixel", slant_range_times)
 
     data = xr.open_dataarray(filepath, engine='rasterio')
     data.encoding.clear()
@@ -168,19 +195,17 @@ def get_chirp_parameters(dataset: gdal.Dataset) -> dict[str, Any]:
     :return: dictionary with chirp parameters.
     """
     metadata = dataset.GetMetadata(domain='RECORDS')
-    params: dict[str, Any] = dict()
+    params: dict[str, Any] = {
+        'zero_doppler_time': utils.get_envisat_time(metadata.get('CHIRP_PARAMS_ADS_ZERO_DOPPLER_TIME')),
+        'attach_flag': bool(int(metadata.get('CHIRP_PARAMS_ADS_ATTACH_FLAG', '0'))),
+        'beam_id': metadata.get('CHIRP_PARAMS_ADS_BEAM_ID'), 'polarisation': metadata.get('CHIRP_PARAMS_ADS_POLAR'),
+        'chirp': {}}
     # Non-float values
-    params['zero_doppler_time'] = utils.get_envisat_time(metadata.get('CHIRP_PARAMS_ADS_ZERO_DOPPLER_TIME'))
-    params['attach_flag'] = bool(int(metadata.get('CHIRP_PARAMS_ADS_ATTACH_FLAG', '0')))
-    params['beam_id'] = metadata.get('CHIRP_PARAMS_ADS_BEAM_ID')
-    params['polarisation'] = metadata.get('CHIRP_PARAMS_ADS_POLAR')
-    params['chirp'] = dict()
     for key, value in metadata.items():
         if 'CHIRP_PARAMS_ADS_CHIRP' in key:
             new_key = key.replace('CHIRP_PARAMS_ADS_CHIRP_', '').lower()
             params['chirp'][new_key] = float(value)
 
-    #params['elev_corr_factor'] = float(metadata.get('CHIRP_PARAMS_ADS_ELEV_CORR_FACTOR'))
     params['cal_pulse_info'] = get_chirp_cal_pulse_info(metadata)
 
     return params