Fix track number/burst id calculation

MANIFEST.in

@@ -1 +1 @@
-include src/s1reader/data/sentinel1_track_burst_id.txt
+recursive-include src/s1reader/data/ *

src/s1reader/s1_burst_id.py

@@ -0,0 +1,147 @@
+import datetime
+from dataclasses import dataclass
+from typing import ClassVar
+
+import numpy as np
+
+
+@dataclass(frozen=True)
+class S1BurstId:
+    T_beam: ClassVar[float] = 2.758273  # interval of one burst [s]
+    T_pre: ClassVar[float] = 2.299849  # Preamble time interval [s]
+    T_orb: ClassVar[float] = 12 * 24 * 3600 / 175  # Nominal orbit period [s]
+    track_number: int
+    esa_burst_id: int
+    subswath: str
+
+    @classmethod
+    def from_burst_params(
+        cls,
+        sensing_time: datetime.datetime,
+        ascending_node_dt: datetime.datetime,
+        start_track: int,
+        end_track: int,
+        subswath: str,
+    ):
+        """Calculate the unique burst ID (track, ESA burstId, swath) of a burst.
+
+        Accounts for equator crossing frames, where the current track number of
+        a burst may change mid-frame. Uses the ESA convention defined in the
+        Sentinel-1 Level 1 Detailed Algorithm Definition.
+
+        Parameters
+        ----------
+        sensing_time : datetime
+            Sensing time of the first input line of this burst [UTC]
+            The XML tag is sensingTime in the annotation file.
+        ascending_node_dt : datetime
+            Time of the ascending node prior to the start of the scene.
+        start_track : int
+            Relative orbit number at the start of the acquisition, from 1-175.
+        end_track : int
+            Relative orbit number at the end of the acquisition.
+        subswath : str, {'IW1', 'IW2', 'IW3'}
+            Name of the subswath of the burst (not case sensitive).
+
+        Returns
+        -------
+        S1BurstId
+            The burst ID object containing track number + ESA's burstId number + swath ID.
+
+        Notes
+        -----
+        The `start_track` and `end_track` parameters are used to determine if the
+        scene crosses the equator. They are the same if the frame does not cross
+        the equator.
+
+        References
+        ----------
+        ESA Sentinel-1 Level 1 Detailed Algorithm Definition
+        https://sentinels.copernicus.eu/documents/247904/1877131/S1-TN-MDA-52-7445_Sentinel-1+Level+1+Detailed+Algorithm+Definition_v2-4.pdf/83624863-6429-cfb8-2371-5c5ca82907b8
+        """
+        # Constants in Table 9-7
+        T_beam = 2.758273  # interval of one burst [s]
+        T_pre = 2.299849  # Preamble time interval [s]
+        T_orb = 12 * 24 * 3600 / 175  # Nominal orbit period [s]
+
+        swath_num = int(subswath[-1])
+        # Since we only have access to the current subswath, we need to use the
+        # burst-to-burst times to figure out
+        #   1. if IW1 crossed the equator, and
+        #   2. The mid-burst sensing time for IW2
+        # IW1 -> IW2 takes ~0.83220 seconds
+        # IW2 -> IW3 takes ~1.07803 seconds
+        # IW3 -> IW1 takes ~0.84803 seconds
+        burst_times = np.array([0.832, 1.078, 0.848])
+        iw1_start_offsets = [
+            0,
+            -burst_times[0],
+            -burst_times[0] - burst_times[1],
+        ]
+        offset = iw1_start_offsets[swath_num - 1]
+        start_iw1 = sensing_time + datetime.timedelta(seconds=offset)
+
+        start_iw1_to_mid_iw2 = burst_times[0] + burst_times[1] / 2
+        mid_iw2 = start_iw1 + datetime.timedelta(seconds=start_iw1_to_mid_iw2)
+
+        has_anx_crossing = (end_track == (start_track + 1) % 175)
+
+        time_since_anx_iw1 = (start_iw1 - ascending_node_dt).total_seconds()
+        time_since_anx = (mid_iw2 - ascending_node_dt).total_seconds()
+
+        if (time_since_anx_iw1 - T_orb) < 0:
+            # Less than a full orbit has passed
+            track_number = start_track
+        else:
+            track_number = end_track
+            # Additional check for scenes which have a given ascending node
+            # that's more than 1 orbit in the past
+            if not has_anx_crossing:
+                time_since_anx = time_since_anx - T_orb
+
+        # Eq. 9-89: ∆tb = tb − t_anx + (r - 1)T_orb
+        # tb: mid-burst sensing time (sensing_time)
+        # t_anx: ascending node time (ascending_node_dt)
+        # r: relative orbit number   (relative_orbit_start)
+        dt_b = time_since_anx + (start_track - 1) * T_orb
+
+        # Eq. 9-91 :   1 + floor((∆tb − T_pre) / T_beam )
+        esa_burst_id = 1 + int(np.floor((dt_b - T_pre) / T_beam))
+
+        return cls(track_number, esa_burst_id, subswath)
+
+    @classmethod
+    def from_str(cls, burst_id_str: str):
+        """Parse a S1BurstId object from a string.
+
+        Parameters
+        ----------
+        burst_id_str : str
+            The burst ID string, e.g. "t123_456_iw1"
+
+        Returns
+        -------
+        S1BurstId
+            The burst ID object containing track number + ESA's burstId number + swath ID.
+        """
+        track_number, esa_burst_id, subswath = burst_id_str.split("_")
+        track_number = int(track_number[1:])
+        esa_burst_id = int(esa_burst_id)
+        return cls(track_number, esa_burst_id, subswath.lower())
+
+    def as_str(self):
+        # Form the unique JPL ID by combining track/burst/swath
+        return f"t{self.track_number:03d}_{self.esa_burst_id:06d}_{self.subswath.lower()}"
+
+    def __str__(self):
+        return self.as_str()
+
+    def __eq__(self, other) -> bool:
+        # Allows for comparison with strings, as well as S1BurstId objects
+        # e.g., you can filter down burst IDs with:
+        # burst_ids = ["t012_024518_iw3", "t012_024519_iw3"]
+        # bursts = [b for b in bursts if b.burst_id in burst_ids]
+        if isinstance(other, str):
+            return self.as_str() == other
+        else:
+            return super().__eq__(other)

src/s1reader/s1_burst_slc.py

@@ -10,6 +10,8 @@
 from osgeo import gdal
 
 from s1reader import s1_annotation
+from .s1_burst_id import S1BurstId
+
 
 # Other functionalities
 def polyfit(xin, yin, zin, azimuth_order, range_order,
@@ -144,7 +146,7 @@ class Sentinel1BurstSlc:
     doppler: Doppler
     range_bandwidth: float
     polarization: str # {VV, VH, HH, HV}
-    burst_id: str # t{track_number}_{burst_index}_iw{1,2,3}
+    burst_id: S1BurstId
     platform_id: str # S1{A,B}
     safe_filename: str # SAFE file name
     center: tuple # {center lon, center lat} in degrees
@@ -660,7 +662,7 @@ def width(self):
     @property
     def swath_name(self):
         '''Swath name in iw1, iw2, iw3.'''
-        return self.burst_id.split('_')[2]
+        return self.burst_id.swath_name
 
     @property
     def thermal_noise_lut(self):
@@ -686,8 +688,9 @@ def eap_compensation_lut(self):
                             f' IPF version = {self.ipf_version}')
 
         return self.burst_eap.compute_eap_compensation_lut(self.width)
-    def bbox(self):
-        '''Returns the (west, south, east, north) bounding box of the burst.'''
-        # Uses https://shapely.readthedocs.io/en/stable/manual.html#object.bounds
-        # Returns a tuple of 4 floats representing (west, south, east, north) in degrees.
-        return self.border[0].bounds
+
+    @property
+    def relative_orbit_number(self):
+        '''Returns the relative orbit number of the burst.'''
+        orbit_number_offset = 73 if self.platform_id == 'S1A' else 202
+        return (self.abs_orbit_number - orbit_number_offset) % 175 + 1