Replace Orbit.load_from_h5 binding with pure Python "load_orbit_from_…

…h5_group" (#1318)

* Migrate orbit deserialization to pure Python

* Replace all usage of Orbit.load_from_h5 with load_orbit_from_h5_group

* Add docstring to Orbit.set_interp_method

* Add docstring to load_orbit_from_h5_group

* Factor ISO date parsing to separate function

* Use public isce3.core imports

python/extensions/pybind_isce3/core/Orbit.cpp

@@ -118,5 +118,24 @@ void addbinding(py::class_<Orbit> & pyOrbit)
             isce3.core.Orbit
                 Orbit object with data containing start & end times)",
             py::arg("start"), py::arg("end"), py::arg("npad") = 0)
+        .def("set_interp_method", [](Orbit& self, const std::string& method) {
+            if (method == "Hermite") {
+                self.interpMethod(OrbitInterpMethod::Hermite);
+            } else if (method == "Legendre") {
+                self.interpMethod(OrbitInterpMethod::Legendre);
+            } else {
+                throw std::invalid_argument("unexpected orbit interpolation method '" + method + "'");
+            }
+        }, R"(
+        Set interpolation method.
+
+        Parameters
+        ----------
+        method : {'Hermite', 'Legendre'}
+            The method for interpolating orbit state vectors (cubic
+            Hermite spline interpolation or eighth-order Legendre
+            polynomial interpolation).
+        )",
+        py::arg("method"))
         ;
 }

python/packages/isce3/core/__init__.py

@@ -6,3 +6,4 @@
 from .poly2d import fit_bivariate_polynomial
 from . import rdr_geo_block_generator
 from .block_param_generator import BlockParam
+from .serialization import load_orbit_from_h5_group

python/packages/isce3/core/serialization.py

@@ -0,0 +1,69 @@
+from __future__ import annotations
+from isce3.core import DateTime, Orbit, StateVector, TimeDelta
+import h5py
+
+def parse_iso_date(iso_date: str) -> DateTime:
+    """
+    Parse ISO date string to DateTime
+
+    This is a direct translation from C++ code in isce3,
+    and could probably be made more robust/intuitive.
+    """
+    utc_ref = ""
+    pos_iso = iso_date.rfind('-')
+    if pos_iso != -1:
+        utc_ref = iso_date[pos_iso - 7:]
+        # remove any trailing whitespace
+        utc_ref = utc_ref.rstrip(' ')
+    return DateTime(utc_ref)
+
+def load_orbit_from_h5_group(group: h5py.Group) -> Orbit:
+    """
+    Load orbit data from a group in an HDF5 file.
+
+    The organization of orbit data in the group is assumed to conform to
+    NISAR product specifications.
+
+    Parameters
+    ----------
+    group : h5py.Group
+        The HDF5 group containing the orbit metadata.
+
+    Returns
+    -------
+    orbit : Orbit
+        The orbit data.
+    """
+    time = group['time']
+    pos = group['position']
+    vel = group['velocity']
+
+    if time.ndim != 1 or pos.ndim != 2 or vel.ndim != 2:
+        raise ValueError("unexpected orbit state vector dims")
+
+    if pos.shape[1] != 3 or vel.shape[1] != 3:
+        raise ValueError("unexpected orbit position/velocity vector size")
+
+    size = time.shape[0];
+    if pos.shape[0] != size or vel.shape[0] != size:
+        raise ValueError("mismatched orbit state vector component sizes")
+
+    # get reference epoch
+    unit_attr = time.attrs['units']
+    # result may be str or bytes, convert to str if needed
+    if type(unit_attr) is not str:
+        unit_attr = unit_attr.decode('utf-8')
+    epoch = parse_iso_date(unit_attr)
+
+    # convert to state vectors
+    statevecs = [StateVector(epoch + TimeDelta(time[i]), pos[i], vel[i])
+                 for i in range(size)]
+
+    # construct Orbit object
+    result = Orbit(statevecs, epoch)
+
+    # set interpolation method, if specified
+    if 'interpMethod' in group.keys():
+        result.set_interp_method(group['interpMethod'][()])
+
+    return result

python/packages/nisar/products/readers/Base/Base.py

@@ -140,7 +140,7 @@ def getOrbit(self):
         '''
         with h5py.File(self.filename, 'r', libver='latest', swmr=True) as fid:
             orbitPath = os.path.join(self.MetadataPath, 'orbit')
-            return isce3.core.Orbit.load_from_h5(fid[orbitPath])
+            return isce3.core.load_orbit_from_h5_group(fid[orbitPath])
 
     @pyre.export
     def getAttitude(self):

python/packages/nisar/products/readers/Raw/Raw.py

@@ -162,7 +162,7 @@ def TelemetryPath(self):
     def getOrbit(self):
         path = f"{self.TelemetryPath}/orbit"
         with h5py.File(self.filename, 'r', libver='latest', swmr=True) as f:
-            orbit = isce3.core.Orbit.load_from_h5(f[path])
+            orbit = isce3.core.load_orbit_from_h5_group(f[path])
         return orbit
 
     def getAttitude(self):

tests/python/extensions/pybind/core/orbit.py

@@ -4,12 +4,12 @@
 import numpy.testing as npt
 
 def load_h5():
-    from isce3.ext.isce3.core import Orbit
+    from isce3.core import load_orbit_from_h5_group
     from iscetest import data
     from os import path
     import h5py
     f = h5py.File(path.join(data, "envisat.h5"), 'r')
-    return Orbit.load_from_h5(f["/science/LSAR/SLC/metadata/orbit"])
+    return load_orbit_from_h5_group(f["/science/LSAR/SLC/metadata/orbit"])
 
 o = load_h5();
 

tests/python/extensions/pybind/focus/backproject.py

@@ -4,6 +4,7 @@
 import numpy as np
 import numpy.testing as npt
 import isce3.ext.isce3 as isce
+from isce3.core import load_orbit_from_h5_group
 from iscetest import data as test_data_dir
 from pathlib import Path
 import json
@@ -20,7 +21,7 @@ def load_h5(filename):
     signal_data = f["data"][()]
 
     # load orbit
-    orbit = isce.core.Orbit.load_from_h5(f["orbit"])
+    orbit = load_orbit_from_h5_group(f["orbit"])
 
     # load Doppler
     doppler = isce.core.LUT2d.load_from_h5(f["doppler"], "doppler")

tests/python/extensions/pybind/geometry/bbox.py

@@ -1,6 +1,7 @@
 import iscetest
 import pathlib
 import isce3.ext.isce3 as isce
+from isce3.core import load_orbit_from_h5_group
 from nisar.products.readers import SLC
 
 
@@ -11,7 +12,7 @@ def load_orbit():
     import h5py
     with h5py.File(path_slc, "r") as h5:
         g = h5["/science/LSAR/SLC/metadata/orbit"]
-        orbit = isce.core.Orbit.load_from_h5(g)
+        orbit = load_orbit_from_h5_group(g)
     return orbit