Baseband RSLC (#749)

* Remove range carrier from RSLC

* Add kludgy function for reading complex float16.

* Check that RSLC is actually baseband.

Author: bhawkins

python/packages/pybind_nisar/types.py

@@ -1,9 +1,28 @@
+import h5py
 import numpy as np
 
 complex32 = np.dtype([('r', np.float16), ('i', np.float16)])
 
+
 def to_complex32(z: np.array):
     zf = np.zeros(z.shape, dtype=complex32)
     zf['r'] = z.real
     zf['i'] = z.imag
     return zf
+
+
+def read_c4_dataset_as_c8(ds: h5py.Dataset, key=np.s_[...]):
+    """
+    Read a complex float16 HDF5 dataset as a numpy.complex64 array.
+
+    Avoids h5py/numpy dtype bugs and uses numpy float16 -> float32 conversions
+    which are about 10x faster than HDF5 ones.
+    """
+    # This context manager avoids h5py exception:
+    # TypeError: data type '<c4' not understood
+    with ds.astype(complex32):
+        z = ds[key]
+    # Define a similar datatype for complex64 to be sure we cast safely.
+    complex64 = np.dtype([("r", np.float32), ("i", np.float32)])
+    # Cast safely and then view as native complex64 numpy dtype.
+    return z.astype(complex64).view(np.complex64)

python/packages/pybind_nisar/workflows/focus.py

@@ -468,6 +468,14 @@ def delete_safely(filename):
         pass
 
 
+def get_range_deramp(grid: isce.product.RadarGridParameters) -> np.ndarray:
+    """Compute the phase ramp required to shift a backprojected grid to
+    baseband in range.
+    """
+    r = grid.starting_range + grid.range_pixel_spacing * np.arange(grid.width)
+    return np.exp(-1j * 4 * np.pi / grid.wavelength * r)
+
+
 def focus(runconfig):
     # Strip off two leading namespaces.
     cfg = runconfig.runconfig.groups
@@ -643,6 +651,8 @@ def temp(suffix):
         if not cfg.processing.is_enabled.azcomp:
             continue
 
+        deramp = get_range_deramp(ogrid[frequency])
+
         for i in range(0, ogrid[frequency].length, na):
             # h5py doesn't follow usual slice rules, raises exception
             # if dest_sel slices extend past dataset shape.
@@ -658,6 +668,7 @@ def temp(suffix):
                             kernel, atmos, vars(cfg.processing.azcomp.rdr2geo),
                             vars(cfg.processing.azcomp.geo2rdr))
                 log.debug(f"max(abs(z)) = {np.max(np.abs(z))}")
+                z *= deramp[None, j:jmax]
                 zf = to_complex32(scale * z)
                 acdata.write_direct(zf, dest_sel=block)
 

tests/python/packages/pybind_nisar/workflows/focus.py

@@ -1,8 +1,11 @@
 import iscetest
 from pybind_nisar.workflows import focus
+import pybind_nisar as nisar
 import os
+import numpy as np
 import numpy.testing as npt
 
+
 def get_test_cfg():
     # Don't just call main() because need to update raw data file name.
     rawname = os.path.join(iscetest.data, "REE_L0B_out17.h5")
@@ -11,6 +14,17 @@ def get_test_cfg():
     cfg.runconfig.groups.InputFileGroup.InputFilePath = [rawname]
     return cfg
 
+
+def slc_is_baseband(filename: str, tol=2*np.pi/100, frequency="A", polarization="HH") -> bool:
+    rslc = nisar.products.readers.SLC(hdf5file=filename)
+    ds = rslc.getSlcDataset(frequency, polarization)
+    # work around h5py/numpy awkwardness with Complex{Float16}
+    z = nisar.types.read_c4_dataset_as_c8(ds)
+    dz = z[:, 1:] * z[:, :-1].conj()
+    return abs(np.angle(dz.sum())) < tol
+
+
 def test_focus():
     cfg = get_test_cfg()
     focus.focus(cfg)
+    assert slc_is_baseband(cfg.runconfig.groups.ProductPathGroup.SASOutputFile)