From 0c3794babb095477cbdcce28078da068029d4075 Mon Sep 17 00:00:00 2001
From: Daniel Pressler <daniel.pressler@govsco.com>
Date: Wed, 5 Aug 2026 21:47:15 +0000
Subject: [PATCH] Add compute_dwelltimes_using_poly to sarkit.cphd

---
 CHANGELOG.md                  |  3 +++
 sarkit/cphd/__init__.py       | 10 ++++++++
 sarkit/cphd/_dwell.py         | 43 +++++++++++++++++++++++++++++++++++
 sarkit/crsd/_computations.py  |  4 ++--
 tests/core/cphd/test_dwell.py | 23 +++++++++++++++++++
 5 files changed, 81 insertions(+), 2 deletions(-)
 create mode 100644 sarkit/cphd/_dwell.py
 create mode 100644 tests/core/cphd/test_dwell.py

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 68dc636..5d25c51 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- `compute_dwelltimes_using_poly` to `sarkit.cphd`
+
 
 ## [1.10.1] - 2026-07-31
 
diff --git a/sarkit/cphd/__init__.py b/sarkit/cphd/__init__.py
index 476d65d..3c90c7f 100644
--- a/sarkit/cphd/__init__.py
+++ b/sarkit/cphd/__init__.py
@@ -121,6 +121,14 @@
 
    compute_reference_geometry
 
+Channel & Dwell Parameters
+==========================
+
+.. autosummary::
+   :toctree: generated/
+
+   compute_dwelltimes_using_poly
+
 Constants
 =========
 
@@ -172,6 +180,7 @@
     SECTION_TERMINATOR,
     VERSION_INFO,
 )
+from ._dwell import compute_dwelltimes_using_poly
 from ._io import (
     FileHeaderPart,
     Metadata,
@@ -262,6 +271,7 @@
     "XyzPolyType",
     "XyzType",
     "binary_format_string_to_dtype",
+    "compute_dwelltimes_using_poly",
     "compute_reference_geometry",
     "compute_t_ref",
     "compute_t_ref_from_pvps",
diff --git a/sarkit/cphd/_dwell.py b/sarkit/cphd/_dwell.py
new file mode 100644
index 0000000..e6c00c3
--- /dev/null
+++ b/sarkit/cphd/_dwell.py
@@ -0,0 +1,43 @@
+import lxml.etree
+import numpy as np
+import numpy.polynomial.polynomial as npp
+import numpy.typing as npt
+
+from . import _xml as skcphd_xml
+
+
+def compute_dwelltimes_using_poly(
+    ch_id: str,
+    iax: npt.ArrayLike,
+    iay: npt.ArrayLike,
+    cphd_xmltree: lxml.etree.ElementTree,
+) -> tuple[np.ndarray, np.ndarray]:
+    """Compute center of dwell times and dwell times for scene points using polynomials.
+
+    Parameters
+    ----------
+    ch_id : str
+        Channel unique identifier
+    iax, iay : array_like
+        Image area coordinates (in meters) of the scene points for which to compute the dwell times
+    cphd_xmltree : lxml.etree.ElementTree
+        CPHD XML
+
+    Returns
+    -------
+    t_cod : ndarray
+        Center of dwell times (sec) for the scene points relative to the CollectionStart time
+    t_dwell : ndarray
+        Dwell times (sec) for which the channel signal array contains the echo signals from the scene points
+    """
+    iax, iay = np.broadcast_arrays(iax, iay)
+
+    ew = skcphd_xml.ElementWrapper(cphd_xmltree.getroot())
+    chan_dt = ew["Channel"].find("Parameters", Identifier=ch_id)["DwellTimes"]
+    cod_poly = ew["Dwell"].find("CODTime", Identifier=chan_dt["CODId"])["CODTimePoly"]
+    dwell_poly = ew["Dwell"].find("DwellTime", Identifier=chan_dt["DwellId"])[
+        "DwellTimePoly"
+    ]
+    t_cod = npp.polyval2d(iax, iay, cod_poly)
+    t_dwell = npp.polyval2d(iax, iay, dwell_poly)
+    return t_cod, t_dwell
diff --git a/sarkit/crsd/_computations.py b/sarkit/crsd/_computations.py
index c0c5245..524bed7 100644
--- a/sarkit/crsd/_computations.py
+++ b/sarkit/crsd/_computations.py
@@ -588,7 +588,7 @@ def compute_dwelltimes_using_poly(
     t_cod : ndarray
         Center of dwell times (sec) for the scene points relative to the Collection Reference Time
     t_dwell : ndarray
-       Dwell times (sec) for which the channel signal array contains the echo signals from the scene points
+        Dwell times (sec) for which the channel signal array contains the echo signals from the scene points
     """
     iax, iay = np.broadcast_arrays(iax, iay)
 
@@ -638,7 +638,7 @@ def compute_dwelltimes_using_dta(
     t_cod : ndarray
         Center of dwell times (sec) for the scene points relative to the Collection Reference Time
     t_dwell : ndarray
-       Dwell times (sec) for which the channel signal array contains the echo signals from the scene points
+        Dwell times (sec) for which the channel signal array contains the echo signals from the scene points
     """
     iax, iay = np.broadcast_arrays(iax, iay)
 
diff --git a/tests/core/cphd/test_dwell.py b/tests/core/cphd/test_dwell.py
new file mode 100644
index 0000000..f496fc1
--- /dev/null
+++ b/tests/core/cphd/test_dwell.py
@@ -0,0 +1,23 @@
+import pathlib
+
+import lxml.etree
+import numpy as np
+import pytest
+
+import sarkit.cphd as skcphd
+
+DATAPATH = pathlib.Path(__file__).parents[3] / "data"
+
+
+def test_compute_dwelltimes_using_poly():
+    cphd_xml = lxml.etree.parse(DATAPATH / "example-cphd-1.1.0.xml")
+    ref_chid = cphd_xml.findtext(".//{*}RefChId")
+
+    srp_cod, srp_dwell = skcphd.compute_dwelltimes_using_poly(ref_chid, 0, 0, cphd_xml)
+    assert srp_cod == pytest.approx(float(cphd_xml.findtext(".//{*}SRPCODTime")))
+    assert srp_dwell == pytest.approx(float(cphd_xml.findtext(".//{*}SRPDwellTime")))
+
+    # broadcast smoke test
+    skcphd.compute_dwelltimes_using_poly(
+        ref_chid, np.zeros((2, 4, 5)), np.zeros((2, 1, 5)), cphd_xml
+    )