Implement more robust PRT gap search

pygac/calibration/noaa.py

@@ -457,14 +457,15 @@ def calibrate_thermal(counts, prt, ict, space, line_numbers, channel, cal):
     # Note that the prt values are the average value of the three readings from one of the four
     # PRTs. See reader.get_telemetry implementations.
     prt_threshold = 50  # empirically found and set by Abhay Devasthale
-    offset = 0
 
-    for i, prt_val in enumerate(prt):
+    for offset in range(5):
         # According to the KLM Guide the fill value between PRT measurments is 0, but we search
-        # for the first measurment gap using the threshold. Is this on purpose?
-        if prt_val < prt_threshold:
-            offset = i
+        # for the first measurement gap using the threshold, because the fill value is in practice
+        # not always exactly 0.
+        if np.median(prt[(line_numbers - line_numbers[0]) % 5 == offset]) < prt_threshold:
             break
+    else:
+        raise IndexError("No PRT 0-index found!")
 
     # get the PRT index, iprt equals to 0 corresponds to the measurement gaps
     iprt = (line_numbers - line_numbers[0] + 5 - offset) % 5

pygac/tests/test_reader.py

@@ -87,6 +87,7 @@ def get_header_timestamp(self):
     def get_telemetry(self):
         """Get the telemetry."""
         prt = 51 * np.ones(self.along_track)  # prt threshold is 50
+        prt[::5] = 0
         ict = 101 * np.ones((self.along_track, 3))  # ict threshold is 100
         space = 101 * np.ones((self.along_track, 3))  # space threshold is 100
         return prt, ict, space
@@ -119,6 +120,15 @@ def get_tsm_pixels(self, channels):
         pass
 
 
+class FakeGACReaderWithWrongPRTs(FakeGACReader):
+
+    along_track = 10
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.scans["scan_line_number"] = np.array([1, 3, 4, 5, 6, 7, 8, 9, 10, 11])
+
+
 class TestGacReader(unittest.TestCase):
     """Test the common GAC Reader."""
 
@@ -253,6 +263,15 @@ def test_get_calibrated_channels(self):
         np.testing.assert_allclose(res[:, 2, 1], 10.23511303)
         assert reader.meta_data["calib_coeffs_version"] == "PATMOS-x, v2023"
 
+    def test_get_calibrated_channels_with_wrong_prts(self):
+        """Test getting calibrated channels."""
+        reader = FakeGACReaderWithWrongPRTs()
+        res = reader.get_calibrated_channels()
+
+        np.testing.assert_allclose(res[:, 1, 0], 8.84714652)
+        np.testing.assert_allclose(res[:, 2, 1], 10.23511303)
+        assert reader.meta_data["calib_coeffs_version"] == "PATMOS-x, v2023"
+
     def test_to_datetime64(self):
         """Test conversion from (year, jday, msec) to datetime64."""
         t0 = GACReader.to_datetime64(year=np.array(1970), jday=np.array(1),
@@ -847,10 +866,6 @@ def test_passing_calibration_coeffs_to_reader_init_is_deprecated():
 
 def test_passing_calibration_to_reader():
     """Test passing calibration info to `get_calibrated_channels`."""
-    #reader = LACPODReader(interpolate_coords=False)
-    #ds = reader.read_as_dataset(pod_file_with_tbm_header)
-    #from pygac.calibration.noaa import calibrate
-
     method = "InvalidMethod"
     with pytest.raises(ValueError, match=method):
         reader = FakeGACReader(calibration_method=method)