Fix antpos units when reading/writing miriad files

pyuvdata/miriad.py

@@ -308,7 +308,8 @@ def read_miriad(self, filepath, correct_lat_lon=True, run_check=True,
 
         nants = uv['nants']
         try:
-            antpos = uv['antpos'].reshape(3, nants).T
+            # Miriad stores antpos values in units of ns, pyuvdata uses meters.
+            antpos = uv['antpos'].reshape(3, nants).T * const.c.to('m/ns').value
             # Miriad stores antpos values in a rotated ECEF coordinate system
             # where the x-axis goes through the local meridan. Need to convert
             # these positions back to standard ECEF and subtract off the
@@ -696,7 +697,8 @@ def write_miriad(self, filepath, run_check=True, check_extra=True,
             antpos[np.where(antpos_length == 0), :] = [0, 0, 0]
 
             uv.add_var('antpos', 'd')
-            uv['antpos'] = antpos.T.flatten()
+            # Miriad stores antpos values in units of ns, pyuvdata uses meters.
+            uv['antpos'] = antpos.T.flatten() / const.c.to('m/ns').value
 
         uv.add_var('sfreq', 'd')
         uv['sfreq'] = self.freq_array[0, 0] / 1e9  # first spw; in GHz

pyuvdata/tests/test_miriad.py

@@ -8,6 +8,8 @@
 import pyuvdata.utils as uvutils
 import pyuvdata.tests as uvtest
 from pyuvdata.data import DATA_PATH
+import aipy.miriad as amiriad
+from astropy import constants as const
 
 
 def test_ReadNRAOWriteMiriadReadMiriad():
@@ -431,3 +433,20 @@ def test_multi_files():
                                            ' pyuvdata.', uv1.history))
     uv1.history = uv_full.history
     nt.assert_equal(uv1, uv_full)
+
+
+def test_antpos_units():
+    """
+    Read uvfits, write miriad. Check written antpos are in ns.
+    """
+    uv = UVData()
+    uvfits_file = os.path.join(DATA_PATH, 'day2_TDEM0003_10s_norx_1src_1spw.uvfits')
+    testfile = os.path.join(DATA_PATH, 'test/uv_antpos_units')
+    uvtest.checkWarnings(uv.read_uvfits, [uvfits_file], message='Telescope EVLA is not')
+    uv.write_miriad(testfile, clobber=True)
+    auv = amiriad.UV(testfile)
+    aantpos = auv['antpos'].reshape(3, -1).T * const.c.to('m/ns').value
+    aantpos = aantpos[uv.antenna_numbers, :]
+    aantpos = (uvutils.ECEF_from_rotECEF(aantpos, uv.telescope_location_lat_lon_alt[1]) -
+               uv.telescope_location)
+    nt.assert_true(np.allclose(aantpos, uv.antenna_positions))

pyuvdata/uvdata.py

@@ -259,7 +259,8 @@ def __init__(self):
                                                spoof_val={}, expected_type=dict)
 
         desc = ('Array giving coordinates of antennas relative to '
-                'telescope_location (ITRF frame), shape (Nants_telescope, 3)')
+                'telescope_location (ITRF frame), shape (Nants_telescope, 3), '
+                'units meters')
         self._antenna_positions = uvp.AntPositionParameter('antenna_positions',
                                                            required=False,
                                                            description=desc,