diff --git a/pygmtsar/pygmtsar/Stack_align.py b/pygmtsar/pygmtsar/Stack_align.py
index 24fe662d..336c78ec 100644
--- a/pygmtsar/pygmtsar/Stack_align.py
+++ b/pygmtsar/pygmtsar/Stack_align.py
@@ -279,7 +279,13 @@ def subswaths_offsets(self, debug=False):
             subswaths = ''.join(map(str, subswaths))
 
         if len(subswaths) == 1:
-            return
+            prm = self.PRM()
+            maxx, yvalid, num_patch = prm.get('num_rng_bins', 'num_valid_az', 'num_patches')
+            maxy = yvalid * num_patch
+            offsets = {'bottom': 0, 'extent': [maxy, maxx]}
+            if debug:
+                print ('offsets', offsets)
+            return offsets
 
         # calculate the offsets to merge subswaths
         prms = []
diff --git a/pygmtsar/pygmtsar/Stack_topo.py b/pygmtsar/pygmtsar/Stack_topo.py
index 9dd318aa..933299eb 100644
--- a/pygmtsar/pygmtsar/Stack_topo.py
+++ b/pygmtsar/pygmtsar/Stack_topo.py
@@ -194,17 +194,8 @@ def block_phase_dask(block_topo, y_chunk, x_chunk, prm1, prm2):
         # immediately prepare PRM
         # here is some delay on the function call but the actual processing is faster
         offsets = self.subswaths_offsets(debug=debug)
-        # reference scene first subswath
-        prm0 = self.PRM()
-        if offsets is not None:
-            # multiple subswaths
-            maxy, maxx = offsets['extent']
-            minh = offsets['bottom']
-        else:
-            # one subswath
-            maxy, maxx = prm0.get('num_valid_az', 'num_rng_bins')
-            minh = 0
-        del prm0, offsets
+        maxy, maxx = offsets['extent']
+        minh = offsets['bottom']
         
         def prepare_prms(pair, *args):
             date1, date2 = pair
diff --git a/pygmtsar/pygmtsar/Stack_trans.py b/pygmtsar/pygmtsar/Stack_trans.py
index e7518151..011cc082 100644
--- a/pygmtsar/pygmtsar/Stack_trans.py
+++ b/pygmtsar/pygmtsar/Stack_trans.py
@@ -17,9 +17,7 @@ def define_trans_grid(self, coarsen):
         # select radar coordinates extent
         #rng_max, yvalid, num_patch = self.PRM().get('num_rng_bins', 'num_valid_az', 'num_patches')
         #azi_max = yvalid * num_patch
-        data = self.open_data(dates=[self.reference])
-        rng_max = float(data.x[-1])
-        azi_max = float(data.y[-1])
+        azi_max, rng_max = self.subswaths_offsets()['extent']
         #print ('azi_max', azi_max, 'rng_max', rng_max)
         # this grid covers the full interferogram area
         # common single pixel resolution