diff --git a/src/s1reader/s1_orbit.py b/src/s1reader/s1_orbit.py
index 2643e30..7bb0d1b 100644
--- a/src/s1reader/s1_orbit.py
+++ b/src/s1reader/s1_orbit.py
@@ -1,5 +1,8 @@
 import datetime
 import os
+import xml.etree.ElementTree as ET
+
+import isce3
 
 # date format used in file names
 FMT = "%Y%m%dT%H%M%S"
@@ -121,3 +124,42 @@ def get_orbit_file_from_dir(path: str, orbit_dir: str) -> str:
         path, [f'{orbit_dir}/{item}' for item in os.listdir(orbit_dir)])
 
     return orbit_path
+
+def burst_orbit_from_file(sensing_start, sensing_stop, osv_list: ET.Element):
+    '''Init and return ISCE3 orbit from element in orbit XML file.
+
+    Parameters:
+    -----------
+    sensing_start : datetime.datetime
+        Sensing start of burst; taken from azimuth time
+    sensing_stop : datetime.datetime
+        Sensing stop of burst
+    osv_list : xml.etree.ElementTree.Element
+        ElementTree containing orbit state vectors
+
+    Returns:
+    --------
+    _ : datetime
+        Sensing mid as datetime object.
+    '''
+    fmt = "UTC=%Y-%m-%dT%H:%M:%S.%f"
+    orbit_sv = []
+    # add start & end padding to ensure sufficient number of orbit points
+    pad = datetime.timedelta(seconds=60)
+    for osv in osv_list:
+        t_orbit = datetime.datetime.strptime(osv[1].text, fmt)
+
+        if t_orbit > sensing_stop + pad:
+            break
+
+        if t_orbit > sensing_start - pad:
+            pos = [float(osv[i].text) for i in range(4,7)]
+            vel = [float(osv[i].text) for i in range(7,10)]
+            orbit_sv.append(isce3.core.StateVector(isce3.core.DateTime(t_orbit),
+                                                   pos, vel))
+
+    # use list of stateVectors to init and return isce3.core.Orbit
+    time_delta = datetime.timedelta(days=2)
+    ref_epoch = isce3.core.DateTime(sensing_start - time_delta)
+
+    return isce3.core.Orbit(orbit_sv, ref_epoch)
diff --git a/src/s1reader/s1_reader.py b/src/s1reader/s1_reader.py
index f469ac4..792c007 100644
--- a/src/s1reader/s1_reader.py
+++ b/src/s1reader/s1_reader.py
@@ -10,9 +10,10 @@
 import isce3
 from nisar.workflows.stage_dem import check_dateline
 from s1reader.s1_burst_slc import Doppler, Sentinel1BurstSlc
+from s1reader.s1_orbit import burst_orbit_from_file
 
 # TODO evaluate if it make sense to combine below into a class
-def as_datetime(t_str, fmt = "%Y-%m-%dT%H:%M:%S.%f"):
+def as_datetime(t_str, fmt="%Y-%m-%dT%H:%M:%S.%f"):
     '''Parse given time string to datetime.datetime object.
 
     Parameters:
@@ -126,7 +127,7 @@ def doppler_poly1d_to_lut2d(doppler_poly1d, starting_slant_range,
     return isce3.core.LUT2d(slant_ranges, az_times,
                             np.vstack((freq_1d, freq_1d)))
 
-def get_burst_orbit(sensing_start, sensing_stop, osv_list: ET.Element):
+def parse_annotation_orbit(sensing_start, sensing_stop, osv_list: ET.Element):
     '''Init and return ISCE3 orbit.
 
     Parameters:
@@ -143,21 +144,18 @@ def get_burst_orbit(sensing_start, sensing_stop, osv_list: ET.Element):
     _ : datetime
         Sensing mid as datetime object.
     '''
-    fmt = "UTC=%Y-%m-%dT%H:%M:%S.%f"
-    orbit_sv = []
+    fmt = "%Y-%m-%dT%H:%M:%S.%f"
+    n_svs = int(osv_list.attrib['count'])
+    orbit_sv = [[]] * n_svs
     # add start & end padding to ensure sufficient number of orbit points
     pad = datetime.timedelta(seconds=60)
-    for osv in osv_list:
-        t_orbit = datetime.datetime.strptime(osv[1].text, fmt)
+    for i, osv in enumerate(osv_list):
+        t_orbit = datetime.datetime.strptime(osv[0].text, fmt)
 
-        if t_orbit > sensing_stop + pad:
-            break
-
-        if t_orbit > sensing_start - pad:
-            pos = [float(osv[i].text) for i in range(4,7)]
-            vel = [float(osv[i].text) for i in range(7,10)]
-            orbit_sv.append(isce3.core.StateVector(isce3.core.DateTime(t_orbit),
-                                                   pos, vel))
+        pos = [float(osv[2][i].text) for i in range(3)]
+        vel = [float(osv[3][i].text) for i in range(3)]
+        orbit_sv[i] = isce3.core.StateVector(isce3.core.DateTime(t_orbit),
+                                             pos, vel)
 
     # use list of stateVectors to init and return isce3.core.Orbit
     time_delta = datetime.timedelta(days=2)
@@ -336,8 +334,13 @@ def burst_from_xml(annotation_path: str, orbit_path: str, tiff_path: str,
         iw2_mid_range = iw2_starting_range + 0.5 * iw2_n_samples * range_pxl_spacing
 
     # find orbit state vectors in 'Data_Block/List_of_OSVs'
-    orbit_tree = ET.parse(orbit_path)
-    osv_list = orbit_tree.find('Data_Block/List_of_OSVs')
+    if orbit_path is None:
+        osv_list = tree.find('generalAnnotation/orbitList')
+        get_orbit = parse_annotation_orbit
+    else:
+        orbit_tree = ET.parse(orbit_path)
+        osv_list = orbit_tree.find('Data_Block/List_of_OSVs')
+        get_orbit = burst_orbit_from_file
 
     # load individual burst
     burst_list_elements = tree.find('swathTiming/burstList')
@@ -369,8 +372,8 @@ def burst_from_xml(annotation_path: str, orbit_path: str, tiff_path: str,
         # get orbit from state vector list/element tree
         sensing_duration = datetime.timedelta(
             seconds=n_samples * azimuth_time_interval)
-        orbit = get_burst_orbit(sensing_start, sensing_start + sensing_duration,
-                                osv_list)
+        orbit = get_orbit(sensing_start, sensing_start + sensing_duration,
+                          osv_list)
 
         # determine burst offset and dimensions
         # TODO move to own method
@@ -425,7 +428,7 @@ def _is_zip_annotation_xml(path: str, id_str: str) -> bool:
         return True
     return False
 
-def load_bursts(path: str, orbit_path: str, swath_num: int, pol: str='vv',
+def load_bursts(path: str, swath_num: int, pol: str='vv', orbit_path: str=None,
                 burst_ids: list[str]=None):
     '''Find bursts in a Sentinel-1 zip file or a SAFE structured directory.
 
@@ -433,12 +436,12 @@ def load_bursts(path: str, orbit_path: str, swath_num: int, pol: str='vv',
     -----------
     path : str
         Path to Sentinel-1 zip file or SAFE directory
-    orbit_path : str
-        Path the orbit file.
     swath_num : int
         Integer of subswath of desired burst. {1, 2, 3}
     pol : str
         Polarization of desired burst. {hh, vv, hv, vh}
+    orbit_path : str
+        Path the orbit file. If None, annotation XML orbit is parsed.
     burst_ids : list[str]
         List of burst IDs for which their Sentinel1BurstSlc objects will be
         returned. Default of None returns all bursts. Empty list returned if
@@ -466,6 +469,11 @@ def load_bursts(path: str, orbit_path: str, swath_num: int, pol: str='vv',
     if pol not in pols:
         raise ValueError(f"polarization not in {pols}")
 
+    if orbit_path is None:
+        warn_str = 'No external orbit given. '
+        warn_str += 'Orbit will be constructed from annotation XML.'
+        warnings.warn(warn_str)
+
     id_str = f'iw{swath_num}-slc-{pol}'
 
     if not os.path.exists(path):
@@ -506,7 +514,7 @@ def _burst_from_zip(zip_path: str, id_str: str, orbit_path: str):
     id_str: str
         Identifcation of desired burst. Format: iw[swath_num]-slc-[pol]
     orbit_path : str
-        Path the orbit file.
+        Path the orbit file. If None, annotation XML orbit is parsed.
 
     Returns:
     --------
@@ -547,7 +555,7 @@ def _burst_from_safe_dir(safe_dir_path: str, id_str: str, orbit_path: str):
     id_str: str
         Identifcation of desired burst. Format: iw[swath_num]-slc-[pol]
     orbit_path : str
-        Path the orbit file.
+        Path the orbit file. If None, annotation XML orbit is parsed.
 
     Returns:
     --------