diff --git a/lisainstrument/instrument.py b/lisainstrument/instrument.py
index 76e5c23d88c815fd00fd266d802be633d83dcea9..e8f0db31ca585c799badfb96e35c2a827a8f7f42 100644
--- a/lisainstrument/instrument.py
+++ b/lisainstrument/instrument.py
@@ -543,9 +543,13 @@ class Instrument:
             return InterpolatedUnivariateSpline(times, values, k=5, ext='raise')
 
         def tps_proper_time_deviations(sc):
-            times = orbitf['tcb']['t'][:]
-            values = orbitf[f'tcb/sc_{sc}']['tau']
-            return InterpolatedUnivariateSpline(times, values, k=5, ext='raise')
+            if self.orbit_dataset == 'tcb/ltt':
+                return lambda x: 0
+            if self.orbit_dataset == 'tps/ppr':
+                times = orbitf['tcb']['t'][:]
+                values = orbitf[f'tcb/sc_{sc}']['tau']
+                return InterpolatedUnivariateSpline(times, values, k=5, ext='raise')
+            raise ValueError(f"invalid orbit dataset '{self.orbit_dataset}'")
 
         try:
             logger.debug("Interpolating proper pseudo-ranges")
@@ -553,12 +557,7 @@ class Instrument:
             logger.debug("Interpolating proper pseudo-range derivatives")
             self.d_pprs = ForEachMOSA(lambda mosa: d_pprs(mosa)(self.physics_t))
             logger.debug("Interpolating proper time deviation from TCB")
-            if self.orbit_dataset =='tcb/ltt':
-                self.tps_proper_time_deviations = ForEachSC(0)
-            elif self.orbit_dataset == 'tps/ppr':
-                self.tps_proper_time_deviations = ForEachSC(lambda sc: tps_proper_time_deviations(sc)(self.telemetry_t))
-            else:
-                raise ValueError(f"invalid orbit dataset '{self.orbit_dataset}'")
+            self.tps_proper_time_deviations = ForEachSC(lambda sc: tps_proper_time_deviations(sc)(self.telemetry_t))
         except ValueError as error:
             logger.error("Missing orbit information at \n%s", self.physics_t)
             raise ValueError("missing orbit information, use longer orbit file or adjust sampling") from error