Use vanishing `d_pprs` for default orbits

lisainstrument/instrument.py

@@ -266,16 +266,13 @@ class Instrument:
             )
             orbitf.close()
         elif orbits is None:
-            logging.info("No orbit file provided, using default set of proper pseudo-ranges and derivatives thereof")
+            logging.info("No orbit file provided, using default set of constant proper pseudo-ranges")
             self.orbit_file = None
             self.pprs = ForEachMOSA({ # Default PPRs based on first samples of Keplerian orbits (v1.0)
                 '12': 8.3324, '23': 8.3028, '31': 8.3324,
                 '13': 8.3315, '32': 8.3044, '21': 8.3315,
             })
-            self.d_pprs = ForEachMOSA({ # Default PPR derivatives based on first samples of Keplerian orbits (v1.0)
-                '12': 3.1977E-9, '23': -5.0241E-9, '31': -3.1977E-9,
-                '13': -3.1977E-9, '32': -5.0252E-9, '21': 3.1977E-9,
-            })
+            self.d_pprs = ForEachMOSA(0)
         else:
             raise TypeError(f"invalid orbits '{orbits}', should be path to orbit file or None")