Refuse to use fplan file if no orbit files used

lisainstrument/instrument.py

@@ -549,6 +549,9 @@ class Instrument:
         elif isinstance(fplan, str):
             logger.info("Using frequency-plan file '%s'", fplan)
             self.fplan_file = fplan
+            # Refuse to use a fplan file if no orbit files are used
+            if self.orbit_file is None:
+                raise ValueError("cannot use frequency-plan for non orbit files")
             # Without a standard lock config, there is no dataset
             # in the frequency-plan file and therefore we cannot use it
             if self.lock_config is None: