Index: trunk/BNC/src/PPP/pppParlist.cpp
===================================================================
--- trunk/BNC/src/PPP/pppParlist.cpp	(revision 9497)
+++ trunk/BNC/src/PPP/pppParlist.cpp	(revision 9498)
@@ -477,4 +477,9 @@
   }
 
+  if (_usedSystems.size() > 1 &&
+      !_usedSystems.contains('G')) { // required to setup ISB w. r. t. GPS
+    return failure;
+  }
+
   // Required Set of Parameters
   // --------------------------
@@ -579,13 +584,19 @@
   // GPS-GLONASS Clock Offset
   // ------------------------
-  required.push_back(new t_pppParam(t_pppParam::offGR, t_prn(), t_lc::dummy));
+  if (_usedSystems.contains('G') && _usedSystems.contains('R')) {
+    required.push_back(new t_pppParam(t_pppParam::offGR, t_prn(), t_lc::dummy));
+  }
 
   //GPS-Galileo Clock Offset
   // ------------------------
-  required.push_back(new t_pppParam(t_pppParam::offGE, t_prn(), t_lc::dummy));
+  if (_usedSystems.contains('G') && _usedSystems.contains('E')) {
+    required.push_back(new t_pppParam(t_pppParam::offGE, t_prn(), t_lc::dummy));
+  }
 
   // GPS-BDS Clock Offset
   // ------------------------
-  required.push_back(new t_pppParam(t_pppParam::offGC, t_prn(), t_lc::dummy));
+  if (_usedSystems.contains('G') && _usedSystems.contains('C')) {
+    required.push_back(new t_pppParam(t_pppParam::offGC, t_prn(), t_lc::dummy));
+  }
 
   // Troposphere