iono free combination

include/gnss_utils/gnss_utils.h

@@ -38,8 +38,10 @@ typedef std::shared_ptr<const Snapshot>     SnapshotConstPtr;
 
 enum Combination
 {
-    CODE_L1,    // only L1 code
-    CARRIER_L1  // only L1 carrier phase
+    CODE_L1,            ///< only L1 code
+    CARRIER_L1,         ///< only L1 carrier phase
+    CODE_IONO_FREE,     ///< iono-free combination for code
+    CARRIER_IONO_FREE   ///< iono-free combination for carrier phase
 };
 
 }