diff --git a/sw/airborne/subsystems/gps.h b/sw/airborne/subsystems/gps.h
index 80b9764699..e0af090f40 100644
--- a/sw/airborne/subsystems/gps.h
+++ b/sw/airborne/subsystems/gps.h
@@ -57,10 +57,6 @@
 #define GpsIsLost() !GpsFixValid()
 #endif
 
-#ifndef GPS_NB_CHANNELS
-#define GPS_NB_CHANNELS 16
-#endif
-
 #define GPS_VALID_POS_ECEF_BIT 0
 #define GPS_VALID_POS_LLA_BIT  1
 #define GPS_VALID_POS_UTM_BIT  2
@@ -69,17 +65,30 @@
 #define GPS_VALID_HMSL_BIT     5
 #define GPS_VALID_COURSE_BIT   6
 
+/* GPS model specific implementation or sim */
+#ifdef GPS_TYPE_H
+#include GPS_TYPE_H
+#ifndef GPS_NB_CHANNELS
+#define GPS_NB_CHANNELS 1
+#endif
+
+#endif
+
 #ifndef GPS_TYPE_H
+
 #define PRIMARY_GPS_INSTANCE 0
 #define SECONDARY_GPS_INSTANCE 1
 
+#ifndef GPS_NB_CHANNELS
+#define GPS_NB_CHANNELS 16
+#endif
+
 #ifdef USE_MULTI_GPS
 #define GPS_NUM_INSTANCES 2
 #else
 #define GPS_NUM_INSTANCES 1
 #endif
 
-
 #define GPS_MODE_PRIMARY 0
 #define GPS_MODE_SECONDARY 1
 #define GPS_MODE_AUTO 2
@@ -148,11 +157,6 @@ extern struct GpsState gps;
 typedef void (*ImplGpsInit)(void);
 typedef void (*ImplGpsEvent)(void);
 
-
-/* GPS model specific implementation or sim */
-#ifdef GPS_TYPE_H
-#include GPS_TYPE_H
-#endif
 #ifdef PRIMARY_GPS_TYPE_H
   void GpsEvent(void);
   /*
diff --git a/sw/airborne/subsystems/gps/gps_piksi.h b/sw/airborne/subsystems/gps/gps_piksi.h
index 89e4a9d8c7..9cba087043 100644
--- a/sw/airborne/subsystems/gps/gps_piksi.h
+++ b/sw/airborne/subsystems/gps/gps_piksi.h
@@ -37,14 +37,21 @@
 #define PIKSI_HEARTBEAT_MSG
 
 #if GPS_SECONDARY_PIKSI
+#ifndef PIKSI_GPS_LINK
 #define PIKSI_GPS_LINK GPS_SECONDARY_PORT
 #define SecondaryGpsImpl piksi
+#endif
 #else
+#ifndef PrimaryGpsImpl
 #define PrimaryGpsImpl piksi
 #endif
+#endif
 #if GPS_PRIMARY_PIKSI
+#ifndef PIKSI_GPS_LINK
 #define PIKSI_GPS_LINK GPS_PRIMARY_PORT
 #endif
+#endif
+
 
 extern void piksi_gps_event(void);
 extern void piksi_gps_impl_init(void);
diff --git a/sw/airborne/subsystems/gps/gps_ubx.h b/sw/airborne/subsystems/gps/gps_ubx.h
index a5a32eefee..6fdb6a81d7 100644
--- a/sw/airborne/subsystems/gps/gps_ubx.h
+++ b/sw/airborne/subsystems/gps/gps_ubx.h
@@ -28,14 +28,20 @@
 #define GPS_UBX_H
 
 #if GPS_SECONDARY_UBX
+#ifndef UBX_GPS_LINK
 #define UBX_GPS_LINK GPS_SECONDARY_PORT
 #define SecondaryGpsImpl ubx
+#endif
 #else
+#ifndef PrimaryGpsImpl
 #define PrimaryGpsImpl ubx
 #endif
+#endif
 #if GPS_PRIMARY_UBX
+#ifndef UBX_GPS_LINK
 #define UBX_GPS_LINK GPS_PRIMARY_PORT
 #endif
+#endif
 
 #ifdef GPS_CONFIGURE
 #warning "Please use gps_ubx_ucenter.xml module instead of GPS_CONFIGURE"