diff --git a/conf/modules/gps_ublox.xml b/conf/modules/gps_ublox.xml
index cbec6341c9..8efe27fd7e 100644
--- a/conf/modules/gps_ublox.xml
+++ b/conf/modules/gps_ublox.xml
@@ -22,9 +22,10 @@
     <file name="gps_ubx.c" dir="subsystems/gps"/>
     <define name="USE_GPS"/>
     <raw>
+      UBX_GPS_PORT_LOWER=$(shell echo $(GPS_PORT) | tr A-Z a-z)
       ap.CFLAGS += -DGPS_TYPE_H=\"subsystems/gps/gps_ubx.h\"
       ap.CFLAGS += -DUSE_$(GPS_PORT) -D$(GPS_PORT)_BAUD=$(GPS_BAUD)
-      ap.CFLAGS += -DUSE_GPS -DGPS_LINK=$(GPS_PORT)
+      ap.CFLAGS += -DUSE_GPS -DGPS_LINK=$(UBX_GPS_PORT_LOWER)
       GPS_LED ?= none
       ifneq ($(GPS_LED),none)
         ap.CFLAGS += -DGPS_LED=$(GPS_LED)
@@ -44,7 +45,6 @@
     <define name="USE_GPS"/>
     <raw>
       sim.CFLAGS += -DGPS_TYPE_H=\"subsystems/gps/gps_sim.h\"
-      jsbsim.CFLAGS += -DGPS_TYPE_H=\"subsystems/gps/gps_sim.h\"
     </raw>
   </makefile>
 </module>