diff --git a/libraries/AP_GPS/AP_GPS_GSOF.cpp b/libraries/AP_GPS/AP_GPS_GSOF.cpp
index f615a766b5..938994a775 100644
--- a/libraries/AP_GPS/AP_GPS_GSOF.cpp
+++ b/libraries/AP_GPS/AP_GPS_GSOF.cpp
@@ -184,7 +184,7 @@ AP_GPS_GSOF::requestGSOF(const uint8_t messagetype, const uint8_t portindex)
 }
 
 double
-AP_GPS_GSOF::SwapDouble(uint8_t* src, const uint32_t pos) const
+AP_GPS_GSOF::SwapDouble(const uint8_t* src, const uint32_t pos) const
 {
     union {
         double d;
@@ -203,7 +203,7 @@ AP_GPS_GSOF::SwapDouble(uint8_t* src, const uint32_t pos) const
 }
 
 float
-AP_GPS_GSOF::SwapFloat(uint8_t* src, const uint32_t pos) const
+AP_GPS_GSOF::SwapFloat(const uint8_t* src, const uint32_t pos) const
 {
     union {
         float f;
@@ -218,7 +218,7 @@ AP_GPS_GSOF::SwapFloat(uint8_t* src, const uint32_t pos) const
 }
 
 uint32_t
-AP_GPS_GSOF::SwapUint32(uint8_t* src, const uint32_t pos) const
+AP_GPS_GSOF::SwapUint32(const uint8_t* src, const uint32_t pos) const
 {
     union {
         uint32_t u;
@@ -233,7 +233,7 @@ AP_GPS_GSOF::SwapUint32(uint8_t* src, const uint32_t pos) const
 }
 
 uint16_t
-AP_GPS_GSOF::SwapUint16(uint8_t* src, const uint32_t pos) const
+AP_GPS_GSOF::SwapUint16(const uint8_t* src, const uint32_t pos) const
 {
     union {
         uint16_t u;
diff --git a/libraries/AP_GPS/AP_GPS_GSOF.h b/libraries/AP_GPS/AP_GPS_GSOF.h
index ec262d35be..717e3a946a 100644
--- a/libraries/AP_GPS/AP_GPS_GSOF.h
+++ b/libraries/AP_GPS/AP_GPS_GSOF.h
@@ -43,10 +43,10 @@ private:
     bool process_message() WARN_IF_UNUSED;
     void requestBaud(const uint8_t portindex);
     void requestGSOF(const uint8_t messagetype, const uint8_t portindex);
-    double SwapDouble(uint8_t* src, const uint32_t pos) const WARN_IF_UNUSED;
-    float SwapFloat(uint8_t* src, const uint32_t pos) const WARN_IF_UNUSED;
-    uint32_t SwapUint32(uint8_t* src, const uint32_t pos) const WARN_IF_UNUSED;
-    uint16_t SwapUint16(uint8_t* src, const uint32_t pos) const WARN_IF_UNUSED;
+    double SwapDouble(const uint8_t* src, const uint32_t pos) const WARN_IF_UNUSED;
+    float SwapFloat(const uint8_t* src, const uint32_t pos) const WARN_IF_UNUSED;
+    uint32_t SwapUint32(const uint8_t* src, const uint32_t pos) const WARN_IF_UNUSED;
+    uint16_t SwapUint16(const uint8_t* src, const uint32_t pos) const WARN_IF_UNUSED;
 
 
     struct gsof_msg_parser_t