diff --git a/libraries/AP_HAL_PX4/Util.cpp b/libraries/AP_HAL_PX4/Util.cpp
index 1155d191ea..29881f1480 100644
--- a/libraries/AP_HAL_PX4/Util.cpp
+++ b/libraries/AP_HAL_PX4/Util.cpp
@@ -12,9 +12,27 @@
 
 extern const AP_HAL::HAL& hal;
 
-static int libc_vsnprintf(char* str, size_t size, const char *format, va_list ap) 
+/*
+  implement vsnprintf with support for %S meaning a progmem string
+ */
+static int libc_vsnprintf(char* str, size_t size, const char *fmt, va_list ap) 
 {
-    return vsnprintf(str, size, format, ap);
+    int i, ret;
+    char *fmt2 = (char *)fmt;
+    if (strstr(fmt2, "%S") != NULL) {
+        fmt2 = strdup(fmt);
+        for (i=0; fmt2[i]; i++) {
+            // cope with %S
+            if (fmt2[i] == '%' && fmt2[i+1] == 'S') {
+                fmt2[i+1] = 's';
+            }
+        }
+    }
+    ret = vsnprintf(str, size, fmt2, ap);
+    if (fmt2 != fmt) {
+        free(fmt2);
+    }
+    return ret;
 }
 
 #include "Util.h"