diff --git a/app/android/src/uk/co/lutraconsulting/PositionTrackingService.java b/app/android/src/uk/co/lutraconsulting/PositionTrackingService.java
index eb5bfb7b7..a9e8d6ce7 100644
--- a/app/android/src/uk/co/lutraconsulting/PositionTrackingService.java
+++ b/app/android/src/uk/co/lutraconsulting/PositionTrackingService.java
@@ -28,6 +28,7 @@
 import java.io.File;
 import java.io.FileOutputStream;
 import java.io.IOException;
+import java.util.Locale;
 
 import uk.co.lutraconsulting.PositionTrackingBroadcastMiddleware;
 
@@ -230,7 +231,7 @@ public int onStartCommand( Intent intent, int flags, int startId ) {
     public void onLocationChanged( Location location ) {
 
         long currentTimeSeconds = System.currentTimeMillis() / 1000; // UTC time
-        String positionUpdate = String.format("%f %f %f %d\n", location.getLongitude(), location.getLatitude(), location.getAltitude(), currentTimeSeconds);
+        String positionUpdate = String.format(Locale.US, "%f %f %f %d\n", location.getLongitude(), location.getLatitude(), location.getAltitude(), currentTimeSeconds);
 
         try {
             if ( positionUpdatesStream != null ) {