diff --git a/handbook/computing/myphone.html b/handbook/computing/myphone.html
index bbf021ac1..378abd262 100644
--- a/handbook/computing/myphone.html
+++ b/handbook/computing/myphone.html
@@ -65,5 +65,11 @@ tbd
     <p>You can buy phone battery backups in Austria, but they are a bit epxensive, and hard to find in Bad Aussee (try the post office phone shop, or the deviously hidden electronics repair shop in central Bad Aussee).
       <h4>How</h4>
 <p>detailed instructions depend on the type of phone  and the version of Android - someone fix this ?!
+  
+  <h3>Calibrate/document your phone's capabilities</h3>
+<div align=right><a href='/handbook/computing/l/gpstest.html'><img width=20% src='/handbook/computing/t/gpstest.jpg' /></a></div>
+Install the "GPS Test" app, this one: <a href="https://play.google.com/store/apps/details?id=com.android.gpstest">GPS TEST</a> and check whether your phone is using SBAS "Satellite-based Augmentation Services", which is another name for EGNOS, the differential GPS system.
+You need to record if SBAS is in operation whenever you record the position of a cave entrance (and ideally whenever you take a photo too, but that would be unrealistic).
+
 <hr>
 Go on to <a href="/handbook/computing/gpxupload.html">Saving your GPS tracks</a></body></html>