mm43: In practical terms, I suspect that the 'pings' happened every 30 minutes, i.e. 11 and 41.
The published chart (
http://www.straitstimes.com/sites/st...325/graphe.jpg) is a little misleading in the way that the dots "measured" are joined: OK for the South and North "predicted tracks", but joining the dots of the pings truly received is going too far (IMHO): anything could have happened between them!