MySQL sort data before using GROUP_CONCAT to concat it together?
You need to get confidence in GROUP_CONCAT with ORDER BY embedded.
Only way to do that is the following
SUGGESTION 01 : Proper Indexing
ALTER TABLE tracks ADD INDEX objectId_posTime_ndx (objectId,posTime);

This will help the query optimizer retrieve your data ordered properly.
SUGGESTION 02 : Display posTime as a Debug Step
SELECT   objectId, 
         group_concat(posTime ORDER BY posTime) as trail 
FROM     tracks 
WHERE    objectId &gt; 0 
GROUP BY objectId;

If the posTime is ordered as desired, then you should be fine with
SELECT   objectId, 
         group_concat(CONCAT(X, ',', Y, ',', hdg, ',', spd) ORDER BY posTime) as trail 
FROM     tracks 
WHERE    objectId &gt; 0 
group by objectId;