<para>
	Video position (relative to beginning of video) for which the corresponding
	GPS timestamp is known.
</para>
<para>
	Format is hhmmss. If omitted, 0:00:00 (beginning of video) is assumed.
</para>
<para>
	The GPS timestamp can be set with the <option>gps_time</option> and 
	<option>gps_date</option> options.
</para>
