P>
 
   As in the case of the wall clock, the length of time associated with a
   tick varies as the clock rate varies.
<P>
 
   Since not all spacecraft clocks are the same, the particular time value
   for one tick varies from spacecraft to spacecraft. For Mars Global
   Surveyor, for instance, one tick is equivalent to approximately four
   milliseconds. For Galileo, it's about 8 1/3 milliseconds.
<P>
 
   In addition to representing spacecraft clock readings, ticks can be used
   to represent arbitrary 