R/V Ewing Data System
Figures for the Ewing's data system.

 

Time Of Day (TOD) synchronization:
    The cpu time of the logging computer is used as the primary source of monotonic time to serialize logged events. The resolution of the CPU time is on the order of 16milliSeconds (mS.) Absolute accuracy is attained by insuring that the CPU time is synchonized to GPS time. For a few special events (such as seismic shot time) better resolution (~1mS) and higher accuracy is attained by using a special purpose interface to tag these events with GPS time of day derived from an on-board GPS station clock.  A detailed explaination of Time Of Day is available.
 
 

Seismic System Interface:

Block diagram of the Ewing's seismic system data flow
Figure 1: Block diagram of the Seismic System interfaces on the Ewing.

Timing diagram of the Time Tagger
Figure 2: Timing diagram of the events associated with the Time Tagger.

    Sending an ASCII "F" (for "fire") to the time tagger will initiate a shot cycle. The delays relative to the receipt of the "Fire" command are identifed by the letters in Figure 2.
 
 

Event Description Delay and jitter (milliseconds)
A Tagger echos the received "F"
B Actual Time Break Event (TBE)
C Tagger emits the IRIG-B derived time message for the TBE Event
D Tagger emits the "T" character
 
 
Time Tagger:
    In normal operation, the time tagger and the Syntrack timing system are handled by 'l_seismic'.

 
 

Timebreak
    This pulse originates in the gun firing box and goes to the following locations:

  1. Syntrak Recording System(Starts the record)
  2. Time-Tagger
  3. Syntrak Interface(Converts to "T")
  4. Gun Depth box(Triggers output)
 

 

 By special arrangement, it is possible to enable external trigging of the seismic system by a visitng system.
 Block diagram of seismic system timing signal distribution

Comments to:
 Dale Chayes  or  Jeff Turmelle
 
Revisions:
   Created: August 3, 1999     Jeff and Dale's best guess from the code
   Revised: August 6, 1999     Clarifications from Joe
   Revised: August 9, 1999     More clarification about the source and routing of  Timebreak from Joe.