The  measurement  and  recording  of  subsurface water   temperature   at   various   depths   is   called   a bathythermograph    observation.    Bathythermograph observations  are  normally  conducted  only  in  ocean depths  of  100  fathoms  (600  feet)  or  greater.  The abbreviation   "BT"   is   often   used   for   the   term bathythermograph. Although most bathythermograph observations are conducted by Sonar Technicians and Aviation Warfare Systems Operators, Aerographer’s Mates may conduct these  observations  while  deployed  aboard  ship  with mobile  environmental  teams.  A  far  larger  number  of Aerographers routinely receive and use the transmitted observation  reports  to  produce  a  variety  of  acoustic analyses   and   forecasts   for   USW   support   and   other mission   briefings. The   input   of   accurate   realtime bathythermograph   data   is   the   critical   factor   in determining   the   sound   velocity   profile   (SVP)   of   a particular  ocean  area. It  is  from  the  SVP  that  the presence  or  absence  of  various  acoustic  propagation paths can be determined and thus exploited. I n    a d d i t i o n    t o    d i r e c t    w a r f a r e    s u p p o r t , bathythermograph  observations  are  also  used  to  analyze the  location  and  structure  of  ocean  fronts  and  eddies. These observations are an important input to numerical oceanographic  models  that  analyze  and  predict  ocean currents,  surface  temperatures,  and  other  features. Bathythermograph  observations  are  also  archived  in climatological data bases used by acoustic predictions systems;   they   are   also   used   by   Research   and Development   (R&D)   activities   to   develop   new oceanographic and acoustic models. To  be  consistent  with  other  environmental observations,  the  World  Meteorological  Organization has  set  standard  bathythetmograph  observation  times  as the synoptic hours—0000, 0600, 1200, and 1800 UTC. Operators should attempt to make all BT observations as  close  to  a  synoptic  hour  as  possible.  USW  ships normally drop shipboard expendable bathythermograph  (SXBT)  probes  every  6  hours,  but may  reduce  observations  to  once  per  day  when operating within the same area for more than 24 hours. At least one BT observation should be taken when a ship enters an area with a differingthermal structure, such as in  the  vicinity  of  ocean  fronts,  eddies,  major  river outflow areas, and differing water masses. REVIEW  QUESTIONS Q4.  What  is  the  primary  purpose  of  conducting bathythermograph  observations? 2-2 Q5.   What   are   some   other   important   uses   of bathythermograph data? Q6.   When   should   routine   BT   observations   be conducted? BATHYTHERMOGRAPH EQUIPMENT LEARNING   OBJECTIVES:   Discuss   the background  and  history  of  bathythermograph observations. Describe the basic operation and m a i n t e n a n c e    o f    t h e    A N / S S Q - 6 1 ( A ) bathythermograph  set  and  the  AN/BQH-7(A) oceanographic data system. Many   different   types   of   seawater   temperature- measuring equipment are in routine use throughout the Navy. S h i p s    a n d submarines conduct bathythermograph   observations   using   both   installed sensors   and   expendable   bathythermograph   probes. Aircraft   deploy   a   sonobuoy   called   an   airborne expendable   bathythermograph   (AXBT).   An   AXBT measures  ocean  temperature  during  the  probes  transit  to the bottom and relays the information to the aircraft via radio signals. Aircraft use different types of recorders that will plot temperature/depth profiles. There are also many  types  of  moored  and  drifting  meteorological/ oceanographic  buoys  that  are  equipped  with  a  sensor cable (“tail”) that can measure the ocean temperature at fixed   depths.   The   following   text   discusses   only shipboard bathythermograph systems. BACKGROUND  AND  HISTORY Through   the   late   1950’s,   Aerographer’s   Mates conducted bathythermograph observations by lowering and   recovering   a   cable-tethered   bathythermograph probe over the side of the ship. A carbon-covered glass slide carried by the probe was removed and evaluated for  each  observation.  In  the  1960’s,  the  electronic bathythermograph recorder AN/SSQ-56 system, using expendable bathythermograph probes, was introduced. Several system modifications were made over the years, the newest of which is designated the AN/SSQ-61(A). Beginning in late 1986, new construction surface ships began   to   receive   the   “next   generation”   of bathythermograph   equipment,   the   AN/BQH-7(A) oceanographic  data  system.  The  AN/BQH-7A,  also called a "bathythermograph/sound  velocimeter," uses the same probes used by the AN/SSQ-(series) sets, but it