Continuous Water Mass Sampling Using STED
The importance of an accurately measured sound velocity profile and its contribution to effective submarine operations have been understood since the days preceding World War II. During that conflict, U.S. Navy submarine crews utilized a mechanical bathythermograph to record water temperatures as the submarine performed a depth excursion. The temperature profile was etched onto a gold-coated glass slide and then read by a microscope within an optical reader.
The advent of the submarine-launched expendable bathythermograph (SSXBT) in the 1960's revolutionized the collection of temperature/depth data. Submarines released the SSXBT probe through a three-inch pressurized launcher, and the probe, housed inside a lifting body, floated to the surface. It then fell free of the lifting body and as it descended through the sea sent temperature information to a recorder on the submarine over a thin, insulated copper wire. The recorder paper moved at a speed calibrated to the drop rate of the probe and provided a temperature profile extending from the surface to depths as deep as 2,500 feet. The SSXBTs in use today are largely unchanged from earlier models, but the shipboard recording systems use modern digital electronics. Also available to the submariner are probes that measure conductivity, temperature, and depth (CTD) for use under the ice (UISSXBT, UISSXCDT). A recent addition to the measurement arsenal is the expendable sound velocity probe (SSXSV).
As useful as these instruments are, they all fall short in one respect: They provide only a single profile for each probe expended. For blue-water, Cold War scenarios, this was acceptable, because except in areas crossed by oceanographic fronts, acoustic conditions do not change rapidly in the open ocean. Thus, a single temperature profile could provide adequate sound velocity information for search planning and weapon presets until the next probe was launched, hours or even days later. As we move into the littorals, however, the characterization of acoustic conditions based on data taken by a single measurement - so called range-independent analysis - can be a limiting factor, because acoustic conditions near coastlines are seldom constant. Additionally, probes that measure only temperature do not take into account the effect that rapidly varying salinity in the near-shore environment can have on sound propagation.
Clearly, there is a need to sample the water mass surrounding the moving submarine continuously. This continuous sampling is necessary to depict accurately the highly dynamic nature of the sound velocity structure found in the littoral regions of the world where we find ourselves operating with increasing frequency. This need stimulated the development of the Submarine Tactical Environmental Display System (STED).
STED was originally a Naval Science Assistance Program (NSAP) project, and early versions of STED were installed on USS Topeka (SSN-754), USS Asheville (SSN-758) and USS Charlotte (SSN-766). The current version of STED, developed in a cooperative effort by COMSUB-DEVRON TWELVE, in Groton, Connecticut, and the Naval Undersea Warfare Center, Division Newport, is operational on USS Alexandria (SSN-757) and was recently re-installed on Charlotte.
STED, in its present configuration, consists of three main parts: the sensors, the processor, and the displays. The two sensors are Sea-Bird Electronics Micro CAT SBE 37-SI scientific-quality CTD sensors located within the sail, aft of the bridge. Seawater is directed into each sensor through reinforced plastic tubing leading from two stainless steel scoops. The scoops are mounted outboard in the port and starboard stanchion holes high up in the sail. The STED processor, installed in the Sonar Room, receives signals from the sensors, along with latitude, longitude, depth, course, and speed from own-ship systems. The processor passes the information to the two display and archiving computers. One of these is located in the Control Room and the other in the Sonar Room. The only non-COTS components contained in STED are the water scoops and the processor itself.
The display and archiving computers are Pentium II-based, running Windows NT. They contain touch sensitive, 12-inch flat panel displays with options that include three and six parameter strip chart screens and a depth profile display. The strip chart parameters are user-selectable and can include sound speed, temperature, salinity, keel depth, sensor depth, own ship's speed, and seawater density (to aid in ballasting). The three parameter and depth profile screens also contain digital readouts of sound speed, temperature, salinity, and density, which are updated every five seconds. Data from the CTD sensors, as well as own-ship systems are archived internally at one-second intervals by each display computer independently. Each computer's hard disk can hold up to 180 days' worth of data. The data can also be exported to 120 MB LS120 "Superdisks," each of which hold approximately 10 days' worth of STED data. This data will be provided to the Naval Oceanographic Office (NAVOCEANO) for inclusion in their high-resolution oceanographic databases. The Naval Research Laboratory (NRL) has already used STED data to measure the fidelity of its ocean modeling effort.
Additional display features include an alarm function to alert operators when user-defined thresholds in any of the display parameters have been crossed. This function can act as a "bellringer" for encountering oceanographic fronts or other tactically significant features. Depth-dependent historical values can be displayed to determine if present conditions have changed to the extent that a system sound velocity profile update is necessary. The strip chart time axes can be set from one to 48 hours.
The STED processor also contains a data port to which a laptop computer can be attached to provide a remote STED display and to act as a back-up in the event of a display computer failure.
Although STED is presently a stand-alone system, plans are underway to integrate its output directly into the combat control system and the Submarine Fleet Mission Program Library (SFMPL) tactical decision aid. STED currently has the ability to output the data taken during a depth excursion to a floppy disk for "sneaker netting" into the SFMPL. In this respect, STED can be used in place of an SSXBT, until now the only means available for submariners to measure vertical temperature profiles in-situ accurately. At a cost of over $600 per BT, using STED to generate a profile can result in considerable cost savings. NRL is in the initial stages of developing a capability that will allow the direct input of STED data into a data assimilation model run onboard the submarine. The model, called the Modular Oceanographic Data Assimilation System (MODAS), will output a high-resolution, three-dimensional estimate of the sound velocity structure of the local water mass. This 3D picture of the acoustic environment surrounding the submarine will be ingested into the SFMPL, which will use the data for all computations requiring sound velocity input, i.e., search planning, ray traces, propagation loss calculations, etc. Submariners presently use MODAS data generated by NAVOCEANO with additional input from a regional Naval Meteorology and Oceanography (METOC) Center. By feeding MODAS directly with STED data, the crew will be able to provide themselves an extremely accurate portrayal of local acoustic conditions, while largely eliminating the need for off-ship communications to acquire that information.
The Program Executive Office, USW, Advanced Systems Technology Office (PEO (USW) ASTO), is presently sponsoring a project aimed at replacing the sail-mounted TR-321 sound velocity measuring head with a new state-of-the-art CTD sensor. This sensor would provide an analog output identical to the TR-321, which the combat control system requires. It would also replace the sail-mounted CTD sensors presently used by STED, and would feed the STED processor and display and archiving computers with digital data directly. A prototype of this new sensor has been built and is undergoing acceptance testing. ASTO plans to install this new sensor on Charlotte and Alexandria for evaluation against the present STED CTD sensors. The ultimate goal is to backfit the new configuration to the entire 688-class submarine fleet. The USS Virginia (SSN-774)-class submarines will contain a CTD sensor, which will give that class STED-type functionality also.
Crew feedback about STED from Alexandria has been extremely positive. Operational considerations permitting, STED-generated sound velocity profiles are the preferred source for input into the SFMPL. By continuously monitoring temperature, salinity, and sound velocity, the crewmembers feel they are able to exploit the environment to much greater tactical advantage.
Mr. Sand is the NAVOCEANO Representative and Staff Oceanographer to Commander, Submarine Development SquadronTWELVE.
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