Lieutenant Norheim’s paper won 1he Naval Submarine League Essay Contest for his class at the Submarine Officers Advanced Course. He is currently the Weapons Officer in USS TENNESSEE (SSBN 754)(Blue).
Why Do We Need Synthetic XBTs?
Submarines rely heavily on their knowledge of underwater acoustics to conduct daily operations which include navigation, tracking contacts or attack. It is important not only to understand the ocean’s acoustical characteristics at the point of the submarine’s location, but also those characteristics distant from the submarine (for as far away from the submarine as 50 nautical miles or more). Typically, submarines obtain general information of the surrounding ocean environment via fronts and eddy messages or from a historical computer database. However, even with the information from these references, today’s modern submarine operates with a limited knowledge of the acoustic environment surrounding it. This situation can be greatly improved by using information gathered from existing satellites which can, in tum, be used to develop a three-dimensional, large area map of synthetic expendable bathythermographs.
What is a Synthetic XBT?
Satellite remote sensing of the world’s oceans can provide rapid worldwide coverage of parameters which are found on the ocean surface. Unfortunately for submarines, these satellite measurements provide no direct sampling of subsurface parameters. New strategies are now being developed which will enable subsurface information to be extracted from ocean surface parameters. These parameters, then, can be measured by satellites on a daily basis.
The most important ocean parameter in undersea warfare is sound velocity. Vertical profiles of sound velocity are typically deduced from vertical temperature profiles which are measured using expendable bathythermographs (XBTs). XBTs are limited in that they are merely a point measurement, whereas satellites have wide area coverage. It is possible to use readily available satellite information to create a three dimensional map of water temperature (therefore sound velocity) over a large area of ocean. These synthetic XBTs are inferred from the height of the water below the satellite, which is measured with a satellite radar altimeter. The availability of this information has both naval and scientific applications. The surface and submarine naval communities can use this information to enhance tactical antisubmarine warfare. The scientific community can utilize synthetic temperature profiles to obtain year round measurements of the world’s ocean temperature profiles without the use of expensive ships and cruises.
How Can Submarines Use This information?
Presently, submarines are limited to launching individual XBTs, typically one per day, and changing depth occasionally to understand the temperature and sound characteristics in the area around them. If submarines have access to temperature profiles of the entire area surrounding them, existing range dependent models (such as those found on TAC-3, the new onboard computer) could be used more efficiently in the detection and prosecution of both surface and subsurface targets. This would be especially advantageous in areas of high mesoscale variability, such as near fronts and eddies or anywhere there are drastic horizontal changes in the temperature profile. It is possible that in the near future the submarine community will be able to come to periscope depth once or twice a day and passively retrieve this type of information from a communication satellite along with the normal submarine broadcast. Onboard computers such as the TAC-3 could, subsequently, be used to accurately map out the sound velocity profiles for a large area surrounding the submarine. This data would give the submariner a tremendous advantage in the tactical use of the ocean environment.
Exactly How is This Done?
Synthetic temperature profiles are obtained by using an algorithm from deWitt  which was statistically developed from a database of all hydrocasts in the U.S. Navy’s archives. The algorithm relates sea surface height to temperature throughout the water column. Water elevation varies throughout the oceans by as much as several feet. The only input to the algorithm is processed data from radar altimeter satellites such as Geosat, Topex/Poseidon or one of the ERS (Earth Remote Sensing) satellites. The raw altimetry data is processed to remove the earth’s geoid, the mean height of water, tidal effects and all atmospheric effects. The remaining variability in the water height (known as Dynamic Height Anomaly) obtained from the satellite radar altimeter is directly related to the density (and thus temperature) of the water column. The algorithm involves the use of variables which change depending on the time of the year and geographic location. These variables have been derived for both the Gulf Stream and the Kuroshio Current for every month of the year. New variables would be needed to apply this method to other locations.
Is This Technology Ready?
Carnes etal (1990] compared the output of this algorithm to air dropped XBTs in transects across the Gulf Stream with results showing good agreement between actual and synthetic XBTs. There are currently some issues with this method which must be addressed. Sea surface temperature (SST) is an important and readily available parameter which should be integrated into the algorithm, however this is currently overlooked. Because most of the thermal variability in the ocean occurs in the upper 100 meters, the use of available SST information can greatly enhance this algorithm. Without SST data the method will be less accurate near the surface, especially in the case of a thin layer of warm water overlying a large mass of cold water. An additional consideration is that the method is location specific, therefore extensive statistical analysis must be performed before the algorithm can work off any specific area. There are areas in the world, parts of the southern hemisphere for example, where we have little or no historical XBT data, making this method worthless. Lastly, the method has been limited to two dimensional profiles of the ocean’s thermal structure along the satellite’s track. In order to provide a dense three dimensional matrix of XBT data, spatial and temporal EOFs (Empirical Orthogonal Functions) would need to be constructed to build data in the voids between successive satellite tracks. The use of EOFs to accurately create detailed maps from previously sparse data fields is known as objective analysis and is widely used by scientists. Altimetry data from more than one satellite can, additionally, be merged to improve the timeliness and the accuracy of the product.
The eventual utility of this algorithm would be to calculate three dimensional thermal profiles of the ocean. Synthetic XBTs would permit the calculation of sound velocity profiles, in three dimensions, over a large area. This would provide input to range dependent acoustic models in the TAC-3 computer. The utilization of this method could enhance conventional methods for locating and tracking important ocean features such as fronts and eddies. Currently SSTs derived from satellite infrared sensors are used to track fronts and eddies. However, the use of satellite altimetry data will allow us to see what is under the surface of the ocean. Synthetic XBTs will provide reliable tracking of a cold core eddy which is masked by a layer of warm water, a task not previously possible. In short, the submariner would have a more detailed grasp of the environment surrounding his submarine with this information available to him.