After a successful maiden deployment, the USS Decatur (DDG-73) returned to home port in San Diego, California. Along with refitting that follows each deployment came a number of equipment changes, including one that would have a subtle yet important effect on the ship's underway operations: installation in the combat information center (CIC) of the Tactical Decision Support Subsystem (TDSS), which would come to replace the Dead Reckoning Tracer (DRT) as a tool for contact maintenance, tactical awareness, and passive submarine tracking.
Intended originally as only a support interface between sonar control and the CIC, the subsystem has a number of "bells and whistles" that assist numerous CIC functions. From calculating wind envelopes for landing helicopters to calculating bearing and range to a drifting man overboard, there seemed to be nothing that TDSS could not do. In its fervor to promote command, control, communications, computers, and intelligence (C4I), the Navy replaced the DRT with the TDSS.
Dead reckoning is the method of navigation and tracking based on known movements from a known position. Just as you can close your eyes and count steps from your doorstep to the car, a vessel can maintain a highly accurate fix of own-ship position based on a record of where it was at a known time—and how far and in what direction it has gone since then. The DRT keeps a continuous log of courses and speeds from the master gyrocompass and the ship's pitometer log. From these, the ship's position in latitude and longitude may be computed and displayed as a position on a chart or in relation to other points and ships.
The TDSS on the Decatur can accomplish many of these tasks. The DRT's primary function of plotting own-ship position in relation to surface and subsurface contacts is largely automatic on the TDSS. Inputs from the ship's command-and-decision (C&D) system filtered through the antisubmarine warfare control system update the surface and undersea warfare tactical pictures continually. The world map and contour interval features give a constant view of own-ship position in relation to land and hydrographic topography in much the same way as does the DRT plotting on a chart. The TDSS accomplishes many of the same functions, but was it ever intended to replace the DRT?
Foremost of the DRT's many functions is its written record of ships' movements. Maintained as legal documents for legibility and operator accountability, these tracings may be used during wartime to reconstruct and debrief an action; during peacetime, their legal function is to determine fault and correct mistakes in the case of groundings, collisions, and other mishaps at sea. It is imperative that such a record always be maintained. The TDSS and C&D system can make continuous data tapes of all information displayed—but it should be noted that the tapes are not easy to use for debriefings and evaluations. Further, they are questionable as legal documents because, unlike a written log, the TDSS has no operater signature.
One advantage of the DRT is its stable and self-contained nature. Without input from the surface tracker or operator, the DRT operator can continue to advance own-ship and plotted contacts along the last-known course and speed. Thus, the CIC watch officer maintains some degree of awareness of the surface and subsurface pictures, even in the midst of radar and sensor failure or certain emission-control conditions. If the ship runs blind, safety of navigation can be maintained based on the last good look.
The DRT is a manual system that allows the plotter to correlate and update information from any number of sources in various ways. The operator is not confined to an automatic input and may demonstrate the effects on the tactical and navigational situations of assumed or hypothetical course changes, lookout reports, and track correlations or decorrelations. Each of these functions requires only a new line drawn with the Parallel Motion Protractor (PMP), a note on the trace paper, or a measurement from the PMP arm or compass rose. Digital DRT (DDRT)—which draws graphic logs of ship movements—and the Naval Tactical Data System (NTDS) have similarly quick functions for visualizing ranges and bearings.
The TDSS requires multiple electronic inputs from a number of sources. Without data from the C&D system, each track must be entered manually by a tedious and slow process for experienced users, let alone sailors who are uncomfortable with computer interfaces. When all contacts are entered, the picture will continue to evolve with the situation, just as a DRT plot can be maintained without new data. The TDSS continues to work automatically without new input or action from the operator: farthest on circles grow, contacts move steadily across the screen on their last known courses and speeds, and other data—such as torpedo danger zones and cones of courses—update and grow with the situation. However, the subsystem quickly becomes reliant on the C&D system and other electronic input, thereby adding to the ship's dependency on the accuracy of tactical data link data and force tracks.
When the C&D and related systems fail for any reason, the TDSS operator is left to begin again from scratch, entering contacts and tracking manually from radar; independent of tactical data link foibles, the DRT operator maintains an accurate picture throughout any number of system failures. On the TDSS, even the track of ownship position is not immune to such failures—it is one more system that requires data from Global Positioning System (GPS) satellites. While GPS units can offer ownship position accuracy within one meter, enemy countersignals can easily compromise their usefulness. Even with inherent cryptographic security, GPS signals are low powered and can be jammed easily. This need not be a problem when using the DRT. The last known good fix of ship's position can be advanced along the intended (or calculated) track while jammed signals are resolved. Using the C&D system and TDSS as the only sources of navigational information and contact management can result in a dangerous loss of awareness when GPS and tactical data link signals are jammed or otherwise lost.
The DRT positions may be plotted on a chart during coastal navigation. The TDSS always displays contacts on a world map and oceanic chart, zoomed to the appropriate scale. Contour intervals and polygonal shapes show landmasses and seafloors. They are far from accurate representations of the earth and were not intended for navigation. The more map functions that are selected, the slower the subsystem runs. Anything more than a few contour intervals can slow it to a near halt during lengthy or busy tracking periods. A deliberate attack on a single subsurface contact can require many assisting unit tracks, dogboxes, and amplifying data. The operator must take care in selecting features for the type of action planned—miscellaneous data might appear useful when the contact picture is relatively clear, but becomes superfluous and wasteful of system resources during battle. The DRT operator, however, is limited only by skill.
The TDSS is a powerful system that automates many of the once burdensome tasks performed by the DRT, DDRT, and NTDS. Its main asset is the speed with which it calculates sonar data. The task of tracking submarines without the use of active sonar requires four target-motion analysis plotters to plot and calculate the many variables. The TDSS makes most of these calculations and accomplishes all of the plotting with just one operator. Although reducing operations specialist positions is a beneficial consequence of increased tracking capability, there still are limitations in replacing the tried and true DRT
As enthusiasm fades, commands must learn to live with what they have—a new system that lacks testing in actual engagements and emergencies, with little (or no) support from the few instructors and technicians having anything more than handbook familiarity. This will change in time as more ships install the system and dependency on NTDS is shifted to C&D systems and TDSSs fleet-wide. Even when the TDSS works perfectly, it cannot do all the DRT once did for ship's safety. It was not intended as a replacement to current systems; rather, it supplements them, easing and automating complex tasks.
The Decatur is deployed with the certain knowledge there is no other force in the world that can match the power projection of a carrier battle group with her at the point of the spear. Yet, she leaves behind a piece of naval tradition: dead reckoning navigation. Will the computers and satellites of the C4I-integrated Navy stand up to the challenge of traditional techniques? Our only consolation may be that the Navy's future course depends more on the experience of its sailors than the precision of its systems.
Petty Officer Inman, a surface warfare specialist, is assigned to the operations division of the USS Decatur (DDG-73).