How to Beat Target Motion Analysis

By Lieutenant Timothy S. Hill, U.S. Navy

The answer to the problem is a computerized tactical aid that combines a powerful programming language with a spreadsheet to eliminate delays and ensure that highly accurate information is provided to the evaluator.

Visual Basic is the language; when combined with a spreadsheet's inherent power, it can tackle a wide variety of engineering and management applications. Because it runs within the Microsoft Excel environment, any CIC that has access to a local area network, desktop, notebook, or even a hand-held computer can access it. Visual Basic runs within the spreadsheet, executing various user-selected code and function procedures that make for easy interaction and logical decision making.

The spreadsheet is set up in the familiar form of a status board. (See Figure 1.) Three successive status board displays accommodate the data entry associated with each ship maneuver. The first five columns are for user input; the remaining cells are calculated by the program. User input includes times, two sets of bearings (one for sensor ambiguity resolution), and two for frequencies. The last two legs include a separate section for range predictions.

Consider a typical event assuming leg 1 data have been gathered, the ship is steady on leg 2, and sensor ambiguity has been resolved. After entering the time, bearing, and frequency data as received from ship sensors, the operator depresses the button labeled "Calculate Leg 1," and the computer starts the following sequence:

1. The raw data are copied to a remote location for possible reuse in the event of a run-time error or other unforeseen circumstance.

2. The user is prompted to enter course (CSE) and speed (SPD) and to select the range of times and bearings (ambiguity is resolved by not selecting ambiguous data).

3. All times and bearings are normalized. This is required because the installed spreadsheet functions will not automatically convert raw data which appear to have target discontinuities. Such a situation could arise from tracking a contact through 2400 (time), or from 359o to 001 True (CSE). This is normally done instinctively by the time-bearing plotter and the actions are transparent to the user because the computer detects, corrects, and reformats all time and bearing information prior to redisplay on the spreadsheet. All times are returned in a date-time group format but are entered on standard 24-hour clock.

4. It calculates and plots faired bearings using the "least squares" method and determines the slope and y-intercept of the time-bearing plot. The faired bearings and raw data are plotted on a small graph adjacent to the status sheet (see Figure 2) to allow the operator to evaluate the faired line and make any deletions or corrections required. To correct an erroneous data point, the operator need only use the "Reset" and "Reload" command buttons, edit the original data, and recalculate.

5. The line-of-sound diagram is then determined. This calculates target angle, mid-bearing and its associated time, ship's speed across the line of sound, as well as speed in the line of sound for each observed time—and then corrects the received frequencies.

If the data computed are either for leg 2 or 3, the operator can choose to calculate the associated Eckland range by depressing the "Calculate Rek" button, which creates the following display:

  • The Eckland range associated with the previous two legs
  • The time of instantaneous turn (t-hat)
  • The best estimate of time of turn (t-star)
  • The correction factor for range (D-star)
  • The corrected Eckland range

Other features include a command button for estimating base frequency by using the evaluator's best estimate of contact course and speed and another that yields the target's minimum speed based on a guess of base frequency.

Once leg 2 is completed, leg 3 is recorded and executed in a similar fashion with another opportunity to calculate an Eckland range. All command buttons available to legs 1 and 2 are available on leg 3. Once leg 3 is completed, the operator depresses a "Reset Sheet" command button which places leg 3 data into leg I and readies the program for another ship maneuver starting in leg 2.

This tactical aid has the following advantages:

Speed—If a proper ship maneuver is conducted, an initial estimate of contact range can be provided immediately after sufficient data have been collected on the subsequent leg. If two such maneuvers are conducted in succession, contact course and speed can be estimated quickly from two estimated positions. Data modification can be accomplished within seconds, if required.

Accuracy—Excel carries all calculations to 13 significant figures, regardless of the format displayed—thus eliminating any errors associated with rounding of data.

Faired bearings—The "least squares" method used by the TREND function in Excel will produce a much more accurate set of faired bearings. Subsequent DRT lines will reflect a more accurate representation for course-speed analysis. Any tendency to plot and use raw data is quickly overcome, owing to the instantaneous availability of accurate faired bearings.

Flexibility—The operator can modify data quickly as needed to fit the circumstance. If insufficient bearing drift is observed because of LOS geometry, for example, he can delete some data points or increase recording interval; or if he would like a rough estimate of range, he could calculate an Eckland on the first few data points and recalculate and refine as more data are gathered.

Decreased manhours—We must be open to any method that can reduce the workload while providing an improved product. There could be benefits associated with elimination of time-consuming calculations. Exactly how this could affect the TMA team composition will depend on the level of training. Since all the "legwork" of the time-bearing and time-frequency plotter is done by the computer, these two stations could be combined; a single operator could plot and maintain track history.

Accessibility—Most ships have some type of personal computer available for use by the watch team. A simple hand-held, off-the-shelf computer can cost as little as $400. Visual Basic is an easy language to learn and ships can customize the application to their needs, or even expand it.

Nothing is perfect. Before you decide to stand down the TMA team, consider the system's limitations:

No long-term record—If a contact does decide to cooperate and stay on a steady course and speed, the valuable history information contained on the time-bearing and time-frequency plots must be retained. The system does reduce significantly, however, the man-hours spent constructing these plots. How this benefit is realized will be dependent upon each watch team's proficiency.

Knowledge and proficiency must be maintained—The evaluator and each member of the TMA team must understand fully the output and be prepared to verify manually the calculation if any doubt exists as to its accuracy.

Garbage in, garbage out—Nothing new here. Although most data entry errors will generate an associated run-time-error, some can go unnoticed. This is why use of this tactical aid is no substitute for knowledge of basic TMA procedures. You must be able to look at the data and say "They make sense."

Previous computerized tactical aids have either been too difficult to master, given the high turnover rate and limited training opportunities afforded by today's operational schedules, or insufficiently automated when compiling input data. This application, however, performs the most common TMA tasks, including all of their required calculations. It provides instantly faired bearings, corrected frequencies, LOS data, and also determines best range-and-time, using only raw sensor data information.

As the demands on our people go up and the man-hours available to perform their most critical tasks diminish, we must be willing to implement the changes that promise big payoffs. As more of our personnel return to the fleet with advanced degrees or extensive software and programming skills, and the availability of personal computers on board today's ships increase, the number of customized time-saving applications currently being developed by personnel will increase. We cannot dismiss the power available in a personal computer.

Lieutenant Hill is the Operations Officer on the USS Underwood (FFG-36).



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