Thursday, March 12, 2009

Chronometer Error (Time Ticks)

Chronometers are set to Greenwich mean time when they are installed and again after each cleaning. They are not reset by the navigator between these times, but as time passes chronometers will acquire error. Determination of the amount of error and the trend of the error is an important duty of a navigator. Knowledge of the error is important so that exact Greenwich mean time is available for use in solving navigational sights.

Chronometer error is determined by comparing the chronometer to a radio time signal broadcast. In most cases this procedure involves tuning in the time signal, listening for the exact GMT, and then using a stop watch or comparing watch to measure the interval between hearing the time signal and the reading of the chronometer. This procedure is termed obtaining a time tick. There maybe some confusion because time signals are in a 24 hour clock format while the chronometer is normally a twelve hour clock, but chronometer error will be relatively small even years after the chronometer is set.

Example: When the chronometer face reads 07h-58m-02s at the 2000 GMT time signal, you should understand that this means the chronometer time is actua!ly 19h-58m-02s. Chronometer error can be either fast or slow. It is also important for the navigator to have a good estimate of the average amount the chronometer error changes each day. This is called the chronometer rate. The rate is determined by obtaining the difference between the error on the first day and last day of a given period, and then dividing by the number of days in the period. The result will be the chronometer rate. The rate is termed "losing" if a slow chronometer error is increasing or a fast error is decreasing. It is "gaining" if a fast error is increasing or a slow error is decreasing. With knowledge of the rate, the navigator can determine the estimated chronometer error on a day when he cannot get a radio time tick.
The following Coast Guard exam problems demonstrate solution of typical "time tick" problems.