Intercept method: Difference between revisions

Revision as of 07:47, 19 April 2016

Revision as of 23:41, 16 June 2016

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:''LHA'' = Local Hour Angle

Since recently theseThese computations can be done easily done using electronic calculators or computers but traditionally there were methods which used logarithm or haversine tables. Some of these methods were H.O. 211 (Ageton), Davies, [[haversine]], etc. The relevant [[haversine formula]] for ''Hc'' is

: <math>\operatorname{hav}(\overline{Hc}) = \operatorname{hav}(LHA) \cdot cos(lat) \cdot cos(dec) + \operatorname{hav}(lat \pm dec) </math>

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When using such tables or a computer or scientific calculator, the navigation triangle is solved directly, so any assumed position can be used. Often the dead reckoning DR position is used. This simplifies plotting and also reduces any slight error caused by plotting a segment of a circle as a straight line.

With the use of astroastral navigation for air navigation, faster methods needed to be developed and tables of precomputed triangles were developed. When using precomputed sight reduction tables, selection of the assumed position is one of the trickier steps for the fledgling navigator to master. Sight reduction tables provide solutions for navigation triangles of integral degree values. When using precomputed sight reduction tables, such as H.O. 229, the assumed position must be selected to yield integer degree values for ''LHA'' (local hour angle) and latitude. West longitudes are subtracted and east longitudes are added to ''GHA'' to derive ''LHA'', so AP's must be selected accordingly. When using precomputed sight reduction tables each observation and each body will require a different assumed position.

Professional navigators are somewhat splitdivided in usage between sight reduction tables on the one hand, and handheld computers or scientific calculators on the other. EitherThe methodmethods isare equally accurate. It is simply a matter of personal preference which method is used. An experienced navigator can reduce a sight from start to finish in about 5 minutes using nautical tables or a scientific calculator.

The precise ___location of the assumed position has no great impact on the result, as long as it is reasonably close to the observer's actual position. An assumed position within 1 degree of arc of the observer's actual position is usually considered acceptable.