Intercept method: Difference between revisions

Revision as of 21:57, 26 December 2010

Revision as of 18:19, 6 January 2011

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== Summary ==

The intercept method is based on the following principle.

The distance from the observer of the geographical position ('''GP''') of a celestial body (where it is overhead) is measured using a sextant, this distance is equal to its zenith distance. The distance is then calculated from where the observer believes himself to be through [[dead reckoning]]. The difference between the two is then calculated by subtracting one from the other. This distance is called the intercept.

[[Image:Diagram showing GP distance = ZD.jpg|thumb|right|500px|]]

The diagram on the right shows why the zenith distance of a celestial body is equal to the angular distance of its GP from the observer's position.

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Taking a sight using the intercept method consists of the following process:

* Observe the altitude above the horizon '''Ho''' of a celestial body and note the time of the observation.

* Assume a certain geographical position (lat., lon.), it does not matter which one so long as it is within, say, 50 NM of the actual position (or even 100 NM would not introduce too much error). Compute the altitude '''Hc''' and azimuth '''Zn''' with which an observer situated at that assumed position would observe the body.

* If the actual observed altitude Ho is smaller than the computed altitude Hc this means the observer is farther away from the body than the observer at the assumed position, and viceversa. For each minute of arc the distance is one NM and the difference between Hc and Ho expressed in minutes of arc (which equal NM) is termed the "intercept". The navigator now has computed the intercept and azimuth of the body.

* On the chart he marks the assumed position '''AP''' and draws a line in the direction of the azimuth Zn. He then measures the intercept distance along this azimuth line, towards the body if Ho>Hc and away from it if Ho<Hc. At this new point he draws a perpendicular to the azimuth line and that is the line of position '''LOP''' at the moment of the observation.

* The reason that the chosen AP is not important (within limits) is that if a position closer to the body is chosen then HCHc will be greater but the distance will be measured from the new AP which is closer to the body and the end resulting LOP will be the same.

==Methodology==