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Some New Dimensions in Sextant-Based Celestial Navigation Aspects of Position Solution Reliability with Multiple Sights

Zevering K. H.

TransNav : International Journal on Marine Navigation and Safety of Sea Transportation

2008

Vol. 2, no. 3

271--278

The traditional approach relying on sight reduction tables, a non-programmatic location of the position fix and an inadequate allowance for observation errors is still widely pursued and advocated. In the late 1970s the programmatic Least Squares method (LSQ) was introduced which determines a random error fix (FixQ) for any multiple sights combination. B.D Yallop & C.Y Hohenkerk (1985) expanded LSQ to incorporate the computation of the random error margin of a fix. Several marketed PDA-based programs apply LSQ, but none have fully incorporated the random error margin as a guide for the navigator. All existing LSQ applications have two drawbacks. One is, all observation error is attributed to random sources, whereas the possibility of systematic error has in fact a long theoretical and practical background in celestial navigation. Systematic error represents a bias in statistical random error theory and can and should be allowed for. A major drawback is that existing LSQ program applications incorporate the running fix technique (RFT) traditionally applied in coastal navigation. It has no general validity in celestial navigation. The position circle of an earlier celestial sight can only be mathematically correctly transferred when its Geometric Position (GP) is transferred for the run data. A final aspect of reliability is the strategy adopted at the sight planning stage. At least during twilight observations, navigators should aim at getting three or four sights with a total azimuth angle >180o, with three successive subsights on each body. In such configurations FixQ and FixS will be relatively close together, generally obviating the need to process the sights for possible systematic error.