?? Sextant vs GPS ???
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Part 1 of 2
GPS is the most accurate around 40 inch or 1 meter.
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The drawback list.? ?
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- Satellite ? and other countries take out
- Satellite and space debre
- Sun spots?
- Thunder ?
- Power aka battery dead?
- About anything effecting electronics.? ? ? ?
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The Sextant?
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While the theoretical maximum accuracy of a celestial fix is 0.1 miles, in reality you will probably never achieve closer than 1 mile.
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How Accurate is a Celestial Position Fix?
The theoretical accuracy of a celestial position fix is based on the accuracy of a sextant. Most sextants let you measure angles to within 0.1 minutes of a degree. 0.1 minutes {6 seconds 0.12 miles 600 feet at equator } on the altitude of a celestial body translates to 0.1 nautical miles on the surface of the earth.
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In reality, however, it is almost impossible to actually achieve an accuracy of 0.1 minutes with a sextant.
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You are taking your measurements from a moving boat, which is floating on a moving ocean. The horizon needs to be clear, and the celestial body needs to be a perfect, bright speck.
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It is far more realistic to get an accuracy of 1 minute with a sextant in day-to-day conditions.
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An accuracy of 1 minute with the sextant implies a final position fix accuracy of 1 mile.?
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With some reading in WW2 the Navey found it 1? with a good operator most was under 5 miles .
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Of course, getting that sort of accuracy relies on minimising every other source of error. Unfortunately, in celestial navigation, there are plenty of potential sources of error: .
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- Errors reading the measurement from the sextant
- Errors identifying a celestial body
- Potential inaccuracies in your chronometer
- Errors in identifying prevailing atmospheric conditions
- Errors in your calculations
- Rounding errors
- Errors plotting the final fix
- operator holding the sextant
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On plus side is?
- no power need?
- No satellites?
- Works with Sun spots
- Works when electronics does not?
- No power need or batteries?
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Fortunately though, practise and precision can be used to reduce or eliminate most of these errors.
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We can therefore say that an experienced navigator can realistically expect an accuracy of around 1 mile when completing a celestial position fix.?
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Beginners, on the other hand, will be lucky to achieve an accuracy of 10 miles the majority of the time.
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Dave?
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