RE: [Paddlewise] GPS

On 29 Aug 2004 at 8:29, PeterO wrote:

> We were using crude angular measurements (eg kamals and thumbs) to
> compare height and horizontal separation of land marks to estimate
> distance travelled, timepieces and an assumed speed to estimate
> distance travelled, compass bearings to landmarks to compare against
> precalculated chart measurements to estimate position and speed
> through the water. Was this pilotage, dead reckoning or coastal
> navigation, or all three?

Dead reckoning and piloting.  

> We used the GPS, i.e timepiece based distance and angular measurements
> to satellites to check our position against a chart. Would this be
> regarded as celestial navigation?

Actually, the GPS doesn't verify angular measurements, it determines 
distances to the satellites.  The satellite transmits its position 
and time information.  The receiver is able to use the time info to 
determine time of travel of the signal and hence the line-of-sight 
distance to the satellite.  Conceptually, if you know the distance to 
four satellites and their exact locations, you can determine your own 
position.  I think that atmospheric refraction and other effects are 
included in the calculations, but I'm not sure.  The real 
calculations are done on earth-centered coordinates IIRC.  BTW, if 
only three satellites are visible, then the receiver will _assume_ 
your current altitude (based on the most recent determination) and 
use that.  Hence the greater uncertainty with only three satellites.

GPS is essentially celestial navigation, however the constellation is 
not the stars, but 24 satellites (min) in six orbital planes.

> Also I'm unclear about the differences between celestial navigation
> and navigation. 

They are the same thing traditionally.

> Also to what extent is the word pilotage mainly used
> to describe guiding ships into harbour as opposed to coastal
> navigation which seems to be used to describe pilotage to find your
> way along the coast?

Historically, they are the same thing.  If a ship entered unknown 
coastal waters, they would use the services of a pilot.  

The pilots for Montreal harbour would board the ships downstream of 
Quebec City harbour in the old days.  That's a couple of hundred 
kilometers outside the harbour proper.  I had a relative that was an 
undercover RCMP officer and he would board Soviet vessels as part of 
the pilot crew back in the cold war days.  It's interesting that the 
pilots were considered critical in those waters, since they were well 
surveyed.  The first accurate survey of the St. Laurence River was 
done by a British officer named Cook.  Seems he visited your part of 
the world, too, Peter!

Mike
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