NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Navigating Around Hills and Dips in the Ocean
From: Trevor Kenchington
Date: 2003 Aug 17, 09:06 -0300
From: Trevor Kenchington
Date: 2003 Aug 17, 09:06 -0300
Dan Allen asked: > Hmmm. How could these dips be known about almost 30 years ago before > GPS? Satellite radar altimetry. I am not sure when that started but it was certainly long before GPS and is far more accurate, with measurements of sea-surface elevations to a precision of centimeters, I think, and certainly a metre or so. The longitudinal and latitudinal precision of the measurements won't be nearly as precise as with GPS but we are not discussing here small elevations with steep slopes (e.g. wind waves) but rather large, low hills and large, shallow dips. I have never understood how the altitude of the satellite above the centre of the Earth is known with sufficient precision for this kind of work, since its orbit must be distorted by the same gravitational anomalies as affect the ocean surface. However, either NASA has gathered the necessary data for corrections or else the speed of the satellite combines with its distance from the nearest large mass of anything such that the error is negligible. Trevor Kenchington -- Trevor J. Kenchington PhD Gadus@iStar.ca Gadus Associates, Office(902) 889-9250 R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251 Nova Scotia B0J 2L0, CANADA Home (902) 889-3555 Science Serving the Fisheries http://home.istar.ca/~gadus