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    Re: Navigating Around Hills and Dips in the Ocean
    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

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