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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


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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

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