Posted on 08/01/2002 3:13:16 PM PDT by RightWhale
Well, it is what I do for a living.....
From your link, the variations in the Earth's gravity are carried in the Geopotential Model, which is a set of coefficients that feed something called a Legendre polynomial. If you feed in the latitude, longitude, and altitude of your vehicle, and the coefficients from the model, the polynomial essentially gives you the gravitiational acceleration at that point.
The model (i.e., coefficients) specified in your link is GEM-T3. The software in the GPS control center uses coefficients from something called the WGS-84 model.
Both of these models were developed years before this change in the bulge was detected. I have no idea of the magnitude of this change, and it may not have a significant effect. Technically, however, this change in the bulge has now rendered both models out of date.
The relevant term for this discussion is the J2 value in the gravity model, which basically describes the gravitational effect of the equatorial bulge. Aside from the central body acceleration, the J2 effect is by far the largest perturbation on the GPS satellite.
If this phenomenon is significant (i.e., if has the same order of magnitude as, say, J4 or J6), then it is something that would merit a change in the gravity used by the orbit estimation and prediction software. And if it's likely to change as rapidly for the next few years, then it will require some means of keeping up with the changes.
It's by no means an impossible problem, it would just complicate things, that's all.
The House Committee on Controlling the Earth's Rubber Ball Effect
And of course, the new Cabinet member, the Department of the Really Interior Interior; no doubt, the Secretary will be an ex-C.E.O. of a major golf ball manufacturer.
A speck in the universe ... the planet Earth in orbit, viewed from space. The Earth has become fatter round the middle in the last four years, according to researchers. Like a pumpkin, the planet is a little bit wider around the equator than a perfect sphere would be. For 19 years its waistline shrank, but since 1998 it has been expanding, according to a study in the journal Science. Photo: AFP
It looks from rock data in the spreading Atlantic rift that this happens at somewhat regular intervals. Does the Van Allen Belt and the rest of the magnetic shield collapse during the flip? If so, would that be a problem?
This is actually a topic of frequent discussion at the office. We're trying to merge GPS readings taken in the field with our GIS, which uses a different geoid. There is a surprising degree of understanding. Residual disagreement between GPS and GIS is compensated for by taking at least one reading at a well-known and agreed upon nearby [within one mile] point on the ground.
I have no idea. I gather that it happens over a very short time period, as I've seen no estimate of the transition duration. If it took an appreciable amount of time, geological records would have the answer to your question.
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