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Gravity Field Measurment / Refresh Rate of Sensors

  1. Jun 16, 2013 #1
    Hi

    Reading another post about the inverse square law by trewsx7 on this forum, I had the idea that maybe the gravity field isn't constant at some parts.

    Imaging a 2-dimensional gravity field produced by a source. Considering ∏, there might be a repeating infinitesimal patterns on ∏ where gravity isn't behaving as it normaly does. I was thinking that the gravity sensors on board of satellites have a refresh rate which is too low to measure this effect, as they are moving too fast at the same time.

    So I was interested in finding out what kind of refresh rate the sensors on gravity satellite have, but couldn't find anything. Maybe someone here knows?

    Regards

    Philipp
     
  2. jcsd
  3. Jun 16, 2013 #2

    D H

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    philipp, I'm a bit leery of responding to your post. We do not tolerate personal theories at this site, and this sure looks like one. I'm closing the thread, pending moderation, with this post.


    That said, Earth's gravitational field does vary with time. The ocean tides and Earth tides change the shape of the Earth and hence of it's gravitational field. So does weather; a very detailed gravity model may even need to account for things such as the winter snow load on Siberia.

    Even earthquakes reshape the Earth's gravity field. The Gravity Recovery and Climate Experiment (GRACE) satellites sensed the gravitational changes that resulted from the 2004 Sumatra, 2010 Chile, and 2011 Japan earthquakes. The final one, the 2011 Japan earthquake, was also sensed by the Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite. These changes were detected after the fact by analyzing before and after gravity models. GOCE also "sensed" the Japan earthquake in a quite different way. The propagating earthquake created sound waves, and GOCE sensed those.

    There is no such thing as a "gravity sensor". The gravitational force itself is unmeasurable. The GRACE satellites measured the distance between the two satellites in the cluster. When the satellites went one by one over a gravitational anomaly (e.g., a mountain), the distance between the satellites changed by a tiny bit. The GRACE gravity models are generated by knowing the satellites' orbits to a high degree of precision and by knowing the relative distance to an even higher degree of precision. No gravity sensors are needed.

    GOCE uses accelerometers configured as gravity gradiometer. While accelerometers do not measure gravity (nothing can), a suite of accelerometers attached to a rigid body can measure the gradient in the gravitational acceleration field. Once again, the gravity field itself can only be recovered with knowledge of the orbit. The accelerometer data enables creation of a high precision gravity model. This is all done after the fact, on the ground. Generating those gravity models requires a lot of computation.
     
    Last edited: Jun 16, 2013
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