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34C3 - Watching the changing Earth

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    34C3 preroll music
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    Herald: Hello everybody to the next talk,
    here at stage Clarke. The next talk will
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    be held in English. And here is a quick
    announcement in German for the
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    translation. Der nächste Vortrag wird in
    Englisch sein. Und wir haben eine deutsche
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    Übersetzung unter streaming.c3lingo.org.
    Und wir haben das auch auf einer Folie.
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    Und es wird auch eine französische
    Übersetzung geben für diesen Vortrag.
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    There will also be a french translation,
    as well as an German translation for the
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    next talk. And you can find everything
    under streaming.c3lingo.org. And, I hope,
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    displayed behind me. The next talk is
    called "Watching the changing Earth".
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    Satellite data and change in the
    gravitational field of the earth can tell
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    us a lot, especially when there's so much
    public domain satelite data coming in from
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    different projects or maybe CC-BY
    satellite data. And how this is done, this
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    new knowledge finding out of this big
    heap of data, this will be explained by
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    Manuel in the talk. He dropped stuff to
    see if gravity still works, or, in fancy
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    words, he does gravimetric methods and
    sensory in geodesy. Is that pronounced
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    right? I'm not sure, but give a big hand
    and a round of applause for our speaker
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    Manuel.
    Applause
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    No Audio
    Manuel: Geiler Scheiß. Oh, das war Sound.
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    So again, so hello and welcome to my
    presentation on watching the changing
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    earth. This year's call for papers for the
    Congress offered me the opportunity to
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    talk about my work in the related fields,
    which is gravity. As far as Congress is
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    concerned, a misunderstood force of
    nature. So in the following couple of
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    minutes, I want to talk about gravity,
    gravitation, about the GRACE satellite
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    mission, which maps the earth gravity
    field every month, about the gravity
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    fields, and I will show good results and
    then we will go forward into the future.
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    That's nice. So it's actually called,
    actually called geodesy. Let me give you a
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    short introduction on geodesy. Friedrich
    Robert Helmert defined it in 1880 at as
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    the science of mapping and measuring the
    earth on its surface, and this still holds
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    up today. It depends on your methods and
    applications, but he was correct. The most
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    known profession is probably land
    surveying, people with colorful
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    instruments and traffic cones. You find
    them on construction sites, on the side of
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    the road, but we actually have a lot of
    applications not only in geodesy but in
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    related fields like geophysics,
    fundamental physics, if you want to build
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    an autonomous car you need geodesists,
    metrology. This talk is specifically about
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    physical geodesy, which is the mapping of
    the gravitational field of the earth, and
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    in this case specifically with satellites.
    So I drop stuff on the earth, which is
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    terrestrial gravimetry, this talk is about
    satellite gravimetry. Now gravity and
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    gravitation, we usually talk about
    gravitational potential. This is a scalar
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    field. Gravitational acceleration is the
    gradient of the gravitational potential
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    and when we talk about gravity in geodesy,
    it's usually the combination of attraction
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    of the masses, gravitation, and the
    centrifugal acceleration, but here we talk
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    mostly about gravitation. And the
    potential can easily be calculated, at
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    least according to this very short
    equation. We have G, which is the
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    gravitational constant of the earth, or
    other planets if you want to do. We have
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    an ugly triple integral about the whole
    earth, and thsis is basically what breaks
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    the neck. We have to integrate about the
    whole mass of the earth, we divide up into
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    small parts and we need to know the
    density of these parts. So, density times
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    small volume, you have the mass of the
    earth if you integrate over it. So what,
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    the density of the whole earth is not
    known. So if you want to calculate the
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    potential sufficiently, you would need the
    density of a penguin on the other side of
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    the world. We don't know that. So, what
    do you do if you cannot calculate the
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    quantity? You write a proposal and get all
    the funding. This is what happened about,
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    let's say, twenty years ago, and the
    result was the gravity recovery and
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    climate experiment, or GRACE for short. In
    this talk, we will only cover gravity
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    recovery, so gravity field of the Earth.
    As we can see, these are two satellites.
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    They are flying in the same orbit, and the
    main instrument is distance measurement
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    between these two satellites, Here we see
    the two satellites prior to its launch in
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    2002, and this K-Band Microwave ranging,
    which is the instrument, gives us a high
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    resolution gravity field of the Earth.
    This is spatial resolution of around 200
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    kilometers (km). You might think 200 km is
    not really high resolution, but we have it
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    for the whole planet and not, let's say,
    for Germany. And also we got the temporal
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    variations. So for 15 years now, we have
    each month, with only a few exceptions, a
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    picture of the gravitational field of the
    earth. The satellites fly in height of
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    about 450 km, 220 km apart, and we see
    here the orbits of a single day. So 15
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    orbits per day, and we take one month of
    data to generate one gravity field. The
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    working principle is quite simple: The
    distance between the two satellites is
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    affected by gravity, so we measure the
    distance and then we calculate gravity. In
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    a homogenous gravity field, this is quite
    simple: Let's say we take a spherical
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    earth, it has only a single density, the
    satellites fly along, and the distance
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    between the two sttellites does not
    change. There is nothing to pull one or
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    another, they just move along, not
    changing the distance. Now we introduce a
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    mass, let's say a mountain, this can be
    any mass change or density change
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    somewhere inside the earth, and the
    leading satellite experiences a
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    gravitational pull by this mass. And as
    gravitation falls off with distance, it is
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    a stronger than the pull experienced by
    the trailing satellite. So the distance
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    between the two satellites increases. Now,
    the satellite, the trailing the leading
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    satellite has passed the mass, and it is
    still feeling its gravitational pull, but
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    now it is being decelerated because the
    mass is behind. And the trailing satellite
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    is still being accelerated towards the
    mass. This means the distance between the
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    satellites decreases. And finally, the
    second satellite passes the mass and it
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    now also feels the gravitational pull
    decelerating the satellite. The leading
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    satellite is feeling less and less
    gravitational pull and once both
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    satellites left the gravitational
    influence of the mass, we will have the
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    same distance as prior to encountering the
    mass. So the gravitational acceleration is
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    a zero sum at this point. So of course,
    the Earth is a little more complex than a
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    single mountain in or a single density
    anomaly in the ground, but this is the
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    basic concept. Now, how do we come from
    these measurements to the actual
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    potential? The formula is basically the
    same as a couple of slides earlier. We are
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    still calculating the potential. It looks
    more complicated, but we don't have triple
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    integrals anymore, and all these
    quantities in here are basically easily
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    calculated. We start with the
    gravitational constant and the mass of the
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    earth, which we can get from a physics
    book, if we like. And then we have a
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    couple of geometric quantities, a and r
    are basically the size of my earth
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    ellipsoid, the major axes and r is the
    distance from a calculating point, let's
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    say this podium, for which I want to know
    the potential value to the center of the
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    ellipsoid. And then we have lambda and
    theta at the end, these are the
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    geographical coordinates of this podium. P
    is short for the associated Legendre
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    functions, also depending solely on
    geometry, not on the mass of the earth,
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    depending on the software where you want
    to implement this formula, it probably has
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    already a function to calculate this, and
    if not, it is easily done by yourself as
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    the formulas look very long, but they are
    quite simple. The interesting part are the
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    two parameters C and S, these are
    spherical harmonic coefficients. They
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    include all the information about the
    mass of the earth, as measured by the
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    satellites. So we have the satellites in
    space, and the user gets just the C and S
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    coefficients, which are a couple of
    thousand for the gravity field. Implements
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    this formula and has a potential value.
    So, these spherical harmonic coefficients
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    are calculated from the GRACE Level 1B
    products. These are the actual
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    measurements done by the satellites. This
    is the ranging information, the distance
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    between satellites, satellite orbits, star
    camera data, and so on. You add a couple
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    of additional models for earth's gravity,
    which you do not want to include in your
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    satellite gravity field, and then you do
    your processing. This is done by a couple
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    of different groups JPL and GFZ, which is
    a German research center for the
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    geosciences. CSR is the center for space
    research at university austin. These three
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    institutes also provides these GRACE Level
    1B data. So they take the raw satellite
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    data processes it to theGRACE Level 1B
    products, which are accessible for all
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    users, and then calculate further these
    coefficients, C and S. But there are also
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    additional groups who provide gravity
    fields who calculate these coefficients,
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    for example, Institute for Geodesy of the
    University of Graz or the Astronomical
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    Institute of the University of Bern. They
    all have slightly different approaches to
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    topic and come to more or less the same
    conclusions. There are countless papers,
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    comparing these different gravity fields
    with each other, but the user usually
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    starts with the coefficients C and S, and
    then it takes a formula like the one on
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    top of this slide and calculates your
    gravity value or whatever you want. Now,
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    I'm talking about potential, I'm talking
    about accelaration. These are not really
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    useful quantities in day to day life. If
    someone told to you in Greenland gravity
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    decrease by 50 microGal, you have two
    choices, you can say, wow, awesome or you
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    can say, oh no, we're all gonna die. It's
    a 50:50 chance you'd say the correct
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    thing. So we are looking for a more useful
    representation of the gravity, changes in
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    gravity field. Now gravity fields reflect
    mass redistributions and the most dynamic
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    redistribution we have is water storage,
    summer/winter, more snow, more rain, less
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    water in summer, so we express our gravity
    change in a unit called equivalent water
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    height. This is the layer of water on the
    surface with a thickness, equivalent to
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    the mass change measured with the
    satellites. This is also easily
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    calculated. This is my last equation, I
    promise, but this looks familiar. The
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    second half of this equation, is basically
    the same we saw one slide prior and the
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    parameters in front of the sum is the
    average density of earth, which is around
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    5500 kg/m^3. We need the density or water,
    let's say it 1000 kg/m^3. And in this
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    fraction in the middle, we need to
    parameter K, which are the so-called Love
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    numbers. Now, this is not a numerical
    representation of mutual attraction, but
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    was put forward by, I think, Albert Love
    in 1911, and they are parameters
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    concerning the elastic response of the
    earth to forces. So, if you put a lot of
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    weight on a part of the earth, the earth
    deforms and these parameters, describe the
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    elastic response of the earth to such
    loading. Now we have calculated our
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    equivalent water height, let's say for two
    months, let's say, in May 2002 and 15
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    years later in May 2017 and we just
    subtract these two gravity fields, these
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    two equivalent waterheights, from these
    two epoches. What we have left is the
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    change in gravity between these epoches 15
    years apart, expressed in water layer
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    equivalent to the change in gravity
    measured. And we can see a couple of
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    features here. There should not be any
    seasonal variations because it's the same
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    month, just 15 years apart. So we see long
    term gravity change between these two
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    epoches. And what we see is, for example,
    mass loss in the northern and southern ice
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    shields, and we see to red blobs, one in
    northern canada and one in northern
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    europe, which are geophysical processes.
    So this is glacial isostastic adjustment
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    and during the last ice age the ice
    shields deformed the earth downward. The
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    material in the "Mantel" had to flow
    aside, and now that the ice is gone, the
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    lead is uplifting and the material in the
    "Mantel" is flowing back. So it's flowing
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    back and the earth is uplifting. This
    process has been going on for 10000 years
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    and will probably a couple of years
    longer. Now how do you get your data?
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    Everyone can get the GRACE lLevel 1B data,
    which are the observations by the
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    satellite, like again, ranging information
    between the satelite, orbits,
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    accelerometer data, star camera data and
    so on. You can get them without hurdles at
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    the ISDC, which is the information system,
    a data center at the geoforschungszentrum
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    Potsdam, or at the Physical Iceonography
    Distributed Active Archive Center run by
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    JPL. And if you'd like, you can calculate
    your own spherical harmonic coefficients
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    for gravity fields. Or you can compare for
    example, satelite orbits they give you
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    with one you integrated yourself using
    your own gravity field, to see if they fit
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    together or not. You can get gravity field
    models, if you'd like. A large collection
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    is at the International Centre for Global
    Earth Models. They have recent and
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    historic gravity models all in the same
    data format. So you only need to implement
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    your software once from the 1970s to
    today. They also have the proper
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    references, the papers you want to read to
    work with them. These are so-called Level
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    2 Products. So, you can take a gravity
    field from there, use the equation, I
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    showed you earlier and calculate your
    equivalent water height, if you'd like. If
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    you don't want to do this, there is
    someone to help you, a service called
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    "TELLUS", which is a play on words I
    don't want to go into detail about. They
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    offer equivalent water heights calculated
    for each monthly solution from the GRACE
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    satelites. This tells a lot about the
    earth, if you look closer into it. In the
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    following, I will use the monthly
    solutions from the ITSG-GRACE 2016,
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    provided by Institute for Geodesy from the
    University of Graz. The previous graph I
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    showed you was also created with that
    gravity model. I will not go into detail
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    about further processing like filtering
    and gravity reductions done to this, not
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    enough time. So here are some results,
    let's start with the most obvious one, the
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    greenland ice shield, which has, as we saw
    earlier, the greatest loss of mass
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    according to the gravity field and we see
    here, a water layer on the whole landmass,
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    describing the loss of mass expressed as a
    water layer of a certain thickness. So
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    let's say in the southern tip, you have
    one meter water layer. This would be
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    equvalent in gravity to the actual mass
    lost in Greenland. But we also see, that
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    the signal is not very localized. So it's
    not bound to the land mass. It's also in
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    the ocean. This effect is called leakage.
    If you do signal processing you will know
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    this. There are methods to reduce leakage.
    My next slide will show such a result, but
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    I have done no reduction to this. So if
    you use my formula I showed you, you will
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    pretty much get a result like this. This
    gives you a trend of around 280 gigatons
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    per year in mass loss over the whole land
    mass of greenland. And now gigatons is
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    also not very useful an expression. One
    cubic meter of water has a weight of a
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    thousand kilos; one ton, one gigaton is
    10^9 tons, if you are familiar with ball
  • 20:17 - 20:25
    sports, one soccerfield with the 140 km
    high water column has the weight of one
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    gigaton, or if you are not fan of sports
    ball, if you're more of a plane guy or
  • 20:33 - 20:43
    girl, the A380-800 has a maximum weight of
    575 tons, so we need 1.7 million of these
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    planes for one gigaton. So this is a more
    beautiful representation of the process in
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    greenland, done by NASA JPL. If you go to
    the website of the GRACE project, they
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    have a couple of these illustrations, they
    obviously worked hard on the leakage. You
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    can see localized where most of the
    gravity, most of the mass is lost on the
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    left and on the right you see accumulated
    over time, the mass which is lost, and
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    which trend it gives you. Also, if look
    closely in the center of greenland, you
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    see black lines, these are the ice flow,
    as determined by radar interferometry. So
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    now pretty much know where ice is lost,
    where mass is lost. This goes into the
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    ocean, and this would be a good idea to
    see, to check our GRACE results, the mass
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    we find missing on earth, so the melted
    ice, and the additional mass in the ocean,
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    does this agree with other methods who
    determine the sea level rise. One of these
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    methods is satellite radar altimetry, that
    started in the 70's, but since 1991, we
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    have lots of dedicated satellite missions,
    which only job is basically mapping the
  • 22:15 - 22:22
    global sea surface. So, they send down a
    radar pulse, which is reflected at the sea
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    surface. They measure the run time and
    then they have a geometric representation
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    of the global sea surface. Now, if we
    compare this with the mass we calculated
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    or we got from the GRACE result, calculate
    a sea level rise rise from this additional
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    mass in the ocean than these two systems
    would not add up. The geometric sea level
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    rise is higher than just the additional
    mass. So there is the second process which
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    is thermal expansion of the water. If
    water gets warm it needs more space. In
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    2000 the deployment of so-called Argo
    floats started. These are free-floating
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    devices in the ocean. Currently, there are
    over 3000 and they measure temperature and
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    salinity between sea surface and a depth
    of 2000 meters. These are globally
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    distributed. So, we have at least for the
    upper layer of the ocean, how much thermal
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    expansion there is. And what we want to
    see is do these components of additional
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    mass in the ocean as determined by GRACE
    and thermal expansion of the upper ocean
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    layer come to the same result as
    geometrical measurements done by satellite
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    altimetry. On the left we see an image
    taken from the last IPCC report on climate
  • 23:53 - 24:00
    change from 2013. In green we see the
    sealevel rise as measured with satellite
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    altimetry in the time span 2005 to 2012
    and in orange we see the combination of
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    additional mass, as measured by GRACE, and
    thermal extension as determined with Argo
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    inside the ocean. And these two follow
    each other quite well. On the right. We
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    see a recent publication by Chen, Wilson
    and Tapley, the latter one being one of
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    the PIs of the GRACE mission, who
    accumulated the data from 2005 to 2011. We
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    basically come to the same conclusion. So
    now if you really don't want to do the
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    math, there are online services who make
    the graphs for you. One of them lotus
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    EGSIEM European Gravity Service for
    Improved Emergency Control. If we can
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    measure how much water stored in a certain
    area, we know that this amount of water
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    has sooner or later to be removed from
    this area. This can be a flood, for
  • 25:05 - 25:09
    example, and with a mission like GRACE, we
    can determine how much mass, how much
  • 25:09 - 25:15
    water is there and are the rivers large
    enough to allow for this water to be
  • 25:15 - 25:23
    flowing away. That was the intention
    behind this service. Oops, no, this is not
  • 25:23 - 25:39
    the future. So, I wanted to do the life
    demo but. So, yeah, the live demo did not
  • 25:39 - 25:47
    work as expected. So, you will be greeted
    with this graphic. You can plot for all
  • 25:47 - 25:51
    areas in the world. The first thing you
    have to do is you change your gravity
  • 25:51 - 25:58
    functional, we want water heights. This is
    what I talked about in this talk. Then you
  • 25:58 - 26:03
    want to look at the data set and at the
    bottom you see a large list of GRACE
  • 26:03 - 26:07
    gravity fields. These are different
    groups, I mentioned, providing these
  • 26:07 - 26:15
    monthly solutions. And so we choose one of
    these groups. Then we choose an area which
  • 26:15 - 26:22
    we are interested in. You can freely
    choose one area like here Finno-Scandia,
  • 26:22 - 26:29
    or you can use pre-determined areas, for
    example, the Amazon river basin or Elbe
  • 26:29 - 26:36
    river or something like that. These areas
    all over the world and you can see the
  • 26:36 - 26:41
    gravity change in this area. So let's look
    here at Finno-Scandia, and then you are
  • 26:41 - 26:47
    greeted with a plot like this. This is
    equivalent water height, even though this
  • 26:47 - 26:52
    is a geophysical process. So we see here
    the laer of water, which would have been
  • 26:52 - 27:02
    added to the region as selected, and we
    see a clear trend upward. Again, this is a
  • 27:02 - 27:09
    geophysical process. This is not
    additional ice or water or anything. Can I
  • 27:09 - 27:20
    return to my...? No, I cannot. So, yeah,
    live demo did not work. If you want to do
  • 27:20 - 27:27
    this yourself. I have uploaded to the
    Fahrplan all my resources, all my links.
  • 27:27 - 27:32
    And the EGSIEM page also includes the
    description of what is done in the backend
  • 27:32 - 27:38
    and were the data comes from and what you
    can see in the various fields. Now I want
  • 27:38 - 27:42
    to give a last impression on the future,
    because unfortunately while I was
  • 27:42 - 27:47
    preparing my abstract for this conference,
    one of the GRACE satellites was turned off
  • 27:47 - 27:52
    due to age. It was launched in 2002,
    planned for a five mission year; it
  • 27:52 - 27:57
    survived 15 years, which is quite good,
    but now we have no more ranging
  • 27:57 - 28:01
    information between these satellites. We
    had ranging information in micrometer
  • 28:01 - 28:09
    accuracy, a couple of micrometer, and now
    we cannot rely on these information
  • 28:09 - 28:14
    anymore. And this means mo more gravity
    fields with high spatial resolution, and
  • 28:14 - 28:18
    I'm not sure about the temporal
    resolution. So, the current work which is
  • 28:18 - 28:23
    done is taking all satellites which are in
    the low-enough orbits and calculate the
  • 28:23 - 28:27
    gravity field from their positions,
    because everything which is in low-earth
  • 28:27 - 28:33
    orbit is affected by the Earth's gravity
    field. So, if I take the satellite orbits,
  • 28:33 - 28:39
    look "how does this orbit change" and the
    reason is gravity, then I can calculate
  • 28:39 - 28:46
    the gravity field. Unfortunately, not in
    this higher resolution we are used to.
  • 28:46 - 28:50
    And... But fortunately, there already is a
    next-generation gravity field mission on
  • 28:50 - 28:58
    its way. It arrived last week in the US,
    where it will be launched in late March,
  • 28:58 - 29:05
    early April by SpaceX. You might look at
    this image and think, "I just saw this
  • 29:05 - 29:11
    earlier" and you are quite correct: The
    mission called "Grace Follow On" is a copy
  • 29:11 - 29:17
    of Grace, which improved components, of
    course, and now with lasers. We see not
  • 29:17 - 29:22
    only the microwave ranging between the two
    satellites, but additionally a laser
  • 29:22 - 29:26
    interferometer. So, from micrometer
    accuracy in the distance measurements we
  • 29:26 - 29:30
    go to nanometer accuracy, hopefully. But
    the main instrument will be theSo from my
  • 29:30 - 29:32
    mitac in the distance measurements we go
    to... yeah, not a metal accuarcy,
  • 29:32 - 29:36
    hopefully, but the main instrument will be
    the microwave ranging. So, in conclusion,
  • 29:36 - 29:42
    I hope I showed you that the gravity field
    can show mass transport on the surface and
  • 29:42 - 29:48
    inside the Earth; that this offers, in
    combination with other methods, new
  • 29:48 - 29:54
    insights and also some kind of new tool...
    verification with several different types
  • 29:54 - 29:58
    of observations coming to the same
    conclusion, none of them can be awfully
  • 29:58 - 30:04
    wrong; and that the access to these
    methods are relatively easy: the data is
  • 30:04 - 30:09
    available, all the methods are described
    in geodesy textbooks and the technical
  • 30:09 - 30:15
    documentation; and there are other
    applications, other than, let's say,
  • 30:15 - 30:22
    climate change; you can look into drought
    and flood prediction; the El Niño–Southern
  • 30:22 - 30:29
    Oscillation you can predict from Grace's
    gravity field data. So, lot's of work to
  • 30:29 - 30:36
    do. So, this would be the end for my talk.
    I thank you for your interest in the topic.
  • 30:36 - 30:39
    applause
  • 30:39 - 30:51
    Herald: Thank you, Manuel, for the talk.
    And I think we have time for one or two,
  • 30:51 - 30:57
    maybe two very short questions. Please be
    seated during the Q&A session. Is there
  • 30:57 - 30:59
    some questions? Okay, microphone 3,
    please.
  • 30:59 - 31:04
    Mic 3: Yeah, hi. In a quiet voice
    Hi, hello? Can you hear me? Now loud
  • 31:04 - 31:08
    Herald: Yeah.
    Mic 3: Okay. Hey. So, my question is
  • 31:08 - 31:14
    regarding acceleration. What's the
    influence of Earth atmosphere and all the
  • 31:14 - 31:17
    planetary bodies, like the moon, and does
    it need to be accounted for?
  • 31:17 - 31:22
    Manuel: The external gravity needs to be
    accounted for, so the tidal effects of sun
  • 31:22 - 31:28
    and moon would be one of those additional
    models you put into the processing of the
  • 31:28 - 31:32
    satellite data. The Earth's atmosphere has
    an effect on the satellites themselves,
  • 31:32 - 31:38
    which is measured onboard by
    accelerometers and then reduced. And the
  • 31:38 - 31:43
    gravitational effect of the atmosphere:
    Part of this is averaged out, because we
  • 31:43 - 31:48
    take a month of time series, and the rest
    are also inclu... provide as extra
  • 31:48 - 31:54
    products; at least by the Institute for
    Geodesy in Graz. So atmosphere... the mass
  • 31:54 - 31:58
    of the atmosphere is... has to be
    accounted for, yes.
  • 31:58 - 32:04
    Herald: Okay. Microphone 2 has vanished
    all of a sudden. Then, microphone 1,
  • 32:04 - 32:09
    please.
    Mic 1: Hi. Is it possible to measure
  • 32:09 - 32:17
    changes in the temperature of the oceans
    or of the ocean streams, like... Can you
  • 32:17 - 32:22
    see if El Niño is active by just measuring
    the gravity... change in gravity fields?
  • 32:22 - 32:30
    Manuel: As a precursor tool, El Niño, as I
    understand it... certain regions of the
  • 32:30 - 32:36
    ocean get warmer; it's a density change;
    and, of course, this would be measured as
  • 32:36 - 32:42
    part of ARGO and it's also in the Grave
    gravity field. There are probably papers
  • 32:42 - 32:48
    on it. So, the last... the extend of the
    last El Niño was predicted by Grace. I
  • 32:48 - 32:51
    don't know to what extend this was
    correct, but...
  • 32:51 - 32:55
    Mic 1: Okay, then.
    Herald: Good. Then, that's all the time we
  • 32:55 - 33:16
    have. A big round of applause for Manuel
    and his talk, please.
  • 33:16 - 33:17
    Applause
  • 33:17 - 33:22
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Title:
34C3 - Watching the changing Earth
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Video Language:
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