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Herald: Hello everybody to the next talk,
here at stage Clarke. The next talk will

be held in English. And here is a quick
announcement in German for the

translation. Der nächste Vortrag wird in
Englisch sein. Und wir haben eine deutsche

Übersetzung unter streaming.c3lingo.org.
Und wir haben das auch auf einer Folie.

Und es wird auch eine französische
Übersetzung geben für diesen Vortrag.

There will also be a french translation,
as well as an German translation for the

next talk. And you can find everything
under streaming.c3lingo.org. And, I hope,

displayed behind me. The next talk is
called "Watching the changing Earth".

Satellite data and change in the
gravitational field of the earth can tell

us a lot, especially when there's so much
public domain satelite data coming in from

different projects or maybe CCBY
satellite data. And how this is done, this

new knowledge finding out of this big
heap of data, this will be explained by

Manuel in the talk. He dropped stuff to
see if gravity still works, or, in fancy

words, he does gravimetric methods and
sensory in geodesy. Is that pronounced

right? I'm not sure, but give a big hand
and a round of applause for our speaker

Manuel.
Applause

No Audio
Manuel: Geiler Scheiß. Oh, das war Sound.

So again, so hello and welcome to my
presentation on watching the changing

earth. This year's call for papers for the
Congress offered me the opportunity to

talk about my work in the related fields,
which is gravity. As far as Congress is

concerned, a misunderstood force of
nature. So in the following couple of

minutes, I want to talk about gravity,
gravitation, about the GRACE satellite

mission, which maps the earth gravity
field every month, about the gravity

fields, and I will show good results and
then we will go forward into the future.

That's nice. So it's actually called,
actually called geodesy. Let me give you a

short introduction on geodesy. Friedrich
Robert Helmert defined it in 1880 at as

the science of mapping and measuring the
earth on its surface, and this still holds

up today. It depends on your methods and
applications, but he was correct. The most

known profession is probably land
surveying, people with colorful

instruments and traffic cones. You find
them on construction sites, on the side of

the road, but we actually have a lot of
applications not only in geodesy but in

related fields like geophysics,
fundamental physics, if you want to build

an autonomous car you need geodesists,
metrology. This talk is specifically about

physical geodesy, which is the mapping of
the gravitational field of the earth, and

in this case specifically with satellites.
So I drop stuff on the earth, which is

terrestrial gravimetry, this talk is about
satellite gravimetry. Now gravity and

gravitation, we usually talk about
gravitational potential. This is a scalar

field. Gravitational acceleration is the
gradient of the gravitational potential

and when we talk about gravity in geodesy,
it's usually the combination of attraction

of the masses, gravitation, and the
centrifugal acceleration, but here we talk

mostly about gravitation. And the
potential can easily be calculated, at

least according to this very short
equation. We have G, which is the

gravitational constant of the earth, or
other planets if you want to do. We have

an ugly triple integral about the whole
earth, and this is basically what breaks

the neck. We have to integrate about the
whole mass of the earth, we divide up into

small parts and we need to know the
density of these parts. So, density times

small volume, you have the mass of the
earth if you integrate over it. So what,

the density of the whole earth is not
known. So if you want to calculate the

potential sufficiently, you would need the
density of a penguin on the other side of

the world. We don't know that. So, what
do you do if you cannot calculate the

quantity? You write a proposal and get all
the funding. This is what happened about,

let's say, twenty years ago, and the
result was the gravity recovery and

climate experiment, or GRACE for short. In
this talk, we will only cover gravity

recovery, so gravity field of the Earth.
As we can see, these are two satellites.

They are flying in the same orbit, and the
main instrument is distance measurement

between these two satellites, Here we see
the two satellites prior to its launch in

2002, and this KBand Microwave ranging,
which is the instrument, gives us a high

resolution gravity field of the Earth.
This is spatial resolution of around 200

kilometers (km). You might think 200 km is
not really high resolution, but we have it

for the whole planet and not, let's say,
for Germany. And also we got the temporal

variations. So for 15 years now, we have
each month, with only a few exceptions, a

picture of the gravitational field of the
earth. The satellites fly in height of

about 450 km, 220 km apart, and we see
here the orbits of a single day. So 15

orbits per day, and we take one month of
data to generate one gravity field. The

working principle is quite simple: The
distance between the two satellites is

affected by gravity, so we measure the
distance and then we calculate gravity. In

a homogenous gravity field, this is quite
simple: Let's say we take a spherical

earth, it has only a single density, the
satellites fly along, and the distance

between the two satellites does not
change. There is nothing to pull one or

another, they just move along, not
changing the distance. Now we introduce a

mass, let's say a mountain, this can be
any mass change or density change

somewhere inside the earth, and the
leading satellite experiences a

gravitational pull by this mass. And as
gravitation falls off with distance, it is

a stronger than the pull experienced by
the trailing satellite. So the distance

between the two satellites increases. Now,
the satellite, the trailing the leading

satellite has passed the mass, and it is
still feeling its gravitational pull, but

now it is being decelerated because the
mass is behind. And the trailing satellite

is still being accelerated towards the
mass. This means the distance between the

satellites decreases. And finally, the
second satellite passes the mass and it

now also feels the gravitational pull
decelerating the satellite. The leading

satellite is feeling less and less
gravitational pull and once both

satellites left the gravitational
influence of this mass, we will have the

same distance as prior to encountering the
mass. So the gravitational acceleration is

a zero sum at this point. So of course,
the Earth is a little more complex than a

single mountain or a single density
anomaly in the ground, but this is the

basic concept. Now, how do we come from
these measurements to the actual

potential? The formula is basically the
same as a couple of slides earlier. We are

still calculating the potential. It looks
more complicated, but we don't have triple

integrals anymore, and all these
quantities in here are basically easily

calculated. We start with the
gravitational constant and the mass of the

earth, which we can get from a physics
book, if we like. And then we have a

couple of geometric quantities, a and r
are basically the size of my earth

ellipsoid, the major axes and r is the
distance from a calculating point, let's

say this podium, for which I want to know
the potential value to the center of the

ellipsoid. And then we have lambda and
theta at the end, these are the

geographical coordinates of this podium. P
is short for the associated Legendre

functions, also depending solely on
geometry, not on the mass of the earth,

depending on the software where you want
to implement this formula, it probably has

already a function to calculate this, and
if not, it is easily done by yourself as

the formulas look very long, but they are
quite simple. The interesting part are the

two parameters C and S, these are
spherical harmonic coefficients. They

include all the information about the
mass of the earth, as measured by the

satellites. So we have the satellites in
space, and the user gets just the C and S

coefficients, which are a couple of
thousand for the gravity field. Implements

this formula and has a potential value.
So, these spherical harmonic coefficients

are calculated from the GRACE Level 1B
products. These are the actual

measurements done by the satellites. This
is the ranging information, the distance

between satellites, satellite orbits, star
camera data, and so on. You add a couple

of additional models for earth's gravity,
which you do not want to include in your

satellite gravity field, and then you do
your processing. This is done by a couple

of different groups JPL and GFZ, which is
a German research center for the

geosciences. CSR is the center for space
research at university Austin. These three

institutes also provides these GRACE Level
1B data. So they take the raw satellite

data, process it to theGRACE Level 1B
products, which are accessible for all

users, and then calculate further these
coefficients, C and S. But there are also

additional groups who provide gravity
fields who calculate these coefficients,

for example, Institute for Geodesy of the
University of Graz, or the Astronomical

Institute of the University of Bern. They
all have slightly different approaches to

topic and come to more or less the same
conclusions. There are countless papers,

comparing these different gravity fields
with each other, but the user usually

starts with the coefficients C and S, and
then it takes a formula like the one on

top of this slide and calculates your
gravity value or whatever you want. Now,

I'm talking about potential, I'm talking
about accelaration. These are not really

useful quantities in day to day life. If
someone told to you in Greenland gravity

decrease by 50 microGal, you have two
choices, you can say "wow, awesome" or you

can say "oh no, we're all gonna die" It's
a 50:50 chance you'd say the correct

thing. So we are looking for a more useful
representation of the changes in

gravity field. Now gravity field reflects
mass redistributions and the most dynamic

redistribution we have is water storage,
summer/winter, more snow, more rain, less

water in summer, so we express our gravity
change in a unit called equivalent water

height. This is the layer of water on the
surface with a thickness, equivalent to

the mass change measured with the
satellites. This is also easily

calculated. This is my last equation, I
promise, but this looks familiar. The

second half of this equation, is basically
the same we saw one slide prior and the

parameters in front of the sum is the
average density of earth, which is around

5500 kg/m^3. We need the density or water,
let's say it 1000 kg/m^3. And in this

fraction in the middle, we need to
parameter K, which are the socalled Love

numbers. Now, this is not a numerical
representation of mutual attraction, but

was put forward by, I think, Albert Love
in 1911, and they are parameters

concerning the elastic response of the
earth to forces. So, if you put a lot of

weight on a part of the earth, the earth
deforms and these parameters, describe the

elastic response of the earth to such
loading. Now we have calculated our

equivalent water height, let's say for two
months, let's say, in May 2002 and 15

years later in May 2017 and we just
subtract these two gravity fields, these

two equivalent waterheights, from these
two epoches. What we have left is the

change in gravity between these 2 epochs,
15 years apart, expressed in water layer

equivalent to the change in gravity
measured. And we can see a couple of

features here. There should not be any
seasonal variations because it's the same

month, just 15 years apart. So we see long
term gravity change between these two

epochs. And what we see is, for example,
mass loss in the northern and southern ice

shields, and we see two red blobs, one in
northern canada and one in northern

europe, which are geophysical processes.
So this is glacial isostastic adjustment

and during the last ice age the ice
shields deformed the earth downward.

The material in the "Mantel" had to flow
aside, and now that the ice is gone, the

lead is uplifting and the material in the
"Mantel" is flowing back. So it's flowing

back and the earth is uplifting. This
process has been going on for 10000 years

and will probably a couple of years
longer. Now how do you get your data?

Everyone can get the GRACE Level 1B data,
which are the observations by the

satellite, like again, ranging information
between the satelite, orbits,

accelerometer data, star camera data and
so on. You can get them without hurdles at

the ISDC, which is the information system,
a data center at the Geoforschungszentrum

Potsdam, or at the Physical Oceanography
Distributed Active Archive Center run by

JPL. And if you'd like, you can calculate
your own spherical harmonic coefficients

for gravity fields. Or you can compare for
example, satelite orbits they give you

with one you integrated yourself using
your own gravity field, to see if they fit

together or not. You can get gravity field
models, if you'd like. A large collection

is at the International Centre for Global
Earth Models. They have recent and

historic gravity models all in the same
data format. So you only need to implement

your software once from the 1970s to
today. They also have the proper

references, the papers you want to read to
work with them. These are socalled Level

2 Products. So, you can take a gravity
field from there, use the equation, I

showed you earlier and calculate your
equivalent water height, if you'd like.

If you don't want to do this, there is
someone to help you, a service called

"TELLUS", which is a play on words I
don't want to go into detail about. They

offer equivalent water heights calculated
for each monthly solution from the GRACE

satellites. This tells us a lot about the
earth, if you look closer into it. In the

following, I will use the monthly
solutions from the ITSGGRACE 2016,

provided by Institute for Geodesy at
University of Graz. The previous graph I

showed you was also created with that
gravity model. I will not go into detail

about further processing like filtering
and gravity reductions done to this, not

enough time. So here are some results,
let's start with the most obvious one, the

greenland ice shield, which has, as we saw
earlier, the greatest loss of mass

according to the gravity field and we see
here, a water layer on the whole landmass,

describing the loss of mass expressed as a
water layer of a certain thickness.

So let's say in the southern tip, you have
one meter water layer. This would be

equvalent in gravity to the actual mass
lost in Greenland. But we also see, that

the signal is not very localized. So it's
not bound to the land mass. It's also in

the ocean. This effect is called leakage.
If you do signal processing you will know

this. There are methods to reduce leakage.
My next slide will show such a result, but

I have done no reduction to this. So if
you use my formula I showed you, you will

pretty much get a result like this. This
gives you a trend of around 280 gigatons

per year in mass loss over the whole land
mass of greenland. And now gigatons is

also not very useful an expression. One
cubic meter of water has a weight of a

1000 kilos; one tonne, 1 gigatonne is
10^9 tonne, if you are familiar with ball

sports, 1 soccer field with the 140 km
high water column has the weight of

1 gigatonne, or if you are not fan of sports
ball, if you're more of a plane guy or girl

the A380800 has a maximum takeoff weight
of 575 tonne, so we need 1.7 mio of these

airplanes for one gigatonne. So this is a more
beautiful representation of the process in

greenland, done by NASA JPL. If you go to
the website of the GRACE project, they

have a couple of these illustrations, they
obviously worked hard on the leakage.

You can see localized where most of the
gravity, most of the mass is lost on the

left and on the right you see accumulated
over time, the mass which is lost, and

which trend it gives you. Also, if look
closely in the center of greenland, you

see black lines, these are the ice flow,
as determined by radar interferometry.

So now pretty much know where ice is lost,
where mass is lost. This goes into the

ocean, and this would be a good idea to
see, to check our GRACE results, the mass

we find missing on earth, so the melted
ice, and the additional mass in the ocean,

does this agree with other methods who
determine the sea level rise. One of these

methods is satellite radar altimetry, that
started in the 70's, but since 1991, we

have lots of dedicated satellite missions,
which only job is basically mapping the

global sea surface. So, they send down a
radar pulse, which is reflected at the sea

surface. They measure the run time and
then they have a geometric representation

of the global sea surface. Now, if we
compare this with the mass we calculated

or we got from the GRACE result, calculate
a sea level rise rise from this additional

mass in the ocean than these two systems
would not add up. The geometric sea level

rise is higher than just the additional
mass. So there is the second process which

is thermal expansion of the water. If
water gets warm it needs more space.

In 2000 the deployment of socalled ARGO
floats started. These are freefloating

devices in the ocean. Currently, there are
over 3000 and they measure temperature and

salinity between sea surface and a depth
of 2000 meters. These are globally

distributed. So, we have at least for the
upper layer of the ocean, how much thermal

expansion there is. And what we want to
see is, do these components of additional

mass in the ocean as determined by GRACE
and thermal expansion of the upper ocean

layer come to the same result as
geometrical measurements done by satellite

altimetry? On the left we see an image
taken from the last IPCC report on climate

change from 2013. In green we see the
sealevel rise as measured with satellite

altimetry in the time span 2005 to 2012
and in orange we see the combination of

additional mass, as measured by GRACE, and
thermal extension as determined with ARGO

inside the ocean. And these 2 graphs follow
each other quite well. On the right. We

see a recent publication by Chen, Wilson
and Tapley, the latter one being one of

the PIs of the GRACE mission, who
accumulated the data from 2005 to 2011. We

basically come to the same conclusion. So
now if you really don't want to do the

math, there are online services who make
the graphs for you. One of those is

EGSIEM European Gravity Service for
Improved Emergency Control. If we can

measure how much water is stored in a certain
area, we know that this amount of water

has sooner or later to be removed from
this area. This can be a flood, for

example, and with a mission like GRACE, we
can determine how much mass, how much

water is there and are the rivers large
enough to allow for this water to be

flowing away. That was the intention
behind this service. Oops, no, this is not

the future. So, I wanted to do the life
demo but. So, yeah, the live demo did not

work as expected. So, you will be greeted
with this graphic. You can plot for all

areas in the world. The first thing you
have to do is you change your gravity

functional, we want water heights. This is
what I talked about in this talk. Then you

want to look at the data set and at the
bottom you see a large list of GRACE

gravity fields. These are different
groups, I mentioned, providing these

monthly solutions. And so we choose one of
these groups. Then we choose an area which

we are interested in. You can freely
choose one area like here FinnoScandia,

or you can use predetermined areas, for
example, the Amazon river basin or Elbe

river or something like that. These areas
all over the world and you can see the

gravity change in this area. So let's look
here at FinnoScandia, and then you are

greeted with a plot like this. This is
equivalent water height, even though this

is a geophysical process. So we see here
the layer of water, which would have been

added to the region as selected, and we
see a clear trend upward. Again, this is a

geophysical process. This is not
additional ice or water or anything. Can I

return to my...? No, I cannot. So, yeah,
live demo did not work. If you want to do

this yourself. I have uploaded to the
Fahrplan all my resources, all my links.

And the EGSIEM page also includes the
description of what is done in the backend

and were the data comes from and what you
can see in the various fields. Now I want

to give a last impression on the future,
because unfortunately while I was

preparing my abstract for this conference,
one of the GRACE satellites was turned off

due to age. It was launched in 2002,
planned for a five mission year; it

survived 15 years, which is quite good,
but now we have no more ranging

information between these satellites. We
had ranging information in micrometer

accuracy, a couple of micrometer, and now
we cannot rely on these information

anymore. And this means mo more gravity
fields with high spatial resolution, and

I'm not sure about the temporal
resolution. So, the current work which is

done is taking all satellites which are in
the lowenough orbits and calculate the

gravity field from their positions,
because everything which is in lowearth

orbit is affected by the Earth's gravity
field. So, if I take the satellite orbits,

look "how does this orbit change" and the
reason is gravity, then I can calculate

the gravity field. Unfortunately, not in
this higher resolution we are used to.

And... But fortunately, there already is a
nextgeneration gravity field mission on

its way. It arrived last week in the US,
where it will be launched in late March,

early April by SpaceX. You might look at
this image and think, "I just saw this

earlier" and you are quite correct: The
mission called "Grace Follow On" is a copy

of Grace, which improved components, of
course, and now with lasers. We see not

only the microwave ranging between the two
satellites, but additionally a laser

interferometer. So, from micrometer
accuracy in the distance measurements we

go to nanometer accuracy, hopefully. But
the main instrument will be the

microwave ranging. So, in conclusion,

I hope I showed you that the gravity field
can show mass transport on the surface and

inside the Earth; that this offers, in
combination with other methods, new

insights and also some kind of mutual
verification. If several different types

of observations coming to the same
conclusion, none of them can be awfully

wrong; and that the access to these
methods are relatively easy: the data is

available, all the methods are described
in geodesy textbooks and the technical

documentation; and there are other
applications, other than, let's say,

climate change; you can look into drought
and flood prediction; the El Niño–Southern

Oscillation you can predict from Grace's
gravity field data. So, lot's of work to do.

So, this would be the end for my talk.
I thank you for your interest in the topic.

applause

Herald: Thank you, Manuel, for the talk.
And I think we have time for one or two,

maybe two very short questions. Please be
seated during the Q&A session. Is there

some questions? Okay, microphone 3,
please.

Mic 3: Yeah, hi. In a quiet voice
Hi, hello? Can you hear me? Now loud

Herald: Yeah.
Mic 3: Okay. Hey. So, my question is

regarding acceleration. What's the
influence of Earth atmosphere and all the

planetary bodies, like the moon, and does
it need to be accounted for?

Manuel: The external gravity needs to be
accounted for, so the tidal effects of sun

and moon would be one of those additional
models you put into the processing of the

satellite data. The Earth's atmosphere has
an effect on the satellites themselves,

which is measured onboard by
accelerometers and then reduced. And the

gravitational effect of the atmosphere:
Part of this is averaged out, because we

take a month of time series, and the rest
are also inclu... provide as extra

products; at least by the Institute for
Geodesy in Graz. So atmosphere... the mass

of the atmosphere is... has to be
accounted for, yes.

Herald: Okay. Microphone 2 has vanished
all of a sudden. Then, microphone 1,

please.
Mic 1: Hi. Is it possible to measure

changes in the temperature of the oceans
or of the ocean streams, like... Can you

see if El Niño is active by just measuring
the gravity... change in gravity fields?

Manuel: As a precursor tool, El Niño, as I
understand it... certain regions of the

ocean get warmer; it's a density change;
and, of course, this would be measured as

part of ARGO and it's also in the GRACE
gravity field. There are probably papers

on it. So, the last... the extend of the
last El Niño was predicted by GRACE. I

don't know to what extend this was
correct, but...

Mic 1: Okay, then.
Herald: Good. Then, that's all the time we

have. A big round of applause for Manuel
and his talk, please.

Applause

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