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Hello and welcome back to the second video 
about GROOPS. In this video I want to show  

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you the workflow of creating a 
script: how do you add programs,  

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how do you get data from one program to the 
next and so on. In the end you will have  

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a script that does multiple things to achieve 
some goal you want. For this example, we want to  

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do a small specific task. We want to simulate 
an orbit for a low Earth orbiting satellite.  

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We then want to visualize the tidal 
accelerations acting on the satellite  

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throughout its orbit and we want to plot this 
in a map. So it's a relatively simple task,  

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which can be done with a few programs in 
GROOPS. I've already created a script file here,  

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it's empty for now except for some global 
variables, which are part of our template.  

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And, yeah, so let's start. First of all we 
want to add a program. You can just hit the  

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"Add" button here and select one. The first 
program we want to add is "SimulateOrbit".  

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We have a filter here, so we type in 
"simulate" and we get "SimulateOrbit".  

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So this is our first program. Our program has 
several settings or input and output fields.  

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Here, everything marked with a red star indicates 
that this is a must-set field. Everything else  

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is optional to some degree or another. Some 
of them have default values, others do not.  

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But "outputfileOrbit" is a must-set value so 
we need to provide a file where the output is  

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written to. So the principle of GROOPS and GROOPS 
programs is that programs possibly read data but  

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they'll usually always write data to some file and 
the next program then can read this data. So let's  

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start with writing our orbit. We just write it 
into a file called "orbit.txt" in the folder where  

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the script is located. "inputfileSatelliteModel": 
This is a model of the satellite, which describes  

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its mass and geometry and so on. So as you can 
see here, the "groopsDataDir" variable is used  

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in the curly braces – and curly braces, 
I'll talk about this more in the next video  

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– this variable will be replaced by 
the global variable "groopsDataDir".  

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So I can select any satellite model. For this 
video let's just use the one from GRACE-1.  

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And, as you can see, in the path the beginning is 
still replaced by the "groopsDataDir" variable.  

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Next, we need a time series, so we need to provide 
the epochs for which the orbit is simulated. As  

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you can see here, this is both mandatory 
and there are ellipses here. This means I  

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could provide multiple fields. So "timeSeries": 
We have several options to choose from. If we  

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want a "uniformSampling", "uniformInterval", if we 
want a "yearly" time series or whatever, or from  

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an instrument file. We will stick with a 
uniform sampling. We will say "timeStart":  

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Here we can choose our global variables, 
"timeStart" and "timeEnd". We haven't set them yet  

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but we can do it, so let's say we want to simulate 
our orbit for 2020-01-01 and for "timeEnd" let's  

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say we want to do one day. We could enter the 
date again but what we can also do is just select  

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"timeStart" and add +1. The time fields are always 
given in modified Julian date (MJD), so it's in  

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float days. We can just select "timeStart+1" 
and GROOPS will know that this "timeStart" is  

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the global variable "timeStart". As you can 
see, in the GUI the date already gets parsed, so  

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we know it is correct. And as you can see here, 
where we link to these global variables, also  

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the dates are already correct. For the sampling 
let's say we want to do a one-minute sampling.  

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Here, "sampling" is also in days. This 
is why it is entered as 1/1440. Okay,  

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so we could add another "timeSeries", but this is 
fine. We want to use these epochs. So this class  

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"timeSeries" will create epochs from "timeStart" 
to "timeEnd" with a one-minute sampling.  

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Okay, "integrationConstants": We can use Kepler 
parameters or "positionAndVelocity". Let's stick  

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with "kepler". Let's say we want to have a 
semi-major axis of around 7 000 kilometers,  

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so about 700 kilometers above Earth's surface. 
"eccentricity": Let's say 0. "inclination":  

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let's say 85 degrees. And for the purpose of 
this video, everything else: Let's set it to 0.  

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But, of course, you could specify 
whatever you need here. Okay, next,  

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"orbitPropagator": We'll just stick with 
"polynomial". There are several others:  

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"euler", "rungeKutta" and so on. Let's stick 
with the default one. Same with Earth rotation,  

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there the default EOP rapid time series from 
the IERS is selected. We are fine with that.  

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"ephemerides": also fine with JPL. What we then 
finally need to provide are our forces that act  

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on the satellite. First of all, we need a gravity 
field. So we can select "potentialCoefficients",  

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and let's use the GOCO06s gravity field. It's just 
a .gfc file which you can download, for example,  

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from ICGEM. "maxDegree": Let's set it to 60. 
So this is the spherical harmonics degree.  

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For this example it doesn't really matter. 
Let's keep it like that. For "tides": Let's add  

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"astronomicalTides", so from planets and 
so on. Let's add "earthTide". The default  

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settings are fine in this case. And let's also 
add "doodsonHarmonicTide", so ocean tides. As  

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you can see, by default, for ocean tides FES2014b 
is already selected. We are fine with that,  

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we just stick to the default settings. And what 
we also going to add are "miscAccelerations",  

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in this case "solarRadiationPressure". This is 
why we need the satellite mode,l which describes  

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the surfaces of the satellite and their optical 
properties, because we want to also simulate  

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"miscAccelerations" in this example, 
so solar radiation pressure.  

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Here, all the settings are fine as well. Okay, 
so this is the first program. This program will  

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integrate an orbit based on the settings we chose 
and it will write the output to "orbit.txt".  

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Next, what we also need is 
the attitude of the satellite.

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We can use a program called "SimulateStarCamera" 
to simulate a basic star camera which observes  

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the attitude. And this program just needs as an 
input file an orbit file and as an output file...  

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let's call the output file "starCamera.txt". 
So it will take each epoch from this orbit  

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and compute the associated star camera file.

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As you can see here in the description, the 
axes of this body frame, which is simulated,  

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are x = along track, y = cross track, and z 
in the radial direction. We're fine with that.  

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Maybe a different satellite needs a different 
system, but in this case we're fine with that.  

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Next, let's stimulate an accelerometer. So this 
is what we wanted to do. We want to visualize  

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the tidal accelerations acting on our satellite. 
So we, again, need an output file, let's call it  

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"accelerometer.txt". We need, again, a satellite 
model. It should be the same as here, so we can  

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just copy-paste it. We need an "inputfileOrbit": 
We can use our orbit. We need the star camera.  

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We can also provide here "earthRotation" and, 
again, it's fine. "ephemerides" are again fine.  

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And for the "forces": This is now which forces do 
we want to simulate, only for this accelerometer.  

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Here we can just use the ocean tides because we're 
only interested in visualizing the ocean tides.  

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In this case, we will only copy the ocean tides, 
the "doodsonHarmonicTide" element from above.  

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And, yeah, that's it, So 
this program will generate  

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or will simulate accelerometer measurements 
from tidal accelerations for the orbit  

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we simulated using the star camera we simulated.

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Now we have all the data we need. We just need 
to visualize it, and we want to do it in a map  

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in the form of a ground track. So we can 
use the program "Orbit2GroundTracks",  

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which does exactly what it says. It needs an 
"inputfileOrbit": Let's, again, copy it from here.  

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And it outputs a gridded data file. So it converts 
data to a grid, which we can plot on a map. Let's  

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just call it "grid.txt". Earth rotation, again, 
is fine. So this program, like this, it only  

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converts the orbit to a ground track. But since 
we also want to visualize our accelerometer data,  

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we can add an additional "inputfileInstrument". 
You can see here, this is an optional file  

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and it has the ellipses, so we can add one or more 
files. So I can add this. We could add additional  

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files. And the values from this accelerometer 
file are going to get appended to the grid file.  

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So it adds the columns from this file to this grid 
file. Okay, so far so good. The last program is  

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"PlotMap". We want to visualize our data 
on a map. Let's call it "groundtrack.png".

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We will skip the title and now we can set 
different "layers" here. First of all,  

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so we have a reference, let's add a "coast" layer 
so we can see, in the background, the continents.  

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I don't particularly like lines, I prefer 
to have a colored ocean. We can say  

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the ocean should have the color light 
blue. "landColor": We will skip it. It will  

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be white or transparent. So this is our background 
layer and then we can add a different layer. This  

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is the same principle as any graphics program, 
like Photoshop and so on. You just add layers  

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and they will get visualized on top of each 
other. Our next layer: We want to visualize  

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our ground tracks. In this case we want to 
use the layer "points". We have one position  

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each minute, so we will visualize them 
as points. As you can see here, we need  

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an "inputfileGriddedData", which is the one we 
created here, you know, with "Orbit2GroundTracks".  

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As a symbol we will use a circle and the color of 
the circle should be determined from the value.  

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So if you set it to "<none>" – you can, of course, 
choose any color – but if you set it to "<none>",  

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the color of the circle will be determined from  

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the value itself. As long as you set a color 
bar, which is enabled by default. And for "data":  

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You can see here on the right it says "input 
columns are data0, data1, ..." one so.  

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This grid file contains coordinates and then the 
additional columns for the accelerometer data.  

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I'll show you later how this file looks 
like, but for now let's say we want to  

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visualize the total acceleration. So we 
can say "sqrt(data0^2+data1^2+data2^2)".

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So this is an equation. GROOPS can parse equations 
and can parse mathematical expressions in most of  

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the input fields. We have three columns for 
x, y, and z, the accelerations along x, y,  

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and z axes. And we just want to compute the 
total. Something else we want to do here is,  

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the values in "SimulateAccelerometer", 
they get output in m/s^2,  

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so it's SI units. Since these will be very small 
values, we want to visualize them in nm/s^2.  

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So we can just, here, multiply times one trillion. 
We will convert the values to nm/s^2. You have to  

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know that here in GROOPS, usually in the Plot 
programs, you have to do this in the values,  

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because then, later, in the color bar, we 
could define our "min" and "max" in nm/s^2.  

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But we will skip the "min" and "max", we 
just add a label "Acceleration [nm/s^2]".  

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Okay, that's pretty much it. We will leave all the 
other settings to our default values. "projection"  

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is fine as a Robinson projection and, yeah,... 
let's see if it runs. One thing I forgot is  

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I go into "options" and set "transparency" 
to "no", since I'm using a black background  

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in my file explorer. Okay, now let's run it.  

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Everything is fine. As you can see, a command 
window opens. The orbit is simulated – one minute  

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sampling – tidal model can take a few seconds. 
It's done. Then, next is "SimulateStarCamera",  

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which finished instantly. The next 
is "SimulateAccelerometer" and then  

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"Orbit2Groundtracks" and "PlotMap", and it's 
already done. So we close it and go to our

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directory. We can see there is now a ground track 
plot created and it is basically what I expected.  

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We can see a map in the background with light blue 
oceans and each point is a point along the orbit.  

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As we had 85 degrees of inclination it's almost 
a polar orbit. It's pretty close to Earth so we  

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have several revolutions for this one day. And 
tidal accelerations are, of course, largest  

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over the oceans, especially close to 
the coast. So this is what I expected.  

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Okay, that's it for this video. 
In the next video we'll talk about  

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the parser in GROOPS, how you can parse 
global variables and other variables  

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and use them in your scripts. Thanks, again, 
for watching and see you in the next video!