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Hello and welcome to the 
third video of this series.  

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In this video, I want to talk about the 
GROOPS parser and how it can make your life  

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easier and how it can make your GROOPS scripts 
more flexible. I've prepared a short script. I'll  

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talk about it in a second, but the purpose of the 
script is to compare kinematic orbits to dynamic  

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orbits of a satellite and visualize this in a 
plot. The result should look something like this.  

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So we have along, cross, and radial axes. The 
kinematic orbits, which are noisy, are compared  

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to a dynamic orbit, which is relatively 
smooth. For this we have some data. We  

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have kinematic orbits and reduced-dynamic orbits 
for four different satellites: GRACE-1, GRACE-2,  

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Sentinel-1A, and TerraSAR-Xe, each for one week. 
Both kinematic and reduced-dynamic orbits. So if  

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we start with our script – I'll go quickly through 
it – it's very simple. We do not have any global  

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variables so far. First of all, we remove a "tmp" 
directory in case it already exists. The next step  

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is to recreate this "tmp" directory and also 
create the "plots" directory. And then the  

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first real step in the script is to synchronize 
our kinematic orbit and our dynamic orbit so we  

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ensure that both contain the same epochs, which 
is necessary because the kinematic orbit might  

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contain some gaps while the reduced dynamic orbit 
does not contain gaps. And as you can see here,  

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I've selected GRACE-1 for 2015-01-01 and the same 
here, I have selected the reduced dynamic orbit.  

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Both orbits are then written to their 
respective files which only contain synchronized  

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epochs, so epochs that are part of both files. 
The next step is "InstrumentMultiplyAdd", where  

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the two temporary files we just wrote  

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– the dynamic orbit is subtracted from the 
kinematic orbit – and the result is written to  

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"orbitDiff.txt". Finally, we want to visualize 
the orbit in a local orbit system so we simulate  

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a star camera file based on the dynamic 
orbit and we rotate the differences from the  

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celestial reference frame to the local orbit 
reference frame. And finally, we plot the result.  

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We have three layers, one for each each 
component, so along, cross, and radial.  

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As you can see, we read the orbit file here and 
"valueX" is "data0", which is the modified Julian  

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date (MJD). The value of "data1" is the first 
value, which is the along-track value. We multiply  

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times 100 to convert it to centimeters 
and add five centimeters to shift it  

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in the plot. That's basically it. We also said 
as the "axisX" we use a "time" axis where we can  

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define the tick spacing and so on. At the end, we 
end up with this plot, where we have along shifted  

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by +5, radial shifted by -5, and the values in 
centimeters, and a time axis. So far so good.  

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As you could see, I just used here the orbits 
from GRACE-1. So now what if we wanted to  

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visualize the orbit for GRACE-2? We could, of 
course, change it here to 2 and also here to 2  

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and run the script and it would work fine. 
But that's already a bit inconvenient. So  

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what we can do in GROOPS is we can create a 
global variable – let's call it "satellite"  

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– we will give it a type "string" and

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just define it as "GRACE-2". And instead of 
having GRACE-2 here, we can now replace this with  

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the variable "{satellite}". Same here.

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And now, when I run the script, "{satellite}" will 
be replaced by "GRACE-2". Another location where  

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we can put it is, for example, in the title. We 
can say, okay, we're going to have the name of  

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the satellite in the title and also, maybe, in 
the file name. So let's say the file name should  

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be "{satellite}_orbitDiff.png". 
And if we now run this...

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Goes by pretty quickly. And now we have a 
file not only called "orbitDiff.png" but  

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"GRACE-2_orbitDiff.png" and also it contains 
GRACE-2 in there and, as you can see,  

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it's different to GRACE-1. The original one 
was GRACE-1. So it's already pretty convenient.  

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As I mentioned in the beginning, we also have data 
for one week but here it's basically hard-coded to  

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use 2015-01-01. We can also circumvent this 
by creating a global variable – let's call  

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it "time" and it's from type "time" – and we can 
just say, okay, we want to visualize 2015-01-02,  

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let's say. As you can see, the modified Julian 
date (MJD) value is this. And I can now replace  

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the date here with the variable "time", but you 
have to be careful here. If I just use "{time}",  

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it would use "57024" as the value, because that's 
the value which represents the time. But of course  

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the file name does not contain the modified 
Julian date but it contains the actual date.  

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Here we can use another feature of our 
parser: We can specify a format. We could do

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"{time:%Y-%m-%d}" or as an 
alternative we can just use  

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"{time:%D}" for date, which will parse 
the date in the ISO format, as shown here.  

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We can do the same here. We can replace it and, 
again, this will be – when I run the script – this  

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will be replaced. What you can also do is add the 
date to the file name and if I now run this...

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I should have another plot – exactly 
– I have now this plot which is called  

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"GRACE-2_orbitDiff_2015-01-02.png". And as 
you can see, again, this is different than  

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2015-01-01 we plotted before. Now it's 
easy for me to just plot, for example,

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"TerraSAR-X" instead of "GRACE-" and 
2015-01-07. So if I run this now...

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Done, and now I have a figure or plot 
"TerraSAR-X_orbitDiff_2015-01-07.png". So,  

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yeah, that's pretty handy! Another thing I want 
to show you is – let's add another layer – and  

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we'll use the same input file "orbitDiff.txt",  

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but instead of – what we now want to do is we 
want to highlight outliers in our time series.  

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Let's define an outlier by the total 
difference being larger than five centimeters.  

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So I can first compute the total difference 
which would be "sqrt(data1^2+data2^2+data3^2)".

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And let's say this should be – or, rather, let's 
now add a condition, so you can write if this  

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distance is larger – let's multiply it by 
100, so let's do it in centimeters – if  

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this is larger than five centimeters, then 
at a "valueY" of 10 – or then use a "valueY"  

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of 10, otherwise use "nan" (not a 
number). And what we want to happen is  

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we want to have a symbol – let's pick a black 
diamond – at the value of 10 on the Y-axis if  

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the total distance between the kinematic and 
the reduced dynamic orbit is larger than 5  

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centimeters. Otherwise we use the value "nan", 
which is this function. So what you can see here  

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is that the parser can interpret mathematical 
expressions. There are quite a few of them:  

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from square root to other mathematical 
operations, comparisons, and also  

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specific functions. We can take a look at the 
GROOPS documentation. If you go to Parser,  

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there's a list of the mathematical expressions 
that are supported: logical expressions, functions  

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with multiple arguments and so on. And there's 
also the text parser, you can see we earlier used  

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"%Y-%m-%d" but also "%D" for the date. 
So here, there's an explanation of  

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these parser options you have. Okay, and if we now 
run it again, we should hopefully end up with –  

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yeah, see, so now in our TerraSAR-X figure we 
have black diamonds at the locations where the  

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total difference between the kinematic orbit 
and the reduced-dynamic orbit is larger than 5  

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centimeters. Of course, there are many more 
options you can use or many more functions you  

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can use with the GROOPS parser, but in principle 
it's super handy to simplify the script, make it  

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more flexible, use the same script for different 
satellites, for different dates and so on.  

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In the next video we will use this 
because we will then extend our script  

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to include loops and conditions and there, 
again, these expressions and the parser will  

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be very convenient. So until the 
next video, see you. Bye bye!