35C3 - Space Ops 101

35c3 Preroll music

Herald: It's an honor to introduce you.
Sven Prüfer who is a professional in the

space business and he's going to give you
a introduction to spacecraft control under

the title of space ops 101.

Applause

Sven: OK. Thank you very much for the kind
introduction. Hello and welcome to space

ops 101. My name is Sven Prüfer, I'm a
mission planning engineer at the German

Space Operations Center which is a part of
the Deutsches Zentrum für Luft und

Raumfahrt and I will give you a slightly
biased introduction to spacecraft control.

It's slightly biased because first of all
I'm working for a particular space agency.

And secondly, because we will look at the
whole thing kind of through the lens of a

mission planning engineering.
Unfortunately the topic is pretty, well..

large so we won't be able to talk about
everything. In particular, we will not

talk about launches, launches are pretty
amazing. I'd love to see one in real life

but we can't really go into that much
detail because that's a very specific and

particular topic. Also we will not talk
much about human spaceflight and neither

about entry descent landing. So for
example, landing on another planet. Of

course the combination of human
spaceflight and landing on another planet

would be very cool to see. But I cant just
talk about it right now. Okay. So instead

we will deal with one of the main segments
of mission operations. So in general you

distinguish three parts. There's one. The
space segments. So this is everything that

actually flies up into space. Some
particular satellite or spacecraft

including its payload, so whatever it is
doing up there. Then there is the transfer

segment which is, well, the launching
business. And then thirdly there is the

ground segment. So we will talk among
mostly about the Ground segment. So this

is everything that actually takes place on
earth in order to command or use the

spacecraft in space. OK. The Ground
segment itself again splits into various

subsystems. So, one of them is the the the
main player when you want to actually talk

to your spacecraft. Those are the ground
stations. OK. So we will definitely need

to talk about those. Secondly we need to
actually know where our spacecraft is and

where it's going. This is actually done or
described by the flight dynamics. Thirdly

space is at the same time very cold and
also very hot. So there's the power and

thermal subsystem. Then there is attitude
and orbit control which are responsible for

telling the spacecraft where it should
look at and for actually figuring out how

it is oriented. Next we need to actually
talk to the spacecraft. This includes

interpreting, well, receiving and
interpreting the data. So this is part of

the TM/TC subsystem or the data system.
And last but not least that's of course

the most important subsystem. That's the
mission planning which is responsible for

scheduling spacecraft activities. OK, so
the talk will kind of follow the lifecycle

of a spacecraft. We will start with the
launch and early operations phase which is

called LEOP for short and that we will
need to talk about orbits and flight

dynamics as well as how to actually
communicate with the spacecraft. After

that we will talk about how we can, well,
test and validate our spacecraft very

quickly and then we will switch to the
routine phase, so when we do the actual

operations for what, of whatever the
spacecraft was designed to do. This

includes data analysis, telemetry and Tele-
commands, so TM/TC and also the mission

planning. And then in the end we will talk
about, well, the end of the mission. So

whatever we are going to do at the end
when we want to dispose of the satellite.

All right. So everything starts with the
launch. Well not quite. Of course before

that we have a pretty lengthy phase of
preparations. I will not actually talk

about this but this might take about
something like two years in advance of the

launch in order to prepare everything to
make sure that everything is running

smoothly. Once the spacecraft is strapped
onto the rocket it will get, well, flown

into space and there it will be separated
from the launch vehicle. From this moment

on then, it's flying by itself. And we
need to actually control it. However we

don't really know right now where the
launch provider will put our spacecraft. It

might actually be on its final orbit. And
so for example if it's a rather low orbit

or it might be a transfer orbit to its
final target orbit if it's actually

further up. Once this is - during this
launch there's actually a second control

center. That's the one for the spacecraft.
This is actually the control room K1 in

the German Space Operations Center. And it
kind of looks like you expect a control

room to look. So in particular, there are
many screens on average everybody has like

four screens. There are large ones for
showing an overview of what's going to

happen. And there are many small yellow
signs. These yellow signs denote the

various positions of the operators and the
engineers. At the back in the center there

is one position that's called the flight
director. The flight director is the

person who is in charge of the operation.
So whenever there's something that needs

to be confirmed, needs to be done, that
needs to be decided and he is the last

operational person to actually confirm the
decision. Now in principle right after the

spacecraft is separated from the rocket.
This control room actually takes over.

However there are few subtleties here. In
particular right after separation the

spacecraft is somewhere we kind of know
approximately where it is because we

planned this beforehand but we don't know
the precise position. We first have to

acquire a signal we have to find it in
space and have to set up a connection in

order to understand this we need to talk a
little bit about orbital mechanics. So

first of all why does a spacecraft not
fall down. Well if you look at the ISS, so

the International Space Station, it flies
at an altitude of about 300 to 400

kilometers where the gravitational force
of the Earth is still about 90 percent of

the one at ground. This means that you
really need some horizontal speed in order

to not fall down to earth. So you need to
go really fast. 7.9 km/s is the speed that

you actually need in order to not fall
down on the ground. So if you're a bit

higher and some orbit then you need a bit
less speed actually. OK because you're

farther away from the from the earth. OK.
So we need to go very fast. Good thing to

know. Secondly we need to know at which
distances we will actually be flying our

spacecraft. So this is Earth obviously, in
particular the following picture will

actually be to scale approximately. So one
thing one possible place where you can put

your spacecraft is low earth orbit. So
that's the region below about 2000 km

altitude above ground. However
2000 are pretty high so very common are

altitudes of 600 kilometres, 500km, 700km.
This is a place where you mostly do

scientific experiments, in particular Earth
observation. OK. So there are many many

satellites, science scientific satellites
that actually try to take pictures at

various frequencies of the earth and also
this is the place where you do

reconnaissance. OK. Then they're actually
a bit higher altitudes for example, this

is medium Earth orbit. So the the drawn
circle is actually at an altitude of 20000

kilometers. And this is mainly used for
navigational satellites. So think GPS or

Galileo, the European version. And then
there's another very common type of orbit

that's the geostationary orbit. This is at
an altitude of about or pretty much precisely

35786 km above ground. This is chosen in
such a way that the orbital period, so the

the time it takes it to fly once around
the Earth, is 24 hours. This has the

advantage that the movement of the
satellite actually sync up with the, are

synchronized with the rotation of the
earth, meaning that's your satellite is

kind of always at the same position as
when seen from Earth. This is a particular

important for TV satellites because well
imagine you would have to actually move

around your TV satellite dish all the time
just because the satellite is moving.

Instead you only have to fix it once and
then it's pointing in the right direction.

OK. And this is also very common place for
communications satellites for the same

reason because we actually want to have a
fixed position in which we have to look.

OK. In order to get there for someone
geostationary orbit it's possible that the

launch provider will actually put us in
some kind of transfer orbit. Yeah. They

usually then don't look like circles but
rather like ellipses and and that in such

a case we would need to do additional
maneuvers. So we are on the red circle. We

will fly outwards but at some point we'll
touch the geostationary orbit, so the black

one. But in order to not well kind of fly
back to Earth we will have to accelerate.

So this is a maneuver that we have to
execute somewhat at the beginning of the

mission in order to reach our
geostationary orbit. OK. So the the system

that actually deals with these
considerations, calculations, Procedura,

that's the flight dynamics department. So
their tasks are in particular orbit

determination. There are various ways to
do this for example. Very often you can

actually ask the satellite where it is
because it has GPS onboard at least

if it's a LEO, so satellite in low earth
orbit so it actually knows where it is or

you can do ranging which we'll talk about
in a few seconds. And from this you can

calculate the orbit. Once you have the
orbit you also want to know where the

satellite is going to be located in the
future. So you will do orbit propagation.

Next thing, well, we have to we might have
to execute some maneuver to actually stay

where we want to be or to get where we
want to be. So we need to calculate which

direction we have to thrust, we have to
turn on our thrusters for how long. This

is also done by flight dynamics. And the
fourth point is, well we have to talk to

the satellite. So we actually need to see
it, in order to do this. And flight

dynamics can actually calculate the times
and the positions or the directions rather

where the satellite's going to be. And you
can see all of these tasks are pretty

numerical nature its really hard core
mathematics numerics. Meaning that you

actually want to use some tools that are
very well battle tested so to speak and

well one of the most common programming
languages for numerical caluclations is of

course Fortran. OK, so that's really a
place where Fortran is still being used,

actively being used because these
libraries are just working the way they're

supposed to work. So nobody really wants
to switch from there because they're just

very good. OK. Now let's go back to the
control room. We have talked to our Flight

Dynamics department who have told us:
Well, the satellite's going to be at a

certain position, at a certain time. Or at
least that's where we expect it. So the

next thing we need to do, is we need to
establish a connection to the satellite.

And for this we need a ground station. The
picture you see here is actually the

ground station in Weilheim. That's in
Bavaria. That's sort of the main ground

station that we use. And well, it knows
where to expect the satellite. So at a

certain direction it should appear, at a
certain time above the horizon and then it

tries to establish a contact. This first
acquisition, as it is called, is the first

contact of the spacecraft after the
separation. And this is of course a

crucial moment. Now, once it has
established a connection, it tries to do

various things. First of all it needs to
downlink some data. So download, but it's

called downlink. This includes telemetry,
so descriptions of the state of the

spacecraft because want to make sure that
the spacecraft is actually still working

after the launch. And then later this also
includes downlink of payload data, for

example. So think pictures, or whatever it
was that the satellite was supposed to

measure or to take. And then it will also
uplink some stuff. So, for example

commands because want to tell the
satellite to do something. But this might

also include for example software updates.
And one other thing that the ground

station can do, is ranging. Ranging means
that you sent a package or a packet to the

satellite from the ground station. This
travels with the speed of light. Then the

satellite will actually reply to that
signal, to that packet, and then the

answer will fly. well, will go back to the
ground station. And if you measure the

time, and if you know how long the
satellite takes to actually react to such

a package, you can calculate the distance
from the ground station. If you do this

several times then you get kind of like a
radio distance profile. And from this you

can really deduce the orbit of the
satellite. OK. So let's look again to

Earth. There's a ground station. It's
actually located at the North Pole here.

So that's on top. And there's a satellite.
The satellite is not to scale, just in

case you were wondering. And it's actually
flying on an orbit which is 600 kilometers

above the ground. This is actually to
scale. Now the signals of the ground

station, they actually have to pass
through the atmosphere, meaning they're

attenuated quite a bit. So you have a
finite range of the ground station's

signal, and this is drawn here. So
the red circle is an approximate range of

the ground station and this intersects the
orbit of the satellite only at a certain

time interval, or a certain interval of
the orbit. In particular, we can look at

some numbers here. If you have a satellite
at 600 kilometers altitude, you get a 90

minutes period, approximately 90 minutes
period, around the Earth. And the portion

of the orbit that you actually see the
satellite from 1 given ground station is

10 minutes long. OK. So this means, we
would expect to see the satellite every 90

minutes for 10 minutes. OK? And this is
when we have to do all the downlink and

uplink. Unfortunately, it's a bit more
complicated because Earth actually

rotates. This map of Earth actually shows
the ground track of the satellite, so

that's the projection of the satellite
onto the ground. So that's the red line.

And the problem is that after 90 minutes
the satellite returns to the position

where it was before. However, Earth has
actually rotated by some amount, like 90

minutes divided over 24 hours. This is why
the ground tracks actually don't close

up. So instead, you get these kinds of
stripes. Over Europe you see WHM, that's

the Weilheim station. It has a certain
range. That's the circle-like black line.

And you can see that usually you have two
contacts with the satellite per rotation.

So the third pass will already be outside
of the range of the ground station. OK. So

we actually have even less contacts than
what I said earlier. This picture actually

shows the same situation from the top, so
from North Pole. You can see that actually

they are circles. So all the distortions
that you've seen on the earlier slide, was

due to the projection that was used for
the map. So this is sort of what it

actually looks like if you look from above
the earth. But the other one is the

typical maps that you see. OK. So now we
have found our spacecraft. We want to talk

to it. So we need to actually send a
signal there. Now let's think about which

kind of frequencies we might use for this
communication. Well, first of all, we

noticed that there is for example water
vapor in the atmosphere which absorbs

parts of the electromagnetic spectrum. So
for example here, at around 23 gigahertz,

there is an absorption peak due to water,
and the higher frequencies we use, the

more actually gets absorbed. This means
that we kind of want to restrict our

frequency usage to actually lower
frequencies, in order to get a higher

range. But then we also have, well, maybe
less data rate. So in spacecrafts, you

usually use actually the lower part of the
graph that's shown here. Usually, even

below what is shown at all. So this starts
at 10 gigahertz, and you use even less

frequencies or lower frequencies. For
example, you might use UHF, so amateur

radio at 430 megahertz, you might use
L-Band, 1 to 2 gigahertz, in particular the

main carrier frequency of GPS satellites
is in this range. OK? Then there is

S-Band. So that's a very typical frequency
range from 2 to 4 gigahertz which is used

for the actual commanding of the
satellite. So this is an important

frequency for us, or band for us. Then
there's also the X-Band. So X-Band is

higher frequency, so we expect even higher
data rates. And this is usually then used

for payloads. OK? So, if you have a lot of
data that you want to downlink, for

example a picture that you just took from
your satellite. Also, this is being used for

deep space missions. Then there is Ku-
Band. Ku-Band is used for TV satellites.

And Ka-Band. So this is now slightly above
the local water vapor absorption maximum.

So this is pretty cool. There, you really
have high data rates. It's been used for

various applications, whenever you need to
a high data rate. However, there are some

mechanical difficulties because you have a
directional antenna, so this is slightly

non-trivial but it's being used more and
more often. Now if you fix such a

frequency, and you talk to the satellite,
you of course need to modulate some signal

on top of that. You need some protocols
which do some level of error correction

etc. I will not talk about this but in
principle there are very specific

standards for space that are being used in
order to assure that signals that you send

or that you receive actually get received.
OK! So, we can now talk to the satellite,

we have acquired a signal. So we switch
back to the control room. In the control

room, we are now very happy. So we have
done the first acquisition, this is

actually when people hear applause. And
then afterwards, there are a few things

that are left to do. Actually, now the
work starts. So for example, the satellite

was actually running on battery during the
launch and afterwards. But it needs of

course some new power. So for this you
need to deploy solar panels. This is done

during the LEOP. Also, you might need to
deploy antennas. I showed you various

frequency bands, and usually satellites
actually have several antennas and use

several bands for different tasks. So the
commanding might be done on the S-Band but

the actual downlink of the payload data
might be on X-Band. For this you need an

additional antenna. So this needs to be
deployed. Also, this is the time when you

do all the other maneuvers in order to
reach your final orbit, and you start

switching on other components of the
spacecraft. This might include for

example, star trackers. Star trackers are
essentially cameras that just take

pictures of the sky, so the stars, and
they compare them to some onboard database

of known star positions. And this way, the
camera can figure out which direction it

is looking. If you know how the star
tracker is actually mounted on a

spacecraft, you can then deduce how the
spacecraft is oriented. This is important

for example if you want to take a picture,
then of course you need to know, where

have you been actually looking at? So you
need something like a star tracker.

Another thing that you would kind of
switch on, or actually spin up during a

LEOP, would be a reaction wheel. So
reaction wheels are essentially gyros that

just rotate very quickly. You spin them
up, and the idea is that, well, this

stabilizes the spacecraft. Because you
actually want to control the rotations in

most cases. OK. So now we hope that
everything was working perfectly. We

launched the spacecraft. But
unfortunately, not always everything goes

perfectly. So let's maybe dig into
some example. This is TV-SAT 1. Well, I

don't have a picture but that was a TV
satellite from 1987. And everything worked

as we described. So we got the first
acquisition. We got some telemetry from

the spacecraft. But unfortunately, the
solar arrays turned out to be only

partially deployed. That's of course a
problem. And we need to diagnose this, and

we need to fix it if possible. So the
first thing you have to know, is that you

kind of can't really necessarily trust all
the data that you get. You have to confirm

that whatever you're seeing, is actually
the case. So you have to use additional

sources. For example, in the case of a
solar array, you can actually check, how

much is the power output? Is it actually
less than expected if it was deployed? And

it turned out: Yes, there's not enough
power. And secondly, once you notice this,

you can actually send the manual
deployment command again, now. So it's

possible that the automatic solar panel
deployment didn't work. So we just tried

again and unfortunately, this did not
work. So it still seemed undeployed. Now

you start thinking, well, what are we
going to do. And you consult usually the

satellite manufacturer. The satellite
manufacturer actually also sits in the

control room during the LEOP because there
happen to be many questions, so you need

somebody on hand. And they suggested, or
the people who operate the satellite, they

suggested various tests to figure out what
was wrong with TV-SAT 1. And I want to

present just 2 of these things you can
try. One is, you can orient the spacecraft

or the satellite such that it is at a 45
degree angle towards the the sunlight, and

then you start rotating it. If you do this
carefully, and you measure the power

output of the solar arrays, you can
actually estimate the angle that the solar

array was deployed. So they did that and
they figured out, well, they're completely

not deployed. So less than 2 degrees
actually. OK. So that's a problem. Then

they did various other tests, and they
came up with one possible problem. And

this is that there might be the actual
stirrups, sort of the black boxes in the

picture which keep the solar array
attached to the satellite during the

launch, that they might still be there. In
principle, they should have been kind of

fired off, or removed, and then the solar
arrays should deploy. But it looked like,

they were actually still there. So one
thing you can then try, is, well, you can

again rotate the satellite in such a way
that the stirrups will cause a small

shadow over the solar array. This will
reduce the power output again just a tiny

little bit. So you might be able to
measure this. And this way confirm that

the stirrups are still there. Turns out,
this was not actually really well

measurable. So this didn't work. However,
they were still able to deduce it. It was

probably the stirrups that are still
there. Once you have diagnosed the

problem, you want to solve it, of course.
So let's see. How can we recover such a

situation? And this is sort of where you
can just follow your creativity and come

up with arbitrary solutions, and see
whether you can actually try them. So one

thing we can do is, we can spin up the
spacecraft. If we do this very fast, we

will have a very strong centrifugal force.
So maybe an acceleration of about 1 G. And

this way, we might hope that we loosen the
stirrups. Another thing you can try to do:

You can use your main engine to actually
accelerate the spacecraft in a pulsed way

in order to excite resonant frequencies
off the stirrups. OK. So hopefully this

might actually loosen the stirrups.
Another thing you can try to do is, you

can comand the spacecraft to heat up and
to cool down in some ways. And this way

actually also losens the stirrups. And the
last thing you can try is, you can kind of

just try to shock the whole thing. So for
example, you could deploy an antenna, in

this particular case that was the main
antenna which was actually stuck beneath

the solar array. So you'd try to deploy
this and hope that the force actually

pushes the solar array out. Yeah.
Unfortunately, none of those worked. And

this was an unsuccessful recovery of a
satellite. So in particular, the main

problem was that, well, this was a TV
satellite, so it really needs the antenna,

but the antenna couldn't deploy because of
the stuck solar array. So in this case,

this did not work. But usually, of course,
this works and people are coming up with

very creative, very interesting solutions
to all kinds of problems and get things

running. All right. So once we have our
spacecraft in some kind of safe state, we

kind of conclude the LEOP and we start
testing the actual properties of the

spacecraft. This is called the
commissioning phase, or in-orbit-testing

of the payload. So this usually takes
longer than a LEOP, might take several

months, depends on what type of mission
you're looking at. This is when you

actually start or switch on the payload,
and when you also verify that the payload

is working as expected. OK. So in the
picture, you see a geostationary

communications satellite. So its main
payload are the communication arrays, or

the antennas, in particular. So, for
example, you might want to actually verify

that the antennas are working properly
after the launch. So during launch, they

all get shaked up and it's really pretty
intense. So you want to make sure that

they are working properly afterwards. So
for example, one thing you might want to

do is, point the satellite at your ground
station, you measure the strength of the

signal that you receive, then you move it
slightly, you measure again the strength,

and this way you kind of get a pattern of
the antenna. OK. And this is a property

of this particular one, this particular
antenna that you might use later. Another

thing that you do during this time is, you
checkout redundant components of the

satellite. So for example, if you have an
Earth observation mission, you, as I

already mentioned, you need to know where
you're looking at. So you need for example

GPS or a star tracker. Now if that fails,
you obviously have a large problem because

now suddenly you don't know where you were
taking photos or images. So usually, there

is quite a bit of redundancy on
satellites. So there are 2 GPS receivers.

And then you can actually switch between
them, and during this phase, you will test

that they are working properly. OK. So
let's suppose, we have done this and

everything is working as expected. Then we
start with the routine phase. The routine

phase is sort of the main phase of the
operation. So that's when you actually do

the science experiments, or you start
offering communication services, or

whatever it is you're doing. This picture
is a picture of the mission TerraSAR-,

TanDEM-X. So those are two radar
satellites flying in low earth orbit. And

they can actually make 3-dimensional maps
of the ground by sending a radar signal

and then receiving it. And because they're
flying in close formation, so something

like a few 100 meters apart from each
other, they actually get this kind of

sterographical 3-D information. And during
the routine phase, a scientist would

actually order a data take or a picture of
this kind somewhere, maybe online. And

then somehow the mission would actually
command this, the command center would

command this data take. It gets
downlinked, and then the result will

actually given to the scientists. OK. So
this is the main phase of the spacecraft's

life. So where we do this payload
operations. But wait. This picture is a

picture of a joint American-German
mission. That's a Grace follow-on mission,

two satellites that have a microwave or a
laser link between them. And they measure

the distances in order to, well,
variations of the distances in order to

deduce the gravitational field of the
Earth. Last year at 34c3, there was

actually a talk about the predecessor
mission. Here, actually, probably in this

room. OK. So this is the time when we do
our science experiments. Furthermore, we

actually monitor the spacecraft of course
because we still need to know what's

happening. Is it working properly? We
will, of course, continue to handle

contingencies. But hopefully there are
none anymore. And we might also adapt to

new mission requirements. So for example,
well, you could actually try to devise new

kinds of experiments on the flying
satellite. And for that, you might need to

upload new software which is also done
during this phase. Another issue is that a

spacecraft actually ages. So for example,
a battery might deteriorate. So its total

capacity actually gets smaller over time.
So you need to adapt to that. For example,

if there's less power available, then you
can actually do fewer data takes,

something like that. And you need to
monitor this and react accordingly. OK. So

how does the monitoring work? Well, that's
part of the TC/TM and the Data subsystem

or system. And the idea is that the
spacecraft actually measures various

properties that it has or that describe
the state, all the time. So we have a time

series of binary data, and also of
numerical values. So for example, here,

the plot shows the temperature of a
certain part of the spacecraft over time.

But remember, we don't have this
information available live. We only get

this once we actually downlink it. OK.
And then we get a huge part of the data at

once. OK. So this describes the state of
the spacecraft. And there can be lots of

parameters. So for example, 20.000
telemetry parameters for one spacecraft is

possible. If you measure something once
every second, you do this for a few years,

20.000 parameters, this means that you
have a lot of data. So obviously, you can

do a lot of data analysis, time series
analysis with that. You can do anomaly

detection, telemetry prediction, or
detecting errors or problems within this

data. Also, what you need to do is, you
kind of need to save this to some kind of

offline database. Because lots of other
subsystems actually need this data.

Because they want to know what is the state
of the spacecraft. So this is an example

for telemetry view. So this is one
software that we use. It's called Geckos.

And you can see here a number of telemetry
pakets. So for example, there are a few

confirmations that some checksum was
correct, and that some pinging was

actually received and was being worked on.
OK. So it was executed. It's time stamped,

and you get some additional information.
And this is sort of the the most basic

thing you can really see. Once you know
the state of your spacecraft, you actually

want to command the spacecraft to do
something. This is done via Telecommands.

And on the picture here, you can see some
commands that have been executed and also

some that are still to be executed. So for
example, in the upper part, you see a few

pings which were not actually answered by
the spacecraft. But the last one was

received and was replied to. And the
operator can, for example, already load a

few Telecommands on the manual command
stack, prepare them and then execute them

very quickly. This is the lower part.
Notice that these Telecommands are very

specific to the spacecraft because they
really need to do something there. So this

is in some way provided by the satellite
manufacturer, and you have to somehow

understand all the possible things you can
do. In particular, you very often don't

really want to do like very atomic things,
but instead, you want to achieve a certain

task. For this you bundle the
Telecommands, you can add for example also

telemetry checks, so conditions on the
telemetry, and you call this a Flight

Operations Procedure. So this will be sort
of a bundled thing that will executed on

the spacecraft, for the purpose of
achieving a specific goal. Another thing

that's important, as I've mentioned
various times, you don't see the

spacecraft all the time, meaning, you
cannot really command it all the time, but

instead what you do is, you send
Telecommands, but you make them time-

tagged, and then they get executed, for
example, when you don't see the

spacecraft. OK. And these kinds of
Telecommands are called TTC. Let's look at

an example. So this might be a set of
time-tagged Telecommands for a maneuver.

OK. So at the time t zero, we want to
execute some maneuver. So we want to turn

on the thrusters, this time and the
position and this, and the duration of the

burn, they were calculated by the flight
dynamics departments of course, but one

hour before that we actually need to
check, for example, that the spacecraft is

in some some fixed state, some prepared
safe state. Eight seconds later we might

actually start heating up the thrusters
because the fuel needs some kind of

operational temperature. Then eleven
minutes before the burn starts, you will

automatically command the switch-on of
some additional telemetry. So this is kind of

like, you turn on the debug
mode, OK? You just tell the spacecraft to

actually tell you, to give you more data.
Then, because the burn will actually

make the the spacecraft shake quite a bit,
there will be lots of alarms going off. So

at some point before the burn, you will
turn off these alarms to safeguards, just

because the reaction of the
spacecraft is actually expected. Then you

start rotating in the right direction of
course. And at some point the burn starts.

Now this should in principle stop
automatically. However you might command

an additional safeguard stop command, just
to make sure that in case the other one

well kind of didn't get
executed, you you stop nevertheless. And

then you kind of reverse the whole
procedure to return to a mode where you

can proceed with your payload operations,
okay? And this would be a sequence of

time-tagged commands that are uploaded to
the spacecraft during an uplink and then

executed whenever t0 was actually taking
place. All right. So there is one other

thing that I want to describe and this is
mission planning, since probably they the...

Yeah. One of the lesser known
subsystems and this is sort of at the

point where you have to wait between
automation and manual commanding. So

suppose you have a scientist that actually
wants to take pictures. So he wants to

have the satellite taking some pictures of
some region. So then he has to sort of ask

if the satellite can do this and has to
make a reservation. This is being taken

care of by the mission planning system,
which will then talk to Flight Dynamics to

see whether this is actually possible,
give feedback to the scientist, and this

will also tell the operators, or the
operating... well, the telecommand

operators to actually execute some command
to take the data take. However, because of

all these kinds of little issues, problems
that you can have all the time, you cannot

really automate everything. There is some
kind of, some amount of manual commanding

that's still being needed. For example due
to those contingencies. So what the

mission planning system internally does is
it schedules activities and it tries to do

this in some consistent and conflict-free
manner. Yeah, si imagine, for example, for

the, for a data take. You need to actually
take the picture before you want to

downlink it. OK, so there those are two
activities and they should actually take

place in some order. OK. From this,
these kind of activities that were

requested by some scientists, they create,
the system creates a timeline which is

then, well, provided to everybody who
needs to know what the spacecraft is going

to do at some point. So here's one
example. So that's one software we

use. So it's called PINTA and it shows on
the x-axis the time, and up on the top,

you see these black-white things. Okay, so
this is, these are actually eclipses. So

whenever the spacecraft is not in the sun
or is in the sun, you can see this there.

And below that, there there are a few
experiments planned, but one of them is

partially planned during an eclipse. But
it has the condition that it must not take

place during the eclipse, so this gives a
conflict. And the mission planning

system is responsible for identifying
these kind of conflicts and

actually supplying that information to the
scientist or the operator to be

resolved. One other thing you can see is
this thing that we talked about at the

beginning. So you need to downlink the
information from the experiment. So you

need some scheduled downlinks, downlink
opportunities, and you can see two of them

actually as the green lines above the blue
ground. So this is when, the next time

when the satellite actually sees the
ground station and it can downlink the

results of the prior experiments. OK. So
now we are doing kind of semi-automated

all our experiments. We gather a lot of
scientific data, but at some point

everything has to end. So there's also the
the end of the mission that you have to

consider. So in general, the mission time
of a spacecraft might depend for example

on the mission goal. Imagine that the,
that you have one specific experiment that

you want to do and this might be finished
at some point in time. Also it might

depend on the orbit itself. So if you have
a spacecraft in an altitude of 300 to 400

kilometers it will actually descend into
the atmosphere within less than a year. If

you have a satellite at an altitude above,
say, 700 kilometers it would take more

than 25 years to actually get down. If you
are in a geostationary orbit you will

actually never come down. So another thing
is, and this is mainly for geostationary

orbit, geostationary satellites, is that
you have a finite amount of fuel. So at

some point you can't really keep your
spacecraft at the position where it is, so

then you have to end the mission, of
course. For geostationary satellites this

might take something like 15 years, for
low earth orbit satellites a few years are

pretty common but very often you can
actually extend the lifetime quite

considerably if you are very careful about
your fuel consumption for example. Now

what are you going to do once it reaches
the end of the mission? Well, this depends

again on the orbit. So for example, if you
have a low earth orbit satellite then you

reserve some fuel, or you might reserve
some fuel in order to actually take it to

a lower orbit such that it deorbits
and disintegrates in the atmosphere within

something like 25 years. These 25 years,
they are now, they nowadays pretty much

mandated by for example the FCC and also
the ESA. So you really need to kind of

dispose of your spacecraft at most 25
years after the end of your mission. So

you can deorbit LEO satellites, but
usually there is not enough fuel to

deorbit a geostationary orbit satellite.
In that case, you will actually raise the

altitude by something like 500 kilometers
and put them on the so-called graveyard

orbit, because that's a place where they
are not disturbing anybody anymore, so you

can put them there and, well, kind of
forget about them. OK. Well and then you

can look back at at your mission, you have
spent quite a few years on that, yeah.

And, well, hopefully it was, everything
was working correctly, you produced a lot

of scientific data, you're happy and with
this I also want to end my talk. So thank

you very much. And enjoy the rest of the
Congress.

Applause

Herald Angel: Thank you. There's about 10

to 15 minutes left for Q and A. This works
pretty simple. You walk to a microphone.

You wave your hand and you may end up with
the opportunity to ask a question, which

gets me to the "asking questions"
bit: Q&A is for questions, not about

statements or how nice a speech was, etc.
So keep it short. And the first question

goes to the Internet, to the Signal Angel
who has been diligently monitoring IRC and

Twitter on the hashtag #hallC, so signal
angel, do have a question?

Signal Angel: Yes, yes, hello. Yes, yes
yes, (in the background: no Mic!) Hello,

hello, hello, hello. (Background: Need mic
on the signal Angel!) Hello check check.

Herald: You need to, you know, use
microphone. Get the microphone, I will

take the Question first to this microphone
over here. (Background: okay)

Question: Hello, is this on? Nope.
Microphone 2, please. it's not on?

(Background: number 5) Is it, is it on
now?

Different Microphone: Okay great, test
test? Ah. Would it be feasible to put like

4 satellites in geostationary orbit as
communication relays, so we have uplink

all the time, and why is it not done?
Sven: Yeah, so, this is feasible, and this

is actually being done. So for example
the, the ISS, as far as I know, actually

does most of its communication via some
relays, relay satellite in geostationary

orbit by NASA, but there also for some
European alternatives, OK, so there's a

European data relay system, for example,
that you can also use for this. This is

being used. However, it's always, I mean,
money is always an important issue. OK so

if you're using somebody else's
communication relay system then you of

course have to pay for that. So you some
very often actually try to find a minimal

solution to your communication
needs. Thank you.

Herald: OK. Next question goes to
Microphone number two.

Microphone 2: Yes. This is the question
from the Internet, which would like to

know about the security of the protocols
and protection or encryption or anything

like that.
Sven: OK. So I mean I can't really give

too many details about this because that's
not my particular area of expertise, but

in principle, the the tele-commanding and,
or the, or at least the telemetry is

usually encrypted. So there is a lot of
effort put into that. However, for the

payload data, this is not always
encrypted. For example, very famously

known are the weather satellites, so you
can just receive the data and it is

transmitted and clear and you can just
receive them. OK. Thank you.

Herald: Okay. Next question is from
microphone number 1.

M1: So just one example, you told an
example of a geosat that inaudible out

and it didn't work. Who does, who has
final decision on, "Oh it's not working,

and we're going to drop this project, and
maybe start anew" - who has the final

decision? And, in particular this geosat,
while it was put on an orbit so long ago,

did they just leave it there? I mean, it's
down and its inaudible. So,

one question: who gets the decision, and
the other one is, did they leave it there?

Sven: OK, so, the decision making process
is kind of involved. I haven't been part

of any mission yet that failed, so I kind
of don't really know the details of that.

But in principle there's not just the
flight director. So first, I mentioned the

flight director but that's actually the
person in charge during the actual

operations. But there's also for example
the project investigators, that the PI,

who's doing the scientific, who's having
the, who's in charge of the scientific

process. There are other kinds of
organizational people, and they decide

this together in some way. Okay so this is
a non-trivial decision. And regarding the

other question, the, So I mean, they could
still, for TVSat 1 they could still

control the satellite. So they were
actually able, as far as I know, they, to

lower the orbit to actually have it burn
up at some point. I think they even tried

to turn it on at some time later and I
think it still worked. But nowadays I

think it has already burned up. So at
least this mentioned somebody, I'm not

quite sure but, yeah, it was still usable.
Well in that sense you could still lower

the orbit, eh, the orbit. So that's not a
problem for the satellite.

Herald: Okay next question for microphone
number 2.

M2: You mentioned a, you had a temperature
time series on your, on your charts. I was

wondering what message do you use to find
anomalies in this temperature time series?

Sven: Pardon, what's the question?
M2: What method do you use to find

anomalies in that temperature series.
Sven: Ah, well, so, I mean, there are

quite a few properties of the spacecraft
that might actually deteriorate over time,

and there might be various indications for
that. And you try to look for hints that

something is wrong, something that you're
not noticing because nothing is failing

yet but you actually want to to see that,
for example, some sliding average is

actually increasing over time. it's still
below some some kind of alarm limit, but

it's actually getting worse. OK. So you,
you try to do Time's series analysis for

that. Yeah. There are very various similar
issues that you want to identify

M2: So it's a moving average, or ARIMA?
Sven: So, this particular example or...?

M2: Yeah, I was wondering...
Sven: Yeah, well, I'm not sure this this

particular example shows anything
particular. So this seemed to work properly,

I guess. Yeah, so.
M2: OK. Thanks a lot.

Herald: Questions inaudible. Sorry, next
question from microphone number 1.

M1: Just talk about commands, sending
commands. Does this command get sent and

interpreted by the server, or at least
some kind of compilation? If you send up

binary or something like that, or an
executable? Do you have a server-side

????? server-side? So, does the satellite
do interpretation or do you send a

compiled command as software?
Sven: OK. Well it's kind of like an API, I

mean, that you define, that actually gets
provided by the, well, satellite

manufacturer. So you really send a binary
command, so I might be, these protocols

are actually very effective. Yes, so they
do just one thing, they make sure that

this is actually transmitted correctly,
and then it gets executed. So this might

be, just switch one of the machines. OK.
So there's just some binary thing that you

need to transmit to the satellite. There's
of course some level of checking going on,

so for example there might be a command
counter that needs to be correct, or some

kind of checksum, but apart from that this
will be executed directly. However,

sometimes you also need to upload some
kind of binary data. For example, imagine

that, for some reason, one of the things
on your satellite moves a little bit then

the orientation is not correct anymore and
you need to somehow fix this in your

internal calculations. For that you need
to actually upload some rotation matrix,

for example, describing this small
distortion. OK. So in that case you would

actually upload some binary data that gets
put at the correct place on the onboard

computer.
M1: OK.

Herald: OK, next question is from
microphone number 4

M4: About the orbits. Is there much
garbage on these orbits, and is this a

problem?
Sven: Is there a what? Sorry.

M4: Is there much garbage, so old
satellites or parts that get lost?

Sven: So so you're talking about space
debris, so stuff that's flying around and

that might actually hit our satellite. Yes
there is quite a bit. So satellites

actually have to do maneuvers to just, to
to be on the safe side to not crash into

some, do not collide with some space
debris. It's getting more and more, in

particular there was a destruction of a
satellite a few years ago by the Chinese

so they tried to blow up their own
satellite, and for example this created a

lot of additional debris. This is,
however, the debris is actually flying on

the same orbit or approximately the same
orbit as it was beforehand. OK. So instead

of large target, you now have many smaller
ones. They are being tracked by various

space agencies. You can actually get their
data online somewhere and I think they

will even write you an e-mail if your
satellite happens to to be on a collision

course with something.
M4: Can I ask a second question? Is there

any idea how to remove this, or...?
Sven: So I'm not too knowledgeable about

this, but in principle there are people
trying to do this. So the ESA actually has

various projects, has done a few
conferences on the question how to deal

with space debris, but I'm not sure
there's any really good and feasible

solution yet, but maybe in a few years,
hopefully.

M4: Thank you.
Sven: Thank you.

Herald: OK. Next question from microphone
number 5.

M5: Yeah. I would like to add to their
question. So she was talking about the

Keppler syndrome in the LEO, right? But
you also talked about the graveyard orbit,

so, will we build a second Keppler
syndrome just a little further out?

Sven: So, I'm not sure I got the last
question but, so, the graveyard orbits,

they are actually for geostationary
orbits, yeah? Because you can't deorbit a

satellite from there. So instead you kind
of move it away from the earth.

M5: Yeah. So my question is: will we
create the same problem on the

geostationary, this.
Sven: Yeah, I mean, in principle this

means that there is also space debris then
there, in geostationary orbit. However, I

mean, if you fixed the orbit then, well,
with increasing orbit, the the, well,

there is more space left. Okay. So the
density actually kind of reduces with a

larger radius. So you're not having the
same problems as with LEO, yeah, so.

Because in LEO they're really, you're
accumulating space debris faster than

you're actually deorbiting it. So you have
to actually go through LEO to get to geo-

transfer orbits. But yeah it's not, it's
not such an urgent issue there, and likely

will never be. But, who knows.
M5: Thank you.

Sven: Also. And maybe also some comment.
Nowadays there's kind of a shift from

geostationary orbit to actually going more
LEO. Yeah. Also for communications

satellites. So this might actually maybe
in long term even reduce the number of

geostationary satellites but I don't know.
Herald: Okay. Next question goes to the

Internet.
Signal angel: So, IRC... Hello? Yes. So,

IRC would like to know if you're concerned
with SpaceX launching 5000 satellites into

low-earth orbit running at 25 000 k/h.
Sven: Pardon, can you repeat that?

Signal: SpaceX is talking about launching
thousands of cell satellites.

Sven: Yeah.
Signal: How is that gonna work with

communications with those buzzing around.
Sven: So I don't know the details about

this project but, so as far as I know,
they talk about something like 4000

communications satellites in low earth
orbit and as far as I remember they're

supposed to communicate via lasers. Okay,
so they will actually spend sort of a

laser communication network and then you
just try to route your, the information

that you have through this network. Okay.
Of course this is a lot of satellites. I

don't know at which altitude they will
operate, whether this will cause problems

for anybody, but as far as I know the FCC
in the U.S. has already said that it's

okay to proceed with this project. So,
yeah, let's see where this will lead. It's

hard to say at the moment I guess.
Herald: Next question is from Microphone

number 3, and this may be the last
question.

M3: I would like to know in regard to
redundancy with antennas, are the

satellites built in a way that an antenna
for one frequency can take over duties

for, that were actually intended for
another frequency especially in two

scenarios, if the antenna receiving
instructions is compromised and cannot

deploy or, for example, if the telemetry
antenna is somehow incapacitated.

Sven: Right, so, on the ground, for
example, an antenna might actually be able

to to serve another frequency, OK? So this
is pretty common, for example in Weilheim

on one of the pictures you've seen a large
antenna that can actually serve multiple

frequencies. On the satellite, I don't
think this is actually done as far as I

know. However, of course, you could try to
route the same kind of information through

another antenna, but it depends a little
bit on the bus I guess. So for example of

the satellite bus. So on some satellites
the additional antennas are actually well

kind of separate from the satellite bus,
and in that case it's not feasible to

actually route the telemetry through that.
But I guess in various cases this is

indeed possible, but I'm not sure, I've
ever heard that it is actually being used.

Herald: OK. Thank you very much. That was
the last question and this was the end of

this talk. A round of applause for our
speaker.

Applause

Sven: Thank you.

applause

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