Welcome to the NASA Goddard Space Flight Center
here in Greenbelt, Maryland where
we're building the flight and ground modems for
the Laser Communication Relay Demonstration.
LCRD is a technology
demonstration mission that will use
infrared lasers to send and receive data
at above a gigabit per second,
in a relay capacity.
Multiple systems are
being incorporated into LCRT to create
this communications relay capability.
In addition to having a flight payload
we'll also develop a ground system for the
payload to talk to.
On both sides will have modems that act as modulators
and demodulators for signals received through
the optical modules.
Today we're going to take a closer look at both our
flight and ground modems and see how we've been developing those.
Come on in.
Hi Eric how're you doing today?
Great!
I'm excited to talk about these two modem
subsystems we've been building.
Wonderful! Rick is the Development Lead for modems
here on LCRD and if you're ready
let's go take a look at the system's.
Absolutely!
So let's get some basic straight.
What are the modems? Where their functions in the
payload in the ground system?
So the basic function of the modem is to take
information and transform it into
another medium. So in our case that means
we take digital data and transition into
light and the reverse. So basically how
the modems work is they receive digital
data, at the ground station, ground modems
transition at the light, goes up through
the ground station telescope on the
spacecraft payload, where it is received
by the flight modem. The flight modem then
takes the light that was received and
transforms an additional data for
further processing by the payload.
I see, now correct me if I'm wrong but you
started out with the ground modems right?
Why is that?
Yeah so if you can see the
flight modem behind me here
it's very compact system but there's a lot going on in there.
There are 17 circuit boards, several hundred splices
and so the first thing we did when we
got the design was wanted to learn how it operated
and so we transition this design
into a rack based design like you can
see behind it.
That gave us a lot more to work
and we were able to use
that room when we were trouble shooting things
learning how to test it
writing our test procedures that sort of thing.
I see.
Can we go check out the codec?
Absolutely!
Awesome! Let's go.
So I've heard that you need the codec to
be able to get through the atmosphere.
What is the codec and
what does it have to do with the modems?
So one of the things that you have you're dealing with
in optical signals of severe atmospheric effects.
Things like wind, rain and
certainly clouds can severely degrade
optical signals so we developed a system
called the codec that would apply
forward correcting algorithm
to the data and
time facial interleaving.
What this does is
allows data, even if it's corrupted,
to be reconstituted on the other end
of the link.
OK. Alright. So then how do data rates
plan into the ground modems?
Well one of the other knobs we have to turn is that
we can slow down the data
this has the effect of increasing our
signal noise ratio.
Which means that we have a better chance of
getting the signals through.
I see. OK.
So I noticed you guys have got the flight modems going on
How's development going with those?
It's actually going very well. We built
up the first flight mode and completely.
It's in requirements testing right now
on our bench.
Soon it's going to go over
to the environmental test facility
that we have here at Goddard,
to complete its requirements testing.
Our second flight modem is also under development,
we've built up, we call them tower A and tower B,
so we built up tower A
and we're in the process of building up tower B.
We expect that work to be complete
in 4 to 6 weeks
and then we'll follow the same
flow is the first flight modem.
That's great! So then how does developing
modems for flight differ from
developing modems for the ground?
So flight modems, flight systems in general,
need to be able to survive two things
that's the launch environment and the
operational environment so what we do a
Goddard is we take our systems that we
design over to the environmental test facility.
We do things like vibration
which ensures that we're going to
survive the launch environment.
We also perform a test called EMIEMC
which ensures that the modems are not susceptible to
or do not interfere with
other systems on the spacecraft.
And finally the big test is thermal vacuum
and that's where we operate the modem
over its
operational temperature range
in a vacuum
to ensure that it meets its
performance requirements.
That's really cool.
So then, what is the space switching unit
have to do with the modems?
The space switching unit is actually the
heart of the LCRD payload.
Not only does it
provide command and telemetry interface
to the modem
it also processes our
digital data and
transmits data between
our two flight modems.
That's great!
Can we go check out the electro-optic components?
Absolutely!
Awesome! Let's go.
So is there special for LCRD that you'll have to
do for the modems?
Actually, typically
when you're designing flight subsystem
you start out with a set of well vetted,
electronic parts that are designed for
spaceflight use.
In our case,
since optical comm is so new
the electro-optic components
are not really designed for space.
So what we have to do is we have
to buy them from commercial houses and
form the of up-screening ourselves
to ensure that they will survive
while in space.
This has been really great Rick.
Thanks so much for letting us come in here
and visit today.
Any time.
Want to learn more about LCRD?
Visit LCRD.GSFC.NASA.GOV