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Welcome to the NASA Goddard Space Flight Center

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here in Greenbelt, Maryland where

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we're building the flight and ground modems for

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the Laser Communication Relay Demonstration.

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LCRD is a technology

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demonstration mission that will use

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infrared lasers to send and receive data

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at above a gigabit per second,

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in a relay capacity.

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Multiple systems are

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being incorporated into LCRT to create

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this communications relay capability.

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In addition to having a flight payload

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we'll also develop a ground system for the

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payload to talk to.

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On both sides will have modems that act as modulators

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and demodulators for signals received through

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the optical modules.

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Today we're going to take a closer look at both our

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flight and ground modems and see how we've been developing those.

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Come on in.

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Hi Eric how're you doing today?

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Great!

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I'm excited to talk about these two modem

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subsystems we've been building.

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Wonderful!  Rick is the Development Lead for modems

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here on LCRD and if you're ready

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let's go take a look at the system's.

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Absolutely!

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So let's get some basic straight.

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What are the modems? Where their functions in the

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payload in the ground system?

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So the basic function of the modem is to take

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information and transform it into

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another medium. So in our case that means

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we take digital data and transition into

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light and the reverse. So basically how

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the modems work is they receive digital

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data, at the ground station, ground modems

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transition at the light, goes up through

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the ground station telescope on the

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spacecraft payload, where it is received

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by the flight modem. The flight modem then

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takes the light that was received and

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transforms an additional data for

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further processing by the payload.

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I see, now correct me if I'm wrong but you

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started out with the ground modems right?

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Why is that?

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Yeah so if you can see the

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flight modem behind me here

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it's very compact system but there's a lot going on in there.

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There are 17 circuit boards, several hundred splices

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and so the first thing we did when we

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got the design was wanted to learn how it operated

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and so we transition this design

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into a rack based design like you can

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see behind it.

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That gave us a lot more to work

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and we were able to use

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that room when we were trouble shooting things

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learning how to test it

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writing our test procedures that sort of thing.

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I see.

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Can we go check out the codec?

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Absolutely!

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Awesome!  Let's go.

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So I've heard that you need the codec to

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be able to get through the atmosphere.

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What is the codec and

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what does it have to do with the modems?

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So one of the things that you have you're dealing with

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in optical signals of severe atmospheric effects.

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Things like wind, rain and

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certainly clouds can severely degrade

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optical signals so we developed a system

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called the codec that would apply

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forward correcting algorithm

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to the data and

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time facial interleaving.

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What this does is

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allows data, even if it's corrupted,

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to be reconstituted on the other end

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of the link.

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OK. Alright. So then how do data rates

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plan into the ground modems?

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Well one of the other knobs we have to turn is that

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we can slow down the data

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this has the effect of increasing our

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signal noise ratio.

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Which means that we have a better chance of

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getting the signals through.

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I see. OK.

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So I noticed you guys have got the flight modems going on

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How's development going with those?

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It's actually going very well. We built

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up the first flight mode and completely.

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It's in requirements testing right now

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on our bench.

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Soon it's going to go over

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to the environmental test facility

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that we have here at Goddard,

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to complete its requirements testing.

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Our second flight modem is also under development,

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we've built up, we call them tower A and tower B,

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so we built up tower A

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and we're in the process of building up tower B.

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We expect that work to be complete

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in 4 to 6 weeks

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and then we'll follow the same

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flow is the first flight modem.

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That's great! So then how does developing

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modems for flight differ from

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developing modems for the ground?

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So flight modems, flight systems in general,

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need to be able to survive two things

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that's the launch environment and the

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operational environment so what we do a

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Goddard is we take our systems that we

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design over to the environmental test facility.

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We do things like vibration

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which ensures that we're going to

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survive the launch environment.

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We also perform a test called EMIEMC

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which ensures that the modems are not susceptible to

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or do not interfere with

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other systems on the spacecraft.

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And finally the big test is thermal vacuum

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and that's where we operate the modem

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over its

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operational temperature range

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in a vacuum

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to ensure that it meets its

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performance requirements.

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That's really cool.

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So then, what is the space switching unit

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have to do with the modems?

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The space switching unit is actually the

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heart of the LCRD payload.

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Not only does it

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provide command and telemetry interface

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to the modem

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it also processes our

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digital data and

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transmits data between

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our two flight modems.

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That's great!

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Can we go check out the electro-optic components?

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Absolutely!

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Awesome! Let's go.

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So is there special for LCRD that you'll have to

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do for the modems?

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Actually, typically

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when you're designing flight subsystem

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you start out with a set of well vetted,

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electronic parts that are designed for

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spaceflight use.

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In our case,

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since optical comm is so new

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the electro-optic components

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are not really designed for space.

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So what we have to do is we have

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to buy them from commercial houses and

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form the of up-screening ourselves

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to ensure that they will survive

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while in space.

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This has been really great Rick.

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Thanks so much for letting us come in here

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and visit today.

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Any time.

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Want to learn more about LCRD?

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Visit LCRD.GSFC.NASA.GOV

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