Lunar

Las

er C

om

munic

atio

n D

emonst

rati

on

Overview of the Lunar Laser

Communications Demonstration

Bryan S. Robinson, Dennis A. Burianek, Daniel V. Murphy,

Don M. Boroson, Andrew S. Fletcher

MIT Lincoln Laboratory

Bernard L. Edwards

NASA Goddard Space Flight Center

Optical Coding and Modulation SIG

at CCSDS Fall 2010 meetings

October 25, 2010

This work is sponsored by National Aeronautics and Space Administration under Air Force Contract #FA8721-

05-C-0002. Opinions, interpretations, recommendations and conclusions are those of the authors and are not

necessarily endorsed by the United States Government.

Lunar Laser Communication DemoSystem

Lunar Lasercom Ground

Terminal

(MIT/LL)

Lunar Lasercom

Space Terminal

(MIT/LL)

LADEE

Mission Ops Center

(ARC)

LLCD Mission

Analysis Center(MIT/LL)

10 cm @ 0.5 W

40-622 Mbps xmt @1550 nm

10-20 Mbps rcv

LADEE

Science Ops Center

(GSFC)

Lunar Lasercom

Ops Center (LLOC)

(MIT/LL)

At White Sands Complex

At GSFCAt ARC

At MIT/LL

In Lunar Orbit

4@15 cm @ 10 W

10-20 Mbps xmt @1558 nm

4@40 cm

Nanowire Detectors

40-622 Mbps rcv

White Sands

Complex

LADEE

Spacecraft

(ARC)

LLGT

LLST

LLOC 2- BSR

LLST Overview

• Optical Module (OM)

– Inertially-stabilized

– 2-axis gimbal

– Fiber coupled to

modem transmit

and receive

• Controller Electronics (CE)

– OM, MM control

– CMD/TLM interface to S/C

• Modem Module (MM)

– EDFA transmitter

– Optically-

preamplified

receiver

– CODECs and data

interfaces

– Data interface to

spacecraft

3- BSR

LLST on LADEE

Optical

Module

Controller

Electronics

Modem

Up

4- BSR

LLCD Downlink Overview

Space Terminal Transmitter

• 10-cm transmit aperture

• Fiber coupled to 0.5-W 1550-

nm Master Oscillator Power

Amplifier (MOPA) transmitter

• Data rates from 40-620 Mbit/s5- BSR

Downlink Apertures

Ground Terminal Receiver

• 4 x 40-cm collection

aperture

• Each telescope fiber-

coupled to photon-

counting detector array

* FIGURES NOT TO SCALE!

6- BSR

LLCD Downlink SignalingModulation and Coding

16-ary Pulse Position Modulation

Average Power

log2M Bits

per Symbol

M Slots

per Symbol

• Pulse position modulation

– Orthogonal, power efficient

– Well-suited for MOPA transmitter architecture

– Well-suited to photon-counting detectors

• Limited count rate (reset time)

• High timing resolution

• Data rate varied by changing slot frequency

– 311 MHz – 5 GHz

• ½-rate serially concatenated turbo code

– Simple encoder

– Iterative decoder

– Operates within 1 dB of theoretical channel capacitySerially Concatenated

PPM Turbo Code

LLCD Superconducting Nanowire

Detector Arrays (SNDA)

• Superconducting nanowire

detects individual photons

– >60% detection efficiency

– <15-ns reset time

– <50-kHz dark count rate

• Detectors coupled to telescopes

using custom multi-mode

polarization maintaining fiber

• Detector array used to achieve

large detection area and high

count rates

– 4 detectors per telescope

– 4 telescopes

7- BSR

(1) Hot spot

formed

(3) Critical current

density exceeded

(4) Resistive

region formed

(2) Current

diverted

Principle of operation

Nanowire superconducting at T < 2 K

4-element LLCD

SNDA

10-1

100

101

102

103

10-1

100

101

Bandwidth Expansion (Channel Usages / Bit)

Receiv

ed

Ph

oto

ns P

er

Bit

8- BSR

LLCD Downlink Power Efficiency

Telecom,

High-Rate

Lasercom

Coherent

Photon

Counting

Quantum

Limit

LLCD Coding +

Modulation Performance

• Efficient detection, modulation, and coding

enables 622 Mbit/s downlink from the Moon

with a 0.5-W transmitter

• LLCD transmitter is 4000x more energy

efficient (per bit) than LADEE RF transceiver

LLCD Uplink Overview

Space Terminal Receiver

• 10-cm receive aperture

• Non-coherent combining of

uplink signals

• Fiber coupled to optically-

preamplified direct detection

receiver 9- BSR

Uplink Apertures

Ground Terminal Transmitter

• 4 x 15-cm transmit aperture

• 10-W per transmit aperture

(40 W, total)

• Data rates from 10-20 Mbit/s

* FIGURES NOT TO SCALE!

10- BSR

LLCD Uplink SignalingModulation, Coding, Detection

• 4-ary pulse position modulation with dead time

– 311-MHz slot rate

– Deadtime provides better match with narrow-band optical filter

– Data rate varied by changing dead time (10 or 20 Mbit/s)

• Pre-amplified direct-detection receiver

• Near-optimal hard-decision 4-PPM decoder

• ½-rate serially concatenated turbo code

– < 1 dB from theoretical channel capacity

Serially Concatenated

PPM Turbo Code

LLGT-LLST Ranging

0 10 20 30 40 50 603.7

3.72

3.74x 10

5

Time (minutes)

LL

GT

-LL

ST

Ran

ge (

km

)

May 01 2012 00:00

0 10 20 30 40 50 60-2

0

2

Ran

ge V

elo

cit

y (

km

/s)

• Ranging is critical for space

navigation

• Fast optical clocks on uplink and

downlink comm signals potentially

enable high-resolution ranging

– 5-GHz downlink clock <1-cm range

resolution

• To demonstrate this potential, LLCD

will measure two-way time-of-flight

to sub-slot precision 11- BSR

LL

GT

LL

ST

Two-Way Time-of-Flight

Measurements

• Duplex communications systems may be easily augmented to

perform two-way time-of-flight measurements

• Additional requirements include

– Common time reference on forward and return links

• LLCD uplink frame duration = 32 x downlink frame duration

– Phase-locked clocks in one of the terminals

• Uplink phase-locked to downlink in LLST

– High-stability time reference for measuring two-way time-of-flight

Uplink

Transmitter

Uplink

Receiver

Downlink

Transmitter

Downlink

Receiver

Frame Clocks

12- BSR

13- BSR

LLCD Clocks for Two-Way Time-of-

Flight Measurements

Duration Distance

Uplink

Slot 3.2 ns 96 cm

Symbol 51.4 ns 15.4 m

Codeword 390 us 117 km

TDM Frame 6.25 ms 1873 km

Downlink

Slot 200 ps 6 cm

Symbol 3.2 ns 96 cm

Codeword 12.2 us 3.7 km

TDM Frame 195 us 58.5 km

• Numerous clocks embedded in uplink and downlink optical signals

• Successful optical communications requires precise synchronization of clocks

– Slot

– Symbol

– Codeword

– Frame

• High frequency clocks provide high-resolution common time reference between transmitter and receiver

• Low frequency frame clocks provide unambiguous measurements of long time scales

– Downlink frame gives 58.5-km ambiguity

– Can be improved to 1873-km ambiguity by observing loopback frame sequence

– Could be further improved by inserting information in loopback data stream

• High-stability GPS-disciplined clock at LLGT used to generate uplink clocks and compare with received downlink clocks

Combination of high-bandwidth modulation and low-frequency

frame clocks enables unambiguous sub-cm ranging.

Acquisition Sequence Summary

Step A: LLST stares,

LLGT scans

Step C: Narrow beams Step D: Transition to

comm signal, fiber

tracking

Step B: LLST acquires

14- BSR

Some Conditions That Can Vary

in Lasercom Links

• Ground Terminal elevation – low / medium / high

• Time of day – dark night / dusk-dawn / bright sky

• Ground Terminal SEP – small / other

• Ground Terminal brightness behind Space Terminal – dark / light / terminator

• Space Terminal background – dark / medium / clouds-snow

• Space Terminal SPE – small / other

• Ground Terminal winds – low / medium / high

• Ground Terminal turbulence – low / medium / high

• Ground Terminal visibility – good / medium / poor

• Space Terminal temperature – near 0 / room temp / hot

• Time since last Ground Terminal calibration

• Time since last Space Terminal boresight

• Hundreds of combinations

• LLCD will try to sample as many of these as possible in its short mission

16- BSR8

Summary

• Optical communications offers the potential for substantial

improvements in deep space communications capabilities

while reducing burden on spacecraft

• LLCD will be NASA’s first step in making lasercom a reality

for future NASA missions