2010 Olin Project Idea

Keith Gendreau

Keith.c.gendreau@nasa.gov

301-286-6188

Jeff Livas

Jeffrey.livas@nasa.gov

301-286-7289

“Solar Jujutsu” Communications

• Jutjutsu – literally the “way of yielding”, is a martial art style that uses the principle of using an attacker’s energy against him/her, rather than directly opposing it

• In this context – try to exploit sunlight or light from another source for communications

“Solar Jujutsu” Communications• Key idea:

– Can we make a low power data transmitter to send “lots” of data from the moon to the earth using a 19th century idea enhanced with 21st century technology?

• Applications– Data downlink for a sensor on the moon– 2-way link from a satellite using a cube-

corner reflector painted by a laser from the ground or another, larger satellite

“Solar Jujutsu” CommunicationsPower Source:Solar constant = 1,367,700 mW/m2 coming at you

Wavelength (microns)

Visible!

(cont)

“Prior Art”

Replace this guy with a high speed optical modulator and an ethernet port.

Replace this guy with a photoreceiver and an ethernet port..

Replacing the guy wiggling the mirror

• Computer monitor (simple and available)– Use simple patterns (“symbol”) to send many bits at

a time– 2 x 2 “Checkerboard” at 1 Hz 24 rate multiplier

• This rate multiplication is almost “free” (very low cost)

• Can investigate other options:– Voltage Controlled LCD displays (KHz Speeds?) (for

example slide displays)– Acoustic Optical Modulators (speeds up to 100 MHz)

Replacing the guy using his eye to see the signal on the

receive end• Webcam (slow, but simple)

– Receive patterns and convert back to bits

• Eventually a segmented photodiode?– Quad cell– Linear array

Simplified Block Diagram

Computer with Olin Student software that prepares the

test pattern for transmission, encodes it

into a pattern for the monitor, decodes the received pattern, and compares received to transmitted. Ideally

produces a BER

cable

Computer monitor

Earth to Moon (“link”)simulator

Sun (or lamp)

Mirror or optics (collection area sets Transmitter power)

Webcam

cable

Project Summary

• Build a “Heliostat” to capture the sun– Optional: start with a lamp or a mirror

• Bounce the light off a spatial light modulator (SLM)– Use a computer monitor to start (find one that works well in reflection– May need some simple optics or telescope

• Implement a simple modulator/encoder to drive the SLM• Use a webcam to receive

– May need some simple optics or telescope

• Implement a simple demodulator to retrieve data

• Predict link performance and compare to Laser Comm.

Optical CommunicationsOptical Communications• With an optical link it is natural to

use it for communications in addition to ranging.

• Potentially higher capacity over large distances than RF communications.

• Several methods currently under development at GSFC.

Parameter Downlink Uplink

Wavelength (µm) 1.55 0.775Data Rate (Mbps) 900 550Tx aperture (cm) 5.00 40.00Rx aperture (cm) 202.50 5.00Code Rate 0.80 0.80

receiver sensitivity (photons/bit) 100 100

BER 1.50E-03 1.50E-03Output power (W) 1 8

Transmitter losses (dB) -3.8 -3.8Net prop loss (dB) -80.78 -88.85

Receiver losses (dB) 2 2

Net Rx power (dBm) -52.58 -51.62Net Margin (dB) 0.86 0.95

There should benefits compared to Laser Comm

• Part of project is to Quantify Benefits– How does such a system compare to RF communication?– How does such a system compare to laser communication?

• Total Power Efficiency– Lasers are ~10% efficient on producing optical output from

electricity it gathers from ~25% efficient solar cells.– Total efficiency from sun = 0.25 * 0.1 = 2.5%– Mirrors are ~90% reflective– Etc.

Other factors in comparison• Angular sensitivity

– Narrow beam (optical vs RF) means more efficient energy transfer

– BUT more precise pointing requirements• Corner cube finesses alignment to some extent

• Mass to moon– Do solar cells and power system with Laser weigh more

than a mirror and heliostat?

• Reliability– Solar panels, motors, AOMs…

– Is dust an issue?

Success means:• A proof of concept demonstration of data

transmission using an external light source– Start with a webcam looking at a monitor

• Estimate of possible performance and benefits over more traditional systems– Focus on efficiency and pointing issues

• Look at supported Data rate• range

– Other criteria as possible: power, mass, volume

Possible Additional Analysis

• 1) quantitative link budget analysis - sensitivity, noise sources, etc

• 2) comparison with laser-based syste• 3) Bit-error-rate performance testing• 4) error correction coding• 5) laser ranging (= time of flight

measurements)