Space-Based Optical Communications with Precision Ranging Capability For Testing Relativity

Stephen M. Merkowitz and Jeff Livas NASA/GSFC May 22, 2006

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Background

Beyond Einstein: From the Big Bang to Black Holes

Retroreflector arrays placed on lunar surface by Apollo 11, 14, & 15 and French Soviet Lunakhod still in use after more than 25 years. Current ranging systems achieve 2-3 cm accuracy, new mm ranging looks promising.

Typical signal loss due to passive reflection is 10 -21 Significant advances in lunar ranging and ranging to other planets will require laser transponders. r 4 -> r 2 Q2C Ranging - 5/22/06 2

Gravitational Physics from Lunar Ranging

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• • • • • Verification of the Equivalence Principle for massive bodies. Relativistic precession of lunar orbit (geodetic precession).

Change in Newton’s Gravitational Constant G with time.

Determination of PPN parameters  of gravity) and  (nonlinearity in superposition (amount of space curvature produced by unit test mass).

Verification of gravitomagnetic and inductive interactions.

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Other Science From Lunar Ranging

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Lunar ranging has greatly improved knowledge of the Moon's orbit, enough to permit accurate analyses of solar eclipses as far back as 1400 B.C.

Revealed a small but constant change in the shape of the Earth. The land masses are gradually changing after being compressed by the great weight of the glaciers in the last Ice Age.

Small-scale variations in the Moon's rotation have been measured. They result from irregularities in the lunar gravity field, from changes in the Moon's shape due to tides raised in the Moon's solid body by the Earth and from the effects of a fluid lunar core.

Sun/(Earth + Moon) mass ratio leading to precision GM Earth Precise positions of the laser ranging observatories on Earth show crustal plate drift.

Moon is receding from Earth at a rate of about 3.8 cm per year due to ocean tides.

And more… Q2C Ranging - 5/22/06 4

Moon to Mars

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• • • • Current Mars ranging achieves only meter level accuracy.

Sun-Earth-Mars-Jupiter system tests SEP qualitatively different from LLR.

SEP polarization effect is ~100 times larger for Earth-Mars orbits than for lunar orbit.

Mass of Jupiter can be determined more accurately than from Pioneer & Voyager data combined.

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Advanced Satellite Ranging

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• Precision satellite ranging would enable advanced LAGEOS, GRACE, and other fundamental physics missions. Q2C Ranging - 5/22/06 6

Combine with Optical Communications

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• • • With an optical link it is natural to use it for communications in addition to ranging.

– Mercury Laser Altimeter instrument on Messenger has demonstrated the basics of laser communication over interplanetary distances.

Mission data requirements are increasing – Free-space optical communications potentially has higher capacity over large distances than RF communications, – Interplanetary missions may stress both range and data rate, – Typically, optical communications is most cost effective at high data rates.

Mars Telesat LaserComm (now canceled) did not include ranging, but it could have been added for little cost had there been justification.

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Possible Roadmap

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• • • • Small, inexpensive ranging instruments based on existing technology can be used to demonstrate techniques and gain experience.

Ranging requirements should be placed on laser communication systems under development.

Laser communication and ranging instrument should be included on one of the first lunar landers. – Multi-spectral imaging, robot control, astronauts surfing the web, etc. will require a high bandwidth link.

Laser communication and ranging instrument included on a future Mars lander.

– Laser communication offers a low-power option for high bandwidth link.

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Low Light Option

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Small inexpensive instruments, Heritage in laser altimeters and current satellite & lunar ranging, Asynchronous allows some loss of pulses, Q-switched or MOPA lasers offer good power, Photon counting detection, Modest communications possible, Ranging limited by clock stability.

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Higher Power Option

Beyond Einstein: From the Big Bang to Black Holes

Stronger signals enable use of phase in comm and ranging measurement, Heritage in telecommunication systems and precision interferometers, Master laser followed by optical amplifier offer reasonable power, Pulse shaping maximizes power usage, Direct detection offers fast response, but requires more light, Fast communications with low bits/photon possible, Encoding ranging signal removes distance ambiguity, End-stations can be phase locked to improve performance, Frequency stabilized master laser can act as precision clock, Very sensitive differential ranging possible using a phasemeter.

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Practical Approach

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Take advantage of commercial telecomm technology – – Externally modulated CW lasers, Low cost parts.

Use master oscillator/power amplifier (MOPA) architecture to separately optimize by function – – – Enables use of quite laser, Clean modulator, High output power.

Use a fast sensitive receiver – Implement a low photons/bit modulation format such as return to zero differential phase shift keying (RZ-DPSK).

RZ can be short period/high energy – – Enable more photons out of amplifier, Improves receiver sensitivity.

Optical phase locking should improve precision.

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Return to zero differential phase shift keying

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 Demonstrated ~25 photons/bit receiver sensitivity at 10 Gb/s Q2C Ranging - 5/22/06 12

MOPA Transmitter

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Laser Modulator Modulator Optical Power Amplifier Telescope Pulse shaper Oscillator FEC/Framer Data In Pulse Carver Data Modulator Power time Phase Shift

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time

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0

13

Telescope

Receiver/Demodulator

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PD1 Erbium Doped Fiber Amplifier Clock Recovery PD2 Balanced Receiver 1 bit time delay Phase Lock Local Laser

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Phase Shift 0 PD1 PD2 0 0 “+” Rx out 1 “-” -1 time Error Correction Data & Ranging Out time

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Summary

Beyond Einstein: From the Big Bang to Black Holes

• • • • • More precise lunar ranging will enable unprecedented tests of Einstein’s theory of General Relativity in addition to providing valuable data on the interior structure of the Moon and Earth-Moon interactions (tidal effects, etc.).

Precision ranging to Mars would provide additional tests of Einstein’s theory of General Relativity, unique data on the structure of Mars, and even provide the most accurate determination of the mass of Jupiter.

Laser communications is likely to be required for advanced interplanetary mission.

Several technology options exist for combining optical communications with precision ranging.

Now is the time to make sure that future planetary missions will support precision ranging.

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