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

1
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

• 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.
  r4 -gt r2

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Gravitational Physics from Lunar Ranging

• Verification of the Equivalence Principle for
  massive bodies.
• Relativistic precession of lunar orbit (geodetic
  precession).
• Change in Newtons Gravitational Constant G with
  time.
• Determination of PPN parameters ? (nonlinearity
  in superposition of gravity) and ? (amount of
  space curvature produced by unit test mass).
• Verification of gravitomagnetic and inductive
  interactions.

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Other Science From Lunar Ranging

• 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 GMEarth
• 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

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Moon to Mars

• 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

• Precision satellite ranging would enable advanced
  LAGEOS, GRACE, and other fundamental physics
  missions.

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Combine with Optical Communications

• 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

• 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

• 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

• 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

• 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

• Demonstrated 25 photons/bit receiver sensitivity
  at 10 Gb/s

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MOPA Transmitter
Laser
Modulator
Modulator
Telescope
Optical Power Amplifier
Pulse shaper
FEC/Framer
Data In
Oscillator
Pulse Carver
Data Modulator
Power
time
?
Phase Shift
0
time
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Receiver/Demodulator
PD1
Erbium Doped Fiber Amplifier
Error Correction
Clock Recovery
Telescope
PD2
Balanced Receiver
Data Ranging Out
1 bit time delay
Phase Lock Local Laser
?
Phase Shift
0

PD1
0
1
Rx out
PD2
-1
0
-
time
time
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Summary

• More precise lunar ranging will enable
  unprecedented tests of Einsteins 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 Einsteins 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.