Asynchronous Laser Transponders: A New Tool for Improved

Fundamental Physics Experiments

John J. Degnan, Sigma Space Corporation

4801 Forbes Blvd., Lanham, MD 20706

From Quantum to Cosmos: Fundamental Physics Research in Space

International Workshop, Washington, DC USA

May 22-24, 2006

Outline

• Heritage: Satellite and Lunar Laser Ranging (SLR & LLR)

• Past Contributions of Laser Ranging to General Relativity

• Interplanetary Laser Ranging with Transponders

• Recent Transponder Experiments to the Mercury Messenger and Mars Global Surveyor Spacecraft

• Two Station SLR: Testing Transponder Concepts Prior to a Mission

• Interplanetary Transponder Design and Flight Readiness

• Summary

Background/HeritageSatellite Laser Ranging (SLR)

• Since 1964, NASA/GSFC has ranged with lasers to spacecraft equipped with retroreflectors

– Over 60 artificial satellites beginning with Beacon Explorer 22B in 1964

– 5 lunar reflectors since the Apollo 11 landing in 1969

• Observable: Roundtrip time of flight of an ultrashort laser pulse to and from onboard reflectors on spacecraft/Moon

• Range precision is presently 1 to 2 mm (instrument limited)

• Absolute accuracy is sub-cm (atmosphere and target limited)

• Single-Ended SLR technique is not applicable much beyond lunar distances since the reflected signal strength falls off as R-4.

NASA’s Next Generation Photon-Counting SLR2000 System

International Laser Ranging Service (ILRS) Network

Lunar Laser Ranging

• Currently five passive retroreflector arrays on the Moon

– 3 NASA (Apollo 11,14, and 15)

– 2 Soviet (Lunakhod 1 and 2)

• Long term LLR data set (1969-present) provided by three sites:

– MLRS, McDonald Observatory, Texas, USA

– CERGA LLR, Grasse, France

– Mt. Haleakala, Hawaii, USA (decommissioned in 1992)

• New LLR systems coming on line:– MLRO, Matera, Italy

– Apollo, Arizona, USA (multiphoton, 3.5 m telescope had “first light” in October 2005)

MLRS ranging to the Moon

Lunar Laser Retroreflector Arrays

Apollo 11, 1969Retroreflector Array Sites

Science Applications of Satellite and Lunar Laser Ranging

• Terrestrial Reference Frame (SLR)– Geocenter motion– Scale (GM)– 3-D station positions and velocities (>50)

• Solar System Reference Frame (LLR)– Dynamic equinox– Obliquity of the Ecliptic– Precession constant

• Earth Orientation Parameters (EOP)– Polar motion– Length of Day (LOD)– High frequency UT1

• Centimeter Accuracy Orbits– Test/calibrate microwave navigation techniques (e.g., GPS,

GLONASS, DORIS, PRARE) – Support microwave and laser altimetry missions (e.g.,

TOPEX/Poseidon, ERS 1&2, GFO-1, JASON, GLAS, VCL)– Support gravity missions (e.g. CHAMP, GRACE, Gravity

Probe B)

• Geodynamics– Tectonic plate motion– Regional crustal deformation

• Earth Gravity Field– Static medium to long wavelength components– Time variation in long wavelength components– Mass motions within the solid Earth, oceans, and atmosphere

• Lunar Physics (LLR)– Centimeter accuracy lunar ephemerides– Lunar librations (variations from uniform rotation)– Lunar tidal displacements– Lunar mass distribution– Secular deceleration due to tidal dissipation in Earth’s oceans– Measurement of G(ME + MM)

• General Relativity– Test/evaluate competing theories– Support atomic clock experiments in aircraft and spacecraft – Verify Equivalence Principle– Constrain parameter in the Robertson-Walker Metric– Constrain time rate of change in G

• Future Applications– Global time transfer to 50 psec to support science, high data

rate link synchronization, etc (French L2T2 Experiment)– Two-way interplanetary ranging and time transfer for Solar

System Science and improved General Relativity Experiments (Asynchronous Laser Transponders)

Univ. of Maryland Airborne Atomic Clock Experiment (C. O. Alley et al,1975)

AIRCRAFT CLOCK

GROUND CLOCK

Pulse detected andreflected at aircraft

Reflected pulsedetected

at ground station

Transmitted pulseleaves

ground station

Pulse Time ofArrival at Aircraftin Ground Time

Gravitational redshift 52.8 ns

Time dilation -5.7 ns

Net effect 47.1 ns

World’s Most Expensive Altimeter

Laser Transponders: Laser Ranging Beyond the Moon

• Given the current difficulty of laser ranging to passive reflectors on the Moon, conventional single-ended ranging to passive reflectors at the planets is unrealistic due to the R-4 signal loss.

• Since double-ended laser transponders have active transmitters on both ends of the link, the signal strength falls off only as R-2 and this makes interplanetary ranging and time transfer possible.

Types of Transponders*

• Echo Transponders (R <<1 AU)– Spacecraft transponder detects pulses

from Earth and fires a reply pulse back to the Earth station.

– To determine range, the delay td must be known a priori (or measured onboard and communicated back to Earth) and subtracted from the measured round-trip time-of-flight at the Earth station.

– Works well on “short” links (e.g. to the Moon) where the single shot detection probability at both terminals is high.

• Asynchronous Transponders (R >1 AU)– Transmitters at opposite terminals fire

asynchronously (independently).– Signal from the opposite terminal must be

acquired autonomously via a search in both space and time (easier when terminals are on the surface or in orbit about the planet)

– The spacecraft transponder measures both the local transmitter time of fire and any receive “events” (signal plus noise) on its own time scale and transmits the information back to the Earth terminal via the spacecraft communications link. Range and clock offsets are then computed.

– This approach works well on “long” links (e.g., interplanetary) even when the single shot probability of detection is relatively small

Earth

tE1

tE2

td

t tEM ME

t tM2

M1 MoontM1 tM2

tE1 tE2

tEM

tME

MARS

EARTH

*J. Degnan, J. Geodynamics, Nov. 2002.

Timing Diagram and Equations for Asynchronous Ranging and Time Transfer

dt

tM1 tM2

tE1 tE2

tEM

tME

R = c(tME +tEM)/2 = c [(tE2-tE1)+(tM2-tM1)]/2

dt = [(tE2-tE1)-(tM2-tM1)]/[2(1+R/c)]

SPACECRAFT

EARTH

Range

Clock Offset

R

Two-Way Transponder Experiment to the Messenger Spacecraft (May/June 2005)*

Messenger Laser Altimeter (MLA) enroute to MercuryGSFC 1.2 Meter Telescope 24.3 Million Km

Science/Analysis/SpacecraftDavid Smith Maria ZuberGreg Neumann John Cavenaugh

Ground Station Xiaoli Sun Jan McGarry Tom Zagwodzki John DegnanD. Barry Coyle

*D. E. Smith et al, Science, January 2006.

Two Way Laser Link between Earth and Messenger Spacecraft

Downlink – Space to Earth Uplink – Earth to Space

One-Way Earth-to-Mars Transponder Experiment (September 2005)

GSFC 1.2 Meter Telescope Mars Orbiter Laser Altimeter (MOLA)

80 Million Km!

Science/Analysis/SpacecraftDavid Smith Maria ZuberGreg Neumann Jim Abshire

Ground Station

Xiaoli Sun Jan McGarry Tom Zagwodzki John Degnan

100’s of pulsesobserved at Mars!

Transponder Link Parameters

Experiment MLA (cruise) MOLA (Mars) Range (106 km) 24.3 ~80.0 Wavelength, nm 1064 1064 Uplink Downlink Uplink Pulsewidth, nsec 10 6 5 Pulse Energy, mJ 16 20 150 Repetition Rate, Hz 240 8 56 Laser Power, W 3.84 0.16 8.4 Full Divergence, rad 60 100 50 Receive Area, m2 .042 1.003 0.196 EA-Product, J-m2 0.00067 0.020 .0294 PA-Product, W-m2 0.161 0.160 1.64

Table 1: Summary of key instrument parameters for recent deep space transponder experiments at 1064 nm.

Where do we go from here?• Messenger and MOLA were experiments of opportunity rather than design.

– Since the spacecraft had no ability to lock onto the opposite terminal or even the Earth image, the spaceborne lasers and receiver FOV’s were scanned across the Earth terminal providing only a few seconds of data.

– Detection thresholds were relatively high due to the choice of wavelength (1064 nm) and analog detectors

– Precision was limited to roughly a decimeter by the long laser pulsewidths (6 nsec) and comparable receiver bandwidths.

– Another two-way transponder attempt will be made as Messenger flies by Venus in June 2007.

• The physical size,weight, and accuracy of future interplanetary transponder experiments will benefit from current SLR technology trends, including:

– Multi-kHz, low energy, ultrashort pulse lasers (10 to 300 psec)– Single photon sensitivity, picosecond resolution, photon-counting receivers– Automated transmitter point ahead and receiver pointing correction via photon-counting

quadrant detectors (NASA’s SLR2000).– The establishment of a Transponder Working Group within the ILRS and the testing of

advanced transponder concepts on passive SLR assets in space via two station ranging.

Dual Station Laser Ranging(both stations lie within each other’s reflected spot)

Station A – Earth Station Simulator

Station B- RemoteTransponder Simulator

Data Flow

Passive Target(e.g. LAGEOS

Equivalent Transponder/Lasercom Range for Two Station SLR

422

sec2

4

4

R

Br

At

AtA

ABrs

At

BqAB

RR

AE

h

Tn

A

22

secsec

4

4

T

Br

At

AtA

BABr

At

BqAB

TR

AE

h

TTn

BA

A

A

B

AsAR

A

B

sARsAT

ThR

T

ThRhR

sec2

sec

sec2

14,

4,,,

Transponder/Lasercom System:

Two-Station Ranging to a Satellite:

Setting gives us an equivalent transponder range forthe two-station SLR experiment

ABR

ABT nn

Link Equations (A to B)

Satellite Simulations of Transponder/Lasercom Links throughout the Solar System

RED (Planets)

Moon Mercury Venus Mars Jupiter Saturn Uranus Neptune Pluto

1 103

0.01

0.1

1

10

100

1 103

Inte

rpla

net

ary

Dis

tan

ce, A

U

BLUE (SLR Satellites)

Champ ERS Starlette Jason LAGEOS Etalon GPS LRE Apollo 15

Red curves bound the Earth-planetary distanceBlue curves bound the equivalent transponder rangeat satellite elevations of 90 and 20 degrees respectively.

Integrated Lasercom/Transponder

Quadrant Detectorfor Transponder/Earth Lasercom

Beacon

532 nm SpectralFilter

Imaging Lens

CCD Array &Readout

(Earth disk &SLR2000 beacon)

LasercomTransmitte

r

To LasercomPointing System

Microchip Laser2kHz Pulse Train

Expander

Annular Mirror

Spatial Filter+ 1o FOVTransponder/

BeaconCPU

SLR2000 StationLocation &

Intermediate Pointing

Dichroic Mirror100% T @ 532 nm

100% R @ 1064 nm

BeamSplitter

Transponder/Beacon

Laser Driver

Start Diode

Spatial Filter+ 10 arcsec

2 kHzTransponder/Beacon from

SLR2000

TransponderReceiver Pulse Epochs

& QuadrantAmplitudes

Lasercom TrackingMount Controller

FrequencyDivider

10 MHz fromRubidium clock

2 kHz

SLR2000 station acquisition verification

& data download enableGimbal Pointing

Corrections

Earth/Moon RangeData to

SpacecraftRecorder

Incoming Pulse(4 channels)

Outgoing Pulse

Laser Enable

Laser Enable

Two-Axis Gimbal Mountshared by

Lasercom &Transponder

GimbalController

Fine PointingCorrection

SLR2000

9

9

9

9/6

9

6

6

9/6

NASA TRL forsub-cm system

NASA’s SLR2000: A Photon Counting Satellite Laser Ranging System

System Characteristics:• Day/Night Eyesafe Operation• Wavelength : 532 nm• Transmitted Energy: 60 J• Laser Fire Rate: 2 kHz• Transmitted Power: 120 mW• Pulsewidth: 300 psec• Telescope Diameter: 40 cm• Mean Signal Strength: <<1 pe per pulse

TOPEX/Poseidon SatelliteAltitude: 1350 kmDaylight Pass: 3/15/05

Closed Loop Tracking of BEC SatellitePhoton-Counting Quadrant Detector and Transmitter Point-Ahead

First closed-loop correction here

Summary• The ability of laser transponders to simultaneously measure range, transfer time

between distant clocks, and indirectly monitor the local gravity field at the spacecraft make it a useful tool for fundamental physics studies within the Solar System.

• Based on the recent successful experiments to the Messenger and MGS spacecraft, the space-qualified technology for decimeter accuracy interplanetary laser transponders is clearly available now; more compact sub-centimeter accuracy photon-counting systems can be made available within 2 to 3 years with very modest technology investments.

• Retroreflectors on international SLR spacecraft are available for simulating interplanetary transponder and lasercom links and testing the ground and spacecraft terminals prior to mission. Sigma has designed an improved array simulator for possible “piggyback” on a future GPS or GEO satellite.

• The International Laser Ranging Service (ILRS) has established a Transponder Working Group which is presently developing hardware and software guidelines for member stations interested in participating in future transponder experiments.

• Next transponder experiment opportunities: – Two Way: to Mercury Messenger during a Venus flyby (June 2007) .– One Way: NASA Lunar Reconnaissance Orbiter (scheduled launch in 2008)