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/Heritage

Satellite Laser Ranging (SLR)

•

NASA’s Next Generation Photon-Counting SLR2000 System

• • • • 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

strength falls off as R -4 . is not applicable much beyond lunar distances since the reflected signal

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

Retroreflector Array Sites Apollo 11, 1969

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(M E + M M ) 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)

AIRCRAFT CLOCK

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

Pulse detected and reflected at aircraft Transmitted pulse leaves ground station GROUND CLOCK Pulse Time of Arrival at Aircraft in Ground Time

Gravitational redshift Time dilation Net effect 52.8 ns -5.7 ns 47.1 ns

Reflected pulse detected at ground station

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.

t EM

Types of Transponders*

t M1 t d t M2 t ME Moon • Earth t E2 t E1

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 t d 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.

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

.

 t M1 t M2 MARS t ME t EM EARTH t E1 t E2 •

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

Timing Diagram and Equations for Asynchronous Ranging and Time Transfer

t M1 t M2 SPACECRAFT dt R t ME t EM EARTH t E1 t E2 Range R = c(t ME +t EM )/2 = c [(t E2 -t E1 )+(t M2 Clock Offset dt = [(t E2 -t E1 )-(t M2 -t M1 )]/[2(1+R/c)] -t M1 )]/2

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

GSFC 1.2 Meter Telescope

Ground Station Xiaoli Sun

24.3 Million Km

Jan McGarry Tom Zagwodzki John Degnan D. Barry Coyle

Messenger Laser Altimeter (MLA) enroute to Mercury

Science/Analysis/Spacecraft David Smith Maria Zuber Greg Neumann John Cavenaugh

*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)

80 Million Km!

100’s of pulses observed at Mars!

GSFC 1.2 Meter Telescope

Ground Station Xiaoli Sun Jan McGarry Tom Zagwodzki John Degnan

Mars Orbiter Laser Altimeter (MOLA)

Science/Analysis/Spacecraft David Smith Maria Zuber Greg Neumann Jim Abshire

Transponder Link Parameters

Experiment Range (10 6 km) Wavelength, nm MLA (cruise) 24.3 1064 MOLA (Mars) ~80.0 1064 Pulsewidth, nsec Pulse Energy, mJ Repetition Rate, Hz Laser Power, W Full Divergence,



rad Receive Area, m 2 EA-Product, J-m 2 PA-Product, W-m 2 Uplink

10 16 240 3.84 60 .042 0.00067 0.161

Downlink

6 20 8 0.16 100 1.003 0.020 0.160

Uplink

5 150 56 8.4 50 0.196 .0294 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) Passive Target (e.g. LAGEOS

1064 nm Data Flow

Station B- Remote Transponder Simulator Station A – Earth Station Simulator

Equivalent Transponder/Lasercom Range for Two Station SLR

Link Equations (A to B) Transponder/Lasercom System:

n T AB

 4 

q B



h



t A



B

sec

r



t T A

  2

A



A T B

sec 

B E t A A r B R T

2 Two-Station Ranging to a Satellite:

n R AB

 4 

q B



h



A t A



s

  

t

2

r B T A

2 sec    2

A E t A A r B R R

4

n T AB AB

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

R T



h

, 

A

, 

s

 

R R

2 

h

, 

A

 4  

s

  sec 

T B

sec 

T A B A

  

R R

2 

h

, 

A

 4  

s

1

T A

sec 

A

Satellite Simulations of Transponder/Lasercom Links

1 .

10 3

throughout the Solar System

Moon Mercury Venus Mars Jupiter Saturn Uranus Neptune Pluto

100

RED (Planets)

10 1 0.1

BLUE (SLR Satellites)

0.01

Champ ERS Starlette Jason LAGEOS Etalon GPS LRE Apollo 15

1 .

10 3 Red curves bound the Earth-planetary distance Blue curves bound the equivalent transponder range at satellite elevations of 90 and 20 degrees respectively.

9 6 Laser Enable Transponder/ Beacon Laser Driver

Integrated Lasercom/Transponder

6 Dichroic Mirror 100% T @ 532 nm To Lasercom Microchip Laser 2kHz Pulse Train Annular Mirror 100% R @ 1064 nm Pointing System Expander Two-Axis Gimbal Mount shared by Lasercom & Transponder Start Diode Outgoing Pulse Quadrant Detector 9/6 for Transponder/ Earth Lasercom Beacon 9 532 nm Spectral Filter 2 kHz Transponder/ Beacon from SLR2000 Beam Splitter Incoming Pulse (4 channels) Gimbal Controller SLR2000 Lasercom Transmitte r Spatial Filter + 10 arcsec Fine Pointing Correction Transponder Receiver Pulse Epochs & Quadrant Amplitudes 2 kHz Spatial Filter + 1 o FOV 9/6 Frequency Divider 10 MHz from Rubidium clock Transponder/ Beacon CPU Imaging Lens Lasercom Tracking Mount Controller Laser Enable 9 SLR2000 Station Location & Intermediate Pointing SLR2000 station acquisition verification & data download enable Gimbal Pointing Corrections Earth/Moon Range Data to Spacecraft Recorder CCD Array & Readout (Earth disk & SLR2000 beacon) 9

NASA TRL for sub-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 Satellite Altitude: 1350 km Daylight Pass: 3/15/05

Closed Loop Tracking of BEC Satellite

Photon-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)