Asynchronous Laser Transponders: A New Tool for Improved Fundamental Physics Experiments John J. Degnan, Sigma Space Corporation 4801 Forbes Blvd., Lanham, MD 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) 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(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)

Univ. of Maryland Airborne Atomic Clock Experiment (C. O. Alley et al,1975) Gravitational redshift52.8 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 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. 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 *J. Degnan, J. Geodynamics, Nov

Timing Diagram and Equations for Asynchronous Ranging and Time Transfer dt t M1 t M2 t E1 t E2 t EM t ME R = c(t ME +t EM )/2 = c [(t E2 -t E1 )+(t M2 -t M1 )]/2 dt = [(t E2 -t E1 )-(t M2 -t M1 )]/[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 Mercury GSFC 1.2 Meter Telescope 24.3 Million Km Science/Analysis/Spacecraft David SmithMaria Zuber Greg NeumannJohn Cavenaugh Ground Station Xiaoli Sun Jan McGarry Tom ZagwodzkiJohn Degnan D. Barry Coyle *D. E. Smith et al, Science, January 2006.

Two Way Laser Link between Earth and Messenger Spacecraft Downlink – Space to EarthUplink – 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/Spacecraft David SmithMaria Zuber Greg Neumann Jim Abshire Ground Station Xiaoli Sun Jan McGarry Tom ZagwodzkiJohn Degnan 100’s of pulses observed at Mars!

Transponder Link Parameters

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 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.

Equivalent Transponder/Lasercom Range for Two Station SLR Transponder/Lasercom System: Two-Station Ranging to a Satellite: Setting gives us an equivalent transponder range for the two-station SLR experiment 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   10 3 Interplanetary Distance, AU BLUE (SLR Satellites) Champ ERS Starlette Jason LAGEOS Etalon GPS LRE Apollo 15 Red curves bound the Earth-planetary distance Blue curves bound the equivalent transponder range at satellite elevations of 90 and 20 degrees respectively.

Integrated Lasercom/Transponder / 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

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)