ABSTRACT

In Satellite Laser Ranging (SLR), a short laser pulse is transmitted from a ground station to an orbiting satellite and reflected back to the station, which measures the roundtrip time of flight and hence the station-to-satellite range. The first laser returns from an artificial satellite were recorded by a NASA team at Goddard Space Flight Center on 29 October 1964.The satellite, Beacon Explorer 22B, was equipped with a Laser Retroreflector Array (LRA), designed to return the laser light to its point of origin. In July 1969, the Apollo 11 astronauts placed the first LRA on the surface of the Moon; the number of lunar LRAs was later increased to 5 by Apollo 14 and 15 and two unmanned Soviet Lunakhod missions. Over the intervening decades, the ranging precision has improved from a few meters to a few mm and the number of stations in the global network has increased to about 40. Today, the International Laser Ranging Service (ILRS), formed in 1998, coordinates the tracking operations and data analysis activities of approximately 30 participating countries. SLR is one of four techniques currently in wide use by the space geodetic community; the others are Very Long Baseline Interferometry (VLBI), Global Navigation Satellite Systems (e.g. GPS) , and Doppler Orbitography and Radiopositioning by Satellite (DORIS).

SLR currently defines the Earth Scale Factor (GM) and the origin of the International Terrestrial Reference Frame (ITRF), i.e. the Earth’s center of mass. Following the launch of the first geodetic satellites in the 1970s, SLR contributed heavily to our early modeling of the Earth’s gravity field, global tectonic plate motion, and regional crustal deformation near plate boundaries. Between VLBI sessions, SLR was also used to interpolate measurements of the Earth Orientation Parameters (EOP, which define the spin axis of the Earth and its time-dependent orientation and speed of rotation within the Celestial Reference Frame. More recently, SLR has been used to transfer time between atomic clocks, located on different continents, at the 50 picosecond level .

The Precise Orbit Determination (POD) capability of SLR has also supported a diverse array of Global Navigation Satellite Systems (GNSS), such as GPS, and international remote sensing satellites. For example, the combination of SLR with spaceborne microwave altimetry has provided spatially resolved maps of: global ocean currents and their velocities, mean sea level (MSL) rise, and even deep sea floor topography. With the advent of spaceborne laser altimeters, which derive much of their technology from the SLR program, high resolution topographic maps of the Earth, Moon, Mars, Mercury, and several asteroids have been obtained. In parallel, Lunar Laser Ranging has made important contributions to Lunar Physics, the Solar System Reference Frame, and General Relativity/Fundamental Physics.

Since the Millennium, a few SLR stations have developed the ability to track satellites in daylight using low energy kHz lasers and single photon returns. This demonstrated capability to extract very low level signals from the solar background has opened the door to precise interplanetary ranging and time transfer through the use of laser transponders, with the promise of further contributions to lunar and solar system science, more precise relativity experiments, and improved lunar and planetary mission operations. A second spinoff of single photon SLR technology has been the development of airborne and spaceborne single photon laser altimeters and 3D imaging lidars, which have demonstrated unprecedented surface measurement rates up to 3.2 million pixels per second.

In October 2013, the Laser Communications Demonstration Experiment (LCDE) on NASA’s lunar LADEE mission demonstrated a 622 Mbit/sec link between the Earth and Moon. Because they share a common need for precision optical tracking telescopes, the globally distributed SLR stations are potential hosts for future high bandwidth optical communications networks and may eventually perform both functions. Furthermore, the current SLR stations and satellite constellation can be used to simulate and test interplanetary transponder and optical communications links through the Earth’s atmosphere without the need for a new and costly space segment.

About the Speaker:

Prior to joining Sigma Space Corporation as Chief Scientist in February 2003, Dr. Degnan accumulated over 38 years of technical, supervisory, and project management experience at NASA’s Goddard Space Flight Center where he led the development of advanced lasers and electro-optical sensors. Supervisory positions at GSFC included: Head, Advanced Electro-optical Instrument Section (1979-1989), Deputy Manager, NASA Crustal Dynamics Project (1989-1993), and Head, Geoscience Technology Office (1993-2003). His GSFC organizations managed and/or provided direct technical, algorithm and software support to the NASA Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI) space geodetic networks, the LAGEOS 2 and TOPEX/Poseidon missions, Mars Orbiter Laser Altimeter (MOLA), Geoscience Laser Altimeter System (GLAS), and Messenger Laser Altimeter (MLA) missions to Mars, Earth, and Mercury respectively. He has authored over 200 publications (including many invited review papers and book chapters) on lasers and laser instrumentation. From 1989 to 1993, he held the post of Distinguished Adjunct Professor of Physics at The American University in Washington DC where he taught a two semester graduate course in Quantum Electronics. In 1998, he led the creation of the International Laser Ranging Service (ILRS) and served as its first Governing Board Chairperson until his retirement from NASA. Dr. Degnan is a Fellow of the International Association for Geodesy (IAG), a Senior Member of OSA and IEEE, a Charter Member of the International Laser Communications Society, and Sigma Pi Sigma National Physics Honor Society. He is the recipient of numerous awards from NASA (including GSFC’s prestigious Moe I. Schneebaum Memorial Award for Engineering in 1987), academia (including Drexel University’s Alumni Circle of Distinction Award in 2005), and international organizations (including the Tsiolkovsky Medal from the Director of the Russian Space Agency Rosaviacosmos in 2002).