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NASA Goddard Geophysical and Astronomical Observatory

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SLR at GGAO

MOBLAS-7

This picture shows the three vans that comprise the MOBLAS-7 system. The laser telescope can be seen in the rear of left-most van. The tower in the background on the left is the satellite communications providing GGAO connections to the Internet.

Satellite laser ranging (SLR) uses lasers to measure ranges from ground stations to satellite borne retro-reflectors to the millimeter level. In Satellite Laser Ranging (SLR) a global network of stations measure the round trip time of flight of ultrashort pulses of light to satellites equipped with special reflectors. This provides instantaneous range measurements of millimeter level precision which can be accumulated to provide accurate orbits and a host of important science products. Analysis of SLR data contributes to the International Terrestrial Reference Frame (ITRF), modeling of the spatial and temporal variations of the Earth's gravitational field, and monitoring of millimeter-level variations in the location of the center of mass of the total Earth system (solid Earth-atmosphere-oceans). In addition, SLR provides precise orbit determination for spaceborne radar altimeter missions mapping the ocean surface (which are used to model global ocean circulation), for mapping volumetric changes in continental ice masses, and for land topography. It provides a means for subnanosecond global time transfer, and a basis for special tests of the Theory of General Relativity.

GGAO is the birthplace of the world's first satellite laser ranging system, Goddard Laser (GODLAS), which became operational May 5, 1964. The knowledge gained from GODLAS spawned a new breed of laser system, the Mobile Laser System (MOBLAS). During the next fifteen years, the second and third generation MOBLAS systems were developed at GGAO and deployed to locations around the world. By 1985, the NASA Satellite Laser Ranging network of eight MOBLAS systems and four transportable laser ranging systems (TLRS) were operational as part of NASA's Crustal Dynamics Project (CDP) making scientific measurements including the determination of the movement of the Earth's tectonic plates.

GGAO is currently home to one of the most accurate SLR systems (MOBLAS-7) in the world. This SLR system is part of the International Laser Ranging Service (ILRS) network, whose primary mission is to support, through satellite and lunar laser tracking data and related products, geodetic and geophysical research activities. In addition to operation of the MOBLAS-7 system, GGAO has hosted several other SLR systems over the years for co-location testing.

MOBLAS-7

This photograph shows the MOBLAS-7 system (with two crew members) performing satellite ranging in the evening at GGAO.

MOBLAS-7, which is the most accurate SLR system in the world, is stationed at GGAO to perform three vital NASA SLR functions: (1) to collect important data on the movement of the North American plate, (2) to serve as a test bed for the implementation of the latest improvements in laser ranging technology, and (3) to serve as a network standard in special collocation tests which compare the performance of new or upgraded systems before they are sent into the field as operational systems.

The MOBLAS-7 system is comprised of three vans that contain various components of the system. Electronics and computer systems required by MOBLAS-7 for targeting, data processing, etc. are located in the vans. Radar systems to detect aircraft flying in the area are mounted on the other two vans.

48-Inch laser ranging system

NGSLR performing nightime ranging at GGAO.

Work continues at this facility on NGSLR, a totally automated satellite laser ranging system for the next decade. NGSLR (formerly known as SLR2000) is an autonomous and eyesafe photon-counting SLR station with an expected single shot range precision of about one centimeter and a normal point precision better than 3 mm. The system will provide continuous 24 hour tracking coverage of artificial satellites at altitudes up to 20, 000 Km. Replication costs are expected to be roughly an order of magnitude less than current operational systems, such as the NASA MOBLAS systems, and about 75% less expensive to operate and maintain relative to the manned systems. Computer simulations have predicted a daylight tracking capability to GPS and lower satellites with telescope apertures of 40 cm and have demonstrated the ability of our current autotracking algorithm to extract mean signal strengths as small as 0.0001 photoelectrons per pulse from solar background noise.The prototype NGSLR system is currently in the testing phase at GGAO.

48-Inch laser ranging system

1.2 meter (48") telescope at GGAO.

At the GGAO multi-user 1.2-meter telescope facility, the Laser Remote Sensing Laboratory (Code 694) is developing the next generation satellite laser ranging systems. These systems will use dual wavelength picosecond pulse-width lasers and picosecond resolution streak cameras to make absolute corrections for atmospheric refraction effects at the few millimeter level. In May 2005, an experiment conducted at this 1.2 meter telescope successfully exchanged laser pulses with the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) spacecraft while the two were millions of miles apart. This activity was the first successful two-way exchange of laser signals over a large (15 million miles) distance from Earth and demonstrated sub-nanosecond timing accuracy. The historic experiment has helped convince mission planners to include the same technology on the Lunar Reconnaissance Orbiter (LRO), which will launch in 2008 to map the lunar surface.

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