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SLR2000 is an autonomous and eyesafe single photon-counting satellite laser ranging 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. Replication costs are expected to be roughly an order of magnitude less than that of current manned systems, and the system will be about 75% less expensive to operate and maintain relative to the current manned systems. Computer simulations have predicted a daylight tracking capability to GPS and lower satellites. Computer and hardware simulations have demonstrated the ability of our current correlation range receiver and autotracking algorithms to extract mean signal strengths as small as 0.0001 photoelectrons per pulse from solar background noise during daylight tracking. The initial SLR2000 system concept was developed in 1994 [1], and the technical approach was refined in later years [2]. However, significant funding for the project was not provided by NASA until August 1997. During the first year of funding, prototypes of several "enabling' components, without which the system is not feasible, were successfully developed. These include: (1) a sensitive, high speed, quadrant microchannel plate photomultiplier; (2) a moderate power microlaser transmitter; (3) a "smart" meteorological station; (4) a high speed range gate generator; and (5) a high speed, high resolution event timer. Once the key specifications on these advanced components were largely met and system feasibility had been established, attention then turned to the detailed engineering design and procurement of more conventional elements of the system such as the shelter and protective dome, arcsecond precision tracking mount, telescope, and optical transceiver. The principal challenge during this second phase was to keep prototype fabrication and replication costs as low as possible to meet our cost goals. Prototypes of the various SLR2000 components and subsystems have either been developed or are well into the detailed design! build phase. The system is scheduled to conduct field tests in the 2000-2001 time frame. The primary driver for schedule is a fixed level of funding available each year to support SLR2000 development. A fairly detailed engineering overview of the SLR2000 system was presented approximately one year ago at the 1 1th International Workshop on Laser Ranging in Deggendorf, Germany, and has recently been published in the Workshop Proceedings [31. In addition, the SLR2000 project maintains a web site at the following URL address: http://cddisa.gsfc.nasa.gov/920_3/slr2000/slr2000.html Thus, only a brief overview of engineering status and a summary of recent developments (i.e. within the past year) on the various subsystems will be given here. The reader is referred to earlier publications [1-3] for more detail on the overall system.
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