Satellite Laser Ranging: Current Status and Future Prospects

This paper is intended to be a nonmathematical tutorial on the subject of satellite laser ranging (SLR) with an emphasis on the characteristics and capabilities of present and future field-hardware and operational methods. Following a brief introduction to the basic concept of SLR and the many science applications of both satellite and lunar laser ranging, we discuss the developmental history of each of the major components which make up the ranging machine, i.e., the laser transmitter, photomultiplier, discriminator, and time interval unit. At the same time, we attempt to identify the sources of range error in each of the devices and present, whenever possible, experimental data which quantifies the magnitude of these errors. We also describe some of the subtleties associated with the operation of these devices in the field. Following the discussions on hardware and system calibration techniques, we briefly describe some error sources external to the basic ranging machine, but highly relevant to SLR, which are introduced by the target, atmospheric channel, ground surveys, epoch timekeeping, geopotential models, and numerical propagation errors. We summarize the description of modern day hardware with samples of actual satellite data, obtained as early as 1981, which show orbital fits with a 1.5 cm single shot rms and normal point rms of less than 3 mm with only 1-6 percent data editing. We conclude the paper with a discussion of ongoing research to develop systems potentially capable of millimeter absolute accuracies over satellite distances.