INNOVATION The DoD's goals for the GPSILRE exper­ Laser Ranging iment are essentially the same as those of the Advanced Clock Ranging Experiment, which NRL originally proposed to DoD's Tri-Ser­ toGPS vice Space Test Program in 1986. In his pro­ posal, which later became the GPS/LRE, NRL principal investigator Ron Beard states Satellites with that a primary objective of the experiment is to provide an independent, high-precision Centimeter measurement of satellite position that, when compared with GPS pseudoranges, can unambiguously separate satellite-position Accuracy errors from on board atomic-clock errors. The accurate determination of satellite position and clock characteristics is essential to pre­ dicting GPS performance and reducing errors John J. Degnan in both the short and long term. The laser Erricos C. Pavlis measurements also support ongoing atomic­ clock evaluations within DoD and their effect Laboratory for Terrestrial Physics on overall GPS system performance. NASA Goddard Space Flight Center NASA's interest in the experiment stems from the agency 's current and future use of In 1960, Theodore H. Maiman, of the Hughes advances in GPS technology and its GPS in a variety of geophysics applications Aircraft Company, successfully operated the applications as well as the fundamentals of that previously had been carried out exclu­ first device to generate an intense beam of GPS positioning. The column is coordinated sively by two more mature space geodetic highly coherent monochromatic radiation. He by Richard Langley and Alfred Kleusberg of techniques: satellite laser ranging and very called his device a laser - for light amplifica­ the Department of Geodesy and Geomatics long baseline interferometry (VLBI). tion by the stimulated emission of radiation. Engineering at the University of New NASA's involvement in all three techniques The laser has become ubiquitous, with literally Brunswick. We appreciate receiving your is pervasive. The Jet Propulsion Laboratory hundreds of uses ranging from optical surgery comments as well as suggested topics for (JPL) in Pasadena, California - with fund­ to precision machining. Lecturers use laser future columns. ing from NASA- serves as both the Central pointers; surveyors use laser distance­ Bureau and an analysis center for the Interna­ measuring devices; police officers use laser On August 30, 1993, the Department of tional GPS Service for Geodynamics (IGS) radar units to catch speeders. Most of us Defense (DoD) launched the GPS-35 and has also been a leader in the engineering unwittingly use a laser each time we listen to (SVN35, PRN05) satellite into orbit, placing development of precise geodetic receivers for our CD players- the light reflected from the it in the GPS satellite constellation's B-4 both ground and spacecraft use. The Goddard microscopic pits on the CD is generated by a orbital plane position. This particular Block Space Flight Center (GSFC), which serves as precisely positioned laser. IIA satellite differs slightly from its prede­ an IGS Global Data Center, is also the pri­ One application of the laser that is not so cessors in that it carries a small panel of opti­ mary Coordination and Data Analysis Center well known is satellite laser ranging. In this cal retroreflectors, enabling it to be tracked for both SLR and VLBI, operating 10 of month's column, John Degnan and Erricos by an international network of centimeter­ approximately 43 stations in the global SLR Pavlis, from the Laboratory for Terrestrial accuracy satellite laser ranging (SLR) sta­ network and three of approximately 30 VLBI Physics (LTP) at NASA's Goddard Space tions. Laser tracking of GPS-35 began on field stations. GSFC hosts IGS GPS receivers Flight Center (GSFC), in Greenbelt, October 17, 1993. On March 10, 1994, GPS- at most of these sites. Maryland, introduce us to satellite laser 36 (SVN36, PRN06) , carrying an identical As GPS demonstrated its ability to mea­ ranging and describe the efforts to track two retroreflector package, was launched into the sure short- to medium-length baselines with of the Navstar GPS satellites using this C-1 orbital slot. Laser tracking of GPS-36 comparable accuracy in the late 1980s, it technique. Dr. Degnan is the head of LTP 's was initiated on April21 , 1994. gradually displaced the more-expensive Space Geodesy and Altimetry Projects Office. The GPS Laser Retroreflector Experiment mobile SLR and VLBI systems as the tech­ He has been employed at GSFC since 1964 (GPS/LRE), which focuses on these two nique of choice in internationally sponsored when, as a coop student from Drexel satellites, is a joint effort by several groups campaigns to measure regional crustal defor­ University, he participated in the first laser­ working in cooperation with the Navstar GPS mation. These early mobile campaigns were ranging experiments to the Beacon Explorer B Joint Program Office. These include the principally conducted in the United States, satellite. Dr. Pavlis is a senior geodesist in Naval Research Laboratory (NRL) in Wash­ . Canada, Mexico, and Europe. the LTP and is affiliated with the Department ington, D.C.; the National Aeronautics and NASA investigators are now comparing of Astronomy at the University of Maryland. Space Administration (NASA) through its global GPS network solutions with those His research interests include satellite Goddard Space Flight Center (GSFC) in obtained from SLR and VLBI. Following orbital dynamics and the analysis of space Greenbelt, Maryland; the University of an appropriate, seven-parameter coordinate geodetic data. Maryland at College Park, Maryland; and the transformation (three translational parame­ "Innovation" is a regular column in Russian Institute for Space Device Engineer­ ters, three rotational, and one for scale), GPS World,featuring discussions on recent ing in Moscow. global station positions obtained by the three 62 GPS WORLD Seplember 1994 ~ /~ ..... ...... ([;;)\~',.tf .~ Reflected / / Satellite time transfer. Another source by the receiver telescope. pulse ,./ ,./ optical of en·or in computing orbits Only a handful of the outgoing photons ..•. .-· ...... •• reflector •• ••·····Tran smitted for the GPS satellites is the makes it back. The telescope focuses the Tracking telescope ,.. •• ··' pulse description of attitude returning radiation onto a high-gain, hi gh­ and detector "' changes, especially during speed (subnanosecond) photodetector th at eclipse seasons when a satel­ stops the time-interval counter. A modern lite spends a significant frac­ SLR counter has a time resolution on the tion of its orbit in the earth's order of 20 picoseconds or less, which corre­ shadow. Better handling of sponds to a single-shot range resolution of 3 the nonconservati ve forces millimeters or less. The epoch time of depar­ Figure 1. A simplified block diagram of a typical satel­ lite laser ranging (SLR) station. By measuring the acting on the satellites (such ture for the laser pulse is also recorded. elapsed time between the transmission of a laser pulse as solar radiation pressure) Because the station master clock is typically and the reception of the pulse reflected by the satellite, can be achieved by using pre­ a cesium standard, periodically updated by a the range to the satellite can be determined. cise SLR tracking and the GPS timing receiver, epoch time is accurate resulting orbits to "tune" to a small fraction of a microsecond. models of these forces to Typically, the photodetector is a photo­ techniques agree at the one- to two-centime­ each satellite individually. multiplier tube. In a photomultiplier tube, ter level at high-quality, collocated sites. This Geodetic positioning can benefit from the incident photons strike a photosensitive sur­ agreement is at a level consistent with the increased orbit accuracy of retroreflector ~ face, causing it to emit electrons through the estimated modeling uncertainties for each carrying GPS satellites in two ways: the photoelectric effect. These so-called photo­ technique. However, there are important geo­ better orbits result directly in better position­ electrons are accelerated by a potential dif­ physical phenomena where the expected ing; and they can also help in the reliable ference and st1ike an electrode, causing it to rates of change of position are on the order of resolution and "fixing" of the ambiguities of dislodge additional electrons. These elec­ a few millimeters per year or less. Such phe­ doubly differenced canier-phase measure­ trons are attracted to another electrode, which nomena include changes in mean sea level, ments. The latter strengthens the estimation in turn ejects more electrons. This process postglacial rebound, and deformations due to procedure because it converts, in effect, the may be repeated 10 or more times, generating atmospheric loading. A better understanding very precise but relative measure of change­ a cun·ent that can readily be measured. of the residual error sources and their magni­ in-range to absolute range (for more on GPS­ When fit to a short orbital arc (less than tudes for each of the positioning techniques canier-phase measurements, see "The GPS one orbital revolution), the single-shot data would be helpful, as would, if possible, the Observables," GPS World, April1993). from ranging to passive orbiting geodetic ability to make reliable measurements with satellites, produced by state-of-the-art SLR millimeter accuracy, especially in the vertical SLR PRINCIPLES field systems, typically exhibit a root-mean­ direction.