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Lasers f or Geodesy in the Year 2 O O O David E. Smith NASA Goddard Space Flight Center INTRODUCTION Laser ranging to earth satellites and the Moon began in the s ixties with range accuracies of several meters and has developed over the last two decades to the one centimeter level to become a maj or geodetic tool for addressing global and regional scale geophysical problems . Maj or contributions have been made to our knowledge and understanding of the Earth' s gravity field, the shape and size of the Earth, the motions of the maj or tectonic plates, the earth and ocean tides, and to our understanding of the orbit of the moon. During the next decade there will be significant advances in the technologies used in laser ranging which will be used to help meet the ever- increasing demands in scientific problems for greater accuracy, greater frequency of measurements, less time required to make measurements and, of course, lower cos t . In 1983 NASA organized a meeting called the Airlie House Conference (Walter, 1984) to discuss the scientific requirements for future space geodetic systems, including laser ranging. The report of this conference spelled out the measurement requirement of millimeters on a local, regional, and global scale for many future scientific problems. In the following sections I briefly describe the technological developments that are expected over the next few years that will influence the growth of laser ranging as a geodetic tool. I will also discuss ranging and altimetry systems that are being developed by NASA as part of its research and development program. TECHNOLOGY DEVELOPMENTS Two main areas of technology development during the next decade that will significantly influence the growth of laser ranging in all its forms are in the transmitter and in the receiver. Present day transmitters consist of a flashlamp pumped, mode-locked, Q-switched Nd: YAG short pulse laser oscillator, a double-pass amplifier, and a frequency-doubling crystal (ref. Cohen and Degnan, 1987~. Because of the need for increased accuracy and more efficient operation, future systems are expected to employ diode pumping and operate at two frequencies (532 and 355 nary). Diode pumping of the laser offers the potential for greater prime power efficiency, longer 91
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92 lifetimes, and a smaller, lighter weight system. Dual frequency operation permits the atmospheric delay to be derived directly from the measurement instead of relying on the modeling of the atmospheric delay based on a knowledge of pressure, temperature, and humidity at a point on the Earth's surface. Diode pumping of the laser is an order of magnitude more efficient than flashlamp pumping, and therefore, results in greater laser lifetime. This is of less importance for ground-based laser ranging but is critical for space-based systems where maintenance is very restricted. Further, great efficiency means less heat is generated so that higher pulse rates can be used and systems can be smaller. In addition, unlike the flashlamp, the method of failure of a diode system is gradual. At the present time flashlamps used in laser ranging systems have effective lifetimes on the order of 106 shots--months of actual operation or days of continuous operation, while the potential for diode pumping is on the order-of 109 shots--decades of actual operation or years of continuous operation. This development of long lifetime lasers makes spaceborne systems viable. A further development, which in part results from the greater efficiency of the diode pumping, is the shortening of the transmitted laser pulse length by two or three orders of magnitude. Present laser tracking systems have pulse lengths of 10 to 100 picoseconds (millimeters), but because of the increased accuracy requirements and the two-color capability, the lasers of the near future will need to have pulse lengths of the order of 10-~5 seconds. Lasers with pulse lengths of 10 x 10-~5 seconds are already operating in laboratories (Degnan, private communication, 1987~. Table 1 shows a comparison of some characteristics of a laser transmitter in 1987 with a system that can be expected before the year 2000. The major development in the receiver will be the introduction of the streak camera which effectively transforms the return pulse from the time domain to the spatial domain. Present-day laser ranging systems employ a photomultiplier to measure the amplitude of the signal and the time of flight. In the two-color lasers of the future the streak camera will act as a sub-picosecond resolution timing system for the green (532 nm) and near-ultraviolet (355 nm) pulses, thus enabling the dispersion caused by the atmosphere to be measured on a pulse by pulse basis (Cohen et al., 1987~. If the separation of the green and near-ultraviolet return pulses can be determined to 200 to 300 x 10-~5 seconds, the atmospheric correction can be determined to 1 mm. This is approximately the capability of present streak tubes, and improvements over the next decade are expected to lead to resolutions of 50 to 100 x lO-is seconds. Atmospheric corrections will then be obtainable at the sub-millimeter level and the overall range accuracy of the tracking system to the one millimeter level.
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93 Finally, in order to measure the range to an object, we must consider the contribution of the cornercube array to the total error. The most precisely configured array in orbit is that of the LAGEOS spacecraft which is believed to be accurate to the few millimeter level (Fitzmaurice et al., 1977~, but the majority of arrays are not this precise. In order to fully utilize the accuracy of the millimeter systems of the future, improved accuracy retro-reflector systems need to be designed to sub-millimeter level. The arrays will need to be smaller, so as to minimize pulse spreading, but large enough to provide sufficient return signal. This may cause pulse rates to increase from their present 5 to 10 pulses per second to the order of 1000 pulses per second, and for return pulses to be averaged to produce an accurate measurement. The design of the laser array and precise knowledge of the array's position with respect to the center of mass of the spacecraft could become a limiting factor in the development of increasingly accurate laser ranging systems. GROUND-BASED LASER RANGING Table 2 shows a comparison of some of the principal components of a ground-based laser ranging system of 1987 with a projected system of the future. The future system reflects the new technology developments previously described and includes the expectation that at least part of the future system will be automated. The degree of automation will depend on the operational philosophy rather than the technology. The development of fully automated ground-based systems will certainly be possible in the near future because to a large degree they will employ the same technology as the spaceborne systems. Fully automated systems could have many advantages, particularly in the cost of operation, but real-time communications systems will be needed for data relay and for routine operation. (For a full description of satellite laser ranging, the reader is referred to Degnan, 1985~. SPACE-BASED LASER RANGING During the next decade we can reasonably expect that significant progress will be made toward putting a laser ranging system into space. In the mid-seventies the preliminary design of such a system was developed for operation from the Space Shuttle and/or a free flying spacecraft (Vonbun et al., 1977; Smith, 1978~. A spaceborne laser is currently scheduled to fly on one of the EOS (Earth Observing System) platforms for launch in the 1997 timeframe (Degnan and Cohen, 1988~. The concept of the spaceborne laser is that precise range measurements can be made from an orbiting space platform to a network of laser retro-reflector arrays on the earth's surface and that from these measurements the relative locations of the retro-reflector arrays can be derived. Figure 1 shows the concept of the spaceborne laser. The general characteristics of the EOS system are shown in Table 3. As the spacecraft passes over the region, the laser makes range measurements to the retro-reflectors according to a pre-determined pattern, remaining on
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94 an array for just a few seconds before moving to the next. The laser has been designed to operate at two frequencies, 532 nm and 354 nm, in order to be able to remove the large effects of atmospheric refraction, and therefore, the need for meteorological sensors at the ground arrays. The basic measurement will be accurate to one centimeter or better, including any remaining atmospheric error. LASER ALTIMETRY A recent development important for geodesy has been the introduction of the laser altimeter. Already operating experimentally from aircraft, the laser altimeter sends a short pulse of radiation to the Earth's surface which is "reflected" back to the receiver in the aircraft. Conceptually, the laser altimeter is similar to the laser ranging system except that it operates without retro-reflectors, using only the radiation scattered back from the surface below. The system can provide topographic information over almost any surface, including land, snow, ice, oceans, and cloud tops. A spaceborne altimetric capability is planned as part of the EOS spaceborne laser facility. The fundamental frequency of the laser (1064 nm) is used for altimetry, and is not needed for ranging. The altimetric component will provide decimeter precision topography of the Earth's surface with horizontal resolutions of 80 and 160 meters (the latter with contiguous spots). This level of horizontal resolution is extremely difficult to obtain from a conventional radar altimeter. A laser altimeter is also being developed for operation around the Moon (Garvin et al, 1987; and 1988~. This developmental system, the Lunar Observer Laser Altimeter (LOLA), could fly on the proposed Lunar Observer orbiting 100 km above the lunar surface and provide decimeter vertical precision topographic profiles with horizontal resolutions of tens of meters. LOLA is designed to operate in two horizontal resolution modes. In the mapping mode, the system would obtain 300 meter spots that would be interpolated to provide a 1-2 kilometer global lunar topographic grid appropriate for geodetic and geophysical studies and regional characterization of all major terrain types. In the high resolution mode, LOLA would obtain 30 meter spots to be used for local- scale geological profiling. The absolute limitation on the accuracy of this system will be determined by the knowledge of the spacecraft orbit and the lunar gravity field in particular. Table 4 shows some of the performance characteristics of the altimetric functions of the EOS spaceborne laser ranger and the developmental lunar altimeter. A point of special importance in Table 4 is the factor of sixty greater power that is planned for the EOS system as compared to the lunar system. This simplifies the lunar system considerably by increasing the life expectancy of the system and reducing its size and weight. The Moon is an ideal object for a laser altimeter because a low altitude orbit is possible and atmospheric absorption is not a problem. In fact, a LOLA-class altimeter could be modified for orbital or flyby missions for any solid planetary body without an atmosphere (Garvin et al, 1987a).
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9s CONCLUSIONS (PREDICTIONS) The next decade will see the introduction of several new technologies into laser ranging, in particular: diode pumped lasers, two frequency ranging, streak cameras, and shorter pulse lengths. These technologies and developments will lead to (1) ground-based laser ranging systems accurate at the 1 mm level, including the effect of the atmosphere; (2) spaceborne laser ranging at the 1 cm level. or better. from the EOS platform or a similar system; and (3) laser altimetry providing topographic information of the Earth and Moon, and possibly the planets, at the 30 cm vertical precision level with decameter horizontal resolution. These capabilities, in conjunction with those occurring in the other areas of geodetic measurement science, will have a profound influence on geodesy and many associated disciplines. ACKNOWLEDGEMENTS I should like to acknowledge the thoughts and ideas of many of my colleagues at GSFC in this forage into the near future. In particular, Dr. Steven Cohen, Dr. John Degnan, Dr. James Garvin, and Dr. Maria Zuber have provided me the benefit of their own crystal balls in this gaze into the next decade plus. However, I take full responsibility for the predictions contained above. In addition, I am pleased to acknowledge the financial support provided for the instrument development by Dr. E. A. Flinn, NASA Geodynamics Program, for the ground-based and space-based ranging; and by Dr. L. Evans, NASA PIDDP Program, for the lunar laser altimeter.
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96 REFERENCES Cohen, S. C., and J. J. Degnan, Spaceborne Laser Ranging from EOS, Proceedings of IGARSS '87 Symposium, Ann Arbor, 18-21 May 1987. Cohen, S. C., J. J. Degnan, J. L. Bufton, J. B. Garvin, J. B. Abshire, The Geoscience Laser Altimetry/Ranging System, IEEE Trans. on Geoscience and Remote Sensing, Vol. GE-25, No. 5, September 1987. Degnan John J. Satellite Laser Ranging: Current Status and Future , , Prospects, IEEE Trans. on Geoscience and Remote Sensing, Vol. GE-23, No. 4, July 1985. Degnan, J. J., and S. C. Cohen, Applications of a Spaceborne Laser Ranger on EOS, SPIE LASER '88 Conference, 1988. Fitzmaurice, M. W., P. O. Minot, J. B. Abshire, H. E. Rowe, Prelaunch Testing of the Laser Geodynamics Satellite (LAGEOS), NASA Goddard Space Flight Center Tech. Paper 1062, October 1977. ~ Garvin, J. B., J. L. Bufton, J. B. Abshire, M. T. Zuber, Laser Altimetry in Planetary Geology, Lunar Planet. Sci. Conf. XVII, 381-391, 1987. Garvin, J. B., M. T. Zuber, J. L. Bufton, Planetary Laser Altimetry, I W G XIX Assembly, 99, 1987a. Garvin, J. B., M. T. Zuber, J. L. Bufton, Lunar Observer Laser Altimeter: Geoscience Applications, Lunar Planet. Sci. Conf. XIX, 379-380, 1988. Smith, D. E., Spaceborne Ranging System, Proc. 9th GEOP Conf., Dept. of Geodetic Science Rept. No. 280, Ohio State University, October 1978. Vonbun, F. O., W. D. Kahn, P. D. Argentiero, D. W. Koch, Spaceborne Earth Applications Ranging System, J. Spacecraft & Rockets, Vol. 14, pp. 492-495, 1977. Walter, L. S., Geodynamics, Proc. of Airlie House Conf., NASA Conf. Publ., #2325, 1984.
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97 Table 1: The Laser Transmitter 1987 1 color flash lamp pumping flash lamp efficiency 1% - 2% laser lifetimes: 106 shots pulse length: 100 x 10-~2 sees Table 2: Accuracy (best systems) Pulse rate Laser Energy Pulse length Data communication Automation 2000 2 colors diode pumping Ground-Based Laser System 1987 1 cm (excl. atm) 5-10 pps Nd:Yag 532 nm 100 mj 100 x 10-12 sees tapes, etc. via mal none 20% - 30% 109 shots 10-15 sees 2000 1 mm (incl. atm) 100-1000 pps Nd:Yag 532 10 mj 10 x 10-15 sees real-time via satellite , 355 nm some --> complete
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98 Table 3: Specifications for the EOS Spaceborne Laser Laser frequency doubled & tripled, mode locked Nd:Yag Pulsewidth 100 picoseconds, FWHM Energy/pulse Beam divergence Maximum pulse rate Receiver telescope diameter (ranging) 120 millijoules (1064 nm) 60 millijoules (532 nm) 20 millijoules (354 nm) 0.1 milliradians 40 pps 18 cm Table 4: Characteristics of the EOS and lunar laser altimeters EOS Orbital altitude Frequency Pulsewidth Energy/pulse Receiver telescope diameter Spot size Lunar 800 km 1064 rim 100 ps 120 mj 50 cm 80, 160 m 100 km 1064 rim 3 us 2 mj 25 cm 30, 300 m Vertical precision 10- 50 cm 30 cm (excl. atmosph . ~ Pulse rate 40 Hz 10, 50 Hz
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99 , Jo J I'm ~ TERRAIN A_ ~,~ ~-FAULT / C!VOTC ~ ~ - - - - L ~- ~ ~ ~.~:F~;-~:::~;~: ~[~ ' I ~ RETROREFLECTOR ~ · ~ ~ C M . ... ..:.:~:-i :;.: ::: :-: At;; ~ it' - ,.~.:?''.,: :,:,~ ,,:.:j': :,:,:: .:,:,:,::: ~:,:.~: :. ./ in : i ::~: :::- :- .:::.:.~:-::-:: ::~:;:~. 7 ;1:.1:::: :.:.: ::. - >~ >I.> ;~ / ~ I~,F .C~F~T /' ~_ Figure 1. Spaceborne laser ranging and altimetry concept.
Representative terms from entire chapter: