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Satellite laser ranging, status and impact for WEGENER
J. Geodwumics
Vol. 25, No. 314. pp. 195-212, 1998
#I;1998 Published
Pergamon PII: SO264-3707(97)00036-7
by Elsevier Science Ltd
All rights reserved. Prmted in Great Britain 02643707!98 $19.00+0.00
SATELLITE LASER RANGING, STATUS AND IMPACT FOR WEGENER ERIK VERMAAT,’ RON NOOMEN4*
JOHN J. DEGNAN,2 PETER and ANDREW T. SINCLAIR’
J. DUNN,3
‘Delft University of Technology, Delft, The Netherlands ‘NASA Goddard Space Flight Centre, MD, U.S.A. 3Hughes STX, MD, U.S.A. 4Delft University of Technology, Faculty of Aerospace Engineering, Kluyverweg 1, 2629 HS Delft, The Netherlands 5Royal Greenwich Observatory, London, U.K.
(Receiwd 25 October 1995: revised 20 August 1996; accepted 2.5 September 1996)
Abstract-The principles of the technique of Satellite Laser Ranging are briefly explained and the current status and outlook for further development are described. Results for the application of this technique in the WEGENER program are reviewed and strategies for continued contributions to this program from stationary and transportable laser ranging systems are presented. 0 1998 Published by Elsevier Science Ltd. All rights reserved
INTRODUCTION In the years 1986,1987, 1989 and 1992 the WEGENER-MEDLAS Project (Working group of European Geo-scientists for the Establishment of Networks for Earth-science ResearchMEDiterranean LASers) organized 4 observation campaigns in the central and eastern Mediterranean area. During these campaigns, some 15 sites were occupied repeatedly by mobile Satellite Laser Ranging (SLR) instruments, facilitating the computation of a timeseries of position solutions and ultimately yielding a consistent model for medium-scale tectonic motions in this region. The determination of these motions is still part of the charter of WEGENER-2, the successor of WEGENER-MEDLAS, although the implementation has changed because of technology developments. At the end of the eighties, mobile SLR and VLBI were the only mature techniques to do such observation campaigns. Since then, the Global Positioning System (GPS) has made significant progress and has turned into the most attractive instrument to observe networks of scales typical for the Med-
*Author to whom tudelfknl.
all correspondence
should
be addressed:
195
Fax:
+ 31-15-2785322;
E-mail:
ron.noomen@
Ir.
196
t. Vcrtnaat of d
iterranean area. in particular because of its cost-etf‘ectiveness and weather independence. Nevertheless. SLR still has a very important role to play within WEGENER. although its focus has shifted. This paper will address the current status of the SLR technique. the quality of the observations and of some of the analysis results, technical developments and its scientific contribution to WEGENER now’ and in the future,
THE
SLR
TECHNIQUE
SLR is the most straightforward in concept of the various space geodetic techniques. Nevertheless, it involves a variety of ditrerent disciplines which need to be pushed to stateof-the-art limits in order to obtain best possible range precision. The basic observable is the two-way travel time of a short laser pulse which is reflected by an orbiting satellite. This observation is corrected for internal delays in the transmission and detection systems, fat the variable propagation velocity in the atmosphcrc. and for the retro-reflector offset to the spacecraft center of maser. The range observation also must be time tagged to an international atomic time standard. The typical Pulaskilasc~r in an SLR system emits pulses with a duration of tens to hundreds of ps FWHM, an energy of IO to l00tn.I. at a rcpctition frequency of usually 5 to 20 HI. Most systems utilize Neodymium YAG laser\ which generate pulses in the infra red (1064nm); these are usually frequency doubled via passive nonlinear crystals IO the green (532 nm). The beam is directed to the satellite via a /~UPI.VII~/~PI. frlrsc~pc with an absolute pointing accuracy of a few arcseconds. The satellite is equipped with retroreflectors which reflect the light to the ground station tracking ~,c,c)irc~r.tc~k~~.sc~opc. which is either the same as, or parallel to, the transmitter telescope. The received light (a few up to several thousands of photons) is detected by a photomultiplier (conventional dynode chain or micro-channel plate type) or by a solid state photo diode. Important ~Ic/cc/o/. characteristics arc its quantum efficiency ( IO to 15’/0) and time response (200 to I500 picoseconds). Titw-o/fflig/Ir is measured by either ;I time intcr\al counter or epoch timer. Time tagging is accomplished with an accuracy of several hundreds of ns by a ( ‘T(’ c/ad driven by a rubidium or cesium frrquenc~ls .stanrkir.o’I.., ving ;I short lerm stability (Allan variance) of a few parts in 10” or 10’3. . The overall range precision of the best contemporary ranging systems is 30 to 70 ps (4 to IO mm) single shot. The error sources which limit absolute range accuracy are the varying propagation velocity through the atmosphere and. for some satellites, the pulse spreading induced by the retro-reflector array in combination with the response characteristics of the detector system. An important advantage of this optical technique is the insensitivity of the propagation velocity to the wet component of the troposphere and to the electron content of the ionosphere. This permits one cm accuracy atmospheric corrections to be made through the application of simple spherical shell models for the atmosphere. which use local meteorological tneasurements at the ground station as input. On the negative side. clouds are opaque to visible light. limiting application to clear sky conditions. Daylight ranging is tnade possible by discriminating against background noise through the use ol spectral. spatial, temporal. and cvcn amplitude dependent filters. Typical spectral bandwidths are I nm or less.
Satellite laser ranging,
197
status and impact for WEGENER
AVAILABILITY
Table 1 summarizes the currently available satellites equipped with retro-reflectors, many of which have been launched in the last three or four years. It also lists prospective satellite missions involving SLR. Because of their high and inherently stable orbits, the LAGEOS (6000 km) and Etalon (19,000 km) series satellites are best suited for geodynamics applications which require precise determination of horizontal and vertical station positions in a geocentric reference frame. A second advantage of these high passive satellites is their practically unlimited lifetime important to long term studies. The application type satellites, e.g. equipped with radar altimeters or gradiometers, require SLR tracking as a supporting technique for precise determination of the orbit. The current network of stationary SLR systems consists of over 40 stations, of which about 20 are high performance stations which produce large quantities of cm accuracy data. Table 2 lists the stations which submitted data in 1994. Because the SLR technique has seen an accuracy improvement of about one order of magnitude per decade since 1964, there is quite a diversity of performance throughout the network, but about 15 stations now operate with single shot precision in the 7 to 20mm range. New systems are under
Table 1. Overview
of satellites
date
that are tracked
Name
Launch
Starlette LAGEOS
February 1975 May 1976
960 5,900
Ajisai Etalon-l Etalon-2 Glonass-49 ERS- 1 Glonass-56 Glonass-57 TOPEX:Poseidon LAGEOS-2
August 1986 January 1989 May 1989 December 1990 June 1991 July 1992 July 1992 September 1992 October 1992
1,500 19.100 19.100 19.140 780 19,140 19.140 1,350 5.900
GPS-35 Fizeau Stella Meteor-3 GPS-36 Glonass-63 MSTI-2 Glonass-66 Glonass-67 GFZ-I ERS-2 TiPS ADEOSRIS LAGEOS-3
August 1993
20,200
Envisat
planned
September 1993 March 1994 March 1994 April 1994 May 1994 August 1994 August 1994 April 1995 April 1995 1996 planned planned
by the network
Height (km)
810
of SLR stations Mission/Role
gravity, tides, orbit crustal deformation, gravity, orbit crustal deformation, crustal deformation, crustal deformation, orbit calibration altimetry, orbit orbit calibration orbit calibration altimetry, orbit crustal deformation. gravity, orbit orbit calibration gravity.
earth rotation.
temporal
gravity, orbit earth rotation earth rotation
earth rotation,
temporal
tides, orbit
20,200 19,140
orbit calibration orbit calibration
19,140 19,140 400 780 1,030
orbit calibration
5.900
(SLR)
gravity, tides, orbit altimetry, orbit physics, dynamics altimetry, orbit crustal deformation, earth rotation. gravity, orbit, relativity altimetry, orbit
temporal
Satellite laser ranging, status and impact for WEGENER
199
development in Saudi Arabia, Italy, the Commonwealth of Independent States (CIS) and the People’s Republic of China (PRC). The geographic distribution (see Fig. I) is far from optimal with dense coverage in Europe, the US and the Far East, and major deficiencies in Africa. the Indian Ocean region and South East Asia. Fortunately this technique can be made transportable, and presently 7 transportable SLR systems are available in the US, Germany, the Netherlands, and Japan, with new transportable systems in development in France, Germany, Australia, the PRC and Japan. One of the most important applications of the transportable systems is to improve the geographic distribution of the stationary network on a global and regional scale. Fig. 2 depicts the stationary SLR stations in Europe, which are of specific significance to WEGENER, although even here the geographic distribution can be improved since the majority of the stations are located in the central part of Western Europe. QUALITY
OF SLR DATA
The SLR data product that is used for virtually all data analyses is the normal point. A typical pass of a high satellite such as LAGEOS may contain several thousands of raw range measurements, which are compressed into normal points by fitting a smoothing function to the raw ranges. This smoothing function may consist of a prediction of this satellite-station range signal, range and/or timing biases, a polynomial or a combination of these signals. After removing any outliers, the pass is divided into contiguous 2-minute (LAGEOS) or 30-second (low satellites) bins. For each bin, the mean value of the difference between the range measurements and the smoothing function is computed next, and this value is added to the smoothing function evaluated at the average observation epoch; this gives the normal points. This process typically reduces the random scatter of the raw data by a factor of 4 to 10. For a few stations the precision of the normal points is now as good as 1 mm, but values of 4 to 5 mm are more typical for many of the stations in the network. The normal points are produced and distributed by SLR data centers, but since September 1990 they are also being produced on-site by the laser stations. These field-generated normal points are now available within one or two days from the time of acquisition. The normal points have very high precision, and careful work is carried out to ensure that they are also free from systematic errors, so that the data are also of high accuracy. One effect that this high precision reveals is the signature that the satellite itself imposes on the data, due to the arrangement of the retro-reflectors. For the spherical satellites this causes a slight skewness of the range measurements towards long ranges, and an effect of this is that the mean value of the measurements can differ from the peak value by several mm. However this effect is now recognized, and appropriate corrections to the data can be made. As the quality and accuracy of the range data have improved over the years, the accuracy of the orbital models has also improved. For a high satellite such as LAGEOS, the accuracy of the computed orbits is now so good that the orbits can be used to monitor the medium and long term stability of laser station performance. Biases introduced by system problems can usually be identified within about one week and corrected. DATA ANALYSIS
STRATEGIES
In the analysis of SLR observations, two fundamentally utilized, i.e. the purely geometric approach and the dynamic
different techniques can be approach. In the geometric
Huah%e
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( i
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.;
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Satellite laser ranging,
201
status and impact for WEGENER
Yendeleevo
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Fig. 2. Stationary SLR stations (bold circles) and platforms for mobile SLR systems (open circles) in Europe.
approach, the satellite serves as a direct intermediate between range observations taken at different stations. Strictly simultaneous observations taken by three different stations to one satellite completely determine the position of that satellite at the time of observation. Simultaneous range measurements by a fourth station provide in principle redundant information which may be used for the computation of the relative coordinates of the tracking stations. However, because of weather, it is often difficult to obtain large numbers of strictly simultaneous SLR observations taken by four stations at a time, and therefore the geometric approach is not appropriate for routine operations. In the dynamic approach, the orbit of the satellite is computed. This is usually accomplished by a numerical integration of an initial satellite state-vector (position and velocity) using refined models which describe very precisely the forces acting on the spacecraft of interest. These forces typically are the gravitational force of the Earth, the direct attraction of other celestial bodies (Sun, Moon, planets), the force which is introduced by the tidal deformation of the Earth in response to the presence of celestial bodies (usually separated into solid Earth tides and ocean tides), direct Solar radiation pressure, atmospheric forces, albedo, etc. In addition, the dynamic approach also requires a detailed knowledge of the position and orientation of the Earth in inertial space, which involves a
precise description of precession, nut&ion, Earth rotation including polar motion, ephemerides of the Earth and other celestial bodies etc. In addition. the deformations of the Earth have to be modeled with great detail. in particular their ctfect on the instantaneous position of the tracking station. This includes phenomena like station uplift, pole tide. atmospheric and ocean loading. and tectonic motions. A good summary of the mathematical models needed for this type of SLR analysis can be found in (McCarthy, 1997). The basic aim of the data reduction using the dynamic approach is to optimize the tit of the actual SLR observations to the theoretical values that are obtained from the mathematical model by solving for a selected set of parameters. The quality of the fit depends on several things. including the arc length. the quality of the mathematical model. the quality of the observations. the parameters that are solved for. etc. The length of the orbital arc. i.e. the period over which the state-vector is integrated. may range from several tens of minutes (the duration of a pass observed by a regional SLR network) to a week. :I month or even a number of years. Since errors in the satellite orbit resulting from deficiencies in the mathematical model tend to build up with increasing arc length. the complexity of the computation model will be driven by the quality demands on the solution. In this context. the aforementioned geometric approach can be seen as a special case of the dynamic approach in the limit of zero arc length. thus entirely eliminating the dynamic behavior of the satellite. In practice the choice of arc length will depend on the purpose of the analysis and on the dataset available. The orbital fit of the observations typically ranges from 1 cm for short data arcs to 2 5 cm for monthly data arch. The SLR analyses simultaneously produce 3 number of useful scientific products including precise satellite orbits and related parameters as well as geodetic or geophysical parameters. e.g. station coordinates plus time derivatives. peocenter. polar motion. length of day, (temporal variations in) gravity field parameters. uplift coeficients, and tidal parameters. Presently station coordinates are produced with ;I typical precision of 3 to 20 mm (Noomen (I[ cl/.. 1996). More detailed papers on the state-of-the-art in the computation of SLR station coordinates can be found in (Smith and Turcotte. 1993). Series of Earth rotation parameter solutions arc described in (Castrique. 1993).
There are ;I number of SLR observatories with dell calibrated systems which have been in operation for as long as eight years. and the ~curacy of the observations from these stations can be assessed by considering their height stability. This can be determined by monitoring the vertical component of a series of three-dimensional station position solutions. obtained from careful analyses of. for example. monthly arcs of LAGEOS data. During the lifetime of each station, continuous improvements are made to the system through upgrades in both hardware and software. Any disturbance at an instrument is monitored with accurate resurveys of the system’s eccentricity with respect to the station’s primary survey monument, as well as of any change in the surveyed distance of the calibration tower used for system delay determination. The eccentricity offsets for the various instruments can be retrieved from the Crustal Dynamics Data Information System (CDDIS) and their accuracy will directly affect the estimated heights, as well as any subsequent estimation of vertical motion of the station. Information concerning calibration characteristics of each system is accessible through the CDDIS, although it has already been used in the processing of the raw range measure-
Satellite laser ranging, status and impact for WEGENER
203
ments and is thus embedded in the normal points which are used in the analysis. As corrections to the calibration procedures are uncovered by subsequent analysis, it is necessary to compensate for any effects that retro-active improvements might exert on station position. Subtle engineering problems in the detection system must be remedied in a pre-processing stage using the original time-of-flight observations, but many of the data corrections can be represented by pass-by-pass or longer term range or timing bias parameters. Range errors of a magnitude of a few cm would significantly affect any estimates of station height, although they would tend to cancel out in the horizontal position components at a station where data has been collected with reasonable sky geometry. The most compelling indication of engineering effects in station position is an abrupt change in station height when compared to a subsequently maintained level. Such effects have indeed been noticed in practice. The independent monthly values of height for Greenbelt, Yarragadee and Arequipa from an analysis performed by the GSFC analysis group are plotted in Fig. 3, and the vertical scales show the least significant figures in millimeters of the estimated height values of Table 3. The quality of these measurements of the distance from an average Earth semi-major axis of 6378136.3 m are given by the error estimates of twice their formal standard deviation based on the final fit of the range observations to each orbital arc. Although the ranges themselves are formally accurate to better than a centimeter, systematic residual signatures of several centimeters in amplitude are observed in the station height due to uncompensated errors in force, measurement and Earth orientation models, but the vertical scales of the plots have been chosen to demonstrate the long-term variation. The effect of atmospheric refraction on the laser ranges is modelled assuming a spherically stratified atmosphere and is primarily based on surface pressure measurements. No zenith correction terms or any other refraction parameters are estimated as they would degrade the vertical resolution inherent in the optical range measurements. However, any long term variation in station barometer accuracy or in the effects of lateral gradients in the atmosphere will directly affect the vertical estimates. The SLR systems could thus be used to calibrate the dry component of atmospheric refraction for nearby microwave instruments, as the microwave measurements are also affected by the wet term, but cannot separate the two components without the use of external information as provided, for example, by a water vapor radiometer. The possibility of errors in the adopted system eccentricities must also be considered, particularly for stations which have undergone changes of system occupation, such as Greenbelt, Quincy and Huahine. The system changes at the North American sites coincided with collocation tests which cross-calibrated each instrument’s ranging machine as well as its eccentricity. The NASA transportable systems are periodically returned to Greenbelt for upgrades and collocation calibration against MOBLAS-7, but they do not usually undergo a collocation test at their working location. Considerable deviation from uniform motion can be noted in the height variation for some stations, but the measures of scatter of the height values about the mean listed in Table 3 are only reduced by a millimeter or two when a linear fit is subtracted, which strongly suggests the absence of a real linear signal. If we consider the scatter of a station’s height about a mean (or uniformly moving) value as a measure of the quality of the station’s performance, we see that it depends as much on system stability and careful calibration as upon the precision of the observations, and the lower values of height scatter at Greenbelt, Yarragadee and Arequipa testify to the reliability of these instruments.
1350
1250
850
.._
VARRAGADEE
.
L
1200
1100
j
,...
FOURTH
_
AREQUIPA ,. .
GENERATION
1
DEVELOPMENTS
The best *third generation’ SLR systems presently in the field exhibit single shot RMS scatters about a short arc LAGEOS orbit of 7 to IO mm and a normal point scatter of 1 to 3 mm (Degnan. 1985: Degnan. 1993). Such systems. when brought together in collocation experiments, typically agree at the few millimeter level over many satellite passes thereby placing a rather stringent upper bound on systematic biases traceable to the laser ranging
Table 3. Average SLR station positions and statistics. The standard mean height, and the standard deviation indicates the uncertainty scatter)
deg Greenbelt Quincy Monument Yarragadee Huahine Arequipa Matera Wettzell Graz RGO Simosato
Peak
Latitude min
39 39 32 -29 -16 -16 40 49 41 50 33
I 58 53 2 44 27 38 8 4 52 34
205
status and impact for WEGENER
Satellite laser ranging,
Longitude min
s
deg
14 30 30 47 I 57 56 42 2 3 40
283 239 243 II5 208 288 I6 I2 15 0 135
10 3 34 20 57 30 42 52 29 20 56
error denotes the formal uncertainty of the of an individual monthly estimate (i.e. the
Height
std. error
std. dev.
s
(m)
(mm)
(mm)
no. of months
20 I9 39 48 32 25 17 41 36 IO I3
19.931 1107.119 1839.746 242.080 46.1 IO 2492.945 536.551 661.842 540.125 76.114 100.175
2 2 2 2 5 2 2 4 3 3 4
I6 18 20 I6 23 I7 I9 24 20 21 25
69 67 73 69 22 52 60 45 55 69 51
hardware. So-called ‘spherical shell’ models presently used to compute the atmospheric refraction correction (e.g. Marini and Murray, 1973) assume that the vertical air density profile is determined under the conditions of hydrostatic equilibrium and that there are no horizontal gradients (latitude or longitude dependence) in the meteorological parameters. Since various analyses suggest that these shortcomings in the atmospheric model can produce systematic range errors of between 6 and 12 mm (Gardner, 1985; Hauser, 1989) the atmosphere can now be viewed as the dominant systematic error source. Fourth generation two color SLR systems, which utilize ultra-short laser pulse widths (< 35 ps FWHM) and streak camera receivers (< 2 ps temporal resolution), are capable of producing range measurements with an absolute accuracy of better than 3 mm. A sample block diagram of a two color system is shown in Fig. 4 (Degnan, 1993). Since the measured differential time-of-flight between the two wavelengths is proportional to the group refractivity integrated over the roundtrip light trajectory, the atmospheric contribution to the optical time-of-flight can be determined and corrected (Abshire and Gardner, 1985). Significant hardware progress has been made in recent years. In 1991, joint Czech/Austrian experiments at Graz recorded the first single color (532nm) streak camera returns from satellite Ajisai (Prochazka et al., 1991). In 1992, NASA’s Goddard Space Flight Center recorded single color streak camera returns (also at 532nm) from up to three retroreflectors each physically separated by 4 cm on the Relay Mirror Experiment (RME) satellite as shown in Fig. 5 (Zagwodzki et al., 1992). These were followed by experiments in which the atmospheric variability near the Greenbelt site was mapped using two color streak camera ranging to a retroreflector mounted on an aircraft (McGarry et al., 1993). Using conventional photomultiplier-based systems, substantial multiple wavelength returns from LAGEOS and lower satellites were successfully recorded by Wettzell (1064 and 532nm) and Graz (1064, 532 and 680nm) observatories and by NASA/GSFC (355 and 532 nm) to Ajisai altitudes. Although these systems had insufficient temporal resolution to adequately determine the differential delay, they demonstrated that adequate signal strength could be achieved using multiple wavelengths. More recently, the NASA group has reported the first two color streak camera returns from a satellite (Zagwodzki et al.. 1994) and have
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status and impact for WEGENER
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since obtained two color streak camera returns (532 and 355 nm) from most satellites at Ajisai or lower altitudes (i.e. lower than Ii00 km). .A sampling is shown in Fig. 6. These experiments have demonstrated that the multiple cube responses and pulse broadening effects typical of most satellites designed for centimeter ranging can substantially decorrelate the waveforms at the two wavelengths. With the recent launch of the Japancsc ADEOS mission in August 19%. the aforementioned R&D stations have proposed two color ranging experiments to the Retroreflector in Space (RIS). z unique single retlector which has a substantial optical cross-section l’or strong signal returns and which induces no pulse spreading. This unique reflector should provide the first true demonstration of millimeter accuracy SLR using two color technique\
Within the I’ramcwork 01‘ the WEGENER-MEDLAS Project and its successor WEGENER-2, the transportable SLR system5 TLRS-I (NASA), MTLRS-I (Institut fiir Angewandte Geodasie) and MTLRS-2 (Delf’t IUnivcrsit) of Technology) have repeatedly occupied some 15 sites in the central and eastern Mediterranean area in the time l‘rame I985 1995. The SLR activity was particularly high during dedicated measurement campaigns in 1986. 1987, I989 and 1993. The analyses of the data taken during these repeat occupations. in combination with global SLR data. hauc Id to ;I clear and consistent picture of the medium-scale deformations in the Meditcrrancan arca. An example of this is given in Fig. 7. which shows an SLR-only solution for horizontal motion vectors relative to stable Eurasia computed at Del0 University of Technology (vectors accompanied by open uncertaint] ellipses). It should be mentioned here that similar results have been obtained by other international research groups participating in WEGENER. At its outset in I9XS. WEGENER-MEDLAS posed the question about the magnitude and orientation of the contemporary crustal deformation in the Mediterranean region. both aspects important for a better understanding of earthquake mechanisms in the area. At the time. qualitative ideas were available only. with uncertainties of block motion estimates on the order of 100’%1. Within less than a decade. WEGENER-MEDLAS has given the scientitic community clear answer\: it has confirmed the northward motion ot Arabia. it has quantified the westward and southwestward motion of Anatolia and Aegea with high reliability. it has established the boundaries of ‘stable’ Eurasia and it has shown that the motion regime in the central (Italian) part of the Mediterranean is dictated by the Africa-Eurasia collision. In addition. the project has demonstrated the capability to organize and perform major international measurement campaigns. Today. re1atiL.e station positions for such networks can be dctcrmined c\ith at Icast comparable precision using geodetic GPS recei\,ers. in ;I much shorter time and at significantly less cost. This has already been proven by the WEGENERYGPS-92 and WEGENER/GPS-94 campaigns. during which almost the entire network was surveyed in 1 7 weeks in I992 and 1994. respectively. As a matter of fact. Fig. 7 also includes a solution for the motions in the Mediterranean based on both SLR and GPS measurements (the vectors accompanied by the solid error ellipses). Clearly. the trends that are evident from SLR-only do not change dramatically when more recently taken GPS data is added; the uncertainty of the solutions does decrease. howcvcr. because of the longer time period now being covered and because ot‘ the high precision of the GPS results in particular. Currently, the deployment of the few transportable SLR systems available (and likewise
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Fig. 6. Sample two color streak camera returns obtained by NASA/GSFC in experiments to the Starlette, show multiple cube returns, The ultraviolet signal at 355 nm is typically much weaker than the green return at 532 nm due to reduced TOPEXIPOSEIDON waveforms reflect the complex signature of the large ring array.
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project (1986,1987. 1989. 1992: open circles) and derived from :I combination SLR and GPS (1989. 1991. 1994) data (filled circles).
Fig. 7. Stationmotions derlvrd from SLR data in the WEGENER-MEDLAS
Punta shwb
n
of thew
Satellite laser ranging, status and impact for WEGENER
211
transportable VLBI instruments) is being reconsidered, not only within WEGENER but within the entire global community. Consensus exists that this new strategy must address all three WEGENER scientific research objectives, and it must optimally exploit the unique capabilities of SLR (accuracy, simplicity of measurement, absence of ambiguity), recognizing the capabilities of GPS. To do so, the deployment of the mobile SLR systems should first of all improve the global distribution of the network of stations; this allows a more even tracking of geophysical and applications satellites, which in turn results in improved orbits and more accurate estimates for absolute vertical station positions and velocities, geocenter variations and mean sea level changes. Some of this has already been accomplished with the temporary visits of MTLRS-I to South Africa (1993) and of TLRS4 to post-glacial uplift areas in Canada (1993, 1994) and revised strategies employed by NASA. A second important new role of SLR is collocation with other space-geodetic measurement devices (GPS, VLBI). This will provide the opportunity to reliably connect different networks and to calibrate measurement techniques themselves. The latter holds in particular for GPS observations, which have inherent absolute uncertainties because of ambiguities and atmospheric delays. Efforts in this respect are currently being conducted in Switzerland (the Zimmerwald station), Great Britain (Herstmonceux) and Germany (the TIGO observatory), amongst others. Finally, it is of importance to preserve the history of the epoch measurements which have been performed in WEGENER up to now. It will be recognized that this can be done (as a matter of fact, has been done twice already) perfectly with GPS. On the other hand, the observation of absolute positions and velocities necessitates regular re-visits of certain sites in the Mediterranean, since the area is located at the southern extreme of the stable Eurasian plate with stationary systems serving as reference here. It must be emphasized, however, that only one or two carefully selected sites will be sufficient for this purpose, and that these fiducial stations will need to be visited at intervals of 334 years. Similarly, this strategy should encompass the measurement of selected fiducial sites elsewhere in boundary zones or uplift areas. The deployment at a limited number of fiducial sites should preferably follow a regular, long-term schedule, irrespective of its purpose being plate boundary, uplift, sea level or geocenter studies. Similar, but complementary, reasonI+ J should be followed concerning the use of stationary and mobile GPS equipment to obtain an overall strategy which optimally engages the various space-geodetic techniques to support the research objectives of the WEGENER project. The typical role of SLR in this programme is to establish and maintain accurate 3dimensional geocentric station positions in a fiducial network made up of stationary stations and selected sites for transportable equipment. Emphasis needs to be placed on the vertical component of these positions, for which SLR is particularly well-suited.
REFERENCES Abshire, J. B. and Gardner, C. S. (1985) Atmospheric refractivity corrections in satellite laser ranging. IEEE Transactions on Geoscience and Remote Sensing GE-23,414425. Castrique, L. (1992) 1991-IERS Annual Report. IERS, Paris. Degnan, J. J. (1985) Satellite laser ranging: current status and future prospects. IEEE Transactions on Geoscience and Remote Sensing GE-23, 398413.
Degnan.
J. J. (1YY3) Millimeter Accuracy Satellite Laser Ranging: A Review. In Conof’Spuce Grorie.s~~ to Gcor!,mmic.s Tec~hnoloy~. ed. D. E. Smith and D. L. Turcotte. AGU Geo~~~muttics Srric.s 25, l33- 162. Gardner, C. S. (1985) Effects of Horizontal Refractivity Gradients on the Accuracy of Laser Ranging to Satellites. Ruditr Scienw 20, 1593 1607. Hauser. J. P. (1989) Effects of Deviations from Hydrostatic Equilibrium on Atmospheric Corrections to Satellite and Lunar Laser Range Measurements. Joutvzu/ o#‘Gmph~~~id Research 94, IO I 82 IO I X6. Husson. V. S. (1991) Collocation Analysis (Looking Back). Paper presented at Crustal Dynamics Principal Investigators Meeting. October I99 I. NASA/GSFC. Marini, J. W. and Murray. C. W. (1973) Correction of Laser Range Tracking Data for Atmospheric Refraction at Elevations Above IO Degrees. NASA Report X-59 I-73-35 I. Goddard Space Flight Center. McCarthy. D. D. (1992) IERS Standards, IERS Technical Note 13. Paris. McGarry. J. F., Cheek. J. W., Millar. P. S. and Abshire, J. B. (1993) Two Color Laser Ranging to a Cooperative Airborne Target. Presented at SPIE OE,‘Aerospace Science and Sensing, April 1993. Orlando, Florida. B. A. c‘.. Herzberger, K., Kuijper, D. C.. Noomen, R., Springer, T. A., Ambrosius. Mets. G. J.. Overgaauw. B. and Wakker. K. F. (1996) Crustal deformations in the Mediterranean area computed from SLR and GPS observations. Journal of’Gcw~~xamic,s 21(l). 73-96. Prochazka. I., Hamal, K.. Kirchner. G.. ScheleL. M. and Postavolov. V. (IYY I) Circular Streak Camera Application for Satellite Laser Ranging. Presented at SPIE Conference on Electronic Imaging. February 24 27. I9Y I. San Jose. California. Smith, D. E. and Turcotte. D. L. (1993) Contributions of Space Geodesy to Gcodynamics: Geodynamics. AGU Gco~~wtttics Scric~s 23. Zagwodzki. T. W.. McGarry. J. F. and Degnan, J. J. (1992) Two Color Satellite Laser Ranging Upgrades at Goddard’s I .2 Meter Telescope Facility. Procwdinys of’thr Ei~yhrh 1ntematiottul Wovk.shop on LNSCI.Rrtnginy In.strtrtt~mtatiott. 7--l 7 to 7 27. May. Annapolis. MD. Zagwodzki, T.. McGarry. J.. Degnan. J., Abbott. J.. Varghese, T.. Oldham, T.. Selden, M.. Chabot. R., Fitzgerald. J., Grolemund. D. and Cheek. J. (1994) Two Color Ranging at NASA’s I.2 Meter Tracking Facility. 0th International Workshop on Laser Ranging Instrumentation, November 7 I I. Canberra. Australia. trihurims
Vol. 25, No. 314. pp. 195-212, 1998
#I;1998 Published
Pergamon PII: SO264-3707(97)00036-7
by Elsevier Science Ltd
All rights reserved. Prmted in Great Britain 02643707!98 $19.00+0.00
SATELLITE LASER RANGING, STATUS AND IMPACT FOR WEGENER ERIK VERMAAT,’ RON NOOMEN4*
JOHN J. DEGNAN,2 PETER and ANDREW T. SINCLAIR’
J. DUNN,3
‘Delft University of Technology, Delft, The Netherlands ‘NASA Goddard Space Flight Centre, MD, U.S.A. 3Hughes STX, MD, U.S.A. 4Delft University of Technology, Faculty of Aerospace Engineering, Kluyverweg 1, 2629 HS Delft, The Netherlands 5Royal Greenwich Observatory, London, U.K.
(Receiwd 25 October 1995: revised 20 August 1996; accepted 2.5 September 1996)
Abstract-The principles of the technique of Satellite Laser Ranging are briefly explained and the current status and outlook for further development are described. Results for the application of this technique in the WEGENER program are reviewed and strategies for continued contributions to this program from stationary and transportable laser ranging systems are presented. 0 1998 Published by Elsevier Science Ltd. All rights reserved
INTRODUCTION In the years 1986,1987, 1989 and 1992 the WEGENER-MEDLAS Project (Working group of European Geo-scientists for the Establishment of Networks for Earth-science ResearchMEDiterranean LASers) organized 4 observation campaigns in the central and eastern Mediterranean area. During these campaigns, some 15 sites were occupied repeatedly by mobile Satellite Laser Ranging (SLR) instruments, facilitating the computation of a timeseries of position solutions and ultimately yielding a consistent model for medium-scale tectonic motions in this region. The determination of these motions is still part of the charter of WEGENER-2, the successor of WEGENER-MEDLAS, although the implementation has changed because of technology developments. At the end of the eighties, mobile SLR and VLBI were the only mature techniques to do such observation campaigns. Since then, the Global Positioning System (GPS) has made significant progress and has turned into the most attractive instrument to observe networks of scales typical for the Med-
*Author to whom tudelfknl.
all correspondence
should
be addressed:
195
Fax:
+ 31-15-2785322;
E-mail:
ron.noomen@
Ir.
196
t. Vcrtnaat of d
iterranean area. in particular because of its cost-etf‘ectiveness and weather independence. Nevertheless. SLR still has a very important role to play within WEGENER. although its focus has shifted. This paper will address the current status of the SLR technique. the quality of the observations and of some of the analysis results, technical developments and its scientific contribution to WEGENER now’ and in the future,
THE
SLR
TECHNIQUE
SLR is the most straightforward in concept of the various space geodetic techniques. Nevertheless, it involves a variety of ditrerent disciplines which need to be pushed to stateof-the-art limits in order to obtain best possible range precision. The basic observable is the two-way travel time of a short laser pulse which is reflected by an orbiting satellite. This observation is corrected for internal delays in the transmission and detection systems, fat the variable propagation velocity in the atmosphcrc. and for the retro-reflector offset to the spacecraft center of maser. The range observation also must be time tagged to an international atomic time standard. The typical Pulaskilasc~r in an SLR system emits pulses with a duration of tens to hundreds of ps FWHM, an energy of IO to l00tn.I. at a rcpctition frequency of usually 5 to 20 HI. Most systems utilize Neodymium YAG laser\ which generate pulses in the infra red (1064nm); these are usually frequency doubled via passive nonlinear crystals IO the green (532 nm). The beam is directed to the satellite via a /~UPI.VII~/~PI. frlrsc~pc with an absolute pointing accuracy of a few arcseconds. The satellite is equipped with retroreflectors which reflect the light to the ground station tracking ~,c,c)irc~r.tc~k~~.sc~opc. which is either the same as, or parallel to, the transmitter telescope. The received light (a few up to several thousands of photons) is detected by a photomultiplier (conventional dynode chain or micro-channel plate type) or by a solid state photo diode. Important ~Ic/cc/o/. characteristics arc its quantum efficiency ( IO to 15’/0) and time response (200 to I500 picoseconds). Titw-o/fflig/Ir is measured by either ;I time intcr\al counter or epoch timer. Time tagging is accomplished with an accuracy of several hundreds of ns by a ( ‘T(’ c/ad driven by a rubidium or cesium frrquenc~ls .stanrkir.o’I.., ving ;I short lerm stability (Allan variance) of a few parts in 10” or 10’3. . The overall range precision of the best contemporary ranging systems is 30 to 70 ps (4 to IO mm) single shot. The error sources which limit absolute range accuracy are the varying propagation velocity through the atmosphere and. for some satellites, the pulse spreading induced by the retro-reflector array in combination with the response characteristics of the detector system. An important advantage of this optical technique is the insensitivity of the propagation velocity to the wet component of the troposphere and to the electron content of the ionosphere. This permits one cm accuracy atmospheric corrections to be made through the application of simple spherical shell models for the atmosphere. which use local meteorological tneasurements at the ground station as input. On the negative side. clouds are opaque to visible light. limiting application to clear sky conditions. Daylight ranging is tnade possible by discriminating against background noise through the use ol spectral. spatial, temporal. and cvcn amplitude dependent filters. Typical spectral bandwidths are I nm or less.
Satellite laser ranging,
197
status and impact for WEGENER
AVAILABILITY
Table 1 summarizes the currently available satellites equipped with retro-reflectors, many of which have been launched in the last three or four years. It also lists prospective satellite missions involving SLR. Because of their high and inherently stable orbits, the LAGEOS (6000 km) and Etalon (19,000 km) series satellites are best suited for geodynamics applications which require precise determination of horizontal and vertical station positions in a geocentric reference frame. A second advantage of these high passive satellites is their practically unlimited lifetime important to long term studies. The application type satellites, e.g. equipped with radar altimeters or gradiometers, require SLR tracking as a supporting technique for precise determination of the orbit. The current network of stationary SLR systems consists of over 40 stations, of which about 20 are high performance stations which produce large quantities of cm accuracy data. Table 2 lists the stations which submitted data in 1994. Because the SLR technique has seen an accuracy improvement of about one order of magnitude per decade since 1964, there is quite a diversity of performance throughout the network, but about 15 stations now operate with single shot precision in the 7 to 20mm range. New systems are under
Table 1. Overview
of satellites
date
that are tracked
Name
Launch
Starlette LAGEOS
February 1975 May 1976
960 5,900
Ajisai Etalon-l Etalon-2 Glonass-49 ERS- 1 Glonass-56 Glonass-57 TOPEX:Poseidon LAGEOS-2
August 1986 January 1989 May 1989 December 1990 June 1991 July 1992 July 1992 September 1992 October 1992
1,500 19.100 19.100 19.140 780 19,140 19.140 1,350 5.900
GPS-35 Fizeau Stella Meteor-3 GPS-36 Glonass-63 MSTI-2 Glonass-66 Glonass-67 GFZ-I ERS-2 TiPS ADEOSRIS LAGEOS-3
August 1993
20,200
Envisat
planned
September 1993 March 1994 March 1994 April 1994 May 1994 August 1994 August 1994 April 1995 April 1995 1996 planned planned
by the network
Height (km)
810
of SLR stations Mission/Role
gravity, tides, orbit crustal deformation, gravity, orbit crustal deformation, crustal deformation, crustal deformation, orbit calibration altimetry, orbit orbit calibration orbit calibration altimetry, orbit crustal deformation. gravity, orbit orbit calibration gravity.
earth rotation.
temporal
gravity, orbit earth rotation earth rotation
earth rotation,
temporal
tides, orbit
20,200 19,140
orbit calibration orbit calibration
19,140 19,140 400 780 1,030
orbit calibration
5.900
(SLR)
gravity, tides, orbit altimetry, orbit physics, dynamics altimetry, orbit crustal deformation, earth rotation. gravity, orbit, relativity altimetry, orbit
temporal
Satellite laser ranging, status and impact for WEGENER
199
development in Saudi Arabia, Italy, the Commonwealth of Independent States (CIS) and the People’s Republic of China (PRC). The geographic distribution (see Fig. I) is far from optimal with dense coverage in Europe, the US and the Far East, and major deficiencies in Africa. the Indian Ocean region and South East Asia. Fortunately this technique can be made transportable, and presently 7 transportable SLR systems are available in the US, Germany, the Netherlands, and Japan, with new transportable systems in development in France, Germany, Australia, the PRC and Japan. One of the most important applications of the transportable systems is to improve the geographic distribution of the stationary network on a global and regional scale. Fig. 2 depicts the stationary SLR stations in Europe, which are of specific significance to WEGENER, although even here the geographic distribution can be improved since the majority of the stations are located in the central part of Western Europe. QUALITY
OF SLR DATA
The SLR data product that is used for virtually all data analyses is the normal point. A typical pass of a high satellite such as LAGEOS may contain several thousands of raw range measurements, which are compressed into normal points by fitting a smoothing function to the raw ranges. This smoothing function may consist of a prediction of this satellite-station range signal, range and/or timing biases, a polynomial or a combination of these signals. After removing any outliers, the pass is divided into contiguous 2-minute (LAGEOS) or 30-second (low satellites) bins. For each bin, the mean value of the difference between the range measurements and the smoothing function is computed next, and this value is added to the smoothing function evaluated at the average observation epoch; this gives the normal points. This process typically reduces the random scatter of the raw data by a factor of 4 to 10. For a few stations the precision of the normal points is now as good as 1 mm, but values of 4 to 5 mm are more typical for many of the stations in the network. The normal points are produced and distributed by SLR data centers, but since September 1990 they are also being produced on-site by the laser stations. These field-generated normal points are now available within one or two days from the time of acquisition. The normal points have very high precision, and careful work is carried out to ensure that they are also free from systematic errors, so that the data are also of high accuracy. One effect that this high precision reveals is the signature that the satellite itself imposes on the data, due to the arrangement of the retro-reflectors. For the spherical satellites this causes a slight skewness of the range measurements towards long ranges, and an effect of this is that the mean value of the measurements can differ from the peak value by several mm. However this effect is now recognized, and appropriate corrections to the data can be made. As the quality and accuracy of the range data have improved over the years, the accuracy of the orbital models has also improved. For a high satellite such as LAGEOS, the accuracy of the computed orbits is now so good that the orbits can be used to monitor the medium and long term stability of laser station performance. Biases introduced by system problems can usually be identified within about one week and corrected. DATA ANALYSIS
STRATEGIES
In the analysis of SLR observations, two fundamentally utilized, i.e. the purely geometric approach and the dynamic
different techniques can be approach. In the geometric
Huah%e
FIaleakala
l
Easter
Cabo
Monument Peak Ensenada
n
Isl&d
I
‘;‘Q
Arequipa
( i
I,
Richmond
;
.;
Komsomolsk-naFArnur&-’
Satellite laser ranging,
201
status and impact for WEGENER
Yendeleevo
Zimmet-wald
Fig. 2. Stationary SLR stations (bold circles) and platforms for mobile SLR systems (open circles) in Europe.
approach, the satellite serves as a direct intermediate between range observations taken at different stations. Strictly simultaneous observations taken by three different stations to one satellite completely determine the position of that satellite at the time of observation. Simultaneous range measurements by a fourth station provide in principle redundant information which may be used for the computation of the relative coordinates of the tracking stations. However, because of weather, it is often difficult to obtain large numbers of strictly simultaneous SLR observations taken by four stations at a time, and therefore the geometric approach is not appropriate for routine operations. In the dynamic approach, the orbit of the satellite is computed. This is usually accomplished by a numerical integration of an initial satellite state-vector (position and velocity) using refined models which describe very precisely the forces acting on the spacecraft of interest. These forces typically are the gravitational force of the Earth, the direct attraction of other celestial bodies (Sun, Moon, planets), the force which is introduced by the tidal deformation of the Earth in response to the presence of celestial bodies (usually separated into solid Earth tides and ocean tides), direct Solar radiation pressure, atmospheric forces, albedo, etc. In addition, the dynamic approach also requires a detailed knowledge of the position and orientation of the Earth in inertial space, which involves a
precise description of precession, nut&ion, Earth rotation including polar motion, ephemerides of the Earth and other celestial bodies etc. In addition. the deformations of the Earth have to be modeled with great detail. in particular their ctfect on the instantaneous position of the tracking station. This includes phenomena like station uplift, pole tide. atmospheric and ocean loading. and tectonic motions. A good summary of the mathematical models needed for this type of SLR analysis can be found in (McCarthy, 1997). The basic aim of the data reduction using the dynamic approach is to optimize the tit of the actual SLR observations to the theoretical values that are obtained from the mathematical model by solving for a selected set of parameters. The quality of the fit depends on several things. including the arc length. the quality of the mathematical model. the quality of the observations. the parameters that are solved for. etc. The length of the orbital arc. i.e. the period over which the state-vector is integrated. may range from several tens of minutes (the duration of a pass observed by a regional SLR network) to a week. :I month or even a number of years. Since errors in the satellite orbit resulting from deficiencies in the mathematical model tend to build up with increasing arc length. the complexity of the computation model will be driven by the quality demands on the solution. In this context. the aforementioned geometric approach can be seen as a special case of the dynamic approach in the limit of zero arc length. thus entirely eliminating the dynamic behavior of the satellite. In practice the choice of arc length will depend on the purpose of the analysis and on the dataset available. The orbital fit of the observations typically ranges from 1 cm for short data arcs to 2 5 cm for monthly data arch. The SLR analyses simultaneously produce 3 number of useful scientific products including precise satellite orbits and related parameters as well as geodetic or geophysical parameters. e.g. station coordinates plus time derivatives. peocenter. polar motion. length of day, (temporal variations in) gravity field parameters. uplift coeficients, and tidal parameters. Presently station coordinates are produced with ;I typical precision of 3 to 20 mm (Noomen (I[ cl/.. 1996). More detailed papers on the state-of-the-art in the computation of SLR station coordinates can be found in (Smith and Turcotte. 1993). Series of Earth rotation parameter solutions arc described in (Castrique. 1993).
There are ;I number of SLR observatories with dell calibrated systems which have been in operation for as long as eight years. and the ~curacy of the observations from these stations can be assessed by considering their height stability. This can be determined by monitoring the vertical component of a series of three-dimensional station position solutions. obtained from careful analyses of. for example. monthly arcs of LAGEOS data. During the lifetime of each station, continuous improvements are made to the system through upgrades in both hardware and software. Any disturbance at an instrument is monitored with accurate resurveys of the system’s eccentricity with respect to the station’s primary survey monument, as well as of any change in the surveyed distance of the calibration tower used for system delay determination. The eccentricity offsets for the various instruments can be retrieved from the Crustal Dynamics Data Information System (CDDIS) and their accuracy will directly affect the estimated heights, as well as any subsequent estimation of vertical motion of the station. Information concerning calibration characteristics of each system is accessible through the CDDIS, although it has already been used in the processing of the raw range measure-
Satellite laser ranging, status and impact for WEGENER
203
ments and is thus embedded in the normal points which are used in the analysis. As corrections to the calibration procedures are uncovered by subsequent analysis, it is necessary to compensate for any effects that retro-active improvements might exert on station position. Subtle engineering problems in the detection system must be remedied in a pre-processing stage using the original time-of-flight observations, but many of the data corrections can be represented by pass-by-pass or longer term range or timing bias parameters. Range errors of a magnitude of a few cm would significantly affect any estimates of station height, although they would tend to cancel out in the horizontal position components at a station where data has been collected with reasonable sky geometry. The most compelling indication of engineering effects in station position is an abrupt change in station height when compared to a subsequently maintained level. Such effects have indeed been noticed in practice. The independent monthly values of height for Greenbelt, Yarragadee and Arequipa from an analysis performed by the GSFC analysis group are plotted in Fig. 3, and the vertical scales show the least significant figures in millimeters of the estimated height values of Table 3. The quality of these measurements of the distance from an average Earth semi-major axis of 6378136.3 m are given by the error estimates of twice their formal standard deviation based on the final fit of the range observations to each orbital arc. Although the ranges themselves are formally accurate to better than a centimeter, systematic residual signatures of several centimeters in amplitude are observed in the station height due to uncompensated errors in force, measurement and Earth orientation models, but the vertical scales of the plots have been chosen to demonstrate the long-term variation. The effect of atmospheric refraction on the laser ranges is modelled assuming a spherically stratified atmosphere and is primarily based on surface pressure measurements. No zenith correction terms or any other refraction parameters are estimated as they would degrade the vertical resolution inherent in the optical range measurements. However, any long term variation in station barometer accuracy or in the effects of lateral gradients in the atmosphere will directly affect the vertical estimates. The SLR systems could thus be used to calibrate the dry component of atmospheric refraction for nearby microwave instruments, as the microwave measurements are also affected by the wet term, but cannot separate the two components without the use of external information as provided, for example, by a water vapor radiometer. The possibility of errors in the adopted system eccentricities must also be considered, particularly for stations which have undergone changes of system occupation, such as Greenbelt, Quincy and Huahine. The system changes at the North American sites coincided with collocation tests which cross-calibrated each instrument’s ranging machine as well as its eccentricity. The NASA transportable systems are periodically returned to Greenbelt for upgrades and collocation calibration against MOBLAS-7, but they do not usually undergo a collocation test at their working location. Considerable deviation from uniform motion can be noted in the height variation for some stations, but the measures of scatter of the height values about the mean listed in Table 3 are only reduced by a millimeter or two when a linear fit is subtracted, which strongly suggests the absence of a real linear signal. If we consider the scatter of a station’s height about a mean (or uniformly moving) value as a measure of the quality of the station’s performance, we see that it depends as much on system stability and careful calibration as upon the precision of the observations, and the lower values of height scatter at Greenbelt, Yarragadee and Arequipa testify to the reliability of these instruments.
1350
1250
850
.._
VARRAGADEE
.
L
1200
1100
j
,...
FOURTH
_
AREQUIPA ,. .
GENERATION
1
DEVELOPMENTS
The best *third generation’ SLR systems presently in the field exhibit single shot RMS scatters about a short arc LAGEOS orbit of 7 to IO mm and a normal point scatter of 1 to 3 mm (Degnan. 1985: Degnan. 1993). Such systems. when brought together in collocation experiments, typically agree at the few millimeter level over many satellite passes thereby placing a rather stringent upper bound on systematic biases traceable to the laser ranging
Table 3. Average SLR station positions and statistics. The standard mean height, and the standard deviation indicates the uncertainty scatter)
deg Greenbelt Quincy Monument Yarragadee Huahine Arequipa Matera Wettzell Graz RGO Simosato
Peak
Latitude min
39 39 32 -29 -16 -16 40 49 41 50 33
I 58 53 2 44 27 38 8 4 52 34
205
status and impact for WEGENER
Satellite laser ranging,
Longitude min
s
deg
14 30 30 47 I 57 56 42 2 3 40
283 239 243 II5 208 288 I6 I2 15 0 135
10 3 34 20 57 30 42 52 29 20 56
error denotes the formal uncertainty of the of an individual monthly estimate (i.e. the
Height
std. error
std. dev.
s
(m)
(mm)
(mm)
no. of months
20 I9 39 48 32 25 17 41 36 IO I3
19.931 1107.119 1839.746 242.080 46.1 IO 2492.945 536.551 661.842 540.125 76.114 100.175
2 2 2 2 5 2 2 4 3 3 4
I6 18 20 I6 23 I7 I9 24 20 21 25
69 67 73 69 22 52 60 45 55 69 51
hardware. So-called ‘spherical shell’ models presently used to compute the atmospheric refraction correction (e.g. Marini and Murray, 1973) assume that the vertical air density profile is determined under the conditions of hydrostatic equilibrium and that there are no horizontal gradients (latitude or longitude dependence) in the meteorological parameters. Since various analyses suggest that these shortcomings in the atmospheric model can produce systematic range errors of between 6 and 12 mm (Gardner, 1985; Hauser, 1989) the atmosphere can now be viewed as the dominant systematic error source. Fourth generation two color SLR systems, which utilize ultra-short laser pulse widths (< 35 ps FWHM) and streak camera receivers (< 2 ps temporal resolution), are capable of producing range measurements with an absolute accuracy of better than 3 mm. A sample block diagram of a two color system is shown in Fig. 4 (Degnan, 1993). Since the measured differential time-of-flight between the two wavelengths is proportional to the group refractivity integrated over the roundtrip light trajectory, the atmospheric contribution to the optical time-of-flight can be determined and corrected (Abshire and Gardner, 1985). Significant hardware progress has been made in recent years. In 1991, joint Czech/Austrian experiments at Graz recorded the first single color (532nm) streak camera returns from satellite Ajisai (Prochazka et al., 1991). In 1992, NASA’s Goddard Space Flight Center recorded single color streak camera returns (also at 532nm) from up to three retroreflectors each physically separated by 4 cm on the Relay Mirror Experiment (RME) satellite as shown in Fig. 5 (Zagwodzki et al., 1992). These were followed by experiments in which the atmospheric variability near the Greenbelt site was mapped using two color streak camera ranging to a retroreflector mounted on an aircraft (McGarry et al., 1993). Using conventional photomultiplier-based systems, substantial multiple wavelength returns from LAGEOS and lower satellites were successfully recorded by Wettzell (1064 and 532nm) and Graz (1064, 532 and 680nm) observatories and by NASA/GSFC (355 and 532 nm) to Ajisai altitudes. Although these systems had insufficient temporal resolution to adequately determine the differential delay, they demonstrated that adequate signal strength could be achieved using multiple wavelengths. More recently, the NASA group has reported the first two color streak camera returns from a satellite (Zagwodzki et al.. 1994) and have
I I
Y w I.
Satellite laser ranging,
.
._‘_
__.’ .._
-
2
.
.
..x._ :. .__
’ : ._._. :<
vnwdmv
a~pp;y
status and impact for WEGENER
208
k
\iermaatC’l
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since obtained two color streak camera returns (532 and 355 nm) from most satellites at Ajisai or lower altitudes (i.e. lower than Ii00 km). .A sampling is shown in Fig. 6. These experiments have demonstrated that the multiple cube responses and pulse broadening effects typical of most satellites designed for centimeter ranging can substantially decorrelate the waveforms at the two wavelengths. With the recent launch of the Japancsc ADEOS mission in August 19%. the aforementioned R&D stations have proposed two color ranging experiments to the Retroreflector in Space (RIS). z unique single retlector which has a substantial optical cross-section l’or strong signal returns and which induces no pulse spreading. This unique reflector should provide the first true demonstration of millimeter accuracy SLR using two color technique\
Within the I’ramcwork 01‘ the WEGENER-MEDLAS Project and its successor WEGENER-2, the transportable SLR system5 TLRS-I (NASA), MTLRS-I (Institut fiir Angewandte Geodasie) and MTLRS-2 (Delf’t IUnivcrsit) of Technology) have repeatedly occupied some 15 sites in the central and eastern Mediterranean area in the time l‘rame I985 1995. The SLR activity was particularly high during dedicated measurement campaigns in 1986. 1987, I989 and 1993. The analyses of the data taken during these repeat occupations. in combination with global SLR data. hauc Id to ;I clear and consistent picture of the medium-scale deformations in the Meditcrrancan arca. An example of this is given in Fig. 7. which shows an SLR-only solution for horizontal motion vectors relative to stable Eurasia computed at Del0 University of Technology (vectors accompanied by open uncertaint] ellipses). It should be mentioned here that similar results have been obtained by other international research groups participating in WEGENER. At its outset in I9XS. WEGENER-MEDLAS posed the question about the magnitude and orientation of the contemporary crustal deformation in the Mediterranean region. both aspects important for a better understanding of earthquake mechanisms in the area. At the time. qualitative ideas were available only. with uncertainties of block motion estimates on the order of 100’%1. Within less than a decade. WEGENER-MEDLAS has given the scientitic community clear answer\: it has confirmed the northward motion ot Arabia. it has quantified the westward and southwestward motion of Anatolia and Aegea with high reliability. it has established the boundaries of ‘stable’ Eurasia and it has shown that the motion regime in the central (Italian) part of the Mediterranean is dictated by the Africa-Eurasia collision. In addition. the project has demonstrated the capability to organize and perform major international measurement campaigns. Today. re1atiL.e station positions for such networks can be dctcrmined c\ith at Icast comparable precision using geodetic GPS recei\,ers. in ;I much shorter time and at significantly less cost. This has already been proven by the WEGENERYGPS-92 and WEGENER/GPS-94 campaigns. during which almost the entire network was surveyed in 1 7 weeks in I992 and 1994. respectively. As a matter of fact. Fig. 7 also includes a solution for the motions in the Mediterranean based on both SLR and GPS measurements (the vectors accompanied by the solid error ellipses). Clearly. the trends that are evident from SLR-only do not change dramatically when more recently taken GPS data is added; the uncertainty of the solutions does decrease. howcvcr. because of the longer time period now being covered and because ot‘ the high precision of the GPS results in particular. Currently, the deployment of the few transportable SLR systems available (and likewise
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Satellite laser ranging, status and impact for WEGENER
211
transportable VLBI instruments) is being reconsidered, not only within WEGENER but within the entire global community. Consensus exists that this new strategy must address all three WEGENER scientific research objectives, and it must optimally exploit the unique capabilities of SLR (accuracy, simplicity of measurement, absence of ambiguity), recognizing the capabilities of GPS. To do so, the deployment of the mobile SLR systems should first of all improve the global distribution of the network of stations; this allows a more even tracking of geophysical and applications satellites, which in turn results in improved orbits and more accurate estimates for absolute vertical station positions and velocities, geocenter variations and mean sea level changes. Some of this has already been accomplished with the temporary visits of MTLRS-I to South Africa (1993) and of TLRS4 to post-glacial uplift areas in Canada (1993, 1994) and revised strategies employed by NASA. A second important new role of SLR is collocation with other space-geodetic measurement devices (GPS, VLBI). This will provide the opportunity to reliably connect different networks and to calibrate measurement techniques themselves. The latter holds in particular for GPS observations, which have inherent absolute uncertainties because of ambiguities and atmospheric delays. Efforts in this respect are currently being conducted in Switzerland (the Zimmerwald station), Great Britain (Herstmonceux) and Germany (the TIGO observatory), amongst others. Finally, it is of importance to preserve the history of the epoch measurements which have been performed in WEGENER up to now. It will be recognized that this can be done (as a matter of fact, has been done twice already) perfectly with GPS. On the other hand, the observation of absolute positions and velocities necessitates regular re-visits of certain sites in the Mediterranean, since the area is located at the southern extreme of the stable Eurasian plate with stationary systems serving as reference here. It must be emphasized, however, that only one or two carefully selected sites will be sufficient for this purpose, and that these fiducial stations will need to be visited at intervals of 334 years. Similarly, this strategy should encompass the measurement of selected fiducial sites elsewhere in boundary zones or uplift areas. The deployment at a limited number of fiducial sites should preferably follow a regular, long-term schedule, irrespective of its purpose being plate boundary, uplift, sea level or geocenter studies. Similar, but complementary, reasonI+ J should be followed concerning the use of stationary and mobile GPS equipment to obtain an overall strategy which optimally engages the various space-geodetic techniques to support the research objectives of the WEGENER project. The typical role of SLR in this programme is to establish and maintain accurate 3dimensional geocentric station positions in a fiducial network made up of stationary stations and selected sites for transportable equipment. Emphasis needs to be placed on the vertical component of these positions, for which SLR is particularly well-suited.
REFERENCES Abshire, J. B. and Gardner, C. S. (1985) Atmospheric refractivity corrections in satellite laser ranging. IEEE Transactions on Geoscience and Remote Sensing GE-23,414425. Castrique, L. (1992) 1991-IERS Annual Report. IERS, Paris. Degnan, J. J. (1985) Satellite laser ranging: current status and future prospects. IEEE Transactions on Geoscience and Remote Sensing GE-23, 398413.
Degnan.
J. J. (1YY3) Millimeter Accuracy Satellite Laser Ranging: A Review. In Conof’Spuce Grorie.s~~ to Gcor!,mmic.s Tec~hnoloy~. ed. D. E. Smith and D. L. Turcotte. AGU Geo~~~muttics Srric.s 25, l33- 162. Gardner, C. S. (1985) Effects of Horizontal Refractivity Gradients on the Accuracy of Laser Ranging to Satellites. Ruditr Scienw 20, 1593 1607. Hauser. J. P. (1989) Effects of Deviations from Hydrostatic Equilibrium on Atmospheric Corrections to Satellite and Lunar Laser Range Measurements. Joutvzu/ o#‘Gmph~~~id Research 94, IO I 82 IO I X6. Husson. V. S. (1991) Collocation Analysis (Looking Back). Paper presented at Crustal Dynamics Principal Investigators Meeting. October I99 I. NASA/GSFC. Marini, J. W. and Murray. C. W. (1973) Correction of Laser Range Tracking Data for Atmospheric Refraction at Elevations Above IO Degrees. NASA Report X-59 I-73-35 I. Goddard Space Flight Center. McCarthy. D. D. (1992) IERS Standards, IERS Technical Note 13. Paris. McGarry. J. F., Cheek. J. W., Millar. P. S. and Abshire, J. B. (1993) Two Color Laser Ranging to a Cooperative Airborne Target. Presented at SPIE OE,‘Aerospace Science and Sensing, April 1993. Orlando, Florida. B. A. c‘.. Herzberger, K., Kuijper, D. C.. Noomen, R., Springer, T. A., Ambrosius. Mets. G. J.. Overgaauw. B. and Wakker. K. F. (1996) Crustal deformations in the Mediterranean area computed from SLR and GPS observations. Journal of’Gcw~~xamic,s 21(l). 73-96. Prochazka. I., Hamal, K.. Kirchner. G.. ScheleL. M. and Postavolov. V. (IYY I) Circular Streak Camera Application for Satellite Laser Ranging. Presented at SPIE Conference on Electronic Imaging. February 24 27. I9Y I. San Jose. California. Smith, D. E. and Turcotte. D. L. (1993) Contributions of Space Geodesy to Gcodynamics: Geodynamics. AGU Gco~~wtttics Scric~s 23. Zagwodzki. T. W.. McGarry. J. F. and Degnan, J. J. (1992) Two Color Satellite Laser Ranging Upgrades at Goddard’s I .2 Meter Telescope Facility. Procwdinys of’thr Ei~yhrh 1ntematiottul Wovk.shop on LNSCI.Rrtnginy In.strtrtt~mtatiott. 7--l 7 to 7 27. May. Annapolis. MD. Zagwodzki, T.. McGarry. J.. Degnan. J., Abbott. J.. Varghese, T.. Oldham, T.. Selden, M.. Chabot. R., Fitzgerald. J., Grolemund. D. and Cheek. J. (1994) Two Color Ranging at NASA’s I.2 Meter Tracking Facility. 0th International Workshop on Laser Ranging Instrumentation, November 7 I I. Canberra. Australia. trihurims