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Geodetic Surveying W¡Th Quosar Radio Inlerferometry

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Geodetic Surveying W¡Th Quosar Radio Inlerferometry ì1">q *f. /b¿ GeodeticSurveying w¡th Quosar Radio Inlerferometry A. STOLZ et al Abstract Front 20 April to 3 May, l,982,five Austalian radio-telescopeswere linked for the first tirne and operated in synchro_nismto fomt a single radiotelescope.The li¡tk-up was the culminatiott of two years of iniensive effort and co-operationto co-ordinatetelàscope modi!ìcøtions and overcomethe logistic problents of urulertakittgsuch an experilnent. The five telescopesare sited at the NASA Deep Space tracking føcility at Tidbinbilla near cønberra, the cslRo Radio Astronomy observatory at parkes,the |lniversitlt of rasmania's Rødio observatory near Hobart, the university of sydney's F-lannobservatory near Sydney, ønd the LANDSAT tracking statìon at Alice Springs. The stutultaneousbut independent operation of two or more widely separatedtelescopes is called Long-Baseline Interferometry. This experiment was set up to provide high-resolutionradio møps of distant Southem Hemisphere quasarsønd galaxies.However, the experiment ß of greit interest to surveyors,since it alsoprovides a meansof møking accurøteposition difference and distance meøsurements.It is expected, for instønce,thøt the distancebetween the telescopesat Parkesand Tidbinbilla will be measur.edto an accurøcyof l0cmwhile the dßtances from the Tidbinbilla telescope to those at Hobart and Alice Springs wilt be determined to an accuracy of I to 2 metres. The experiment is describedin this paper. The basicprinciples of the VLBI techníqueare ølsoreviewed in generalterms. lntroduction Astronomershave developedradio-interferometers with whichthe baselines between antennascan be measuredwith high precisionover both short and very long distances (Barc et al., 1967; Broten et al., i9í:I;Gubbay t: s!., !974). Thesetechniques, calÌeci very-Iong-Baseline Interfe¡ometry (vLBÐ have significant applicationsto geodesy (Gold, 1967; shapiro and Knight, 1970). In a generalway, the radio signalsfrom a common sourceare receivedat two or more antennasat the end of a more or lesslong basetne.The signalsare brought together,and correlatedto determinethe differencein phaseor the differencein time in which a particularburst of energyis received,at the two antennas.This quantity relatesto the length and the direction ofthe baselinerelative to the source.In conventionalinterferometry the two signalsare brought togetherthrough cablesbut in VLBI the signalsare recordedindependently at each site, againsthighÌy precise time and frequency standardsand later brought togetherin a computer. Thi ìr A. STOLZ, B.Sürü¿,Ph.D.(N.S.IV.), School of Surveying,University of New So,rrt ililJ- B. HARVEY, Schôol of Su¡veying,University of New South Wales. D. L. JAUNCEY,Rrdiophysics Division, CSIRO. A. NEILL, D. MORABITO,R. PRESTON,Jet PropulsionLaboratory. B. GREENE, Division of National Mapping. K. LAMBECK' Researchschool of Earth sciences,Aust¡alian National university. A. TZIOUMIS, School of Physics,Sydney University. A. WATKINSON, Departmentof Eþctrical Engineering SydneyUniversity. G. W. R, ROYLE, Depattment of Physics,University of Tasmania. D. JOHNSON,School of Earth Sciences,Macquàrie University. The AustralìanSurreyo¡, March, 1983, Vol. 31, No. 5 305 ''i*, ',1-.,.::r':..¡'+ . ' . - r''r .'.;,.+ÞI-.å{.,. ' ,";, i '-' *;;,,:,' i".,,,i.1,''':i . ,,t, . ,'. ",þ '',;,:,it.'';' i*'¡.4,;r:i A. STOLZ et al antennad'ìSeparationis now limited only by the requirementthat the sourceismutually visiblefrom the two stations. The first geodetic VLBI experimentswere conducted in the United States in the early 1970's (Hintereggeret ø1., 1972; Shapiroet al., 1974} Sincethen numerous experimentshave been performedby groupsin the United States,Canada and Europe. Transcontinentalbase[nes, for example,have been measuredwith a repeatability of better than 5 cm (Shapiro,1978) and a 1.2 km baselinevector has been determined with a repeatabilityof 5 mm (Rogerset al., 1978). Two transportableantennas, 9 and 4 metres in diameter have been developedat the Jet Propulsion l:boratory (JPL) in Pasadena specifically for geodesy.Using these transportable antennas in conjunction with fixed radio-telescopesin California, triangle closuresto within 10-20 cm have been achieved over a perimeterof about 1000km (Niell et al., 1979). A number of conceptshave been proposedto use the Global PositioningSystem (GPS) sateltitesfor accuratemeasurements of distancein the range 100-1000km (Fell, t9E0). Theseinclude the interferometricmode in which the GPSsignals are used as a noise source, that is, essentiallylong baselineinterferometry techniques are used. The receiverscan be made quite small.Field deployable GPS receivers are expected to become commercially availablein 1984 (Bossler,1981). Moreover,it now appearslikely that measurementscan be made with 1 ot 2 cm açcuracyin times as short ashalf an hour or less if the system is completed as planned. The VLBI technique is thus of considerable interestto surveyors. Geodetic VLBI has not beenactively pursuedin Australia in the past. This changed in April, 1982 when five Australian telescopeswere linked for the fìrst time to measure the position differericesand.baselines. between them. We describethe experiment in this paper. The basic principles'of thlVLBI technique are also reviewed, in generalterms. Detailsmay be found in Counselman(1976) and Carter(1981). GeodeticVLBI Radio-interferometryhas been reviewed byseveral authors (e.g. Counselman,19T6). Here we shall discussonly those aspectswhich pertain to geodeticmeasurements. Observables The basic principles of the VLBI techniqueare illustrated in Fig. I which showsthe geometry of a geodetic VLBI measurementconfìguration. Two widely separatedradio- telescopesobserve radiation from an extra-galacticradiosource, typically a distant quasar or radio galaxy. The Australian geodeticobservations were made at S-band(2.3 GlIz) and X-band (8.4 GHz). The sequenceof observationsconsists of about 100 separatemeasure- ments ingolving 10 to 20 different radio sourcesspread across the sþ. The signal received from eaclibource is amplifìed and mixed with a signalfrom a local oscillator. lndependent phase.stabtÇ.localoscillator signalsat each antenna are obtained from ultra-stableatomic oscillators,¡{ypically Rubidium standards or preferably Hydrogen masers. Practical stabilities of about I part in 1013over a spanof 103 secondsare achievable with Rubidium standardswhile Hydrogen masersyield about I part in 101s. The video frequency signal produced after the local oscillator is mixed with the radio frequency signal is then sampledat a fixed rate (4 MHz in our case)and recorded on a high-speedrecorder. The final result is a set oftapes, one from eachantenna, càrry- ing a sequenceof ones and zeroes,called bits, and each carrying its own time tags. The tapes carry a "noise" contributed by the different receiversat each site as well as the source information. These tapes are then transported to a special central processing 306 The Aus*alian Sunteyor,March, 1983, Vol. 31, No. 5 I I I l I ':i ,r1r ''.' ii,i¡..,- : i : , ., . ,.r,,¡:1,'t,,'t;Ì¡,';;i:'!ii':'1.,,- :"ii¡.,;'1 ,-rt",a;ffi CEODETICSURVEYING WITH QUASARRADIO INTERFEROMETRY facility (there are at presentthree such facilities in the world, two in the United States and one in Europe)where they areall replayedunder central computer control and are cross-relatedin pairs. During the cross-correlation,each tape nìust be appropriately delayed,since the signalwas receivedat the more distantantenna at a time z sec<>nds after it was receivedat the nearantenna. This delayis the basicobservable of geodetic VLBI measurements.Fig. I showsthat, as the earth rotatesthe delaywill vary as the anglebetween the sourceand the baselinevaries. This quantity,called the delayrate, is the secondobservable. TapeCorrelation In essence,the two stringsof bits a¡e multipliêdtogether, bit by bit, and the productsare summedover an appropriateinterval. The procedure is repeatedin step-wise fashion for a range of offsetsabout the predictedtime delay. This is done on a special correlator.The sum reachesa maimum at the correctdelay. In order to achievebetter precision in the delayobservable than can be obtained from observationsat a single frequency,a techniquecalled bandwidthsynthesis,is used in which the delay is obtainedas the changein interferometerphase across two or more frequenciesspanning up to severalhundred megaHertz.More than two frequenciesare desirablein o¡der to removethe integer-cyclephase ambiguities, analogous to the useof multiple modulatingfrequencies to resolveambiguíties in EDM. Geometryof VLBI Observables From Fig. l, the geometricaldelay is =-l's , c whereB is the baselinevector between the observingantennas, S is the unlr vecrorpolnt- ingin the direction of the sourceand c is the velocity of light. In the following discuìsion, c is setto unity for convenience. ln a referenceco-ordinate system where the sourceco-ordinates are fixed. the vector B will changein orientation but not in magnitudeas the earthrotates. For simplicity,-of assumethe co-o¡dinateaxes are arrangedso that the z-axis is in the direction the earth's instantaneousspin axis. The baselinevector É can then be split into two parts,a polar or z'componentparallel to the spinaxis and an equatorialor *y-.ornponentperpen- dicular to the spin axis.Thus - 1É,+ B*v) .(3" * 3*y) =:*n - (bzsz* Ê*v'\v) whereb, and Sj are the magnitudeof the polar conponents. By definitlon, the polar componentcannot changewith the earth'sdiurnal rota- tion. Accorrlingly, part of the geometricdelay is constant and dependsonly
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