The Giant Metrewave Radio TelescopeA. Pramesh Rao and S. AnanthakrishnanNational Centre for Radio Astrophysics, Tata Institute of Fundamental Research,Post Bag 3, Ganeshkhind, Pune 411007, India

Abstract: The Giant Metrewave Radio Telescope (GMRT)is a major new facility available for radio astronomicalobservations at metre wavelengths. This summary describesthe current state of GMRT with emphasis on the featuresthat are of relevance to a potential user.

IntroductionOne of the most challenging problems faced by

radioastronomers has been the poor angular resolution ofradiotelescopes. Since all radio telescopes are diffractionlimited, the angular resolution is equal to λ/D where λis the wavelength and D is the size of the aperture. Theradio band covers frequencies from 30 MHz to 300 GHzwith wavelengths going from 10 m to 1 mm. Comparedto an optical telescope of the same size, (optical wavelengthis about 0.5 micrometer), the angular resolution of radiotelescopes at metre wavelengths, is about a million timespoorer. Building bigger radio telescopes (increasing D),while easier than in the optical case, is not a solution sincea radio telescope operating at 1 m should have a diameterof 200 km to have the same angular resolution as a 10cm optical telescope. Since telescopes of such sizes arenot technically feasible, radioastronomers have followed theroute taken by optical astronomers when they wantedtelescopes larger than what was technically possible(Michelson 1920) and have perfected interferometry tech-niques.

If one wants to form an image as in a camera ora telescope, the obvious way to proceed is to use one ormore lenses or curved mirrors. But even if one does nothave any lenses or mirrors, one can still form an image.The simple pin hole camera forms an image but suffersfrom poor angular resolution since the aperture D isnecessarily small. However, if one makes a second pinholeand makes the light source quasi-monochromatic (eitherintrinsic or by using a filter), then in the image plane onesees an inteference fringe pattern, from which one canmeasure the Visibility amplitude [(Imax–Imin)/(Imax+Imin)]and phase (position of the observed fringe maximacompared to that expected for a planewave normal to thescreen). When the field of view is small, it can be shownthat the complex fringe visibility is proportional to the 2-dimensional Fourier transform of the incident angularspectrum corresponding to the spatial Fourier componentu=x/λλλλλ and v=y/λλλλλ, where x and y are the separation ofthe two pinholes, z being perpendicular to the screen (see“Principle of Optics” by M. Born & E. Wolf, Cambridge).Thus, if one has an array of screens with 2 pin holes at

all possible relative orientations upto a maximum separationof d and one measures all the complex Visibility functions(Fourier components), then one could feed all the Fouriercomponents into a computer, do an inverse Fouriertransform and produce an image of the object as wouldhave been seen with a lens of diameter d. One has avoidedlenses but paid a price in complexity, but in the radio casewhere the technology does not support building “lenses”larger than a 100 m or so, this is the only way to go.

The radio equivalent of the 2 pinhole system is the‘2 element adding interferometer’ which consists of 2 an-tennae tracking a source in the sky. The electric fieldsreceived by the antennae are brought on cable to a commonplace where they are added and detected. Representing theamplitude and phase of the electric field by complexnumber E, the power output of the interferometer is

I = (E1 + E2) (E1 + E2)*= |E1|2 + |E2|2 + 2|E1||E2| cos(φφφφφ1 – φφφφφ2)

The third term, which corresponds to the interferenceterm can be used to determine the complex fringe visibility.The 2 element interferometer measures one spatial Fouriercomponent of the source. Due to the rotation of the earth,the orientation of the source with respect to the antennaseparation changes with time and the interferometermeasures different Fourier components and traces a trackon the 2-D Fourier plane. If there are N such antennaesuitably located, the electric fields from all the antennascan be brought to a central location, with appropriateelectronics so that each antenna forms an interferometerwith every other antenna giving N(N–1)/2 pairs, each ofwhich measures the visibility function on a track in theFourier plane.

All these Fourier components are recorded, calibrated,Fourier transformed and processed to produce an imageof the source, with a resolution equivalent to the largestseparation between the antennas, which could be manykilometers. Inspite of the fact that all the Fouriercomponents have not been measured, good quality imagescan often be made by appropriate image processing. Thiswhole process is referred to as Earth Rotation ApertureSynthesis and is the principle behind the most powerfulradio telescopes in the world.

The basic aim while building GMRT was to realisea very sensitive and state of the art aperture synthesis radiotelescope array in the frequency range of 30–1500 MHz,which was lacking in the world astronomical scene. Thereare advantages in building such an array in India. Because

of the low frequencies, (i) the surface accuracy require-ments are not so stringent and the construction labourintensive resulting in lower cost; (ii) low frequency elec-tronics is relatively inexpensive; (iii) very high angularresolution is not required, since the interesting science atlow frequencies is in diffused extended regions. Thedisadvantages of low frequencies are that (i) sky back-ground noise is higher; (ii) ionospheric distortion increasestowards lower frequencies and (iii) man made radio noiseis higher.

GMRT — SpecificationsThe GMRT consists of thirty 45 m diameter antennae

spread over 25 km. Half the antennas are in a compact,randomly distributed array with a diameter of about 1 km.The remaining antennae are on 3 arms of length 14 km(NorthWest, NorthEast and South) with 5-6 antennas oneach arm. The longest baseline is 26 km and the shortest,100 m which comes down to about 60 m with projectioneffects. The array configuration is shown in Fig. 1 and theu-v coverage for some declinations are shown in Fig. 2.The telescope (Latitude = 19.1o, Longitude = –74.05o) islocated near Khodad village, which is about 80 km northof Pune. The facility is run by the National Centre forRadio Astrophysics, Pune, which is a part of the TataInstitute of Fundamental Research, Mumbai, India.

Antennas and FeedsThe GMRT antennas are 45 m alt-azimuth mounted

dishes (Fig. 3). The reflecting surface is formed by wiremesh and the efficiency of the antennas varies from 60%to 40% from the lowest to the highest frequency. The total

weight of the moving structure, excluding the counter-weight of 34 tonnes, is only 82 tonnes, making it one ofthe lightest antennas of its size. The dishes can go downto an elevation of 16o, giving a declination coverage from–55o to +90o. The slew speed of the antennas is 30o/minin azimuth and 20o/min in elevation and they are notoperated when winds are higher than 50 km/h. There isa rotating turret at the focus on which the different feedsare mounted. The feeds presently available are the 151, 325,610/235 and the 1000–1420 MHz feeds.

Both the orthogonal polarizations are brought fromeach antenna. The orthogonal polarizations are circular forall feeds except the 1420 MHz feeds which are linear. TheL-Band feed is a corrugated horn that was designed andfabricated by the Raman Research Institute, Bangalore,India. The 610/235 MHz feed is a dual concentric coaxialcavity, and with both feeds at the focus at the same time,the system allows simultaneous observations at 610 and235 MHz with one polarisation at each frequency. The 325MHz feed is a Kildal feed (half wave dipole over a groundplane with a beam forming ring in front, Kildal 1982) andthe 151 MHz feed consists of two orthogonal pairs ofdipoles in front of a plane reflector. A 50–70 MHz feedfor the system is under development.

Control SystemThe control of the whole array is done by a

distributed system of microprocessors that communicatewith and are controlled from a workstation in the controlroom. At the base of each antenna is the Antenna BaseComputer (ABC) which controls all functions at theantenna. The ABC generates tracking commands for the

Fig. 1: Layout of the antennas of GMRT. The central square isshown in the inset.

Fig. 2: u–v coverage for the GMRT at 4 declinations for 1 mwavelength.

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GMRT ARRAY CONFIGURATION

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Servo Computer and issues instructions to the variousMonitor and Control Modules (MCMs) that control thestate of the RF, IF Local Oscillator and feed positioningsystems. The servo system, designed by the Reactor ControlDivision of the Bhabha Atomic Research Centre, ensuresthat a source is tracked with an rms error of ~1′. TheABC collects monitoring signals from the various sub-systems and periodically sends them back to the controlroom. The control instructions and return telemetry in-formation are sent on the same optical fibres used for theradio signals and there are various displays for the userto ensure that the system is healthy. User generated controlfile can also instruct the main computer.

ElectronicsAt the focus of each antenna, each feed has 2 low

noise amplifiers, (one for each polarization), followed bya noise injection facility where the user can select 4 noisevalues. The two RF signals are down converted to two IFbands of 32 MHz width at the bottom of each antennaand sent to the receiver room on optical fibre links. Inthe central receiver room, each signal is converted to 2baseband signals with user selectable bandwidths that gofrom 16 MHz to 62.5 KHz in binary steps. The basebandsignals are fed to a digital FX type spectral correlator whichactually multiplies the electric fields from all the antennasin pairs, to give the visibility function as a function offrequency.

The GMRT can also be at present used for a singleantenna VLBI observation at frequencies up to 1665 MHz.The flexible local oscillator frequencies as well as timingis based on VCXO’s, GPS and Rubidium standards.

Digital Back EndThe 2 baseband signals are handled by 2 nearly

independent correlator systems. The GMRT correlatorwhich is an FX type correlator, runs at 32 MHz but canbe programmed to use only every 2nth sample so that theeffective sampling interval is larger and the bandwidthnarrower. The basic components of the system are thesamplers, delay system, the FFT pipeline and the multiplierand accumulator (MAC). The baseband signal is sampled(4 bits) and compensated for propagation delays. The datastream is Fourier transformed to give 256 frequencychannels across the band but while multiplying andaveraging, adjacent frequency channels are averaged givingeffectively 128 channels across the band. The channel widthdepends on the base band filter and the effective samplinginterval and can range from a maximum of 128 KHz to0.5 KHz with the total bandwidth ranging from 16 MHzto 64 KHz in each sideband for each polarisation. Thevoltage spectrum from each antenna is multiplied with thatfrom all antennae (including itself) and the resultant(30×31/2)×128 signals for each polarisation are averagedfor 128 ms in the hardware, before being read out andfurther integrated. The hardware provides a variety offacilities like fringe stopping, delay compensation, Walshand noise switching demodulation, window functions beforeFourier transforming and the possibility of blanking thesignal for interference rejection. The 2 sideband correlatorsystem gives 4 outputs, 2 RR and 2 LL, effectively doublingthe bandwidth of the system. However, it can also bereconfigured to give the 4 Stokes parameters for one ofthe 2 baseband systems. The full spectral line data isrecorded with user selectable integration time that can bea multiple of 8×128 ms=1.024 s. The default integrationtime is 16 s which generates 53 Mbytes of data every hour.There are options to record only a subsection of thebaseline and frequency channels if one needs much shorterintegration times.

The output of the FFT pipeline can also be sent tothe GMRT Array Combiner (GAC) (built at the RamanResearch Institute, Bangalore) where signals from differentantennas can be combined to give a single time series ofspectra with a sampling interval of ~16 ms. The data fromthe antennas can be combined either coherently (voltage)or incoherently (power) depending on the users require-ments. The output of the GAC can be recorded for VLBI,pulsar search etc or sent to the pulsar backends where onehas facilities for dedispersion (both coherent and incoher-ent), polarimetry, etc. The measured system parameters ofGMRT are shown in Table 1.

Current StatusThe GMRT has been in operation since 2000 and

currently all the subsystems are operational except the 50-70 MHz system that is yet to be installed. Routineobservations are being made at all the other GMRT bandsthough the 150 MHz band is heavily affected by interference

Fig. 3: A few of the GMRT antennas in the central square.

for which remedial actions are being taken. Except for aweekly maintenance period from Wednesday morning toThursday evening, the GMRT is used 24 hours of the day,seven days of the week. It has been used for a varietyof observations ranging from the Sun, Jupiter, galacticobjects like pulsars, x-ray binaries, the Galactic centre, HIIregions and supernovae and extra galactic obhects likenearby galaxies, radio galaxies, clusters of galaxies, spectralline observation of the 21 cm Hydrogen line and OHmaser lines at a variety of redshifts and so forth. An imageof the nearby spiral galaxy NGC 2997 is shown in Fig. 4.

Being the most powerful radio telescope in the worldin the metre wavelength band, there is considerable interestin the radioastronomy community in using the GMRT. Forthe optimal utilisation of the GMRT, the GMRT TimeAllocation Committee (GTAC) has been set up, currently

chaired by Prof. G. Srininvasan of the Raman ResearchInstitute, Bangalore. The GTAC periodically informs theinternational astronomy community of the capabilities ofthe GMRT, invites proposals for projects utilising GMRT,reviews them and if scientifically viable, allocates GMRTtime. These projects are scheduled on the telescope overthe next 3-4 months and the astronomer carries out hisobservation and either analyses the data locally or takesthe data home for analysis. Since 2002, 4 such cycles havebeen completed and the 5th GTAC cycle will start shortly(Dec 2003). For each GTAC cycle, 50 to 60 proposals arereceived and many of them get scheduled. Fig. 5 gives asummary of the statistics of the received proposals in thefirst 4 GTAC cycles.

For further details of GMRT and information onhow to apply for GMRT observing time, please log in toour website http://www.ncra.tifr.res.in

AcknowledgementsSuccessful completion of GMRT has required the

participation of a large number of scientists and engineersand supporting staff. Many other institutions in India, suchas the Raman Research Institute, Bangalore and the BhabhaAtomic Research Centre have also built significant part ofthe hardware. We would like to thank all of the abovepersons and institutions that have made GMRT into areality.

ReferencesAnanthakrishnan, S. & Pramesh Rao, A. 2002, Multicolour

Universe, p233, Ed. Manchanda, R. & Paul, B., TIFR,Mumbai.

Swarup, G., Ananthakrishnan, S., Kapahi, V.K., Rao, A.P.,Subrahmanya, C.R. & Kulkarni, V.K. 1991, CurrentScience, 60, 95

Kildal, P-S 1982, IEEE Trans. Antennas Propagation, AP-30,529

Michelson, A.A. 1920, Astrophysical Journal, 51, 257.

UK, 26, 9%

USA, 23, 8%

Other Countries, 64, 23%

India, 169, 60%

Fig. 5: Chart showing proposals received for observationswith GMRT during the first 4 GTAC cycles.

Fig. 4: GMRT radio contour of the nearby spiral galaxyNGC 2997at 21 cm, superposed on false colour optical image.

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Table 1: Measured parameters of GMRT.