GMRT Status : January 1996
GMRT is an array of thirty fully steerable antennas, each 45 metres in diameter. Twelve of these are located in compact central array, about 1 km x 1 km in size. The remaining 18 antennas are placed along the three arms of an approximately Y-shaped configuration with each arm extending to about 14 km from the array centre. There are six operating frequency bands centred at 50, 153, 233, 327, 610 and 1420 MHz. Dual polarisation operation is supported at all these bands. The feeds are designed such that dual frequency operations will be possible with some combinations of the bands. Optical fibre links are being used to distribute the local oscillator and control signals to the antennas from the central laboratory and to bring back the IF signals. The main digital back-ends being built are an FX correlator and a pulsar receiver.
Mechanical Construction : The contractors have completed the erection of the structural parts for all of the thirty antennas of GMRT. This milestone was achieved on November 10, 1995. Fourteen of the thirty antennas have been fully commissioned so far, eleven in the central array and three in the Y arms of the array. This includes installation and tuning of the servo drive systems.
Work on servo drive installation for six more antennas, three each in the central array and the Y arms, is in progress and these will be commissioned soon. Of the ten remaining antennas, four will soon be handed over by the contractors, and the other six are in various stages of completion. All construction work on the antennas is expected to be finished by May 1996, and the installation of servo drives by October, 1996.
Analog Electronics Systems : On the analog electronics side, RF front-ends for all bands (except for the 50 MHz and the 1400 MHz bands) have already been installed on twenty antennas, and will soon be installed on three more. All these front-ends allow signals of 32 MHz bandwidth to be brought down to the antenna base as a set of two circularly polarised signals. Front-ends for the 1400 MHz band have been installed on four antennas and will soon be installed on two more; the new front-ends will have the full bandwidth of about 500 MHz (1000-1500 MHz) brought to the antenna base. These signals will be linearly polarised. The 1400 MHz feeds and front-ends have been designed and are being built by the Raman Research Institute, Bangalore. IF systems, baseband units and local oscillator synthesisers for twenty antennas have been assembled and tested at Pune and sent to the site. Fourteen of these systems have been installed, three are under ``continuing burn-in and regular test'' at the central electronics building at the site, and three are in queue for this test.
Optical-fibre connections have been made from the central electronics building at the site to fifteen antennas. On the telemetry and control front, systems for fifteen antennas are ready. More than 90 \% of the Monitor and Control Modules (MCMs) that will be finally required have been produced and distributed to various groups that will be using them for monitoring and control at various locations of the electronics chain.
Correlator : The prototype version of the correlator has been working at the site for quite some time now; it is an FX correlator that can handle signals from four antennas with two polarizations each. This has been extensively used for the initial testing, debugging and calibration of GMRT. The next version of the correlator, which will have a capacity of eight antennas, is expected to be released by the end of March, 1996. This system will work at 32 MHz and will have 128 spectral channels over bandwidths ranging from 64 kHz to 16 MHz, in the RR-LL polarization mode. Fringe stopping and fractional-sample-time corrections will be done in real time in the hardware. The final correlator for the thirty-antenna system is expected to be ready later this year.
Pulsar Receiver : The prototype version of the pulsar receiver, built jointly by the Raman Research Institute, Bangalore and NCRA, has been installed at the site. Like the prototype correlator, this system can handle signals from four antennas, with two polarisations each. The input to this receiver is the output from the Fourier Transform engines of the correlator and its final output is the sum of the signals from the four antennas after detection and required pre-integration, resolved into 256 spectral channels spanning the band. The fastest sampling rate available with this version is 0.15 millisecond. This system has been used extensively for test observations of pulsars with the GMRT antennas. An expanded and improved version of the pulsar receiver that can combine signals from eight antennas is under construction at the Raman Research Institute and is expected to be installed by the end of March, 1996. This will later be upgraded for signals from thirty antennas. Other improvements to the pulsar receiver, such as a polarimeter and a coherent dedispersor, are expected to be ready by the middle of 1996.
Measurement of System Parameters : The antenna efficiencies and system temperatures of a number of GMRT antennas have been measured at 327 MHz and 1420 MHz, using standard sources like Virgo-A, Cas-A, and Crab. At 327 MHz, the typical values for the measured antenna efficiency are 0.5 to 0.6 and the system temperature is 100 - 120 K. At 1420 MHz these numbers are $\approx$ 0.35 and 70 K, respectively. For some antennas, the efficiency should improve with further adjustments of the wire mesh reflecting surface and better feed positioning. Pointing measurements and calibration have been carried out for several antennas. The pointing offsets are found to be stable over a wide range of antenna azimuth and elevation.
Testing the Interferometer Mode : Baseline calibration has been done for a few of the GMRT antennas in the central array and one in the western arm. This was done by using subarrays of three or more antennas to observe point sources at 327 MHz. The coordinates of these antennas are now known to better than 20 cm.
The phases of various point sources have been monitored continuously for 6-8 hours on several days. Drifts of a few tens of degrees are seen over several hours. The most likely source of these drifts is the ionosphere, though changes due to varying gains in the electronics cannot be ruled out yet. Rapid phase variations of 10\deg-20\deg\ are sometimes observed; these are almost certainly due to problems in the electronics, which are being corrected.
For strong sources ($>$ 10 Jy at 327 MHz) closure phase and amplitude errors are routinely found to be less than a few degrees and 5-10 \%, respectively. This indicates that in spite of ionospheric instabilities, the array can be phased over baselines of 2-3 km.
Since the GMRT has a spectral (FX) correlator rather than a continuum (XF) correlator, accurate determination of antenna-based fixed delays is essential for coherent averaging of the band to generate continuum visibilities. There are significant variations in the measured fixed delays within the four-antenna correlator now being used. This problem is expected to be fixed in the new eight-antenna correlator now being tested.
Gain stability of the antennas is being monitored. The differences in gain between antennas are found to be significantly larger than expected, a problem that is being looked into.
Software libraries for off-line data analysis have been developed and tested. Off-line programs for fringe stopping, data display and primary calibration as well as for importing continuum, single-source data into AIPS have also been tested.
Spectral Line Testing : Some observations of the Galactic plane were made at 1420 MHz. Absorption features were detected against a strong source (Cas-A) using the four-antenna correlator. The spectra were found to have some undesirable features, e.g. drifting patterns in the time-frequency plane and very narrow spurious spectral features. The most likely sources of these problems are local radio frequency interference and the number of bits used in the trig tables of the correlator. These problems are expected to be corrected by the use of Walsh switching and the new eight-antenna correlator.
Testing the Phased Array Mode : The plan is to use GMRT in the ``coherently phased array mode'' (addition of the pre-detection signals from the antennas followed by total power detection) or in the ``incoherently phased array mode'' (total power detection followed by addition), depending on the nature of the observation. Observations such as all sky pulsar searches will benefit from the incoherently phased array mode, whereas observations like polarimetry and microstructure studies of known pulsars will need the coherently phased array mode. The Pulsar Receiver is being used to test the incoherently phased array mode of operation.
Extensive test observations of single antennas in the total power mode using continuum radio sources in conjunction with the calibration noise sources have been done. For many of the observations carried out during day time, the measured signal to noise ratio is significantly less than expected. This can be traced to the higher levels of interference due to ongoing construction activities such as welding, walkie-talkie signals etc. Incoherent addition of the signals from more than one antenna is not found to give the expected increase in signal to noise ratio and this problem is being looked into presently.
Early observations with known pulsars showed pulsar profiles which were affected by the operation of the Automatic Gain Control (AGC) circuitry in the signal path. This problem has been taken care of for all the new antenna electronics that is being released. Good quality profiles have been obtained for several pulsars now and they agree with published results in the literature.
The pulsar receiver has also been interfaced to a GPS receiver, which is the time keeping standard for the observatory, to obtain accurate time stamps for the acquired data. Using this facility, timing observations of pulsars have also been initiated.
First Results using a GMRT antenna
Simultaneous ORT-GMRT Observations: The inter-planetary scintillation (IPS) group of NCRA has taken up a project to measure the solar wind velocity by recording inter-planetary scintillations of compact sources observed simultaneously with the Ooty Radio Telescope (ORT) and some antennas of GMRT. The ORT-GMRT baseline is about 900 km and is almost North-South, making it one the longest baselines used for such observations and with the advantage that it can study the solar wind at high heliographic latitudes. The idea for this project came up during discussions between Prof. W. A. Coles of the University of California San Diego and the IPS group. An initial run of observations in March, 1995, using one of the GMRT antennas in the total power mode, showed promising results, but the data quality was not fully satisfactory due to interference and initial problems with the electronics, which was really not designed for such total power observations. Subsequently, many of the problems were rectified and an observing run was made in December, 1995, where the data quality from GMRT was quite satisfactory. In addition, a fast sampling mode was implemented in the prototype four-antenna correlator, where one could record data from 128 frequency channels from two self and two cross correlations with an integration period of 22 ms. The correlator output was also synchronised with the GPS clock to give accurate time stamps on all the correlator data. IPS data recorded with the correlator is less prone to interference and of much better quality than the total power data. In the December-95 run, data was recorded in both modes and is being analysed. A fresh coordinated observing run is being carried out in Feb-Mar, 1996.