NCRA – TIFR, Pune, India
Outline of the talk :
• Introduction to the GMRT
• Description of correlator and pulsar receivers at the GMRT
• Upgrade plans

•
The GMRT is a low frequency (50 -
1450 MHz), multi-element radio telescope built by NCRA (TIFR)
•
Located in India : about 70 km N of Pune, 160 km E of Mumbai.
•
Latitude : 19 deg N
•
Longitude : 74 deg E
1 km x 1 km
•
30 dishes; 45 m diameter
•
12 dishes in central compact array
•
Remaining along 3 arms of
Y-array, extending ~ 14 km from centre
14 km
•
Longest baseline : ~ 25 km
•
Modes of operation :
•
Aperture synthesis
•
Array mode


40 – 60 MHz
120 – 180 MHz
225 – 245 MHz
300 – 360 MHz
580 – 650 MHz
1000 – 1430 MHz
Antenna primary feeds are placed on a rotating turret
Near the focus of the
45-m dishes.


Dual polarized feeds

Low Noise Gas - FET amplifiers at front-end

Super-heterodyne receiver chain : IF & baseband sections

Tunable LO (30
– 1700
MHz) with low phase noise (~2 o @1 GHz ;
-50 dBc/ Hz at 1 Hz offset) ; locked to a reference from central receiver room

Optical fibre link from each antenna to central receiver room

Maximum IF bandwidth:
32 MHz


Main components :
 FX Correlator
 Pulsar Receiver can operate simultaneously

Common signal processing stages: sampling, delay correction, fringe stopping and
FFT

Input data rate : 1.9 Gsamples/s

8 bit sampling but input to FFT stage is 4 bits

Integer delay correction to +/-
16 ns sample and fractional sample delay correction to 2 ns using phase gradients across the FFT


Input data rate : 1.9 Gsamples/s

Output visibilities at 128 ms rate

256 spectral channels across 32
MHz BW for total intensity mode; full polar mode at half the BW

Supports sub-array mode of
GMRT with different sources / frequencies for each sub-array

Total compute power :
~ 100 GCops

Uses mostly ASICs + some
FPGAs


Delay and phase corrected data from the FFT outputs is given to the
GMRT Array Combiner (GAC)

The GAC allows any user selected set of antenna signals to be added to get the array output

Supports 2 modes (simultaneous operation) :
 incoherent array -- power sum
 phased array -- voltage sum

Can do simultaneous multifrequency observations with a single pulsar receiver -- trade-off
BW for different sub-arrays

Pulsar Receiver uses more of
DSPs (flexible signal processing)

Significant amount of real-time processing on general purpose computer network : shared memories, socket communications etc.


Phasing the array requires calculations of the phase of each antenna (with respect to a reference antenna) from observations of a suitable calibration source

The computed phases are then inserted into the signal path of the antennas so that the signals from each antenna are in phase and can be added together
Phases of different antennas as a function of time
Before phasing After phasing


The process of phasing also corrects for phase variations across the frequency band for each antenna
Phases of different antennas as a function of frequency
Frequency Frequency
Before phasing After phasing

150
Frequency (MHz)
235 327 610 1420
Primary Beam (deg) 3.2
2.0
1.4
0.8
0.4
5 Synthesized Beam (arcsec)
Effective Area (m 2 )
System Temp ( o K)
20
450
Usable Band (MHz)
Best RMS sensitivity achieved from
2 - 3 hrs using full available BW (mJy)
150-
156
3
13 9
180
30,000
110
232-
244
2
310-
350
1
100
590-
650
0.1
2
18,000
70
1000-
1450
0.05
Typical Dynamic Range achieved 1000 1000 1200 > 2000 > 5000

 Effective Area : 30,000 sq. m. for 610 MHz and below
20,000 sq. m. for 900-1400 MHz
 Maximum Bandwidth : 32 MHz
 Sampling time : > 16 microsec for incoherent dedispersion
< 1 microsec for coherent dedispersion
 Polarization : All 4 Stokes parameters for coherent phased array
Total intensity for incoherent array
 Sensitivity :
 For pulsar search mode (using incoherent array mode) :
S av
= 1 mJy
[f = 325 MHz; all 30 dishes added; 2 polarisations;
100 microsec sampling; 10 minute scans]
 For single pulse mode (using phased array mode) :
S av
= 35 mJy
[f = 325 MHz; all 30 dishes added; 2 polarisations;
500 microsec sampling]


Although the GMRT is likely to remain a competitive instrument for some more years, plans are already underway for upgrading the GMRT

Main targets :
 Provide, as far as possible, seamless frequency coverage from ~ 100
MHz to ~ 1700 MHz, instead of the limited bands at present
 Provide instantaneous maximum bandwidth ~ 400 MHz

Required improvements :
 New, broadband feeds : maximum frequency coverage per feed, while avoiding the major RFI sources. e.g. ~1000 to 1700 MHz, ~500 to 900
MHz, ~300 to 500 MHz + narrower bands around 150 and 230 MHz
 Redesign of the receiver system and of the transport over the fibre link
 New digital back-ends (correlator and pulsar receiver) to handle 400 MHz of bandwidth (in modules of 50 to100 MHz)
 Accompanied by a revamped control and monitor system, and also an upgraded servo system

Parameter
Number of Stations
Number of polarisations
Maximum instantaneous BW
Max number of spectral channels (across full BW)
Number of bits (at digitisation stage)
Fastest dump time for visibilities
Support for full polar (4 Stokes) mode
Support for sub-array mode
Existing GMRT
Back-ends
30
Upgraded GMRT (planned)
32 (spare inputs for future applications)
2 2
32
256
4
400
8192
8
128 millisec 32 millisec (less for reduced # of channels)
Yes (but for half BW) Yes, for full BW
Yes Yes
Number of phased array beam outputs
Number of incoherent array beam outputs
1
1
Fastest dump time for array outputs 128 microsec
Ability to record raw output of phased array beam limited
Real-time RFI detection and flagging
Technology approach
Not available
ASICs + DSPs
4
2
4 - 16 microsec
Yes, full BW for at least 1 beam
Yes, both time domain & spectral
?? (ASICs , FPGAs, software, mixed)


Development of a software correlator for the GMRT :
 back-up for existing correlator;
 niche applications like better RFI immunity at low frequencies;
 applications benefiting from raw dumping of voltage signals from each antenna, followed by off-line processing

Application of beam forming techniques for a pulsar observing mode that synthesizes multiple beams within the primary beam, using the central square antennas

Preliminary plans for focal plane array on one face of the feed turret

Existing GMRT Back-ends
The Upgraded GMRT (planned)

Number of stations : 30

Number of polarisations per station : 2

Maximum instantanious BW : 32 MHz

Number of bits (at digitisation stage) : 4

Max number of spectral channels : 256

Fastest dump time for visibilities : 128 msec

Supports full polar mode : Yes (but with half the BW)

Supports sub-array mode : Yes

Number of phased array beam outputs
: 1

Number of incoherent array beam outputs : 1

Fastest dump time for array outputs :
128 micrsosec


Number of stations : 30

Number of polarisations per station : 2

Maximum instantanious BW : 400 MHz

Number of bits (at digitisation stage) : 8

Max number of spectral channels : 8192

Fastest dump time for visibilities : 32 msec
(with capability for faster dump times for reduced # of spectral channels)

Support for full polar mode : Yes

Support for sub-array mode : Yes

Number of phased array beam outputs : 4 (can be pointed anywhere within the primary beam)

Number of incoherent array beam outputs : 2

Independent selection and weighting of indvidual antennas in array sum

Fastest dump time for array outputs : 8-16 microsec

Ability to record raw output of phased array beam

Ability for real-time RFI detection & flagging, both in time domain & spectral domain

Technology approach : FPGAs, custom hardware