GMRT Observer`s Manual - Tata Institute of Fundamental Research
Contents
1. 6 2 1 A Flagging and Calibration Pipeline FLAGCAL is a flagging and calibration program meant for use with GMRT data The input to FLAGCAL is araw GMRT FITS file and its output is a calibrated and flagged FITS file which can be directly used for imaging and further processing This can be accessed by saying flagcal at the UNIX prompt on any computer at the observatory FLAGCAL is flexible and highly configurable information and help on using 19 it is available in the NCRA library dspace technical report collection viz http nceralibl ncra tifr res in 8080 jspui handle 2301 581 6 3 Publication Guidelines A copy of the final paper preprint should be mailed to the Secretary Operations at secr opsATncraDOTtifrDOT res DOTin and library ATncraDOTtifrDOTresDOTin or sent to the following address Secretary Operations National Centre for radio Astrophysics NCRA TIFR Post Box 3 Ganeshkhind P O Pune University Campus Pune 411007 INDIA 6 3 1 Acknowledging the GMRT All publications resulting from GMRT data should have an acknowledgment of the form We thank the staff of the GMRT that made these observations possible GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research 6 3 2 Dissertations We request students whose dissertations include observations made with GMRT to provide copies of their theses to secr opsATncraDOTtifrDOT res DOTin or sent to the fol2. 5 7 Splitting the Target Source TASK SPLAT SOURCES PROG SRC DOCAL 1 GAINUSE 2 DOBAND 3 BPVER 1 FLAGVER 1 GO This would make a new UVDATA file PROG SRC SPLAT 1 containing the calibrated data for the program source that is free from bad data Alternative one could also use AIPS task SPLIT 5 8 Imaging Continuum Here I am assuming the imaging is being performed at 610 MHz and we would perform the wide field imaging TASK IMAGR SOURCES PROG SRC BCHAN 10 ECHAN 246 NCHAV 237 CELLSIZE 0 8 0 8 IMSIZE 1024 1024 RASHIFT 750 0 750 750 0 750 750 0 750 0 DECSHIF 750 750 750 0 0 0 750 750 750 0 DO3DIMAG 1 OVERLAP 2 NITER 1000000 DOTV 1 GO Here we are doing wide field imaging in the so called manual interactive mode We would stop the task once we have performed the complete deconvolution CLEAN process Also this would make nine BEAMs and nine CLEAN fields as the image catalog files These are shifted from the phase centre using RASHIFT 17 and DECSHIF defined above One can stitch these nine CLEAN fields using TASK FLATN GETN m NFIELD 9 NMAPS 1 IMSIZE 3000 3000 GO where m is the first image file comprising nine CLEAN fields Note that instead one could use a SETFC to make a BOXF ILE for input to IMAGR In order to present your imaging results use TASK KNTR TASK IMEAN TASK IMSTAT etc 5 9 Imaging Spectral
3. and the quick start guide to GMRT proposal submission is available at http www ncra tifr res in 8081 yogesh quickstartguide html After a proposal has been allotted time and scheduled further queries regarding travel logistics see Section 6 1 can be addressed to the Secretary Operations secr ops ATncraDOTtifrDOT res DOT in Any queries related to the observing schedule should be sent to gmrtscheduleATncraDOTtifrDOTresDOTin and regarding other aspects of the observations should be sent to gmrtoperationsATncraDOTtifrDOTresDOTin In the event of potential overlap of interests of different proposers GTAC will try and encourage collabora tion or sharing of data by bringing the concerned individuals into contact with each other Note that starting from 2012 GMRT has entered into an intensive phase of the ongoing upgrade see Section 6 and this requires additional day time slots for engineering activity The observatory tries its best to maintain the availability of 26 antennas for all the scientific observations Some of the existing observation modes and available flexibilities of settings for observations may be partially affected by the changes implemented as part of the upgrade activities In the regard the users are advised to read the GMRT status document available at http www ncra tifr res in ncra gmrt gtac which is updated at every proposal deadline 4 1 0 1 Proposing for GMRT Observations Checklist i R
4. used for quick check of the data quality operate on the LTA format First run the program 1istscan to create O1TSTO1_OBJ log file Edit the log file to select normalisation type a subset of the data provide your favourite name UVDATA FITS to the output file etc Finally run gvfits it provides UV data in J2000 epoch on the edited log file For example bash gt listscan PATH DATE 01TSTO1_OBJ lta bash gt vi O1TSTO1_OBJ log bash gt gvfits O1TSTO1_OBJ log Note that these programs e g listscan gvfits etc are observatory based programmes which are available on all GTAC computing machines at GMRT and NCRA 14 5 Analysing Data from the GMRT Straw Man Recipe Although one can use FLAGCAL to flag and calibrate GMRT data see Section 6 2 1 for more details and move straight to Section 5 8 and perform imaging here we present a quick recipe for the analysis of GMRT data from scratch using classic AIPS The users program source is named PROG SRC The interferometer phase is calibrated by observations of PH CAL The flux density scale and the bandpass is calibrated by observing AMPBP CAL The data file containing this observation is named as UVDATA FITS containing 256 spectral channels at 610 MHz 5 1 Setting up NRAO arps Classic AIPS is installed on all machines at the observatory Or alternatively install AIPS on your machine from http www aips nrao edu index shtml Next for bourne shell bash gt export
5. 1 1 Interferometric Observations Pre observation Checklist e contacted the observing assistant the Operator who would assist and run your observations identified selected a suitable bandpass calibrator flux density calibrator phase calibrator checked that all sources are visible during observing time prepared a source file in the right format prepared a command file using http gmrt ncra tifr res in gmrt_hpage Users Help sys setup html and that this has been checked by the GMRT operator e calculated how much data the observations will generate and obtained enough disk space to accom modate this Operator s too warn you of this 4 2 1 2 Observatory Supported Modes Below in the Table 3 we list the default settings for a contin uum observations at a given frequency and system parameters including usable frequency recommended available respectively 4 2 2 Pulsar The GSB beamformer pulsar receiver allows incoherent array IA and phased array PA mode observa tions using all the 30 antennas with a maximum bandwidth of 32 MHz i For the IA mode observations the actual number of antennas that can be added depends on the qual ity of the signals from the antennas including effects of receiver instabilities and RFI These effects vary with time the radio band being used and depends on the choice of antennas i e from central square antennas to arm antennas Due to the current limitations of the data acquisition system since obser
6. FITS data aips FITS bash gt source data aips LOGIN SH or alternatively for csh tsch shell csh gt setenv FITS data aips FITS csh gt source data aips LOGIN CSH and start AIPS via bash gt data aips START_AIPS Typically at the AIPS prompt gt TASK taskname gt INP to review the input parameters called ADVERBS of taskname gt GO runs the task taskname gt EXPLAIN taskname prints helpful information on the terminal or on the line printer gt UCAT lists cataloged UV files gt MCAT lists cataloged map files gt GETNAME n assigns the INNAME INCLASS INSEQ and INTYPE values corresponding to file with catalog no n gt ZAP delete a catalog entry and its extensions files gt RESTORE 0 initializes AIPS and restores defaults gt KLEENEX exit from AIPS wiping the slate kleen 5 2 Loading Data TASK FITLD DATAIN FITS UVDATA FITS GO Say this makes the catalog no 1 5 3 Indexing TASK INDXR GETN 1 CPARM 3 1 GO This creates NX and CL ver 1 tables 15 5 3 1 HEADER Information GETN n IMHE It lists header information from the catalog file no n selected It also lists tables that have two letter names which are HI Human readable history of things done to your data Use AIPS task PRTHI to read it and task PRTAB to read other tables AN Antenna location and polarisation tables NX Index into visibility file based source name and observation time SU S
7. node33 34 node33 34 node47 48 with XNET node47 48 with XNET Output data is in spectral voltage form needs one inverse FT to get voltage time series time resolution of 15 nsec for 32 MHz mode and 30 nsec for 16 MHz mode Table 2 Available modes of the GSB for continuum spectral pulsar and raw dump observations All details including standard operating procedure etc are available at GSB SOP 2 9 GMRT Calibrators Amplitude Bandpass and Phase Calibration The flux density calibrators 3C 48 0137 331 J2000 3C 147 0542 498 J2000 and 3C 286 or 1331 305 J2000 are used for both amplitude and bandpass calibration Together these three calibrators almost cover the entire 24 hr observing run The flux density scale used for for the observing bands at GMRT is based on the Baars et al 1977 Astron Astrophys 61 99 scale A suitable list of phase calibrators selected from the complete VLA calibrator list that are appropriate for GMRT is being collated at 610 and 240 MHz Once it is ready it would be made available Till then users can consult the VLA calibrator list for appropriate phase calibrator for their observation Since the mapped field of view Section 2 1 4 are larger at low frequencies users should map the calibrator field and use these solutions instead of assuming it to be a field with only one dominant point source at the phase centre 3 Radio Frequency Interference Radio frequency interference RFI signals are
8. the phase calibrator TASK GETJY SOURCES PH CAL CALSOUR AMPBP CAL SNVER 0 GO These values too are now written in the SU table 16 5 5 4 CLCAL Interpolate the gains TASK CLCAL SOURCES AMPBP CAL CALSOUR AMPBP CAL OPCODE CALI SNVER 0 INVER 0 GAINVER 1 GAINUSE 2 REFANT X GO Now the gains have been interpolated for the time instances of the program source The solution SN CL tables can be plotted using the AIPS task SNPLT 5 5 5 Bandpass Calibration TASK BPASS CALSOUR AMPBP CAL DOCAL 1 DOBAND 1 SOLINT 0 REFANT X BPASSPRM 5 1 ICHANSEL Y Y 1 0 GO The bandpass solutions BP table can be plotted using the AIPS task POSSM TASK POSSM APARM 8 2 NPLOTS 6 GO Again ICHANSEL Y is a RFI free channel which was used as a reference channel for calibration and antenna X is assumed to be good Now CL table ver 2 with the UVDATA file can be deleted using INP EXTDEST INEXT CL INVERS 2 EXTDEST 5 5 6 CLCAL Interpolate the gains TASK CLCAL SOURCES CALSOUR AMPBP CAL PH CAL OPCODE CALI SNVER 0 INVER 0 GAINVER 1 GAINUSE 2 REFANT X GO Now the gains have been interpolated for the time instances of the program source The solution SN CL tables can be plotted using the AIPS task SNPLT 5 6 Saving your Tables TASK TASAV OUTNA INNA OUTDI INDI INP GO
9. 12 Usage Mode Input IF BW MHz Acquisition BW or Final o p BW MHz Number of Channels Output Time Resolution 1 Interferometry Continuum Total Intensity 32 MHz mode 32 16 6 512 256 2 4 8 sec Total Intensity 16 MHz mode Full Stokes 32 MHz mode Full Stokes 16 MHz mode 16 6 16 512 256 2 4 8 sec 0 5 1 2 sec 0 25 0 5 1 sec an ee 2 4 8 sec 2 Interferometry Spectral Line Total Intensity 16 MHz and lower BW modes Total Intensity 32 MHz mode Total Intensity 16 MHz mode 32 16 6 16 6 16 N N 4 8 16 128 viz 4 2 1 0 5 0 25 0 125 MHz 16 N N 4 8 16 viz 4 2 1 MHz 2 4 8 sec Pre Post 1 2 4 Total Intensity 32 MHz mode 32 16 6 32 Pre Post 1 2 4 Total Intensity 16 MHz mode 16 6 16 N N 4 8 16 viz 4 2 1 MHz Pre Post 1 2 4 Full Stokes 16 MHz mode Voltage Beam 32 MHz mode Voltage Beam 16 MHz mode Raw voltages from all antennas 32 16 6 16 32 16 6 15 nsec O 30 nsec O 30 nsec at 4 bits per sample 4 Raw Dump 16 6 16 Notes Released on trial basis 245 76 uSec Regarding beam integration and beam data host set lta visibility pre integration to 4sec and beam output IA TI IA TI IA TI node33 34 node33 34 node33 34 node33 34 IA TI PA TI IA TI PA TI
10. Contents GMRT Observer s Manual Dharam Vir LAL dharamATncraDOTtifrDOTresDOTin November 18 2013 1 An Overview of the GMRT 2 Specifications of the GMRT D AMERA lt a dama A a a ab 2 1 1 Observing Frequency Bands o c es a roa emae aa eee ee ee 242 BandWidth os ooo kee bee bbe Ge ae ae a dad 2453 Resolotiom s es ote tE dnt pbaw eta hoe he heh ba eas mA FENG Vew s 2 6 bie ck be EG ee Ae we Eaters fe es LRS GAN o Pca ie ee a oe ee a eB A ee ALO SERBIO pr eee OP ee i eS ee E peh er bee a 22 System Parameters s soo osos de s ee oe a RY Se ee ee 23 Doa TAS toss ae eat Ql WE Aw Ae Be eh ee So Be es Bee Se ey SURES Ph ec dnc be Boul a Beals ie Ge Se A Brae bode gee ow Ao ete ara iat Bie 2 5 Pointing Accuracy and Pointing Correction 2 2 o e ee eee 26 POLSGTISat n o gae c he be ee dea od Ba bole we Eee Eee ee ERE ea eee a 2 GSB COMPRUTANOUS 2 aa eae eA a aa ee ee A A eA ee a LLL AMONG we ee a A Se Rain a ewe Cee Se ew es LL Beamon ay pear a a wie pie i ee e ie PRES ee A ee ee eS Do Snapshots s nba Beek Ma a ea a a Y E a bo be ae 2 9 GMRT Calibrators Amplitude Bandpass and Phase Calibration 3 Radio Frequency Interference 4 Observing with the GMRT 4 1 Regular Observing Proposals for TAC 2 o o e o 4 1 0 1 Proposing for GMRT Observations Checklist 4 1 1 TAC Approved Proposals 2 ce ba oie ee 4 1 2 Director s Discreti
11. RT antennas In order to provide a wideband feed for the GMRT to cover low frequencies a dipole feed with a conical reflector has been designed fabricated and successfully tested on the GMRT antenna This feed gives uniform sensitivity for the frequency range of 280 520 MHz 7 1 4 Fiber Optics Team members S Sureshkumar P Raybole S Lokhande M Gopinathan The optical fibre link to each antenna will be modified to provide additional wavelengths to bring back the broadband RF signals directly to the receiver room without disturbing the existing narrow bandwidth path that brings back the two IF signals As part of the GMRT upgrade Wavelength Division Multiplexing based RF Fiber Optic links have been designed fabricated and installed on six GMRT antennas The system uses four wavelength channels operating at the 1550 nm window Two channels are used to bring the full RF band 10 1500 MHz for the two orthogonally polarised signals directly from the Front End system without any frequency translation to 22 the central station for further signal processing A third channel is used to support the existing GMRT return link from the antenna thereby allowing for coexistence of the old and new systems 1 Gigabit Ethernet links from the central station to the GMRT antennas have been installed on 4 antennas of the GMRT for control and monitor applications As a part of the RFI mitigation and modernisation efforts at the GMRT a Fiber Optic LAN ha
12. a The RMS noise sensitivity equation is V2 k Tsys Na Ne A y np np Av T AS and the on source integration time 7 is computed via eet V2 k Toys a 1 AS X Ma Me A np nip Av where AS is the required RMS noise Jy Tsys is the system temperature K and _ A 2k G is the antenna gain K Jy where na see Section 2 1 1 and ne 1 being aperture and correlator efficiencies respectively A is the geometrical area of the antenna ny is the number of baselines nyp is the number of IF channels and Av Hz is the bandwidth of each sideband or channel width for spectral line observations NIF Npol NsB Where Npo is the number of polarisations 2 for the GMRT and ngp is the number of sidebands 2 2 System Parameters Below in the Table 1 we list the system parameters It also lists best RMS sensitivities and typical dynamic ranges achieve in a mapped field which are known to us Frequency MHz 151 235 325 610 1420 Primary beam HPBW arcmin 186 6 114 5 81 4 43 3 24 2 x 1400 f System temperature Tsystem K Receiver temperature Tr 295 106 53 60 45 Typical Ty off Galactic plane 308 99 40 10 4 Typical Terouna 12 32 13 32 24 Tsystem Tr Tsky Teround 61 5 237 106 102 73 Antenna gain K Jy Antenna 0 33 0 33 0 32 0 32 0 22 Synthesized beam FWHM Full array arcsec 20 13 9 5 2 Central square arcsec 420 270 200 100 40 Largest detectable st
13. among the main factors limiting the performance of radio telescopes RFI constricts the available frequency space effectively increases system noise and corrupts calibration solutions The effect is particularly strong at frequencies below 1 GHz Although the RFI envi ronment at GMRT is fairly decent especially at frequencies below 235 MHz band it can be quite alarming While the strongest source in a field is typically only a few Jy the RFI can occasionally reach several hun dred Jy in some channels in the 150 and 235 MHz bands However broadband RFI of up to several tens of Jy causes even more damage currently available tools including the AIPS task FLGIT provide some solution As mentioned above the RFI environment can be bad at 150 MHz and is sometimes a problem at 235 MHz the situation is usually better at night time and on weekends For these two bands it is rec ommended to use the solar attenuators in the common box to minimize the possibilities of saturating the downstream electronic chain due to RFI Observations at 325 MHz can be hampered sometimes due to RFI from nearby aircraft activity especially during day time Due to increasing interference from mobile phone signals around 950 MHz the all pass mode at 1400 MHz band is no longer supported as an official mode and consequently observations below 1000 MHz in this band are not supported Even for the lowest of the 4 sub bands of the 1400 MHz band covering 1060 60 MHz the user
14. area perform this with an appropriate choice of ADVERBS of the image Apart from presenting your imaging results discussed above Section 5 8 in order to present your spectral line results use TASK MOMNT TASK IMEAN TASK IMSTAT etc 18 6 Miscellaneous 6 1 Making the Observations Logistics 6 1 1 Accommodation Travel amp Help for Visitors to the GMRT NCRA The telescope is located near Khodad village which is about 80 km north of Pune The telescope site houses laboratories complementing facilities like guest house canteen library etc The observatory can be reached using the daily shuttle service starting from NCRA Pune at 0700 Hrs in the morning or by direct taxi from Mumbai or Pune The closest town Narayangaon is about 14 km from the observatory and is connected to Pune and Mumbai by public bus transport system If information is given in advance transportation arrangements can be made for the observers from the Narayangaon bus stand to the observatory See http www ncra tifr res in ncra gmrt gtac for more details on various general aspects about observing at the GMRT including a road map for travel to the observatory Several general and administrative guidelines for visitors to help them organise are available at administrative info general info including handy information available at handy info and road map information available at raod map 6 1 1 1 Absentee Observing All users are expected to be present
15. e particularly those associated with the GMRT We express our thanks to everyone of them ver 3 This version ver 2 2 M S Jangam N G Kantharia amp S S Sherkar ver 2 1 M S Jangam amp D J Saikia ver 2 D V Lal A P Rao M S Jangam amp N G Kantharia ver 1 Judith Irwin Compared to previous versions this draft includes updated specification of the GMRT straw man s GMRT data analysis recipe and lists ongoing GMRT upgrades 6 6 1 Acknowledgments Format and style are also based on manuals from other observatories e g ATCA Users Guide VLA Observational Status Summary etc for uniformity In case of questions on the material or suggestions that would enhance the clarity of this guide please contact D V Lal dharamATncraDOTtifrDOTresDOTin 7 Ongoing Developments 7 1 GMRT Upgrades A major upgrade of the GMRT is currently underway The main goals of this upgrade are to provide i as far as possible seamless frequency coverage from 50 MHz to 1500 MHz ii improved sensitivity with better quality receivers iii a maximum instantaneous usable bandwidth of 400 MHz iv a revamped and modern servo system v anew generation monitor and control system vi improvements in the antenna mechanical structure and vii matching improvements in infrastructure and computational facilities The upgrade will result in significant changes to almost all aspects of the GMRT receiver chain and other systems Ho
16. e o o o oa e ee eee eee ae Dae eel aes 5 8 Imaging Comtmpum lt o occo 2 Poe ead ea eee Ee Eee ee ERE ee ees 59 Imagine Specttal ine o insis aa eee ee wee a Ee ee eA ee S91 Con nuum Image 242 454 86 oa dea eee awed Gade ow oe oo 592 Subiractmp the Contant 4 oi sce eee eae ya e wR Ee a A ee 3939 Making Special Cube 2 ind gr ew bee oe i ee hae ee ee 594 Extrac ne the SPECIE lt a sc ew a a ea ee de ww Miscellaneous 6 1 Making the Observations Logistics aoaaa ee 6 1 1 Accommodation Travel amp Help for Visitors to the GMRT NCRA 6LLI Absentee Observing ce 00 2 we ee eae Da TOO a Gan aoe eh a Rak Be pel pes i selena ee iaa Wolk a ew bowed 6 2 1 A Flagging and Calibration Pipeline o o o G4 Publication Guidelmes s eosi ae haw a ra a A A a a a h 63 1 Acknowledging the GMRT o e ee 2a db bev dw bed hee o eae he ded 6 3 2 DISsertalions o se ue eo eR Ew ERR RO ee a ee 64 Key Personnel 2 226 ec be hee eee ee kaa ee bd aes 6 3 Documentation cdas od Ee ed ee A ee we Eee aa 66 Dicdincation History lt lt ei aie me ed A a ee ek A eR A GOL Acknowledgments s lt c e toet 228 Meee eee See ew he bee ar eee a Ongoing Developments TA AGMRT Uperades si vbw Qo ea Sedat eh een She BE ae Re a A PLA CNO Sy ICMR a ie es hs em ee A ee Dlg EE Brontend Systems ac a Oe MA Ee la Go bha Low NoBeAMpIBRGP o icr ed A ek a eS ee TLS FADCPODUCS sopo pret a ee ee ei eae Ee ee ee hee a TiS Back end Syst
17. egister and login at the heipes Maps Nera Eirr resin ii choose Create Proposal and follow necessary steps 111 Remember to attach a self contained Scientific Justification not exceeding 3000 words Preprints or reprints will be ignored unless reporting previous GMRT observations A brief status report not exceeding 150 words on each previous proposal should be appended to the scientific justification Note when your proposal is scheduled the contents of the cover sheets become public information Any supporting pages are for refereeing purpose only 4 1 1 TAC Approved Proposals 4 1 2 Director s Discretionary Time In addition to regular GTAC proposals users can also avail i Target of Opportunity Target of Opportunity TOO time is primarily intended for short lived unexpected unpredicted phenomena or time dependent astronomical phenomena ii Exploratory Proposals Here the time is primarily meant for pilot observations or feasibility studies which might lead to future GTAC proposals and confirmatory observations Both types of time allocation are subject to availability of time and will be evaluated on the basis of scien tific merit by the relevant referees and are cleared by the Centre Director NCRA Requests for allocation of TOO and DDT should also be submitted using the NAPS interface code DDT Such observations will be scheduled in the empty white slots http www ncra tifr res in 8081 gmrtoperatio
18. ems iaa hw o Ba eR Be Se ee ee a 15 15 15 15 16 16 16 16 16 16 17 17 17 17 17 17 18 18 18 18 18 19 19 19 19 19 19 20 20 20 20 21 21 21 1 An Overview of the GMRT The Giant Metrewave Radio Telescope GMRT is located near Khodad village which is about 80 km north of Pune The GMRT consists of an array of 30 antennae each of 45 m diameter spread over a region of 25 km diameter Amongst the multi element earth rotation aperture synthesis telescopes operating at meter wavelengths the GMRT has the largest collecting area The interferometer has a hybrid configuration with 14 of its 30 antennas located in a central compact array with size 1 1 km and the remaining antennas dis tributed in a roughly Y shaped configuration giving a maximum baseline length of 25 km The baselines obtained from antennas in the central square are similar in length to the VLA D array configuration while the baselines between the arm antennas are comparable in length to the baseline lengths between VLA A array and VLA B array configurations Hence a single observation with the GMRT samples the u v plane adequately on both short and long baselines with reasonable sensitivities The GMRT can also be config ured in array mode where it acts as a single dish by adding the signals from individual dishes This mode of operation is used for studying compact objects like pulsars which are effectively point sources even for the largest in
19. for observations However in case of short observations of routine nature users can request remote observing by writing to the GMRT Operations at gmrtoperationsATncraDOTtifrDOTresDOT in with a copy to Y Gupta ygupta ATncraDOTtifrDOTresDOTin 6 2 Tools Various tools have been developed to visualise the data in quasi real time Some of the most used tools are mon displays real time snapshots of cross correlations self correlations antenna bandshape phase and amplitude etc tax gives offline plots of cross correlations self correlations antenna bandshape phase and amplitude etc gdp gives information on non working antennas bad baselines phase jumps delay jumps etc These tools are available for all users and can be run with some help from on duty telescope operators Additionally several useful online scripts available are GMRT online archive access A portal to find data from the GMRT archive https naps ncra tifr res in goa mt search basicSearch GMRT observing command file creator Create your observing command files http gmrt ncra tifr res in gmrt_hpage Users Help sys setup html GMRT frequency calculation Calculate your TPA values http gmrt ncra tifr res in gmrt_hpage Users Help sys freq html GMRT source rise transit and set times Rise transit and set times for the source s at GMRT http gmrt ncra tifr res in gmrt_hpage Users Help sys time php which can be used for optimising the observing strategy
20. is advised to check for effects of RFI Please refer to Sections 2 1 2 and 4 2 1 2 for usable bandwidth and default settings respectively Users are also informed that since cycle 21 night time scintillation has been occasionally seen even at the 1400 MHz band and the probability of scintillation is higher close to the equinoxes 4 Observing with the GMRT 4 1 Regular Observing Proposals for TAC The GMRT Time Allocation Committee GTAC invites proposals for two Cycles April to September and October to March The deadline for receiving these proposals is 17 00 HRS IST UTC 5 5 hrs January 15 and July 15 All proposals are to be submitted online via the NCRA Archive and Proposal Management System NAPS available at bttp naps nera tifr res i which provides a password authenticated web browser based interface for proposal submission Note that proposal may be submitted only by the PI All co I s also need to be registered users of the system All pro posals are processed by GTAC with external refereeing as needed with inputs from the GMRT Observatory on technical issues and the proposers are sent intimations of the time allocation The call for proposals the 10 status of the GMRT and a set of guidelines for GMRT users can be found at http www nora Litr res in norafgmrt gtao Additionally frequently asked questions on GMRT proposal submission are available at http www ncra tifr res in 8081 yogesh proposalfaq html
21. is now ready for mass production Synthesiser circuits for Frequency con version before digitisation is also finalised These circuits will allow different observing bands in different antennas The phase noise of the circuit was tested and found to be 90 dBc Hz 1 KHz offset which is 30 dB better than the existing GMRT receiver Digital Back end System The digital back end digitizes the data and implements the correlator and beamformer for the interferometry and array outputs respectively A four antenna FPGA based correlator was completed and the first light signals were obtained with this It is now in final stages of test and release The design has a modular approach so it can be easily expanded to a 30 antenna design by adding additional processing boards The system uses 8 Virtex 5 FPGA boards 4 ADC boards one 10 GbE switch and a control PC The design incorporates coarse fine delay and fringe corrections and has been tested using GMRT antennas We have also developed a hybrid design using a CPU GPU based approach In this design a FPGA board is used to send the digitized data to a PC in 10 GB packets GPU boards hosted by the PC are used to process these data and provide correlated outputs A two antenna 200 MHz bandwidth 8 bit correlator based on this concept has been implemented and tested 23
22. line Here I am again assuming the spectral imaging is being performed at 610 MHz and the wide field imaging is necessary Also I am assuming that first 20 channels are free from spectral line information and rest of the channels from 21 to 256 have spectral line information 5 9 1 Continuum Image Making a continuum image of the line free channels TASK IMAGR SOURCES PROG SRC BCHAN 8 ECHAN 17 NCHAV 10 CELLSIZE 0 8 0 8 IMSIZE 1024 1024 RASHIFT 750 0 750 750 0 750 750 0 750 0 DECSHIF 750 750 750 0 0 O 750 750 750 0 DO3DIMAG 1 OVERLAP 2 UVTAPER 60 60 UVRANGE 0 90 NITER 1000000 DOTV 1 GO 5 9 2 Subtracting the Continuum Now subtracting the continuum emission from all channels TASK UVSUB GETN n GET2N m NMAPS 9 GO where n and m are catalog no for PROG SRC SPLAT 1 and PROG SRC ICLOO1 1 respectively to generate PROG SRC UVSUB 1 Alternatively AIPS tasks UVLIN IMLIN could also be used to subtract continuum Note that m is the first image file comprising nine CLEAN fields 5 9 3 Making a Spectral Cube Imaging the continuum subtracted data set TASK IMAGR BCHAN 21 ECHAN 246 CELLSIZE 0 8 0 8 IMSIZE 1024 1024 UVTAPER 60 60 UVRANGE 0 90 NITER 0 DOTV 1 GO This would provide the image cube 5 9 4 Extracting the Spectrum Finally extracting the spectrum TASK ISPEC GO gives the spectrum averaged over a chosen
23. lowing address Secretary Operations National Centre for radio Astrophysics NCRA TIFR Post Box 3 Ganeshkhind P O Pune University Campus Pune 411007 INDIA for inclusion and maintenance at the NCRA GMRT library 6 4 Key Personnel Note queries should be made directly to the Secretary Operations at secr opsATncraDOTtifrDOTresDOT in However you may e mail any of the following C H Ishwara Chandra ishwar ATncraDOTtifrDOTresDOTin N G Kantharia ngk AT ncra DOT tifrDOTresDOTin Y Gupta yguptaATncraDOTtifrDOTresDOTin with copies to Secretary Operations secr opsATncraDOTtifrDOTresDOTin and GMRT Operations gmrtoperationsATncraDOTtifrDOTresDOTin 20 6 5 Documentation GMRT primary beam shapes http www ncra tifr res in 8081 ngk primarybeam beam html and http www ncra tifr res in 8081 ngk primarybeam report_lband_beam ps Technical report on finding the beamshape parameters http www ncra tifr res in 8081 ngk primarybeam report_lband_beam ps A real time software backend for the GMRT Roy et al 2009 http arxiv org abs 0810 1517 FLAGCAL A flagging and calibration pipeline for GMRT data http ncralibl ncra tifr res in 8081 jspui handle 2301 581 NCRA annual report 2010 2011 http www ncra tifr res in library annual report 10 11 paf 6 6 Modification History This user s manual is based on the previous versions i e the document involves inputs from several peopl
24. made public three months after the date of observation 4 2 3 1 GMRT Data Archive Since GMRT observing cycle 1 year 2002 observations the entire archive is available via the web The following url allows to view the data https naps ncra tifr res in goa mt search basicSearch Presently this archive provide FITS u v format files and native LTA format files Data base searches may be made based on a large number of user specified criteria and one can download data via ftp protocols Similar to all other observatories GMRT data too are restricted to proprietary use by the proposing team for a period of 18 months from the date of the last observation in a proposal 4 3 GMRT User Tool A GMRT User Tool GUT has been released for users It is a graphical interface to several existing GMRT software that are regularly used before observations e g command file maker rise and set time calculator during observations e g ondisp for monitoring the antenna status etc and after observations e g tax gvfits and includes a few other useful utilities This can be accessed by saying gut at the UNIX prompt on any computer at the observatory 4 4 Converting the Native LTA Format to FITS Format Assuming that telescope operators gave you your data in the the native LTA format 01TSTO1_OBJ lta file this file is in the raw telescope format where Ita stands for long term accumulation programs available at the observatory e g XTRACT TAX etc
25. news about the GSB can be found at http gmrt ncra tifr res in gmrt_hpage sub_system gmrt_gsb Table 2 presents the available modes of the back end Briefly from cycle 20 onwards the GSB is the back end at the observatory the older hardware back ends have been decommissioned and the details of available modes are as follows The new GSB back end supports a maximum bandwidth of 32 MHz However users should consult the observatory regarding usable RF bandwidth at the lower frequencies because of RFI For 150 MHz and 235 MHz the recommended IF bandwidth is 6 MHz however 16 MHz is also usable albeit with some caution Currently the interferometre polarisation observations are supported on an experimental basis and the analysis requires a lot of hard work Walsh and Noise Cal modulation for real time Tsys measurements are currently not supported and hence absolute flux calibration in regions where the system temperature varies like the galactic plane is not automatic Normal integration times used are 8 or 16 s but more rapid sampling down to 2 s can be done subject to the availability of enough disk space for recording the larger data volume Users requiring such modes should consult the Operations group at the observatory 2 7 1 Correlator It presently supports the following modes of interferometric observations i Full bandwidth non polar and full polar interferometric observations in the 16 MHz and 32 MHz modes wi
26. ns gtac request html 11 without affecting the scheduled GTAC observations as far as possible the guidelines followed at the obser vatory for allocating these white slots are available at http www ncra tifr res in ncra gmrt gtac 4 2 Planning the Observing Run If you are ready to observe visit the following site for preparing your observations http www ncra tifr res in gmrt_hpage Users Help help html Also check that the following tasks see Section 4 2 1 1 and 4 2 2 1 have been accomplished 4 2 1 Continuum Spectral line From cycle 20 onwards the GSB is the only back end available at the GMRT The older hardware back ends have been decommissioned All 30 antennas are in use and feeds for all frequencies are available At a given time the observatory will try to make at least 26 antennas available for any observation Programs that critically depend on the short spacing antennas C05 C06 and C09 with 100 m separation must indicate this in the GTAC proposal Rotation of the feed system to change the frequency of observation is now possible fairly routinely and requires about 2 hours time for rotation and for set up at the new frequency of operation including antenna pointing Section 2 1 6 gives the measured system parameters of the GMRT antennas and some useful numbers for estimating the required observation time Additional information can be found at http www ncra tifr res in ncra gmrt gtac 4 2
27. ntenna primary beam 8 oa C28 a 103 107 1010 1013 where a b c and d are PBPARM 3 PBPARM 4 PBPARM 5 and PBPARM 6 respectively These can be directly plugged into AIPS task PBCOR are 150 MHz PBPARM 1 0 1 PBPARM 2 1 PBPARM 3 4 04 PBPARM 4 76 2 PBPARM 5 68 8 PBPARM 6 22 03 PBPARM 7 0 235 MHz PBPARM 1 0 1 PBPARM 2 1 PBPARM 3 3 366 PBPARM 4 46 159 PBPARM 5 29 963 PBPARM 6 7 529 PBPARM 7 0 325 MHz PBPARM 1 0 1 PBPARM 2 1 PBPARM 3 3 397 PBPARM 4 47 192 PBPARM 5 30 931 PBPARM 6 7 803 PBPARM 7 0 610 MHz PBPARM 1 0 1 PBPARM 2 1 PBPARM 3 3 486 PBPARM 4 47 749 PBPARMI 5 35 203 PBPARM 6 10 399 PBPARM 7 0 1280 MHz PBPARM 1 0 1 PBPARM 2 1 PBPARM 3 2 27961 PBPARM 4 21 4611 PBPARM 5 9 7929 PBPARM 6 1 80153 PBPARM 7 0 1 a More details such as the fitted polynomial methodology etc are available at http www ncra tifr res in 8081 ngk primarybeam beam html 2 1 6 Sensitivity The system temperature is 150 MHz 615 K 235 MHz 237 Ki 325 MHz 106 K 610 MHz 102 K 1280 MHz 73K The system temperature measurements are with Solar attenuator switched on and the antenna gain is 150 MHz 0 33 K Jy Antenna 235 MHz 0 33 K Jy Antenna 325 MHz 0 32 K Jy Antenna 610 MHz 0 32 K Jy Antenna 1280 MHz 0 22 K Jy Antenn
28. onary Time o gt o o cs caca Se eRe ee eee 4 2 Planning the Observing Run 2 ea 0 2 ee ee 421 Continuum Spectral line 2 242024 4c ice ded swage eae wee ees 4 2 1 1 Interferometric Observations Pre observation Checklist 4 2 1 2 Observatory Supported Modes o o e BLA PUE o ps ra a Sa ee ed 4 2 2 1 Pulsar Observations Pre observation Checklist 42 9 DAR ACCESS scada e ee Ae 4231 GMRT Data Archive o ss cr a a a da GMT User Tool s s sa eek a sd da e he ae wh Gk we oe eS 4 4 Converting the Native LTA Format to FITS Format 0 Analysing Data from the GMRT Straw Man Recipe 21 Setunesip NRAO AIS cias pa Pa ea A wae ee ee er D2 Loading DAA eo sa gg doe we dd ee es we Oana ee de Be a ed Sia MOB o ee te a ed a do Ca ed he de we Ph a ee ee ew i 53 1 HEADER Information lt 0 eH Re ee 34 Data Editing FLAGging Viewing de GRRE EERE DEES ORDERS 33 Gain Calibration lt sos c esre ee ss wR REE EE OH ERE Bol o nada be DOS Ghee eh eee ORDERS RHE Se CARER on a id dace dod dd od de ek aha at NO Sook SER ae woke dude Ae OR AO ade eed Se ee ak a eed Ae BORA de a ee SOMA oon oad we Gee ph ee Go Oe we E a E E ee Se we he 339 Bandpass Calibration 2 ik 6 ee a eR EE ew eee a DO CAD cs bon hte we A a Bad ok ede ew Bs we ee Bee eo 30 Saving Your TADES oec e SG ghee A A bed a A et gee eae Se 5 7 Splitting the Target Sourc
29. ource table contains the list of sources observed and indexes into the frequency table The flux den sities Of the calibration sources are entered into this table FO Frequencies of observation and bandwidth with index into visibility data CL Calibration ver 1 table describing the antenna based gains The goal of calibration is to create a good CL ver 2 5 4 Data Editing FLAGging Viewing TASK QUACK OPCODE BEG APARM 0 5 GO This FLAGs the bad points at the beginning of each scan and creates a Flag Table FG Again TASK UVFLG ANTEN 0 TIMER OUTFGVER 1 INP GO FLAGs bad UV data TASK LISTR OPTYP SCAN DOCRT 1 SOURC INP GO This LISTR task lists your UV data in a variety of ways and the above inputs show the summary of the sources in the input data file 5 5 Gain Calibration 5 5 1 SETIY Enter the absolute flux density of the primary calibrator in the SU table TASK SETJY SOURCES AMPBP CAL OPTYPE CALC GO 5 5 2 CALIB Solve for the antenna based amplitudes and phases for AMPBP CAL and PH CAL TASK CALIB CALSOUR AMPBP CAL PH CAL BCHAN Y ECHAN Y REFANT X SOLINT 2 GO Here channel Y is a RFI free channel from the dataset and antenna X is assumed to be good This will generate the solution SN table with ver 1 The calibrated data can now be plotted using the AIPS task UVPLT 5 5 3 GETJY Bootstrapping the flux density to
30. rding of the digitized voltage signals from each antenna 4 bit samples for 16 MHz mode followed by limited capability for offline playback and correlation beamformer the frequency of usage of this mode of operation will be restricted by the total volume of disk space available for recording as well as the time taken for the offline analysis The observatory offers very limited capabilities for long term back up of the raw voltage dump data and the user will be responsi ble for clearing the large volumes of data from the disks within a stipulated time typically a week or less Users desirous of using this mode should check with the observatory well in advance 2 8 Snapshots The two dimensional geometry along with a hybrid configuration with 14 of its 30 antennas located in a central compact array with size 1 1 km and the remaining antennas distributed in a roughly Y shaped configuration giving a maximum baseline length of 25 km of the GMRT allows a snapshot mode whereby short snapshot observations can be made to map detailed source structure with good angular resolution and with reasonably good sensitivity Before considering snapshot observations using GMRT users should first determine if the goals desired can be achieved with the existing TIFR GMRT Sky Survey TGSS http tgss ncra tifr res in which is an all sky survey at 150 MHz Observation Type Available modes of the GSB Release Ver 2 3 18 January 20
31. rface is formed by wire mesh and the efficiency of the antennas varies from 60 to 40 from the lowest to the highest frequency Both the orthogonal polarisations are brought to the control room from each antenna The polarisations are circular for all feeds except the 1420 MHz feeds which are linear The 610 MHz and 235 MHz feeds are coaxial allowing simultaneous dual frequency observations to be carried out at these two frequency bands albeit for only one polarisation per band 2 1 2 Bandwidth The new GMRT Software Backend GSB supports a maximum bandwidth of 32 MHz Users should consult the observatory regarding usable RF bandwidth at the lower frequencies because of RFI For 150 MHz and 235 MHz the recommended IF bandwidth is 6 MHz however 16 MHz is also usable albeit with some caution 2 1 3 Resolution The approximate synthesized beam size full width at half maximum for a full synthesis observation at each of the observing bands is given below 150 MHz 20 235 MHz 13 325 MHz 9 610 MHz 5 1280 MHz 2 2 1 4 Field of View The field of view diffraction limited response of the individual antennas is 25 GH Pre pes V gt where 0 the half power beam width is in arcmin and v is the frequency These are 150 MHz 186 6 235 MHz 114 5 325 MHz 81 4 610 MHz 43 3 1280 MHz 26 2 2 2 1 5 Primary Beam The coefficients of an eighth order polynomial fit to the a
32. rs Pulsar PULSAR_MANUAL pdf a 1 a 11 4 2 2 1 Pulsar Observations Pre observation Checklist e contacted the observing assistant the Operator who would assist and run your observations e calibrator for gain equalisation 9300 count close to target ph cal will work but gt 5 Jy e perform phasing on the phase calibrator e goto target pulsar e configure A PA GAC 13 e start IA and or PA ACQ acquisition Process chain e start data recording to the disk 4 2 3 Data Access The data from the GMRT can be converted to FITS files using locally developed software and analysed in standard packages like AIPS see Section 4 4 These can be backed up on DVDs or external hard disks written by Linux machines Users are requested to bring their own media for backing up the data Facilities for analysing GMRT data are available both at the Observatory and at the NCRA Pune Users requiring extensive computing facilities or large amounts of disk space for data storage or analysis should make their requirements known well in advance The online archive contains all GMRT data since 2002 Cycle 01 and will also serve the user commu nity with GMRT data The entire archive is now on disk and is available via the Archive Access Tool at https naps ncra tifr res in goa mt search basicSearch Data from all standard observations with the GMRT will be made public 18 months after the date of obser vation and the TOO observations will be
33. ructure arcmin 68 44 32 17 7 Usable frequency range observatory default MHz 150 156 236 244 305 345 580 640 1000 1450 allowed by electronics MHz 130 190 230 250 305 360 570 650 1000 1450 Fudge factor actual to estimated time for short observations 10 5 2 2 for long observations 5 2 2 1 1 Best RMS sensitivities achieved mJy 0 7 0 25 0 04 0 02 0 03 Typical dynamic range achieved gt 1500 gt 1500 gt 1500 gt 2000 gt 2000 t With default solar attenuator 14 dB For spectral observations fudge factor is close to 1 t So far known to us Table 1 Measured system parameters of the GMRT 2 3 Data Rates For the correlator configurations discussed in Section 2 7 integration time At and recording both self and cross correlation coefficients the data rate is Nant X Nant 1 2 sec MB sec o a 1 86 x Nant X Nant 1 x C Zj 30 x 31 At which corresponds to 0 1 TB day when using the full 30 element array with 2 sec integrations 2 RR and LL total intensity polarisations and 512 spectral channels 3 35 x GB hr il 2 4 Subarrays GMRT supports multiple subarrays for users where the array is divided into smaller groups of antennas It can be configured in array mode where it acts as a single dish by adding the signals from individual dishes Note that up to a maximum of five subarrays are possible at any given time and it is not possible to have different correla
34. s been implemented at the GMRT Observatory building This network has 144 user locations connecting all the laboratories and offices in the building 7 1 5 Back end Systems Team members B Ajit Kumar S Gupta N D Shinde D K Nanaware A Ganla S V Phakatkar P J Hande A Vishwakarma S C Chaudhari M V Muley H Reddy I Halagalli K Buch G J Shelton LS Bhonde S S Kudale In the receiver room a separate and parallel signal path will convert the broadband RF signals to base band signals with a maximum bandwith of 400 MHz which will be processed with a new digital back end system correlator plus beamformer plus pulsar receiver that can handle the full 400 MHz bandwidth This entire chain will run in parallel with the existing 32 MHz bandwidth receiver chain without affecting its performance in any way The team is using the CASPER Centre for Astronomy Signal Processing and Research based design approach for implementation of the digital back ends We also plan to add facilities for RFI cancellation in both analog amp digital domain and raw data recording and software based back ends Analog Back end System A versatile analog back end has been developed to process the RF signal received from antennas using high dynamic range circuits The circuit has been analysed and optimised for appropriate gain flatness and one set of final prototype has been wired and tested in the lab The test results are as expected and the circuit
35. terferometric baselines The maximum instantaneous operating bandwidth at any frequency band is 33 MHz Each antenna provides signals in two orthogonal polarisations which are processed through a heterodyne receiver chain and brought to the central receiver building where they are converted to baseband signals and fed to the digital back end consisting of correlator and pulsar receiver l screen use the Print INK next to tne map Go gle zz TT 5 2 KHOD AD Figure 1 Location of the GMRT array as seen on the Google maps latitude and longitude are provided in Section below 2 Specifications of the GMRT 2 1 Antennas The GMRT antennas are alt azimuth mounted parabolic prime focus dishes While the dishes can go down to an elevation of 16 at present the elevation limit has been set at 17 giving a declination coverage from 53 to 90 The slew speed of the antennas is 20 min on elevation axis and 30 min on azimuth axis and they are not operated when winds are higher than 40 km h The reference antenna C02 of the array is located at latitude 19 1 N longitude 74 05 E altitude 588 m 2 1 1 Observing Frequency Bands There is a rotating turret at the focus on which the different feeds are mounted The feeds presently available are the 150 325 610 235 and the 1000 1450 MHz feeds The 1000 1450 MHz feed L band feed is subdivided into 1060 1170 1280 and 1390 MHz sub bands The reflecting su
36. th a choice of 256 or 512 spectral channels across the full band and with integration time of 2 sec or larger Spectral zoom modes for spectral line observations where the input band is filtered and decimated by factors 4 8 16 32 64 128 while keeping the number of spectral channels across the reduced bandwidth fixed at 256 or 512 this mode will only work within the 16 MHz mode Variable spectral resolution with a choice of 64 128 256 512 or 1024 note 1024 channel mode has not been tested and debugged thoroughly spectral channels across the band of observation Reduced spectral channels will allow for correspondingly faster dump times for the visibility data a 1 a 11 2 7 2 Beamformer The beamformer is especially used for pulsar observations and it presently supports the following modes of array beamformer i Incoherent array IA and phased array PA beam modes with total intensity output here at present the fastest sampling time is 60 usec ii PA beam mode with full polar output at correspondingly reduced time resolutions see Table 2 iii Full time resolution voltage beam data with facility for offline coherent dedispersion of the same iv Multi subarray beam modes IA or PA with full bandwidth for up to two subarrays with indepen dent GMRT Array Combiner GAC antenna selection control for each subarray Additional a special feature of the GSB being raw voltage dump data 1 e v raw voltage reco
37. tor settings for these subarrays This mode of operation is used for studying compact objects like pulsars which are effectively point sources even for the largest interferometric baselines 2 5 Pointing Accuracy and Pointing Correction The typical pointing accuracy of a GMRT antenna is 1 arcsec and the typical tracking accuracy is 10 ar cmin A pointing model has been in use since observing cycle 15 which can be applied online during the observations it updates the antenna pointing offsets every half an hour or so during an observing run using commands included in the user s observe file Elevation dependent offsets are seen in a few antennas which can again be corrected using a pointing model If a user does not want to apply the dynamic pointing model the control room should be informed before the start of the observing run 2 6 Polarisation Polarisation observations are now possible using GMRT and several users have carried out full polar obser vations at 610 MHz Interested users could look at the following article http arxiv org abs 1309 4646 Despite the on axis instrumental polarisation at GMRT being of large amplitude and highly frequency dependent this article demonstrates the viability of the GMRT for full polarisation wide field spectropo larimetry 2 7 GSB Configurations The current GMRT Software Back end GSB offers relatively more options as compared to its earlier hardware counterpart Current status and latest
38. v For the polar mode the fastest sampling times are slower by a factor of 4 than these numbers i e 122 usec and 245 usec for the 256 channels and the 512 channels respectively for the corresponding GSB modes Here both IA and PA modes are available with a time tagging facility that is accurate to 240 x 107 sec with respect to the GPS clock and the timing observations can be carried out The format of the beamformer data from the GSB is identical to that provided by the older hardware pulsar back end The final multi channel data from the IA beamformer output can be recorded either on disk or on tapes SDLT or LTO The PA data from the GSB can also be recorded on disk or tapes SDLT or LTO User can choose either the total intensity signal or the four polarisations signal to record Additionally the PA mode of the GSB also includes capability for recording the full time resolution voltage beam data with a facility for off line coherent dedispersion Finally the GSB also allows a mode where the single IA and PA beams can be replaced by two IA beams or two PA beams with independent antenna selections for each This facilitates simultaneous dual frequency observations of pulsars using the subarray mode see Section 2 4 of the GMRT Note that such observations involve special scheduling considerations due to feed rotations and related overheads A Pulsar observing manual for more details is available at http gmrt ncra tifr res in gmrt_hpage Use
39. ving cycle 20 the fastest sampling achievable in the IA mode is 30 usec and 61 usec for the 256 12 RF LO1 IF BW tpa MHz MHz MHz 150 218 16 156 156 218 218 62 62 235 304 16 242 242 304 304 62 62 325 255 32 306 306 255 255 51 51 610 540 32 591 591 540 540 51 51 1060 990 32 1041 1041 990 990 51 51 1170 1100 32 1151 1151 1100 1100 51 51 1280 1210 32 1261 1261 1210 1210 51 51 1390 1320 32 1371 1371 1320 1320 51 51 DUAL 680 304 32 16 629 253 680 304 51 51 Table 3 Supported defaults for a typical continuum observation channels and 512 channels for both 16 or 32 MHz modes of the GSB For the PA mode of observations there is an algorithm that can phase any selected number of avail able working antennas using interferometric observations of a point source calibrator The phasing for central 14 antennas works well for time scales ranging from one hour to a few hours de pending on the operational frequency and the ionospheric conditions For the arm antennas or for the longer baselines de phasing can be more rapid Polarimetric pulsar observations are possible with the GSB but instrumental polarisation effects for the GMRT are not fully characterised and observers will therefore need to carry out their own calibra tion iv For total intensity the fastest sampling rate currently supported is 30 usec and 61 psec for the 256 channels and the 512 channels respectively for both 16 and 32 MHz modes of the GSB
40. wever full care is being taken in the design of the new systems to ensure that the performance of the existing GMRT is not affected as the upgrade is implemented Below we present salient features of the upgrades of individual systems Many of these upgrade ac tivities are now past the prototype development and testing stages and are entering mass production and 21 commissioning stages These latter phases are expected to go on for the next 1 2 year or so 7 1 1 Servo System Team members S Sabhapathy S K Bagade A Kumar S M Burle A Bhumkar S J Bachal B Thiya garajan D D Ghorpade T S Haokip A Poonattu D D Temgire As part of the upgrade of GMRT servo system extensive tests for validation of new Brush less DC motors and drives were carried out on C04 antenna The new system was integrated with the existing station servo control and was commissioned in a Radio Frequency Interference free enclosure in March 2011 The system performance was compared with the brushed motor systems in other antennas and was found to be similar The system is performing satisfactorily since its commissioning The mass production of this system for three more antennas has begun and procurement for six more antennas is in progress A new interface was designed and validated for PC 104 based digital position loop and tests on C02 antennas are continuing The interlock system on all antenna except C12 has now been upgraded to solid state interlock s
41. ystem Likewise feed position system is also in the process to be upgraded on more antennas 7 1 2 RF Front end Systems Team Members S Kumar A Raut V B Bhalerao S Ramesh M Parathe Ankur In feeds and front end electronics the existing 325 MHz feed will be replaced with a broadband feed operating from 250 to 500 MHz along with a broadband low noise amplifier with improved noise tempera ture A new feed operating from 550 to 900 MHz will replace the existing 610 235 MHz co axial feed also with a matching LNA with an improved noise figure The 150 MHz feed will also get replaced with a wider bandwidth feed As far as possible it is being ensured that the new feeds will cover the frequency range provided by the existing narrow band feeds that they are replacing with similar or better level of sensitivity The final upgraded L Band 1000 1500 MHz receiver for the GMRT has been installed on 22 antennas of the GMRT This improved receiver has high dynamic range amplifiers incorporated for better immunity against higher levels of radio frequency interference RFI Rest of the feed design has not been modified In addition band reject filters have been placed to suppress the RFI from GSM 900 and CDMA cellular mobile RF carriers 7 1 3 Low Noise Amplifier Team Members A P Kumar B Hanumanthrao A N Raut G Sankarasubramanian A final prototype of the 550 900 MHz Low Noise Amplifier has been designed and successfully tested on the GM
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