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RFI at GMRT: Detection, Suppression and Co-existence

RFI at GMRT: Detection, Suppression and Co-existence . Pravin Raybole, S.Sureshkumar GMRT, India. Acknowledgement. Yashwant Gupta, GMRT Chief Scientist A. Praveen Kumar S. Suresh Kumar B. Ajith Kumar Suresh Sabhapathy M. R. Sankararaman Field Team members

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RFI at GMRT: Detection, Suppression and Co-existence

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  1. RFI at GMRT: Detection, Suppression and Co-existence Pravin Raybole, S.Sureshkumar GMRT, India

  2. Acknowledgement • Yashwant Gupta, GMRT Chief Scientist • A. Praveen Kumar • S. Suresh Kumar • B. Ajith Kumar • Suresh Sabhapathy • M. R. Sankararaman Field Team members • AmitSawant, ManojChavan, RahulDurgude, DhairyashilDhamdere, MukundDeshmukh, MilindThorat and Suresh Kale

  3. Contents • Introduction to GMRT • RFI Environment at GMRT • Interference from TV boosters • Interference from Cell Phone towers • Power line interference • RFI detection using the GMRT Software Backend

  4. 1 km x 1 km 14 km The GMRT • GMRT is designed and built by NCRA – TIFR Pune. Located 80 km N of Pune and 160 km E of Mumbai. • 30 antennas of 45 meter diameter covering 50MHz to 1450 MHz • 12 dishes in central compact (1km) array • Remaining along 3 arms of Y-array, longest baseline (25kms)

  5. RF Spectrum at GMRT : 0 – 500 MHz Day 12:00 hours Night 22:00 hours TV FM SW POLICE WIRELESS SATELLITE ATC

  6. RF Spectrum at the GMRT : 500MHz – 1GHz Day 12:00 hours Night 22:00 hours TV CDMA GSM

  7. RF Spectrum at the GMRT : 1GHz – 2GHz Day 12:00 hours Night 22:00 hours GSM

  8. RFI environment at GMRT

  9. Monitoring the RFI environment at the GMRT • Regular monitoring of the RF spectrum. • Regular field survey to check radiation from new installations. • Close contact and a shared knowledge base with government agencies, local industries, cellular phone operators, electric companies and individuals. • Regular studies of internal ( i.e. self generated ) RFI, e.g. from GMRT computers, instruments.

  10. Important RFI sources at the GMRT

  11. Interference from TV boosters

  12. Locating TV Booster • Did a field survey of TV dipole antennas. • Visited every location for RFI measurement. • Measurement setup • Log periodic antenna • Spectrum analyzer • 20dB Amplifier • Generator for backup.

  13. RFI from TV Boosters : Broad-band Frequency 40MHz to 650MHz

  14. RFI from TV Boosters : Broad-band Frequency 314MHz to 375MHz

  15. RFI from TV Boosters : Narrow-bandFrequency 40MHz to 650MHz

  16. Method adopted Problem : Saturation and oscillation of TV booster amplifiers Solution : New resistor added in series with existing variable resistor Resistance Values : 900 Ohm to 1.2KOhm A 25 paise solution Variable Pot Before Putting Resistor After Putting Resistor

  17. Corrected TV booster Frequency 40MHz to 650MHz FM TV SIGNAL

  18. Broadband RFI from TV boosters • Field survey of 25 nearby villages in and around GMRT with the help of the local administration. • Inspected nearly 1800 TV boosters over a 3 years period. • Reduced the gain of the amplifier to prevent saturation and oscillation, thereby ensuring good signal reception. • Follow-up monitoring studies found no cases of recurring problem over a 3-year period. • Recent field surveys have found that 66% of users have switched to Direct-to-Home services which have Ku–band downlink frequencies. These do not produce any RFI at GMRT frequencies. The situation is likely to continue to improve over the next year.

  19. Interference from Cellular Phone towers

  20. Co-existing with cellular operators • Cell phone towers use CDMA (824-896 MHz), GSM (935-960 MHz) and (1811-1880 MHz) and they operate with 20 Watt and 40 W transmitters. • After detail discussions, most of the GSM operators within a radius of 20 km of GMRT, agreed to shift GSM transmissions to the 1800 MHz band, which does not affect GMRT observations. A few towers beyond 20 km have also been asked to change to 1800 MHz GSM band. • CDMA operators operating at 824-896 MHz are still a serious source of RFI for the GMRT especially with the planned extensions in frequency coverage. At present, the CDMA operator have been request to use low power transmitters and to focus their transmitters away from GMRT antennas. Longer-term solutions are now being examined.

  21. GSM transmission at 950 MHz band shifted to 1800 MHz GSM Transmission at 950 MHz GSM transmission at 950 MHz

  22. Power Line Interference

  23. 11KV/415V AC 3 Phase Transformer : Power line interference measured using Ultrasound detector Transformer installation Ultrasound detector Accurate and efficient in pin- pointing problems with disc, pin insulators, fuse links and cut joints at transformer sites.

  24. Survey of transformers along the GMRT West arm W06 W03 W05 W04 W02 W01

  25. Survey of 3 Phase Transformer installation along GMRT antennas ( 16th March 2010 )

  26. Uses measurements made with the GMRT for the localisation of RFI.

  27. An RFI map with the GMRT Software backend at 150 MHz A Technique developed by Ue-Li Pen and collaborators for Epoch of Reionization work at the GMRT. Uses measurement made with Interferometer for localization of RFI

  28. Location of RFI sources are marked in blue Antenna location are marked in red

  29. Hanging stray wire on 440 KV AC EHT line

  30. Hanging stray wire on 440 KV AC EHT line

  31. Abandoned Telephone line near HT line

  32. Summary The GMRT is located in a region of high population density, and must hence co-exist with multiple sources of RFI. The primary sources of RFI at GMRT are from cellular phone transmissions, power line signals, and TV boosters. Regular field surveys have been carried out to locate sources of strong RFI. Direct repairs of individual TVs were used to solve the problem of malfunctioning TV boosters. Close interaction with the Cell phone operators have resulted in a significant fraction of these transmission being shifted to 1800MHz, out side the GMRT bands. Similar interactions with electric companies have been used to repair malfunctioning power line transformres. o

  33. Summarycont… • Close interaction with the Cell phone operators have resulted in a significant fraction of these transmission being shifted to 1800MHz, out side the GMRT bands. • Similar interactions with electric companies have been used to repair malfunctioning power line transformers • RFI environment at GMRT is still quite acceptable for cutting-edge low frequency radio astronomy. The frequency range is now being extended to cover 250-500 MHz and 550-900 MHz and there are no obvious show-stoppers from the RFI point of view that will limit system performance.

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