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

2. Acknowledgement Yashwant Gupta, GMRT Chief Scientist Yashwant Gupta, GMRT Chief Scientist A. Praveen Kumar A. Praveen Kumar S. Suresh Kumar S. Suresh Kumar B. Ajith Kumar B. Ajith Kumar Suresh Sabhapathy Suresh Sabhapathy M. R. Sankararaman M. R. Sankararaman Field Team members Amit Sawant, Manoj Chavan, Rahul Durgude, Dhairyashil Dhamdere, Mukund Deshmukh, Milind Thorat and Suresh Kale Amit Sawant, Manoj Chavan, Rahul Durgude, Dhairyashil Dhamdere, Mukund Deshmukh, Milind Thorat and Suresh Kale

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

4. 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) 14 km 1 km x 1 km

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

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

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

8. GMRT : Operating Frequency Bands Frequency Radio Astronomy Band System Bandwidth Source of Interference observed near Radio Astronomy band. 150 MHz 150 – 153.6 MHz 130 – 230 MHz Air Traffic control, Fixed Mobile, TV Transmission, 235 MHz 230 – 235 MHz 213 – 253 MHz Planned: Planned: 250 – 500 MHz 250 – 500 MHz Digital Audio Broadcast ( DAB ), Aeronautical Radio Navigation, Leakage from Cable TV signals. 327 MHz 322 – 328.6 MHz 295 – 365 MHz Fixed Mobile, Aeronautical Radio Navigation 610 MHz 604 – 616 MHz 570 – 660 MHz Planned Planned 550 – 900 MHz 550 – 900 MHz TV Transmission 1060 – 1420 MHz 1400 - 1427 885 – 1450 MHz Fixed Mobile Broadcasting, Satellite

9. RFI environment at GMRT Intentional Radiation Unintentional Radiation TV and FM TV and FM Power Line RFI Police Wireless Communication TV Boosters Air Traffic Control Industries Mobile Communication Electronics Instruments, Computers Air Force Networking Devices

10. Monitoring the RFI environment at the GMRT Regular monitoring of the RF spectrum. Regular monitoring of the RF spectrum. Regular field survey to check radiation from new installations. 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. 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. Regular studies of internal ( i.e. self generated ) RFI, e.g. from GMRT computers, instruments.

11. Important RFI sources at the GMRT Sources Nature of RFI TV Boosters Broad-band, Narrow band or very narrow lines Cellular phones Narrow-band Power lines Broad-band

12. Interference from TV boosters

13. Locating TV Booster Locating TV Booster Did a field survey of TV dipole antennas. Did a field survey of TV dipole antennas. Visited every location for RFI measurement. Visited every location for RFI measurement. Measurement setup Log periodic antenna Spectrum analyzer 20dB Amplifier Generator for backup. Frequency Range VHF and UHF band Gain 40 – 60dB 40 – 60dB No of stages 2 – 3 cascaded stage 2 – 3 cascaded stage

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

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

16. RFI from TV Boosters : Narrow-band Frequency 40MHz to 650MHz

17. Method adopted Before Putting ResistorAfter Putting Resistor Variable Pot A 25 paise solution 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

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

19. Broadband RFI from TV boosters Field survey of 25 nearby villages in and around GMRT with the help of the local administration. 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. 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. 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. 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. 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.

20. Interference from Cellular Phone towers

21. 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. 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. 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. 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.

22. GSM transmission at 950 MHz band shifted to 1800 MHz GSM Transmission at 950 MHzGSM transmission at 950 MHz

23. Power Line Interference

24. 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.

25. W06 W05W04 W03 W02 W01 Survey of transformers along the GMRT West arm

26. Survey of 3 Phase Transformer installation along GMRT antennas ( 16 th March 2010 ) SR.ANTENNASTRANSFORMERSPROBLEMATICAVERAGEGOOD NO. ATTENDED 1EAST ARM5E-02,E-05 E-03,E-04,E-06 2WEST ARM6W-01,W-03 W-02,W-04,W-05,W-06 3SOUTH ARM5S-01 S-02,S-03,S-04,S-06 TOTAL165 11 SR. NO.DETAILS OF RADIATIONEASTWESTSOUTH 1 BAD 11KV DISC INSULATOR W-01 2BAD DOE-02W-01 3BAD TRANSFORMERE-02,E-05W-01, W03S-01 4BAD H.T. BUSHING PIN INSULATOR W-01 5BAD L.T. BUSHING PIN INSULATOR W-01

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

28. 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

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

30. Hanging stray wire on 440 KV AC EHT line

32. Abandoned Telephone line near HT line

33. Summary The GMRT is located in a region of high population density, and must hence co-exist with multiple sources of RFI. 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. 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. 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. 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. Similar interactions with electric companies have been used to repair malfunctioning power line transformres. o

34. Summary cont… Summary cont… 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. 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 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. 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.