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Living with RFI at AO(aka rfi snooping) Puerto Rican Spectrum Users Group 23sep10 meeting Phil Perillat
Talk Outline • Frequency bands covered at the Arecibo Observatory (AO) • Why we want large frequency coverage • Hydrogen line, doppler shift, expansion of universe • How RFI comes to our attention. • Users • Ao monitoring of data • Where is the RFI coming from? • Minimizing the effect of RFI. • A look at some RFI Online: http://www.naic.edu/~phil/talks/talks.html sept10 PRSUG
Frequency coverage • Receiver sensitivity • Amplifiers cooled to about 15 Kelvins • System temp receiver+sky ~ 25-30 Kelvin • 1 Kelvin = -198 dbm/Hz • 30K=-183 dbm/1Hz, -130dbm/(20Khz) • Rfi usually scattered in via sidelobes so no telescope gain.
Why we want large freq coverage(a little astronomy) • Hydrogen is the most abundant element. • Emits radiation at 1420.4058 Mhz .. • Doppler shift: • Freq shifts because of relative motion emitter and observer • (freqEmit-freqObs)/freqEmit = RelativeVelocity/C • Or relativeVel=(freqDif)/freqEmit*C • Our galaxy: • Need about a Mhz about 1420.4 to get doppler info.
Why we want large freq coverage • Velocity of light c is finite:300,000 km/sec • Looking far away also looks back in time (because it takes the light time to get here). • 1 light yr.=distance light travels in 1 year=9.5*10^12 km • Universe expanding from big bang • Things move apart at a high speed. • Over time gravity slows down the expansion. • Looking far away (back in time) things were moving faster than they are now • Freq Shift from slowdown of expansion: • Hubbles law:70 Km/sec every 3.2*10^6 light years • Galaxy UGC10721 at 1406.62 Mhz 2911 km/sec
Why we want large freq coverage • Example: observing galaxies at 1220 Mhz • (1420-1220)/1420 *C velocity is 42241 Km/sec • 42242/70 * 3.32e6 = 2 billion light years • Universe age : 13 billion years so looking back 2 billion years is a fraction 1/6 of the universe lifetime. • Received signals get weaker as 1/distance squared • Need long integration times and sensitive telescopes to be able to do this. • Arecibo is the most sensitive radio telescope in the world for lband observing. • Disclaimer: Observing galaxies using hydrogen is not the only interesting thing we do here… (but I’ve only got so much time for the talk….
How rfi comes to our attention • Users of the telescope see it in their data • AO monitoring of telescope data for rfi • Hilltop monitoring system • The grapevine .. • ITU, WRC, CORF, PRSUG • google
Users looking at their data: • Most sensitive way, but: • Could be large time lag between users taking the data and analyzing it. • We may not use a freq range for a long time period. About 7 experiments are done daily. These may last from days to years. • So user input is good but not sufficient to cover all the bands in a timely manner.
Ao monitoring of telescope data • Lots of data.. • Need to automate the identification of rfi in spectra.. • Have users dump spectra fast (1sec, 1millisec..) • Compute the (rms/mean) along time axis for each freq channel. • The resulting noise is only a function of the integration time and the channel width: • Radiometer Equation: • deltaT/T=1/sqrt(channelBandwidth*integrationTime)
Ao monitoring of telescope data • Why rfi sticks out in rms/mean: • Rfi will usually have a time variability that is different from Gaussian noise (either inherently or because it is moving through our side lobes. • Using rms/mean gets around having to remove the band pass. • Set a threshold of 2 or 3 sigma above the expected noise level. Everything above this is called rfi. • Drawbacks: • Sensitivity limited by the single record integration (in this case 1 second, not 600 seconds). • Can also include real signals if they vary with time (eg sky drifting through telescope: (see HI, galaxy)
Ao monitoring of telescope data • Once a month: • process most of the data taken at arecibo computing rms/mean by channel. • For each freq range make a histogram of the fraction of time we saw rms/mean with a value 3 sigma above the expected noise value. • Call this rfi and publish it on the web: http://www.naic.edu;/~phil/rfi/rms.html • Histogram of 1250-1450 for 2010 jan-aug • Also have monthly plots • Punta salinas 1280-1330 occurred only in mar10 (we probably shifted our schedule without telling them).
Hilltop monitoring system • Rfi monitoring system running on hilltop 24 hours a day • Setup: • Omni directional ant 0 to lband • Log periodic 1.8 GHz to 10 GHz • Filter bank, amplifiers (programmable) connected to spectrum analyzer • Peak hold 60 seconds, dump, then next band • 20 steps to cover 0 to 10 GHz.
Hilltop monitoring system • Data Products: • Interactive tool to access 1 minute records. • Daily web output: • 24 hour average over each band • Images power time by frequency • Rms/mean over the day for each band • Usage: • Good for strong, wide (order 100 khz) signals. • Tsys > 1000 K, 401 channels over bandwidths 100 to 1GHz • Looks at the horizon (hard to see satellites). • Can tell you when something started eg.. • WCS 1305-1320 appeared 7:30am 23jun10. • Look at statistics over long periods of time. • How often was the 1330 or 1350 radar down for repair a few years ago
Where is the rfi coming from? • Are we generating it: • 1/distance^2 birdies originating in the dome will be strong. • Swing the azimuth arm by 360 deg. No az dependence points to birdie in the dome. • If a tone, measure it’s stability. Many of our signals are locked to a hydrogen maser. High stability (1 part 10^12 or better us).
Intermods, harmonics • Is the birdie entering the horn, or is it an intermod, harmonic created in our system? • Move the first LO by delta F and see how much the birdie changes.. • Compute the harmonics, intermods for the system in use to see if it could come from a known source. • If possible select an rf filter (before 1st mixer) that will cut down on out of band birdies. • Tv stations: 14 (471.25) 22 (519.25) created 430 birdie in: 2*f1-f2. Placed a cavity filter in front of dewar to get rid of it (worsened Tsys by a little). • Check if it is a harmonic of a known signal. • Chan 54 (711.25) was generating a birdie at 1422.5 from their transmitter. • Radio station 107.3 was generating a 13th harmonic at 1394.9 MHz
Periodic signals, radars • Periodic signals • Ipps 1-3millisecs • Rotation rates 12 secs (4 sec for ships). • Pulsed signals • Duty cycles .005 to 10 % • Including multipath scattering increases the duty cycle.. • Different radars: • FAA 1330,1350. 5usec pulse*2 • aerostat:1241,1246,1256,1261 2*150usec pulses • Punta salinas. 1220-1390. 11 frequencies to hop • Remy radar: 1270,1290 5 usec pulse • Hawkeye airborne: around 430 MHz. • Weather radar: 5610 • Pico del este: 2900-3100
Satellites • Global navigation satellites • Gps: L1 1574.2, L2-1227.6, L3=1381.05, L5=1176.45 • Glonass: • L1: (1598-1609.3) 20 chan .5625 MHz spacing • L2: (1241.94-1251.98) 20 chan, .4375 MHz spacing • Compass (Chinese) • 1589.74,1561.1,1268.52,1207.14 • Galileo (European) • E1=1575.42, E5a:1176.45,E5b:1207.14, E6:1278.75 • Communications: • Iridium. 1621.35 (1618) – 1626.5 • Satellite radio: 2320-2345.
Minimizing the effect of RFI. • If AO generated try to find and eliminate it. • Examples: • Laser rangers: put wire mesh over windows • But cuts down on the signal • Huffman boxes, seal with conductive gaskets. • But conductive gaskets lose conductivity with time. • Position encoders of telescope. • Buy better shielded devices, put filters inside the encoders. • Beware of water tight devices. May use rubber gaskets that cut down on conductivity between parts.
External sources: spend $$$ • Can we spend some money to help the outside users cut down on out of band emission? • Chan 54. Helped screen their transmitter room to get rid of 1422.5 harmonic • Low power TV station chn 67 at 789.25 • Helped them buy equipment so they could switch to digital transmission outside the 700-800 Mhz Band.
Co existing with other users • See if we can time multiplex use of the band • Punta salinas moves to mode A (4 freq) when we do lband observing. When not using lband they use all of their frequencies • Punta borinquen. They try to use one of their 2 frequencies 1270,1290 • Blank the radar in our direction to limit compression in our system: • Punta salinas and aerostat both blank their transmitters when they point at us.
Co existing.. • Gps L3 at 1381.05 • Coordinate the testing of gps l3 and our observing schedules at lband. Shortened 4 days of testing for new svn 25 because is conflicted with AO observing • Iridium satellite. • Coordinate traffic levels with our observing schedules at OH. • FAA future upgrade • Try to blank new transmitters when they point at AO (but now 8 rather than 3 frequencies!!).
Some examples of rfi • Gps L2-1228,glonass L2-1248, compass-1268 • Faa 1330,1350, iq images • Boriquen 1290 radar. • Punta salinas mode A • Freq vs time of power:1220-1395 Mhz • 1 second spectra, 21 Khz resolution • Freq vs time of power 1335-1425 Mhz • Gps L3 and external galaxy • 1340,1380,1405,1410 generated from faa radar • GPS L3 and external galaxy spectra • Shows difference in strength • And this is a nearby strong galaxy.
Last example of rfi • In rfi sleuthing, things never come out the way you expect them to.. Always try and duplicate the findings… • Example: punta salinas test blanking in our direction. • Online it looked like they had a gigantic sidelobe 120 degrees from pointing at us. • Offline I noticed that the line was curved… • More Info: http://www.naic.edu/~phil/rfi • Thanks..