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The First Radio Astronomer
…And discovers the Galactic Center!
1932: Karl Jansky given task to find sources of radio frequency interference (RFI) to transatlantic radio communications
First Sky Maps at Radio Frequencies1938-48: Radio engineer Grote Reber builds the first modern radio
telescope (in his backyard!) and makes the first sky maps
First Sky Maps at Radio FrequenciesAnd may also have been the first to grapple with RFI…
Oh no…RFI !!!
1967: While studying interstellar scintillation, Antony Hewish and Jocelyn Bell puzzle over mysterious interference
…And realize that they have discovered a pulsar!
Sometimes interference is…something new
1967: While studying interstellar scintillation, Antony Hewish and Jocelyn Bell puzzle over mysterious interference
…And realize that they have discovered a pulsar!
Sometimes interference is…something new
But, most RFI is just that – interference.
A Present Day RFI Problem: GLONASS vs. OH
Hydroxyl (OH)– Important for the study of evolved stars & Galactic dynamics– Spectral lines in at least three bands between 1600-1800 MHz– Bands protected world-wide by ITU regulation
GLONASS– The Russian Version of the US Global Position System (GPS)– 24 carrier frequencies over 1602-1616 MHz
Narrowband (500 kHz) DSSS emission
Wideband (5 MHz) DSSS emission
Typical GLONASS Power Spectrum
By the way:INR < −27 dB
in occupied bandwidth
What GLONASS does to Aperture Synthesis Imaging
What GLONASS looks like to a Radio Astronomer
Up to 50% of data collectedin this band has to be
thrown out!
Why Not Just Throw Away Bad Data?
A typical major radio telescope costs on the order of US$20,000 per day to operate
Post-observation editing & scheduling is possible, but is – labor intensive– subject to selection effects
There are only a few major radio telescopes, so telescope time has value that transcends simple dollar estimates
In summary: Real-time in-line RFI mitigation is highly desirable
Adaptive Canceling Approach
Don’t need to knowanything about interferencewaveform
Requires large INR
Injects referencenoise into output
Courtesy R. Fisher (NRAO)
Adaptive Canceling Approach
Don’t need to knowanything about interferencewaveform
Requires large INR
Injects referencenoise into output
May be possible to patch this up…current research topic
Otherwise, best hope is to exploit waveform knowledge
Courtesy R. Fisher (NRAO)
Ellingson, Bunton, & Bell (2001), ApJS, 135, 87
GLONASS Canceller
GLONASSDemodulator
Noise-Freecopy of GLONASS
as transmitted
The Hard Part:Modeling the effect
of antennas &receiver passbands
Noise-Freecopy of GLONASS
as received
Ellingson, Bunton, & Bell (2001), ApJS, 135, 87
GLONASS Canceller
Estimate System Response
By Correlation
GLONASS Canceller
Ellingson, Bunton, & Bell (2001), ApJS, 135, 87
Australia Telescope Compact Array (ATCA)Narrabri, NSW
Observations of OH MaserIRAS 1731-33
(4 sec integration)
About 24 dB suppression,
Limited by weak INR
Another Problem: L-Band Radar• The band 1215-1400 MHz is important for:
• Spectroscopy of redshifted HI• Continuum & Pulsar work• Earth climate & geophysical studies: Brightness temperatures infer ocean
salinity, soil moisture, ...
• AND this entire band is allocated primarily to aviation radars!
Waveform: Fixed-frequency (CW) or chirped,& pulsed
Pulse length: 2-400 µsPulse spacing: 1-27 ms (typical duty cycle ~0.1%)Bandwidth: ~1 MHzTx pwr: 103 - 106 WAntenna: Highly directional,
rotation rate ~10 s
Another Problem: L-Band Radar• The band 1215-1400 MHz is important for:
• Spectroscopy of redshifted HI• Continuum & Pulsar work• Earth climate & geophysical studies: Brightness temperatures infer ocean
salinity, soil moisture, ...
• AND this entire band is allocated primarily to aviation radars!
Waveform: Fixed-frequency (CW) or chirped,& pulsed
Pulse length: 2-400 msPulse spacing: 1-27 ms (typical duty cycle ~0.1%)Bandwidth: ~1 MHzTx pwr: 103 - 106 WAntenna: Highly directional,
rotation rate ~10 s
I’m focusing on radio astronomy,but virtually all comments areequally applicable to all forms
of microwave radiometry,including remote sensing.
L-Band Interference Surveyor/Analyzer (LISA)LISA co-observes with existing
passive microwave sensors to identify sources of damaging RFI
• Spectrum analyzer for full-bandwidth monitoring of power spectral density
• 14 MHz (8+8 bit @ 20 MSPS) coherent sampling capability for waveform capture and analysis
Nadir-lookingcavity-backed spiral
antenna w/ custom LNA & calibration electronics
in tail radome
Spectrum analyzer,electronics rack &control console
mounted in cabin
RF distribution, antenna unit control &
coherent sampling subsystem
20,000 ft over Virginia/Maryland Coast
OSU NASA/IIP Wideband Digital Receiver
200 MSPSA/Ds
FPGAs implementing Receiver, FFT, RFI Mitigation, PC Interface
What Arecibo Sees:
Measured Radar Waveform Characteristics
Derived Transmit Pulse Waveform(Based on receive data taken at Arecibo
and lots of post processing)
Magnitude
Phase
PSD
Ellingson & Hampson (2003), ApJS, 147, 167.
Derived Channel Impulse Response
Pico de Este to Arecibo
Max hold
Mean
Ellingson & Hampson (2003), ApJS, 147, 167.
Every detected pulse infers the presence of many delayed
(possibly undetected) pulses
Asynchronous Pulse Blanking (APB)Continuously estimates mean/variance of incoming time domain signal
A sample > β standard deviations above the mean triggers blanker
Blanking operates on down-stream data exiting a FIFO; blanking window extends before and after triggering sample
APB blanking decision
Triggering pulseMultipath copies
APB in Action @ Arecibo
Total power in 50 MHz x 42-ms integrations
APB offAPB on
APB in Action @ Arecibo
Total power in 50 MHz x 42-ms integrations
APB offAPB on
Works great for continuum & spectroscopy,also total power remote sensing
(But still INR-limited)
Blanking is Not For Everyone…Blanking is problematic for “time-domain” radio astronomy
– Pulsars– Asynchronous or one-time astronomical transients
“Giant” Pulses from pulsarsPrompt emission from GRBsPrompt emission from SNeCoalescing neutron star & black hole binariesExploding black holes
In dealing with these things, we would really like to be able just to “look through” the interference; i.e., back to canceling
Possible answer is Pulse Canceling: Estimate and subtract pulses, as opposed to simply blanking
Pulse Canceling at Arecibo
Before
After (~16 dB suppression)
Ellingson & Hampson (2003), ApJS, 147, 167.
Pulse Canceling vs. Pulse Blanking
Before
Pulse Canceling
Pulse Blanking
Ironically, it is thedetector - not the
waveform estimation -that limits performance.
Ellingson & Hampson (2003), ApJS, 147, 167.
A Brief Comment on Pulse Searches…
A Brief Comment on Pulse Searches…
Will an RFI pulse canceller eat astronomical pulses too?
Dispersive Interstellar Medium
A Brief Comment on Pulsar Observing…
Asymptotic vs. Known Dispersion Measures
Cordes & Lazio (2003)
So Could Interesting Pulses Be Thrown Out?
DM < 20 or so: Risk decreases withdecreasing DM
DM=15
DM=5DM=0.5
DM > 20 or so: No, if time abovethreshold is taken into account
DM=71
DM=25
DM=∞
Radar
DM ~ 20: Dispersed pulse looks like radar at output of matched filter.At Arecibo, dispersed pulses greater than about 0.1-1.0 Jy are detected.
In general: Risk is greatest when dispersed pulse exhibits the sametime-frequency occupancy as radar pulse. (In this case, 7 ms x 150 kHz)
Ellingson & Hampson (2003), ApJS, 147, 167.
Other Annoyances…
FPS-117 radar received at Arecibo(Linear FM Waveform)
Typical Pulsar Pulse
Ellingson & Hampson (2003), ApJS, 147, 167.
“RFI-Challenged” ScienceAsynchronous & one time pulses (already discussed)
Deuterium – Very long integration at 327 MHz– Detection, Measurement
The 21-cm Signature of the Epoch of Reionization (EoR)– Very long integration in 75-225 MHz band– Detection, Mapping– Critical (justifying) science for LOFAR
21-cm emission from galaxies at very-high redshift– 1420 MHz ends up in the 1215- 1400 MHz Air Traffic Control Radar band– Critical (justifying) science for SKA
“RFI-Challenged” ScienceAsynchronous & one time pulses (already discussed)
Deuterium – Very long integration at 327 MHz– Detection, Measurement
The 21-cm Signature of the Epoch of Reionization (EoR)– Very long integration in 75-225 MHz band– Detection, Mapping– Critical (justifying) science for LOFAR
21-cm emission from galaxies at very-high redshift– 1420 MHz ends up in the 1215- 1400 MHz Air Traffic Control Radar
band– Critical (justifying) science for SKA
These experiments are unlikely to succeed unless effective RFI mitigation
technology is worked out!
