Allen Telescope Array & Radio Frequency Interference Geoffrey C. Bower UC Berkeley
Allen Telescope Array Large N design 350 x 6.1m antennas Sensitivity of the VLA Unprecedented imaging capabilities Continuous frequency coverage 0.5 to 11.2 GHz Wide field of view 3.5 degrees at 1 GHz Excellent survey instrument Compact Configuration 1 arcmin at 1.4 GHz (350), 1 arcmin at 5 GHz (42) Simultaneous observing with multiple backends Correlator: 2 x 100 MHz x full Stokes x 1024 channels Phased array beams: 32 indepent at 4 frequencies Joint project of UC Berkeley/SETI Inst. Privately funded construction $30M from PGAF and others NSF support for correlator & operations ($2.2M) USNO support for construction ($4M)
EVLA
ATA Science All Frequencies Transient Signals SETI GC survey: 20 sq. deg, 1.4-1.7 GHz, 6 months 10 6 stars over 1-10 GHz Arecibo radar detectable at 300 pc Simultaneous with correlator Radio continuum science Static & transient surveys Extragalactic HI survey Neutral gas SDSS equivalent L* at z=0.1 over the entire sky Galactic magnetic fields, HI, long-chain molecules Pulsars Can Confine to Protected Bands Transient Signals Deep integrations Out of band observations Deep integrations Out of band observations Deep integrations Out of band observations Transient signals
ATA 42 Commissioning Construction Complete July 2007 Commissioning Summer 2007 Internal science Summer 2007- Public call for proposals early 2008
Wide Field Mosaic >30 antennas 7 pointing mosaic ~20 square degrees >170 sources 15 mjy minimum source flux
First 12 Antennas Phased Up By Beamformer 7/19/07 Inmarsat Coarse resolution ~1kHz Fine resolution ~1 Hz
Tsys 2K-Y-11.10.06-moon Tsys in K 200 180 160 140 120 100 80 60 40 20 0 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 Frequency in GHz
Σ 2D FT Flexible, upgradable digital signal processing FT DSP
RFI Strategy Overview Know the enemy Put many arrows in the quiver No Magic Bullets Exploit large N capability Exploit flexible digital electronics Implement at all levels Antenna design Array configuration Headroom in signal path 8-bit digitization
RFI Modeling: Hat Creek Terrain
Increased Path Loss due to Terrain --- ITU-R 526
RFI Monitor Simple design Discone antenna Broadband amp Spectrum analyzer Linux PC on net 0.1 to 10 GHz 0.3 to 3 GHz currently Isotropic sensitivity
(One of) the Enemies is Us RFI generated by BIMA array Offset +25dB for clarity
Aircraft DME
Multiple Time Scales: Radar
Broadband Spectrum Interferometric Detection with ATA
1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 Parkes 1995 Green Bank 1996-8 Project Phoenix 1200-3000 MHz 10 MHz Blocked Bands 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 Arecibo 1998-2004 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 1.2 3.0 Fractional Occupancy 1.5 0 3C 48 spectral occupancy May 4-5, 2007 1 1 Prelude RFI Survey December 2006 2 MHz Block Bands 1 1 0 0 0 0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 Frequency (GHz)
Real Time RFI Mitigation Monitoring Artificial horizons around known interferers Real time RFI Databases Multiple beams for rejection Time Blanking Adaptive canceling Interferometric Nulling Postcorrelation Analysis
Time Blanking Implementation Done in backends Correlator will have 100 μsec precision External & Internal Triggers
Adaptive Canceling
Rapid Prototyping Array
Cancellation of Glonass
Cancellation of GPS
Phased Array Beam Nulls with the ATA
Multiple Nulls
Nulls in Frequency Space
The Costs of Nulling Requires large-n to be effective Loss in SNR with number of nulls Must be able to update complex gains at ~100 Hz Matrix inversion is computationally expensive
Postcorrelation Analysis Manipulate output of correlator/visibility matrix Data rate: 350x349/2 4 Stokes 1024 Channels 16 bit 100 Hz sampling 400 Gbits/sec: too much!
Where to Use the Tools Blanking Radar, Aircraft, intermittent signals Adaptive Canceling Reference antenna(s) on transmitter Interferometric Nulling Predictable trajectories: satellites, fixed transmitters Postcorrelation Analysis Broadly adaptable
Square Kilometer Array * 100
ATA & RFI We have made extensive surveys to characterize RFI We plan to use numerous tools Many of these tools are untested All carry a significant cost in engineering, observing efficiency, etc.