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The Allen Telescope Array: The First Widefield, Panchromatic, Snapshot Radio Camera for Radio Astronomy and SETI
Jack Welch, Don Backer, Leo Blitz, Douglas Bock, Geoffrey C. Bower, Calvin Cheng, Steve Croft, Matt Dexter, Greg Engargiola, Ed Fields, James Forster, Colby Gutierrez-Kraybill, Carl Heiles, Tamara Helfer, Susanne Jorgensen, Garrett Keating, John Lugten, Dave MacMahon, Oren Milgrome, Douglas Thornton, Lynn Urry, Joeri van Leeuwen, Dan Werthimer, Peter H. Williams, Melvin Wright Jill Tarter, Robert Ackermann, Shannon Atkinson, Peter Backus, William Barott, Tucker Bradford, Michael Davis, Dave DeBoer, John Dreher, Gerry Harp, Jane Jordan, Tom Kilsdonk, Tom Pierson, Karen Randall, John Ross, Seth Shostak Matt Fleming, Chris Cork, Artyom Vitouchkine Niklas Wadefalk, Sander Weinreb
TL;DR
The ATA addresses the challenge of conducting conventional radio-astronomy surveys and SETI searches commensally at centimeter wavelengths. It combines many small dishes with wide frequency coverage and flexible processing, and its wide-field capability is demonstrated by an 800-square-degree mosaic detecting several thousand sources.
Problem
The design challenge is to conduct conventional radio-astronomy surveys and SETI searches commensally at centimeter wavelengths.
Source: §I · p. 1Method
The ATA uses many small dishes, wide frequency coverage, offset Gregorian antennas, and multiple backends including correlators and beamformers.
Source: §I · pp. 1–2, §II · p. 2Results
An 800-square-degree, 1.4-GHz mosaic from 350 one-minute pointings detected several thousand sources.
Source: §VIII · p. 8Takeaways & Limitations
Up to 32 beams may feed SETI signal detectors or radio-transient processors.
Source: §I · p. 2
Abstract
from arXiv · showhide
The first 42 elements of the Allen Telescope Array (ATA-42) are beginning to deliver data at the Hat Creek Radio Observatory in Northern California. Scientists and engineers are actively exploiting all of the flexibility designed into this innovative instrument for simultaneously conducting surveys of the astrophysical sky and conducting searches for distant technological civilizations. This paper summarizes the design elements of the ATA, the cost savings made possible by the use of COTS components, and the cost/performance trades that eventually enabled this first snapshot radio camera. The fundamental scientific program of this new telescope is varied and exciting; some of the first astronomical results will be discussed.
I. INTRODUCTION
The ATA is a planned 350-dish array designed for commensal centimeter-wave astronomy and SETI surveys. Its wide frequency coverage, correlators, and beamformers support flexible survey and signal-processing programs.
I. INTRODUCTION
- For surveys requiring multiple pointings, point-source sensitivity scales with ND rather than collecting area ND^2, motivating 350 six-meter dishes.The array's many baselines also provide rich uv-plane sampling for snapshot imaging.Source: §I · p. 1
- Four independently tunable 600-MHz back-end bands, 100-MHz correlators, and up to 32 beams support spectral imaging, SETI, and transient processing.The correlators have 1024 channels with adjustable overall bandwidths.Source: §I · pp. 1–2
- The design targets continuous 0.5–10 GHz coverage, a cost improvement approaching a factor of 10, and collecting area approaching one hectare.The design uses a cooled MMIC receiver, log-periodic feed, commodity components, and mass manufacturing to lower costs.Source: §I · p. 2
- The ATA was operating with 42 antennas at Hat Creek, with frequency agility, low sidelobes, wideband receivers, and correlator and beamformer backends.The completed array was planned to contain 350 antennas.Source: §I · p. 2
II. THE ANTENNAS
The ATA antenna design combines an offset Gregorian reflector, shielding, and a broad-beam feed arrangement with low spillover and reproducible optics. Measurements characterize beam sidelobes and surface accuracy, while the construction process is designed for low-cost, rapid manufacture.
II. THE ANTENNAS
- The offset Gregorian allows a larger secondary without aperture blockage, providing a clear aperture with lower sidelobes and thermal background.A shroud protects the feed from ground interference and thermal noise; the primary is approximately 6 m across and the secondary is 2.4 m.Source: §II · p. 2
- The aperture-efficiency estimate removes geometric blockage for the offset design but retains edge diffraction, assigning one quarter of diffraction losses to ground coupling because of the shroud.For the chosen parameters, Cd is 0.454 and edge illumination Ao is 0.211 (-13.5 dB).Source: §II · pp. 2–3
- Shroud reflection at the feed input is about -40 dB at the longest wavelength, and echoes from the primary and secondary are also small.The reflection estimate is based on a stationary-phase calculation.Source: §II · p. 3
- The fabric radome and shroud protect the feed, while the mount permits slewing up to 3°/s in azimuth and 1.5°/s in elevation.Ground spillover is generally small, rising to 1% of integrated gain near zenith.Source: §II · pp. 2–3
- Nighttime surface RMS error is 0.7 mm and RMS variation among 11 antennas is 0.5%; daytime surface RMS reaches 1.5 mm, still adequate at the shortest operating wavelength.The daytime value is 1/20 wavelength; residual sunlight absorption causes differential heating.Source: §II · p. 3
- At 3877 MHz, the average first sidelobe level is -30 dB and small sidelobe peaks are -24 dB; two antennas have similar measured patterns.The patterns show x-polarization on the left and y-polarization on the right.Source: Figure 3 · p. 3, §II · p. 3
- A stamped mirror takes less than half an hour to form, and final antenna assembly takes eight person-days.The construction uses a welded tower structure and a machined-casting alidade.Source: §II · p. 3
III. THE FEEDS AND RECEIVERS
The ATA feed and receiver design spans 0.5–10 GHz, using a pyramidal log-periodic feed and cooled low-noise amplifiers. Measured system temperatures agree with predictions through 5 GHz but rise at higher frequencies.
III. THE FEEDS AND RECEIVERS
- The dual-linear-polarization feed was chosen to pair with very wideband low-noise MMIC receivers.Source: §III · p. 3
- A pyramidal log-periodic feed covers about 500 MHz–10 GHz; focusing near 6.25 GHz provides access to the full band with about 1-dB gain degradation at the edges.The measured feed gain varies with frequency, as expected for a log-periodic antenna.Source: §III · pp. 3–4, Figure 5 · p. 4
- The feed’s central metallic pyramid places the low-noise amplifiers directly behind the antenna terminals, reducing cable losses and total effective receiver noise temperature.The LNAs are cooled to about 60 K.Source: §III · p. 4
- Measured Tsys agrees with predictions at ~40 K up to 5 GHz, then rises to 80 K above 8.5 GHz.The authors suggest higher-than-predicted input losses at higher frequencies.Source: §III · p. 4, Figure 6 · p. 5
IV. THE ARRAY
The array section describes the ATA-42 and the baseline layouts considered for expansion to 98 and 350 elements. The final ATA-350 layout aims for a Gaussian beam pattern and an ideal point spread function.
IV. THE ARRAY
- The present ATA-42 has a snapshot baseline distribution, alongside a possible 98-element intermediate stage and the final ATA-350 layout.Source: §IV · p. 5, Figure 7 · p. 5
- The ATA-350 baseline directions are uniform in azimuth, and its baseline lengths approach a Gaussian distribution.Source: §IV · p. 5
- The nearly Gaussian ATA-350 distribution in uv coordinates yields a Gaussian beam pattern and an ideal point spread function.Source: §IV · p. 5
A. Antenna Nodes
Antenna nodes connect electronics to the antennas and use buried pipes to supply airflow cooled by the ground. The pipe depth and airflow design limit temperature variation at the antennas.
A. Antenna Nodes
- Buried PVC pipes connect each node to its antennas and carry optical fiber, control fiber, and mains; nodes typically serve 10 antennas.Ambient-temperature air is also blown from each node through the pipes to the antennas.Source: A. Antenna Nodes · p. 5
- At one meter, daily ground temperature is approximately constant, and the ground and pipes exchange heat with the air supplied to each antenna.The daily and 12-day temperature components have diffusion depths of 0.11 m and 0.38 m; these rapid components motivate the one-meter pipe depth.Source: A. Antenna Nodes · pp. 5–6
- Turbulent airflow in the pipes transfers heat through the pipe walls to the ground, while rapid environmental temperature variations are completely filtered out.The reported Reynolds number is about 20,000, indicating effective turbulent heat transfer.Source: A. Antenna Nodes · p. 6
- About 0.06 m^3/s of airflow cools antenna components; with average 30 m pipes, the typical residual temperature variation is about 1°C.For average Hat Creek soil with 10% moisture, γ is approximately 16 m.Source: A. Antenna Nodes · p. 6
V. THE ANALOG ELECTRONIC SYSTEM
The analog system carries each antenna’s 500 MHz–10 GHz signals to the processor building, where the RFCB selects and converts bands for digital processing. Separate tunings support simultaneous observations, while phase switching helps control cross-talk.
V. THE ANALOG ELECTRONIC SYSTEM
- Four separately tuned first local oscillators select four simultaneous 600-MHz bands from the 500 MHz–10 GHz antenna input, then convert them to 630 MHz ± 300 MHz.A 200-MHz antialiasing filter centered at 630 MHz precedes digital processing.Source: §V · p. 6
- Walsh-function phase switching on the second local oscillators avoids cross-talk between antennas and is removed after digitization, before correlation or beamforming.Source: §V · p. 6
- The RFCB duplicates the signal path for the other polarization, and its output provides four dual-polarization IF channels.Figure 9 depicts one IF channel’s dual-polarization path through the RFCB and RF converter board.Source: Figure 9 · p. 6, §V · p. 6
- The ATA-42 rack includes 42 primary RFCB boards plus two spares, two dual-polarization spectral-imaging correlators, and three beamformers; a third beamformer was planned.The stated goal was eight beamformers across the two LO tunings routinely used for SETI observations.Source: §V · p. 6, Figure 10 · p. 6
VI. MONITOR AND CONTROL
The monitor and control system coordinates antennas and signal-processing equipment through a distributed Linux environment. It supports multiple commensal backends, remote operation, and low-level antenna control.
VI. MONITOR AND CONTROL
- The M&C system integrates antenna, RF, correlator, and beamformer control while supporting concurrent astronomy, SETI, pulsar, and other backend processes.Source: §VI · p. 6
- About 30% of observations are initiated remotely, and the system provides full remote-control support over the Internet.Source: §VI · pp. 6–7
- Java/Linux servers communicate over a proprietary message bus with network-booted antenna computers, which relay commands to six microcontroller boards for positioning, focus, amplifier settings, and monitoring.Source: §VI · p. 7
VII. DIGITAL SIGNAL PROCESSORS
Multiple digital backends can process the same or different independently tunable IF channels at once. The digital front end uses adjustable ADC inputs and FPGA processing to support dynamic range in the presence of RFI.
VII. DIGITAL SIGNAL PROCESSORS
- Spectral-imaging correlators, per-antenna fast spectrometers, and beamformers serving SETI and pulsar processors can operate simultaneously on the same or different IF channels.Source: §VII · p. 7
- The ADCs and FPGA board for digital downconversion and fringe-rate/delay tracking come from the CASPER project and use eight-bit quantization.Source: §VII · p. 7
- Per-antenna input levels are precisely adjusted to ensure adequate ADC dynamic range in the presence of radio-frequency interference.Source: §VII · p. 7
A. Digital Correlators
The ATA correlator uses an FX architecture with a polyphase filter bank to form spectral cross-correlations while reducing channel crosstalk and narrowband RFI effects. Its current configuration produces 1024 spectral maps, with snapshot maps spanning 250 independent pixels over the primary beam.
A. Digital Correlators
- The FX correlator channelizes antenna signals before cross-multiplying each antenna pair to form cross-correlation spectra.A polyphase filter bank reduces channel crosstalk and the effects of narrowband RFI.Source: A. Digital Correlators · p. 7
- Per-channel bit selection across 1024 channels permits a large dynamic range in signal level despite four-bit-limited cross-correlation products.The corner turner handles transposition of the filter-bank channels.Source: A. Digital Correlators · p. 7
- 250 independent pixels fill the primary beam in 42-antenna snapshot maps, and earth-rotation synthesis can increase that number by a factor of a few.The present correlators have 100 MHz overall bandwidth and 1024 spectral channels, producing 1024 spectral maps.Source: A. Digital Correlators · p. 7
B. Beamformers
The beamformers add antenna outputs phased toward a chosen sky direction over 100 MHz. A synthesized null can be used to avoid radio-frequency interference.
B. Beamformers
- A synthesized null can avoid RFI in the beamformer pattern, with the illustrated null placed at -28 dB offset from the primary beam direction.The beamformer adds antenna outputs phased for a particular sky direction over 100 MHz.Source: B. Beamformers · p. 7, Figure 12 · p. 7
B.1 The Prelude SETI System
The ATA's deployed systems support SETI detection, pulsar processing, wide-field transient searches, and imaging. Early imaging includes a 1.4-GHz mosaic spanning 800 square degrees.
B.1 The Prelude SETI System
- Voyager 1's faint carrier signal is readily detected by algorithms that sum power along possible drift paths in frequency-time plots.For a precompensated single-tone signal, the Prelude system would detect a Doppler drift rate of about 0.1 Hz/s; signals near zero drift can be discarded as interference.Source: §B.1 · pp. 7–8, Figure 13 · p. 8
- The Prelude system is slated for replacement by SonATA, a commodity-server cluster with software-only detection and automated follow-up processing.The planned system is intended to accommodate more antennas, bandwidth, and detection algorithms for more complex signal classes.Source: §B.1 · p. 8
B.2 Pulsar Processor
- The 100-MHz dual-polarization pulsar processor applies a 2048-point polyphase filter bank, cross-correlates orthogonal polarizations, and forms Stokes parameters.The results are sent to an offline computer for storage and subsequent processing at kHz rates.Source: §B.2 · p. 8
C. The Fly’s Eye System
- The Fly’s Eye system points antennas in different directions, with a de-dispersion spectrometer on each searching for sharp background pulses across a wide field.A recent use searched for rare, highly dispersed transient pulses.Source: C. The Fly’s Eye System · p. 8
VIII. EARLY IMAGING SCIENCE RESULTS
- 800 square degrees were mosaicked at 1.4 GHz from 350 one-minute pointings, detecting several thousand sources.A single observation of the field takes 8 hours, and repeated observations survey for transient and variable sources.Source: §VIII · p. 8
- M31 and M33 were imaged in neutral-hydrogen emission; M31 shows an atomic-gas ring beyond its optical image.These galaxies are extended on the sky and difficult to image with conventional arrays of large antennas.Source: §VIII · p. 9, Figure 16 · p. 9
- The 25-square-degree 1.4-GHz mosaic subset shows good positional and flux-density agreement with NRAO VLA Sky Survey sources.The grayscale represents ATA flux density, while circles mark catalogued source positions and scale with their flux density.Source: Figure 15 · p. 9
IX. FURTHER DEVELOPMENTS
- A feed redesign aims for system temperature below 40 K up to 24 GHz by narrowing the input cross-section and cooling the final four inches to 60 K.The further-development plan also includes building out to 350 antennas and adding correlator and beamformer backends.Source: §IX · p. 9