Grant A. Hampson

Australian SKA Pathfinder: A High-Dynamic Range Wide-Field of View Survey Telescope

The Australia SKA Pathfinder (ASKAP) is a new telescope under development as a world-class high-dynamic-range wide-field-of-view survey instrument. It will utilize focal plane phased array feeds on the 36 12-m antennas that will compose the array. The large amounts of data present a huge computing challenge, and ASKAP will store data products in an archive after near real-time pipeline processing. This powerful instrument will be deployed at a new radio-quiet observatory, the Murchison Radio-astronomy Observatory in the midwest region of Western Australia, to enable sensitive surveys of the entire sky to address some of the big questions in contemporary physics. As a pathfinder for the SKA, ASKAP will demonstrate field of view enhancement and computing/processing technology as well as the operation of a large-scale radio array in a remote and radio-quiet region of Australia.

Mitigation of Radar Interference in L-Band Radio Astronomy

The 1215-1400 MHz band is important for spectroscopy of H I at high redshift, pulsar work, and SETI. Observations at these frequencies are complicated by pulsed interference from ground-based aviation radars. In this paper, we characterize one such radar received at the Arecibo Observatory using coherently sampled data sets obtained during a recent observation. Using these data, we demonstrate some simple methods for detection and removal of the radar pulses. One of these uses a coherent subtraction technique that has not previously been applied to the radar problem. This new technique provides an alternative to blanking, which is undesirable in pulsar and SETI work. We demonstrate that the radar studied in this paper can be suppressed by at least 16 dB in integrated spectra using the coherent subtraction technique. The maximum single-pulse power observed at the output of the canceler is ~15 dB less. The primary limitation appears to be the detector performance; as a result, the performance using blanking is about the same. Also, we demonstrate that the matched detector for pulses from this radar is relatively insensitive to astronomical transients (e.g., giant pulses), and we quantify the risk of such transients being falsely identified as radar pulses. The techniques described in this paper can easily be adapted to radar waveforms other than the one examined here.

The Australian Square Kilometre Array Pathfinder: System Architecture and Specifications of the Boolardy Engineering Test Array

This paper describes the system architecture of a newly constructed radio telescope - the Boolardy Engineering Test Array, which is a prototype of the Australian Square Kilometre Array Pathfinder telescope. Phased array feed technology is used to form multiple simultaneous beams per antenna, providing astronomers with unprecedented survey speed. The test array described here is a 6-antenna interferometer, fitted with prototype signal processing hardware capable of forming at least 9 dual-polarisation beams simultaneously, allowing several square degrees to be imaged in a single pointed observation. The main purpose of the test array is to develop beamforming and wide-field calibration methods for use with the full telescope, but it will also be capable of limited early science demonstrations.

Airborne radio-frequency interference studies at C-band using a digital receiver

Corruption of C-band microwave brightness observations by radio-frequency interference (RFI) has been reported in recent data from orbiting radiometers; methods for mitigating these effects are of great importance for the design of future spaceborne microwave radiometers. One approach that has been suggested involves the use of multiple subchannels at C-band as opposed to a single channel; the use of multiple subchannels allows RFI to be detected and mitigated by analyzing relationships among subchannel brightnesses. While this approach has been utilized in previous airborne measurements, demonstrations of the RFI mitigation performance achieved have been difficult to obtain. To address this issue, an enhanced airborne system for observing radio-frequency interference effects on C-band microwave radiometers was developed, and is described in this paper. The system includes a traditional microwave radiometer with four C-band subchannels, so that RFI removal is possible using a subchannel mitigation algorithm. In addition, the system includes a digital receiver with the capability of providing high temporal and spectral resolution observations of interference. This high-resolution data allows improved understanding of RFI sources to be obtained, and also allows analysis of subchannel mitigation algorithm performance. Observations using the system in a test flight near Wallops Island, VA are described. Results show the four subchannel approach generally to be effective in mitigating the observed RFI sources, although examples are also illustrated using the digital receiver data to demonstrate failure of this approach. While studies of the digital receiver data alone could be performed to demonstrate further improvements in RFI mitigation, issues with this initial dataset limit the extent of such studies. Nevertheless, the results obtained still demonstrate qualitatively the improved RFI mitigation that can be achieved in brightness observations through the use of digital receivers

The Australian Square Kilometre Array Pathfinder: Performance of the Boolardy Engineering Test Array

We describe the performance of the Boolardy Engineering Test Array, the prototype for the Australian Square Kilometre Array Pathfinder telescope. Boolardy Engineering Test Array is the first aperture synthesis radio telescope to use phased array feed technology, giving it the ability to electronically form up to nine dual-polarisation beams. We report the methods developed for forming and measuring the beams, and the adaptations that have been made to the traditional calibration and imaging procedures in order to allow BETA to function as a multi-beam aperture synthesis telescope. We describe the commissioning of the instrument and present details of Boolardy Engineering Test Array’s performance: sensitivity, beam characteristics, polarimetric properties, and image quality. We summarise the astronomical science that it has produced and draw lessons from operating Boolardy Engineering Test Array that will be relevant to the commissioning and operation of the final Australian Square Kilometre Array Path telescope.

Design and demonstration of an interference suppressing microwave radiometer

Microwave radiometers operating outside protected portions of the frequency spectrum can be adversely impacted by radio frequency interference. In this paper, we describe a new radiometer which coherently samples 100 MHz of spectrum and applies real-time RFI mitigation techniques using FPGAs. Experiments currently in progress to demonstrate the system are also detailed

Airborne radio frequency interference studies at C-band using a digital receiver

Corruption of C-band microwave brightness observations by radio-frequency interference (RFI) has been reported in recent data from orbiting radiometers; methods for mitigating these effects are of great importance for the design of future spaceborne microwave radiometers. One approach that has been suggested involves the use of multiple subchannels at C-band as opposed to a single channel; the use of multiple subchannels allows RFI to be detected and mitigated by analyzing relationships among subchannel brightnesses. While this approach has been utilized in previous airborne measurements, demonstrations of the RFI mitigation performance achieved have been difficult to obtain. To address this issue, an enhanced airborne system for observing radio-frequency interference effects on C-band microwave radiometers was developed, and is described in this paper. The system includes a traditional microwave radiometer with four C-band subchannels, so that RFI removal is possible using a subchannel mitigation algorithm. In addition, the system includes a digital receiver with the capability of providing high temporal and spectral resolution observations of interference. This high-resolution data allows improved understanding of RFI sources to be obtained, and also allows analysis of subchannel mitigation algorithm performance. Observations using the system in a test flight near Wallops Island, VA are described. Results show the four subchannel approach generally to be effective in mitigating the observed RFI sources, although examples are also illustrated using the digital receiver data to demonstrate failure of this approach. While studies of the digital receiver data alone could be performed to demonstrate further improvements in RFI mitigation, issues with this initial dataset limit the extent of such studies. Nevertheless, the results obtained still demonstrate qualitatively the improved RFI mitigation that can be achieved in brightness observations through the use of digital receivers

Characterization of L-band RFI and implications for mitigation techniques

We describe measurements of radio frequency interference (RFI) in the 1200-1800 MHz band as observed from NASAs P-3 research aircraft during a flight along the Mid-Atlantic coast of the U.S. at altitudes of 2,000 and 20,000 ft. Both power spectra and coherently-sampled waveform data were obtained. Our results indicate that the spectrum below 1400 MHz is typically dominated by pulses from ground based radars, which, while very strong, individually have transmit duty cycles of on the order of 0.1%. We detect but are unable to identify some relatively weak intermittent RFI inside the 20 MHz protected band centered at 1413 MHz. We detect no significant RFI above 1420 MHz, but the limited sensitivity of this particular ex- periment makes it impossible to rule out the presense of RFI at levels damaging to total power radiometry. Im- plications for radiometer design, including possible active countermeasures for RFI mitigation, are discussed. I. Introduction Radio frequency interference (RFI) can impair the op- eration of radiometers operating at L-Band, even for sys- tems that operate in the protected 26 MHz frequency band around 1413 MHz. While bandwidths on the order of 100 MHz at L-Band are desirable for improving sensitiv- ity in soil moisture and ocean salinity applications, the use of active RFI mitigation techniques is required to achieve these bandwidths. To be effective, such techniques must take into account the spectral and temporal properties of the RFI. This paper presents some findings from an initial sur- vey of RFI in the frequency range 1200-1800 MHz, as ob- served from an airborne platform. The data were collected using a portable instrument known as the L-Band Inter- ference Surveyor/Analyzer (LISA), which was developed at the Ohio State University ElectroScience Laboratory in 2002. LISA includes two complementary subsystems: an off-the-shelf computer-controlled spectrum analyzer and a custom wideband high-dynamic-range coherent-sampling receiver. The former is useful for understanding the dis- tribution of RFI over large frequency spans and long time periods, whereas the latter provides waveform capture ca- pability with high temporal resolution. Our analysis of data captured during an initial test flight above the mid- Atlantic coast of the U.S. reveals some properties of the RFI which are helpful to know in the design of new air- borne and space-based radiometers, and especially in the development of mitigation techniques.

ASKAP Redback-3 — An agile digital signal processing platform

The third generation of high end Digital Signal Processing (DSP) platforms for the Australian Square Kilometre Array Pathfinder (ASKAP) is called Redback-3. It is a purely digital board consisting of three primary components; FPGAs for DSP, DRAM for bulk memory storage and optical communications for inter-board communications. The motivation and strategy for the third generation hardware is significantly different from its predecessors and this is largely reflected in its implementation. It is moving towards a more agile DSP platform for radio astronomy instrumentation and Software Defined Radio (SDR) applications. This paper describes the hardware and firmware in the context of the ASKAP beamformer and correlator subsystems.

Development of multi-stage filter banks for ASKAP

Radio astronomy instrumentation is continually seeking greater processing bandwidths whilst maintaining output frequency resolutions of order one kHz per channel. With the recent commercial availability of multi Giga sample per second analog to digital converters, instantaneous processing bandwidths of the order of hundreds of MHz to many GHz are now viable for radio astronomy applications. This is driving the design of more efficient filter-bank architectures to manage the significant frequency channelisation task. In this paper we present a new multi-stage filter-bank for the ASKAP instrument. We cover the filter-bank architecture, some of the design constraints governing its implementation, and considerations relating to the filter-bank outputs.