The FASR Reference Instrument

1. The FASR Reference Instrument Tim Bastian (tbastian@nrao.edu)1, Dale E. Gary(dgary@njit.edu)2, Steven M. Gross (smgross@umich.edu)4, Gordon J. Hurford(ghurford@ssl.berkeley.edu)3, Hirofumi Kawakubo (kawakubo@umich.edu)4,Christopher S. Ruf (cruf@umich.edu)4, Stephen M. White (white@astro.umd.edu)5, Thomas H. Zurbuchen (thomasz@umich.edu)4 1National Radio Astronomy Observatory, 2Center for Solar-Terrestrial Research, New Jersey Institute of Technology, 3Space Sciences Laboratory, University of California, Berkeley 4Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, 5Department of Astronomy, University of Maryland, College Park 2006 Joint Assembly Meeting May 23-26, 2006 SH33A-08 Abstract The Frequency Agile Solar Radiotelescope (FASR) is a unique, solar-dedicated radio facility slated for completion by 2012. The instrument will address an extremely broad range of solar and space weather science, including routine measurement of coronal magnetic fields, imaging coronal mass ejections near the solar surface, quantitative diagnostics of energy release and particle acceleration in flares, and the extension of the solar corona into the heliosphere. Although the precise details of the FASR design are still being developed, we present the high-level design referred to as the FASR Reference Instrument. The Reference Instrument meets the science requirements and will serve as the basis for cost estimates for construction and operation of the instrument. This paper gives an overview of the FASR Reference Instrument, describes the science goals and objectives, and gives the flow-down of science goals to engineering specifications. The innovative aspects of the FASR design are highlighted, as are prototyping and test activities. A summary of the current status is given. III. Antenna Configuration FASR will comprise three arrays of antennas – A, B, and C – that must each image the Sun with high dynamic range and fidelity over bandwidth ratios of a decade. This suggests the use of scale-invariant configurations that yield a power-law distribution of antenna baselines. One such configuration is the log-spiral (Conway 1998, ALMA Memo 216). In Fig. 3, we show an illustrative example of a log-spiral configuration with parameters suitable to FASR A (3-30 GHz) sited in the southwest United States. In particular, 100 antennas are distributed in three log-spiral arms to a maximum baseline of 3 km (Fig. 3). To illustrate the imaging properties of this array, we use model brightness distribution based on images from the TRACE satellite, which have an angular resolution of 1”. The example shown in Fig. 5 is a 171 A full disk mosaic was from 2000 Sep 11 (orange). Using the AIPS software package, the TRACE image was Fourier transformed and sampled by the model array to produce model visibility data. The simulated data base corresponds to 2 hrs of time and bandwidth synthesis of 10%. We show the resulting “dirty” synthesis maps (4096 x 4096 pixels) at 5 GHz (green) and at 15 GHz (blue); that is, no deconvolution of the instrument response function from the raw inversion of the visibility data has been performed. TRACE 171 A Figure 1: FASR Block Diagram I. FASR Prototyping Several prototyping efforts are underway, including the FASR Subsystem Testbed (FST) shown in Figure 2. Signals from three of the antennas of the Owens Valley Solar Array are passed through a down-converter, digitized at 109samples per second, and recorded to a PC for offline software correlation. This enables 500 MHz bandwidth of digitized data, tunable throughout a 1-9 GHz range, to be filtered, cross-correlated and otherwise digitally processed to simulate multiple versions of the FASR digital processor design in a realistic environment. The goals of FST are to study radio frequency interference (RFI) mitigation techniques, the use of satellite signals for calibration, analog phase stability issues, and so provide a testbed for the design of FASR’s digital processing system. In its present form, the FST can also observe solar bursts with unprecedented time and spectral resolution. FASR Image 5 GHz (6 cm) FASR Image 15 GHz (2 cm) Table 1: Science Requirements and Goals Figure 5: Simulated FASR A images VI. Current Status FASR development and planning activities are underway at a number of partner institutions, including the NRAO, NJIT, University of Michigan, University of Maryland, and University of California, Berkeley. The work is supported by a FASR Design and Development Plan grant from NSF/ATM, as well as through the NSF MRI and ATI programs. Activities include definition of the FASR Reference Instrument, software and data management planning, operations and maintenance planning, site evaluation and array configuration studies, and science planning. In the coming year, R&D will be directed toward ultra-wideband front ends, data transmission, and RF conversion. The details of the digital backend and correlator will also be refined. Science simulations will be directed toward key science goals and developing the inversion tools to recover key observables from FASR data. It is anticipated that the proposal for construction will be prepared in 2007-2008. II. Digital Back-End and Correlator The FASR correlator (See Fig 1) is an FX frequency decomposition correlator, providing 500 MHz of instantaneous bandwidth. A single Correlator Unit services FASR A, B, and C in a programmable time-sharing mode. It provides frequency resolution of 1% and time resolution of 100 ms across the 50 MHz to 20 GHz frequency range . The correlator supports up to 128 antennas per band, each with 2 polarization channels. This translates into as many as 8128 antenna baselines processed simultaneously. Since much of FASR’s wide frequency range lies outside of protected bands, potential radio-frequency interference (RFI) from aircraft, satellites and fixed and mobile communications systems must be identified and removed., The FASR correlator uses real-time statistical analysis of received signals to distinguish between natural and man-made signals. The basis for the distinction is the Gaussian nature of natural signals which implies a Kurtosis of 3.0, whereas that of manmade signals deviates from this value (see Ruf et al. 2006, IEEE Transactions on Geoscience and Remote Sensing, Vol. 44, Issue 3, 694). We are testing the feasibility of the RFI detection and mitigation methods by analyzing the data from the FASR Subsystem Testbed (Fig 4.). This correlator will be implemented in either commercial field-programmable gated array (FPGA) or application-specific integrated circuit (ASIC) technology. a) Antenna Layout b) u-v configuration a) b) c) c) GPS Satellite signal on three antennas c) Beam pattern d) u-v configuration of inner region d) a) The FASR Subsystem Testbed (FST) is attached to three of the antennas (b) of the Owens Valley Solar Array Table 2: FASR Reference Instrument Specifications Figure 3: Antenna Array Analysis d) Phase plot showing first fringes on a GPS satellite • For more information • http://www.ovsa.njit.edu/fasr Figure 4: RFI Detection and Mitigation Figure 2: FASR Subsystem Testbed at Owens Valley Radio Observatory