0912 0204v1

0912 0204v1

(Parte 1 de 6)

arXiv:0912.0204v1 [astro-ph.IM] 1 Dec 2009

Phased Array Feed Calibration, Beamforming and Imaging

Jonathan Landon, Michael Elmer, Jacob Waldron, David Jones, Alan Stemmons,

Brian D. Jeffs, Karl F. Warnick

Department of Electrical and Computer Engineering

Brigham Young University 459 Clyde Building, Provo, UT 84602, USA

J. Richard Fisher

National Radio Astronomy Observatory1 Charlottesville, VA, USA

Roger D. Norrod

National Radio Astronomy Observatory1 Green Bank, WV, USA

Phased array feeds (PAFs) for reflector antennas offer the potential for increased reflector field of view and faster survey speeds. To address some of the development challenges that remain for scientifically useful PAFs, including calibration and beamforming algorithms, sensitivity optimization, and demonstration of wide field of view imaging, we report experimental results from a 19 element room temperature L-band PAF mounted on the Green Bank 20-Meter Telescope. Formed beams achieved an aperture efficiency of 69% and system noise temperature of 6 K. Radio camera images of several sky regions are presented. We investigate the noise performance and sensitivity of the system as a function of elevation angle with statistically optimal beamforming and demonstrate cancelation of radio frequency interference sources with adaptive spatial filtering.

1. Introduction

The generationof radio astronomy instrumentsnow coming on line will fill much of the observational parameter space—frequency coverage, instantaneous bandwidth, system noise temperature, angular resolution, and time and frequency resolution. The two remaining open-ended parameters where fundamentally new science will be explored are collecting area and field of view. More collecting area will allow us to observe known phenomena much deeper in the universe, and greater fields of view open the possibility of finding new phenomena, such as transient radio sources

1The NRAO is operated for the National Science Foundation

(NSF) by Associated Universities, Inc. (AUI) under a cooperative agreement.

and rare types of pulsars, by making large-area sky surveys much more efficient. Several major instrument development projects are underway worldwide to develop radio cameras, variously referred to as active, phased, beamforming, or smart arrays to distinguish them from the more conventional independentpixel feed-horn arrays which sample less than 1/16th of the available sky area within the array’s field-ofview. These instruments will utilize focal plane phased array feeds (PAF) which can electronically synthesize multiple, simultaneous far field beams for complete coverage of the field of view without loss of sensitivity in each beam. PAFs are in development or in planning stages for single dish instruments including the Green Bank Telescope (GBT) as well as the Westerbork Synthesis Radio Telescope (WSRT)

(Verheijen et al. 2008) and the Australian Square Kilometer Array (ASKAP) Pathfinder synthesis imaging array (S. G. Hay et al. 2007).

As the cost of signal processing for beam-forming and the correlation of multiple beams in aperture synthesis arrays allows, wider fields of view for SKA and its predecessors will open up new science, such as surveys of neutral hydrogen at high redshifts to study the evolutionary history of the universe. First science that will be enabled by PAFs with modest signal processing bandwidthsinclude observationsof the trajectories and abundance of high-velocity HI clouds interacting with the disk of the Milky Way (Wakker & van Woerden 1997; Putman 2006; Lockman et al. 2008), surveys of gas clouds outside other galaxies covering large areas (D. A. Thilker et al. 2004; Braun & Thilker 2004; M. Grossi et al. 2008), and studies of the kinematics of extended HI clouds contained by galactic groups (S. Borthakur et al. 2008; Braun & Thilker 2004; et al. 2008a). These types of observations are time consuming with single-pixel instruments and could be conducted more rapidly and to greater depth using a PAF with wide field of view.

Demand for use of the GBT to study the inner parts of the Milky Way disk is extremely high. Wider fields of view would enable additional studies of the galactic center and objects such as the physical conditions and structure of the recently discovered “far” counterpart to the 3-kpc arm (Dame & Thaddeus 2008). A PAF on the GBT would be of immediate use in analyzing the complex chemistry and relationship to dust evolution of the diffuse interstellar medium using 18cm emission from OH, and in understanding the medium to minimize the effect of foreground matter on cosmological investigations (Black & Dalgarno 1977; Gilmon & Shull 2006; Liszt & Lucas 1996; et al. 2008b).

As compared to single-pixel instruments, PAF beam formation requires a substantial amount of signal processing for array calibration, operational beamforming, and image formation. Calibration and beamforming algorithms and performance criteria for array feeds were surveyed by Jeffs et al. (2008b). Radio frequency interference (RFI) mitigation using adaptive nulling algorithms was demonstrated by Nagel et al. (2007). Other recent results demonstrating feasibility of PAF-based interference mitigation in astronomical applications include (Jeffs & Warnick 2008, 2007; Jeffs et al. 2008b; Landon et al. 2008a,b; Warnick et al. 2007).

Fig. 1.— 19 element single polarized PAF and front end box containing analog downconverters and receiver front ends, mounted on on Green Bank 20- Meter Telescope (October, 2007).

In this paper, we present results from an L-band prototype PAF on the Green Bank 20-Meter Telescope. Early radio camera images obtained with the PAF were reported by Warnick et al. (2008). A more detailed study of performance metrics including sensitivity and system noise are given here. A major concern with PAF development has been optimizing the interface between the array and front end receiver electronics (Warnick & Jensen 2007; Ivashina et al. 2008). Due to mutual coupling between array antenna elements, standard impedance matching techniques used for single-port antennas must be extended to multiport systems using the theory of active impedances (E. E. M. Woestenburg 2005; Warnick et al. 2009b). The performance improvement that could be realized using these methods are explored.

PAFs offer the potential for optimizing beam patterns to improve sensitivity as the noise environment changes or to maximize system performance for a given type of observation. We study the dependence of sensitivity and efficiency of formed beams on the reflector tipping angle as sky noise increases near the horizon. Results of RFI mitigation experiments are also shown, in order to demonstrate the potential for operational use of RFI nulling algorithms in observations. These results are strong evidence for the feasibility of a high sensitivity, wide field of view PAF.

2. Experiment Description

The characterization techniques, calibration methods, and beamforming algorithms used in the reported

Fig. 2.— Ground shield and PAF in sky noise measurement facility with absorber being lowered over the array for Thot measurement measurement(July, 2008).

Fig. 3.— Dipole element diagram.

work were developed with a prototype 19 element array (Fig. 1) of single polarized, thickened dipoles backed by a ground plane, with a 20 channel noncryocooled receiver and data acquisition system for real time sampling and streaming to disk. The array was initially characterized using the BYU “Very Small Array” 3-meter prototype platform and the NRAO antenna range in Green Bank, WV. In October 2007 and July 2008 the prototype PAF was mounted on the Green Bank 20-Meter Telescope to measure aperture and spillover efficiency, demonstrate multiple beam formation, and test RFI mitigation algorithms.

2.1. Array Feed and Data Acquisition

The PAF was located at the focus of the Green Bank 20-Meter 0.43 f/D reflector. Array elements were balun-fed half-wave dipoles tuned to 1600MHz λ/4 above a ground plane. The dipoles achieved a 570 MHz impedance bandwidth (-10 dB reflection coefficient) or a 35% bandwidth relative to the center frequency, which is modest but adequate for initial tests. The elements are arranged in two concentric hexagonal rings around a center dipole. The hexagonal grid permits 0.6λ spacing between dipoles, which is slightly farther apart than the typical λ/2 for a rectangular grid. This increased inter-element spacing reduced mutual coupling without undersampling or producing grating lobes. A diagram of the dipole element is shown in Figure 3. Additional details were given by Nagel et al. (2007).

The dipoles were connected through the array ground plane to uncooled LNAs (Ciao Wireless, Inc., Camarillo, CA) with 3 K noise temperature and 41 dB gain at 1600 MHz. The LNAs were measured by L. Belostotski (U. Calgary) to have the following noise

Receivers in the front-end box at the dish focus performed a two-stage downconversion to a final IF centered at 2.8125 MHz with a 3 dB bandwidth of 425 kHz. IF signals were fed by coaxial cable into the pedestal room of the telescope to a data acquisition system in a shielded rack. IF voltages were synchronously baseband-subsampled using 12-bit quantization for high dynamic range at a sample rate of 1.25 Msamples/sec per channel for 20 channels, using five commercial data acquisition cards with four channels per card. A single network server class PC housed all acquisition cards and a high speed array of hard drives for real-time data streaming to disk. Beams were formed in post-processing on the raw voltage samples.

2.2. Isotropic Noise Measurement Facility

Measurements of the array isotropic noise response can be used to determine the aperture efficiency and system noise temperature associated with formed beams (Warnick & Jeffs 2008). The array was mounted in a facility with a retractable roof to produce a noise field that is approximately isotropic at the temperature of cold sky over the array’s significant response directions. A copper screen connected to the

Fig. 4.— The primary advantage of FPA telescopes is increased field of view provided by multiple, simultaneously formed beams. Spatial cancelation of interfering signals is also possible.

array ground plane at a slope of 30◦ provided partial shielding of the array from noise radiated by ground, horizon, and nearby buildings. This provided the cold load for a Y -factor array isotropic noise temperature measurement. For the hot load, a 2.4m × 2.4m sheet of RF absorber was lowered over the copper screen (Fig. 2). The computed array output voltage correlation matrices for the two configurations were used to characterize the system noise for the results given in Section 5.1.

3. PAF Beamforming

In this section we discuss principles and methodologies for practical beamforming with an astronomical phased array feed instrument. Figure 4 illustrates how digital beamforming techniques enable a PAF to form multiple simultaneous beams to increase the instantaneous field of view for the telescope.

3.1. Signal Model

We assume that the array output signals are processed in narrow subbandssuch that B D/c, where B is bandwidth, D is the PAF diameter, and c is the speed of light. After downconversion and sampling, the M element PAF produces a length M ×1 complex baseband data vector at time sample n: x[n] = as[n] + n[n] (1) and where s[n] is the signal of interest (SOI) and n[n] is the total system noise vector seen at the array. In the presence of interference, n[n] will also include components due to a man-made detrimental signal d[n] as illustrated in Fig. 4. Vector a is the normalized array response to a unit amplitude point source in the far field direction corresponding to s[n].

Assuming zero mean wide-sense stationary signals and statistical independence between distinct signal and noise sources the array covariance is

Rs = σ2saaH Rn = Rrec + Rsp + Rsky + Rloss + Rint (3) where σ2s is SOI signal power (variance), and Rrec,

Rsp, Rsky, Rloss and Rint are receiver, spillover, sky, antenna resistive loss, and interference noise covari- ance matrices, respectively. Rsky refers to non-SOI noise sources in the beam main lobe, including at- mospheric,cosmic microwavebackground(CMB) and galactic background (GB) radiation. Models for each of these terms, including their spatial structure across the array and effective noise temperatures seen at the beamformer output, will be introduced in Section 4. Except in Section 7 where we consider interference canceling, we will assume Rint = 0.

R is estimated for the jth short term integration (STI) window with sample covariance matrix n=jL where Xj is the L sample long STI data window. The required length L depends on the signal scenario and desired operational mode. If adaptive cancelation is used to suppress moving interference as discussed in Section 7, beamformer weights (computed from R(j)) must be updated rapidly with L short enough that Rint doesnot changeoverthe STI window. A slower update rate, and thus longer L, is used to adapt to spillover and sky noise due to pointing changes, as discussed in Section 6. Finally, in stable signal conditions, L can be very long (on the order of minutes or hours) to reduce sample estimation error and yield accurate calibration or high sensitivity beamformer solutions. In these cases superscript (j) will be dropped for simplicity when this does not lead to an ambiguous interpretation.

3.2. Calibration

Since multiple simultaneousbeams are formedwith a PAF as shown in Fig. 4, a calibration for the sig-

nal array response vector ai must be performed for each direction, Ωi, corresponding to each formed beam’s boresight direction, and any additional direc- tions where point constraints in the beam pattern response will be placed. Periodic re-calibration may be necessary due to strict beam pattern stability requirements, to correct for differential electronic phase and gain drift, and to characterize changes in receiver noise temperatures.

We proposed a calibration procedure in (Jeffs et al. 2008b), which is improved here with a lower error, noise whitening approach in step 3.

Calibration Procedure:

1. Noise covariance, Rn: Steer the dish to a relatively empty patch of sky so x[n] ≈ n[n], and collect a long term (large L, e.g. 10 minutes) sample covariance estimate for the noise field

(Parte 1 de 6)