Jonathan Landon

Signal Processing for Phased Array Feeds in Radio Astronomical Telescopes

Relative to traditional waveguide feeds, phased array feeds (PAFs) for radio telescopes can increase the instrument field of view and sky survey speed. Unique challenges associated with PAF observations, including extremely low signal levels, long-term system gain stability requirements, spatially correlated noise due to mutual coupling, and tight beamshape tolerances, require the development of new array signal processing techniques for this application. We propose a calibration and beamforming strategy for PAFs including interference mitigation with power spectral density (PSD) estimation bias correction. Key efficiency metrics for single-feed instruments are extended to the array case and used to verify performance of the algorithms. These techniques are validated using numerical simulations and experimental data from a 19-element PAF on the Green Bank 20-m telescope.

Beamforming and imaging with the BYU/NRAO L-band 19-element phased array feed

An experimental 19-element L-band phased array feed was installed on the Green Bank 20-Meter Telescope in October 2007 and July 2008 to measure sensitivity and effciency and demonstrate signal processing algorithms for array calibration, multiple beam formation, imaging, and adaptive spatial filtering methods for interference mitigation. System noise performance was characterized using a warm absorber/cold sky Y-factor setup. The peak beam aperture effciency was 69% and the minimum beam equivalent system temperature was 66K. With a single reflector pointing, a high sensitivity image of a field of view approximately two half-power beamwidths in diameter can be produced. Measured figures of merit compare well to numerical simulations, indicating that complicating effects such as mutual coupling are understood well enough to enable the next phase of array feed development to proceed on firm grounds.

Model-Based Subspace Projection Beamforming for Deep Interference Nulling

This paper considers the problem of adaptive array processing for interference canceling to drive very deep nulls in difficult signal environments. In many practical scenarios, the achievable null depth is limited by covariance matrix estimation error leading to poor identification of the interference subspace. We address the particularly troublesome cases of low interference-to-noise ratio (INR), relatively rapid interference motion, and correlated noise across the receiving array. A polynomial-based model is incorporated in the proposed algorithm to track changes in the array covariance matrix over time, mitigate interference subspace estimation errors, and improve canceler performance. The application of phased array feeds for radio astronomical telescopes is used to illustrate the problem and proposed solution. Here even weak residual interference after cancellation may obscure a signal of interest, so very deep beampattern nulls are required. Performance for conventional subspace projection (SP) is compared with polynomial-augmented SP using simulated and real experimental data, showing null-depth improvement of 6 to 30 dB.

Design and Characterization of an Active Impedance Matched Low-Noise Phased Array Feed

An L-band low-noise phased array feed antenna was designed, fabricated, and characterized experimentally. The element design was optimized for an active impedance matching condition with a given set of nonuniform amplitude beamformer coefficients associated with a formed PAF beam. Using measured array S-parameters and modeled array element patterns, the predicted boresight beam equivalent system noise temperature and aperture efficiency at prime focus with uncooled low-noise amplifiers were 42 K and 61% at 1600 MHz. Experimental measurements of receiver noise using an array Y-factor technique matched modeled predictions. The array feed was mounted on the Arecibo radio telescope for on-reflector characterization. The measured boresight beam sensitivity figure of merit (Tsys/ηap) was 88 K and the 1 dB sensitivity bandwidth was 450 MHz.

Phased Array Feed Calibration, Beamforming and Imaging

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 a system noise temperature of 66 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.

Phased array antenna design and characterization for next-generation radio telescopes

Instrumentation for radio astronomical observations is currently undergoing a transition from large reflectors with single-pixel feeds to multi-pixel phased arrays. Research efforts aimed at realizing the benefits of phased arrays for highly sensitive, calibrated astronomical measurements have opened up interesting challenges for antenna designers. This paper surveys recent theoretical and experimental results on characterization of beam equivalent noise temperature and efficiency, impedance matching to minimize front end noise, and beamforming and calibration algorithms for phased array feeds.

Towards a high sensitivity cryogenic phased array feed antenna for the Green Bank Telescope

Efforts have been underway for the last several years to develop phased array feed antennas for large reflectors such as the Green Bank Telescope (GBT) as well as mid-size reflectors for Square Kilometer Array demonstration instruments. We report on recent work on a cryogenic L-band phased array feed with the goal of reducing the beam equivalent system noise temperature so that it is competitive with that of traditional horn type feeds.

Active impedance matching, calibration, and interference mitigation for the BYU/NRAO L-band phased array feed

This paper is a summary of recent research progress towards scientifically useful low noise, high sensitivity phased array feeds for radio telescopes. Developments include techniques for designing phased array antennas to realize an active impedance matching condition that minimizes receiver noise over the array field of view, array beamforming algorithms that allow a controlled tradeoff between side-lobe control and peak beam sensitivity, and spatial filtering algorithms that achieve unprecedented levels of interference suppression for moving sources of unwanted radio signals.

Modeled and measured mutual impedances, element patterns, and sensitivity for a 19 element focal plane array

The dipole array has an approximate analytical electrical model, which facilitates the development and testing of signal processing algorithms for the FPA. To verify the accuracy of the analytical approach, we have also modeled the array using the finite element method (FEM). In this paper, we compare the approximate analytical and FEM models to preliminary experimental measurements. Results are given for mutual and self impedances, element patterns for the bare array (no reflector antenna), and the sensitivity of the FPA on the Green Bank 20 Meter Telescope.