IAA-01-IAA.9.1.02 ARRAY2K: MULTIPLE DISHES, MULTIPLE MODES H. Paul Shuch, Ph.D. Executive Director, The SETI League, Inc. PO Box 555, Little Ferry NJ 07643 USA ABSTRACT THE NEED FOR MULTIPLE MODES For the past two years, the SETI community has It has long been recognized by those skilled in the marveled at the development of the ambitious Paul Allen art that multiple antennas can be combined together for Telescope, a mini-Cyclops consisting of up to a thousand increased receiver performance. The advantages are phased satellite TV-type dishes. While saluting the numerous and well known. Various forms of prior art efforts of our California colleagues, The SETI League technology exist for combining the antennas. has been hard at work on its own phased array design, For example, in astrophysical research and the more modest in scope but quite as technologically electromagnetic Search for Extra-Terrestrial Intelligence audacious. When completed, Array2k will employ a (SETI), it has been the common practice to combine unique mix of analog and digital techniques to operate in multiple dish antennas into an array, optimized to five distinct modes simultaneously. Optimized as a drift- produce a specific beam geometry. Beam geometries scan sky survey instrument in the proud tradition of Ohio tend to be highly application-specific. For example, drift- State's Big Ear, its multi-mode capability will enable scan SETI receiving stations are best served by an Array2k to serve as its own Follow-Up Detection antenna pattern that is somewhat broader in the Device, verifying its own findings in real time. declination axis than it is in right ascension. This very type of beam pattern was implemented by the late Ohio INTRODUCTION State University "Big Ear" radio telescope, circa 1964 – 1997, which was one of the great pioneers in SETI. The SETI League, Inc. launched its Project Argus Total power studies of the galactic core favor an opposite all-sky survey in April 1996, with the ambitious goal of antenna pattern (that is, a geometry which is broader in real-time all-sky coverage (Shuch, 1997). Our right ascension than it is in declination). Targeted experience in implementing a global network of small searches of individual stars and quasi-stellar objects radio telescopes (Shuch, 2000) has underscored the require a spot beam, narrow in both planes. importance of developing larger scale telescopes with At this time, in order to achieve a given beam improved sensitivity. Due to negative economies of pattern distinct, application-specific arrays of antennas scale, we early decided to explore the arraying of a are used. Obviously this approach has limitations when quantity of the very type of antennas used in the current funds are limited, and only one array is practicable. Project Argus network -- that is, extrapolating from our Alternatively, antennas may be physically relocated. area of greatest expertise. Obviously this approach is difficult, and sometimes The technological breakthroughs described here may proves impractical. For example, the twenty-seven dish be applied generally to radio astronomy, and the antennas at the 78 million dollar Very Large Array microwave antenna arrays and systems utilized in such (VLA) in Socorro New Mexico each weigh 230 tons. To installations. More particularly, the present invention change this array between operating configurations, each describes a multi-dish antenna array primarily adapted of its dishes is moved along approximately thirty miles for astrophysical research and the Search for Extra- of railroad track. Terrestrial Intelligence (SETI). We have named our It has long been accepted that diverse beam proposed antenna Array2k, not for the year past, or the geometries tend to be mutually exclusive. An adaptive much-feared computer crisis, but rather in recognition of antenna array, one that can operate in multiple geometric its 2,000 square feet of collecting area, which can be modes simultaneously, would be highly advantageous. expected to yield performance equivalent to that of a single 50-foot dish, but at perhaps a tenth the cost. REVIEW OF PRIOR ART _____________________________________________ The advantages gained by combining multiple Copyright © 2001 by H. Paul Shuch. Published by the antennas into an array are well known, and fall into two American Institute of Aeronautics and Astronautics, Inc., broad categories: (a) improving sensitivity, and (b) with permission. Released to IAF/IAA/AIAA to publish improving resolution. The two most common ways of in all forms. connecting multiple antennas into an array are (a) as a 1 radiometer, and (b) into correlation detectors. (Burke simultaneously. The multiple operating modes and Graham-Smith, 1997). envisioned will allow the array to achieve both high In the case of the radiometer connection, a single sensitivity and high angular resolution, allowing it to detector is connected to all of the antennas in the array fulfill a variety of research objectives. via a branched feedline, which maximizes sensitivity by producing a single beam. The best known (though never ARRAY2K DESIGN OBJECTIVES: implemented) example of this configuration is Project Cyclops (Oliver et. al., 1973). Array2k is an array of small, dish antennas all In an interferometer (Ryle, 1952) resolution is interconnected to accomplish specific beam patterning. improved by combining the signals of two antennas, As initially envisioned, the array comprises 16 individual which are separated by a specified distance (called the parabolic dish antennas, each four meters in diameter. baseline). With dish antennas, the resulting gain is Four sub-arrays, each with four individual antennas, are simply that which would be achieved by a single dish established in a cross-like formation, with one sub-array with a surface area equal to the sum of that of the two each running north, south, west and east of the array's antennas. However, the angular resolution of such an phase center. interferometer is equivalent to that of a single dish with a We propose a means for electronically changing a diameter equal to the baseline. Thus, interferometers complex multiple-antenna array into different provide a modest improvement in sensitivity with a configurations for producing different beam patterns. In much greater increase in resolution. other words, radio signals derived from the sub-arrays A multiple-antenna interferometer array may be can be analog-processed and combined into control constructed using a technique known as aperture signals that are useful for generating steering parameters. synthesis. Each possible pairing of antennas in the array Control signals are synthesized through a combination of is accomplished by applying the outputs of the antennas analog signal quadrature techniques, combined with to a multitude of correlator circuits. The correlator digital conversion and software correlation. As different outputs may be combined to produce multiple beams, individual antennas forming each sub-array monitor at making it possible to image distant astrophysical objects least portions of overlapping sky viewed by others, with high levels of detail. quadrature processing of signals derived from each Well-known multiple-antenna interferometers individual antennas can be processed not only to yield include the Very Large Array (Napier et. al., 1983) and the composite observed target sought by the radio the Giant Meter-Wave Radio Telescope (Swarup et. al., telescope, but can be correlated to generate the required 1991). Both of these arrays arrange their antennas (27 in steering parameters to observe the desired beam patterns. the case of the VLA; 30 at the GMRT) in a "Y" configuration with extremely wide baselines, and use The basic objectives of this design are: digital correlators to combine the signals from the multiple dishes. 1. to provide an adaptive antenna array system The Mills Cross arrangement (Mills, 1963) consists which can operate in multiple geometric modes of two line-type antennas, one oriented North-South and simultaneously. the other East-West. The former antenna produces a 2. to provide an adaptive antenna array of the beam pattern which is narrow in declination and broad in character described that can achieve a beam right ascension. The latter produces a beam pattern pattern that is broader in the declination axis which is broad in declination and narrow in right than in right ascension, and at the whim of the ascension. When signals from the two antennas are user, be quickly changed to a beam pattern that combined, a beam is produced which is narrow in both is broader in right ascension than in declination. axes. Bracewell and Swarup (1961) produced an array 3. to utilize sensed antenna data and parameters to of 32 small parabolic dish antennas, oriented in a Mills develop steering signals for the array. Cross, to produce a pencil-beam interferometer with 4. to aid drift-scan SETI receiving stations by micro-steradian resolution. deriving antenna patterns that are somewhat All of the antenna arrays described above achieve broader in the declination axis than in right stated design goals of high sensitivity or high angular ascension. resolution. In each case, one and only one of these 5. to provide an electronic means of creating a design objectives can be achieved, and invariably at the beam geometry broader in right ascension than expense of the other. it is in declination. The present invention resembles the Bracewell and 6. to be able to electronically convert a radio Swarup array in physical configuration. Unique circuitry telescope and switch it between beam patters. is added to allow it to operate both as a total-power 7. to provide an array that can be switched to a radiometer, and as a correlated interferometer, spot beam, that is narrow in both planes, to aid 2 in the study of individual stars, quasi-stellar FEED ASSEMBLY: objects, and other deep space targets. 8. to simplify the changing from one beam pattern Figure 3 shows the prime-focus feed assembly to another.