SUMMARY OF POSSIBLE USES OF AN INTERSTELLAR INTRODUCTION Radio astronomical investigations of great scientific interest can be carried out with the wide range of SETI antenna systems presently under discussion. This range includes both the SETI programs planned for the near future with existing antennas, and larger ground-based or spaceborne antenna systems that might be built in the future. The effect of SETI technology on radio astronomy can be broadly broken down into two classes. In the first class we have the extension or improvement of existing microwave technology: 1. Receiver design: A class of receivers to be developed for SETI is characterized by near optimum noise figures (~10 K at a room temperature waveguide flange), broad instantaneous bandwidth (300 MHz), and octave bandwidth tuning ranges. This technology will probably be rapidly adopted by radio observatories so that the possession of such receivers will not make SETI systems unique, but would be a SETI spin-off. 2. Collecting area: Eventually, a SETI receiving system may vastly surpass radio astronomy facilities, existing or projected, in collecting area. (Compare the VLA at twenty-seven 25-m antennas to even two 100-m antennas.) In the second class of SETI impact we have the development of a new generation of signal processing facilities: 3. Data Processing Hardware: On the basis of SETI requirements, it is possible to predict the general properties of such hardware. To make a microwave search tractable, it will be necessary to utilize fully the entire receiver bandwidth (300 MHz) while retaining high spectral resolution: a processor of 106 channels. An integrating spectrometer with such characteristics is an impressive scaling-up of present radio astronomy technology, but a significant development is called for when we admit that we have no a priori knowledge of the nature of SETI signals. Then, we require a fully flexible data processing system that can measure all properties of a signal (e.g., frequency distribution, time structure, polarization) continuously in real time. The ability to fully characterize radio signals offers hope for recognizing and rejecting various kinds of interference. If the processor is eventually to be used with an antenna array, the ability to operate several array subsets or several array beams simultaneously would be very useful. 4. Data Acquisition Management: The management of a 106 channel signal processor and the extraction and sorting out of various kinds of scientific data in real time represents another breakthrough area. The following kinds of data are some that need to be managed, preferably simultaneously: Time averaged spectrum (106 channels; recognize and extract spectral lines for astronomy; Doppler compensate for space motion of observatory) Dynamic spectrum (106 channel with high time resolution; search for Doppler patterns perhaps characteristic of ET transmitters; recognize interfering signal patterns; e.g., solar bursts in sidelobes, satellite transmitters, etc.) Polarization (four Stokes parameters in 106 channels needed for signal recognition; i.e., a weak, narrow, unpolarized signal would probably be an unknown spectral line rather than an ET signal; an ET signal might be polarization modulated) Dispersion removal (an ET signal might consist of broadband pulses that would be dispersed in the interstellar medium) Thus, this second class of SETI impact would represent a major new way of handling data. It would permit astronomers to engage in survey projects of a scope that has only been attempted a few times in the past, and then only with a large dedication of scientific manpower. In this capacity, a SETI system would likely be unique for a considerable period of time. As SETI activities widen in scope and increase in sensitivity, the utility of SETI facilities for radio astronomical investigations will surely increase. In particular, aspects (2-4) above present enormous potential for improvement. It is also apparent that immediate SETI efforts utilizing currently achievable advances along the lines of (1) and (3) will yield new results of significant radio astronomical interest. This complementary document discusses specific scientific benefits that would arise from SETI efforts. This treatment is by no means exhaustive. For instance, serendipity is a vital factor attendant to any major leap in instrumentation. It is, however, impossible to discuss benefits that derive from new and unexpected discoveries. As in the case of the 200-in. Hale Observatories telescope, there will surely be many that derive merely from each significant increase in collecting area. In addition, extensive sky and frequency coverage with high frequency resolution (several Hz, or ~0.001 km sec1 at 1.5 GHz), wide instantaneous bandwidth (~300 MHz), and possible sensitivity to pulsed signals will surely result in new discoveries of scientific importance. Remaining within the domain of foreseeable scientific benefit, we present here likely applications of several near- and far-term SETI systems to radio astronomy. Three different scales of system complexity are represented: an optimally equipped single 26-m antenna, the equivalent of the full Cyclops array of 1026 antennas each of 100-m diameter, and an intermediate case. POTENTIAL SCIENTIFIC APPLICATIONS OF A 26-METER SETI SYSTEM1 Astronomical investigations of individual radio sources have achieved higher sensitivity levels than could be obtained with a 26-m antenna in a survey program. As a survey instrument, however, a 26-m SETI facility with an optimum front end compares favorably in sensitivity with surveys that have been done, but with the added advantages of higher spectral resolution, greatly expanded frequency coverage, and complete coverage of the visible sky (see Section I-2). In considering potential programs, we will assume a minimal sensitivity system consisting of a 15 K system on a 26-m antenna. We will assume a dual-polarization receiver, although the same sensitivity can be achieved with a single-polarization system operating twice as long. In comparing various observations, we will compute the minimum detectable flux density from where S = flux density, Jy; K = detection limit factor≈5; KR = receiver mode factor, √ for receiver switching, π/2 for autocorrelation spectrometer, √ if 7 includes both on and off measurement; T, = system temperature, K; D = antenna diameter, m; B = bandwidth, Hz; 7 = integration time, sec; and ng = antenna aperture efficiency. For studies of extended objects, such as the larger interstellar clouds, the antenna resolving power may not be an important factor. For each sky position, we have a minimum detectable brightness temperature of is the beam efficiency. A higher sensitivity can be achieved by averaging adjacent sky positions so that the effective beamwidth is larger. where nB Radio Source Surveys from SETI: Number-Flux Density Relationship A natural consequence of the SETI program will be a number of very sensitive radio source surveys over the frequency range 1.4-23 GHz covering all the visible sky. The sensitivity that will be achieved in the constant beamwidth surveys extends beyond the confusion limits for nearly all frequency intervals which would be observed with the 26-m telescope. Thus it will be possible to 1This section includes contributions from S. Gulkis, M. Janssen, T. Kuiper, and E. Olsen of Jet Propulsion Laboratory, Pasadena, Calif. |