NASA's
High
Resolution
Microwave
Survey

Targeted Search and Sky Survey Status
Quarterly Report -- June 1993

BACKGROUND

The High Resolution Microwave Survey (HRMS) is part of the Toward Other Planetary Systems (TOPS) program in NASA's Solar System Exploration Division. The HRMS searches for evidence of planets orbiting other stars through radio emissions that may be produced by technological civilizations. The HRMS has two search modes, a Sky Survey and a Targeted Search. The Sky Survey is managed by the Jet Propulsion Laboratory and uses 34-meter antennas in NASA's Deep Space Network to sweep the entire sky over a wide range of frequencies for the presence of strong signals. The Targeted Search uses the largest available radio telescopes to observe nearby Sun-like stars over a narrower range of frequencies for weak signals. The Targeted Search is managed by NASA's Ames Research Center, which is also the lead center for the HRMS. The combination of the two search modes is millions of times more comprehensive than the sum of all previous search programs. The observational phase of the HRMS was inaugurated at 1900 UT on 12 October 1992 at the NASA Goldstone Deep Space Communications Complex in California and the Arecibo Observatory in Puerto Rico. The Arecibo Observatory is part of the National Astronomy and Ionosphere Center operated by Cornell University for the National Science Foundation. In a coordinated initial observation program, the Arecibo antenna pointed at the star Gliese 615.1A and the Goldstone antenna began to scan the area of sky that included the targeted star. This report presents an overview of the observations, project activities, and results to date.

THE OBSERVATIONS

Sky Survey

Initial observations began using the Sky Survey Prototype System (SSPS) with the new 34-meter antenna and an existing low noise microwave receiver at the Venus Development Station at Goldstone. The SSPS spectrum analyzer divides a single-polarization, 40 MHz wide radio frequency band into slightly more than two million channels of 19 Hz resolution. The SSPS observatory control software drives the antenna very rapidly in a precision scan pattern lasting about 90 minutes. The pattern systematically covers 1.4 degrees high by 30 degrees wide rectangles, called "skyframes," which are fixed on the celestial sphere. The skyframes therefore appear to move across the sky as the Earth rotates. While observing, the SSPS real time signal detection subsystem excises channels contaminated by terrestrial signals and stores data from uncontaminated channels whose power exceeds a specified threshold. The scan pattern is designed so that each point in a skyframe will be scanned by the antenna at least twice (with slightly different position offsets) separated by about eight minutes in time. At the completion of each skyframe, the SSPS post processing subsystem analyzes the stored data and selects 20 candidates for the first level verification tests. These tests employ the SSPS in a different mode to perform more sensitive and localized reobservations.

Forty-six single-polarization skyframes have been completed (i.e., both the initial skyframe and candidate reobservation phases were performed). To date, a single event has reappeared in a first level verification reobservation, albeit at a much weaker power level than in the original skyframe. The event did not survive additional verification tests, and the appearance of one such false alarm in 46 skyframes is consistent with statistical expectations arising from small fluctuations caused by the thermal noise of the receiver itself. The SSPS is now observing for approximately 30-40 hours per week on the 34-meter Goldstone antenna.

The SSPS has completed a series of special observations on a nearby 26-meter antenna at lower microwave frequencies. These observations were designed to enhance radio astronomy spinoffs from ongoing Sky Survey operations and to improve interference excision algorithms. Three skyframes covering half the galactic plane visible from the northern hemisphere were repeatedly observed in frequency bands that include the natural emission lines produced by hydrogen atoms and hydroxyl radicals (OH). Results of these observations, including maps of the detected emission, will be published later this year.

Work continues at JPL on the development of the Sky Survey Operational System (with sixteen times the bandwidth of the SSPS) and of the wide band receiver systems to be used by both search modes.

Targeted Search

The Targeted Search System (TSS) used the Arecibo 305-meter antenna, the world's largest radio telescope, during its initial campaign to observe 25 stars within 100 light years of the Earth. Receivers provided by the observatory allowed observations in four frequency bands covering a total of about 300 MHz within the range from 1300 MHz to 2400 MHz. The TSS processes a dual-polarization 10 MHz wide radio frequency band into more than 14 million channels, each only 1 Hz wide. It simultaneously processes the same band into channels with 7 Hz and 28 Hz resolution. Real time signal detectors scrutinize the data for the presence of continuous wave and pulsed signals that may drift in frequency by as much as 1 Hz per second.

An "observation" of a star in a 10 MHz frequency band consisted of three steps: pointing the antenna at the star, then away from the star, and then back at the star. Each step lasted either 92 seconds or 299 seconds. Signals that were present only when the telescope was pointed at the star (and not previously seen in other observations) were selected as candidates for further verification tests. Signals that were present both "on" and "off" the star were rejected as terrestrial interference. During the 200 hours of assigned telescope time the TSS made 436 observations of the target stars and a variety of test observations. Many interference signals were detected and cataloged while fifteen signals satisfied the basic candidate selection criteria. These candidates were immediately subjected to further verification tests, but all proved to be intermittent terrestrial interference. Analysis of the data collected at Arecibo continues with the goal of developing better techniques for quickly identifying, classifying, and perhaps even avoiding interference signals.

Since returning from Arecibo, the TSS has been reassembled in the development laboratory at NASA Ames. This is part of a planned system expansion timed to coincide with major upgrade activities at the Arecibo Observatory. The "lessons learned" through operational experience are also being implemented as modifications to several circuit boards and improvements in the software. Over the coming months, the TSS will expand to process two dual-polarization 10 MHz radio frequency bands with additional resolutions. The Mobile Research Facility that houses the TSS for transportation to and operation at observatories has received additional cabling and electronics racks needed for the 20 MHz system. The expanded TSS is required for the 1994 Targeted Search Campaign, during which nearby Sun-like stars in the Southern Hemisphere will be observed using the 64-meter antenna of the Parkes Observatory in Australia. Parkes is part of the Australian Telescope National Facility operated by the Commonwealth Scientific Industrial Research Organization. A Memorandum of Agreement has been negotiated between NASA and CSIRO through the Australian Space Office for the use of this facility.

RESULTS

No signals of extraterrestrial intelligent origin have been detected. It is encouraging that most of the terrestrial interference signals encountered so far have been immediately recognized as such by the Sky Survey and Targeted Search Systems. A few signals required additional verification tests to determine that they were of human origin or due to thermal noise. In summary, the HRMS has successfully inaugurated its observational phase. Both the Targeted Search and the Sky Survey components are using the lessons learned in the initial observations to improve their hardware, software, and observation techniques as they expand their systems by stages to the final configurations required to pursue the full-scale search.

Other Information

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