LO-BUDGET ETI SEARCH
Compiled from papers by Bob Dixon and Russ Childers
Additions By Herb Johnson
LOBES and the Horn Cart
From our "WOW!" experience and similar research by others, we knew that any SETI candidate signal needed to be observed for a longer period than several seconds once a day. But to study the signal for a longer period, we had to track it across the sky, in its apparent track as the Earth rotated the telescope past the signal's location. It would be impossible to move the telescope to follow. Fortunately, the focus of the telescope is so broad we can actually shift where we receive the focussed signal and continue reception of the signal. It's like looking "sideways" through a window to see past the normal edge.
This system avoids the problem encountered by other SETI programs where interesting signals are found after-the-fact as part of a systematic search, but are no longer there when further observations are attempted.
In 1992, Assistant Chief Engineer Russ Childers wrote his masters thesis on the development of a system called LOBES, which incorporated the movable horn cart, the ZOOM technique, and the PDP-11 system along with a IBM-PC compatible. The system began its survey in September of 1993, and will complete its work sometime in 1997. It is fully automatic and only requires a human to move the telescope to scan the next portion of the sky: an operation requiring 15 minutes of careful manipulation of the 100-foot antenna "flat". Natural radio sources, previously charted by this telescope decades ago, are automatically detected, logged, and ignored.
A bank of 50 receivers of 100 kHz bandwidth are "hopped" across the entire "waterhole" band of 1420 MHz to 1720 MHz, in effect as a 3000 channel receiver. The output of the receivers go to phase-sensitive detectors, synchronized to the Dicke-switched dual horns, to measure the received power in each channel. The output of the detectors is digitized by the PDP-11, and the digital values are sent to the IBM-PC where LOBES performs its analyses.
The survey started on September 29,1993, with the telescope pointed at +62 degrees, 20 minutes declination: the Radio Observatory's northernmost limit A circle of sky is observed each day by using the earth's rotation to point the telescope across the sky. Every few days, the telescope's flat is moved 1/3 of a degree lower in declination to observe another circle of sky. In addition to a SETI search, the natural radio sources detected by this survey are noted, compared and verified against the Ohio Survey data, and a new map of the radio sky will be developed.
When a likely signal is found, the LOBES software commands the telescope to move its receiving apparatus to actually "follow" the source for up to an hour. Rather than attempt to move the huge telescope, we move the large metal "horns" that intercept the telecope's focus on a truck-like cart under computer control. This system eliminates the problem of earlier surveys by large telescopes that could not follow a possible signal as the sky passes over. Also, an online catalog of known Radio Frequency Interference sources was developed, to be used by the computer to ignore them.
Meanwhile, additional algorithms in the PDP-11 portion of the LOBES system monitor the continuum channel, the original 8 MHz bandwidth system used in the Ohio Survey. Algorithms match the received signal to the shape of the antenna pattern: a point source would achieve this match. If a match occurs, a message is printed showing the degree of "fit" and the intensity of the source, and compares it to similar sources as found by the Ohio Survey to determine the most likely match.
Finally, as a separate system, an IBM-PC is set up to monitor this 8 MHz channel and to produce an analog chart. Angelo Campanella designed a program to take the digitized analog signal and to print a time-stamped strip-chart on an ordinary impact printer and plain paper. This chart provides a printed record and convenient monitor of the antenna and receiver system without requiring data analysis.
Astrophysical Results from LOBES
The LOBES system and movable horn cart has operated successfully, as of 1996 completing two-thirds of its all-sky program under Russ Childers' supervision. It has confirmed the persistance of most of the natural radio objects first found by the Ohio Survey over thirty years ago with remarkable accuracy. Generally the results are identical to the original survey. We are creating new maps which are comparable to the Ohio Survey maps of 30 years ago. But, we were recently surprised to find that we had "missed" a source - it fell in between two maps of the old survey!
On January 1994, LOBES detected what has been called the "Whoa!" signal, a small narrow-band "poof of energy" which, while not a SETI candidate, was certainly a dramatic event as most natural sources are not so narrowband.
On June 9th 1994, the LOBES program announced a "SETI strike!" at 1612.540 MHz at 5PM. Russ Childers called Bob Dixon and hastily explained the source's characteristics. It satisfied all the criteria: matching the antenna pattern (a point source); it moved at the sidereal rate (not an airplane); it was narrowband, close to the galactic plane; it was constant. But, it was visible in more than one channel, and it was unusually powerful. Bob was not phased by the suggestion "we might not be alone". He said, " look at the OH emission line [frequencies]". Lobes had detected one of the first known "radio masers", natural sources of narrowband radiation which, like pulsars, were also briefly considered artificial but are understood to be a natural phenomena.
Some years prior to LOBES, volunteer observer Tom Van Horne looked through old SETI records and made the discovery of a narrowband celestial radio source, detected in 1980. It did not look like a point source, but was rather distributed. Its shape and frequency suggested a cloud composed mainly of hydrogen. Hydrogen clouds are common in and around the galaxy. With the development of LOBES in 1993, Tom hoped a detailed narrowband survey of the area would reveal the extent and origin of the phenomena. Consideration of both the old and new surveys suggested that this source would be detectable in channels 25 and 26 in one band of the LOBES survey range.
During March and April of 1993, Steve Brown and Russ Childers moved the antenna to the appropriate declination. Steve had written a program for the PDP-11 to collect data for this event. The source was observed on another channel, 1 MHz away from the expected frequency. Observations were made with one horn, as the source was so broad that two horns would produce signals from two sections of the cloud at once. Two weeks of observations were made at one declination, to confirm the observing process, and then the antenna was moved every three days, scanning in different declination across the source. After the power of the source dropped considerably with position, the declination was moved well north and the declinations south of that point were scanned.
It appeared to be two sources seperated by 10 minutes RA. There may in fact be several sources, seperated by frequency and positions. Analysis of the data from the 100 kHz channels showed a weak, diffuse source about 12 minutes RA from the strong source, but detections of it in the 10 kHz channels were difficult. A graphic of the primar source, with colors representing frequency, dramatically show the slight variation in frequency, which implies variations in velocity, across an apparently torus-like rotating cloud viewed edge-on.
In 1994 we gained a huge increase in the telescope's sensitivity. New HEMT amplifiers have been put together by Angelo Campanella which cause the same improvement as if the telescope had been made three times larger. The LOBES survey was restarted in 1995, and will complete a sky survey late in 1997: the fourth survey conducted by Big Ear in thirty years. The American edition of the Guinness Book of Records notes Big Ear as the "longest coninuously-running SETI program in the world".
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