Classical SETI 4. The SETI Search Space

What exactly is SETI looking for, and how is a SETI search conducted?

The everyday experience of tuning in a radio station is a good starting point for understanding SETI. What needs to be grasped is that a radio and a radio signal from a commercial broadcast station are designed for each other. One says, the receiver is matched to the signal it is receiving, and this matching is important to the communications process. But in SETI, virtually no prior agreement on transmitter and receiver characteristics can exist. Rather, an elaborate guessing game takes place. We, on the receiving end, must guess what transmitting societies would do if they believed we were trying to guess what they would do.

If we guess correctly what a transmitting society thinks we will guess, and design our receiver accordingly, we will maximize the probability of detecting their signal. SETI researchers believe that any given transmitting society will design its signal to be very simple in some way, so as to minimize the number of parameters that we, the receiver, would have to guess when designing our receiving system.

It is also expected that they would design their simple signal so as to be unlike anything that would be generated by a natural source.

The Simplest Signals

It turns out that there are two perfectly simple forms of signals, and they are utterly unlike each other.4 These are

  1. a pure, monochromatic (single frequency) sine wave, and
  2. a single pulse, of near-zero duration. This single pulse would be a brief outpouring of all the energy the transmitting society had saved up for the great event.
The signal of type 1, in its ideal form, exists throughout all time so that one time is as good as any other, as far as a search is concerned--but one must guess the frequency correctly. Conversely, the type 2 signal occupies all frequencies (subject to transmission windows along the path) but only during a single brief instant of time can it be received.

Because signals of these two types are so different from one another, any receiver perfectly matched to one of them will be utterly unable to detect a signal of the other type. SETI experimenters are faced with the choice of which type of signal they will search for, or they may choose to spend their resources on the deployment of two separate receiving systems. For a signal that is not quite either of the ideal forms, there is a possibility that one of the idealized receiver types can detect it.

Concerning the type-2 signal (one brief pulse in all of eternity) no SETI scientist believes that a transmitting society would actually choose to construct a beacon that way, for reasons that are probably so obvious that there is no need to elaborate on them. However, a modified type-2 in the form of a sequence of pulses--we'll call this a pulsing signal--has seemed plausible to some. The pulsing signal does away with the necessity to guess the single moment of time to receive it, but it replaces that with a different parameter to guess: the pulse repetition rate. For the receiver to maximize its probability of detection, it must be designed to match the transmitter's pulse repetition rate.

Rather than attempting to guess the single frequency for type 1 (i.e., monochromatic) signals, receivers search simultaneously many possible or plausible frequencies. Likewise for the pulsing signals, many possible/plausible pulse repetition rates are tried.

Choosing the Target Signal Type

SETI theorists have always agreed that they should be searching for the simplest signal types, but there has been much confusion over which type would be the likeliest for an ETI society to use for a beacon.

A rationale for monochromatic signal searches was developed early on by Dr. Bernard M. "Barney" Oliver, who was for many years Director of Research at Hewlett Packard, and one of the founders of the SETI field. In his Project Cyclops study Oliver and Billingham (1972), ("The SETI Bible"), Oliver argued that if the signal consists of a series of pulses, and an appropriate matched filter is used to optimize the received signal-to-noise ratio (SNR), SNR is proportional to the energy per pulse. He went on to say,

"The energy of the signal can be increased, of course, by increasing the radiated power, but once a practical limit of power has been reached, further increase is only possible by increasing the signal duration. This narrows the signal spectrum. In the limit, therefore, we would expect interstellar contact signals to be highly monochromatic."

Oliver seemed to be referring to a limitation in instantaneous power. But if a society were power-limited, the limitation would be in the average power that it could deliver to the transmitting system. That is because energy could be stored between pulses and delivered during the pulse time.

It is the average power that ultimately translates into energy resources expended over time. Within the limitation of any given average power, the transmitting society can achieve any arbitrary instantaneous power desired, just by delivering it within arbitrarily short pulses.

Thus the argument appears to be erroneous. Nevertheless, although Cyclops was never built, the design study and Oliver himself were very influential for many years (for an example of this influence, see "Dr. Jill Tarter: Looking to Make 'Contact'"), and Oliver's rationale had and still has been the major guidance for SETI receiver design at least for US-based programs.

Scientists in the former Soviet Union believed that pulsed signals should be given at least equal attention to monochromatic, and in fact they conducted searches for pulsed signals at the Gor'kii Radiophysics Institute, the Institute for Space Research, and the Sternberg Astronomical Institute. (Soviet CETI Report)

But SETI astronomers in the West, following Oliver's lead, generally denigrated the possibility that a transmitting society would build a pulsing signal beacon. For many years, and even today, SETI receivers have been designed as spectrometers, best suited for the monochromatic signal search, in which frequency is the key search variable. SETI receiver development has always emphasized a push to an ever greater capability to scan multiple frequencies simultaneously.

Notwithstanding that, in a review article published in 1985, Cullers, Linscott and Oliver conclude that there is a "gain in energy efficiency obtainable with pulses over CW signals [that] leads us to expect pulsatile signals from beacons."

They appear to have discovered the earlier mistake. But by that time, with a total investment in "multi-channel spectral analyzers" (MCSAs), the authors had to suggest a dubious post-processing algorithm that would attempt to detect intermittent CW signals in the MCSA frequency bins. The algorithm is in use today as a secondary process, with the emphasis remaining on the monochromatic search. If SETI programs were to devote appropriate resources to searching for pulsed signals, they should make use of adaptive filter methodologies operating in the time domain. These are well developed for other applications. See for example, a 1977 patent, Adaptive pulse processing means and method.

Details of the Monochromatic Search Method

SETI receivers searching for monochromatic signals need to scan the range of practical values of signal frequency. While it is important to scan as many frequencies as possible, it is also important to spend as much time as possible on each frequency. This is called integrating over time, and the longer the time integration, the easier it will be to distinguish any stable signal that is present, against the background noise.

But there is a problem here: relative motions between the transmitter and receiver will create Doppler shifts of the received signal frequency during the integration time. This must be corrected in the receiver, or integration will not work. Unfortunately, the amount of relative motion, and hence the required Doppler shift correction, is unknown. Therefore all likely Doppler shift rate corrections must be applied. (This is accomplished by means of massive parallel signal processing.)

Additional Dimensions of the Search Space

Another important variable is the direction or celestial coordinates of the transmitter site. These too must be searched systematically.

And finally, there is the matter of polarization - which has to do with certain geometrical aspects of transmitting and receiving antennas. For our purposes it will suffice to state that, in the absence of knowledge of transmitter signal polarization, a minimum of two receiving polarizations is required, in order to avoid the possibility of totally locking out the incoming signal.

These various search parameters comprise the dimensions of what is called the search space.6

The Endless Search

If the search space has been thoroughly covered, and somebody finds a way to construct a more sensitive receiver, then really the entire search must be conducted again, because it has now become possible to find weaker signals, emanating either from points farther away than before, or from weaker transmitters at points previously searched. In other words, signal intensity is really another dimension of the search space.

From all of this, it can be seen that until ETI is found, SETI is never conclusive. It is always possible to argue for a larger antenna or a better receiver, to expand the search space, endlessly.

SETI's Roadmap

Jill Tarter (2001), laying out "a roadmap for SETI research for the next few decades" reaffirms the intention of orthodox SETI to persist in an almost exponentially expanding search, as long as support can be found. SETI is apparently no longer sure of the type of signal ETI would use for their beacons. To the basic monochromatic beacon target model, SETI would add searches for "very fast optical and infrared pulses". It would still be a "needle in a haystack" proposition, looking for faint electromagnetic beacon signals associated with distant stars.

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