SP-419 SETI: The Search for Extraterrestrial Intelligence

[185] COMPLEMENTARY DOCUMENT 8

Prepared by:
Mark A. Stull
Charles L. Seeger
SETI Program Office
Ames Research Center

[186] blank page

An Interstellar Search System (ISS) is necessarily exceptionally vulnerable to interference by man-made coherent radiations, and the terrestrial radio spectrum is crowded. Therefore an extensive and complex strategy will be required in order to hold interference to an acceptable level. This complementary document discusses the matter only with the intention of demonstrating the seriousness of the problem.

It is convenient to discuss the interference problem in terms of the propagation paths whereby unwanted signals can enter an ISS. The most important paths are:

1 . Line-of-sight propagation

2. Reflection from flying or orbiting objects

3. Refractive, diffractive, scatter propagation modes

We first consider line-of-sight propagation. The free-space transmission equation is

(1)

where

Pt = transmitted power,W

= wavelength, m

gr = receiving antenna gain relative to an isotropic radiator

gt = transmitting antenna gain relative to an isotropic radiator

Assuming = 0.2 m (or, v = 1.5 GHz), gr = gt = Pt = 1, and converting to R, km, we have

Pr = 2.53 x l0-10 R-2 (W)

which, on converting to decibel notation, gives

Pr(dB) = -96 - 20 log R (2)

Equation (2) is convenient for this discussion. It represents the received power in dB relative to

1 W, as a function of R. (Should, add 10 log (Ptgrgt) to the right-hand side of eq. (2).)

The lowest received power flux density above which interference will be caused can be calculated from the probable characteristics of an ISS receiving system and is approximately (±10 dBW) independent of the antenna design. The signal-to-noise power ratio (S/N) out of a square-law detector is given by

[188] S/N = n(Pr/PO)2 /(1 + 2Pr/PO) (3)

where

P0 = total system noise power, W

n = = number of independent samples that are averaged

= averaging time, sec

Setting (S/N) = 1, we define the minimum detectable power

(4)

Now

P0 = kTB (5),

where

k = 1.38 x 10-23 JK-1/K-1 (where K is the Boltzmann constant)

T = equivalent system noise temperature, K

Combining equations (4) and (5) and inserting reasonable values for the parameters (T= 5 K, B = 0.01 Hz, n = 100 which corresponds to), we have

Pmin= 7.6 x 10-26 W= -251 dBW (6)

Note that Pmin is at least 50-110 dB less than that common in terrestrial communication services.

Setting Pr = Pmin as calculated in equation (6), we find

R = 5.7 x 107 km

149 x the distance to the Moon

That is, using isotropic antennas whose effective cross section is 0.003 m2, an ISS receiver could detect a 1-W transmitter at this distance. At 2 x 104 km, the same arrangements would produce a receiver output 69 dB above the detection limit; at 200 km, 109 dB.

Clearly, line-of-sight transmissions in and near the ISS reception pass band are a serious matter, whether on the Earth's surface or in distant Earth orbit. Equally, too, out-of-band and harmonic radiations from transmitters in line-of-sight are matters of serious concern.

[189] We can illustrate the problem with respect to reflection from flying or orbiting objects by considering the bi-static radar situation. For simplicity assume the same transmitter and receiver as above, and assume that they are Earth-based and over the horizon from each other. Further, assume there is some object with a scattering (radar) cross sectionof 1 m2, equidistant from the transmitter and the receiver but in line of sight of each. The appropriate relation is

(7)

where R is in kilometers. Substituting Pmin as before, we find R to be 126 km. Further calculation shows that Sky-Lab would return a good signal. In fact, if the experiment were really carried out, Sky-Lab would return many signals to this receiver. And probably many satellites and large pieces of space debris would also do so (see fig. 1).

Figure 1. Bi-static "radar" range for an ISS receiver with S = N = - 238 dBW at 1.5 GHz.

Discussion of atmospheric-scatter propagation modes is complex. Suffice it to say that an ISS may be able to function in conjunction with properly engineered Earth-bound communication circuits using the same frequencies. In any case, the matter needs careful study.

The nature of the entire directivity pattern of the antennas used in an ISS affects the response of the system to an interfering radiation field. When the physical collecting area of an ISS [190] antenna is greater than some tens of 100-m dishes, the capital and the operating costs of the antennas overwhelm the corresponding electronic and data-handling costs; and a large ISS is costly. Thus there is a strong economic imperative to design for maximum antenna efficiency ( = A effective/A geometric). How well one may maximize while optimizing the antenna response away from the main beam, is a matter not well understood at the present time for lack of theoretical and, in particular, experimental studies. Satisfactory experimental techniques for examining the overall response of an antenna are known. As a result, it is recognized that for a few large antennas with(which is undesirably low in an ISS context), the off-axis response is down to that of an isotropic antenna at angles from the main beam on the order of 10°-20° when operating at about 1.5 GHz. Only at angles from the main beam of approximately 60° or more does it decrease to more than 10 dB below isotropic (see fig. 2, taken from CCIR Rept. 391-1). Even if novel antenna designs succeed in reducing the off-axis response by an additional 10 to 20 dB, it will remain sufficiently close to isotropic over a large enough range of angles from the main beam that the -251 dBW level for harmful interference set by the characteristics of the receiving system remains in good approximation. However, with respect to interfering signals propagated by reflection and scatter modes, which are likely to be incident to an ISS primarily at large angles from the main beam and which will probably not generally exceed the -251 dBW threshold by large amounts, such a reduction in off-axis gain could be of major importance.

Figure 2. Peak side lobe levels of radiation patterns for large antennae.

The terrestrial origin of an interfering signal can probably be recognized to a high degree of certainty by a properly operated ISS. But, each detectable interfering signal removes that part of the spectrum from examination for interstellar signals; and strong signals, on the order of -150 dBW at the receiver may totally paralyze an ISS by saturating the masers.

[191] The preceding technical discussion may be summarized as follows. Although improved antenna designs are possible and needed, there is not a hint yet that theISS antenna response can ever be reduced to the range 60-90 dB below the response of an isotropic antenna. Thus operation of an Earth-based ISS is incompatible with transmissions to and from satellites using frequencies in and near the ISS passband. Further studies are needed to determine to what degree the ISS frequencies may be used simultaneously in a variety of services where the signals are kept essentially close to the surface of the Earth. Certainly, an ISS is compatible with a large use of the same frequency band if this use is confined to the surface of the Earth.

The feasibility of constructing a suitable ground-based ISS has been established, and reasonably good cost estimates are possible (see ref. 1). An ISS could also be located in orbit or on the far side of the Moon (see Section III-7). A far-side lunar ISS would need RFI protection only against transmissions originating beyond the lunar orbit; its cost at present, however, appears prohibitive and it probably will not even be possible to build for many years. On the other hand, an orbital ISS might be less costly than an Earth-based system, but it is not clear that cost estimates are yet reliable even to within an order of magnitude. Furthermore, an orbital ISS, unless located far above the geostationary orbit and shielded (an unknown additional cost) is far more vulnerable to RFI than a ground-based system. Indeed, an unshielded ISS would require a total clearing of the observation band (see Section III-9) because all Earth-based transmitters in the hemisphere facing it, as well as satellite transmitters, would be line of sight. Of course a shielded ISS in orbit would require only about the same protection as an ISS on the far side of the Moon.

Thus, whatever the ultimate ISS location, some kind of RFI protection seems desirable. Furthermore, it is likely that preliminary Earth-based searches with existing large radio telescopes, as well as perhaps a small ISS (say, five to ten 100-m dishes) will be conducted for at least a decade prior to the start of operations with larger systems, both to search for strong signals and to perfect techniques, receivers, and data processors. These preliminary searches will require essentially the same degree of protection as a full-scale Earth-based system, because that is dictated by the value of Pmin (eq. 6) and not the antenna collecting area. Thus the development of a SETI program in the reasonably near future depends upon reaching an accommodation with the way in which the electromagnetic spectrum is to be used. Fortunately, the use of the 1400 to 1727 MHz band is not yet fully developed and a move starting now to accommodate ISS requirements would forestall large investments now in the planning and conceptual stages.

In 1979 there will be a general World Administrative Radio Conference (WARC). Such meetings are held by the member countries of the International Telecommunications Union (ITU) approximately every 20 years to determine radio spectrum allocations among the various competing users. It is the purpose of the upcoming WARC to review, thoroughly, existing allocations across the entire spectrum and revise them in accordance with the envisaged needs of the remainder of the century. Thus if an ISS is to be built to operate in the 1400 to 1727 MHz band (or in any other band), it will be necessary to obtain formal international protection for it in 1979.

At present, satellite use of the 1400 to 1727 MHz band is relatively small; however, the AEROSAT system is currently being brought into operation there, and the AEROSAT and NAVSTAR systems will probably use the band starting at some time in the 1980's; and the FCC [192] has long-range plans to open the band to television broadcasting. Beyond this, it can be anticipated that plans for other satellite systems will be developed, and that as a result, an enormous equipment investment will be made in the 1980's and 1990's, both in the 1400 to 1727 MHz band and elsewhere in the spectrum, up to about 30 GHz. The effect of this huge prospective investment-perhaps tens of billions of dollars-will be to create great economic pressure against any attempt at a subsequent WARC to revise whatever allocations are devised in 1979. Certainly any protection given the 1400 to 1727 MHz band for SETI after 1979 will result in much greater economic dislocation.

On the other hand, much present use of the 1400 to 1727 MHz band is compatible with the operation of an ISS, or could be if properly engineered. In contrast to bands below 1400 MHz and above 1727 MHz, where one finds powerful aircraft acquisition radars with EIRP ranging from 107 to more than 109 W, terrestrial transmitters in this band are primarily low-power. Satellite users are still relatively few, and satellite equipment is still designed to be amortized generally in about five years, although construction for much longer lifetimes is planned for the 1980's and thereafter. The necessity of replacing equipment as it becomes obsolete provides an opportunity to engineer the new equipment to operate at a different frequency without incurring excessive costs. Then, too, a large fraction of present equipment can be cheaply modified for operation in a neighboring or other appropriate band. Since a large ISS would probably not be operational before 1985 at the earliest and large-scale preliminary searches will not be made before the early 1980's, satellite transmissions in the band can be tolerated until then, and perhaps longer if need be, to avoid economic hardships. Thus, if prompt action is taken, a plan that will protect the 1400 to 1727 MHz band with minimum economic impact can be developed and implemented. This plan would be in compliance with United States telecommunications law, which requires that, in the allocation of frequencies, public interest considerations, such as those that motivate a SETI program, prevail over economic ones; it should be based on the resolution of the Science Workshop (see Section I, Conclusion 2, p. 19, and Section III-9).

REFERENCES

1. Oliver, B. M.; and Billingham, J.: Project Cyclops, A Design Study of a System for Detecting Extraterrestrial Intelligent Life. NASA CR114445,1972.