Acta Astronautica Vol. 26, No. 3/4, pp. 213-217, 1992 0094-5765/92 $5.00 + 0.00 Printed in Great Britain. All rights reserved Copyright © 1992 Pergamon Press Ltd

SETI AND THE RADIOSPECTRUM#

H. C. KAHLMANN Westerbork Radio Observatory, Westerbork 9433, The Netherlands

(Received 29 November 1991)

Abstract--The distribution of radio bands to various users is a very difficult task internationally. Any signal emitted from outside the Earth will be badly deteriorated by any emission by man--several WARC (World Administrative Radio Conferences), set by ITU (International Telecommunication Union, an agency of the United States), attempt to minimize this effect.

I. WAVELENGTH, WAVELENGTH OF THEM ALL. WHICH IS THE BEST ONE FOR THE CALL

Our knowledge of the universe outside this planet Earth is almost entirely based on information which is carried towards us by electromagnetic waves. Among the many possibilities to gather information from the outside, the photon based systems have until now shown to be the most practical. The fact that the electromagnetic wave reception technique generated two types of astronomy coincides with two principal transparent bands for electromagnetic waves in the earth's atmosphere and ionosphere. These bands are often refered to as the optical window with wavelengths between 0.4-0.8/~m (I octave) and the radio window with wavelengths between I mm and I0 m (over I0 octaves).

Based on the experience in astronomy and radioastronomy in particular it seems reasonable to expect that the most practical approach to SETI is the search for radiosignals from other civilizations. The search for signals in the radio domain immediately leads to the question: what frequency band in the radiospectrum are we going to use for this search, because signals from other civilizations may be broadcasted at any frequency.

This consideration leads to the doublet:

Where do we have the greatest chance to detect them? Where do we have the greatest ability to detect them?

Several authors answered these questions in a different way with different arguments for their choices.

The "classical" choice is around the hydrogen line (1420 MHz) and the band between the hydrogen line and the hydroxyl line (1638MHz), the so called "waterhole"[l ,2]. Another choice has been the l l .7cm (2.5568 GHz) band with arguments based

"['Paper IAA-90-579 presented at the 41st Congress of the International Astronautical Federation, Dresden, Germany, 8-12 October 1990.

213

on universal fundamental constants. Also the 13.5 mm (22.2 GHz), the second waterhole, has been proposed. There are arguments given for search above 100GHz. The band between 1400 and 1700 MHz, the waterhole, however seems to be of unique value for SETI. The band 1400-1727MHz is a classic radioastronomy band with the highest protection level (see Appendix 1).

2. OZMA WAS CARRIED OUT IN 1960, IT IS 1990 NOW, IT WILL BE 1992 SOON

The story starts with the publication of the article by Cocconi and Morisson [1]. This happened at a time that in Geneva a WARC (see Appendix 2) was going on. This WARC convened to review the International Frequency Allocation table made in the 1947 Atlantic City Conference was of extreme importance to the science of radioastronomy and space-research. These scientific "users" of the spectrum were not yet recognized as services in the ITU sense. Therefore proposals were made to create the services: radioastronomy and space research.

Thanks to the work of many the results were positive. Also some bands were reserved for the new services, in particular the 1400-1427 MHz band was allocated to the radioastronomy.

The situation in practice was that between 1400 and 1750MHz, although bands were allocated to different services, this band was usable for radioastronomy without noticeable interference. This situation was only slightly different when in 1979 a next general WARC was convened.

Again and specially with the eye on the future developments, a complete review of the Radio Regulations was the goal. Radioastronomy went into the conference with an agreed international position to present a united front. They had adopted the position that the protection should be gained

• for continuum bands (broadband obser- vations) at octave intervals throughout the spectrum above 10 MHz with at least a one percent bandwidth in each band.

214 H.C. KAHLMANN

• for a large number of lines emitted from molecules and atoms which are considered of prime astrophysical importance.

In general it can be said that radioastronomy came from WARC 79 in a better position than it went into it.

SETI got a footnote (see Appendix 1) in 1979 saying:

No. 722: In the bands 1400-1727MHz and 101- 120GHz and 197- 220GHz, passive search is being conducted by some countries in a programme for the search of intentional emissions of extra-terrestrial origin.

After the WARC 1979 and its date of implementation 1982 there was still no real problem for radioastronomy in the practice of all day. But suddenly this changed. Had the use of the bands in the so called waterhole by satellites been negligible, this was no longer the case. In a short time satellite systems appeared, causing a lot of nuisance to radioastronomy in these bands. The most prominent are the satellite system NAVSTAR or GPS of the U.S. and the GLONASS system of the U.S.S.R. And all this is only the start of what is already visible of what is going to come in the near future.

As a result of pressure on the radiospectrum for numerous new and developing services, the International Telecommunication Union (ITU) has scheduled a World Administrative Radio Conference (WARC) for the first quarter of 1992. The agenda for this WARC is set in June 1990 and only parts of the radiospectrum outlined below will be involved.

lines of methanol, formaldehyde, water, ammonia and other molecules occur in this frequency range.

(d) Above 20GHz

Possible new space services will be accommodated.

The WARC will have the authority to make extensive reallocations and it is impossible to predict the extent of changes that may be made. Even if radioastronomy loses none of the existing allocated bands, it is possible that presently passive allocations may have to be shared with active (transmitting) services. More intensive use of the spectrum will make it more difficult to find interference-free bands for such observations as redshifted hydrogen below 1400MHz. Allocations involving satellite transmissions could cause problems of low-level sidebands falling within nearby radioastronomy bands.

Work is now in progress within the administration of nations participating in the WARC to establish their positions with regard to making new allocations and preserving existing ones. Delegations of a few countries may include a radioastronomer, but the great majority of participants in the WARC will be primarily concerned with active services. The most important thing that astronomers can do is to contact those people in their national telecommunications administrations who are involved in preparations and make sure that they understand the requirements of radioastronomy and SETI. Much of the preparation will be completed at least a year before the WARC begins, so there is little time for delay if we are to ensure that all delegates to the WARC are fully cognizant of the needs of radioastronomy and SETI.

(a) 2-30 MHz

More allocations to HF broadcasting have been requested. Any action in this range will mainly affect radioastronomers who are interested in the longest wavelength.

(b) 500 M H z - S GHz

Frequencies are sought for the Broadcast Satellite Service (sound) and for personal mobile communications systems (mobile telephones, etc.). For the latter, an allocation of 300 MHz bandwidth somewhere between 1.7 and 2.3GHz has been discussed. The frequency range is of great importance to radioastronomy and SETI with the hydrogen and OH lines, continuum hands at 610 and 2695 MHz and the deep space band at 2 .3GHz which is routinely used for geodetic VLBI observations.

(c) 12.7-23GHz

An allocation for High Definition TV (HDTV) is sought. Something like 600 MHz will be required. Radioastronomy allocations of 14.47-14.5GHz (secondary), 15.35-15.4 and 22.21-22.5GHz and

3. CONTRARY TO THE BIBLICAL DICTUM: IN CASE OF RADIOASTRONOMY AND SETI IT IS BETTER

TO RECEIVE THAN TO SEND

In the 1959 WARC radioastronomy was recognized as a service. In the discussion during WARC 79 it became clear that this service in the radiocommunication fields is unique in the sense that it is entirely passive, it only receives and does not transmit.

Radioastronomy is defined as: astronomy based on the reception of radiation of cosmic origin.

In the realm of communication engineering, communication systems are understood to consist of a transmitter (source), a channel and a receiver (destination) and that in general all these components can be controlled. In most active systems this is the actual situation. If, for example the signal to noise ratio in a communication link is not good enough the signal power at the transmitter can be increased.

In radioastronomy we can control neither the transmitter nor the channel: these are set by nature. This results in an increased vulnerability for interference given the characteristics o f radio- astronomy.

SETI and the radiospectrum 215

To a large extent this also applies to SETI where our control of transmitter and channel is non existent as well.

As a result of the "passiveness" of radioastronomy and SETI there are some characteristics which apply to both:

(A) Radioastronomy is interested in the entire electro- magnetic spectrum

Different physical processes produce electro- magnetic radiation at different frequencies. For all different domains of the spectrum there are telescopes available. The natural limitations for the usability of the different sections of the spectrum are given by the ionosphere, which becomes opaque below 3 MHz and the absorption due to molecular constituents of the atmosphere at frequencies higher than 350 GHz.

So is SETI because we do not know where to search. It can be anywhere.

(B) Receiver bandwidth

There are two major goals in radio astronomy:

(a) Broadband: detection of continuum emision from thermal as well as non- thermal extra-terrestriai radio source. In this application the sensitivity is improving with increasing bandwidth.

(b) Narrowband: in use for spectral line studies, i.e. of the Doppler-shifted line emission, which informs us about the kinematics within extra-terrestrial radio s o u r c e s .

For SETI the bandwidth requirement is very narrow-band.

Equipment. The equipment is continually evolving to better sensitivity and better angular resolution. System temperatures of 10-20 K for cm wavelengths and angular resolutions of milliseconds of arc are obtained in daily practice. The interference problems in radioastronomical observatories can also be divided into different categories with relation to their distance from the source. Already mentioned are the local problems, that require a local solution like e.g. a radio-quiet zone around the radioastronomical observatories. T.V.-broadcasting by adjacent channel or in-channel use of the radioastronomical reception band is regarded as a regional problem. These problems are different for Europe, America, Asia, Africa and Australia. They require a regional consideration.

The worst for radioastronomy are the global problems mainly caused by satellites and satellite systems.

4. INTERFERENCE SITUATION

The interference situation is worsening every day by increasing usage of spectrum for a multitude of applications.

Radio and television broadcasts make daily use of a considerable fraction of the spectrum in most regions of the world. As a rule, centrally generated programs are emitted by transmitters, often located on elevated platforms (e.g. masts or towers). They radiate output powers up to hundreds of kilowatts and can be received using simple aerials placed on the roof of a house or even a car. Reception with portable receivers is also possible. The carrier frequencies used in broadcasting are, in general, below 1 GHz.

The development of satellite communication makes it technically possible to broadcast radio and television programs directly from satellites. The carrier frequencies for direct satellite broadcasting are around 12GHz. Reception is possible with dish antennas of 0.6-m diameter. For collective reception, 3-m dishes are used.

Mobile communication involves the transmission and reception of radio waves between moving vehicles, such as cars, ships and aircrafts or between moving vehicles and a fixed station (e.g. a fleet of taxicabs and a dispatch station). Radio is clearly the only means of communication in these cases. The carrier frequencies used for mobile communications are, in general, under 1 GHz and the typical power used by the emitting station is 10 W. Mobile communications, particularly cellular car telephone service, is in a phase of tremendous increase. The technical developments aim to provide the car-telephone user with the same quality com- munication and diversity of service possibilities as those available to the home-telephone subscriber.

The mobile service also supports a large number of closed networks. These are used (e.g. by cab and other dispatch services; police; firefighters; ambulances; trains; army; etc.) Maritime and aeronautical communications between, or to and from, ships or aircraft must be of a very high standard because safety aspects play an important role. This is, in particular, the case for navigation and communication in situations of emergency. Ground- based microwave links and satellite links are import- ant parts of the infrastructure used for the public telephone networks. For international and inter- continental connections, satellite channels are exploited. Microwave links, as well as satellite links, are used for the international distribution of television signals.

Microwave links make use of carrier frequencies between 1 and 20 GHz. Usually a limited amount of power (of the order of 10 W) is transmitted by high-gain antennas. The receiving and transmitting equipment is, in general, located on masts or towers. Satellite communication channels usually also use carrier frequencies between 1 and 20 GHz. These frequencies are ideal because of the transparency of the atmosphere. The ground stations use large-dish antennas, up to diameters of 30 m, which have a large antenna gain. The receivers are often suppfied with cooled first amplifiers to bring down the noise power.

216 H.C. KAHLMANN

Radar (radio detection and ranging) was primarily developed for military applications but has many civil applications as well. For example, large aircraft are usually equipped with weather radars which enable them to observe and thus avoid bad weather areas from large distances. Airports are unthinkable without their radar installations. Also, in ocean and seaports, radar systems are in use to assist ships to enter the harbors in bad weather conditions (such as fog and mist). Ships often are equipped with radar for navigation purposes. Police departments use Doppler radars for speed control along the roads and highways. Radar systems often use carrier frequen- cies, between 1 and 30 GHz and very large powers. Radar antennas often are of the rotating type to enable them to scan around the horizon.

The common factor in all these radio communi- cation systems is that they use a transmitter, as well as receiving equipment. The existence of a receive-only user can easily be overlooked in the process of planning the frequency bands for these users. This is particularly true if the congestion forces users to share a frequency band. Sharing a frequency band is highly impractical for a receive-only user like a radioastronomy observatory and SETI stations.

5. ONE MAN'S SIGNAL IS THE OTHER MAN'S INTERFERENCE

Radioastronomy SET! have to work under circumstances which are "'alien" to the active spectrum users. They do not control the character o f the transmitted signal and the transmitted signal cannot be varied to increase detectability. Furthermore the sig- nals are extremely weak. A typical signal to noise ratio in radioastronomy is - 3 0 d B compared to lOdB in active spectrum use. With no control over the transmit- ter nor the channel radioastronomy and SET! must try to control the electromagnetic environment at the receiver. Herein lies a potential incompatibility with active use o f the spectrum.

6. OUR TRANSMITTERS MAY MAKE EXTRATERRESTRIAL CIVILIZATIONS AWARE OF US, BUT IF WE HAVE NO CLEAR CHANNELS ON WHICH TO LISTEN, WE WILL NEVER BE AWARE OF THEM

After the radioastronomy service was introduced in the Radio Regulations at 1959 WARC, a Commission was set up, sponsored jointly by the Union Radio Scientifique International (URSI), the International Astronomical Union (IAU) and the Committee on Space Research (COSPAR). The Commision, called the Interunion Commission on the Allocation of Frequencies (IUCAF) was charged to: study and coordinate the requirements for the radio frequency allocations for radioastronomy and space science and make these requirements known to the national and international bodies responsible for frequency allocations; and take action aimed at ensuring that harmful interference is not caused to

radioastronomy or space science operating within the allocated bands by other radio services. IUCAF operates under the aegis of the International Council of Scientific Unions (ICSU). The work of this Commission can be described as follows:

(A) The first task requires IUCAF to work with various national and international bodies and with individual scientists, to define the scientific requirements. These then have to be presented to WARCs, convened by the ITU from time to time. To be considered at a WARC, proposals for frequency allocation must be presented by one or more administrations and must be technically sound and acceptable to most of the administrations participating in the work of the WARC. Thus the first task may involve considerable national and international discussion to reach a satisfactory consensus.

(B) The second task is one of protection and is one which has, in recent years, become more important and perhaps, more difficult to attack. The radiospectrum now has a number of bands allocated to radioastronomy and space research (though the degree of protection varies widely from band to band).

The growth in number and increasing power of emissions of satellite-borne transmitters increases the threat to radioastronomy observatories. IUCAF (representing URSI, the IAU and COSPAR) is the "recognized" voice of the scientific world at WARCs (IUCAF enjoys observer status at these Conferences). Input to IUCAF is provided by two Committees that operate on a regional basis. The elder is the United States' National Academy of Sciences' Committee on Radio Frequencies (CORF). CORF is composed of scientists from various fields who use radio frequencies in conducting their research. CORF's primary objective is to limit the level of harmful interference in the bands used by scientists as much as possible. The Committee retains legal counsel to protect the interests of radioastronomers and the other scientists that it represents. CORF also transmits the views of the scientific community it serves to IUCAF.

The younger Committee is the European Science Foundation's Committee on Radio Astronomy Frequencies (CRAF). This Committee was created as a result of an initiative of Dutch radioastronomers to coordinate the efforts on frequency protection for radioastronomical bands in Europe. The Committee started its work in 1988. It is composed of scientists who are active in the field of frequency management at radioastronomical observatories in Europe. It also provides input to the IUCAF on issues that deserve global attention. IUCAF, CORF and CRAF strive to maintain good relations with other organizations

SETI and the radiospectrum 217

working in spectrum management, in the first place, the local agencies that act as representatives of the various administrations to the ITU. Within the U.S., the CORF has to deal with two agencies, the FCC and the NTIA, while CRAF deals with many countries and consequently, with many agencies. To simplify matters, CRAF tries to deal directly with the European Post and Telegraph Commission (CEPT) when possible.

Technical matters related to protection of radioastronomy from harmful interference are contained in the CCIR documentation. This documentation is periodically revised and updated by CCIR Study Group 7 which deals with Space Research and Radioastronomy.

7. WHAT CAN RADIOASTRONOMY AND SET! EXPECT IN THE FUTURE?

There can be no doubt that interference-free bands are as essential for the future development of passive search as dark skies are for optical astronomy. In practice, interference-free sites may no longer be realized by locating radio observatories at remote locations. The main reason for this situation is, of course, that the sources of interference are becoming increasingly space borne.

The passive services may, under certain circum- stances, be able to share with some active services (e.g. the fixed service and even the terrestrial broad- casting service) because geometrical separation may be used to meet the harmful interference levels. For the mobile services, such coordination is much more complicated and it is nearly impossible to do so as soon as satellite services come into the picture.

A host of emerging technologies demand portions of the radiospectrum. Among these are:

The Mobile Satellite Service MSS The Radio Determination

Sat. Service RDSS The Broadcasting Satellite Service BSS

Yet, another relative newcomer to the spectrum is the so-called low-power, or nonlicensed, device. The use of these devices (e.g. cordless phones, garage door openers, computer games, etc.) make of the spectrum are regulated according to local rules, but, in practice, they may be found everywhere and are extremely troublesome for highly sensitive instruments such as radioastronomical receivers.

The interference-free bands in the radiospectrum, which are necessary for passive observations, can only be obtained and maintained by international agreements, insuring that certain bands remain free from man-made signals. Radioastronomy's survival as a science requires a few relatively narrow bands which

are totally free of man-made emissions. Unfortunately, one of the regions of the spectrum which is most important to astronomy is the 0.5- to 3-GHz range which is also being sought and fought for by many other services and commercial interests.

REFERENCES

1. G. Cocconi and P. Morisson, Nature 184, No. 4690 0959).

2. B. Oliver, The search for extra terrestrial intelligence. NASA SP-419 (1977).

APPENDIX 1

Levels of Protection

The International Radio Regulations provide several levels for protecting radio services from mutual interference. The highest level of protection is a primary, exclusive allocation in a frequency band; that is, only one service is authorized to operate in a given band. The second level is also a primary status, but the same band is shared on an equal basis with one or more additional services. The third level is called secondary where a service is authorized to operate, but may not interfere with other services in that band which have primary status. The fourth level offers no formal protection, but is a notification that a service is using the band. This last type is often used for spectral line observations and it simply calls attention to the use of the band by radioastronomers and asks for protection when practicable.

APPENDIX 2

RR, ITU, WARC, CCIR

The Radio Regulations (RR) are administered by the International Telecommunication Union (ITU), which is a specialized agency of the United Nations. Reviewing the Radio Regulations, either partly or complete, is done at World Administrative Radio Conferences (WARCs). Part of the Radio Regulations is the Table of Frequency Allocations. In this table the spectrum from 9 kHz to 275GHz is portionized into frequency bands for use by thirty-oue (31) communication services such as broad- casting, aeronautical radio communication, aeronautical radio navigation, maritime radio communication, maritime radio navigation, radio location, land mobile communi- cation, fixed point to point communication, amateur and so on. One of these is radioastronomy. The technical arm of the ITU is the International Radio Consultative Committee (CCIR) which defines such items as levels of harmful interference. The Radio Regulations have the force of an international treaty for a country once the Final Acts of a WARC are signed (and for some countries: are ratified by the senate). These regulations however, only provide for the allocation of frequency bands to radio services on the international level. Additional protection for passive fre- quency use is necessary and can be achieved by enlightening assignment of frequencies within a band to stations of an authorized service, by effective compliance checking and by adequate technical standards for equipment. All of these areas are tasks Of the individual national administrations. Thus, the radioastronomers' efforts did not end with WARC 79. Representation must also be maintained in the daily regulatory process in each country.