A long-term radio frequency interference monitoring program is now in progress at the Ohio State University Radio Observatory. Current observations encompass the 1-2 GHz band, measuring the signal strength in each 150 KHz subband. Preliminary results show many strong and persistent signals, and wide bands in which no signals are detected. Daily and weekly variations appear to be correlated with periods of maximum human activity such as “rush hours”. It may be that aircraft reflections are causing many of the signals to be received, and that airline schedules correlate with “rush hours”. The program is being upgraded to automatically measure the direction of arrival of each signal, as an aid to further identify it. This directional information will also be used in designing rolled edges and side shields for the co-located OSU Radio Telescope, to further reduce its vulnerability to RFI.

With so many satellites (current and anticipated) broadcasting all sorts of signals in all sorts of bands, a group of radio astronomers came together in 1987 to discuss the problems. This group has become the European Science Foundation Committee on Radio Astronomical Frequencies (ESF-CRAF). The terms of reference of this Committee, together with its future activities and possibilities within the European legal context were discussed in the oral paper.

The problems of protecting passive bands in Australia, India, and Japan reflect the variety of research activities and radio telescopes in those countries, colored by the degree of user friendliness of the frequency management authorities.

In India, it is important to protect frequencies below 1400 MHz (for high redshift hydrogen line absorption or emission) and continuum bands at 327 MHz and 150 MHz (the latter currently allocated to cordless phones, paging systems, and rural communication).

In Japan, protection from harmful interference has been sought and refused at 4.8 and 5 GHz (microwave network), 10 and 15 GHz (mobile relay service), and 22 GHz (mobile data relay service). But extensive radio astronomy usage of mm to sub-mm bands has established priority for their use and allocation.

In Australia, there are major problems at 408 MHz (telephone links), 1.6 GHz (GLONASS, RDSS, and Land Mobile Service), 4.9 to 5 GHz (RDSS), and 22 GHz (satellite broadcasting and high definition TV service).

The degree of user friendliness of the frequency management administration appears to rate: 1. Australia, 2. India, 3. Japan on a diminishing scale of cooperation and concern. This affects the awareness of the radio astronomy community of conflicting allocations and the level of input into CCIR and the WARC.

A Giant Meterwave Radio Telescope (GMRT) is being set up at Khodad about 80 km north of Pune in India for operation in the frequency range of about 30 to 1500 MHz. It is to be completed by 1992 and is being designed to investigate many outstanding problems in the fields of galactic and extragalactic astronomy. We present here measurements of man-made radio frequency interference (RFI) conducted at the GMRT site in 1985 and 1988. It is seen that highly sensitive radio astronomy observations can still be made at selected bands in the above frequency range because of the relatively low level of RFI in India. However, this advantage may not remain for more than a decade or two.

Many papers in the Colloquium describe the difficulties which now exist when radio astronomers try to make highly sensitive observations in various regions of the spectrum. The author has been involved for about 30 years in attempts to get parts of the radio spectrum “protected” so that radio observations could be made in these regions free from damaging interference. Thus it may be interesting in this paper to look backward to the work done by many people before and during the International Telecommunications Union (ITU) World Administrative Radio Conference (WARC) of 1959, and to describe briefly the outcome of that WARC.

Occasional interference experienced in the channels of communications satellites has prompted an analysis to see how radio frequency interference (RFI) might be detected from space. RFI may be experienced on any type of satellite, including commercial and scientific satellites. For a satellite in geostationary orbit that interference may come from anywhere in the hemisphere under the satellite. Because the location of an interfering transmitter is so uncertain, traditional means for geolocating it is not effective. “Down-looking” detectors are needed to detect the “up-looking” interference. Moreover, a low cost, simple solution to the problem – one in which the cost to geolocate is small relative to the cost of the downtime in the channel – is required in order to make the solution tractable.

Geostar Radiodetermination Satellite Service (RDSS) measures the position of vehicles on the earth using geostationary satellites and time-difference ranging techniques. One of the RDSS frequency bands overlaps with a radio astronomy frequency allocation. The Federal Communications Commission (FCC) mandated a unique spacetime frequency sharing rule to minimize interference.

The Radiodetermination Satellite Service (RDSS) provides radiolocation and limited communication for users of aeronautical, marine and terrestrial vehicles. A system of the RDSS is being developed in the United States by the Geostar Corporation (Rothblatt, 1988). In this system signals from a control center are transmitted to transceivers on users’ vehicles through one of a series of two or three satellites spaced along the geostationary orbit, and responses are received through each of these satellites. If three satellites are used, locations of vehicles can be determined from the measured times of transmission from the control center to the user and back via different satellite paths. If two satellites are used the timing data must be supplemented by information such as altitude data from a terrain map, or an altimeter reading from the user’s vehicle, which can be encoded in the response transmitted back. Computation of the vehicle location is performed at the control center. The equipment at the vehicle can be relatively simple since its basic function is that of a transponder, and intervention by the vehicle operator is not necessarily required. However, provision can also be made for the inclusion of short coded messages from the vehicle, so the unit on the vehicle is usually referred to as a transceiver. The system is designed to meet the needs of various users; for example, trucking companies which must periodically monitor the locations of vehicles in a widely distributed fleet. Position information can be relayed to the company headquarters as well as to the vehicle operator. The role played by the central control computer in determining the locations requires that all vehicles using the service identify themselves by a transmitted code. Thus developers and operators of RDSS systems can lease their services to vehicle users.

Measurements at the 1612 MHz OH line are necessary to identify and study an important class of highly evolved stars whose properties cannot be determined by other means. Observing at this frequency over an 18 month period has been a frustrating experience in “sharing” a small portion of the 1610-1626.5 MHz frequency allocation on a secondary basis with the Aeronautical Radionavigation Service. Transmissions from the GLONASS satellite system are so powerful that they are received in the side lobes and back lobes of the radio telescope, no matter where in the sky it is pointing as long as one satellite is above the horizon. On most days about half the observations had to be discarded to prevent spurious satellite signals from masquerading as astronomical spectral lines.

Solar microwave burst observations with high time resolution (~ 1 ms) are important but difficult to make. It is shown by the experiments of radiometer at wavelength 21 cm on 1 ms time scale that some ultrafast time structures in microwaves, which includes spike impulses, switch-on and switch-off structures, etc., may not be from solar emission but from RFI (radio-frequency interference) or from radiometer itself. Because of the uncertainty at 21 cm and other several wavelengths, we suggest that joint observations of the solar microwave bursts on 1 ms time scale should be carried out on the peak years of the 22nd solar activity cycle.

Plans are currently being made to carry out a comprehensive, all-sky search for radio signals of extraterrestrial origin. The survey will employ the Goldstone tracking station near Barstow, California, and other sites in the northern and southern hemispheres. The principal parameters of this survey are given in Table 1. In preparation for this search, we have constructed a radio spectrum surveillance system (RSSS), and made a series of measurements of the RFI environment at the Goldstone-Venus tracking station. We describe in this paper the receiving system used (Crow et al. 1985), and the results of a low-sensitivity survey performed during February 16-24,1987.

As competition for radio spectrum space continues to increase, radio astronomers can expect to put more technical effort into ways of observing in the presence of interference. Much of the spectrum outside of exclusive radio astronomy frequency bands will continue to be available to the science if receivers and antennas are designed to make efficient use of times, frequencies, directions, and coherence envelopes that do not contain sources of interference. The paper outlines the state of the art in antenna sidelobe reduction, high dynamic range spectrometers, and receiver designs for handling large signals. Techniques for excising pulsed interference on very short timescales and a few thoughts on signal canceling techniques are discussed.

Radio astronomical observations are increasingly affected by man-made interference of various kinds. Sophisticated software tools have to be developed to cope with interference effects in the data. A calibration procedure is described in which the effects of different categories of errors are separated. If this separation can be performed for interference effects.

Interference effects remaining in the data are erroneously interpreted as source structure by self-calibration techniques. This is due to the fact that self-calibration alone does not properly take into account this separation. Some results for the Westerbork Synthesis Radio Telescope are given.

Determining whether a particular radio transmitter will produce harmful levels of radio-frequency radiation at a location of interest (the “receiver,” be it a nearby home or a distant radio telescope), has two steps. The first is to determine which standard for harm applies: Section II reviews those for human exposure, for interference with electronic devices, for interference with optical and infrared astronomy, and for interference with radio astronomy.

The second step is to estimate the propagation losses between the transmitter and the “receiver.” Many factors, several highly time variable, contribute to such losses - including atmospheric refraction, diffraction by obstacles, tropospheric scattering, and atmospheric absorption - and are discussed in Section III.

The radio regulations often require frequency bands to be shared between radio astronomy and services which transmit. This poses severe problems in a small island, given the sensitivity of radio astronomy receivers. The survival of radio astronomy in these circumstances depends on wide awareness of the problems. Some of the current sharing problems in the UK are discussed.

If the funding that has been requested in NASA’s FY89 budget is forthcoming, the Search for Extraterrestrial Intelligence (SETI) will inaugurate its microwave search program on Columbus Day, 1992. This systematic search for evidence of narrowband microwave signals generated by another technology will continue at several sites around the world until at least 1998. The search will be conducted in two modes: a sensitive targeted search of selected solar type stars covering 1 to 3 GHz, and an all sky survey that covers 1 to 10 GHz. This region of the spectrum is the quietest in terms of natural background noise from astrophysical sources and/or the terrestrial atmosphere, which is why it was chosen. Unfortunately, at the same time that terrestrial technology has enabled this systematic search, it also threatens to render the search all but impossible from the surface of the Earth. Terrestrial transmissions generated for an enormous number of different purposes will be the signals most frequently detected by the SETI systems! This paper describes the design decisions and observing plans that the SETI Program is now making in order to have a chance of successfully “listening” throujgh this heavily utilized portion of the spectrum. We do not yet know precisely how heroic our efforts to discriminate against and identify RFI may have to be, but we have been accumulating observational experience and data that demonstrate convincingly that the situation is rapidly growing worse. Because of the real-time nature of the signal processing being planned, SETI systems will provide an excellent opportunity to monitor, and perhaps identify, sources of narrowband interference within the radio astronomy bands at each site where observations are being conducted.

The International Radio Consultative Committee (CCIR) is a permanent organization within the International Telecommunication Union (ITU). The purpose of the CCIR is to provide technical advice to the ITU and its various organs and members on the characteristics of the radio services which are governed by the International Radio Regulations, a product of the ITU. This is accomplished through reports and recommendations which may result from the regularly scheduled meetings of the CCIR or from meetings convened to prepare for a special event such as an upcoming World Administrative Radio Conference (WARC). The CCIR is divided into Study Groups. Study Group 2 covers Space Research and Radioastronomy. This paper describes the interactions of radio astronomers with Study Group 2. The radio astronomy related Study Questions currently before this Study Group are delineated, and the nature of the active reports and recommendations are discussed.

The World Administrative Radio Conference for the Mobile Services (WARC Mob-87), held in Geneva in September-October, 1987, took several actions that will have an impact on radio astronomy. Worldwide frequency allocations were made for the Radiodetermination Satellite Service (RDSS) in the 1610 to 1626.5 MHz band. The secondary allocation to radio astronomy in this band has received strong protection, however. The 1660 to 1660.5 MHz band, which radio astronomy shared on a primary basis with the Aeronautical Mobile Satellite Service has been reallocated to the Land Mobile Satellite Service, shared with radio astronomy on a similar basis. The impact that this reallocation will have on radio astronomy is not clear. Since the Radioastronomy Service is primary in the band, Mobile Satellite Systems that evolve will have to provide adequate protection. Perhaps most significantly, WARC Mob-87 recommended that a conference be convened to reallocate all bands in the 1 to 3 GHz range, no later than in 1992. Several other conferences dealing with particular services have been proposed. At first glance, some of these some of these proposals may not appear to have an impact on radio astronomy. They will, however, increase the pressure on the entire radio spectrum. Finally, it is highly likely that a General WARC will be proposed to be held before the end of the 1990s. The radio astronomy community will have to prepare for these conferences.

Radio pollution of space is connected with general ecological conditions resulting from world development. A practical aspect of the problem is related to the possibility of making radioastronomy observations of cosmic emissions and searching for signals from extraterrestrial civilizations over almost all of the entire radio spectrum. For this purpose, it was decided at WACR-79 to keep a quiet zone on the shielded side of the moon. Here we shall discuss the problem as it relates to an observatory in orbit. The vicinity of the L2 Lagrangian point of the earth-sun system is of particular interest as a location for such an observatory.

The purpose of this paper is to describe one program of the National Academy of Sciences’ Committee on Radio Frequencies (“CORF”). This program uses legal means to protect radio astronomy frequencies from radio interference.

CORF is composed of eminent scientists from the fields of radio astronomy, space research, remote sensing, meteorology, and wildlife tracking who use radio frequencies in conducting their research. CORF’s primary objective is to limit the level of harmful man-made interference in the bands used by these scientists as much as possible.