1350 PROCEEDINGS OF THE IEEE, VOL. 61, NO. 9, SEPTEMBER 1973

Spectrum Control Procedures For the National Radio

Astronomy Observatory

JAMES L. DOLAN

Absftacf-TheGreen Bank site for the National Radio Astronomy Obsematory m 0 ) and the Sugar Grove site for the Naval Radio Research Laboratory (NRRL) wereiocated fairly close together with the hope that both could share in the locally relatively low radio noise environment that existed when the installations were &st built. The procedures which have been followed to maintain, as far as possible, the scientific value of this environment are described, and some examples of the degree to-which this effort is succeeding are given. Similar problems wiil arise at the site chosen for the very large array (VLA) in New Mexico and tlxeseare dkcrlssed briefly.

INTRODUCTION

T H E National Radio Astronomy Observatory (NRAO) at Green Bank, W. V a , has now beerr in. continuous operation for about 16 years: The-site was selected-

with several considerations in m i d , onewf -which- was the existing ambient radio noise level in the Green Bank vicinity. At the time that Findlay [l] prepared his paper on the sub- ject, the Observatory was in its infancy. However, i t was dear at that time that some attempts should be made to maintain and possibly improve the radio-freque’ncy spectrum for the kinds of research using very sensitive receivers which were planned for both NRAO a t Green Bank and for the Naval Radio Research Laboratory (NRRL), a facility of the Naval Research Laboratory (NRL) of Washington, D. C., located about 30 mi east of Green Bank near Sugar Grove. The measurements made during the initial site selec- tion process by Jansky and Bailey, Inc., Washington, D. C. [l], indicated that the vicinity around Green Bank was rela- tively free from high-power transmitters and man-made inter- ference of various types. The measurements showed that the VHF bands such as television, FM, andaircraft communica- tions were occupied, as one would expect, though television and FM broadcasts were generally weaker than in other areas of the country. Above approximately 400 to 500 MHz the spectrum was relatively clean and there were few man- made signals large enough to be observed. Findlay pointed

THE RADIO-QUIET ZONE The zone is bounded by a rectangle described by lines 01

latitude and longitude and is about 34 OOO km2 in area situated on the West Virginia-Virginia border. The details of operation of the quiet zone have been described more fullJ in other places [ 2 ] , but, to summarize, all new applicants ir the quiet zone, except amateur, temporary, and Class E: Citizens Band, are required to notify the director of tht NRAO of their intentions, and the NRAO and the NRL a r c permitted to comment on the impact of t h e p ~ o p o s d t r a n s mitter system before the FCC will issue a license. Similarly a Government agency proposing to install a new systen within the zone must notify the NRL, which in turn notifie: the director of the NRAO, and comments are jointly mad( to the IRAC. In case of conflict, the FCC and the IRAC an the final authorities. In practice, over the years, many case have been resolved by the design of systems acceptable to a1 concerned parties.

Initial control procedures have mainly consisted of mak ing power density predictions based on path loss derive( from hand-drawn path profiles. In some special cases thl transmission loss has also been measured. Path loss analysi has been done mainly by using methods outlined by Bulling ton [3] and Rice and Longley [4] of the National Bureau o Standards. Measurements have been made using calibrate( communications receivers, or in some cases single-ban1 radiometer-type receivers, constructed at the NRAO labora tory. As the radio telescopes have used more sensitive ampli fiers such as masers, parametric amplifiers, and tunnel diod amplifiers, better methods of spectrum control have becom necessary, the first essential being to define more carefull: the levels of interfering signals which could damage the radio astronomical observations.

THE SENSITIVITY OF RADIO TELESCOPES Out steps that be taken to preserve the The sensitivity of a radio~astronomy receiver is a furrctio trum around Greenk Bank, and at the time of his report some had already been taken. The director of the National Science width. I t is to generalize about sensitivity for a:

of effective noise temperature, integration time, and band

Foundation and the president of Associated Universities, Inc., had requested the FCC to limit the radiation from cases. However, in order. to set up a more meaningful con

licensed transmitters within a zone surrounding NRAO and trol program, a model receiver was assumed to have a nois temperature of 50 K, an integration time of 1000 s, an

NRRL* The FCC with rule-making procedure No* bandwidths of either 100 kHz or 10 MHz. These 11745, dated November 19, 1958, and the Interdepartmental are realistic and are achieved in modern radio-astronom Radio Advisory Committee (IRAc) agreed to systems. Using these parameters, a curve of maximum allom certain restrictions to new Government transmitter facilities within the zone around NRAO and NRRL. able interfering power density can be plotted and is shown i

Fig. 1. I t can be seen from this curve that up to loo0 MH a maximum desirable level of 10-1’ W/m* has been adaptec

rekvd February 1, 1973; M~~ 1, 1973. The Actually, the curve continues downward lower than le: National Radio Astronomy Obsematory is operated by A8sociated W/mf in the spectrum below 1ooO- MHz, but there are i Universities, Inc., under Contract with the National Saence Foundation. signals, originating mainly outside the zone, tha

Green Bank, W. Va. 24944. The author is with the National Radio Astronomy Observatory,

exceed the maximum desirable level in the high-frequenc:

DOLAN: SPECTRUM CONTROL PROCEDURES 1351

, I I , ,-go ! UAXIMUM D E S I ~ A E L E POWER DENSITY AT N R A O ,

GREEN 8ANK.W.VA.

10-0 I 1

1 6 ’ ~ -150

IO+ -160

16” -170

10- -180

10-l~

[y]

IO -190

1000 10000 20000 100 f [“HZ]

Fig. 1 . Maximum desirable power density at NRAO.

range. Thus 10-17 W/m2 was chosen as a good compromise in this area of the spectrum. Above lo00 MHz the sensitivity decreases and the maximum desirable level increases until the tolerable power density a t 10 GHz is about W/m*. These curves were made with the assumption that the receiv- ing antenna is isotropic, a good approximation for large parabolic reflectors a few degrees off the main beam. All the predictions of interference at Green Bank are made assuming an isotropic receiving antenna.

INTERFERENCE MEASUREMENTS At the NRAO a mobile surveillance system capable of

observing the radio spectrum from 50 MHz to 8 GHz in approximate octave bandwidths has been built. The equip- ment is designed along classical radio-astronomy receiver lines and is capable of making either short- or long-term spectrum analysis measurements. The mobile system is used in the Green Bank area to locate sources of interference with radio telescoges such as power line faults, oscillating TV boosters, and other defective electrical devices. The inter- ference-checking vehicle canelocate most line faults within a few minutes and report them to the local power company for corrective actions. This equipment has also been used for long-term spectrum analysis ad site surveys. Over the last 15 years, radiometer sensitivity has increased three or four orders of magnitude to about 10-” W/mf. This increase in sensitivity has brought with i t increased susceptibility to lower levels of intentional and unintentional radiation. The problem is compounded by an increase in the number of radio transmitters in the zone. There are at present about 1500 licensed transmitters in the zone-a large number con- sidering the controls and the rural nature of most of the area. Most of this equipment is in the point-to-point business

FREQ. Yhr

Fig. 2. A spectrum survey at Green Bank, 150-200 MHz.

Fig. 3. A spectrum survey at Green Bank, 200-250 MHz.

bands: police and emergency. All new transmitters since the inception of the quiet zone have been processed by the NRAO and the NRL, and in some cases qualifications have been placed on the proposed stations such as the use of direc- tional antennas, lower power, and sometimes the relocation of antenna sites.

Although the number of broadcast and point-to-point transmitters has increased, the increase has been in an area of the spectrum that was somewhat congested before the quiet zone was created. Man-made interference in the quiet zone above about 500 MHz is most often caused by faulty switches, electric motors, power lines, airborne radar, or automobile ignition systems. Automobile ignition systems have become increasingly more troublesome due to the in- creasing number of vehicles passing the site and the higher sensitivity of radio-astronomy receivers. Most on-site main- tenance vehicles have been converted to diesel, which pro- vides some relief from much of this type of impulse radiation.

The samples of spectrum measurements shown in Figs. 2 and 3, although not directly comparable to the measurements made by Jarrsky and Bailey, indicate the conditions in the VHF areas of the spectrum. Another compatibility problem that has arisen at the NR4O is interference caused by on-

1352 PROCEEDINGS OF TEE IEEE, SEPTEMBER 1973

site equipment. The mobile interference detection system has been used many times to locate sources of interference such as an on-line computer generating wide-band noise, local oscillators radiating into adjacent systems, etc. To minimize this problem, some of the new buildings at the NRAO have had shielding built in during construction. These shields, consisting of wire mesh within the walls of the build- ing, provide about 20 to 30 dB of shielding in the frequency range mostly used for observations at the NRAO.

THE PREDICTION OF INTERFERENCE LEVELS Along with the new surveillance equipment and measuring

techniques, there has been an improvement in analytical procedures for predicting power density at the site caused by proposed transmitters in the protected area. Through the facilities of the Electromagnetic Compatibility Analysis Center (ECAC) a t Annapolis, Md., most of the radio-quiet- zone topographical data have been placed in computer memory. This program allows automatic profile plotting with an accuracy better than 15”, and when used in conjunction with diffraction analysis programs developed by the National Bureau of Standards [SI, computerized-path loss calculations can be made. When the coordinates and physical parameters of the proposed transmitter are known, the computer pro- gram provides the predicted transmission loss and power density a t Green Bank caused by the proposed transmitter. If the predicted level exceeds the values shown in Fig. 1, the application is subject to disapproval by the NRAO. In most such cases the engineering staff a t Green Bank aids the applicant in designing a system which will meet his needs and still be compatible with the requirements of the NRAO and the NRL.

SPECIAL CASES This is how the system presently works, except for some

applications warranting individual attention, such as police and emergency services. These are handled as special cases. An area below 50 MHz has been set aside for special emer- gency services within the radio-quiet zone, and it has been the policy of the NRAO to work around this particular band (44.61-46.6 MHz). Also,-police services are given every con- sideration in the zone sa as not to impede efforts to preserve life and property.

The spectrum above 1 GHz is still relatively free from interference except for a few areas occupied by high-power radar between 1 and 1.4 GHz. Many of these radar systems are located outside the perimeter of the zone but possess sufficient signal strength to be observed a t Green Bank- There have been some cases of interference in-assigned bands such as the hydrogen line band 1400-1427 MHz, apparently caused by mistuned military radar or possibly some electronic countermeasure (ECM) equipment on board military air- craft. These cases are relatively isolated and can often be handled by proper communications with military authorities.

At the time of Findlay’s report, the hydrogen spectral line was the only detected line in the microwave region. Since then, other substances such as formaldehyde, am- monia, and water have been discovered in interstellar space. There have also been some band assignments to t h e radio- astronomy service such as the exclusive assignment of 1400- 1427 MHz to cover the hydrogen line. Nature does not al- ways agree with human decisions, and many of the newly discovered spectral lines do not fall in one of the already assigned radio-astronomy bands. This makes i t necessary to make observations outside the allocated bands, and the

radio astronomer must coexist with services legally occupying these bands. At Green Bank observations can still be made outside the assigned radio-astronomy bands with a high de- gree of success, but in many parts of the country it is not possible to carry on radio-astronomical observations outside assigned bands. As far as is known, the radio-quiet zone around NRAO and NRRL is unique, and efforts of the kind described here will be continued to preserve its future value.

THE VERY LARGE ARRAY SITE The construction of a very large array (VLA) for radio

astronomy on the Plains of St. Augustine near Magadelena, N. Mex., is just beginning. The site chosen has many ad- vantages for this project but presents a special problem in electromagnetic compatibility. The large Dep. of Defense range locate,d at White Sands, N. Mex., approximately 125 miles southeast of the VLA site, is heavily instrumented with electronic equipment. Measurements are currently under way to determine spectrum characteristics in the vicinity of the VLA, and an investigation is being made of all frequency assignments in the area to determine what the electromag- netic environment is in the vicinity of the VLA. Since no quiet zone has been established in New Mexico to protect the VLA from interference, the instrument will be forced to operate in the existing environment. The VLA is basically a large number of interferometers, and this fact, combined with the use of a good antenna design and filtering tech- niques, can make it somewhat less susceptible to radio inter- ference relative to a single-dish radio telescope, but inter- ference will still be an important factor in the successful use of this instrument. Distance and terrain shielding will help when the transmitters are ground based, but airborne trans- mitters such as ECM’s and radar present special problems. In some cases i t will be possible to schedule telescope opera- tions in areas of the spectrum not being used for this type of electronic operation, but this requires prior knowledge of band use, which is not always available. Spectrum utiliza- tion will be an integral part of .the operating plans for this instrument.

CONCLUSION I t is interesting to speculate on what might have hap-

pened to the spectrum around Green. Bank if there had been no controls. There would have- been many point-to-point transmitters within the line of sight of the facility. At least one communications.satellite ground station would have been located within 50 mi (80 km). Television stations:operoting on subharmonics of the hydrogen line would have been located in the zone. There may well have been applications which were not submitted in view of quiet-zone restrictions. But the-foresight of those responsible for the quiet zone has been borne out; With continued control the area should be useful for radio research for many years.

REFERENCES (11 J. W. Findey, ‘Noise levels at the National Radio Astronomy

[Z] J. L. Dolan, ‘EMC in radio astronomy,” in 1970 IEEE Elcctro- Observatory, Proc. IRE, vol. 46, pp. 35-38, Jan. 1958.

[3] K. Bullington, “Radiopropagation fundamentals,” Bell Syst. Tmh. J . , magnetic Compatibility Symp. Rec., pp. 365-375.

[4] P. L. Rice, A. G. Longley, K. A. Norton, and A. P. Barsis, “Trans- vol. 36, pp. 593626, May 1967.

mission lass predictions for tropospheric communicatian circuits,r NBS Tech. Note 101 (revised;, May 1, 1966.

151 A.G. Longley and P. L. Rice, Prediction of tropospheric radio trans- mission loss over irregular terrain: A computer method-1968,’ U. S. Dep. of Commerce/Environ. Sci. Sew. Admin., Boulder, Colo. ESSA Tech. Rep. ERL 79-ITS 67, July 1968.