Radio Spectrum Wars

Jose Ribeiro

HET608 20 Sep. 03

In a little more than one century, the use of electromagnetic radio waves became the main media for communications. The experimental work of Heinrich Hertz in the second half of the 19th century, proved in practice the theories of James Clerk Maxwell on the electromagnetic fields. Guglielmo Marconi gave the technological meaning to the works of Hertz by developing the first telecommunication system based on the emission and reception of information transported through an electromagnetic wave. He performed the first transatlantic wireless communication in 1901.

More than a quarter of century later, radio astronomy was taking its first steps with the discovery by Karl Jansky in 1932 of a strange “static” that changed position, located in the direction of the constellation of Sagittarius. Jansky’s discovery was studied by Grote Reber, who developed the first single dish radio telescope.

A war was about to begin, other than the WW2: the war for radio frequency allocation. The major part of radio services are active, emitting and receiving radio waves. Radio astronomy is mainly a passive service, only receiving radio signals, with the exception for radar astronomy, which is not the issue of this essay.

With the growth of the use of radio services, the radio spectrum became more and more saturated in many of its bands. On the other hand, radio technology has advanced and the upper part of the radio spectrum including the micro waves is beginning to be routinely used by many services, some of which in massive civilian industries, such as the short-range radar (SRR) in the automobile industry, and the mobile phones.

Definitions:

One consequence of this radio spectrum usage is the radio frequency interference (RFI) between services. According to the International Telecommunications Union (ITU), the international organisation that rules the radio spectrum by dividing it into bands, interference is defined as “The effect of unwanted energy due to one or a combination of emissions, radiations, or inductions upon reception in a radio communication system, or loss of information which could be extracted in the absence of such unwanted energy”. [5].

Interference is classified in several degrees of importance, being the harmful interference the worst one. In this case, it may be due to:

- Sharing services, the case in which more than one service shares the same frequency band [5].

- Out-of-band emission, the emission of frequencies which exceeds the necessary bandwidth, at extending the emission to its neighbour bands.[6].

- Spurious emission, emission of frequencies outside the allocated bandwidth, mainly due to bad filtering, harmonic emissions, and parasitic emissions [7].

Radio astronomy

In radio astronomy the signals of interest reach us millions of times weaker than the radio sources generated by men. There are two kinds of observation of radio astronomy:

continuum observations and spectral line observations. Continuum observations are normally wide-band observations. They are important to characterise the shape of the emission spectra of the radio sources, thermal or non-thermal. That information is useful in order to assess some physical parameters of the radio sources, such as magnetic fields, temperatures, and densities. Spectral line observations do not need wide-band hardware, except for shifted lines due to the Doppler effect. These observations are extremely important in the assessment of the element/molecule abundance of the radio sources, as well as their dynamics.

Everything that produces electric sparks may interfere with the arriving signals seriously degrading them. Devices that we use everyday such as electrical motors, personal computers, and mobile phones, may be at this precise moment jeopardising an important observation of a gamma-ray burst afterglow. RFI is so serious that occupies today an important place in the radio astronomy work. Many international institutions are dealing with this problem. The Radio astronomy Commission 40 of the International Astronomical Union (IAU) working in co-operation with the ITU, allocated some bands specific for the use in radio astronomy. Some of these bands are reserved, meaning that all emissions are prohibited, others are shared with other services, some passive and some not at all. For example, the 22 GHz frequency so important due to the signature of the water molecule only has one shared allocation in the 22.21-22.50 GHz band, being the neighbour bands of 22.01-22.21 GHz and 22.81-22.86 GHz not even allocated. This means that redshifted or blueshifted H₂O line could never be detected due to harmful interference [8].

Even the reserved bands are not free from harmful interference, due to out-of-band emissions or to spurious emissions. For example, the most important band of 1400-1427 MHz is threatened by out-of-band emissions from Aeronautical Radionavigation service, Fixed service, Mobile service, and Space Operations service, and by spurious emission from Broadcasting service and Broadcasting Satellite service, both in the band 1452-1492 MHz. [9].

ITU reserved the following frequency bands to radio astronomy, where all emissions are prohibited [4]:

1400-1427 MHz

This band is the 21 cm neutral atomic hydrogen emission line. It is due to the electron spin-flip transition. The neutral hydrogen atom is normally in the lower energy configuration, where the proton and the electron have opposite spins. Due to the influence of some external causes such as collisions, the spins of the atom may become equal (parallel spin) and the atom changes to a higher energy configuration. When the atom returns to its lower energy configuration, then its emits a photon with a wavelength of 21cm (1420.406 MHz) [2]. As hydrogen is the most abundant element in the Universe, this band has the major importance in mapping, the hydrogen distribution through the interstellar medium in our Galaxy as well as in other galaxies and in studying galactic dynamics by measuring the Doppler shifts of this line.

2690-2700 MHz

This band is used in continuum measurements. At this band the Galactic background is weak, thus allowing studies of extragalactic radio sources, mainly in non-thermal processes.

10.68-10.70 GHz

This band as well as the next one marks the limits of detectability of radio synchrotron radiation, a radiation whose amplitude decreases with increasing frequency. The band is used by the Very Long Baseline Interferometry networks, mainly to estimate the size of quasars. [1].

13.35-15.4 GHz

This band has the same astronomical interest as the previous one.

23.6-24 GHz

This band covers three ammonia (NH₃) [3].

31.3-31.5 GHz 31.5-31.8 GHz

Continuum observations.

48.94-49.04 GHz

The more important line in this band is carbon monosulphide (CS), that is found in “giant molecule clouds and cool dark clouds” [4], clouds difficult to detect and map.

50.2-50.4 GHz

Continuum observations.

52.6-54.25 GHz

Continuum observations.

86-92 GHz

Continuum observations and spectral line detection. The most important molecular lines are: two lines of silicon monoxide (SiO), formylium (H¹³CO⁺), Ethynyl radical (C₂H), and hydrogen cyanide (HCN).

100-102 GHz

Continuum observations. Spectral line of cyanoacetylene (HC₃N).

109.5-111.8 GHz

Continuum observations. Some varieties of carbon monoxide, deutered ammonia and methyl cyanide lines.

114.25-116 GHz

Continuum observations. One important line, carbon monoxide (CO), is found at 2.6 mm. There is a relatively constant ratio of CO abundance to molecular hydrogen (H₂) of 1 to 10000 molecules. Molecular hydrogen is hard to detect because it is symmetric and its radiation due to rotation is weak, so CO is the beacon for the study of giant molecular clouds.

148.5-151.5 GHz

Continuum observations. Nitric oxide (NO) and formaldehyde (H₂CO) lines.

164-167 GHz

Continuum observations.

182-185 GHz

Continuum observations. A water vapour line can be found at 183.31 GHz.

200-202 GHz

Continuum observations. Water vapour (H₂¹⁸O) and sulphur monoxide (SO) lines.

226-231.5 GHz

Continuum observations. Cyano radical (CN) and carbon monoxide (CO) lines.

250-252 GHz

Continuum observations. Nitric oxide (NO) and sulphur dioxide (SO₂) lines.

Among the important bands that are not allocated to radio astronomy, one can find:

1330-1400 MHz, important to study redshifted atomic hydrogen.

3260-3267 MHz, 3332-3339 MHz, 3345.8-3352.5 MHz, important for the methyladyne (CH) molecule whose presence may suggest the presence of methane (CH₄), a basic component to life.

22.01-22.21 GHz which importance was explained above.

Until now, ITU has only allocated frequencies up to 275 GHz.

Many telecommunication services threaten the bands allocated to radio astronomy. The reserved ones are threatened by out-of-band and spurious emissions while the only allocated ones are also threatened by the sharing services. The majority of the RFI sources to radio astronomy come from Aeronautical and Satellite services [12]. This is logical as the signals of interest also come from above, to where the directional antennas are aimed. Even the far side lobes of the antenna may be the door to satellite RFI [11].
In land, mobile services, including mobile phones, and broadcasting services, are among the more active RFI sources to radio astronomy.

In the future higher frequency bands will be occupied either by telecommunication services, either by radio astronomy. Reserving bands for radio astronomy will be essential, basically for the study of molecular lines. Yet, radio astronomy must develop techniques in order to suppress or minimise RFI.

Radio Frequency Interference Mitigation

The RFI mitigation techniques will have in the future of radio astronomy the same relevance that active, adaptive, and tumography techniques have in the actual-future optical astronomy. As a big aperture optical terrestrial telescope is meaningless without atmospheric turbulence correction, so an ultra-sensitive, ultra-wide-band, ultra-large-array radio telescope system will be useless is RFI countermeasures are not taken, independently of frequency reservation by international authorities.

Some techniques to mitigate RFI are being studied by IAU [10]:

- Choice of site

In order to minimise RFI, the physical place where the receiving antennas will mounted must be very well characterised for interference sources. Sometimes, a small terrain feature may work as a radio reflector changing the behaviour of the antenna.

- Good engineering

The antennas and structure must be projected in order to minimise the side lobes, a major door for RFI entrance. Good shielding of the observatory’s electronic devices must be done in order to prevent unwanted radiation. A difficult task in electronics engineering in the design of high-gain amplifiers free from saturation. In the future radio astronomy better high-gain amplifiers will be required.

- A priori and adaptive techniques in real-time

These techniques are based in real-time monitorisation of RFI signals and consequent action of mitigation. The actions may be time blanking, where the signal acquisition is interrupted during a RFI event; frequency blanking by rejection of a specific band interval, when the RFI source has an unchanged known frequency; adaptive cancelling by sampling the RFI source and subtracting it from the signal of interest; interferometric nulling, in interferometer systems, by detecting the position of the RFI source in the array and nulling the gain at that point. Some of these techniques may be applied in later analysis.

As in the optical telescopes, the real-time procedure to correct the signals of interest depends a lot on the computation capacity and the future measures will benefit from the improvement of computers. One thing is for sure: RFI mitigation techniques will take a major place in the near future of radio astronomy.

Conclusions

Mankind live with their heads turned to the ground. It is impressive the ignorance of the majority of people about the Nature that exists above their heads. Light pollution is an example. It is important that the astronomers who have their heads mainly turned to the sky indulge in actions to sensitize people that the sky is patrimony of the humanity. This should begin as soon possible, preferably at the elementary school. If mentalities are more receptive, perhaps the RFI war will be over. The continuous stress in defending the radio astronomy bands will no more exist in the future. As an example, recently the automobile industry began equipping cars with SRRs as a help for parking. These radars were to work in a frequency important in radio astronomy. Fortunately, radio astronomy institutions managed to persuade the automobile industry to change the frequency of the SRR. In future, situations like this one should not be considered small victories, but the natural behaviour for the respect amongst services (that service everybody).

References:

[1] Report to the IAU General Assembly, by Division X/Commission 40: Radio

Astronomy, Sydney 2003

[2] Universe 5th Ed., Kaufmann and Freedman, ISBN 0-7167-3495-8

[3] IAU List of Important Spectral Lines

http://www.astron.nl/craf/iaulist.htm

[4] Astrophysical Importance of Bands in Radio Astronomy

http://www.astron.nl/craf/freq.htm

[5] Frequency Sharing

http://www.astron.nl/craf/shardef.htm

[6] ITU-R definition of Out-of-band Emission

http://www.astron.nl/craf/oubdef.htm