DECADES ago, the ether was a peaceful wilderness. With a sensitive radio ear, you could just about pick up the faint sounds of nature: the hiss of ancient cosmic microwaves, the delicate drumbeats of spinning neutron stars, the crackle and low whistle of lightning strikes. Then came radio and TV, their hulking transmitters filling the atmosphere with electromagnetic noise.
Now they have been joined by mobile phones, GPS receivers, CCTVs, wireless broadband connections and all manner of other stuff, fracturing the once-pristine airwaves into a crazy cacophony. Even cars have Wi-Fi hotspots that can’t be disabled and wireless sensors that constantly communicate their status with the dashboard computer. “There are transmitters in everything,” says astronomer . “And they’re mobile.”
Liszt works at the US National Radio Astronomy Observatory in Virginia, and he knows the problems this causes. His kind need pin-drop quiet to detect emissions from across the cosmos, produced by interstellar molecules that might be early stages in the emergence of life, or clouds of hydrogen that will soon form stars, or giant galaxies where black holes generate plumes of hot gas. As the airwaves fill up with chatter, that quiet is becoming increasingly difficult to find. Right now Liszt and his colleagues are squaring up to do battle for a fundamental right – their right to silence.
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The clamour for spectrum space affects us all. It would be no good if your TV signal cut out every time you made a phone call, or if car radars wiped out air traffic control. Everyone has to agree what radio waves belong where, across the whole planet. “Every country is sovereign, they could do what they want, but radio waves don’t respect borders,” says Liszt.
“It would be no good if your TV signal cut out every time you made a phone call”
That’s the starting point for one of those unknown, essential processes that determine the smooth functioning of modern life. It comes to a high point every four years at the (WRC), held in Geneva under the auspices of the International Telecommunications Union. Around 3000 or so delegates representing commercial, government and scientific interests from more than 180 nations attend. Similar meetings have been , and so far they have apportioned frequencies from 10 KHz – in the land of very low frequency, where wavelengths are measured in tens of kilometres – right up to 275 GHz near the top of the extremely high frequency band, with wavelengths around a millimetre.
Thanks to these internationally agreed rules we have things such as the international aircraft mayday frequency at 121.5 MHz. These agreements flow down to national bodies who fill in the gaps, producing detailed plans of who can use what bandwidth where (see “Tune in your radio“). The result is a set of accepted frequencies on which television and radio are beamed, where mobile phone and broadband signals can be broadcast, on which amateur radio hams can communicate, and where GPS, climate monitoring satellites, RFID tags, car parking radar, radar for detecting obstacles on railway level crossings and all manner of other signal-beaming devices can operate.
Tune in your radio
Taking into account international rules, different nations allocate radio spectrum frequencies differently.
- USA: National Telecommunications and Information Administration
(PDF download) - Australia: Australian Communications and Media Authority
(PDF download) - UK: Ofcom
For radio astronomers, the WRC process means a quadrennial battle to defend their territory: “reserved bands” where quiet from other radio sources is particularly essential. One instance is the interval from 1400 to 1427 MHz in which hydrogen atoms emit a spike of radiation. This 21-centimetre wavelength reveals clouds of interstellar gas, and can be used to map our galaxy’s spiral arms and follow its rotation. More reserved bands cover frequencies where other crucial cosmic molecules such as water or ammonia give out energy in the form of radio waves (see “The universe in radio“).
Such frequencies lie well above the KHz and MHz frequencies of traditional broadcast radio and television. But the higher a radio wave’s frequency, the greater its bandwidth, which quantifies the amount of information it can transmit in a given time. Frequencies from a few hundred MHz to a few GHz represent a sweet spot for mobile 3G, 4G and broadband services. Now, with the drive for even more data-hungry 5G services, telecoms companies are encroaching on higher-frequency territories that radio astronomers once called their own.
Don’t interfere
At the last WRC meeting in 2015, the agenda item on mobile broadband amounted to “find some bands between 400 MHz and 6 gHZ” says , chair of Europe’s (CRAF), which works to prevent interference in reserved bands. This dangerously vague proposal included the important hydrogen band around 1420 MHz. “It got crazy – an enormous circus,” says Van Driel. “We had meetings with 500 people attending. But it concerned the 21-centimetre band, which is sacrosanct to us, so we were very vigilant.”
Based on what governments have been able to demand auctioning various parts of it to commercial interests, the entire radio spectrum is worth trillions of dollars. Perhaps surprisingly, though, telecoms companies can’t play purely on profit. Diplomacy and good intent tend to carry the day, and the voices of the astronomers have “surprising weight”, says Liszt. “If people try to come up with monetary arguments, they are whistled down by the chair – that is not the way the game is played,” says van Driel.
At CRAF, spectrum manager is in charge of studying any potential threats to astronomers’ interests. If there’s a serious risk of interference, she might suggest “guard bands” – buffer zones to prevent overspill reaching an astronomy band. If that’s not enough? “We ask that no allocation be given,” she says. In the case of services proposed in 2015 threatening the 21 centimetre band, CRAF produced studies showing there could be no ground stations within 500 kilometres of any radio observatory. The proposals were eventually dropped.
“One climate satellite saw how things can go wrong in 2011: all of a sudden, all of Japan lit up”
Another win in 2015 concerned orbiting high-resolution radars operating at frequencies around 9.6 GHz, which are used for ground mapping, including covert surveillance. “The spooks use this because most structures are transparent to 9.6 GHz,” says Liszt. At the meeting, operators agreed not to point these satellites at radio astronomy sites. “They are incredibly powerful. If we looked at one of these when it was pointed at us, it would burn out our receivers in a flash.”
Often, the solution is an uneasy coexistence, creating shared bands where two or more uses are permitted. “When it says there is a shared band, practically speaking we can use it as long as telecoms aren’t using it,” says Liszt. Again, fair play rules the day. Once reserved or shared bands are agreed, nobody tries to pull a fast one, even though the whole agreement is voluntary. “There are no international cops, going round with a robot enforcing electromagnetic compatibility,” says Liszt.
A more insidious problem is broadcasts spilling over from their allotted territory. The Iridium satellite communications network is supposed to operate between 1617.775 and 1626.5 MHz, but a design flaw means that since their launch almost 20 years ago the satellites have been generating interference at other frequencies. Slap bang next door is a band that radio astronomers use to detect hydroxyl. This was the first radio-emitting molecule identified in interstellar space, and reveals the inner structure of the gas clouds there even better than the 21 cm hydrogen band. Or at least it would, were it not for the interference. “It’s not bad in the southern hemisphere,” says Liszt. “But over Europe and the US it’s toxic. If you want to do cutting-edge research in this band, forget it.” The problem should finally be resolved with the new generation of Iridium satellites now being launched.
It’s not just astronomers who suffer: researchers studying Earth from space also need clean airwaves, for example, so they can bounce radio waves off clouds to study global air circulation. One climate satellite saw what happens when things going wrong in 2011. “All of a sudden, all of Japan lit up,” says Liszt. It wasn’t until 2015 that the source was found: a new satellite TV station. It was broadcasting at the right frequency, but equipment used to receive the signal in individual homes was generating interference in a band reserved for Earth observation satellites.
For the next WRC meeting in 2019, Japan has already submitted plans to use real-time video transmitted at various frequencies between 1 and 100 GHz as part of a train control system. “It’s astonishing how much of the spectrum they would use for that – parts of the spectrum currently used for satellites looking down at clouds, for climate research. They assume there’s nothing near those bands.”
The 2019 meeting is likely to see battles spread to still higher frequencies. One of the main flash points will probably be between 24 and 86 GHz. This is ideal for short-range radar that driverless cars will use, while the vast bandwidth available makes it promised land for data-hungry new 5G services. “While you are walking to your airplane, your phone can download a movie,” says Liszt. “The people promoting 5G are trying to put their hands on as much spectrum as they can.”
Onwards and upwards
At these higher frequencies, sharing the airwaves is easier: radio waves are absorbed quickly by the atmosphere, especially in the tens of GHz and above. As long as people coordinate where and when they are blasting out transmissions, radio telescopes can avoid being overwhelmed. Still, with several bands crucial to radio astronomy standing squarely in the way, there are squatters’ rights to be defended, for example in reserved bands around 23 GHz, where emission from ammonia molecules can reveal the temperature of gas in star-forming regions.
A whole new swathe of virgin territory will be on offer at the 2019 meeting. Delegates will slice up and hand out the chunks stretching from the existing boundary at 275 GHz all the way up to 450 GHz. The radio telescopes operating in this band tend to be in isolated spots at high altitude, such as Mauna Kea in Hawaii, so they should be safe from interference. An exception could be the dish on Pico Veleta in southern Spain, which has a direct line of sight to the city of Granada.
As the technological pressure increases, Liszt wants to extend the concept of radio quiet zones. He lives next to a huge quiet zone around the Green Bank dish in West Virginia, the world’s largest steerable radio telescope. Another zone protects the ALMA radio array in Chile. Although effective for the moment, under current rules these zones don’t have to be observed by satellite broadcasts or aircraft – a particular worry as it becomes possible to use mobile phones on planes.
Another option for astronomers is to keep on trekking, heading out across the spectral plane to explore new frontiers and find new signatures for what’s out there. “We are always out front, going to higher and higher frequencies, until technology catches up,” says Liszt. “Things get more difficult but we continue to operate somehow.” And of course there are new frontiers in spatial terms too, with dishes being built on the most remote mountains and at the South Pole – and perhaps someday on the far side of the moon.
But still this most civilised of conflicts continues on Earth, and astronomers can’t afford to stop fighting their corner. “Our colleagues sometimes say – you spend your whole time going to these meetings – what successes do you have to show for it?” says van Driel. “I reply: once we stop going then you will see what happens. They want your frequencies – they know what a hertz is worth.”
The universe in radio
Radio waves provide a unique window on the cosmos. Distant, extreme objects such as pulsars, quasars and black holes send out radio signals at many different frequencies, as does the coldest object in the universe – the cosmic microwave background, the relic radiation of the big bang. In our cosmic backyard, meanwhile, specific objects and chemical molecules send out radio waves at set frequencies – determining which bands radio astronomers are particularly keen to keep clear of noise.
This article appeared in print under the headline “Right to silence”
