A giant battle has emerged between mobile phone operators ready to deploy new 5G phone equipment near airports, and aircraft operators landing at those airports. The battle exists because the airlines are afraid that signals from the 5G towers in a particular radio band might interfere with the radar altimeters that planes use to measure how high they are above the ground when visibility as poor. The altimeter is important and so they are threatening to not fly to certain airports if the 5G is on, or having to abort landings at them if weather changes.
This issue was known about some time ago. In the USA, the FCC is the agency which tries to resolve conflict between people using radio signals. It took a variety of steps to avoid any problem, but not enough to satisfy the aviation folks, who made threats of canceling flights. This caused a delay in the rollout, but that delay is over and the rubber is meeting the tarmac. Who’s wrong or right, and what can be done and what should have been done?
Since the dawn of radio, we’ve resolved radio conflicts by partitioning the radio spectrum. Just as broadcast radio and TV are divided into “channels” or “stations” which get exclusive use of a particular part of this radio spectrum, so are applications and classes of users. Spectrum is allocated according to the frequency of the old-fashioned carrier wave. When you tune your FM radio to 101.5 mhz, it uses a “filter” so that it only listens for radio signals close to 101.5. Another station can be at 101.7 mhz, but if people’s radios have good filters and transmitting power is manged, your filter will stop the broadcast at 101.7 from interfering with the station you are listening to at 101.5, and so will somebody else’s filter. We call a range of frequencies a band, and the size of the range the bandwidth. All old-school radio and TV worked on the same principle, which could be called the “analog” way of sharing the ether.
Later in the 20th century we developed better ways to share the radio spectrum — digital ways. Instead of allocating frequency ranges exclusively to specific users, we developed ways that all users can share a large frequency band, but digital technology can separate the signals in a variety of other ways. We call this multiplexing, and the new methods are vastly superior to the old approach of allocating permanent channels. It lets there be large numbers of cell phones in the same area, all talking to the same cell tower at the same time. Another version lets there be 50 computers in the same room sharing the same wifi. The old versions of that forced the 50 computers to all take a piece of the available “bandwidth” resource, so that if there was 40 megabits of bandwidth, all the computers sharing it would add up to less than 40 megabits when put together. Newer approaches go past this, and can let the total bandwidth be much greater. It’s amazing and it keeps getting better. Sharing, it turns out, is caring, and today we know how to do it. Frequencies still matter a bit in that they propagate differently, and antennas and coils are tuned to them, but they matter a lot less.
The airplanes are using radio in one of its oldest forms, for radar. It’s not the most basic radar of the 1940s, where you send out a burst and wait to hear it bounce back. Modern radars send out a wave whose frequency constantly changes in a specific pattern, and they look at the frequency they receive back. It’s better, and helps prevent some interference from other signals, but you still need good filters so you don’t get interfered with by other transmitters working near your “band.”
International authorities allocated the band from 4.2ghz to 4.4ghz for aircraft activities and the radar. This has to be international because aircraft fly over borders every day. 4 gigahertz is 40 times the frequency of those FM radio channels described above. Radio waves with very high frequencies act differently from lower ones, which is why your AM radio (1mhz) works when you go through a tunnel but your Wifi (2.4ghz) can’t get to the next room. 4ghz works mostly in a straight line, which is good for radar use.
The FCC and other world governments allocated the bands just under 4ghz for use in 5G cell phones. To protect the aircraft, they left a “guard band” from 3.8ghz to 4ghz empty — nobody gets to use it, except satellites and a few remaining fixed microwave links which don’t interfere the same way. This was only to protect the aircraft radios. Because they are used in safety critical functions, they get this huge protective buffer. These bands are very useful — recently the cell companies paid $81 billion at auction for the rights to use the 280mhz slice from 3.7ghz to 3.98ghz, and they are leaving the 220mhz slice above that empty to protect the aircraft radios from having noisy neighbors, even thought it would presumably be worth another $60 billion and the total of the guard band (which can be used for satellite) and aviation band might be valued at $120B just in the USA.
The problem is, the aviation radars in some planes are poorly made. They don’t have the proper filters. When they made them, the only neighbors they had were satellites which are very far away and don’t interfere. So they made them badly. Even with the buffer zone protecting them, there is worry that some of them might still get some interference from a 5G tower near an airport approach using the band the FCC sold to the cell companies. You can read former FCC chairman Tom Wheeler describe some of the history and studies on interference. In a rare worst case scenario, where a badly made radar is coupled with an automated aircraft in a landing zone with the wrong configuration, there could be a safety risk.
From a radio allocation standpoint, this is 100% the fault of the aircraft radars. They had lots of warning that this was coming, but they didn’t want to spend the money to get better radio technology to avoid the interference.
There’s also some blame for the FAA. They published specs, last updated in 1983(!) telling the companies it was OK to have crappy radars. Those specs should have never been written, and should have been updated long ago, and the order put out to do the upgrade.
That doesn’t stop the fact that these planes are still flying, and it is possible one of their altimeters could fail due to a 5G tower. Well, that actually gets debated. Those on the aviation side published a study showing how it’s possible. Those on the cell company side say those studies are flawed and show only extreme situations that are very unlikely to happen. Indeed, in a large number of countries, the 5G has been turned on without reported problems, though in some cases they have larger guard bands or other different regulations. The airlines argue the USA is different in that way and a greater risk.
We expect the airlines to act with an abundance of caution. We put our lives in their hands. That motivates them to be worried about the potential, even if unlikely. For some reason, though, it didn’t motivate them to just spend the money to get properly operating radars that won’t suffer interference. It’s been a tough two years for them, and this may have motivated them when this deadline was known long ago.
There are also a lot of other ways aircraft could use to measure their altitude. The truth is, this technology is pretty ancient.
The Mistake of Spectrum Allocation
The truth is, the 20th century concept of spectrum allocation itself is obsolete. Back when the only way we knew to separate radio signals was to put them in bands, it made sense. Agencies like the FCC gave “ownership” to ranges of frequencies and handed monopolies to these owners on use of them. They declared that if somebody else dared interfere in your band, the government would punish them. It’s like the government said Verizon owns blue, and AT&T owns red, and nobody else can use those colours. That’s actually a decent description, not hyperbole.
Monopolies are rarely a good idea, but the primitive technology of the past made them happen. Today, governments have been seduced by the fact that they can auction off these monopolies for very huge sums. In fact, the ether should belong to nobody, and improving technology should allow us to share it in productive ways.
The greatest revolution in radio came when it was decided that nobody wanted the small band around 2.4ghz. Microwave ovens leak signals in this band, making it less useful for somebody to own a monopoly on it. That’s when the FCC decided to declare it “open.” Anybody could use it, as long as they stuck to some rules limiting power and a few other things. Instead of the old system of monopoly, they declared that if you were getting interference, it was your problem not the interferer’s. They wouldn’t help you, so radio designers learned how to help themselves.
The result was an explosion of innovation that gave us wifi, bluetooth and many other wonders. It was so good that people clamored to get more open bands, and a few appeared but people would love even more. Unfortunately, once governments learned they could sell the monopolies for billions the temptation to do so was too great.
In a brief moment of sanity, the same philosophy was applied to all the old TV channels. TV station owners got free monopolies on these in the old days, but they were mostly sitting unused. Changes were made to allow use of the unused TV channels. Of course, the monopolists have slowed down the exploitation of this wonderful idea, even though only a tiny fraction of people watch TV over broadcast antennas today.
In reality, if we were designing from the ground up, we wouldn’t have an FCC that allocates bands, except in a few special places where physics demands it for very special applications. The fewer the better. Even if people argued that monopolies made sense to spur investment, they should only be leased, not sold, and every few years the monopolists should need to pay again, and new technologies should be given a chance so that innovators keep winning.
This applies even to things like emergency services and safety critical aircraft radios. Fire and police radios are made in small quantities, and thus at high prices, and their price/performance is a fraction of consumer equipment today. Yet famously, on 9/11, they couldn’t even talk to one another. By giving them their monopoly, rather than forcing them to share and create high reliability in the shared bands, we crippled them.
And yes, the aircraft should have been told that interference was their problem too. That they should, after a few years warning, replace their old-school equipment to make use of the latest technology.
Don’t be selfish
The reality is, spectrum is not a scarce resource. It’s only scarce because we made it one. It’s only scarce because some players are selfish, and use more than they need — much, much more. A requirement to not be selfish is the main principle we need. This means:
- Don’t use old technology that can’t do a better job of sharing once its obsolete
- Don’t transmit through the ether at all if you could go via the ground
- Don’t use more power than you need to
- Don’t use omnidirectional antennas when you could use directional ones, including MIMO and phased arrays.
- As long as somebody else is not being selfish, if you are getting interference, it’s your problem, not theirs.
The governments and telcom companies are being selfish by perpetuating and paying for the idea that they should get monopolies over a free public resource. The airlines are being selfish for not even bringing their gear up to 20th century quality levels after getting a huge protective buffer to begin with.
But aviation safety is so important, for now we’re going to let them get away with it, it seems. One reasonable approach (which the airlines would not like in current economic times) would be for the 5G operators to shut off their airport towers when aircraft with broken radar are approaching, and somebody should have to pay the 5G operators for all lost revenue due to this — including wasted investments. Now the airlines don’t have much money, so it could be argued that the FCC owes the telcos money because they can’t use the spectrum monopoly they paid for. Or perhaps the FAA owes the airlines money for having allowed them to include broken radars so the airlines can pay the telcos to shut off their towers. Mistakes were made by multiple parties here, and nobody will want to be the one to pay.