From ultra-wideband (UWB) to software defined radio, spectrum issues remain front and center in aviation. Old threats and problems remain, while new spectrum-hungry technologies compete for real estate. Avionics Magazine visited Oscar Alvarez, FAA’s acting director of air traffic control (ATC) spectrum engineering services with the agency’s Air Traffic Organization shortly after Hurricane Katrina hit the Gulf of Mexico. Alvarez reviewed FAA’s major spectrum challenges now and in the longer term.
Avionics: You’ve been busy in the wake of Katrina. Could you fill us in?
Alvarez: So far we have coordinated frequencies for mobile VORs being put into place. I know of at least one at Gulfport, Miss. Also we are coordinating frequencies for portable air/ground communications towers. We also are coordinating air-to-air communications frequencies for military helicopter operations, and we have received requests for air/ground communications frequencies from the Federal Emergency Management Agency.
We also just coordinated a request through NTIA [the National Telecommunications and Information Administration, charged with managing federal spectrum] from Time Domain [a UWB technology developer], which wanted a special temporary authority to use ground penetrating radar for some of the recovery efforts. We told NTIA it’s OK, provided we get a contact point, so that if we get reports that helicopters are getting interference, we can call that person and…shut it down.
Avionics: What are the big spectrum challenges?
Alvarez: In my opinion it is new technologies. They are coming in any part of the spectrum, so we have to be vigilant to make sure they will not encroach on the aviation bands. For example, ultra-wideband, GPS reradiators [satellite signal amplifiers] and broadband over power line [BPL].
Avionics: How would you rank these issues?
Alvarez: UWB still would be one of our main concerns. We’ve done studies, and limits have been set. But we haven’t been able to see [consumer-style] products that we can get our hands on and measure. What worried us the most is if people brought those things on board. We’re waiting for commercial products to come out that we can get hold of and measure to see what the emissions are.
Last April we completed an engineering study on the effects of UWB devices below 960 MHz. We concluded it would be advisable to reduce the current general Part 15 limits for UWB devices [tighten the emission constraints] using continuous wave modulation, specifically in the 108-to-137-MHz band. Communications are more susceptible in that band. That band is used by the FAA for localizers, VORs and air/ground air traffic control communications.
As part of that study, we did some operational tests on a noncompliant ground penetrating radar. We noticed that it would cause interference with air-to-ground communications. [The radar was nonconformant to Part 15 rules, but it was legal because it had been grandfathered in.]
Avionics: What tests did you conduct?
Alvarez: It was more of a subjective test. We measured ground penetrating radar according to the FCC [Federal Communications Commission] test method. We turned it on and moved it around our experimental RCAG [remote control air-to-ground] site at the Technical Center in Atlantic City, N.J. It’s ground equipment…this is what the controllers would use.
What happens is that when the pilot talks to the controller [voice was simulated in the tests], the controller would hear the pilot, and this noise from the ground penetrating radar [was] mixed in with it. It came to a point where you could not hear the pilot. Then we moved it farther and farther away from the antenna. We could detect interference to the helicopter as far away as 600 feet [183 m]. We detected the signal on our ground receivers at up to 100 feet (30 m). Our policy is to have no interference.
We sent the report to FCC via NTIA, but we haven’t received any replies or comments. A concern was that everybody was concentrating on [UWB’s] effects on an aircraft. We discovered that this ground penetrating radar could affect the ground.
Avionics: What’s happening with onboard cell phones? [See page 32.]
Alavarez: FCC thinks that the technology is there so that the ground infrastructure of cell phones won’t be impacted. FCC is trying to lift their ban. But they have at no time said, you can use cell phones on aircraft. They have deferred to FAA.
RTCA came up with a Phase 1 report last year. It gave some guidelines to manufacturers and airlines on how to test, not cell phones, but PEDs [personal electronic devices] to see if they interfere with any aircraft systems. Phase 2 was going to look into transmitting PEDs like cell phones. Phase 2 is to come in two parts. The first part will simplify the Phase 1 results. So they’ll make it a little simpler for existing aircraft–the measurement methodology. The second part will deal with new aircraft–how can manufacturers design aircraft so that they would be immune to certain transmitting PEDs.
Avionics: Are cell phones inevitable?
Alvarez: I wouldn’t say inevitable. I think that the industry will keep pushing, but I’m not sure how successful it’s going to be. One of the biggest issues is just the annoyance factor. That’s probably more of an issue than the technical issue. Technically, I think it could be resolved, using picocell technology. But the whole system has to be designed in such a way, so if the picocell fails, the phone will be shut down.
People may think they can use their cell phones. But in my view–and I don’t know if that’s going to be the case–it’s going to have to be a special cell phone. And maybe by using the picocell, [passengers] will be charged a different fee, instead of using their own free minutes. It’s not a simple issue.
Avionics: How do you see software defined radio?
Alvarez: From a spectrum point of view, "Jitters" [the Joint Tactical Radio System], the military version of software defined radio, our main concern is that the software that is put into those [devices] has to come from the depot. It can’t be something that final end users can change. If software comes from the depot with specific bands and specific power [limits], then we’re OK. Our concern is if anybody can change that [software] to make this radio operate anywhere in the band.
Avionics: But next-generation software radio users want spectrum agility.
Alvarez: They want to operate anywhere, but it has to be coordinated. If you’re in the United States and you want to tune to the 136-MHz band for air traffic control, the waveform has to be double sideband AM, which it is now, and you have to conform to that. You cannot put extraneous waveforms there or use it for something other than air traffic control. So [for software defined radios] the software has to be controlled, and we will probably say, stay away from the aeronautical bands.
Avionics: Are GPS reradiators an issue?
[GPS reradiators, which amplify the signal to allow GPS receivers to lock on inside a building, can also be viewed as spoofers. Selling them is currently illegal.]
Alvarez: It could be a serious threat because the pilot could get the wrong position if the pilot is flying a GPS approach and gets a position from a satellite, and all of a sudden gets a signal from this pseudo GPS. If the GPS [receiver] from the aircraft locks onto that signal, the aircraft may think it’s where the antenna of the reradiator is. It may not happen a million times, but it could happen one time. It could actually trap GPS, and you could lock onto the wrong signal. There are things in there I don’t feel comfortable about because I don’t know the exact answers.
We have asked the Tech Center to test the effects of a GPS reradiator on our WAAS [wide area augmentation system] reference stations. The main purpose for us is to understand the potential problems and figure out how to mitigate [them], how to locate these things and come up with some more engineering data.
The reason we are allowed to test these things is that NTIA…came up with some guidelines for federal use. [Agencies] can get a license to use a reradiator. The guidelines are very specific. One requirement is for GPS reradiators to be used specifically for testing GPS receivers or systems that use GPS. We don’t have a way to test whether our aircraft GPS receivers are working unless we bring them out of the hangar. FAA is in the process of securing a frequency assignment from NTIA. It’s my understanding that FCC will probably work on some restrictions for private [sector use].
Avionics: You also mentioned broadband over power line?
Alvarez: The president’s desire is for broadband to be available and affordable to everyone. It is my understanding that BPL is being tested in various cities. One of the bands being proposed is the HF band, parts of which are used for air/ground communications. We petitioned the FCC through NTIA to notch out that portion of the band in their rulemaking, and that was done. The ham radio operator community is having interference problems, and they have performed numerous measurements.
Avionics: Has aviation gained anything from the president’s spectrum initiative?
Alvarez: I haven’t seen any benefits yet, but there are a couple of recommendations that may help aviation. Each agency is being encouraged to come up with a long-term spectrum plan, so that would be our opportunity to put our aviation requirements in writing. Another recommendation was the establishment of a policy and plans steering group that will help resolve major contentious spectrum policy issues that affect the use of spectrum by federal and non-federal users.
Avionics: What challenges are beyond the immediate horizon?
Alvarez: One challenge is the depletion of the VHF air-to-ground frequency band. We’re not running out of frequencies, but in some areas of the country it’s very, very congested. It has become very difficult to find frequencies to satisfy the requirements of air traffic. We’re refining our engineering criteria. We’re trying to find more spectrum. We’re trying to figure out a way to use some [other dedicated] channels for air traffic control. It has become an engineering policy challenge to us.
Until a future communications system comes into place–around 2015–it’s becoming more difficult every day. FAA needs to make a decision pretty soon. One alternative until we transition to a new system is to go to 8.33-KHz [channel spacing]. It wouldn’t be too difficult to implement, but the problem is equipage.
UWB Primer
Ultra-wideband (UWB) is not new. The principle was used by the Italian inventor, Guglielmo Marconi, when he first operated spark-gap transmitters to send pulse-code messages wirelessly through space in the 1890s. Had Marconi persevered, he might have invented the UWB technology that is used by the military and is being developed for consumer electronics. Instead radio was developed, using carrier waves of set frequencies, onto which message content is modulated by varying amplitude, frequency or phase.
In contrast, UWB, in its impulse form, transmits short pulses of energy spread out over a wide swath of the spectrum. Though spectrally profligate, UWB operates at low power–mostly below ambient radio noise–and is designed for communication over ranges of no more than a few meters. UWB is supposed to be able to coexist with conventional spectrum users. And since it makes no demands for spectrum allocation, it need not be licensed–like Bluetooth, for example.
Impulse-type systems typically emit pulses less than half a nanosecond (a billionth of a second) long, maybe millions of times per second. Data can be superimposed on the pulses by shifting their timing slightly to represent "1s" or "0s," or by modulating them in amplitude or polarity. High data rates can be achieved: up to 50 Mbits/s initially, then to 500 Mbits/s and perhaps to Gbit/s rates ultimately. Enough, in fact, to satisfy the most ardent mobile Internet user or downloader of streamed video and multimedia content. This, plus the fact that UWB transmitters can be tiny, solid state and cheap, accounts for UWB’s being the emergent darling of the data-rich end of the consumer electronics industry.
Not all UWB devices will be the impulse type. The competing multiband orthogonal frequency division multiplexing (MB-OFDM) concept is not pulse-based. Both forms of UWB, however, will be extremely power-efficient. But existing radio service users with particular spectrum allocations do not welcome the interloper. The aviation industry fears that cheap UWB transmitters embedded in personal electronic devices would spew out pulses occupying 2 to 3 GHz or more of radio spectrum within the 3.1-to-10.6-GHz range, on which most aeronautical radio services rely.
Nor is aviation alone. Radio astronomy, satellite-based services and emergency services may suffer from any rise in the general noise floor.
UWB proponents say that the power from each emitter is less than the stray RF emanations from an electric razor, computer chip or car ignition system, and a tiny fraction of what a mobile phone emits, and that therefore it will not interfere with anyone. But, while random use of individual devices, mostly indoors, may not constitute a problem, dozens of them switched on together could impair a victim receiver’s ability to pick out low-power signals from a higher noise background. And energy from even a single device reaching a high-gain antenna along its boresight could be greatly amplified, substantially degrading signal-to-noise ratios.
The aviation industry dislikes the upsetting of long-established convention in RF spectrum management. It argues that any new technology has to prove that it will not harm established spectrum users, as a condition of approval, and that in this case approval was given before the proof. So far the U.S. Federal Communications Commission (FCC) has cleared UWB for unlicensed use mainly in the 3.1-to-10.6-GHz band. The clearance two years ago was subject to the use of a protective mask, a technology layer that would prevent transmission at certain frequencies considered sensitive, such as that of GPS. FCC, however, was concerned that the mask was too restrictive and was going to remeasure the impact of UWB on GPS and potentially change the mask.
– Reported by George Marsh