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Waveforms Lead the Charge as OEMs Aim to Tackle Helicopter ‘Through the Blades’ Connectivity Problems

Has the "through the rotor blades" problem been resolved for helicopter satcom?

Helicopters have a unique connectivity problem caused by the same mechanism that helps them fly: the rotor blades. These blades cause interference every time they rotate, interrupting the signal from its source.

“The historic challenge with putting communications on helicopters, especially satellite communications, is that the satellites are up in the sky and the equipment's on the aircraft,” Mark Goodman, director of product management at Honeywell, told Avionics International. “Typically, you have to look through the rotors which are basically just wings that rotate and block the signal.”

Helicopters have historically always had issues obtaining reliable uninterrupted satellite signal connectivity because of the disruption caused by the spinning rotor blades while airborne.

Honeywell’s Aspire 200 satellite communications system installed on a helicopter.Honeywell Aerospace

When it comes to helicopter connectivity problems, the current answer seems to lie in waveforms, according to leading industry professionals who spoke to Avionics. OEMs are working to develop waveform technologies, along with other solutions such as antenna mounting positions and error connection codes, to provide solutions to this challenge.

Honeywell’s solution is the Aspire 200 Satellite Communications System which allows aircraft to connect to a SwiftBroadband service, Goodman said. It uses HDR or high data rate, which is a special waveform, and an interleaver.

“In 2015/2016 we introduced the Aspire 200 product, which is an immersat SwiftBroadband based solution,” Goodman said. “In that solution we introduced what we call HDR or high data rate, which is a special waveform that has been constructed to do two things: one, to provide higher modulation, so, higher data rates on the existing SwiftBroadband channels…and then two, it added an interleaver. What the interleaver does is it mixes up the information, the bits, coming off the aircraft and puts them into separate packets. Then it has the intelligence on the other side so that if any of those bits get lost because of blockage to the rotors, it's able to reconstruct the missing bits.”

Before this solution, helicopters could only get voice communications at about 2.4 kilobits per second onto the aircraft, Goodman said.

Viasat also uses a unique waveform to counter connectivity problems. Joe Johnson, director of business development at Viasat, told Avionics International that Viasat uses an Arclight waveform with Ka-band satellite technology that includes extensions for small antennas.

The AMT-700 antenna for Honeywell’s Aspire 200 system. Honeywell Aerospace

“When you have small antennas and they're looking at the geostationary arc of satellites, you have to make sure you don't interfere with adjacent satellites, and the smaller the antenna the more interference it can cause,” Johnson said. “So, what you have to do with that waveform is use a technique called spreading. So, you actually spread across a certain amount of bandwidth.”

In February, Viasat announced the completion of a successful demonstrations of a new Ka-band satellite technology that will allow through the blades beyond line of sight (BLOS) communications on a U.S. Army UH-60M rotary-wing helicopter. However, the hardware platform used in the demonstration was previously operated on a Marine Corps MV-22 with a different waveform.

The waveform itself is also hardware agnostic, according to Viasat.

Johnson said Viasat also uses an interleaver to deal with blockages caused by the rotor blades. In most digital communications scenarios, an interleaver is a hardware device that contains error correcting codes capable of the occurrences of errors between a transmitting and receiving device or medium.

In this case, Viasat is using an interleaver to counteract errors generated by the spinning rotor blades between the satellite signal and helicopter antenna.

“Your rotors are spinning and every time they spin and pass over or in between the antenna and the satellite, they create errors,” Johnson said. “They create a blockage. How do you deal with that blockage, so that you have still error-free communications? The way we do that is those periodic blockages, they interrupt the signal, we use something that's that standard in communications theory called an interleaver that turns those long blockages into shorter bursts of errors that then we correct using, again, standard error correction techniques. So that's kind of the secret sauce.”

Advances in satellite technology are enabling more options for helicopter communications, according to SKYTRAC. SKYTRAC

Hughes is using unique antenna mounting positions coupled with waveform technology to achieve connectivity, according to Rick Lober, vice president and general manager of the defense and intelligence systems division of Hughes Network Systems.

“Hughes has come up with an approach where we can actually mount the satellite antenna on the fuselage underneath the rotor blades, and even though those rotor blades are spinning at a couple hundred RPM chopping up the signal, we have an algorithm that kind of puts it all back together again,” Lober said. “We call it HeloSat, and we've been able to get rates as high as 17 megabits per second out of the helicopter and into the helicopter to allow wideband communications from the helicopter to really almost anywhere in the world.”

The algorithm uses a two-step process involving the waveform and error correction code, Lober said. The waveform helps to relock onto the signal after the blade interrupts it and the error correction code makes up for the blockages lost during the disconnection.

Hughes has already demonstrated HeloSat’s ability to transmit real-time full motion video to a live global audience directly from a UH-60A Black Hawk during a flight test last year. The flight demonstration included a Hughes HM series modem, the Ku-band frequency Intelsat Galaxy 18 (G-18) satellite, and a 22-pound fuselage-mounted GetSat Milli-H antenna. The Ku-band frequency allows higher data rate transmissions than L-band and is cheaper to use than the L-band satellite frequency, as the latter charges by per minute usage, according to Hughes.

While the current solutions for this problem are mostly concentrated on waveforms the future solutions seem to center on advancements in the satellites themselves.

SKYTRAC Systems is the outlier of this group. Instead of using waveforms to avoid or correct for blade interference, they are avoiding it altogether. SKYTRAC is using the network of Iridium Next satellites, which give global coverage and provide a net around the planet, by connecting from underneath the rotor blades instead of through them, Jan Van der Heul, vice president of sales at SKYTRAC, told Avionics International.

“In order to connect with those satellites, you need to go through the rotors and that's an obvious issue,” Van der Heul said. “And so, the unique advantage that we have when using Iridium, is that we do not need to go through the rotors in order to establish that connectivity.”

SKYTRAC’s system uses LEO satellites to avoid the interference to helicopter connectivity caused by rotor blades. SKYTRAC

The Iridium satellites are referred to as LEO or Low Earth Orbit satellites because they sit at about 800 kilometers above Earth instead of at about 35,000 kilometers where geostationary orbit satellites sit. They also are in motion and provide a latency of 500 milliseconds instead of 2 seconds.

The new Iridium Next terminals are launching this year and they will increase the kilobits per second by more than 150 times what the legacy solutions offer, Van der Heul said.

Goodman also said that Honeywell was in the process of developing avionics technology to support Iridium service that could be certified by the end of this year and provide 700 kilobits per second per system.

Lober said Hughes is also interested in developing technology in this area.

“Well because those satellites are closer to the earth, you can use a smaller antenna because the signal is usually stronger and you have less latency, particularly for the LEO satellites,” Lober said. “You might have applications where latency or delay is important and that's where you would want to use the LEOs. Also, the LEOs have polar coverage, which is another thing that's becoming more important.

This is the Ka-band SATCOM antenna used by Viasat to provide through the blades broadband connectivity. Viasat

Another area for technology development in the future could be different band frequencies. Goodman said Honeywell is also looking into other band frequencies such as the Ku and Ka with hardened waveforms for future development and use.

“By introducing a custom waveform in with the jet wave system, as an example, we may be able to achieve the error-free robust operation that we see today with our L band systems,” Goodman said. “The challenge continues to be, though, to get the Ka-band equipment down to a size that would be compatible with the halo platforms.”