Astronics AeroSat

Fresnel lens-horn design coupled with Ku-band transmission works well for bizav internet reception.

By Shannon Forrest
President, Turbine Mentor
ATP/CFII. Challenger 604/605,
Gulfstream IV, MU2B

FliteStream connectivity systems enable seamless productivity and entertainment while in flight.

Satellite communication (SatCom) technology is a means of delivering voice, data and entertainment to nearly every location on the planet. Nowadays it's taken for granted that broadband internet and hundreds of television channels are available instantaneously with the aid of a small antenna and processor. However, just a few decades ago these conveniences were sparse, cumbersome and costly.

Baby boomers and those born into generation X may recall what home satellite systems looked like in the late 70s and throughout the 80s. The receiving antenna was an enormous parabolic dish at least 10–12 feet wide. The logistical challenge was positioning the dish so that it maintained a clear line of sight with a geostationary (above the equator) satellite while at the same time not provoking the ire of the homeowners association or neighbors.

Size and footprint of old equipment really made the installation an eyesore. But the behemoth antenna was necessary to capture the low power C-band signals being delivered by the television providers of the day.

Historical background of commercial tv-dish signal reception

The knowledge underlying the technical requirements for a home dish was made public by Taylor Howard, an electrical engineering professor at Stanford. In 1976, Taylor, a licensed pilot who worked on radio astronomy for several NASA programs from Apollo to Voyager, built his own 16 foot personal satellite dish. He also published a guide for others to do the same. At 1st, commercial television signals were not encrypted, so anyone with the financial means to install a dish (around $3000 in 1981) could intercept the feed and watch content for free.

Howard even sent HBO a check for $100 in an unsolicited attempt to pay for programming he consumed. The check was promptly returned because the company only dealt with large cable companies, not individuals.

Years later, HBO became the 1st television provider to scramble the downlink from the satellite in an attempt to end what it considered poaching of proprietary material. Other companies soon followed, necessitating satellite dish enthusiasts to purchase a decoder and subscription to watch.

Today, home satellite antennas are approximately the size of a large pizza. The equipment costs are negligible when compared to past iterations, and the technology has advanced exponentially.

Why are the costs high for aircraft reception

FliteStream T-Series tail mounted connectivity system for business aviation.

Given the ease of obtaining broadband internet and television at home and the office, why is it so complex and correspondingly costly to do the same on an aircraft? The enormous cost of certification (for aviation use) is a big driver when it comes to price. And the technological challenges of installing and then keeping airborne antenna aligned with satellites while moving in 3 dimensions and also crossing satellite boundaries are other salient factors.

Nearly all communication satellites (which include voice, data and entertainment) are parked in orbit over the equator. By definition, a satellite over the equator that remains over the same geographical location is known as geostationary. Line of sight has to be maintained for signal reception to be continuous.

However, unlike a ground receiver that can remain stationary and locked at a specific azimuth, aircraft receivers must have the ability to constantly track the satellite as the aircraft moves. Additionally, physics plays an important role in designing an antenna. The home satellite system previously mentioned consists of a parabolic dish connected to a protrusion that extends outward from the center, or an attachment that wraps up from underneath. In either case, the hardware–known in industry parlance as a feed horn–serves an important purpose.

When receiving a signal, the curved parabolic dish reflects radio waves inward to the feed horn, much like the concave mirror behind a flashlight bulb focuses light into a concentrated beam. And when transmitting, the controller 1st sends a signal to the feed horn which bounces the signal off the dish, transforming it into a narrow outgoing beam.

A side effect of a traditional open-ended feed horn is that, by the nature of its placement, it blocks a portion of the incoming satellite signal–much like an umbrella blocks overhead sun. One solution to increase gain and efficiency is to just make the parabolic antenna enormous, which is not a problem if the corporate aircraft happens to be an 800 ft long Zeppelin with a top speed of 75 knots. But for a midsized business jet flying at 80% of the speed of sound, there's a finite limitation contingent on fuselage space and willingness to accept parasitic drag.

The Astronics AeroSat approach

Astronics AeroSat FliteStream T-Series system is a tail-mounted internet and live tv connectivity solution ideal for business aircraft. FliteStream T-Series is composed of an antenna (GAU), transceiver (LPT & PAU), and satellite modem & antenna controller (ACMU).

Astronics AeroSat has developed a novel approach to the feed horn issue. AeroSat, which was acquired by Astronics in 2013, was founded in 1997 with the purpose of creating satellite connectivity for aircraft. The company rapidly excelled in the field of antenna performance so it wasn't long before it developed and patented a lens-horn technology.

The gist is that in lieu of a using a traditional feed horn to serve as the radio wave focal point for transmission and reception, the lens covering the antenna performs that function. The lens itself is composed of Rexolite, an extremely lightweight (15% lighter than acrylic) cross-linked polystyrene plastic with superior properties that make it an excellent choice for satellite dishes and other antennas.

The lens material is important, but the key to it functioning like a traditional feed horn is in the way the lens is engineered. If the word Fresnel sounds familiar it's probably because it brings back memories of 8th grade science class. A Fresnel lens (named after French mathematician and physicist Augustin Fresnel in 1822) is made of concentric rings with each groove cut at a different angle so that it focuses light towards the center. It's the principle behind a magnifying glass lens which is thick in the middle and thinner towards the outside.


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