Avionics News September 2012 - 22

AIRCRAFT AS ANTENNA FARM
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and both requiring antenna for the datalink receiver. The most-recent addition is so new, few are flying. But, there absolutely will be thousands by 2020: the receivers, transmitters and transceivers of automatic dependent surveillance-broadcast, or ADS-B. Even within these various roles, we have seen antenna serving multiple roles, such as the GPS receivers in some electronic flight instrument systems that serve to round out the functions and sensors used in some primary flight display systems. Think of Aspen Avionics Evolution 1000 Pilot PFD and MFD packages, as well as similar stacks based on Garmin’s G300. Without the data of the integral GPS receivers, these two packages lose some of their functions. In the cases of these two systems, position and attitude data depends on input from a functional WAAS-compliant GPS navigator working hand-in-hand with attitude and air-data sensors to round out the packages’ ability to sense turns, bank and pitch angles. Now, create an image of all the receivers and the antennae crowded onto the skin of a fuselage or other surface. It looks and is crowded, or it would be if not for advances in antenna technology that let some serve combined functions. Even with those advances, the antenna crop puts more pressure than ever on the care required to determine where on the aircraft to mount some of them. The issue could become less complicated in time as technology solves more of the problems associated with mounting decisions. In the meantime, careful attention to the role of each antenna, manufacturers’ instructions and multiple-role aspects of some demand careful planning and execution when moving or adding an antenna. Challenges of Proliferation: Like Real Estate, Location is Everything While aircraft and avionics provide new services and functions not thought of just a few years ago, including satellite navigation, satellite weather, ground-based weather and ADS-B, the proliferation of antenna needs presents the same age-old challenges of the past: mounting location. Mounting locations must be chosen with the avoidance of external interference in mind. Some systems require greater care than others, in large
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part because of their sensitivity to interference and the low signal-to-noise ratio of the transmission. Spherics devices, in particular, are sensitive to electronic noise sources external to the system. To hear lightning strikes with sufficient signal strength for the computer to calculate direction and distance, the antenna must be mounted on a spot with minimal electronic noise from other devices. The cable should be routed away from other wiring – even other coaxial cables – so the faint cry of a distant lightning strike gets heard loudly and clearly. Technicians have long worked to map the aircraft skin to find the quietest spot for the spherics antenna. Separation standards also must be followed between VHF and microwave antennae – think nav/comm and transponder – to avoid the prospect of a transmission bleeding through to a GPS receiver. It’s the issue of interference of GPS navigators traced to an adjacent microwave-spectrum source that brought down Lightsquared’s ambitious plan to launch an independent 4-G smartphone service to resell to cellphone companies. Too much power from even miles away simply drowned out the relatively weak signal of the navigation satellites. Avoiding the same problem on a single-airplane scale drives the location decisions for mounting everything, from the GPS antenna to the transponder and CAS antennae, VHF nav/comm antennae and datalink-receiver antennae. The issue also strikes at the heart of some avionics equipment that uses GPS as one of several references used to determine attitude, heading and air data provided by some of the newest primary flight displays. But, some of these issues have changed, thanks to some very clever engineers who undertook the design challenge of new-technology antennae and help rectify the issues of cross- and intra-spectrum interference, while reducing the crop in the antenna farm. Multitasking Comes to the Antenna Farm Multitasking has come to aviation radio antennae, and not too soon. “Just when you think this is getting too complicated to complete, someone comes along with a solution,” offered an avionics technician at the Sun ‘n Fun Fly-In earlier this year. The complicated problem: how to mount – and where – a collection of antennae for dual GPS receivers, a PFD with a GPS sensor, an anti-collision system box, dual nav/comms and a transponder. If you’re counting, that’s nine different antennae: two for the VHF comm transceivers, one for the two VHF nav receivers, another for the glideslope receiver, another GPS

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Avionics News September 2012

Table of Contents for the Digital Edition of Avionics News September 2012