Avionics News January 2021 - 20

ADS-B ROTORCRAFT STYLE

Continued from page 19

antenna from the aircraft, it's not unusual for that connection
to be lost. Throw in the obstructions from trees or structures,
the strength of that connection isn't always optimal.
In some parts of the world, the ADS-B network comes
courtesy of a private provider rather than a government entity
like the FAA. Complying with the access rules, conditions
and equipment requirements may vary from those standards
for public networks like the FAA's.
In some parts of the world, including those which have
yet to require ADS-B Out, geography and budgets render the
type of ground-based network used by the FAA impractical
- or simply unaffordable. For those parts of the world -
including the vast expanses of airspace over the world's
oceans - satellite-based ADS-B like Aireon's system offers a
practical alternative. As Avionics News explains in a feature
story beginning on page 58, ADS-B diversity transponders
have been proven to work with the Aireon network to provide
worldwide coverage.
Employing a diversity-capable system on helicopters may
raise challenges not seen with non-diversity ADS-B solutions.
Standard installs: Bottom-mounted antennae
The standard for most aircraft with transponder, both
ADS-B bottom-mounted antennae provide a line-of-sight
view of the 800-odd ADS-B transceiver ground stations the
FAA spent the better part of a decade establishing and testing
after concluding the years-long proving under the Capstone
Project in Alaska.
For helicopters, like most aircraft, the belly of the aircraft
worked well for the need of radar transponders to connect
with radar stations. As it was for radar, so it is for most
aircraft equipped to comply with the ADS-B Out mandate.
In fact, ADS-B solutions often use the exact same spot to
mount the antenna for ADS-B Out, as well as for the ADS-B
In system. ADS-B In rebroadcasts aircraft position reports
to other participating aircraft under the traffic information
services-broadcast, or TIS-B, as well as weather from the
FAA's flight information services-broadcast, or FIS-B.
The connection issue for helicopters mirrors the same
issue for fixed-wing aircraft when they descend below the
line-of-sight line to the nearest ADS-B ground station. The
connection may be lost, depriving controllers and nearby
ADS-B In-equipped aircraft of TIS-B from the ground
station, as well as the direct traffic broadcasts of other aircraft.
In built-up areas, obstructions - buildings, grain elevators
and other tall structures - can similarly interfere with ADS-B
20

avionics news

january

2021

In service and the connection to air traffic control facilities.
The same issue comes into play for air traffic control
authorities with authority over airspace lacking a groundbased network like the FAA built in the U.S.
Thankfully, Aireon's new satellite-based ADS-B network
offers a solution - one which may be challenging to
implement on some helicopters.
Diversity, in brief:
It's a matter of staying on top of things
As detailed elsewhere in this issue, the scope of available
ADS-B options continued to grow in 2020. And with an
increasing number of air traffic service providers recognizing
the benefits of satellite-based ADS-B, more options came in
the form of systems capable of ADS-B diversity.
In brief, diversity in ADS-B Out requires a 1090ES Mode
S transponder capable of broadcasting its data blocks through
two antennae, one mounted in the usual place on the aircraft's
belly, the other via a second antenna mounted on top of the
aircraft's fuselage. The top-mounted antenna provides a
line-of-sight view of Iridium satellites in low-Earth orbit with
ADS-B transceivers.
This orbiting ADS-B network is operated by Aireon, a
joint venture of Iridium Communications, Nav Canada,
the Irish Aviation Authority, Italian air navigation service
provider ENAV, and Naviair - which provides air navigation
services in Denmark, Greenland, and the Faroe Islands. Nav
Canada is the lead air navigation service provider in the
Aireon joint venture.
As Aireon notes on its website, " Iridium hosts Aireon's
specially designed receivers on each Iridium satellite,
providing coverage over 100% of the globe. "
With only a bottom antenna - the vast majority of U.S.
systems - ADS-B transponders have a clear view of the
FAA's ground network of about 800 stations when the
aircraft is high enough. The bottom antenna, however,
is largely blocked from " seeing " the Aireon ADS-B
transceivers riding on the Iridium satellites. So accessing
the Aireon satellite-based system requires diversity - with a
top-mounted ADS-B antenna.
Avionics makers already offer options for diversity-capable
ADS-B systems, among them Garmin, uAvionix, Collins and
Honeywell.
Adding a top-mounted antenna to a helicopter may incur
issues with interference from the helicopter's rotor blades,
depending in part on the material used in the rotor blades.
The greatest potential for interference, according to avionics
engineers, comes with metal rotor blades. Rotor blades made
of composite materials may be less prone to interfering with

Avionics News January 2021

Table of Contents for the Digital Edition of Avionics News January 2021