Avionics News July 2015 - 42

CURRENT STATUS OF ADS-B
Continued from page 40

Questions also have been raised whether portable equipment could be used for ADS-B compliance, or not. The
rule specifically requires the equipment to be installed. The
issue, however, is not portables versus installed or certified
versus uncertified, but that any equipment used in ADS-B
airspace must meet the performance requirements that have
been established based on close to 20 years of work between the FAA and industry.
One could speculate that a portable receiver that meets
the required safety performance might even be more costly
than an installed solution when considering some of the
points below. This is what the Equip 2020 group has concluded as a result of its recent meetings with a vast array
of technical experts. The following are some examples of
issues that portable equipment would encounter:
* The antenna is a major issue. The ADS-B Out transmitter could conceivably be portable, but nobody
knows how to handle a portable antenna. A portable
antenna has several technical issues in terms of effectiveness depending on where an operator puts it
(bank angles, pitch angles, etc.), which can all disturb the transmission. This could cause the transmitted signal to come and go, causing problems for both
ground and airborne receivers. Portable antennas
also are susceptible to electromagnetic interference,
stability and other problems. Furthermore, a portable
antenna is an impractical solution for rotorcraft
given the physics of the rotor blades. Finally, a portable antenna is not viable because it needs a ground
to have any chance of being able to reliably transmit.
In short, an installed antenna is needed.
* Another issue is the configuration of the unit in
terms of the N-number. If equipment is portable, it
implies that the unit can be moved from airplane to
airplane - but then how would the N-number match
with the airplane? If the pilot is given the chance to
enter it manually, then the pilot could be flying one
airplane and squawking another N-number.
* One of the biggest technical issues is the transponder
code. The ADS-B transponder code from the notional portable device has to match the transponder
code of the installed Mode A/C or S transponder
so that air traffic control sees only one from the
airplane. The certified units do this with direct wiring, or "sniffing" the transponder itself, which is a
complex process. Some people will say "just manually enter the code one at a time" at each device, but
the FAA prohibits this based on experience gained
in the CAPSTONE ADS-B testing in Alaska. It just
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AVIONICS NEWS

JULY

2015

didn't work. Manual entry sounds simple, but if the
pilot delays entry or, worse, gets distracted and forgets, ATC starts seeing two different codes from the
airplane, which will cause an instant alarm that the
ATC controller will need to address. Finally, if the
codes mismatch, then the pilot has to be informed
(cockpit annunciation) and have a procedure to either resolve the difference or "exit" the system without tying up ATC.
* Another issue: What happens if the pilot accidentally leaves the equipment on, lands, puts the portable device in his or her bag, and drives home? For
example, if the pilot takes the portable unit out of
the airplane and forgets to turn it off, he or she will
be transmitting to the system, and ATC will think the
equipment is still "active" and so will other aircraft
that pick up the transmission when above 40 knots.
* Integrity and availability of the GPS source also is
key. Current portable GPS devices simply ignore
this issue because small "outages" are no big deal
for an iPad, but ultimately are a big deal to the ATC
system. Imagine how disconcerting it would be for
a pilot in another aircraft with an onboard ADS-B
In display to see a target come and go or even dance
around. The argument has been made that you don't
need the stringency on a small, slow, piston airplane.
But remember that ADS-B is cooperative surveillance, so it is the other aircraft's ability to see you
that comes into play. Even a Piper Cub would have a
closing speed of 300 to 500 knots with a business jet
or other airplane when coming head-on. A glitch in
the system would have detrimental consequences.
* Finally, for most aircraft, the ADS-B Out equipment
becomes the transponder. Portable transponders
have never been allowed to meet the requirements
of 91.215 because of safety, so why think that a shift
to a cooperative surveillance environment would
lift the requirement of a compliant and installed
device? ADS-B Out is more critical than a transponder. The transponder just gives ident and altitude to
a target that is primarily detected and tracked today
by a multimillion-dollar certified ground infrastructure. Once ADS-B Out is installed, it becomes the
primary position source used by ATC and other
traffic, so the idea of everybody in the NAS relying
on something you put in your pocket or flightbag is
simply a nonstarter.
Given the challenges stated above, there is not a single
avionics manufacturer we are aware of that sees portables
as a practical solution for ADS-B Out - including every
one of the manufacturers that is currently, or soon plans to,
provide portable capability for ADS-B In. q

Table of Contents for the Digital Edition of Avionics News July 2015