Avionics News July 2016 - 44

ADS-B COMPLIANCE FOR EXPERIMENTAL AIRCRAFT
Continued from page 43

For the GPS-2020, Dynon went through each of the
TSO performance requirements and determined how it
would comply, and how it would evaluate compliance
with each one. It documented every aspect of each
performance evaluation, but the FAA does not audit
this process, and Dynon doesn't need to share it with
anyone. "The FAA guidance is that you are allowed
to install something that doesn't have a TSO if that
equipment has a statement of TSO compliance from the
manufacturer," Jordan said.
"A TSO is essentially a performance checklist,"
added Robert Hamilton, Dynon's president, "and how
you show compliance with every requirement to a level
that Dynon, as a business, supports its willingness
to make that statement to its customers. If the FAA
discovers that it's wrong, resolving the issue is between
the manufacturer and the customer, so the statement of
compliance puts our relationship with our customers as
our insurance that we did it right."
"The hard thing about an ADS-B-compliant GPS
position source is knowing when you're wrong, not
the 99.99999999 percent of the time you're right,"
Jordan continued. "It is analyzing all of the signals and
knowing when the chance is too high that the output
is wrong. GPS is a known technology, so there's a lot
we can use that the world has figured out in the last 20
years."
The integrity itemized in §91.227 and specified in the
TSO is the unique challenge in developing an ADSB-complaint GPS. There are two primary levels of
integrity, Jordan explained. Source Integrity Level, or
SIL, is the overall chance that the position source will
tell you something that isn't true at the time.
"Where it is is one of these things," Jordan said.
"GPS never knows exactly where it is. The bigger issue
is the containment radius; how close you are to where it
says you are." Two numbers provide this information.
The first is the digital location and the second says it is
within so many feet of that position.
"That second number is the harder calculation, and
it's the one that needs to be right because it is what the
rest of the world uses to determine if it can trust you or
not," he said. "Saying I'm here, but saying I'm within
100 miles of here isn't very useful. There are deeper
levels of numbers that do the same thing for vertical
position and velocity, and they all have different metrics
for how much you trust them."
System Design Assurance, or SDA, is the other
primary level of integrity, according to Jordan. It is
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2016

the probability that number produced by the GPS is
transmitted correctly, that it hasn't been corrupted
by other components in the ADS-B system, like the
transponder.
Dynon conducted its performance evaluations on the
bench. "The world knows how GPS gets distorted, and
you can build scenarios using GPS signal generators
that broadcast RF that the device receives like a satellite
source and test it to make sure its operating properly,"
Jordan said.
On the transponder side of the ADS-B system,
Dynon partnered with Trig Avionics for its TSO'd
1090 ES unit, the SV-XPNDR-261. "Transponders
are pretty reasonably priced, and good competition
and certification haven't pushed the price that much,"
Jordan said. "With a TSO-certified transponder, there
is no question on performance. ADS-B capability
required new software, which is less expensive than new
hardware."
With the GPS-2020, "our purpose was to innovate on
price (less than $600)," Jordan said. "The world already
knows how to do integrity checking, so the goal was
not to reinvent that process. We combined an affordable
hardware receiver chain with the software that's
already proven its ability to meet the integrity checking
requirements. This commercial GPS engine doesn't
meet the more minute aspects of the TSO like lightning
strikes, which is where a lot of the costs get driven into
the hardware."
Working to TSO performance only allowed Dynon
to combine all of the hardware with the antenna in its
shell, which reduced manufacturing costs, saves the cost
of a separate certified GPS antenna, and saves hours of
installation costs.
Despite its ADS-B compliant integrity, the GPS-202
cannot drive IFR navigation. As the rules are written,
that requires TSO certification. The GPS-2020 also has
a 400-mph speed limit. "The simplest thing you can
do to improve integrity is give the device more time
to figure out where it is," Jordan said. This allowed
Dynon to increase the position output beyond ADS-B
requirements to four times a second, which increased
the performance of other systems, such as smoother
synthetic vision in Dynon's SkyView PFD.
Many experimental, amateur-built aircraft fly with
good IFR GPS navigators, Hamilton said, but research
suggests that they represent less than half the fleet.
"So all these VFR airplanes needed an ADS-B position
source" with an affordable integration cost. Focusing on
compliance with ADS-B performance requirements has
allowed manufacturers to develop affordable position
sources necessary for NextGen operations. q

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