Avionics News January 2015 - 60

GETTING THE ADS-B SYSTEM INSTALLATION RIGHT
Continued from page 59

tion is, what will it display on? Garmin has adopted
a proprietary display interface for its ADS-B In GDL
88, for example. The rest of the industry has adopted
a different display interface called the Capstone Interface as its standard. This is the original interface used
by the FAA's Alaska Capstone project that proved the
concept of ADS-B. Avidyne, Aspen Avionics, FreeFlight
Systems, BendixKing and NavWorx have all agreed to
support the Capstone interfaces because they are open
interfaces and provide for interoperability.
POSITION SOURCE
An approved position source must be used as part of
an ADS-B installation. In fact, it must have been approved for use by an STC/TC for use with the specific
ADS-B Out equipment. Position sources may be standalone equipment, an existing GPS or integrated into the
ADS-B equipment itself. As a practical matter, the position source will normally be a TSO C145/146 WAAS
system, but non-WAAS systems that meet TSO C129 or
TSO 196 may be used if they comply with AC 20-165A
guidance and are approved via the STC/TC process.
The lowest-cost options for position sources for many
customers are based on an already installed WAAS GPS
such as the Garmin GNS 400W/500W series or the Garmin GTN series of WAAS navigators, for example. The
Garmin ADS-B position source provided by these units
is a proprietary interface and not normally disclosed
by Garmin. There are vendors who have reverse engineered the Garmin interface and support it via STC with
their own ADS-B Out equipment. Two vendors to have
recently accomplished this include Trig Avionics with its
TT31 Mode S transponder and BendixKing with its KT
74 Mode S transponder. These STCs are tied to a specific
version of the Garmin GPS software, and there is always
some risk that Garmin can update the software, requiring
a new round of testing to verify future compliance.
The other low-cost option currently available only on
UAT-based ADS-B Out systems consists of a built-in,
optional position source. The option list price point on
these varies between $1,000 and $1,500 when you factor
in the price of the antenna. Examples of UAT ADS-B
units available today for installation that have an internal
position source option include the FreeFlight RANGR series, the Garmin GDL 88 and the NavWorx 600BG. It is
obvious there will be 1090 ES transponder-based systems
that will have a built-in position source in the future.
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january

2015

CONFIGURATION AND COMPLIANCE VERIFICATION
The FAA has developed a monitoring system to
determine compliance with the mandate. Currently,
about half the aircraft with ADS-B Out systems
flying today have issues with compliance. As avionics
shops gain more experience with these installations,
the expectation is that these numbers will improve.
Many shops do not have the equipment necessary to
verify that the system is operating as it is required.
The equipment is either an expensive addition ($6,000
to $12,000) to existing test gear or totally new test
equipment, which can be more than $25,000.
In addition to following the instructions for
interfacing to an approved position source, the
configuration information also must be properly set up
in accordance with the STC installation manual. One
common error is to accept the default values for the
source integrity level parameter. The SIL parameter
defines the probability that the position source will
not generate an alert that the containment radius of the
position is not satisfactory for use. A SIL value of 3 is
required, which is the code for the probability being less
than 1 x 107 per hour.
Upgrading an existing installation that has a Garmin
GTX 330 Mode S transponder already installed to one
updated to ES by the factory is more than a slide-in
replacement, as wires need to be added to connect to the
position source, software may have to be upgraded on
the GPS, and both the GPS and the transponder must be
configured. Personally, I see many such upgrades done
improperly. Often, the avionics shop does not ask the
pilot if they have a portable ADS-B In receiver, and the
configuration bits for ADS-B In are not set to reflect the
pilot's portable capabilities.
Some shops might say the regulation only permits
these configuration elements to reflect the status of
"installed ADS-B In equipment." I raised this issue with
the FAA chief counsel and recently received a letter of
response with the opinion quoted below:
The FAA's intent was for this message element
to indicate that the aircraft has the capability to
receive ADS-B In services, not necessarily that
this capability be installed. The use of the word
"installed" in this section is confusing, given the
specific terminology in the above referenced RTCA
documents. The FAA did not intend to restrict
the designation of ADS-B In capability if using
portable ADS-B In receivers.

Avionics News January 2015

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