Avionics News July 2015 - 40

CURRENT STATUS OF ADS-B
Continued from page 39

scope, limitations and types of projects allowed. Nevertheless, a PNL continues to be required for ADS-B projects
that involve a change to the certification basis (14 CFR
21.101 significant change, special condition or exemption),
a new method of compliance, airworthiness directive, or
approval of any ADS-B and position source pairings not
previously qualified.
Privacy in the NAS: Some operators have raised concerns
that equipping with ADS-B would have privacy and security implications for their aircraft. The Equip 2020 working
group has reviewed this issue and determined that equipping
aircraft with ADS-B does not materially change the ability
to track aircraft, because aircraft that currently operate with
a Mode S transponder already transmit their ICAO 24-bit
code. This will not change with ADS-B. Additionally, UATequipped aircraft have the ability to operate using an anonymity mode when squawking 1200. The FAA, however, has
recognized that the general aviation community has specific
privacy and security concerns, and the agency has committed
to consider mechanisms to mitigate the risk of identifying the
real-time location of a specific aircraft that is transmitting its
identity on the 1090 megahertz frequency.
Why are the requirements in the rule what they are?
Recently, there have been some discussions about how the
rule and associated guidance were written, and whether
the associated ACs and requirements are appropriate or too
stringent. The following summary provides an overview of
both the current requirements and some of the history of
how these requirements were developed in cooperation between the FAA and industry.
The FAA created two paths for the ADS-B link: TSOC154c on 978 MHz (UAT) and TSO-C166b on 1090 MHz.
The TSO-C154 standard was developed specifically for GA
in response to the community's input that general aviation
wanted UAT. General aviation wanted UAT as an option to
1090 MHz because the bandwidth of the UAT link gives GA
more functionality by also enabling weather and traffic along
with future applications that could not be offered within the
limitations of the 1090 frequency bandwidth.
The FAA has invoked a graduated set of performance requirements for the different classes of equipment based on the
size and complexity of the aircraft. There are six equipment
classes (A1, A1S, A2, A3, B1S and B1 for the 1090 link and
similar for UAT) identified in the rule. The B1 class is specifically for small GA airplanes.
Questions have been raised about the required "9 feet of accuracy" for small GA airplanes and whether it is too stringent.
If accuracy was the real issue, the aviation industry could use
a commercial off the shelf GPS for ADS-B, because a typi40

AVIONICS NEWS

JULY

2015

cal unaugmented GPS receiver has about 10 feet of accuracy
most of the time without WAAS. But accuracy is not the issue;
rather, it is integrity and availability. The FAA does not say
that WAAS is required, but identifies TSO-C129 GPS, TSOC145/C146 GPS/WAAS, and TSO-C196 GPS as acceptable,
provided they have been qualified against the requirements in
AC 20-165A, Appendix 2.
Manufacturers of GA receivers choose GPS/WAAS for a
number of reasons, including:
* TSO-C129 equipment is mostly out of production, and
expending precious engineering resources to update the
software to meet Appendix 2 is not good for a company
getting return on the investment.
* TSO-C145/C146 is readily available and used in current designs (LPV equipment) and can be leveraged
simply for the ADS-B equipment.
* Manufacturers are making use of commercially available certified GPS/WAAS cards that meet the requirements already.
* GPS/WAAS has the highest availability in meeting the
ADS-B Out NIC and NACp requirements.
Manufacturers could take existing GPS/WAAS chips and
"dumb them down" to be TSO-C196 compliant. But that
would involve cost and result in an operational limitation on
those aircraft equipped that way because of the pre-departure
requirement to show that the aircraft can meet the performance of the rule.
Several GAMA members expended resources in support of
the FAA's development of TSO-C199 TABS requirements that
looked at uncertified COTS GPS chipsets in attempts to determine whether these GPS chips could be used as a viable position source for ADS-B. A key finding of that work was that
uncertified COTS GPS chipsets had some ability to exclude
faulty satellites, but not to the level required by the GPS TSOs
or the 91.227 rule. A few lessons from this testing include:
* The chipsets had the ability to detect step errors. (This
is typically done outside of RAIM/FDE.)
* Most chipsets could keep ramp error effects to minimal
level when stationary, but under dynamic tests would
fail to exclude faulty satellites.
* Most chipsets would exclude faulty satellites marked as
"do not use" by WAAS.
* All of the COTS receivers would use satellites that
were unmonitored by WAAS, which means that if
WAAS wasn't monitoring a faulty satellite, the COTS
GPS could be using it in a position solution that could
contribute to an inaccurate position.
The conclusion of these tests is what has supported the
FAA's decision that the TSO-C199 TABS equipment is unsuitable for 91.227 compliance.
Continued on page 42

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