Avionics News December 2020 - 63

aircraft, ADS broadcasts the four-digit code, a unique
identifier assigned to the aircraft, position, velocity (both
vertical and horizontal), pressure altitude, whether the
aircraft is on the ground or airborne, emergency status, and
quality indicators.
No original ATC transponder and none of the first
generation of Mode-S transponders can do all of these things.
One choice for compliance with the ADS-B mandate is with
a Mode-S extended squitter transponder, which is also known
as a 1090ES transponder. The " 1090 " is a reference to the
frequency on which the transponder transmits.
An important difference between previous transponders
and those that satisfy the ADS requirement is position
reporting. For position reporting to work, the 1090ES
transponder must include or be connected to a GPS receiver
augmented by the wide area augmentation system. Some
aircraft owners who wish to upgrade their system need to
purchase a WAAS-enabled GPS along with the 1090ES
transponder. Many general aviation owner/operators
wanted a less-expensive compliance solution. Fortunately
for owners in the United States, there is a less-expensive
option, which can be used below 18,000 feet.
For aircraft in the United States, a device called a
universal access transceiver, known widely as a 978

UAT, is another way to comply with the ADS mandate.
Like the transponder described previously, the " 978 " is
a reference to the frequency on which the unit transmits.
Knowledgeable readers will realize this is one of the
frequencies used for distance measuring equipment.
Fortunately, DME and 978 UAT use different code formats.
In other words, they speak completely different languages.
As a result, it's unlikely the two systems could interfere
with each other. Like the 1090ES transponder, the 978 UAT
must include or be connected to a WAAS-enabled GPS. In
addition, the 978 UAT must sense the transponder outputs.
The Federal Aviation Administration requires systems to
be unable to transmit conflicting data. One can imagine
the confusion and danger, which would occur if the 978
UAT broadcast 1200 at 3,500 feet, while the transponder
broadcast 0461 at 4,000 feet. I had a conversation with an
aircraft owner who told me about a flight when he spent
a considerable amount of time looking for an aircraft
reported to be following him closely. As it turned out, the
other aircraft was a second instance, offset, of his own
data. There was no other aircraft. This story points out the
importance of everything working together properly.
Overall, ADS works better than SSR. Air traffic controllers
get updated information on their screens nearly five times
faster than the older
system. In addition,
the ADS data is more
precise, which enables
controllers to allow
air traffic to fly closer
safely.
Within the United
States, for air traffic
control purposes,
the FAA uses SSR
along with ADS
receivers to collect
data from traditional
ATC transponders,
1090ES transponders
and 978 UATs. This
data is provided
to the air traffic
controllers.

Flight Information Services-Broadcast is part of ADS-R for 978 UAT equipped aircraft.

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Avionics News December 2020

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