82 SA Flyer Magazine
WORDS:
CHRIS
MARTINUS
Several years ago,
there was considerable
excitement when ATNS
told us about ADS-B
(Automatic Dependent
Surveillance-Broadcast) and
PBN (Performance Based
Navigation). Is it going to
meet expectations?
T
HESE ‘modern’ schemes
are facilitated by satellite
navigation systems like
GPS, GLONASS, Galileo
and Compass, which,
together with a ground-
based augmentation system, promised
low-cost Global Navigation Satellite System
(GNSS) IFR approaches at small airelds
and the freeing up of airspace due to tighter
and more direct navigation – which would
also have the benet of lower fuel burn and
shorter commercial ights.
It became attractive for private pilots to
consider putting in the effort and expense
to obtain and maintain IF ratings, since this
would dramatically enhance the utility of
their aircraft. The CAA also embarked on
developing a new and simplied IF rating.
Things looked rosy indeed. Furthermore,
with ‘ADS-B In’ equipment installed in light
aircraft, with the prospect of having trafc
information and weather displayed, general
aviation would see a new era of innovation
and development.
Before GPS, both commercial and
general aviation were largely dependent
on ground-based navigational aids such as
NDB, VOR and ILS. Collision avoidance
relied entirely upon the naked eye and, in
proximity to major centres, ATC (air trafc
controllers) who possibly had radar to
monitor trafc in their areas.
ATNS then embarked upon
decommissioning dozens of NDBs and
VORs, many of which were unserviceable
anyway. But the promised cheap GNSS
approaches did not materialise and
there are only declarations of intent from
CAA and ATNS to require ‘ADS-B Out’
equipment to be made mandatory for
aircraft operated under IFR or in controlled
airspace by 2020. This equipment would
simply be Mode S transponders which are
capable of broadcasting on 1090ES.
STANDARDS AND TECHNOLOGIES
ADS-B and PBN are not really
technologies themselves, but are standards
which have a ‘layered’ approach, much
like the Internet standards which were
developed in the early 1970s. Instead of
standards being based upon technologies,
ADS-B and PBN are objective based.
PBN is ‘performance based’, meaning that
the standard hinges on the accuracy and
performance required, rather than the older
bottom-up approach of the standards being
dened by technologies like ADF and VOR.
The technological layers upon which these
more modern standards rest may then be
developed, changed or otherwise altered
without changing the standards themselves.
The internet owes its success to the big
picture approach, where user applications
are dened as the top layer, regardless
of whether the data itself is delivered by
a physical layer of bre cables, Wi-Fi,
ADS-B ... OR NOT TO BE
AOPA BRIEFING AIRCRAFT OWNERS & PILOTS ASSOCIATION – SOUTH AFRICA
How ADS-B works compared to radar.
Airways
83 SA Flyer Magazine
ethernet, LTE or even a man wearing a
loincloth carrying the data as a paper note
in a cleft stick.
ADS-B and PBN rest largely upon a
physical layer of satellite communications
and location, supported by ground stations
in some instances.
HOW IT WORKS
Present and historic ATC surveillance
and information relies upon radar
technology which was developed during
World War 2. Primary radar surveillance
relies upon transmission of a radio signal
and the time and direction of a reection of
that signal off the hull of an aircraft. Primary
radar gives no other information other than
the location of the target and suffers from
clutter from weather, objects on the ground
and radio noise.
Secondary radar was developed
to overcome the clutter problem and
provide identication of the aircraft. This
was achieved by tting a transponder to
the aircraft which receives radar signals
on 1030 MHz and replies with a series
of encoded pulses known as a ‘code’ or
‘squitter’ on 1090 MHz.
Mode A transponders reply with a code
containing a four-digit octal ‘squawk’ code
which is entered by the pilot. The code then
allows ATC to identify and separate aircraft
by assigning them different squawk codes.
Mode C transponders extend the
response code by adding the aircraft’s
pressure altitude to the code. This requires
input from an altitude encoder or encoding
altimeter which enables ATC to provide
vertical separation between aircraft.
Mode A and C transponders have
proven to be inadequate where there is
high trafc density and several transponder
responses overlap, causing corrupted
information to be received by the radar
ground station.
Mode S (select) is a transponder
standard which allows the ground
radar station to interrogate individual
transponders. In order to make the
transponders individually addressable,
each transponder must be given a unique
callsign. Mode S also provides altitude
information.
Mode S has been modied to lengthen
the code to carry a considerable amount
of additional information. This modication
is known as ‘1090ES’, meaning that an
‘extended squitter’ is sent on the usual
response frequency of 1090 MHz. For the
purposes of ADS-B, this extended code
carries location information. ADS-B ground
stations can then forward this aircraft
identity and location information to other
stations via landlines and communication
satellite links.
1090ES data is also transmitted
automatically once every second, rather
than in response to receiving a radar
signal. This is where the word ‘automatic’ in
ADS-B comes from.
In order to transmit location, velocity
and vector information, the transponder
requires this data from a GPS to which it
is connected. The transponder is therefore
‘dependent’ on such GPS data, bringing
us to the full meaning of ADS-B: Automatic
Dependent Surveillance-Broadcast.
A more recent scheme is to use satellite
receivers to receive 1090ES directly
from aircraft and then feed the data via
a downlink and terrestrial data networks
to ATC. For this to work properly, it is
necessary for the transponder antenna to
be mounted on top of the aircraft, rather
than the usual position on its belly.
This chain of developments and
extensions of the old transponder
technology provides a workable physical
layer for ‘ADS-B Out’ information that can
be used by ATC for trafc surveillance with
far wider coverage than ordinary terrestrial
radar.
IS THAT ALL THERE IS TO IT?
Well, from the documents that have
thus far been published by ATNS and CAA
(which lack a great deal of clarity), that
seems to be as far as things are planned to
AOPA BRIEFING AIRCRAFT OWNERS & PILOTS ASSOCIATION – SOUTH AFRICA COLUMNS
There are cheap functional ADS-B gadgets on the market, but the question is
whether they will be accepted if not certified or paired to a certified GPS.
The evolution of transponder capabilities.
84 SA Flyer Magazine
go in South Africa.
The bi-directional physical layer
standard for ADS-B, known as Universal
Access Transceiver (UAT) which operates
on 978 MHz is specically excluded.
Furthermore, since there is no requirement
for belly-mounted transponder antennas,
there seem to be no plans to have any
ADS-B ground stations in the foreseeable
future.
It appears that the objective of the
roll-out of ADS-B by April 2020 is solely
to provide ATNS with a substitute for their
ground-based navaids and radar, with no
benet accruing to aircraft owners and
pilots, but at great expense to aircraft
owners who will be required to upgrade
their equipment.
In this regard, a 1090ES-capable
Mode S transponder is not in itself a hugely
expensive piece of equipment, but it will
likely require a certied GPS device which,
we are given to understand, must be WAAS
(Wide Area Augmentation System) capable.
WAAS has only been implemented in the
USA, so this system that adds additional
precision to basic GPS signals is unlikely
to add signicant value for most general
aviation purposes.
An additional problem is that there
appear to be no low-cost all-in-one GPS/
Mode S devices that are certied for an
antenna on top of the aircraft. There are
‘diversity’ transponders which allow for two
antennas (one on top and one underneath
the aircraft), but these are rare and
expensive.
ADS-B IN
‘ADS-B In’ is optional in most parts of
the world and allows signals carrying useful
information to be received by aircraft. Using
UAT transceivers, position information is not
only sent to ground and/or satellite stations,
but between aircraft themselves. This
provides an effective collision avoidance
feature where other trafc can be displayed
on the aircraft’s EFIS.
UAT also provides the basis for TIS-B
(Trafc Information Service-Broadcast),
which receives other trafc information from
ground stations. FIS-B (Flight Information
Services-Broadcast) also uses the UAT
physical layer. FIS-B provides weather text
and graphics, NOTAMs, ATIS and other
useful information.
However, since there appear to be
no plans for UAT and ground stations in
South Africa, there is little point in installing
‘ADS-B In’ capable equipment in aircraft
that do not y to Europe or the USA. Such
equipment is not going to provide even the
level of information that is available from the
Internet, where there is weather information
as well as trafc information from sites such
as FlightRadar24 and FlightAware which
tap into the ADS-B system.
THE HOLY GRAIL
At the IAOPA World Assembly held in
New Zealand at the end of March this year,
ICAO appeared very enthusiastic about
the roll-out of ADS-B and PBN (which I will
cover in more detail in a future article) and
believed it to be a solution which should
be embraced by the general aviation
community. Also, ADS-B is seen as a
potential answer to the intractable problem
of separating trafc between manned and
unmanned aircraft.
However, the GA community outside
the USA seems a bit sceptical. Different
countries are implementing limited and
varied portions of a system that is already a
hodgepodge of ideas, and is largely based
on ancient technologies.
Most important, not all aircraft will
be able to implement ‘ADS-B Out’, like
balloons, gliders, vintage aircraft without
electrical systems and small drones. Pilots
who become psychologically dependent
upon such technologies will certainly fail
to take the effort to still visually look out for
unequipped aircraft, and incursions into
controlled airspaces by unequipped aircraft
will similarly go undetected.
It seems that during any transition
period, it may be necessary for GA aircraft
to be equipped with both older Mode C
transponders to be visible to ground radar
as well as ADS-B satellite-compatible
equipment, while at the same time requiring
pilots to use the old Mk 1 eyeball to
maintain separation from aircraft that are
exempt. It just doesn’t seem practical.
There are other little problems creeping
out. For example, several AOPAs in Europe
raised the point that ADS-B broadcasts
unencrypted location and other personal
information that conicts with the new
European privacy laws that came into effect
in May this year. It will be interesting to see
how it all unfolds.
AOPA BRIEFING AIRCRAFT OWNERS & PILOTS ASSOCIATION – SOUTH AFRICA
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