Page 8 - Index

Figure 4: Aircraft taxiing practices at a particular airport.

sponse times. From this basis, improve- cussed here. Let us sort out our squawks
ments to the infrastructure—provision from our squits. A “squawk” is a response
of routes or removal of obstructions— from a transponder that is made upon
can be considered. I digress slightly from interrogation by ATC equipment, the
the issue of investigations to offer an secondary surveillance radar. Starting
example of what I mean. An airport’s from the Mode C transponder, the ma-
airside infrastructure was such that a jority of IFR and VFR traffic can squawk
conflict between taxiing aircraft and their identity, position, and speed. Using
ground vehicles at a busy hold led to the a Mode S transponder, the vehicle—be it
construction of an extra length of service an aircraft or ground vehicle—can squit
road to deconflict the traffic and reduce its ID and position information without
the chance of delay and incidents. This, being interrogated by ATC equipment.
from a safety point of view, is the differ- This reduces the back-and-forth interro-
ence between a reactive and a proactive gation/response cycles and thus mini-
approach. mizes the number of messages transmit-
So far I have mentioned ADS-B; the ted, increasing the system’s surveillance
use of multilateration data with Mode S capacity. The more modern ADS-B units
works equally well. There are, of course, use an extended squitter (ES). This just
limitations to the data. For a start, it is means more data per squit. So instead
not as accurate as FDR or FDM data, of ATC equipment having to correlate
recording data at 1 hertz. This is, howev- the Mode S message against radar data,
er, better than the data rate of primary the aircraft’s GPS sends its data message
radar. once a second. An ES message can carry
There are a number of signal accuracy 49 datapoints compared to 7 from a ba-
issues. As a rule of thumb, do not expect sic Mode S transponder and just 3 from a
a positional accuracy that is better than Mode C transponder.
about 10 meters. Trying to demonstrate Most of the traffic at airports handling
if a vehicle has or has not crossed from commercial aircraft will have 1090 ES
the apron to the maneuvering area is transponders, potentially delivering a lot
not really viable. However, some aircraft of data. However, the expense of these
and some vehicles perform better than units may be a disincentive to airport
others. Either way, tailor what you do to operators to install them on ground
the data’s accuracy. An aircraft or vehicle vehicles, especially vehicles that do not
just about to encroach on the runway usually enter the maneuvering area.
strip is not what you want to analyze. Using low-power radio transmitters and
Regular shortcuts across the runway end receivers, it is possible to develop a local
safety area will be traceable. triangulation system to track vehicles for
Let us just take a quick pause to look a much lower cost than for 1090 ES tran-
at the basic technology that is being dis- sponders. Somewhere in the middle lies

8 • January-March 2022 ISASI Forum