Metro Rail News October 2017

would double the capacity of current ERTMS2
systems.
ADS technology is called Radar Positioning
System. RPS equipment consists of a highfrequency
radar installed on the underbody of the
train pointed to the ground and scanning 1 cm2 of
surface. The radar, whose dimensions are similar
to a shoebox, measures, at a rate of 150,000
times per second, the distance to the surface with
an accuracy of 100 µm. Thus, the radar obtains a
detailed ground profile as the train moves
through the railway infrastructure. Signal
processing of the profile allows the sleeper
detection, since they will protrude from ground
level and present a smooth surface in contrast to
the irregular profile of the ballast. By a simple
count of the sleepers, it is possible to determine
the position of the train with accuracy equivalent
to the spacing between two consecutive sleepers.
This technique suffers from the possible
cumulative positioning error that occurs by false
detections or loss detection sleepers.
Grouping a set of 32 bits (32 different
thicknesses), logical words are formed that are
able to uniquely encode more than 4,000 million
track segments. Once the radar has decoded a
complete word, the position of the train can be
associated to the encoded location.
Since the scanned area interrogated by the radar
is one square cm, the bit length may be just a cm
long and thus the information can be compacted.
RPS uses 16 types of plastic beacons, whose
dimensions are similar to a tobacco box, to
encode information. Each balise contains 4 bits of
information, which are what limit the balise set to
16 types, simplifying the installation and its
maintenance. Appropriately combining these
balises into sets, balise words of 8, 16, 24, 32 or
64 bits can be formed. Two additional bits are
added at the edges of each balise, corresponding
to s tart and end bits, so the radar is able to
detect the reading direction and reorder the bits
if it is reading in reverse way.
However, RPS solves this problem because its
patented design includes reading information
encoded in plastic balises installed in the railway
infrastructure. The millimeter-wave radar signal
penetrates all plastics, so the radar is able to
measure the thickness of the balise. By
associating certain thicknesses to certain logic
levels, it is possible to encode information in the
plastic that the radar can read. For example, it is
possible to encode a 0 bit by installing a 2 cm
thickness plastic laminate over a sleeper and
encode a 1 bit by installing a 4 cm laminate.
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