Page 68 - Misconduct a Reference for Race Officials

that can record and display the track of each boat throughout a race. Often marks of the
course are fitted with a tracker. Depending on the data transmission method used, boat
positions may be logged at intervals as short as one or two seconds or as long as hours
between log points.
AIS tracks. Many racing yachts are equipped with AIS transponders that transmit GPS
derived information over VHF. Websites exist that display this, essentially public, information
in the form of vessel positions and/or tracks. Class B AIS systems, the type most often
installed on racing boats, attempt to transmit a position message every three minutes when
the vessel speed is less than two knots or every 30 seconds for higher speeds. However, in
busy areas, class B transmissions are not guaranteed and it is not unusual for a class B
track to contain segments with very long track intervals.

Limitations of GPS Tracks
Important issues to consider for tracks are the logging interval (note 2) and the degree of
smoothing carried out by software.
Changes of course and/or speed significantly shorter in duration than the logging interval are
not usually identifiable. A high frequency non-smoothed track display will give more
information about a boats course than a low frequency highly smoothed track.
The boat ‘icon’ displayed on the screen is not normally to scale with the boat and is usually
displayed with the icon centred on the reported position. The latter can be particularly
misleading for larger boats (see below).
When comparing multiple tracks, the track points will not usually be synchronized in time.
In some cases, it may be necessary to view the raw log data.

Installation Factors
Location of antenna. The position reported by a GPS is the position of the antenna. It is
important to know the location of the antenna within the boat. This is particularly important
for larger boats, or when the GPS is carried by a crew member rather than fixed to the boat.

Incident Related Considerations

When evaluating GPS evidence, it is important to carefully consider the inherent uncertainty
of the GPS data in relation to the facts required to decide the case, the nature of the incident
and the size and speed of the boats involved.
Incidents between boats. GPS tracks and positions are not usually sufficiently accurate on
their own to decide close quarters incidents such as establishing/breaking an overlap,
keeping clear, brief changes of course, etc.
Marginal OCS calls. GPS tracks and positions are not usually sufficiently accurate on their
own to decide marginal OCS calls. Specific important factors to consider are the position of
the antenna on the boat, the method the boats used to identify the position of the line (e.g.
manually ‘pinging’ the ends of the line, GPS trackers on the marks, etc. which introduce their
own regions of uncertainty) and the ability to synchronize the boat’s track position to the
starting signal

Sailing the course. GPS data and tracks will often be sufficient to show that a boat has
approached close to a given mark of the course, has crossed the starting or finishing lines or
even that a mark has been rounded or passed on the correct side when the margin of
passing or rounding is large.

Especially on smaller boats, GPS data is not usually sufficiently accurate on its own to show
that a boat has passed close on either side of a mark.

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