Page 67 - Misconduct a Reference for Race Officials

RACING RULES GUIDANCE

GPS EVIDENCE IN HEARINGS

GPS is almost universally used for navigation by yachts and the use of GPS based ‘tracking’
is becoming common at events for boats of all sizes. As a result, parties frequently seek to
present GPS based evidence in hearings.
GPS evidence can often be very helpful, but it must be considered in the light of the
limitations of the technology used, the GPS installation and the circumstances of the
incident.

Limitations of GPS
Position accuracy. GPS positions and tracks give an illusion of precision. Positions are
usually quoted to three decimal places of a minute of latitude and Longitude; for latitude, this
is equivalent to about 1.85 metres. However, most modern manufacturers quote an accuracy
of three to five metres to a 95% certainty level (note 1) . This means that there is a 95%
probability that the GPS is located somewhere inside a circle of that radius with the quoted
‘position’ at the centre. There is a 5% chance that the GPS is outside that circle. If the
evidence is comparing the position of two GPS units, this circle of uncertainty must be
applied to both units, which increases the uncertainty of the distance between the units.
Higher accuracy is possible using techniques such as differential GPS, but the equipment
and technical infrastructure required is expensive and is not currently common in marine
navigation or fleet tracking systems.
Speed accuracy. The reported speed at which a GPS receiver is moving is typically
accurate to within 0.1 to 0.2 knots.
SOG not speed through the water. The speed reported by GPS is the speed over the
ground not boat speed through the water. These can be significantly different if there is a
strong current.

Course Accuracy. The accuracy of the course reported by GPS depends on the speed.
Typically, around +/- 2.5 degrees at 2 knots, improving to +/- 0.5 degrees at ten knots.

COG not heading. The course reported by a GPS is the course over the ground (COG) not
the heading of the boat (the direction in which it is pointing) unless the tracking information is
supplemented by compass information from an external system.
GPS datum. The geographic coordinate system used to reference the position. The most
common standard in Europe is WGS 84. Positions can vary by hundreds of metres when
referred to different datums. When comparing positions reported by two different GPS units
(e.g. on different boats), or a reported GPS position to a waypoint or charted position, it is
important to ensure that all positions are referenced to the same datum.

GPS Tracks
Many GPS enabled devices can record a track to show a boat’s course over time. This is
usually derived from a log of timestamped positions and converted by software into a
graphical display on a PC or dedicated chart plotter.

Fleet tracking systems. Increasingly used at regattas, employ GPS devices on the boat or
on the sailor to transmit timestamped position and course information to a master system

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