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Dangerous problem with a GPS receiver.

This is to alert users of the handheld GPS receiver, types SP24 and SP24Y, (and perhaps others), made by the French firm MLR, now part of Thales Navigation.

There is a potentially dangerous defect, which occurs only if a compass variation has been entered in order to provide magnetic courses.

As the unit comes new, in its box, then the compass variation has been preset to zero. The course to a waypoint at the North Pole is then 000 degrees, and all is well.

However, if the unit is being used in a location where the magnetic variation is shown on the chart to be 10 degrees West, and if the user makes the entry "10W" at the label "COMPASS VAR.", he will then find that the course shown to the North Pole becomes 350, not 010 as would be expected. All courses shown are in error by twice the magnetic variation, because the unit makes the correction for variation in the wrong direction.

To get the correct magnetic course to be shown, the user, if he is an area with variation of 10 degrees West, must make an entry of "10E" for his "COMPASS WAR." (and vice versa). More wisely, he will avoid correcting for magnetic variation altogether, and work entirely with true courses.

There is no indication of this problem in the manual. It appears to have affected the entire production of these units since they first appeared, a year or so ago. Distributors appear to be well aware of it, and have long ago informed the makers, who until now seem to have denied any defect.

The SP24 has had its performance commended in sailing journals, and I can only conclude that the testers, like me, chose to work in true courses rather than magnetic.

Once the user is aware of this defect, much of the danger will disappear, which is my reason for publicising the matter.