NASA Marine CLIPPER SPEED AND DISTANCE LOG, Manual

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Any changes are automatically applied to both speed and distance
measurements, and are always permanently stored in the unit even
when it is switched off.
If the factory-fresh unit (with a pre-set calibration factor of 100) is
found to under-read by, for example, 6% the calibration factor must
be increased by 6% to 106 to corect the under reading, The
calibration factor is increased by pressing the
TOTAL button, and
decreased by pressing the SPEED button.
The general rule is that if the unit over-reads, the factor must be
reduced, and if it under reads, the factor must be increased. The
percentage error in the readings is the same percentage change
which must be entered. Any changes affect both speed and
distance correctly.
When the correct calibration factor has been entered, press TRIP
to return to the “SET” display, and to store the new calibration
factor for future use.
13
CALIBRATION EXAMPLE
Calibration is never easy because of the effects of currents and tides.
Because the unit measures the distance travelled through the water,
there can be significant errors in measured speed and distances
over the ground if currents are flowing. Calibration can be done by
measuring a test speed with the unit and comparing its reading with a
known speed. Another method is to traverse a know distance
(preferably twice in opposite directions to cancel the effect of currents)
and to compare the unit’s measurement with the known distance.
However, suppose, for example, a go-and-return test distance
measured by the unit was found to be 12.5 miles, and the distance is
known( from charts) to be 11.9 miles. The unit is over-reading. Doing
the division (measured divided by known) gives 100 X (12.5/11.9) =
105. This value is the effective calibration factor. To Correct the over-
reading, the unit`s calibration factor must be reduced by 5 to force
the unit`s calibration to match the outside world.