Accuracy - Suunto G9 Instruction Manual

2.4.3. Accuracy

To calculate a position, a GPS receiver usually requires simultaneous reception from at
least four satellites. The position accuracy typically increases with the number of
satellites being tracked but the most important factor is actually the relative geometric
position of the satellites. The best position estimate is obtained when satellites from
different directions and angles can be tracked. In other words, the less obstructed the
receiver's view of the sky, the better the accuracy.
The accuracy also improves with the number of measurements made on the same
spot, as some errors will be averaged out. Under all-in-view satellite conditions a
global average horizontal positioning error is 7.8 meters (95% confidence level). How-
ever, depending on the satellite constellation and the distortion the GPS signal experi-
ences when traveling through the ionosphere, the instantaneous accuracy can vary
from a couple of meters to over ten meters. The vertical position estimate is roughly
two times more inaccurate than the horizontal. Because of this inevitable vertical
inaccuracy of GPS positioning, Suunto G9 encases a separate altimeter sensor with
a 1-meter resolution that should be used instead of the GPS estimate when measuring
small changes in altitude.
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