GPS Measurement 1-8 Situations where errors occur in the current
location and direction Errors sometimes occur when the accuracy of the
GPS satellite unit is poor.
The GPS satellites are controlled by the United
States Department of Defense, so sometimes the
satellite itself intentionally sends incorrect position
data. The measurement error will increase in this
kind of situation.
Sometimes sufficient accuracy is not possible
because the distribution of the GPS satellites is poor
(when the satellites are aligned in approximately the
same direction or height). (The best accuracy for
GPS measurement is when signals are received from
multiple satellites; a satellite directly over the vehicle
and others just over the horizon to the north, south,
east and west.)
Regarding the height and directional accuracy of
GPS measurement, errors are slightly more likely in
the horizontal direction.
Electromagnetic waves from satellites higher than the
vehicle height can be received, but electromagnetic
waves cannot physically be received from satellites
positioned lower than the vehicle height (on the far
side of the Earth). This means that an adequate
height comparison is not possible.
3D MeasurementWhen valid electromagnetic waves can be received
from at least four GPS satellites, the 3D positions of
latitude, longitude and height are calculated.2D MeasurementWhen valid electromagnetic waves can only be
received from three or less GPS satellites, the height
is assumed to be unchanged from the previous
measurement and the 2D positions of latitude and
longitude are calculated. If this happens, the accuracy
of the position is less than with the 3D measurement.No MeasurementIf no valid electromagnetic waves can be received from
the GPS satellites, there is no measurement.
