•  When using the navigation system for the first time after purchase or for the first time after a long period during which it was not used, it may take about 5—15 min. until the current position is measured. Also, even during normal use, it may take about 2—3 min. for measurement.

•  The GPS antenna may be unable to receive GPS satellite signals when a vehicle passes through tunnels, valleys between tall buildings, or in the mountains.

•  Placing an object above the GPS antenna may prevent the navigation unit from taking measurements.

•  When GPS measurement conditions are bad, the navigation unit may be unable to compute dimensions or correct to the proper position.

•  The position measurement error for GPS information can be reduced by reception conditions, the time band, and by deliberate reduction in satellite accuracy by the United States Department of Defence. Also, under the following conditions, interference with satellite signals may make it temporally impossible to receive signals from GPS satellites.

•  The navigation unit can locate absolute position only when the vehicle is in motion. Therefore, the navigation unit does not correct direction when the vehicle is not moving.

•  The navigation unit computes three positions (latitude, longitude, and altitude) using radio signals from four or more satellites, called three-dimensional positioning. The more GPS satellite signals received, the more accurate the three-dimensional positioning is performed. The navigation unit can receive a maximum of eight satellite signals to compute a vehicle's position.

•  If only three satellite signals can be received, the navigation unit uses two positions (latitude and longitude) and the altitude calculated while in three-dimensional positioning to compute a vehicle's position. This is called two-dimensional positioning.

•  There can be as much as a 30-m +/- factor in the position detection system, even using the three-dimensional positioning, which is highly accurate.

•  The position detection system is affected by positions of the GPS satellites which send signals.