In recent years, positioning devices used in a vehicle navigation apparatus generally determine a position of a vehicle in an estimating manner by a dead-reckoning method. The dead-reckoning method calculates the position of the vehicle, as the vehicle is driving, by using the current position of the vehicle provided by a GPS, and corrects the detected position using the speed and direction of the vehicle, which can be detected by a speed sensor and a gyro sensor, respectively.
Due to the detection errors of the speed and gyro sensors, the error of the positioning accumulates as time lapses, and accuracy of the vehicle's position deteriorates. Therefore, absolute positioning coordinates derived from the GPS receiver, which are free from accumulated positioning error, is ideal for the improvement of accuracy and correction of the current position of the vehicle.
In addition, in terms of its positioning accuracy, the GPS receiver is susceptible to the influence of multi-path reception of GPS signals, such as, when the vehicle is traveling in a valley of high-rise buildings. Therefore, a conventional method of tracing a vehicle based on the dead-reckoning method and correcting the trace of the vehicle by the GPS is considered as practical and is widely used.
In addition, along with other electrical devices in a vehicle, navigation apparatuses must also be energy efficient without sacrificing performance, such as position accuracy. One method of providing an energy efficient device with position accuracy is disclosed in Japanese Patent JP-A-8-512163 (JP '163). In JP '163 the power supply for the GPS receiver is turned off when the vehicle is stopping, which may be determined by the speed and gyro sensor (i.e. sensors other than the GPS receiver).
However, such operation of the navigation apparatus may cause a problem when the vehicle is stopping for a long time or when the vehicle frequently stops, both of which may lead to a continuation of power loss for the GPS receiver, thereby disabling the acquisition of the vehicle position from the GPS receiver. Such operation scheme is also problematic when the vehicle continues to travel for a long time, because the power supply for the GPS receiver may seldom be turned off, thereby disabling power saving of the navigation apparatus during such travel.