Method of acquiring error correction value of reference frequency, terminal device, and recording medium

A terminal device that receives satellite signals from a plurality of satellite positioning system (SPS) satellites and locates a position, the terminal device comprising:

• an oscillator that outputs a reference frequency, an oscillation frequency of the oscillator being continually corrected;
• a local oscillator that outputs a frequency signal used to receive the satellite signal, the local oscillator being different from the oscillator;
• a first drift calculation section that periodically calculates a drift of an output frequency of the local oscillator as a first drift using the reference frequency and the output frequency of the local oscillator;
• a search range calculation section that calculates a search range of a reception frequency of the satellite signal using a transmission frequency of the satellite signal, a Doppler shift, and the first drift;
• a second drift calculation section that calculates a drift of the output frequency of the local oscillator as a second drift using an estimated reception frequency that is estimated based on the transmission frequency of the satellite signal and the Doppler shift and a reception frequency when the satellite signal has been received;
• a reference frequency error calculation section that calculates a difference between the first drift and the second drift as a reference frequency error that is an error of the reference frequency;
• an error correction value calculation section that calculates an error correction value of the reference frequency based on a plurality of the reference frequency errors;
• an update method selection section that selects a search range update method using an amount of change in the first drift and the error correction value;
• a search range update section that updates the search range using the search range update method that has been selected by the update method selection section;
• a reception section that searches the satellite signal within the search range, and receives the satellite signal;
• and
• a positioning section that calculates a position of the terminal device based on the satellite signal.

A computer-readable recording medium storing a program that is executed by a computer included in a terminal device that receives satellite signals from a plurality of satellite positioning system (SPS) satellites and locates a position, the program causing the computer to:

• periodically calculate a drift of an output frequency of a local oscillator as a first drift using a reference frequency that is output from a given oscillator different from the local oscillator and the output frequency of a local oscillator, an oscillation frequency of the oscillator being continually corrected;
• calculate a search range of a reception frequency of the satellite signal using a transmission frequency of the satellite signal, a Doppler shift, and the first drift;
• calculate a drift of the output frequency of the local oscillator as a second drift using an estimated reception frequency that is estimated based on the transmission frequency of the satellite signal and the Doppler shift and a reception frequency when the satellite signal has been received;
• calculate a difference between the first drift and the second drift as a reference frequency error that is an error of the reference frequency;
• calculate an error correction value of the reference frequency based on a plurality of the reference frequency errors;
• select a search range update method using an amount of change in the first drift and the error correction value;
• update the search range using the search range update method that has been selected;
• search the satellite signal within the search range, and receive the satellite signal;
• and
• calculate a position based on the satellite signal.