1. Field of the invention
The present invention relates to a signal receiving method for a user's device in a global positioning system (abbreviated as GPS, hereinafter), which receives signals from GPS satellites and determines a precise position of a user, and especially to an improved signal receiving method capable of reducing the amount of calculation necessary for determining satellites to be used for the positioning.
2. Description of the related art
As is well known, a GPS is a universal positioning or navigation system. In the fully implemented system, three GPS satellites are arranged in each orbit and total 18 satellites are in six orbits, whereby signals from some of the GPS satellites are always available anywhere on the earth.
A user of the GPS selects plural satellites available to him at that time and determines three or four from among the available satellites. The user carries out the calculation for the positioning on the basis of signals transmitted from the three or four satellites.
It is also known that signals from four satellites are needed in order to determine the three-dimensional position of a user, and signals from three satellites are sufficient in the two-dimensional positioning; typical examples of the former are satellite and airplane, and those of the latter are ship and car. However, a car often requires the three dimensional positioning, when it travels in a mountain district.
The determination of three or four satellites to be used from among the available ones is usually performed, based on a value of a geometric dilution of precision (abbreviated as GDOP, hereinafter), which has been already known in a navigation technique, especially in a hyperbolic navigation, and is an index of a positioning accuracy depending on the spatial arrangement of the three or four satellites to be used with respect to a user.
In other words, a GDOP value can be said to indicate a degree of scope of an error, which can occur when a user's precise position is specified. A combination of satellites, which has a smaller GDOP value, can provide the more precise positioning.
The GPS is discussed, for example, in the article "The development of NAVSTAR/GPS and its system" by Shoichi Kimura on pages 41 to 79 of the magazine "Zosen Gijutsu (Shipbuilding Technique)", published May, 1987, or the article "Analysis of the usable time and the positioning accuracy for the phase II GPS" by Shuji Nishi on pages 57 to 65 of the transactions (February, 1982) of the Institute of Navigation of Japan.
In a conventional user's device of the GPS, including examples discussed in the aforesaid articles, GDOP values are calculated with respect to all of the combinations possible, each comprising three or four selected from among satellites available to a user at that time, and a best combination of satellites, which usually has a smallest GDOP value, is selected; namely, satellites included in the selected combination are determined as satellites to be used for the positioning. Signals transmitted from the thus determined satellites are used in the processing for the positioning.
By the way, since GPS satellites are not geostationary satellites, GDOP values of combinations of satellites change every minute. Therefore, GDOP values are always monitored, and a combination of satellites, which has come to have a GDOP value larger than a desired value, is changed by another combination of satellites, which has a GDOP value smaller than the desired value. In the conventional user's device, such calculation of GDOP values is always carried out before execution of the processing for the positioning.
When the calculation of GDOP values is carried out, a microprocessor in a user's device of the GPS is devoted only for that purpose, and in the mean time, it is not allowed to execute the processing for the positioning. The calculation of GDOP values is considerably time-consuming, and therefore, if it is carried out so often as in the conventional user's device, not only the load of the microprocessor increases remarkably, but also the operation time of the microprocessor, which is occupied by the calculation of GDOP values, becomes considerably long, compared with the operation time for execution of the processing for the positioning. As a result, there occurs an undersired problem that a time duration, in which the positioning is impossible, increases.
In addition, for the communication between GPS satellites and a user, there is usually used a spread spectrum modulation method, which consumes considerable time for preparing a receiver in a user's device to receive satellite signals. If, therefore, the switchover of satellites occurs often, this also causes further increase of the positioning impossible time.