Method for setting search band width of a GPS receiver

Abstract: A GPS receiver searches for a satellite radio wave over a predetermined search band width to allow for frequency shift of the received signal (the Doppler effect) and fluctuations in the oscillating frequency of the oscillator. When a high accuracy oscillator is used, the search band width can always be set relatively narrow. The invention provides a system in which the search band width can be set relatively narrow when using lower accuracy oscillators. A search centre frequency f s is used as a start point, and an offset value Δ f osc of an oscillator is calculated when the receiver is turned off on completion of the position measurement, and the offset value is stored as a backup value into a memory. When the power source of the receiver is turned on at the next position measurement, the backup offset value Δ f osc is read out and an initial search band width W o at the start of the position measurement is set in accordance with the magnitude of the offset value. According to another feature, when the offset value of the oscillator which was calculated during the position measurement exceeds a predetermined reference value, the time of the calculation is backed up into a memory. If a predetermined reference time has not elapsed from the next measurement, the initial search band width at the start of the position measurement is set to a wide band.

Description: The present invention relates to a method for setting search band widths in a GPS receiver. A Position measuring system using artificial satellites is known as Global positioning System (GPS). The system employs six orbits set around the earth at an altitude of about 20200 km. Into each of the orbits, a plurality of satellites are put up so that GPS signals including navigation data for the position measurement are transmitted from the satellites toward the earth by a spread spectrum method. The radio waves of three or four satellites which are necessary for the position measurement are received by a GPS receiver on the ground (or on the sea or in the air), thereby enabling the calculation of necessary position information such as latitude, longitude, altitude of the reception point in a real-time manner by using the navigation data included in the GPS signal from each satellite. The GPS signal which is released for non-military use is called C/A code, and is transmitted at a frequency of 1575.42 MHz. Therefore, to capture the satellite signal, in principle, it is sufficient to wait for the reception of the radio wave at 1575.42 MHz in a standby mode. However, since the GPS satellite is not a geostationary satellite, fluctuation of the received frequency will occur due to a Doppler effect. Furthermore, the oscillating frequency of an oscillator of the GPS receiver also fluctuates due to an offset of such an oscillator. To capture the satellite signal in practice, accordingly, a search center frequency of the satellite radio wave is set by a frequency shift corresponding to a deviation amount of the received frequency due to the Doppler effect (hereinafter, referred to as "Doppler frequency deviation") and an offset amount of the oscillator. Furthermore, an appropriate search- band width is set, so that the satellite signal is searched for while shifting the search frequency in the high and low frequency directions within the search band width, with a search start point being set to the search center frequency. Thus the capture of the satellite signal is completed at a point in time when the oscillator is locked to the satellite radio wave. By repeating the above capturing operations, signals from the satellites of the number necessary for the position measurement are captured. In the operation of capturing the satellite signal, the narrower search band width the shorter capturing time of the satellite signal. Also, the smaller the offset value of the oscillator, the narrower the search band width becomes narrower. Therefore, in order to set the search band width as narrow as possible, it is desirable to minimize an error of the offset value of the oscillator of the GPS receiver. In conventional GPS receivers, accordingly, a crystal oscillator with a temperature compensation device of an extremely high accuracy (hereinafter, such an oscillator is referred to as "TCXO") is used as an oscillator. Furthermore, those receivers are designed to store into a memory the offset value of the oscillator calculated lastly when the power source of the receiver is turned off upon completion of the position measurement, as a backup value. The backup value is used as an initial offset value of the oscillator at the beginning of the next position measurement. The TCXO exhibits, by the compensation of temperature characteristics of the quartz resonator by an electrical circuit, an extremely high accuracy of about ±2 to ±5 ppm (parts per million) (±3 kHz to ±8 kHz in the frequency) in a wide temperature range from -40°C to +85°C. On the other hand, the accuracy of oscillators used in an ordinary communicating apparatuses lies within a range of about ±50 to ±100 ppm and, preferably, about ±20 ppm. In conventional GPS receivers, since the high accurate TCXO is used, there has been no need to consider the change in offset value of the oscillator due to a temperature change in the receiver for a period of time when the power source of the receiver is OFF. The offset value backed up in the memory is directly used as an initial offset value at the start of the next position measurement. In the event that the data stored in the memory is extinguished or the backup data is made invalid due to an error of the memory or the like, the search range is determined as wide as the whole fluctuation range of the offset value of the TCXO. If an oscillator of a low accuracy is used, on the other hand, a change in offset value due to a temperature change is large and the offset value backed up in the memory cannot directly used as an offset value of the oscillator at the start of the next position measurement. Therefore, when an oscillator of a low accuracy is used, it is necessary to search for the satellite radio wave in a wide band over a whole range of the offset change of the oscillator and there arises a problem that a long time is required to capture the satellite signal because of the necessity of the search through such a wide band. Such a problem also occurs even if a backup value of the offset value is used. The present invention is made in consideration of the above circumstances and it is an object of the invention to provide a method of setting a search band width in a GPS receiver, in which a capturing time of a satellite signal can be reduced even if an oscillator of a relatively low accuracy is used. To accomplish the above object, according to a first feature of the present invention, there is provided a method of setting a search band width in a GPS receiver which searches for a satellite radio signal over a predetermined search band width with a search start point set at a search center frequency, characterised in that an offset frequency value of an oscillator most recently calculated at a time point when a power source of the receiver is turned off upon completion of a position measurement is stored in a memory as a backup offset frequency value, said backup offset frequency value is read out when the power source of the receiver is turned on at a next position measurement, and in that an initial search band width upon starting of the position measurement is set in accordance with a magnitude of said backup offset frequency value. To accomplish the above object, according to a further feature of the present invention, there is provided a method of setting a search band width in a GPS receiver which searches for a satellite radio signal over a predetermined search band width with a search start point set at a search centre frequency, characterised in that an offset frequency value of an oscillator calculated during a position measurement is stored in to a memory as a backup value, when an initial search band width is set at the start of a subsequent position measurement, in case where said backup offset frequency value exceeds a positive side reference value, an upper search band width for a reference receiving frequency is set to be a maximum range of an offset change of said oscillator and a lower search band width is set to be smaller than the maximum range of the offset change, and in case where said backup offset frequency value exceeds a negative side reference value, the lower search band width for the reference receiving frequency is set to be the maximum range of the offset change of the oscillator and the upper search band width is set to be smaller than the maximum range of the offset change. Fig. 1 is a diagram showing an example of temperature characteristics of an oscillator; Fig. 2 is an operation flowchart according to an embodiment of a method of the present invention; Fig. 3 is a diagram for explaining a switching operation of a search band width in the embodiment in the case where a back-up offset value is small; Fig. 4 is a diagram for explaining the switching operation of a search band width in the embodiment in the case where the back-up offset value is large; Fig. 5 is an operation flowchart according to further another embodiment of a method of the invention; Fig. 6 is a diagram for explaining the switching operation of a search band width in case of uniformly searching the frequencies in an initial search band width in the embodiment; Fig. 7 is a diagram for explaining the switching operation of a search band width in case of mostly searching an ordinary use temperature range in the initial search band width in the embodiment; and Fig. 8 is a diagram showing an example of temperature characteristics of the oscillator. Prior to explaining embodiments in detail, the principle of a search band width setting method according to the first feature of the present invention will be first explained. Fig. 1 shows an example of temperature characteristics of an oscillator which realizes an offset value of ±20 ppm in a temperature range from -40°C to +85°C. As will be readily understood from Fig. 1, a change in offset value is large at positions near the right and left ends of the temperature range. In other words, it will be understood that when (the absolute value of) the offset value is small, the temperature change of the offset value is also small and, when (the absolute value of) the offset value is large, the temperature change of the offset value is also large. When (the absolute value of) the offset value backed up in the memory is small, since it is presumed that a change in offset value for the temperature change is also small, it is sufficient to set an initial search band width to a narrow width. When (the absolute value of) the offset value backed up in the memory is large, it is presumed that a change in offset value for the temperature change is also large. It is, therefore, sufficient to set an initial search band width to a wide width. By searching for a satellite radio wave in accordance with the backup value of the offset value as mentioned above, even in case of using an oscillator of a low accuracy, a situation such that the wide frequency range must be always searched as in the conventional method can be avoided, so that a capturing time of the satellite signal can be reduced. Fig. 2 is an operation flowchart of an embodiment according to the first feature of the present invention. Fig. 3 is a diagram for explaining the switching operation of a search band width in the case where the backup offset value is small in the above embodiment. Fig. 4 is a diagram for explaining the switching operation of a search band width in the case where the backup offset value is large in the above embodiment. According to the embodiment, in accordance with the result of judgment regarding whether (the absolute value of) the offset value of the oscillator which has been backed up in the memory is larger or smaller than a predetermined reference value at the end of the preceding position measurement, an initial search band width W 0 at the start of the next position measurement is switched to either one of the wide and narrow bands as shown in Figs. 3 and 4. The processing operation in this instance will now be described in detail hereinafter. In the case of the present invention, reference values on the positive and negative sides to judge the magnitude of the offset value which has previously been backed up in the memory are set. By way of the temperature characteristics of Fig. 1 as an example, the operation will now be described. In a range from -20°C to +70°C which is considered to cover the ordinary use temperature range, the offset value is equal to or less than ±5 ppm. For instance, therefore, +5 ppm is set as a positive side reference value and -5 ppm is set as a negative side reference value. When the power source of the GPS receiver is now turned on to perform the position measurement, the receiver reads out an offset value (hereinafter, abbreviated to a backup value) Δf osc of the oscillator which has been backed up at the end of the preceding position measurement from the memory. A check is made to see if the backup value is used as an initial offset value of the oscillator at the present position measurement or not (step S101 in Fig. 2). In the following description, for simplicity of explanation, it is assumed that a unit of the backup value Δf osc is not displayed by a unified value but is set to ppm or [Hz] according to circumstances. It is needless to say that the processing unit should be unified in the actual processes. In the case where the read-out backup value Δf osc is not used as an initial offset value for the present position measurement, a predetermined initial search band width, for example, the maximum change range of the offset value of the oscillator is set as an initial search band width (step S107). A search center frequency f s is set by using a Doppler frequency deviation Δf d of the satellite which has been calculated from orbit data and a desired offset value a which has been prepared (step S108). After that, the searching operation of the satellite radio wave is started (step S109). Although the desired offset value α can be set to a fixed value, it is desirable to change the offset value α in accordance with the use environment of the receiver. In case of using the backup value as an initial offset value for the present position measurement, a check is made to see if the read-out backup value Δf osc is larger than the predetermined positive side reference value (for example, +5 ppm) or not (step S102). When it is detected that the backup value Δf osc is smaller than the predetermined positive side reference value, a check is further made to see if the backup value Δf osc is smaller than negative side reference value (for instance, -5 ppm) or not (step S103). In step S103, when it is detected that the backup value Δf osc is larger than the negative side reference value (-5 ppm), it will be understood that the backup value Δf osc lies within a narrow range of ±5 ppm (in a temperature range from -20°C to 70°C) in the temperature characteristics of Fig. 1. When the backup value lies within a predetermined range as mentioned above, there is no need to set the initial search band width W 0 to a fairly wide width. In step S105, accordingly, the initial search band width W 0 is set to a narrow width as shown in Fig. 3. After that, in step S106, the search center frequency f s is set on the basis of the Doppler frequency deviation Δf d of the satellite calculated from the orbit data and the backup value Δf osc in step S106. After that, the searching operation is started (step S109). In the case where the target satellite signal cannot be captured even by repeating the searching operation a predetermined number of times (three times in Fig. 3) within the set initial search band width W 0 , the searching operation is successively executed while gradually widening the search band width like W 1 to W 3 . On the other hand, when it is detected in step S103 that the backup value Δf osc is smaller than the negative side reference value (-5 ppm), it will be understood that the backup value Δf osc lies within a range of the large absolute value (-20°C or less in a temperature range) on the lower side than -5 ppm in the temperature characteristics of Fig. 1. When it is detected in step S102 that the backup value Δf osc is larger than the positive side reference value (+5 ppm), it will be understood that the backup value Δf osc lies within a range of the large absolute value (+70°C or higher in a temperature range) on the upper side than +5 ppm in the temperature characteristics of Fig. 1. As mentioned above, when the absolute value of the backup value is large, the fluctuation of the frequency is also large and it is also necessary to set the initial search band width W 0 to a large width in accordance with such a large frequency fluctuation. In step S104, therefore, the initial search band width W 0 is set to a wide band as shown in an example of Fig. 4. After that, in step S106, the search center frequency f s is set from the Doppler frequency deviation Δf d of the satellite calculated from the orbit data and the backup value Δf osc in step S106. After that, the searching operation is started (step S109). By switching the initial search band width W 0 in accordance with the magnitude of the offset value which has been backed up in the memory as mentioned above, even when an oscillator of a low accuracy is used, the initial search band width can be set to a narrow width in an ordinary use temperature range. The capturing time of the satellite radio wave can be reduced by a time corresponding to such a narrow initial search band width. Although the above embodiment has been described with respect to the case of switching the initial search band width W 0 to either one of the two band widths shown in Figs. 3 and 4, the number of switching values of the initial search band width W 0 can be also set to an arbitrary number. The initial search band width W 0 can be also continuously switched in proportion to the magnitude of the backup value. As will be readily understood from the above description, according to the method of the present invention, the offset value of the oscillator which has been most recently calculated when the power source of the receiver is turned off upon completion of the position measurement is stored as a backup value into the memory. When the power source of the receiver is subsequently turned on at the time of the next position measurement, the backup offset value is read out and the initial search band width at the start of the position measurement is set in accordance with the magnitude of the offset value. Even in case of using an oscillator of a low accuracy, therefore, the capturing time of the satellite can be reduced. A search band width setting method according to the second feature of the present invention will now be described hereinafter. As will be readily understood from Fig. 1, (the absolute value of) the offset value has a large value only in both edge portions (near +85°C and -40°C) of the operation temperature range. The receiver is seldom set to such a high or low temperature in the ordinary use state. It is considered that the occurrence of such a situation is concentrated to special cases in terms of the season and district. That is, in locations and times at which such a state does not occur generally, it can be regarded that such a situation never occurs. On the other hand, when such a situation once occurs, it can be presumed that such a state continues for a while. According to the second feature of the present invention, in the case where it is necessary to search a wide band as in case of using the oscillator of a low accuracy, the search operation is not of the type to always search the whole range of the temperature change of the offset value as in the conventional method. When the backup offset value exceeds the preset reference value, the setting state of the initial search band width is changed depending on whether the offset value has exceeded the reference value in the positive direction or in the negative direction. By using such a setting method, even in case of using the oscillator of the low accuracy, the initial search band width can be set to a width as narrow as possible and the capturing time of the satellite can be reduced in correspondence to such a narrow band width. An embodiment of the method according to the third feature of the present invention will now be described hereinafter with reference to the drawings. Fig. 5 is an operation flowchart of an embodiment of the method of the present invention. Figs. 6 and 7 are diagrams each for explaining the switching operation of the search band width in the embodiment. In the case of the present invention, a positive side reference value and a negative side reference value to judge the magnitude of the offset value backed up in the memory are preset. For example, the operation in the case of temperature characteristics of Fig. 8 will now be explained. The offset value lies within a range of ±5 ppm in a range from -20°C to +70°C which is considered to almost cover the ordinary use temperature range. Therefore, + 5 ppm is used as a positive side reference value and - 5 ppm is used as a negative side reference value. When the power source of the GPS receiver is now turned on to execute the position measurement, the receiver discriminates whether the offset value (hereinafter, abbreviated to a backup value) Δf osc of the oscillator which has been backed up in the memory at the end of the preceding position measurement lies within a range of the reference values ±5 ppm or not (step S301 in Fig. 5). When the backup value Δf osc lies within the range of the reference values ±5 ppm, the oscillator lies within the ordinary use temperature range (-20°C to +70°C) and it is presumed that the change in offset value is not so large. The initial search band width, therefore, can be set to a narrow band. In this case, therefore, the processing routine advances to step S302 and the initial search band width is set to W 01 in Fig. 12 (step S302). On the other hand, when the backup value Δf osc is out of the range of ±5 ppm, the oscillator is out of the ordinary use temperature range (-20°C to +70°C) and it is presumed that the change in offset value is large, so that the initial search band width cannot help being set to a wide band. In this case, consequently, the processing routine advances to step S303 and a check is made to see if the read-out backup value Δf osc exceeds which one of the positive side reference value (+ 5 ppm) and the negative side reference value (- 5 ppm). When it is detected that the backup value exceeds the positive side reference value (+ 5 ppm), the processing routine advances to step S304 and the initial search band width is set to W 02 in Fig. 8. When it is detected that the backup value exceeds the negative side reference value (- 5 ppm), the processing routine advances to step S305 and the initial search band width is set to W 03 in Fig. 8. After either one of the initial search band widths W 01 , W 02 , and W 03 was set in accordance with the backup value Δf osc at that time as mentioned above, the search center frequency f s is set by using the Doppler frequency deviation Δf d of the satellite calculated from the orbit data and the desired offset value α in step S306. After that, the searching operation is started (step S307) as shown in Fig. 6 or 7. Each of figs. 6 and 7 shows an example of the switching operation of the search band width in the case where the backup value Δf osc exceeds the positive side reference value (+ 5 ppm). Fig. 6 shows the case where the search center frequency f s is set to the center of the initial search band width W 02 and the inside of the search band W 02 is equivalently searched only the same number of times in the upper and lower sides. Fig. 7 shows the case where search center frequency f s is set to the upper edge position of the search band width W 01 in the ordinary use temperature range and the inside of the search band width W 01 as an ordinary use temperature range is searched a larger number of times. When the backup value Δf osc exceeds the positive side reference value (+ 5 ppm) as in the above example, it is considered that the present frequency of the oscillator is largely offset to the positive side. In case of the invention, as shown in Figs. 6 and 7, with respect to the positive side which is considered to be offset, the satellite is searched up to the maximum fluctuation range (maximum frequency deviation) of the offset value. On the other hand, with regard to the negative side which is considered not to be offset, the satellite is not searched up to the maximum fluctuation range. It is, therefore, sufficient to set the search range on the negative side to a narrow value and the capturing time of the satellite is reduced in correspondence to such a narrow range. In the case where the satellite signal cannot be captured even by repeating the searching operation only a predetermined number of times (three times in Fig. 6, two times in Fig. 7) or only for a predetermined time within the initial search band widths W 01 , W 02 , and W 03 , it is considered that the satellite doesn't exist in such a frequency range. After that, therefore, it is sufficient to repeat the searching operation while widening the search range to the whole range of the offset value change of the oscillator in a manner similar to the conventional method. The value calculated lastly can be also used as a backup value Δf osc . When the offset value exceeding the reference value is calculated, the backup value can be also updated. It is also possible to set a term of validity to the backup value and not to use the value beyond such a term. As will be obviously understood from the above description, according to the method of the invention, when the initial search band width is set at the start of the next position measurement, in the case where the backup value exceeds the positive side reference value, the upper search band of the reference received frequency is set to the maximum range of the offset change of the oscillator and the lower search band is set to a band smaller than the maximum range of the offset change. When the backup value exceeds the negative side reference value, the lower search band of the reference received frequency is set to the maximum range of the offset change of the oscillator and the upper search band is set to a band smaller than the maximum range of the offset change. Even in case of using the oscillator of a low accuracy or the like, therefore, even when an error of the offset value is so large that the search band must be set to the whole range of the offset change of the oscillator, the initial search band width can be set to a width as narrow as possible. The capturing time of the satellite can be reduced in correspondence to such a narrow band width.

Claims: A method of setting a search band width in a GPS receiver which searches for a satellite radio signal over a predetermined search band width with a search start point set at a search center frequency, characterised in that an offset frequency value of an oscillator most recently calculated at a time point when a power source of the receiver is turned off upon completion of a position measurement is stored in a memory as a backup offset frequency value, said backup offset frequency value is read out when the power source of the receiver is turned on at a next position measurement, and in that an initial search band width upon starting of the position measurement is set in accordance with a magnitude of said backup offset frequency value. A method of setting a search band width in a GPS receiver which searches for a satellite radio signal over a predetermined search band width with a search start point set at a search center frequency, characterized in that an offset frequency value of an oscillator calculated during a position measurement is stored in to a memory as a backup value, when an initial search band width is set at the start of a subsequent position measurement, in case where said backup offset frequency value exceeds a positive side reference value, an upper search band width for a reference receiving frequency is set to be a maximum range of an offset change of said oscillator and a lower search band width is set to be smaller than the maximum range of the offset change, and in case where said backup offset frequency value exceeds a negative side reference value, the lower search band width for the reference receiving frequency is set to be the maximum range of the offset change of the oscillator and the upper search band width is set to be smaller than the maximum range of the offset change.

Inventors: ENDO ISAO, ENDO, ISAO, ENDO, ISAO, C/O PIONEER ELECTRONIC CORP.
Applicants: PIONEER ELECTRONIC CORP, PIONEER ELECTRONIC CORPORATION
Application year: 1993
Main classification: G01S5
IPC codes: G01S1