System for measuring position by using global positioning system and receiver for global positioning system

Abstract: A GPS position measuring system includes at least one mobile station and a base station associated with the mobile station. The mobile station is provided with an antenna (30) for receiving one or more GPS signals from the corresponding GPS satellites, an amplifying and frequency converting circuit (31) for amplifying the received signal and converting the amplified GPS signal into a signal on an IF band, an analog-to-digital converter (32) for converting the signal on the IF band into a digital signal, and a writing unit (33) for writing the digital signal into a memory card in order to transfer the digital signal to the base station.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a system for measuring a current position of an automobile or the like by using a global positioning system (GPS) and a receiver for the GPS system. 2. Description of the Related Art A great remark has been placed on an NVSTAR (Navigation System with Time and Range)/GPS which has been developed and used in U. S. Army, because this system is capable of measuring a position of a target object at any place and time in the world. In recent days, a GPS receiver unit as well as a car navigation system provided with the GPS receiver are made commercially available. In near future, the GPS receiver may apply to more various fields. The GPS known by the present inventors is designed to capture an electromagnetic wave transmitted from a GPS satellite going around the earth as keeping the orbit highly accurate, measure the time when the wave reaches the GPS, and measure the distance between the GPS satellite and the receiving mobile station (user) based on the measured time. The similar measurement is carried out in the other two GPS satellites for deriving the position of the mobile station. The unknown position of the mobile station can be derived by the positions of the three GPS satellites obtained-from the orbit information and the measured distances from the station to each the GPS satellite. Even if the foregoing process is carried out with respect to these three GPS satellites, the measured time has a uncertain component. Hence, by treating a time lag Δ t as one variant and capturing the positions of the four GPS satellites, it is possible to accurately measure the position of the GPS mobile station. The GPS has a capability of measuring the position with high precision of about several tens to hundreds meter. Hence, the mobile station is capable of grasping the position of itself and the base station for managing two or more mobile stations loaded on a car, for example, is also capable of managing the position of each mobile station. The managing result is used for managing an everyday operation range or providing a service for a user. The description will be directed to the known GPS position measuring system. A GPS signal is received through a receiving antenna. The received signal is sent to an amplifying and frequency converting circuit in which the signal is amplified and then is converted into a signal on an IF (Intermediate Frequency) band (referred to as an IF signal). The resulting signal is sent to an analog-to-digital converter. In the A/D converter, the signal is directly converted into a digital signal or the base-band signal is converted into a digital signal. The digital signal is sent to a digital signal processing circuit in which a spectrum back-diffusion process, a digital demodulating process and a position measuring process are carried out with respect to the digital signal for deriving the data about the measured distance and the satellite orbit. Four GPS satellites are required for measuring the position of a target object. Hence, at first, four satellites are selected from all GPS satellites which can be captured. As a determining criterion for the selection, a GDOP (Geometrical Dilution of Precision) is used. The GDOP is a coefficient standing for a geometrical precision degradation. In the selecting process, at first one GPS satellite is captured so that a signal received from the captured GPS satellite is converted to the corresponding digital signal and collecting pieces of data about orbits of all the GPS satellites is performed. The collective data being referred to as an almanac. From the almanac, the GDOP is derived. On the derived GDOP, the four satellites are allowed to be selected so that the measuring error may become minimum in the selected combination. From the selected four satellites, the GPS signals are received for collecting ephemerides (data about orbits of the selected satellites). In the case that at least one of the GPS signals is not allowed to be received from the corresponding satellite through the adverse effect of an obstacle, the combination of the satellites bringing the GDOP to the second smallest value is selected. Then, the foregoing process is carried out with respect to the selected combination. Next, with a correlation pulse, a pseudo-distance between the GPS receiver and each satellite is derived. To improve the measuring precision, it is better to use the GPS signals received simultaneously for deriving each pseudo-distance. Hence, the recent GPS receiver is required to provide the digital signal processing circuits corresponding to four or more channels. The data about the pseudo-distances and satellite orbits obtained by the above process is applied to a position deriving circuit for calculating a position of each satellite, operating an quartic equation, and performing the coordinate conversion, the statistical processing and the like. As a result of performing the processing, the measured position (latitude, longitude and height, for example) of the GPS receiver itself can be obtained. In such a GPS position measuring system applied for managing a promotion of an automobile, the position data is sent to a writing unit in which the data is written to a memory card (or IC card) together with a measuring time. The position data and the measuring time are sequentially written on the memory card. The memory card containing the position data and the measuring times accumulated thereon is transferred to the base station. The memory card is inserted to the reading unit for reading the position data and the measuring times from the card. The data is sent to a computer having a operation managing program built therein. In the computer, for example, the one-day operation management of the car is derived from the data. The foregoing related art provides a circuit for processing a GPS signal and a circuit for deriving the position of the mobile station on each mobile station. It means that each mobile station is required to have a numerical control oscillator (NCO), a digital signal processor (DS?), a 16-bit central processing unit (CPU), a numerical processor and the like. Further, for measuring the position of the mobile station, it is necessary to prepare the four or more channels or use a high-speed IC for operating one channel at high speed. As a result, each mobile station has so large and costly hardware. It is difficult and disadvantageous in light of the cost to provide such a mobile station or a GPS receiver in each one of a lot of cars. The GB-A-2 241 623 discloses a vehicle location system with a base station and a receiver in a vehicle. In the base station a control and calculating apparatus calculates the position of the vehicle by using ephemeris data and propagation delays. Here only the base station knows the position of the vehicle and not the vehicle itself. Next, the description will be directed to a data format of a GPS signal sent from a satellite. The GPS data is composed of repetitive master frames. One master frame consists of 25 pages (main frames). One page consists of five sub frames. Each sub frame is arranged to have 300 bits. For example, the first sub frame contains an amending coefficient about a timer of the satellite, the second and the third sub frames contain the orbit data of the satellite, and the fourth and the fifth sub frames contain the almanac and an amending coefficient about an ionospheric layer, ant etc.. Since the first to the third sub frames are completed at each page, all the orbit data of the GPS satellite is contained in one page. On the other hand, the fourth and the fifth sub frames are arranged to be completed at each frame master. That is, the remaining part of the almanac, or the orbit data of another GPS satellites is arranged to be dispersed in each page. It means that the orbit data about all the satellites can be obtained over 25 pages. Since the bit rate is 50 bps, 30 seconds are needed for reading one page. That is, for reading one master frame, 12.5 minutes are needed. It means that for obtaining the almanac, it is necessary to receive a GPS signal for as long a time as 12.5 minutes. Consider a portable GPS receiver. To receive the almanac data, the receiver is required to support its receiving antenna directed toward the sky for 12.5 minutes or more. This is unpractical. Further, it takes 12.5 minutes or longer to measure the position of the target object. This is a long time. To overcome the foregoing shortcomings, it has been considered that the digital signal processing circuits corresponding to the satellites (6 to 8) located within a signal-receivable field are prepared, the one-frame data is received from all the satellites located within the receivable field, and the DGOP is derived by using the data in place of the almanac. To employ this arrangement, it is possible to reduce the receiving time to about 30 seconds. However, it is technically unpractical to provide such number of channels in the receiver in light of the hardware arrangement. If such channels may be provided, the resulting receiver becomes so costly. Hence, this arrangement is unpractically in light of the hardware and the cost. SUMMARY OF THE INVENTION It is therefore the object of the present invention to provide a GPS receiver which is capable of greatly reducing a time needed for measuring a position of a GPS receiver itself without having to increase the hardware amount. This object is solved with a receiver for a Global Positioning System according to claim 1. Preferred embodiments of this receiver are defined in the sub-claims. In operation, the GPS signal received from the GPS satellite is converted into an IF signal. The IF signal is quantized for converting the signal into a digital signal. The digital signal corresponding to any period of time is temporarily stored in the storing mechanism. Then, the digital signal read from the storing mechanism is processed on the time series for performing the measurement of the position. Such time-series processing is equivalent to preparation of a multi-channel digital signal processing unit and allows the data about each satellite to be collected, resulting in eliminating the necessity of the almanac. Hence, the GPS receiver of this invention does not need to consume a large period of time for collecting the almanac. This results in reducing the measuring time to a quite short time without having to increase the hardware amount. By making the reading speed of a digital signal from the storing mechanism faster than the writing speed of the digital signal to the storage mechanism, it is possible to reduce the measuring time to a far shorter time. Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing an arrangement of a GPS receiver according to an embodiment of the present invention; and Figs. 2 and 3 are flowcharts showing a flow of data process done in the embodiment show in Fig. 1, respectively. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the invention will be described in detail as referring to the drawings. Fig. 1 is a block diagram schematically showing an arrangement of a GPS receiver according to the embodiment of the present invention. As shown, The GPS receiver is provided with an antenna 60 through which a GPS signal is received from a GPS satellite, an amplifying and frequency converting circuit 61, an analog-to-digital (A/D) converter 62, a writing circuit 63, a memory 64, a reading circuit 65, a digital signal processing circuit 66, and a position deriving circuit 67. The amplifying and frequency converting circuit 61 has a preamplifying section for amplifying the received GPS signal and converts the amplified signal into an IF (intermediate frequency) signal. The IF signal is sent to the A/D converter 62 which serves to convert the IF signal into a digital signal. The writing circuit 63 serves to write the digital signal extending for a certain period of time, for example, 30 seconds (corresponding to about one page (one main frame)) into the memory 64 such as an IC memory. The reading circuit 65 serves to read the written digital signal from the memory 64. The digital signal processing circuit 66 operates to perform the reverse diffusion of the written digital signal by using a diffusion code for each GPS satellite located within a receivable field for the purpose of demodulating the data from the signal. The demodulated data contains ephemerides (data about a satellite orbit) of all the satellites and a pseudo-distance from each of all the satellite. The position deriving circuit 67 serves to derive the position of a GPS receiver itself from the derived pseudo-distance and the data about a satellite orbit. Though not shown, a local oscillating signal is applied to the circuit 61. The local oscillating signal is synthesized in an oscillator (not shown). Fig. 3 is a flowchart showing a flow of data process done in this embodiment. As shown in Fig. 2, at the step S1, the signal is received from each GPS satellite located within the signal-receivable field through the antenna 60. At the step S2, the RF (radio frequency) signal is converted down to the IF signal. The IF signal converted in the circuit 61 is a signal obtained by modulating a carrier in a BPSK (Bi-Phase Shift Keying) system. The GPS receiver employs the spectrum diffusion communication system in which the GPS signals sent from the GPS satellites are located on the band in an overlapping manner. The IF signal composed of all the overlapped signals is sampled and quantized in the A/D converter 62 for obtaining a digital signal at the step S3. Proceeding to the step S4, the reading circuit 63 serves to write a digital signal to the memory 64 for a certain period of time, for example, 30 seconds (corresponding to about one page). Next, the operation shown in Fig. 3 is executed. At a step S40, the reading circuit 65 serves to read the digital signal from the memory 64. Proceeding to a step S41, a diffusion code for one of all the GPS satellites located within a receivable field is multiplied by the read digital data. The reverse diffusion is carried out with respect to the multiplied result for sensing the synchronous phase of the satellite (capturing the satellite). At a step S42, it is determined whether or not the satellite is properly captured. In case of yes, at a step S43, the ephemeris (data about a satellite orbit) of this satellite is collected from the data for one page. Then, the process goes to a step S44. In case that the satellite is not properly captured, the process skips the step S43 and goes to a step S44. At the step S44, it is determined how many times the process from the steps S41 to S43 is carried out and whether or not the time K (for example, 8) is more than the number of all the GPS satellites. If the time K is not more than the number of all the GPS satellites, at a step S45, K is incremented by one and then the process at the steps S41 to S43 is repeated. After the ephemerides all the receivable GPS satellites are collected, the process goes to a step S46. At this step, the GDOP is derived from those ephemerides. On the derived GDOP, the four satellites are selected so that a measuring error may become minimum. Proceeding to a step S47, each pseudo-distance between a GPS receiver and each of the selected four satellites is derived on the basis of the data about each satellite. The processes done at the steps S41 to S47 are carried out in the digital signal processing circuit 66. At a step S48, the position deriving circuit 67 serves to operate the position of the GPS receiver based on the derived pseudo-distance and the ephemerides (data about satellite orbits). As set forth above, this embodiment is arranged to process the data on time series in a digital signal processing circuit for one channel. Hence, the function of the multi-channel digital processing circuit can be achieved by one-channel hardware. Further, the embodiment does not need collection of the almanac, resulting in reducing the time needed in measuring the position of the GPS receiver to a quite short time. Since the processing time consumed at the steps S41 to S43 of Fig. 11 is as short as 0.5 minute, assuming that the number of the receivable GPS satellites is 8, the processing time needs only four minutes (= 0.5 minute X 8). This processing time is a small fraction of the time of 12.5 minutes consumed in collecting the almanac in the known PGS system. The time needed in keeping the antenna directed toward the sky corresponds to one page of the GPS signal, that is, about 30 seconds. Hence, the time needed in supporting the GPS receiver is greatly reduced to a quite short time, resulting in greatly reducing the burden of the user. The data stored in the memory 64 such as an IC memory is allowed to be read from the memory 64 at any speed. Hence, by reading the data from the reading circuit 65 at an n-multiplied clock of the sampling frequency, the processing time consumed at the steps S41 to S43 is reduced to 1/n. For example, by reading the data at an 8-multiplied clock of the sampling frequency, the time consumed in processing the GPS signals sent from the eight satellites is as short as 0.5 minute X 8/8 = 0.5 minute (30 seconds). As the memory, a magnetic medium such as a magnetic tape or a magnetic disk, or a recording medium except the magnetic and semiconductor medium may be used. The digital signal processing circuit may provide two or more channels. Many widely different embodiments of the present invention may be constructed without departing from the scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.

Claims: A receiver for a Global Positioning System (GPS) comprising, receiving means (60) for receiving a plurality of GPS signals transmitted from a plurality of GPS satellites located within a receivable field and transmitting in a common frequency band in an overlapping manner, first converting means (61) for converting said GPS signals into an intermediate frequency (IF) signal, second converting means (62) for converting said IF signal into a digital signal, storing means (63, 64) for storing said digital signal at a predetermined interval of time, and reading means (65) for reading said digital signal stored in said storing means, characterized in that said GPS receiver further comprises, capturing means (66, S41) for capturing each of said plurality of satellites from said digital signal read by said reading means (65), collecting means (66, S43) for collecting the ephemeris data corresponding to each of said captured satellites, calculating means (66, S47), coupled to said collecting means, for calculating distance data and accuracy data from said ephemeris data, after collection of said collecting means, respectively, selecting means (66), for selecting a predetermined number of distance data and corresponding ephemeris data, in accordance with respective values of said accuracy data, and position deriving means (67, S48), for deriving a position data of said receiver, by using said distance data and corresponding ephemeris data selected by said selecting means, and in that said selecting means (66) selects said distance data and corresponding ephemeris data so as to minimize any error in said derived position data. A GPS receiver according to Claim 1, characterized in that said collecting means (66) omits collection of said ephemeris data of one of said GPS satellites whenever said detecting means (66) fails to detect said ephemeris data of said one of said GPS satellites. A GPS receiver according to Claim 1 or 2, characterized in that said storing means (63, 64) stores said digital signal at 30 seconds intervals. A GPS receiver according to any one of Claims 1 to 3, characterized in that said selecting means (66) selects four distance data and corresponding ephemeris data, in accordance with said values of said accuracy data preferentially selecting those data having the smallest error. A GPS receiver according to any one of Claims 1 to 4, characterized in that said reading means (65) reads said digital signal stored in said storing means (63, 64) at a rate which is an integer multiple of a storing rate at which said storing means stored said digital signal. A GPS receiver according to Claim 5, characterized in that said reading means (65) reads said digital signal stored in said storing means at a rate which is 8 times the storing rate at which said storing means stored said digital signal. A GPS receiver according to any one of Claims 1 to 6, characterized in that said storing means (63, 64) includes an integrated circuit memory. A GPS receiver according to any one of Claims 1 to 7, characterized in that said collecting means (66) collects said ephemeris data for each satellite within a period not exceeding 30 seconds and said position deriving means (67) completely derives said position within a period not exceeding n times 30 seconds, where n is the number of satellites from which ephemeris data were collected. A GPS receiver according to Claim 8, characterized in that said receiver uses time-series processing in a single-channel device to simulate a multiple-channel digital processing circuit, and derives a Geometrical Dilution of Precision (GDOP) coefficient within a period not exceeding n times 30 seconds.

Inventors: MIYAZAKI MASAO, OHTA TOMOZO, OKAMOTO NAOKI, OKAMOTO TAKESHI, MIYAZAKI, MASAO, OHTA, TOMOZO, OKAMOTO, NAOKI, OKAMOTO, TAKESHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1992
Main classification: G01S5
IPC codes: G07C5, G01S5