Abstract:
A positioning signal reception device receives a positioning signal transmitted from a position information satellite, and includes: a signal reception portion that obtains a reception signal by receiving the positioning signal; an A/D conversion portion that generates a digital signal by converting the reception signal into a digital form; a filter portion that subjects the digital signal to frequency separation, a characteristic of the filter portion being changeable; a filter characteristic change portion that changes the characteristic of the filter portion according to an operating state of the positioning signal reception device and an external environmental condition of the positioning signal reception device; and a code synchronization establishment portion that establishes synchronization of spreading codes contained in the reception signal.

Description:
CROSS-REFERENCE TO THE RELATED APPLICATIONS 
     This is a continuation application of U.S. patent application Ser. No. 11/295,513 filed Dec. 7, 2005, now pending, which claims priority to Japanese Patent Application No. 2004-356421 filed on Dec. 9, 2004. The entire disclosure of U.S. patent application Ser. No. 11/295,513 and Japanese Patent Application No. 2004-356421 is hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a positioning signal reception device and a control method of the positioning signal reception device. 
     2. Related Art 
     A GPS (Global Positioning System) device has been used as a positioning system that determines a position using an artificial satellite. 
     A positioning signal transmitted from a GPS satellite is a radio frequency signal modulated by the CDMA (Code Division Multiple Access) scheme. After it is received by a receiver, it is demodulated and decoded by processes, such as frequency conversion, A/D conversion, frequency separation by a filter, and synchronization of PN (Pseudo Noise) codes, so that a navigation message is obtained. 
     Various proposals have been made for the characteristic of the filter that performs frequency separation, because it has influences on the performance as the receiver, for example, a time needed for synchronization of PN codes and the positioning accuracy. 
     JP-A-5-157826 (see FIG. 9) discloses a configuration to shorten the synchronization time before the synchronization of PN codes is established by broadening the bandwidth of the filter, and to enhance the stability of synchronization after the synchronization is established by narrowing the bandwidth. 
     Recently, the GPS receiver is incorporated into a general-purpose small terminal, such as a cellular phone, as one of its functions. Positioning is therefore performed in a place where the radio wave reception environment is poor, such as an indoor place, more often than in a case where it is incorporated into a traditional, single-purpose positioning machine. In addition, the GPS receiver is exposed to adverse effects from the other components forming the terminal that incorporates the GPS receiver, for example, noises from the CPU (Central Processing Unit) as the source of release, interference by radio waves transmitted from a cellular phone unit, etc. more frequently than before. 
     The configuration in the related art, however, is not able to set an appropriate filter characteristic in response to the external environment of the GPS receiver as described above, because the characteristic of the filter is changed in response to the operating state of the GPS receiver alone. This raises a problem that the positioning takes a longer time or the positioning accuracy becomes poor depending on the external environment. 
     SUMMARY 
     An advantage of some aspects of the invention is to provide a positioning signal reception device capable of performing highly accurate positioning in a short time by setting an appropriate filter characteristic in response to the external environment and a control method of the positioning signal reception device. 
     A first aspect of the invention provides a positioning signal reception device that receives a positioning signal transmitted from a position information satellite. The positioning signal reception device includes: a signal reception portion that obtains a reception signal by receiving the positioning signal; an A/D conversion portion that generates a digital signal by converting the reception signal into a digital form; a filter portion that subjects the digital signal to frequency separation, a phase characteristic and a frequency characteristic of the filter portion being changeable; a filter characteristic change portion that changes the characteristic of the filter portion according to an operating state of the positioning signal reception device or an external environmental condition of the positioning signal reception device; and a code synchronization establishment portion that establishes synchronization of spreading codes contained in the reception signal. 
     According to the first aspect of the invention, the positioning signal reception device has the signal reception portion, and is therefore able to obtain a reception signal by receiving a positioning signal transmitted from a position information satellite, for example, a GPS satellite. The reception signal is normally a radio frequency analog signal, and contains many noises besides the transmission signals. Also, the reception signal is a signal modulated by the spread spectrum-frequency hopping scheme, for example, the CDMA scheme, so that the reception signal can transmit signals from more than one position information satellite on a carrier wave at the same frequency. 
     The reception signal is subjected to sampling and quantization to be converted to a digital signal in the A/D conversion portion. 
     The digitalized reception signal is subjected to frequency separation in the filter portion, and as many signal components as possible are removed except for a desired positioning signal. The frequency characteristic as one of the characteristics of the filter portion, more specifically, a characteristic as to, of the input signals, in which frequency band a signal is allowed to pass and in which frequency band a signal is removed, can be changed by the filter characteristic change portion. 
     The reception signal, from which unwanted signal components have been removed by the filter portion, is sent to the code synchronization establishment portion to isolate a transmission signal from a particular position information satellite by synchronizing the spreading codes. For example, in a case where the CDMA scheme is used for transmission, the code synchronization establishment portion establishes the code synchronization by calculating the correlation from multiplication of the spreading codes generated inside the reception device and the reception signal. 
     A preferable characteristic for the filter portion depends on cases. This is because the preferable characteristic differs with the operating state of the positioning signal reception device, the external environmental condition of the positioning signal reception device, etc. For example, when the characteristic of the filter portion has been set on the assumption that the reception environment is satisfactory and the intensity of the reception signal is strong, the positioning may fail in an environment where the intensity of the reception signal is not sufficiently strong, such as an indoor place, because the positioning may take a long time, and in an extreme case, sufficient navigation data may not be acquired. 
     The positioning signal reception device of the invention, however, is configured to be able to change the characteristic of the filter portion. In addition, because the positioning signal reception device also has the filter characteristic change portion, it is possible to set the characteristic of the filter portion to best suit the operating state or the external environment of the positioning signal reception device. 
     The positioning signal reception device of the invention is thus able to perform highly accurate positioning in a short time. 
     It is preferable that the filter characteristic change portion changes the characteristic of the filter portion so that a bandwidth of a signal outputted from the filter portion becomes narrower during the time period from when the signal reception portion obtains the reception signal until the synchronization of the spreading codes is established and the synchronization comes to be maintained in a stable manner, and changes the characteristic of the filter portion so that the bandwidth of the signal outputted from the filter portion becomes broader after the synchronization of the spreading codes is established and the synchronization is maintained in a stable manner. 
     When configured in this manner, the filter characteristic change portion changes the characteristic of the filter portion on the basis of judgment as to the operating state of the positioning signal reception device, that is, whether the synchronization of the spreading codes is maintained in a stable manner. More specifically, the characteristic of the filter portion is changed to a narrow band before the code synchronization is stabilized, and the characteristic of the filter portion is changed to a broad band after the code synchronization is stabilized. 
     It is thus possible to reduce the noise components contained in the reception signal before the code synchronization is stabilized, which can in turn enhance the sensitivity of the positioning signal reception device. Also, after the code synchronization is stabilized, the phase characteristic of the reception signal is flattened over a broad frequency band, which can in turn enhance the positioning accuracy. 
     It is preferable that the filter characteristic change portion assumes the external environmental condition of the positioning signal reception device on the basis of signal intensity of the reception signal, and changes the characteristic of the filter portion according to the assumed environmental condition. 
     When configured in this manner, the filter characteristic change portion changes the characteristic of the filter portion according to the environmental condition, which is assumed on the basis of the signal intensity of the reception signal. 
     For the positioning signal reception device, it is difficult to directly know the environment under which the positioning signal reception device is present, for example, whether the current position is in an outdoor place with a good view in the environment under which the positioning signal is received satisfactorily, or the current position is in an indoor place in the environment under which the reception signal is not readily received. Hence, for example, an additional configuration, such as portion that checks the rough positioning result against the map database, is necessary. 
     However, it is possible to roughly assume that the positioning signal reception device is in an outdoor place under the environmental condition that makes it easy for waves to arrive when the intensity of the reception signal is strong, and that the positioning signal reception device is in an indoor place under the environmental condition that makes it difficult for radio waves to arrive when the intensity of the reception signal is weak. The positioning signal reception device of the invention changes the characteristic of the filter portion by utilizing these assumptions. It is thus possible to set an appropriate filter characteristic in response to the environmental condition without having to add a special configuration. 
     It is preferable that the filter characteristic change portion changes the characteristic of the filter portion so that a bandwidth of a signal outputted from the filter portion becomes narrower when the signal intensity of the reception signal is weak, and changes the characteristic of the filter portion so that the bandwidth of the signal outputted from the filter portion becomes broader when the signal intensity of the reception signal is strong. 
     When configured in this manner, the filter characteristic change portion changes the characteristic of the filter portion to a narrow band when the signal intensity of the reception signal is weak, and changes the characteristic of the filter portion to a broad band when the signal intensity is strong. 
     It is thus possible to enhance the sensitivity when the signal intensity of the reception signal is weak, that is, when it is assumed that the external environment is poor, and to enhance the positioning accuracy when the signal intensity of the reception signal is strong, that is, when it is assumed that the external environment is satisfactory. 
     It is preferable that the filter characteristic change portion changes the characteristic of the filter portion to remove a disturbing signal coming from an external of the reception device. 
     When configured in this manner, the filter characteristic change portion changes the characteristic of the filter portion to remove a disturbing signal. 
     The absence or presence of a disturbing signal from the external of the positioning signal reception device is one factor of the environmental condition. Examples of the disturbing signal include a signal transmitted for a purpose other than positioning for communications over cellular phones, noises induced from another device incorporated into a casing together with the positioning signal reception device, etc. 
     In a case where the occurrence of such a disturbing signal is anticipated, or where a disturbing signal is actually detected, the positioning signal reception device of the invention is able to remove the disturbing signal contained in the reception signal by changing the characteristic of the filter portion. It is thus possible to perform highly accurate positioning in a short time. 
     It is preferable that the filter characteristic change portion changes the characteristic of the filter portion to lower an output level of a signal outputted from the filter portion at an occurrence of the disturbing signal. 
     When configured in this manner, the filter characteristic change portion changes the characteristic of the filter portion to lower the output level of the signal at the occurrence of a disturbing signal. The output level of the signal is one of the characteristics of the filter portion. 
     It is thus possible to relieve the adverse effects on the positioning from burst noises, such as switching noises and a burst transmission of a cellular phone, that are induced irregularly for a short time. 
     It is preferable that the filter portion is formed of a digital filter, the digital filter has a register that stores a numerical value used to determine a characteristic of the digital filter, and the filter characteristic change portion changes the characteristic of the digital filter during operation of the positioning signal reception device by changing the numerical value stored in the register using software. 
     When configured in this manner, because the filter portion is formed of a digital filter having a register that stores a numerical value used to determine its characteristic, the filter characteristic change portion changes the characteristic of the digital filter by changing the numerical value stored in the register of the digital filter using software. The filter characteristic change portion can change the characteristic of the filter portion during operation of the positioning signal reception device. 
     Because the filter portion of the positioning signal reception device of the invention is formed of a digital filter, it is easy to achieve the filter characteristic change portion that changes the numerical value used to determine the characteristic of the filter portion stored in the register in the form of software. 
     The characteristic, such as the one to broaden/narrow a pass band of the digital filter or to remove a specific frequency in response to the use state of the positioning signal reception device, can be therefore changed flexibly. In addition, because the digital filter can change the characteristic faster than an analog filter, the filter characteristic change portion is able to follow an abrupt change in the external environment at a high speed. 
     A second aspect of the invention provides a method of controlling a positioning signal reception device that receives a positioning signal transmitted from a position information satellite. The control method includes: obtaining a reception signal by receiving the positioning signal in a reception signal reception portion; generating a digital signal by converting the reception signal in a digital form in an A/D conversion portion; subjecting the digital signal to frequency separation in a filter portion having a changeable characteristic; changing the characteristic of the filter portion according to one of an operating state of the positioning signal reception device and an external environmental condition of the positioning signal reception device in a filter characteristic change portion; and establishing synchronization of spreading codes contained in the reception signal in a code synchronization establishment portion. 
     As with the first aspect, the control method of the positioning signal reception device according to the second aspect of the invention enables highly accurate positioning to be performed in a short time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a view schematically showing a GPS receiver and the like according to one embodiment of the invention. 
         FIG. 2  is a view schematically showing the major configuration of the GPS receiver of  FIG. 1 . 
         FIG. 3  is a conceptual view showing the configuration of a digital filter. 
         FIG. 4  is a flowchart detailing an example of the operation of a filter characteristic change program. 
         FIG. 5  is a flowchart detailing another example of the operation of the filter characteristic change program. 
         FIG. 6  is a flowchart detailing an example of the operation of the GPS receiver of  FIG. 1 . 
         FIG. 7  is a view schematically showing a portable terminal and the like according to another embodiment of the invention. 
         FIG. 8  is a view schematically showing the major configuration of the portable terminal of  FIG. 7 . 
         FIG. 9  is a flowchart detailing an example of the operation of the filter characteristic change program. 
         FIG. 10  is a flowchart detailing another example of the operation of the filter characteristic change program. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, preferred embodiments of the invention will be described in detail with reference to the accompanying drawings. 
     Because the following embodiments are preferred examples of the invention, they have various technically preferable limitations. It should be appreciated, however, that the scope of the invention is not limited to these embodiments unless specified so in the descriptions below. 
     First Embodiment 
       FIG. 1  is a view schematically showing a GPS receiver  10  and the like according to one embodiment of the invention. 
     The GPS receiver  10  as one example of the positioning signal reception device has a GPS antenna  14  as one example of a signal reception portion. The GPS receiver  10  performs positioning by receiving positioning signals S 1  through S 4  transmitted, respectively, from GPS satellites  12   a  through  12   d  as one example of position information satellites. The positioning signals S 1  through S 4  are radio frequency analog signals at a carrier frequency of 1.575 GHz. Also, these signals are transmitted by the CDMA scheme. 
     The number of the GPS satellites the GPS receiver  10  used for the positioning is not limited to four as shown in  FIG. 1 , and it can be three or less or five or more. 
     Configuration of GPS Receiver  10   
       FIG. 2  is a view schematically showing the configuration of the GPS receiver  10 . Referring to the drawing, a solid line arrow indicates the flow of the processing on the reception signal, and a thick outline arrow indicates the flow of the control. 
     The positioning signals S 1  and so forth received at the GPS antenna  14  are sent to an RF (Radio Frequency) portion  16 . The reception signal sent to the RF portion  16  is processed to the form suitable for the processing performed in a BB (Base Band) portion  18 . The BB portion  18  demodulates and decodes the signal, and a navigation message is consequently obtained from the reception signal. 
     A down converter  20  down-converts the reception signal at the frequency of 1.575 GHz to the frequency of the order of several hundred KHz, for example. This is because it is difficult to perform subsequent processing when the signal stays at a radio frequency. When down-conversion takes place, another peak of an unwanted signal is generated at a symmetric position with respect to the frequency of the reception signal in addition to an aimed output frequency. Such an unwanted signal is removed by an RF portion filter  22 . 
     The RF portion filter  22  removes radio frequency components from a signal inputted from the down converter  20 . Because the purpose of this filtering is to enable the sampling to be performed correctly in a sampling portion  36 , frequency components larger than half the sampling frequency are removed. Because the positioning signals transmitted from the GPS satellites have bandwidths of about 2 MHz, the RF portion filter  22  is configured to have the bandwidth of about 4 MHz, for example. 
     An A/D converter  28  as one example of an A/D conversion portion digitalizes an analog signal that has passed through the RF portion filter  22  to enable digital processing in the BB portion  18 . 
     The sampling portion  36  samples the signal turned to the digital form in the A/D converter  28 . The sampling frequency is set to at least twice the bandwidth of an input signal, and more preferably, it is set to a higher frequency, for example, 2.5 times the bandwidth of the input signal. 
     The sampling portion  36  outputs the sampling result to a digital filter  38 . The signal at this stage, however, contains many noises besides the positioning signals. 
     The digital filter  38  as one example of a filter portion removes as many signals as possible from the input signal except for the GPS signals by subjecting the digital signal inputted from the sampling portion  36  to numerical processing. 
       FIG. 3  is a conceptual view showing the basic configuration of the digital filter  38 . The digital filter  38  has multiplication portions  38   a  with register and delay portions  38   b . It is therefore possible to change the characteristic of the digital filter  38 , such as the frequency characteristic and the output level, by varying numerical values set in the registers of the multiplication portions  38   a  with register. Also, the digital filter  38  of  FIG. 3  is the basic form, and a more complex characteristic may be achieved by connecting these filters in series in several stages or the characteristics may be switched for a satellite from which the signal is received by disposing these filters in parallel. 
     A PN code synchronization portion  40  as one example of a code synchronization establishment portion establishes the synchronization between the PN code contained in the signal extracted from the digital filter  38  and the PN code generated locally in the GPS receiver  10 . The phrase “the synchronization is established” means a state where the phase of the PN code at the transmitter end and the phase of the PN code generated inside the GPS receiver  10  coincide with each other, and a signal transmitted from a particular GPS satellite can be extracted from the reception signal. A state where the synchronization is established may be also expressed as “acquisition of the satellite succeeds”. 
     Even after the synchronization is established, there may be a case where the synchronization is lost due to fluctuations in the signal reception situations. A state where the synchronization is maintained over a certain time is therefore expressed as “synchronization is stabilized”. 
     A computation portion  42  obtains a navigation message by decoding the reception signal separated for each GPS satellite. The computation portion  42  next calculates the current position of the GPS receiver  10  according to the navigation message thus obtained. An output portion  44  outputs the current position thus calculated to a display device comprising, for example, a liquid crystal display, in the form of an image. 
     A control portion  30  comprises, for example, a micro processor, and controls operations of the entire GPS receiver  10 . It performs controls using control signals sent to the respective components forming the GPS receiver  10 , such as the RF portion  16 , by running the OS (Operating System) and application programs pre-installed in a storage portion  32 . 
     The storage portion  32  as one example of a storage device comprises, for example, a RAM (Random Access Memory), and it stores programs run by the control portion  30 , such as a filter characteristic change program  34 , as well as the navigation data obtained in the computation portion  34 , the current position of the GPS receiver  10  calculated by the computation portion  34 , etc. 
     The storage portion  32  is not limited to a RAM, and it can be any type of memory capable of storing information for the control portion  30  to read out, such as a ROM (Read Only Memory) and a flash memory. 
     The filter characteristic change program  34  as one example of a filter characteristic change portion is a program that sets the characteristic of the digital filter  38  by changing the numerical values set in the multiplication portions  38   a  with register in the digital filter  38  (see  FIG. 3 ). An example of the operation of this program will now be described. 
       FIG. 4  is a flowchart detailing an example of the operation of the filter characteristic change program  34  when the filter characteristic is changed in response to the operating state of the GPS receiver  10 . 
     According to the filter characteristic change program  34 , whether the synchronization of the PN codes has been established is judged first (ST 10 ). This judgment is made, for example, by referring to the flag periodically. When the synchronization of the PN codes has not been established, the characteristic of the digital filter  38  is set to a narrow band (judgment result in ST 10  is NO, ST 13 ). 
     When the synchronization of the PN codes has been established, whether the synchronization is stabilized is judged (ST 11 ). This judgment is made, for example, as follows. That is, in a case where the synchronization has been established over a period longer than a pre-set threshold, the synchronization is judged as having been stabilized. 
     When the synchronization has not been stabilized, the characteristic of the digital filter  38  is set to a narrow band judgment result in ST 11  is NO, ST 13 ). The purpose to set the characteristic of the digital filter  38  to a narrow band until the code synchronization is stabilized in this manner is to enhance the sensitivity of the GPS receiver  10  by reducing noise components contained in the signal. 
     When the synchronization of the PN codes has been established and is now stabilized, the characteristic of the digital filter  38  is set to a broad band (judgment result in ST 11  is YES, ST 12 ). This is to enhance the positioning accuracy by flattening the phase characteristic over wide frequencies of the reception signal. 
     Due to the operation of the filter characteristic change program  34  as described above, the GPS receiver  10  changes the characteristic of the digital filter  38  before and after the synchronization of the PN codes is stabilized, and is thus able to achieve both fast code synchronization (acquisition of satellite) and highly accurate positioning. 
       FIG. 5  is a flowchart detailing an example of the operation of the filter characteristic change program  34  when the filter characteristic is changed in response to the intensity of the reception signal. 
     Initially, whether the intensity of the reception signal is equal to or higher than a certain threshold is judged (ST 20 ). This judgment is made by allowing the filter characteristic change program  34  to directly obtain the intensity of the reception signal. Also, in a case where the GPS receiver  10  has more than one positioning mode, for example, an outdoor mode and an indoor mode, and the operation modes are switched in response to the intensity of the reception signal, the judgment on the intensity of the reception signal may be made indirectly by allowing the filter characteristic change program  34  to obtain the positioning mode that is currently used. 
     When the intensity of the reception signal is higher than the pre-set threshold, it is assumed that the external environment of the GPS receiver  10  is satisfactory. The characteristic of the digital filter  38  is therefore set to a broad band to enhance the positioning accuracy (judgment result in ST 20  is YES, ST 21 ). When the intensity of the reception signal is lower than the threshold, it is assumed that the external environment of the GPS receiver  10  is poor. The characteristic of the digital filter  38  is therefore set to a narrow band to enhance the sensitivity (judgment result in ST 20  is NO, ST 22 ). The operation as above is repeated until the calculation of the position is completed (ST 23 ). 
     Example of Major Operation of GPS Receiver  10   
     The GPS receiver  10  of this embodiment is configured as described above, and an example of the operation will be described in the following. 
       FIG. 6  is a flowchart schematically showing an example of the major operation of the GPS receiver  10  of this embodiment. 
     A reception signal is obtained by receiving the positioning signals S 1  and so forth transmitted from the GPS satellites  12   a  and so forth at the GPS antenna  14  (ST 1 , an example of receiving the signal). The reception signal is a radio frequency analog signal at the frequency of 1.575 GHz. 
     The reception signal is sent to the down converter  20 , and is down-converted to a frequency of the order of several hundred KHz, for example (ST 2 ). 
     The down-converted reception signal is then sent to the RF portion filter  22 , and frequency components larger than half the sampling frequency are cut so as not to cause a trouble in the subsequent sampling step (ST 3 ). 
     The reception signal is then digitalized and sampled by the A/D converter  28  and the sampling portion  36 , respectively (ST 4 , an example of subjecting the reception signal to the A/D conversion). 
     Subsequently, filtering by the digital filter  38  is preformed to remove noises from the reception signal (ST 5 , an example of filtering the digital signal). Because the synchronization of the PN codes has not been established at this stage, the characteristic of the digital filter  38  is set to a narrow band to enhance the sensitivity. 
     Because transmission signals from more than one GPS satellite are mixed in the digital signal obtained in the steps from ST 1  through ST 5 , the digital signal is sent to the PN code synchronization portion  40  for the synchronization of the PN codes (ST 6 , an example of establishing the code synchronization). When the code synchronization is established, a signal from a particular GPS satellite can be isolated. The signal at this stage, however, still contains noises. 
     After the synchronization of the PN codes is established, the characteristic of the digital filter  38  is changed to a broad band to enhance the positioning accuracy (ST 7 , an example of changing the filter characteristic). This change can be made by allowing the control portion  30  to run the filter characteristic change program  34  pre-installed in the storage portion  32 . 
     Filtering is performed again after the filter characteristic is changed to remove noises from the reception signal (ST 8 ). 
     The navigation data can be thus acquired from the reception signal that now contains the transmission signal from a single GPS satellite alone. The current position of the GPS receiver  10  is then calculated using the navigation data thus acquired (ST 9 ). 
     As has been described, the GPS receiver  10  is able to set the characteristic of the digital filter  38  appropriately in response to the external environment of the GPS receiver  10  and the operating state of the GPS receiver  10  by running the filter characteristic change program  34 . Also, because the filter characteristic can be set by changing the numerical values stored in the registers of the digital filter  38  by running the filter characteristic change program  34 , it can be set at quite a high speed. 
     The GPS receiver  10  is therefore able to perform highly accurate positioning in a short time. 
     Second Embodiment 
       FIG. 7  is a view schematically showing a portable terminal  100  and the like according to another embodiment of the invention. 
     The portable terminal  100  as one example of the positioning signal reception device has a GPS antenna  14  and a GPS portion  110 , and is able to perform the positioning by receiving positioning signals S 1  through S 4  transmitted from GPS satellites  12   a  through  12   d , respectively. 
     The portable terminal  100  also has a cellular phone portion  120 , and thereby enables communications with another cellular phone terminal or the like by establishing a connection to a base station  130  by transmitting/receiving radio waves at a cellar phone antenna  122 . Because the signal intensity of the radio wave transmitted/received to/from the cellular phone portion  120  is far stronger than the signal intensity of the positioning signals S 1  and so forth from the GPS satellites  12   a  and so forth received at the GPS reception portion  110 , the radio wave transmitted from the cellular phone portion  120  may possibly become a disturbing signal to the GPS reception portion  110 . 
     Configuration of Portable Terminal  100   
       FIG. 8  is a view schematically showing the configuration of the portable terminal  100 . Referring to the drawing, a solid line arrow indicates the flow of the processing on the reception signal, and a thick outline arrow indicates the flow of the control. The configurations of the GPS antenna  14  and the GPS portion  110  are the same as their counterparts in the GPS receiver  10  of  FIG. 2 ; however, the filter characteristic change program  34  operates in a different manner. 
       FIG. 9  is a flowchart detailing an example of the operation of the filter characteristic change program  34  when the filter characteristic is changed in response to the absence or presence of a disturbing signal as one example of the external environmental condition. 
     Initially, whether a disturbing signal is contained in the reception signal is judged (ST 30 ). 
     When the disturbing signal is detected, the filter characteristic change program  34  changes the characteristic of the digital filter  38  by changing the numerical values stored in the multiplication portions  38   a  with register in a direction to cut the detected disturbing signal (judgment result in ST 30  is YES, ST 31 ). 
     When no disturbing signal is detected, the characteristic of the digital filter  38  is set to the normal characteristic by changing the numerical values of the multiplication portions  38   a  with register (judgment result in ST 30  is NO, ST 32 ). 
     The filter characteristic change program  34  is kept run until the GPS portion  110  ends the positioning (ST 33 ). 
     Because the portable terminal  100  has the cellular phone portion  120 , it is anticipated that a signal transmitted from the cellular phone portion  120  is received at the GPS antenna  14 , and this signal becomes a disturbing signal to the GPS portion  110 . 
     When the occurrence and the frequency of the disturbing signal are known, by setting the digital filter to the characteristic such that selectively attenuates the frequency of the disturbing signal, it is possible to reduce the influences of the disturbing signal without impairing the reception performance of the GPS. 
       FIG. 10  is a flowchart detailing another example of the operation of the filter characteristic change program  34  when the filter characteristic is changed in response to the absence or presence of a disturbing signal as one example of the external environmental condition. 
     Initially, whether a disturbing signal is contained in the reception signal is judged (ST 40 ). 
     When the disturbing signal is detected, the filter characteristic change program  34  changes the characteristic of the digital filter  38  by changing the numerical values stored in the multiplication portions  38   a  with register in a direction to lower the level of an output signal from the digital filter  38  (judgment result in ST 40  is YES, ST 41 ). 
     When no disturbing signal is detected, the characteristic of the digital filter  38  is set to the normal characteristic by changing the numerical values in the multiplication portions  38   a  with register (judgment result in ST 40  is NO, ST 42 ). 
     The filter characteristic change program  34  is kept run until the GPS portion  110  ends the positioning (ST 43 ). 
     The output level of a signal outputted from the filter is lowered in the presence of the disturbing signal also in the related art. However, because a level adjustment circuit operating at a frequency of the order of several tens Hz is formed using an analog circuit, it is impossible to follow an abrupt change in environment, such as interference caused by switching noises or a burst transmission of a cellular phone. 
     The portable terminal  100  of the invention, however, adjusts the output level using the digital filter  38 , which is a digital circuit and is thereby able to operate at a high speed. It is thus possible to maintain the characteristic of the digital filter  38  to be optimal by following an abrupt change in environment. 
     As has been described, according to the portable terminal  100 , the filter characteristic change program  34  detects the occurrence of the disturbing signal, which allows the frequency characteristic or the output level of the digital filter  38  to be set appropriately. 
     It is thus possible to perform highly accurate positioning in a short time even in the environment under which a disturbing signal occurs. 
     It should be appreciated that the invention is not limited to the embodiments above. Further, the respective embodiments above may be combined with each other.