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 . fig1 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 fig1 , and it can be three or less or five or more . fig2 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 . fig3 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 fig3 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 fig3 ). an example of the operation of this program will now be described . fig4 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 . fig5 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 ). the gps receiver 10 of this embodiment is configured as described above , and an example of the operation will be described in the following . fig6 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 . fig7 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 . fig8 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 fig2 ; however , the filter characteristic change program 34 operates in a different manner . fig9 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 . fig1 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 .