SPECIFICATION Title of the Invention SATELLITE BASED AUGMENTATION SYSTEM AND SATELLITE BASED AUGMENTATION METHOD Technical Field The present invention relates to a satellite based augmentation system and a Satellite based augmentation method that use only high-reliability GPS positioning signals artiong signals from GPS satellites. Background Art In a global positioning system such as the GPS (Global Positioning System) or the like, defining as a pseudorange a value obtained at a user's GPS receiver by multiplying the difference between a time of transmission of a signal such as a ranging signal (LlC/A), which is a GPS positioning signal from a GPS satellite, and a time of reception of the signal at a receiving station (monitoring point) by the speed of light, a position of the receiving station is measured on the basis of the pseudorange. Accuracy of the pseudorange is degraded when C/No (carrier to noise power density ratio) decreases because of power reduction of the ranging signal, a clock error, an error due to the ionosphere, that due to the troposphere, noise due to multiple paths or the like, trouble of the GPS receiver itself and the like. In cases accuracy of the pseudorange is degraded, a user incorrectly determines his/her own position. Accordingly, self-position determination based on a low-accuracy pseudorange results in dangerous information for a high-speed mobile object such as an airplane. In this respect, in usual global positioning systems, in order to remove noise due to multiple paths or the like from a measured pseudorange, smoothing processing is performed on the pseudorange. However, because such real time smoothing processing is simple one, its accuracy cannot be said to be high, and thus some portion of the noise may remain, and accordingly, there may be cases where the positioning accuracy is finally decreased. In this respect, Japanese Patent Application Laid-Open No. 2005-249653 discloses a pseudorange evaluation system comprising a data processing device which, when an abnormal value is detected in pseudorange values after real time smoothing processing used for positioning computation, determines whether or not a pseudorange value after post-process smoothing processing measured at the same monitoring time as that of the abnormal value is normal value. If it is found that the pseudorange was determined to be an absolute value in spite of its being a normal value, owing to the real time smoothing processing, this data processing device determines that the cause of the abnormal value detection is residual noise due to a limit of the real time smoothing processing. By this way, the cause of a decrease in pseudorange accuracy can be recognized. Alternatively, International Publication No. WO2006-132003 discloses a GPS receiver device comprising a position correction data calculation means which calculates the position of a base station on the basis of satellite signals and thus calculates data for position correction indicating a deviation of the calculated position from the absolute position of the station, and a base station receiving-intensity measurement means which measures receiving intensity of the satellite signals at the base station. This base station transmits to a mobile station the data for position correction calculated by the position correction data calculation means and the receiving intensity of the satellite signals measured by the base station receiving-intensity measurement means. The mobile station compares the receiving intensity of the satellite signals at the base station, measured by the base station receiving-intensity measurement means, with receiving intensity of the satellite signals at the mobile station (own station) measured by itself. By this way, recognition of a satellite signal influenced by multiple paths and correction of a measured position of a mobile station was made possible. Disclosure of invention While, as described above, determination of a self-position based on a low-accuracy pseudorange could rather lead to a dangerous situation, above-described Japanese Patent Application Laid-Open No. 2005-249653 and International Publication No. WO2006-132003 have a problem of no consideration about reliability of the ranging signal from a GPS satellite itself, which is the base of the pseudorange. Accordingly, even if various processings are performed, self-position determination cannot be performed correctly when the reliability of a base signal is low. In this respect, the primary objective of the present invention is to provide a satellite based augmentation system and a satellite based augmentation method, which reinforce GPS satellite navigation with respect to a high-speed mobile object such as an airplane, by performing position calculation using only GPS signals giving a highly accurate pseudorange, by the use of a device equipped with a signal monitoring program capable of properly determining the reliability of a received signal from a GPS satellite. A satellite based augmentation system according to the present invention is characterized by its comprising a threshold value calculation unit which calculates a monitoring threshold value for determining whether or not a value of carrier to noise power density ratio (C/No) of when a pseudorange is measured on the basis of a signal from a GPS satellite is proper, and a pseudorange determination unit which, by comparing a C/No value with the monitoring threshold value, determines whether or not a pseudorange is of proper accuracy. According to the present invention, a GPS receiver used by a user becomes able to properly determine the reliability of a signal from a GPS satellite, and thus accuracy and reliability of self-position determination using GPS satellites are improved. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a satellite based augmentation system according to a first exemplary embodiment of the present invention. Fig. 2 is a flow chart showing a calculation procedure of a monitoring threshold value according to the first exemplary embodiment. Fig. 3 is a frequency distribution table of C/No values divided by seven bins according to the first exemplary embodiment. Fig. 4 is a frequency distribution table of C/No values divided by an increased number of bins according to the first exemplary embodiment. Fig. 5 is a diagram showing a frequency distribution of measurement errors in C/No values according to the first exemplary embodiment. Description of Embodiment An exemplary embodiment of the present invention will be described below. Fig. 1 is a block diagram of a satellite based augmentation system 2 according to a first exemplary embodiment. The satellite based augmentation system 2 comprises a base station 10 which receives signals from GPS satellites 4 (4a to 4e), a signal monitoring device 30 which measures ranging information on the basis of a signal from the base station 10 and outputs a result of determining the properness of the ranging information as reliability information, and an information output device 20 which outputs the reliability information from the signal monitoring device 30 to a ranging information utilization apparatus 6 such as an airplane. Here, the ranging information includes a pseudorange, a C/No (carrier to noise power density ratio) value, a time of their measurement and the like. This pseudorange is a value obtained by multiplying the difference between a time of transmission of a signal such as a ranging signal (L1C/A) from a positioning satellite and a time of reception of the signal at a base station by the speed of light. The C/No value is a value of carrier to noise power density ratio at the time of acquiring the pseudorange. The base station 10 includes a receiving antenna 11 and a receiver 12 for receiving a signal from each of the GPS satellites 4. The information output device 20 includes a transmission antenna 21 which outputs the reliability information from the signal monitoring device 30 to the ranging information utilization apparatus 6 such as an airplane. The signal monitoring device 30 includes a measurement unit 31, a threshold value calculation unit 32, a pseudorange determination unit 33 and a reliability computation unit 34. Here, the measurement unit 31 may be installed in the receiver 12 at the base station 10. The measurement unit 31 measures a pseudorange value and a C/No value at predetermined measurement time intervals, on the basis of a ranging signal (L1C/A) included in a signal received by the base station 10. Then, the pseudorange and C/No value are stored, along with the time of their measurement, in a storage unit not illustrated in the diagram. A value of 0.5 seconds may be suggested as an example of the predetermined measurement time interval. The threshold value calculation unit 32 calculates a monitoring threshold value, which is used at a time of determining whether or not the pseudorange value measured by the measurement unit 31 is proper, on the basis of the C/No value measured by the measurement unit 31. On the basis of the monitoring threshold value, the pseudorange determination unit 33 determines whether or not the pseudorange value is proper, in terms of each of the GPS satellites 4. On the basis of the determination result by the pseudorange determination unit 33, the reliability computation unit 34 computes reliability information. This reliability information, along with the pseudorange value, is outputted as monitoring information to the information output device 20. The information output device 20 comprises the transmission antenna 21, such as a VHF data broadcasting equipment antenna, which outputs the monitoring information from the signal monitoring device 30 to the ranging information utilization apparatus 6 such as an airplane. Because, by this way, on the basis of the monitoring information, the ranging information utilization apparatus 6 such as an airplane becomes able to determine its self-position or the like according to only ones of the GPS satellites 4 which were determined to be proper, the self-position determination can become correct. Thus, the monitoring threshold value is an important value for determining the accuracy of a pseudorange. Fig. 2 is a flow chart showing a calculation procedure of the monitor threshold value. Monitoring information outputted by the satellite based augmentation system 2 is required to be of a higher accuracy than a predetermined one (hereafter, referred to as a minimum desired accuracy). Of course, the accuracy is preferred to be as high as possible, but in order to achieve higher accuracy, the use of expensive hardware and the like becomes necessary, and a load of various sorts of data processing becomes large. Therefore, an acceptable accuracy range is set here. Since monitoring information is created on the basis of a C/No value, a C/No value range corresponding to an accuracy range of monitoring information is defined. The upper limit of this C/No value range is expressed as a CN range upper-limit value CN_max, and the lower limit as a CN range lower-limit value CN_min. Further, the probability of a situation where the properness of a pseudorange cannot be determined owing to the measurement error in a corresponding C/No value being smaller than the monitoring threshold value (missed detection) is expressed as a missed-detection probability Pmd. Additionally, the probability of a situation where a pseudorange is determined to be improper, in spite of its satisfying an accuracy required of it, owing to the measurement error in a corresponding C/No value being smaller than the monitoring threshold value (false alarm) is expressed as a false alarm probability P_fa. These C/No value range, missed-detection probability P_md and false alarm probability P_fa are registered by a user or a system constructor in advance in the threshold value calculation unit 32. Under such settings, the measurement unit 31 measures pseudoranges and C/No values from ranging signals (L1C/A) at predetermined intervals over a predetermined time period (Step SI). The measured pseudoranges and C/No values, along with the time of their measurement, are stored in a storage unit not illustrated in the diagram. Hereafter, the above-described predetermined time period during which the measurements are carried out is expressed as a measurement time period, and the predetermined interval as a sampling time. A time of 0.1-1 second may be suggested as an example of the sampling time. For example, if the measurement time period is 3 months, the sampling time is 0.5 seconds, and the number of GPS satellites whose signals are received is eight, then the number of measurements is calculated as approximately 1.24 x 108 (= 3 [months]x 30 [days]x 24 [h]x 3600 [s] x (1/0.5 [s])x 8 [satellites]). In the next step, the threshold value calculation unit 32 creates a frequency distribution table (histogram) of the stored C/No values (Step S2). At that time, when the interval of a frequency distribution table is 1 [dB-Hz] and the value range the C/No values M may take is 20 [dB-Hz]-60 [dB-Hz], for example, it results that the frequency distribution table is divided into forty bins. That is, in the frequency distribution table, bin 1 becomes 20[dB-Hz]