A satellite based augmentation system comprising: a threshold value calculation unit which calculates a monitor threshold value for determining whether or not a value of carrier to noise power density ratio (C/No) of a received signal from a GPS satellite at a time when a pseudorange is measured on the basis of the signal from the GPS satellite is proper; and a pseudorange determination unit which determines whether or not the pseudorange is of proper accuracy, by comparing the C/No value with the monitor threshold value, characterised in that the threshold value calculation unit calculates a frequency distribution of the C/No values with respect to a plurality of measured pseudoranges, and calculates the monitor threshold value, for which a probability P_CN_min of the C/No value is equal to the sum of a missed-detection probability P_md_cno and a false alarm probability P_fa_cno, wherein P_CN_min = frequency of C / No values being within a range CN_min ± 3 σ_cno / total frequency, the CN_min is a C/No range lower-limit value, the σ_cno is the estimated standard deviation of measurement errors in the C/No values, the missed-detection probability P_md_cno is a probability of situations where the properness of the pseudorange cannot be determined owing to the standard deviation of measurement errors in the C/No values in the frequency distribution being smaller than the monitor threshold value; and the false alarm probability P_fa_cno is a probability of situations where the pseudorange is determined to be improper, in spite of its satisfying an accuracy required of the pseudorange, owing to the standard deviation of measurement errors in the C/No values in the frequency distribution being smaller than the monitor threshold value. a threshold value calculation unit which calculates a monitor threshold value for determining whether or not a value of carrier to noise power density ratio (C/No) of a received signal from a GPS satellite at a time when a pseudorange is measured on the basis of the signal from the GPS satellite is proper; and a pseudorange determination unit which determines whether or not the pseudorange is of proper accuracy, by comparing the C/No value with the monitor threshold value, characterised in that the threshold value calculation unit calculates a frequency distribution of the C/No values with respect to a plurality of measured pseudoranges, and calculates the monitor threshold value, for which a probability P_CN_min of the C/No value is equal to the sum of a missed-detection probability P_md_cno and a false alarm probability P_fa_cno, wherein P_CN_min = frequency of C / No values being within a range CN_min ± 3 σ_cno / total frequency, the CN_min is a C/No range lower-limit value, the σ_cno is the estimated standard deviation of measurement errors in the C/No values, the missed-detection probability P_md_cno is a probability of situations where the properness of the pseudorange cannot be determined owing to the standard deviation of measurement errors in the C/No values in the frequency distribution being smaller than the monitor threshold value; and the false alarm probability P_fa_cno is a probability of situations where the pseudorange is determined to be improper, in spite of its satisfying an accuracy required of the pseudorange, owing to the standard deviation of measurement errors in the C/No values in the frequency distribution being smaller than the monitor threshold value. the CN_min is a C/No range lower-limit value, the σ_cno is the estimated standard deviation of measurement errors in the C/No values, the missed-detection probability P_md_cno is a probability of situations where the properness of the pseudorange cannot be determined owing to the standard deviation of measurement errors in the C/No values in the frequency distribution being smaller than the monitor threshold value; and the false alarm probability P_fa_cno is a probability of situations where the pseudorange is determined to be improper, in spite of its satisfying an accuracy required of the pseudorange, owing to the standard deviation of measurement errors in the C/No values in the frequency distribution being smaller than the monitor threshold value.