As for the baseline measurement using geodetic GPS receiver, this paper puts forth the effective measurement range of a low-priced GPS system receiving Ll frequency only. Compared with the data of baseline eliminated the ionospheric effects with L1/L2 frequencies, the results with Ll frequency approximate to those with dual frequency within the range of 15 km (the baseline discrepancy of the two methods is less than 2 cm). The results with Ll frequency are better within the range of 18 km. Based upon the analysis of data, we find that the convergence limit of integer ambiguity-applicable to double difference fixed solution- is about 18 km and the ionospheric effects have a great influence on the results when the baseline are longer than 20 km. This study also presents a standard which makes it possible to select the favorable data from the results of the GPS relative distance measurements.