Abstract: In the condition of weak signals,GPS receiver's baseband cannot lock on the modulated data in L1 signal to get accurate transmit time,but only get the fraction under 1 ms,or get 1 ms and the fraction under 1 ms but can not get 20 ms navigation-bit stream. On the other hand,while GPS module is embedded into mobile or other devices with time maintaining functional designed,GPS receiver starts working with a roughly initial time. With this initial time,we can estimate GPS signal transmit time's ambiguity (number of 1 ms or 20 ms) instantly as long as we get fraction time in GPS baseband processing and ephemeris from GPS signal or outside aiding information. At this time,the 5-parameter-model is used for positioning.This paper first addresses the algorithm in detail. Then we design two groups of starting test experiment using software receiver and log the middle-frequency data from RF output. Post-processing technology simulates the starting test with two groups of 950 trials and 600 trials,corresponding to 42 dBHz and 32 dBHz. The results show that even if in high power status,the N bit resolution with acceptable positions is successful at a rate of 90%. In middle-or low-power level (about 32 dBHz),the success rate is about 60%. These results indicate that strict quality control algorithm should be introduced into engineering development for the 5-parameter solution.