A POSITIONING METHOD OF BDS RECEIVER UNDER WEAK SIGNAL CIRCUMSTANCES BASED ON COMPRESSED FRACTIONAL-STEP

Original scientific paper This article puts forward the compression fractional-step method which applies to the positioning of BDS receivers under weak signal circumstances. It then analyses the features of BDS constellation and improves the original algorithm based on various satellites’ dynamic conditions of mixed constellation. With respect to the ambiguity of the decimal of milliseconds arising from the clock error of receivers in the process of calculation, this article advances a processing mode of extending search dimension and a fractional-step method of employing the implied elevation information of receivers which is then substituted into the equation as the observed quantity to reduce the calculation amount of ambiguity. It concludes that in the most adverse circumstances this method can reduce nearly half of the calculation amount of the whole second fuzzy search of signals’ emission time. This article also brings forward the compression calculation method to compress the range between the approximate position and clock error based on the feature that the dynamic of non-MEO satellites among BDS satellites is small. Taking non-MEO satellites as calculation, satellites can reduce the accuracy requirements of the local clock of receivers. In theory, calculation through using pure GEO satellites can broaden the requirement from 187,5 s to 1500 s. In the light of simulation results, it turns out that when the elevation information is known and precise, the first step of fractional-step calculation is capable of meeting the requirement of positioning. Otherwise, the second step of calculation can obtain the accurate positioning and achieve the rapid positioning under weak signal circumstances.