Observed time difference (OTD) estimation for mobile positioning in the presence of BTS clock drift

The Enhanced Observed Time Difference (E-OTD) method is one of the emergency positioning technologies that has been standardized for use with GSM systems in the United States. In this paper, we apply the E-OTD approach to a (30 kHz) IS-136 TDMA system in order to study the effect of a linearly increasing BTS clock drift on RMS OTD estimation accuracy. We investigate performance of the OTD estimation algorithm as a function of the SNR, C - I and the magnitude of the BTS clock drift error. Our simulation results indicate that the RMS OTD estimation error can initially be reduced by increasing the observation interval, but that beyond a certain number of measurements (depending on the SNR and C - I), the BTS clock drift becomes the dominant error source and causes the RMS OTD error to rise.