Low-cost software-defined GPS simulator with the capability of time synchronization

In this paper, a low-cost software-defined Global Positioning System (GPS) simulator for indoor positioning is proposed. The purpose of the proposed GPS simulator is to generate GPS signals so that the users can obtain the desired positions in an indoor environment with conventional GPS receivers. Since some commercial GPS receivers cannot properly provide position solutions if the calculated time based on the simulated signals is earlier than the last measured time based on the actual signals, the proposed GPS simulator generates GPS signals that do not cause this problem. In order to achieve this, an outdoor receiver was utilized to retrieve live ephemeris data, and the proposed GPS simulator generated baseband GPS signals that can provide a desired indoor position and time based on the ephemeris data. During the process of baseband GPS signal generation, time delay in the simulation process was compensated. Then, the baseband signals were modulated to GPS L1 frequency and were broadcast as radio frequency (RF) signals using a software-defined radio platform. The proposed GPS simulator was implemented using a u-blox M8T as an outdoor receiver and a USRP N210 as a software-defined radio platform. The performance of the proposed simulator was validated with another u-blox receiver as a user. The u-blox receiver successfully acquired the desired position with the simulated RF signals that were transmitted via a coaxial cable to the receiver.