The main objectives of our work are to implement an indoor positioning system using "Grin-Locs", a kind of pseudolite with dual transmitting antennas, and to evaluate its performance, using Software Defined Radio (SDR). "Grin-locs" architecture allows to perform measurements of the carrier phase without the need to synchronize the various pseudolites, and to obtain a typical positioning accuracy of a few decimeters. The first implementation uses GNSS signals, but the use of an SDR is likely to allow a change in the frequency bands used, in ISM bands for example. The paper describe the SDR choices and the way we implement both the transmitters and the receivers, also using SDR.