|M.Sc Student||Ladin Geinitz Yoel|
|Subject||GNSS Code Differential Positioning in Smartphones|
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Gilad Even-Tzur|
|Full Thesis text|
The location of a Global Navigation Satellite System (GNSS) receiver is determined using radio signals from satellites in space. An error in the derived position occurs due to errors in the received signals. Some of those errors are mitigated, by means of hardware and/or software and algorithmic techniques. High performance GNSS receivers, e.g. geodetic receivers, yield an accuracy of a few centimeters. Those are dual-frequency receivers, equipped with high quality antennas, and can perform phase and code measurements. In addition, differential GNSS (DGNSS) techniques are used, what provides a more accurate solution than a standalone absolute positioning, since measurement errors are reduced in the differentiation process. On the other end of the spectrum, single-frequency, code-only receivers, can achieve an accuracy of roughly 5-10 meters. Such is the case with smartphones, which are embedded with a low cost (~1 USD) GNSS chip. Several characteristics dictate limitations on positioning performance:
• Single frequency - limits the capability of ionospheric delay corrections (with dual-frequency, the ionospheric delay Is almost eliminated);
• Code only measurements - dictates lower accuracy than achieved with phase measurements;
• No access to raw data - limits the ability to implement ordinary differential GNSS corrections;
• Low grade GNSS Antenna - dictates poor multipath suppression.
This work tries to overcome some of these limitations. It aims to enhance the phone’s positioning accuracy, by performing DGNSS corrections. Those corrections are based on GNSS measurements from nearby permanent base stations and are received in the form of Radio Technical Commission for Maritime services (RTCM) messages, via cellular internet.
The legacy correction method performs corrections in the range-domain, i.e. the pseudorange from each satellite is corrected, based on data from the RTCM messages. But performing range-domain corrections is not feasible in smartphones, since the raw data are not accessible. Instead, this work proposes a method for translating the available pseudorange corrections to a position-domain correction-shift, applied to the phone’s position. Thus, the positioning accuracy is improved by means of additional software alone, without changing the phone’s hardware. Three experiments were performed to evaluate the performance of the proposed method. The first two experiments validate the algorithm with measurements from a geodetic GNSS receiver, and present satisfying results. The 2D-position mean error in these experiments is reduced from about 1 m, to around 0.1-0.3 m (a reduction rate of 70%-90%). The third experiment implements the proposed algorithm, performing corrections to the phone’s position. An accuracy improvement is achieved, although not as significant as expected. The 2D-position mean error is reduced from 1.54 m to 0.96 m (a reduction rate of 38%). The largest obstacle for accuracy improvement is the lack of knowledge of the exact positioning algorithm, performed inside the phone’s GNSS chip. This limits the possibility to adjust the correction algorithm to the phone’s solution in an optimal way. Nevertheless, future research suggestions are outlined, that might further improve the accuracy of GNSS positioning in smartphones. Higher accuracy with a low-cost GNSS chip is relevant not only in smartphones, but also in other applications, such as drones, autonomous vehicles, etc.