Bringing RTK to Cellular Terminals Using a LowCost Single-Frequency AGPS Receiver and Inertial Sensors

Today an ever-increasing number of handsets come equipped with a GPS receiver and some even with inertial sensors. Moreover, an even higher number of terminals are already capable of connecting to an add-on device with such capabilities. However, the full potential of these devices is not yet exploited. This paper introduces the mobile RTK (mRTK) solution, which can be included in the wireless standards to enable high-precision double-difference carrier phase positioning in handsets at no extra hardware cost. mRTK differs from the current OTF/RTK solutions in that it is a software-only solution using the hardware and wireless connections already existing in handsets. Moreover, the mRTK solution can utilize information from on-board inertial sensors. These are the key differentiating factors compared to the previous solutions. The paper shows that the sensors supplying information on baseline changes during the ambiguity initialization significantly assist the ambiguity resolution. A new communication protocol and messaging was defined in order to be able to exchange information between mRTK-capable handsets. The protocol includes reservations for additional GPS frequencies as well as for other Global Navigation Satellite Systems (GNSSs), such as Galileo. This protocol can be directly included in the wireless standards. Challenges in the current implementation include using only the L1 frequency for ambiguity resolution. Utilizing an L1-only receiver necessarily leads to penalties in the baseline accuracy due to inherent problems in the ambiguity resolution and validation. However, this paper shows that the baseline obtained is still better than the plain difference of positions. This paper shows that the mRTK solution significantly improves A-GPS performance. The mRTK solution also brings near-professional-quality positioning performance to the mass market. It would, therefore, be beneficial to include mRTK in wireless standards in order to expand A-GPS use cases in the short term and A-GNSS use cases in the long term.