A mobile device in a moving vehicle initializes a Global Navigation Satellite System (GNSS)-Inertial Navigation System (INS) system while the vehicle is in-motion with the orientation of the mobile device with respect to a global reference frame. The mobile device uses gyroscope measurements made while the vehicle is turning to determine a gravity vector. The gravity vector and accelerometer measurements may be used to determine a forward vector for the mobile device. A north vector is determined using the GNSS measurements. After the GNSS-INS system is calibrated, the mobile device may be positioned with respect to the vehicle. The orientation of the mobile device, prior to repositioning, may be compared to a current orientation, determined while the vehicle is in motion, in order to determine whether the GNSS-INS system should be re-initialized.