  A global navigation satellite system (GNSS) is a sensor that can acquire 3D
position and velocity in an earth-fixed coordinate system and is widely used
for outdoor position estimation of robots and vehicles. Various GNSS/inertial
measurement unit (IMU) integration methods have been proposed to improve the
accuracy and availability of GNSS positioning. However, all these methods
require the addition of a 3D attitude to the estimated state to fuse the IMU
data. In this study, we propose a new optimization-based positioning method for
combining GNSS and IMU that does not require attitude estimation. The proposed
method uses two types of constraints: one is a constraint between states using
only the magnitude of the 3D acceleration observed by an accelerometer, and the
other is a constraint on the angle between the velocity vectors using the
angular change measured by a gyroscope. The evaluation results with the
simulation data show that the proposed method maintains the position estimation
accuracy even when the IMU mounting position error increases and improves the
accuracy when the GNSS observations contain multipath errors or missing data.
The proposed method could improve positioning accuracy in experiments using
IMUs acquired in real environments.
