  A pose-graph-based optimization technique is widely used to estimate robot
poses using various sensor measurements from devices such as laser scanners and
cameras. The global navigation satellite system (GNSS) has recently been used
to estimate the absolute 3D position of outdoor mobile robots. However, since
the accuracy of GNSS single-point positioning is only a few meters, the GNSS is
not used for the loop closure of a pose graph. The main purpose of this study
is to generate a loop closure of a pose graph using a time-relative real-time
kinematic GNSS (TR-RTK-GNSS) technique. The proposed TR-RTK-GNSS technique uses
time-differential carrier phase positioning, which is based on
carrier-phase-based differential GNSS with a single GNSS receiver. Unlike a
conventional RTK-GNSS, we can directly compute the robot's relative position
using only a stand-alone GNSS receiver. The initial pose graph is generated
from the accumulated velocity computed from GNSS Doppler measurements. To
reduce the accumulated error of velocity, we use the TR-RTK-GNSS technique for
the loop closure in the graph-based optimization framework. The kinematic
positioning tests were performed using an unmanned aerial vehicle to confirm
the effectiveness of the proposed technique. From the tests, we can estimate
the vehicle's trajectory with approximately 3 cm accuracy using only a
stand-alone GNSS receiver.
