A Time Alignment Method for Multiple Sensing Systems with GNSS Timing and IMUs with Frame-Sync Input

Abstract

In this paper, we propose a method to align time stamps for sensed data among multiple data logging devices. In the method, we use the timing signals from the global navigation satellite systems (GNSS) and inertial measurement units (IMU) that have a frame-synchronous-input. The frame synchronous input is an external input, and its value will be attached to sensed data as the timing label bit. In most of the data logging method, the timestamp is generated and attached to the data by its data logging device. However, the timestamps contain a clock jitter about 100 ppm or more due to the internal clock inferior quality in a low-cost devices. It is difficult to sample data continuously time interval being fixed and to attach timestamps with high reliability. Therefore, we estimate the actual sampling timing of sensed data measured by IMUs using the timing signals from a GNSS module. Our key idea is to utilize the fact that actual sensors' internal oscillator is slightly out of alignment with the precise clock, which can be provided by the navigation satellites. We applied the proposed method to 1 kHz sampling data obtained from a commercially available sensing module of several dollars equipped with a quartz crystal. We have confirmed that the proposed method improves the precision of such sampling timing from around 350 microseconds to 150 microseconds.