GPS-synchronization of a seismic accelerometer and a ground-based interferometric radar for Structural Health Monitoring

This paper presents a seismic accelerometer with GPS-synchronization based on the pulse per second (PPS) signal provided by the GNSS satellite network. This feature is essential for a Structural Health Monitoring (SHM) network that includes different sensors as for example accelerometers, cameras and a ground-based radar. Aligning data recorded from different sources is important to provide an accurate sensor fusion and to correlate the acquired data to time-based events (for example the passage of a truck on a bridge). The PPS signal is widely used in applications of timing synchronization because of its high accuracy and long-term stability. A common time base between devices allows the internal time of the devices to be resynchronized every second, avoiding drift. In this work we developed and tested a hardware with its own firmware able to provide synchronized data recorded by different acquisition systems, by imprinting the data with a reliable timestamp based on the universal time (UTC) provided by GNSS. The timestamp is updated every second by the PPS signal, while the fractional seconds are derived from the internal clock of the measurement device. Before saving the data, the time information is combined with data. To achieve this, a firmware architecture has been designed to acquire UTC. It incorporates a PPS counter that increments with each PPS signal and a timer that tracks fractional seconds until the next PPS, after which it resets. The developed hardware/firmware has been tested with a simultaneous acquisition with two device GPS-synchronized: a seismic accelerometer in MEMS technology and a ground-based interferometric radar. Both systems are equipped with high precision GNSS receiver u-blox F9, that provide the UTC data and the PPS signal. About the architecture of the accelerometer system, an acquisition system based on ΣΔ ADC is designed, which samples the data on the three axes simultaneously, while the firmware architecture is implemented on a Nucleo STM32 microcontroller, which receives the data from the ADC and associates it with the corresponding time as previously described. About the radar, the firmware architecture is implemented on a programmable FPGA inside the sensor. In order to validate the synchronization system, the accelerometer and the ground-based radar have been operated simultaneously to detect the oscillations of a moving target (a metallic swinging rod).