# A High-Precision Energy-Efficient GPS Time-Sync Method for High-Density Seismic Surveys
## 2 Development of the GPS Time-Synchronization Method
### Time-Synchronization Method  
It can be seen from Figure 4 that the time synchronization system should be established during the data acquisition task. Therefore, the data collection workflow and hardware architecture of the system should be analyzed in detail. Based on the GEIWSR-III hardware units and its workflow, we designed a multi-node time synchronization system based on GPS and crystal oscillator. The overall architecture of the time-synchronization system is shown in Figure 5. The MCU with UCOS-III operating system is responsible for maintaining and managing the normal operation of the system. High precision crystal oscillator provides time service and a standard time reference for ADC and DAC. When the system is turned on, it performs a self-test. The MCU controls the DAC to complete the short-circuit noise test, harmonic distortion test, and geophone impedance test. When the self-test is completed, the node starts to complete the acquisition of seismic data. When the data reaches the size of the data buffer, the MCU writes the seismic data to a file and then stores it in a large-capacity SD card in the form of a file block. Apparently, the seismic data file is also transmitted in the file format through the built-in FTP server in the MCU when the seismic data is acquired via the Ethernet. Generally, GPS-disciplined ADC for high-precision time stamping is used to a data acquisition unit that synchronously samples multiple channels in a manner [22]. However, this method is usually applied to keep GPS turned on continuously. This method may not be applicable in long-term high-density seismic surveys. In this paper, a GPS file is used to record the exact time and the number of sampling points to time stamp the seismic data accurately. As shown in Figure 5, the seismic data file and the GPS time and data-point file are recorded in the data acquisition process at the same time. After the seismic data is collected, the server updates and marks the seismic data according to the GPS time and data-point file, and then performs multi-node seismic data files in the time-sharing interception, so as to achieve the purpose of time synchronization of multiple nodes. Figure 4: The simplified system working flowchart of GEIWR-III. The crystal oscillator provides counting reference for ADC during data acquisition. The data acquisition unit adopts a high-resolution 32-bit ADC named ADS1282. ADS1282 has a data-ready output pin DRDY, that is, MCU read the data on DRDY falling edge. We connect DRDY to the interrupt pin of the MCU. When the data is ready, the interrupt is generated, and then AD outputs the 32-bit data to MCU. The SYNC input pin and the SYNC command are the two sources for synchronization in the ADS1282. In actual application, we use the continuous-sync mode for synchronization with the SYNC command source. Figure 6 shows the timing for continuous-sync mode. In continuous-sync mode, there is a continuous clock (System Clock) applied to the SYNC pin with a frequency equal to the integer multiple of the output data rate. When SYNC is applied, the device can run freely only if the period matches an integer multiple of the output data rate; otherwise, the device will resynchronize. Thus, the accuracy of AD conversion synchronization depends on the accuracy of the system clock. The system clock of ADC and DAC mainly rely on the crystal oscillator. Considering the energy-efficient design of the instrument, it is necessary to take the travel time accuracy and power consumption into consideration when selecting a crystal oscillator. The crystal oscillator is mainly divided into a voltage-controlled crystal oscillator (VCXO), a temperature-compensated crystal oscillator (TCXO), and an oven-controlled crystal oscillator (OCXO) [23]. The parameters are shown in Table 1.  
Figure 5: Architecture of the time-synchronization system. Figure 6: The timing for continuous-sync and data output mode. As can be seen from Table 1, the main parameter items of various types of crystal oscillators are different.