# 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
The sensor node uses an active crystal oscillator whose frequency is 16.384 MHz \(\pm\) 0.2 PPM. The maximum deviation \(\Delta f_{max}\) can be calculated as  
\[\Delta f_{max}=16.384\;MHz\times 0.2\;PPM=3.2768\;Hz \tag{2}\]  
therefore, the cumulative time error \(\Delta t_{max}\) of any two nodes in the time period T can be expressed as follows:  
\[\Delta t_{max}=\frac{2\times T\times\Delta f_{max}}{f_{0}} \tag{3}\]  
where \(f_{0}\) is the nominal oscillation frequency of the crystal. When the sensor node collects seismic data at the sampling rate of \(F\). The time required for each data point difference can be calculated as follows:  
\[\Delta\text{T}=\frac{1}{\Delta t_{max}\times F} \tag{4}\]  
There are five sampling frequency settings for the ADC of the acquisition node involved in this paper, which are 250, 500, 1000, 2000, and 4000 sps. When calculated at 4000 sps, the time required for a data point difference is 625 s. There may not be such a large error in the same time period because the calculations are based on the maximum error. However, the error of the data points may not be neglected as time accumulates. In order to ensure that the data points are strictly aligned at the same time, this paper proposes the T3I method to reduce the data point errors generated by all nodes. This method aligns the seismic data by GPS time shot in a data stream with a basic time stamp. The GPS time snapshot is aligned when ensuring that the data collected by multiple nodes does not differ from the time period of one data point. Then, the data is truncated according to the GPS time snapshot in the data stream after the seismic data is collected. Due to the high time accuracy of GPS, it ensures that the seismic data required for each segment is strictly aligned. The key to this method is to choose a reasonable time to record the GPS time snapshot. The node uses the NEO-M8N GPS receiver, which delivers high sensitivity and minimal acquisition times when maintaining low system power. This makes it perfectly suited for high-density seismic surveys with complex environments (some places with weak GPS signal) for long periods. Protocols and interfaces of the NEO-M8N mainly include the UBX with a binary format and NMEA 0183 with ASCII format [25]. In UBX format, GPS is defined by week number and seconds-of-week. This protocol uses 8-bit binary data, a low-overhead checksum algorithm, and a 2-stage message identifier (Class and Message ID), which transmit fewer bytes when getting valid information compared to the NMEA format. The UBX protocol is selected as the message output format to transmit GNSS data to GEIWSR-III. According to the launch strategy, the time synchronization operation designed in this paper is divided into periodic synchronization at a certain time interval, continuous-time synchronization, and forced time synchronization. Periodic synchronization at a certain time interval starts the time-synchronization operation at a specified sampling point interval. This requires setting a counter and takes a snapshot of the GPS time and the amount of ADC data points when the sampling points meet a certain number. Then we use this snapshot to align the data later. When encountering a failure of the current GPS timing, we use the continuous-time synchronization mode, which continuously monitors the GPS timing and performs the GPS time snapshot operation as soon as the timing is successful. Forced time synchronization uses the wireless monitoring system of the node for time synchronization. When the node receives the mandatory GPS time snapshot event instruction, it searches for timing information immediately. If the timing is successful, it executes the GPS time snapshot and returns a success message to the control center. If the timing fails, it returns a GPS time snapshot failure message and then enters continuous synchronization mode. GEIWSR-III generally only takes periodic synchronization at a certain time interval mode when in a fine GPS signal. However, setting GPS to continuous mode consumes a lot of energy.