Study and Implement on GPS Time Synchronization of the Transient Electromagnetic Detection System
|Course||Electronic Science and Technology|
|Keywords||Transient electromagnetic detection GPS time synchronization Frequency drift of crystal oscillator Adaptive IIR filter Least squaremethod|
The transient electromagnetic detection system is a kind of geophysicalinstrument developed according to the principles of time dom ain electromagneticmethod, with the penetrating high resistance ability, random interference effect,probing depth, high resolution, rich information and other advantages, which has beenwidely used in mineral exploration, oil exploration, engineering geo logicalexploration fields.To improve the precision of time synchronization of transientelectromagnetic sounding system separation structure transmitter and receiver is thekey to the detection performance.The cable or wireless synchronization scheme isadopted by the traditional transient electromagnetic detection system whose timesynchronization precision is low, and not suitable for the complex circumstance.TheGPS time synchronization scheme which adopts the1PPS signal to correct theoscillator frequency is more and more wildly used, but is complex now, and when theGPS is unlocked, the time synchronization precision will be lower.In allusion to the problems mentioned above, a scheme of high precision GPStime synchronization which adopts medium precision active crystal oscillator andGPS, is designed based on embedded microprocessor STM32F407VGT6tosynchronize the transmitter and receiver of transient electromagnetic detection systemin this paper.The1PPS signal is used as a reference to obtained the oscillator s actualworking frequency, and then the time synchronization signal is output bySTM32F407VGT6timer counter s Pulse width modulation mode in GPS scheme,compared with the traditional time synchronization scheme which after using1PPSobtaining the oscillator s actual working frequency and then adjust the voltagecontrolled crystal oscillator s voltage to modify the oscillator output frequency toobtain synchronization signal, the GPS time synchronization in this paper improve theaccuracy of the synchronization signal and the synchronization signal adjustment ismore flexible, more universal, more suitable for the transient electromagneticdetection system which works in the field using.The theory of the GPS time synchronization scheme included the model ofcrystal frequency drift, the principle of GPS timing and1PPS signal error is firstlyanalyzed in this paper. Based on the theory, the GPS time synchronization principle isresearched, and then the hardware and software design of GPS time synchronization of transient electromagnetic detection system is completed, finally GPS timesynchronization algorithm is implemented. T he random errors in1PPS signal isfiltered out by sliding mean filter, the crystal frequency drift rate of temperature andaging factors is separated by adaptive IIR filter, the crystal frequency drift agingtemperature trend is recognized and calculated by least squares method, the crystalactual operating frequency is calculated by signal synthesis.Finally, the GPS timesynchronization accuracy performance is analyzed through the experiment, and theexperiment results show that the time synchronization precision of transientelectromagnetic detection system can reach1us in GPS locked and unlocked status,and the synchronous signal accuracy and stability have been significantly improved,which has good application prospects.