Semi-airborne transient electromagnetic time synchronization method and system

Abstract

The application belongs to the technical field of semi-airborne transient electromagnetic exploration. A semi-airborne transient electromagnetic time synchronization method and system are provided. Altitude data and satellite original second pulse signals are obtained. A feedforward compensation voltage component is generated based on a height-temperature-frequency physical mapping model. A phase difference measurement result between a local signal and the original second pulse is obtained, and a feedback control quantity is generated. The feedforward compensation and the feedback control quantity are fused to output an adjusted digital control word. The phase residual of a transmitter and a receiver relative to the original second pulse is quantified based on the adjusted clock frequency. The phase residual is embedded as synchronization metadata in an electromagnetic sampling signal data frame. The ground-air node sampling data is reconstructed on a time axis and differentially aligned. The application suppresses dynamic frequency deviation through feedforward compensation, and improves the time synchronization accuracy of the semi-airborne transient electromagnetic system in a complex flight environment by combining phase residual quantification reconstruction.

Description

Technical Field

[0001] This invention relates to the field of semi-airborne transient electromagnetic exploration technology, specifically to a semi-airborne transient electromagnetic time synchronization method and system. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] Semi-airborne transient electromagnetic exploration employs a ground-based high-power transmission and airborne platform reception operation mode, and its data inversion quality is highly dependent on the time synchronization accuracy between ground and air nodes. Currently, mainstream technologies utilize global navigation satellite systems to discipline cryogenic crystal oscillators, combined with phase-locked loops to maintain local clock stability, providing a time reference for data acquisition. With the widespread application of mobile platforms such as UAVs in geophysical exploration, the operating environment is becoming increasingly complex, especially under flight conditions involving frequent takeoffs and landings and drastic temperature changes. Maintaining high-precision synchronization performance in dynamic environments and ensuring the accuracy of electromagnetic signal acquisition has become a key technological focus in this field.

[0004] In the highly dynamic flight environment of semi-aerial operations, the drastic altitude changes caused by the take-off and landing of UAVs lead to rapid fluctuations in the external ambient temperature. This thermal shock acts on the temperature-controlled crystal oscillator through physical heat conduction. However, the thermal inertia of the crystal oscillator's temperature control system causes a lag in adjustment, making it difficult to compensate for the frequency fluctuations caused by instantaneous temperature drift in real time. This decrease in frequency stability directly results in a phase offset between the transmitter's physical actions and the receiver's sampling time relative to the satellite's absolute reference. Existing technologies mostly focus on the physical accuracy of the second pulse signal itself, lacking precise quantification and recording of the phase residual across the entire link from signal triggering to the completion of the physical action. This makes it difficult for subsequent data processing to effectively eliminate the relative time deviation between ground and air nodes, affecting further improvements in synchronization accuracy. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a semi-airborne transient electromagnetic time synchronization method and system. It constructs a frequency feedforward compensation system based on altitude prediction, solves the problem of environmental thermal shock caused by frequent take-off and landing of UAVs in semi-airborne geophysical exploration operations, realizes hardware-level real-time quantitative monitoring of action signal phase residuals, establishes a data calibration path based on "synchronization metadata piling" and algebraic difference, and significantly improves the inversion resolution of early weak signals in semi-airborne transient electromagnetic systems.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a semi-aeronautical transient electromagnetic time synchronization method.

[0007] A semi-airborne transient electromagnetic time synchronization method includes the following process: Acquire altitude data of the target work area and raw second pulse signals output by the global navigation satellite system, calculate the altitude change rate based on the altitude data, and generate feedforward compensation voltage components by combining the altitude-temperature-frequency physical mapping model; The phase difference measurement result between the local second pulse and the original second pulse is obtained, and a feedback control quantity is generated based on the phase difference measurement result. The feedforward compensation voltage component and the feedback control quantity are linearly weighted and fused to output the adjusted digital control word. Based on the clock frequency corresponding to the adjusted digital control word, the phase residuals of the physical action time of the transmitter and the sampling time of the receiver relative to the original second pulse are quantized. The phase residual is embedded as synchronization metadata into the data frame of the electromagnetic sampling signal, and the sampling data of the ground-to-air node is reconstructed and differentially aligned based on the synchronization metadata.

[0008] In one implementation of the first aspect of the present invention, the altitude change rate is calculated based on altitude data, and a feedforward compensation voltage component is generated by combining an altitude-temperature-frequency physical mapping model, including: The absolute altitude is extracted from the Global Navigation Satellite System (GNSS) messages and fused with the high-frequency relative altitude change rate output by the barometric pressure sensor to obtain the filtered altitude. Calculate the difference between the current filtered height and the previous filtered height to obtain the height change rate; Based on the product relationship between the altitude change rate and the feedforward control gain, the predicted frequency drift caused by altitude change is calculated. The expected frequency drift prediction is converted into a feedforward compensation voltage component and then smoothed by a first-order low-pass filter.

[0009] As a further limitation of the first aspect of the present invention, the process of establishing the height-temperature-frequency physical mapping model includes: Based on the vertical lapse rate of tropospheric temperature under standard atmospheric conditions, a linear relationship model between external ambient temperature and altitude is constructed. Based on the temperature sensitivity coefficient of a specific crystal oscillator, a linear relationship model between frequency deviation and temperature change is constructed. By coupling the linear relationship model between external ambient temperature and altitude and the linear relationship model between frequency deviation and temperature change, the transfer function from the rate of change of altitude to frequency drift is obtained.

[0010] In one implementation of the first aspect of the present invention, obtaining the phase difference measurement result between the local second pulse and the original second pulse, and generating a feedback control quantity based on the phase difference measurement result, includes: Receive the phase difference count value output by the high-resolution time-to-digital converter. The phase difference count value represents the time interval between the rising edge of the local second pulse generated by the local temperature-controlled crystal oscillator frequency division and the original second pulse. A Kalman filter is used to estimate the state of the phase difference count values, and the trends of frequency offset and aging rate are extracted. The filtered phase deviation is input into a gain-adaptive proportional-integral-derivative (PID) regulator to calculate the feedback control quantity.

[0011] In one implementation of the first aspect of the present invention, the feedforward compensation voltage component and the feedback control quantity are linearly weighted and fused, including: In timekeeping logic mode, when the global navigation satellite system signal is lost, the system stops receiving feedback control inputs and maintains output solely by relying on the aging slope and temperature drift compensation characteristics learned before the loss of lock. In the acquisition logic mode, when the global navigation satellite system signal is locked, the feedforward compensation voltage component is superimposed with the feedback control quantity; the final regulated digital control word is output to the digital-to-analog converter to drive the voltage control terminal of the temperature-controlled crystal oscillator.

[0012] In one implementation of the first aspect of the present invention, the phase residual between the physical action time of the transmitter and the sampling time of the receiver relative to the original second pulse includes: At the transmitting end, the logic flip signal output by the current zero-point detection circuit is acquired to capture the absolute moment when the current is completely turned off. At the receiving end, the sampling start pulse of the analog-to-digital converter is acquired; The absolute interval between the physical action times of the transmitter and receiver and the original second pulse is recorded to obtain the picosecond-level phase residual.

[0013] In one implementation of the first aspect of the present invention, the phase residual is embedded as synchronization metadata into the data frame of the electromagnetic sampling signal, including: Construct a structured data frame that includes a frame header, global navigation satellite system absolute time, latitude and longitude information, altitude information, phase residual, and electromagnetic field sampling voltage sequence; A synchronization frame is inserted at the beginning of each second to record the precise offset of the starting sampling point of that second relative to the original second pulse; using a double buffer mechanism and direct memory access technology, the synchronization data frame containing the phase residual is written to the storage medium.

[0014] In one implementation of the first aspect of the present invention, time-axis reconstruction and differential alignment of the sampled data of the ground-to-air nodes based on synchronization metadata includes: The Global Navigation Satellite System second tags in the storage media of the ground transmitter and the air receiver are analyzed to achieve initial pairing of data packets; Retrieve the transmitter phase residual and receiver phase residual from the metadata area, calculate the algebraic difference between the two, and obtain the relative phase deviation between nodes; The receiver time axis is shifted by relative phase deviation to force the sampling zero point to align with the moment of transmission shutdown.

[0015] In one implementation of the first aspect of the present invention, the time-axis reconstruction and differential alignment of the sampled data of the ground-to-air nodes based on synchronization metadata further includes: Obtain the fixed system deviations generated by the hardware link, which include signal transmission line delay, opto-isolator response time, and analog-to-digital converter digital filter group delay. The offset of the final calibration model is obtained by superimposing the relative phase deviation with the fixed system deviation. Based on the final calibration model, the sampling points at both the ground and air ends are projected onto the global standard time axis. Secondly, the present invention provides a semi-aeronautical transient electromagnetic time synchronization system.

[0016] A semi-airborne transient electromagnetic time synchronization system, characterized in that it comprises: The altitude feedforward unit is configured to: acquire altitude data of the target operating area and the raw second pulse signal output by the global navigation satellite system, calculate the altitude change rate based on the altitude data, and generate feedforward compensation voltage components in combination with the altitude-temperature-frequency physical mapping model; The phase feedback unit is configured to: acquire the phase difference measurement result between the local second pulse and the original second pulse, generate a feedback control quantity based on the phase difference measurement result, linearly weight and fuse the feedforward compensation voltage component and the feedback control quantity, and output the adjusted digital control word. The residual quantization unit is configured to quantize the phase residual between the physical action time of the transmitter and the sampling time of the receiver relative to the original second pulse, based on the clock frequency corresponding to the adjusted digital control word. The data reconstruction unit is configured to: embed the phase residual as synchronization metadata into the data frame of the electromagnetic sampling signal, and perform time axis reconstruction and differential alignment on the sampling data of the ground-to-air node based on the synchronization metadata.

[0017] Compared with the prior art, the beneficial effects of the present invention are: This invention effectively addresses temperature shocks caused by rapid altitude changes during semi-aerial operations by introducing an altitude-feedforward compensation mechanism. Utilizing an altitude-temperature-frequency physical mapping model to predict crystal oscillator frequency drift trends, it outputs a compensation voltage component before thermal hysteresis occurs in the temperature control system, significantly reducing frequency fluctuations in dynamic environments. By combining linear weighted fusion of phase difference feedback control and feedforward compensation, this invention improves the short-term stability of the local clock under severe temperature variations while ensuring long-term clock accuracy, reducing phase offsets between physical action moments and received sampling moments relative to the satellite reference. This invention quantifies the phase residual across the entire transmitter and receiver link and embeds it as synchronization metadata into the sampled data frame. This design allows subsequent data processing to reconstruct the time axis and differentially align the ground-to-air node sampled data based on precisely recorded phase deviations, effectively eliminating time errors caused by hardware link delay differences and dynamic frequency offset accumulation. Through the dual effects of feedforward suppression of frequency drift and residual quantization reconstruction of the time axis, the invention improves the time synchronization accuracy and data inversion reliability of the semi-aerial transient electromagnetic system in complex dynamic flight environments.

[0018] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0019] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0020] Figure 1 A flowchart illustrating a semi-aeronautical transient electromagnetic time synchronization method provided as an exemplary embodiment of the present invention; Figure 2 A schematic diagram of a semi-airborne transient electromagnetic time synchronization system provided as an exemplary embodiment of the present invention. Detailed Implementation

[0021] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0022] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0023] Semi-airborne transient electromagnetic exploration employs a ground-based transmission and airborne reception mode. Sub-microsecond time alignment between ground and air nodes is the core foundation for ensuring inversion accuracy. Currently, the industry commonly uses GPS phase-locked crystal oscillators (GPSDOs) to obtain synchronization pulses. However, in complex dynamic operating environments, system performance is still limited by the following factors: First, thermal conduction hysteresis caused by environmental temperature differences. Semi-airborne operations involve frequent altitude changes, resulting in drastic fluctuations in external temperature, causing continuous thermal shock to the OCXO. Due to the physical thermal conduction inertia of the crystal oscillator temperature control system, there is significant hysteresis in the internal monitoring and adjustment loop. This hysteresis feedback cannot offset the frequency fluctuations caused by instantaneous temperature drift in real time, directly leading to a decrease in frequency stability under high-dynamic flight conditions. Second, the quantitative recording of physical action phase deviation is lacking. Affected by hardware circuit response, ADC sampling timing, and interrupt processing delays, there is a system deviation and random jitter of hundreds of nanoseconds between the transmitter action and the receiver sampling time relative to the GPS absolute reference. Current solutions mostly focus on the physical accuracy of the second pulse signal itself, lacking real-time monitoring of the phase residual throughout the entire process from signal triggering to the completion of physical action. Because the synchronization parameters are not included in the sampled metadata for storage, it is difficult to eliminate the relative offset between nodes in the later processing, making it impossible to achieve nanosecond-level data reconstruction, which restricts the inversion resolution of early weak signals.

[0024] In view of the problems existing in the current solution, this implementation proposes a semi-aeronautical transient electromagnetic time synchronization method, which involves building a temperature-controlled crystal oscillator (OCXO) frequency control unit. As the core hardware foundation of the high-precision GPSDO system, the success or failure of the OCXO frequency control unit design directly determines the stability and accuracy of the system's output frequency. In this implementation, a hardware architecture using a high-quality temperature-controlled crystal oscillator (OCXO) as the central frequency source is employed. The OCXO, through its internal precision temperature control circuit, places the crystal in a constant hot bath, greatly reducing the impact of ambient temperature fluctuations on the physical properties of the quartz crystal slice, thus providing short-time stability and ultra-low phase noise far exceeding that of ordinary compensated crystal oscillators (TCXOs). The OCXO selected in this invention must possess excellent voltage tuning linearity. Its frequency control terminal receives the voltage-controlled voltage signal from the main control unit to achieve fine-tuning of the frequency. To ensure the purity of the voltage-controlled signal, a high-performance RC low-pass filter network is designed at the voltage input terminal to suppress power supply ripple and high-frequency noise introduced by the control link, preventing them from being converted into phase noise in the output signal.

[0025] like Figure 1As shown, after the system powers on and completes environmental baseline adaptive calibration, the GNSS module acquires the raw PPS second pulse and altitude message data, which are then sent to the dual-loop control logic. Branch A feedback loop measures the phase deviation between the local PPS and GPS PPS through TDC, performs phase state estimation after Kalman filtering, and obtains the feedback gain through PID adjustment. Branch B feedforward loop analyzes the altitude change and calculates the altitude change rate. After heat conduction model matching filtering, it calculates the altitude compensation voltage control voltage feedforward component, fuses the feedback gain and feedforward component, updates the high-level DAC to adjust the frequency of the temperature-controlled crystal oscillator OCXO, and then captures the phase residual of the action shutdown / start relative to GPS PPS through TDC. The UTC, altitude, residual, and EM sampling data are encapsulated with metadata staking and stored in the SD card. In offline mode, the stored data undergoes ground-to-air system residual algebraic differential cancellation processing to complete the nanosecond-level alignment of the time axis and the phase coherence reconstruction of the induced signal, and finally outputs the calibrated semi-airborne transient electromagnetic secondary field waveform.

[0026] In this implementation, a high-resolution time-to-digital converter (TDC) is introduced as the core measurement unit of the feedback loop. The TDC chip directly receives the raw pulse-per-second (PPS) output from the GNSS module and the local pulse-per-second (Local PPS) generated by the local OCXO after frequency division. Its measurement essentially records the phase difference between the rising edges of the two pulses on the time axis. Assuming the local clock frequency is... The phase difference count value measured by TDC is The corresponding picosecond-level time deviation It can be expressed as follows: (1); In the formula, This is the internal reference clock of the TDC. This represents the integral nonlinearity error of TDC. This is determined by analyzing multiple consecutive periods. During observation, the STM32 main control unit can calculate the frequency drift rate of the local clock relative to the absolute time of GPS. This phase-accumulation-based measurement method avoids the insufficient resolution problem caused by quantization errors in low-frequency signal measurements using traditional frequency meters.

[0027] In this implementation, after acquiring the phase deviation, the main control unit needs to convert the digital compensation command into an analog voltage signal using a high-precision digital-to-analog converter (DAC). To achieve nanosecond-level frequency alignment, a monotonic DAC with 24 bits or higher resolution must be selected. Because the frequency adjustment range of the OCXO is extremely narrow (typically only a few ppm), even a small voltage jump in the DAC can cause significant frequency fluctuations. Therefore, establishing a precise mathematical mapping between the DAC control word and the frequency change is crucial. Frequency offset With DAC output voltage The relational model can be described as: (2); In the formula, The center frequency is 10MHz. This refers to the voltage-controlled sensitivity of the crystal oscillator. This is the center voltage-controlled voltage of the crystal oscillator at the standard frequency. Using this model, the STM32 can accurately calculate the amount of voltage adjustment required to compensate for the current phase drift.

[0028] Simple PID control is prone to loop oscillation or overshoot when dealing with random jitter in the raw GPS PPS signal. Therefore, the STM32 (i.e., the main control unit) first uses a Kalman filter to perform state estimation on the raw phase deviation sequence measured by the TDC. The state-space equation of the Kalman filter can be abstracted as synchronous tracking of frequency offset and aging rate: (3); In the formula, represent Phase deviation at time, represent Frequency deviation at time, The sampling period is represent Phase deviation at time, represent Frequency deviation at any given moment. Using this recursive algorithm, this invention can extract the true drift trend of the OCXO from the noisy raw GPS signal. Subsequently, the filtered stable deviation is fed into a gain-adaptive PID controller to calculate the final DAC update value. The proportional, integral, and derivative coefficients of the PID controller need to be dynamically adjusted for the highly dynamic environment of semi-aerial operations to ensure that this invention has rapid response capabilities during takeoff and climb, and extremely high steady-state accuracy during level flight.

[0029] OCXOs and DACs are analog-sensitive components; any power supply ripple will be directly modulated onto the output frequency through the control link. The design of this invention requires independent power supplies using multi-stage linear regulators (LDOs), and strict isolation between digital and analog grounds must be implemented in the PCB layout. The reference voltage source for the high-resolution DAC must be a dedicated reference chip with low temperature drift and low noise to ensure that the reference voltage itself does not generate perceptible temperature drift during drastic temperature changes in a semi-aviation environment. This hardware-level stability guarantee, combined with the aforementioned altitude prediction feedforward algorithm, constitutes a complete control framework for addressing complex environmental challenges.

[0030] When the satellite positioning module loses signal lock due to changes in flight attitude or obstruction, the main control unit must immediately stop closed-loop feedback and enter timekeeping mode. At this time, the invention no longer adjusts based on instantaneous TDC measurements, but instead utilizes the aging slope and temperature drift compensation characteristics learned over a period prior to the lock loss to continuously output the predicted voltage-controlled voltage. In this way, even in a signal-off state, the OCXO can still provide a highly reliable synchronous clock signal thanks to its internal constant-temperature environment and the inertia of the main control algorithm, supporting the continuity of semi-aerial reconnaissance missions.

[0031] To address the environmental thermal shock caused by frequent takeoffs and landings of UAVs in semi-aerial exploration, the core of this invention lies in breaking away from the traditional responsive feedback control mechanism and establishing a predictive feedforward compensation model based on altitude data. In actual operations, the UAV platform experiences drastic altitude changes during takeoff, climb, and landing. According to the dry adiabatic lapse rate in atmospheric physics, the ambient temperature drops rapidly with increasing altitude. Even with the use of an oven-controlled crystal oscillator (OCXO), the drastic external temperature difference still disrupts the thermal balance within the oven through heat conduction, causing minute thermal stress deformation of the quartz crystal. Due to the significant physical lag in heat conduction, by the time the temperature sensor inside the OCXO detects the temperature fluctuation and initiates PID control, the crystal oscillator's output frequency has often already shifted significantly. Therefore, using altitude information from GPS messages to intervene in control in advance is the key logic for maintaining nanosecond-level synchronization accuracy in highly dynamic environments.

[0032] A precise altitude-temperature-frequency physical mapping model is established as the theoretical basis for the algorithm's predictions. Under standard atmospheric conditions, the external ambient temperature... T ext With altitude H The linear relationship between them can be described as follows: (4); In the formula, Temperature at the ground reference point. This represents the vertical temperature lapse rate (approximately 0.0065℃ / m). The reference elevation is used. Based on this model, the algorithm can predict the evolution trend of the external environmental thermal field in real time. Due to the frequency deviation of the crystal oscillator... The temperature sensitivity coefficient of a specific crystal oscillator can be obtained through experimental calibration, as it is strongly correlated with temperature changes. By coupling these two parameters, the predicted frequency drift caused by height variations can be obtained. This method transforms the originally uncontrollable random thermal interference into deterministic compensation parameters based on location information.

[0033] The STM32 main control unit constructs a multi-source heterogeneous elevation data fusion architecture at the software level. Because a single GNSS signal experiences instantaneous jumps due to multipath interference during dynamic flight, and traditional moving average filtering introduces significant phase lag, compromising the real-time performance of feedforward compensation, this invention introduces a high-precision barometric pressure sensor at the hardware level. This sensor obtains the high-frequency, low-noise relative altitude change rate by measuring barometric pressure changes in real time. The invention utilizes a Kalman filter to fuse the absolute altitude extracted from GNSS messages (such as GPGGA) as a low-frequency reference with the high-frequency relative altitude change rate output from the barometer for estimation. The feedforward compensation voltage component is then calculated from this. It possesses both absolute position accuracy and achieves a transient response with zero phase lag, completely eliminating the prediction delay introduced by filtering single GNSS data. Let... The current filter height. Given the altitude at the previous moment, this invention calculates the rate of change of altitude. Based on this, the feedforward compensation voltage component is calculated. : (5); In the formula, The feedforward control gain, physically represented as the DAC control voltage correction per unit altitude change rate, is obtained through system identification during flight experiments. This compensation accurately counteracts the frequency shift tendency of the crystal oscillator under temperature variations. Furthermore, considering the significant physical heat conduction delay between the temperature bath receiving external environmental temperature changes and the internal wafer undergoing thermal stress deformation, this invention calculates the original feedforward component... Subsequently, instead of acting directly on the control loop, it is input into a first-order low-pass filter that matches the heat transfer time constant of the thermostat. The transfer function of this stage is modeled and matched with the thermal response characteristics of the OCXO, enabling smoothing of sudden increases in altitude change rate. This design effectively prevents frequency overcompensation caused by excessively rapid feedforward voltage intervention that exceeds the heat conduction rate during rapid ascent or descent of the UAV, ensuring precise synchronization between the electrical compensation and the physical thermal imbalance process in the time domain.

[0034] During the control loop integration phase, the feedforward compensation component needs to be linearly weighted and fused with the existing TDC phase feedback loop. Traditional GPSDO relies solely on the phase error measured by the TDC for closed-loop adjustment. This single-loop structure performs excellently in stable environments, but during the takeoff and climb phase of semi-aerial operations, the feedback loop often accumulates significant phase residuals due to thermal hysteresis. By introducing an altitude feedforward branch, this invention forms a composite control architecture of feedforward + feedback. The digital control word ultimately acts on the high-level DAC. The calculation model is as follows: (6); In the formula, The real-time phase deviation measured by TDC; the first three terms constitute a standard PID feedback control, and the last term... This is based on altitude prediction and feedforward intervention. The advantage of this architecture is that when the drone begins to ascend, the feedforward term instantly adjusts the DAC output, causing the OCXO frequency to compensate in advance, thereby greatly suppressing frequency drift caused by the temperature drop due to altitude increase.

[0035] Because semi-airborne transient electromagnetic exploration (SEM) covers a wide area and spans different seasons, the initial surface environment varies greatly. The algorithm implementation takes into account the initial surface temperature in different seasons and regions. To address the physical differences, this invention introduces a benchmark-adaptive calibration mechanism. Specifically, this invention records the initial altitude of the device when it is powered on (or when the drone is stationary before takeoff). The system automatically calibrates the initial reference point of the altitude-temperature-frequency mapping model in relation to ambient temperature. This adaptive initialization adjustment mechanism eliminates the static bias effect of the base ambient temperature on the feedforward gain coefficient under different operating conditions. This makes the feedforward compensation model effective not only in the theoretical altitude range but also adaptable to the actual operating conditions of various extreme geophysical exploration environments, from sea level to high-altitude mountainous areas and from extreme cold to extreme heat. This further enhances the system's robustness to the initial state of the external thermodynamic environment.

[0036] Stability verification of the feedforward compensation algorithm is crucial for ensuring reliable system operation. While altitude feedforward control can improve dynamic response speed, excessive feedforward gain may introduce low-frequency oscillations. This invention incorporates a feedforward contribution threshold limit in the software logic. When a decrease in altitude data confidence or an abnormally large altitude change rate (such as coordinate jumps caused by satellite signal loss) is detected, the system automatically reduces the feedforward weight, relying on the OCXO's own isothermal inertia and long-term feedback loop for maintenance. Through this hardware-software collaborative strategy, feedforward compensation not only solves the problem of heat conduction lag caused by drastic altitude changes in semi-aerial operations but also forms a technical closed loop with the backend TDC residual recording system, providing a solid clock physical foundation for nanosecond-level alignment of semi-aerial transient electromagnetic data.

[0037] The measurement and storage of phase residuals in action signals is a crucial link between real-time hardware control and subsequent data processing. Although high-precision GPSDO can provide a stable local frequency reference, in practical engineering applications, there is a significant time lag between the controller issuing commands and the actuator completing physical actions. For ground transmitters, the turn-off process of high-power semiconductor devices such as IGBTs is affected by drive circuit delays, charge storage effects, and loop inductance, often resulting in a shift of hundreds of nanoseconds between the moment the current truly returns to zero and the synchronization pulse. For air receivers, the ADC conversion start signal also generates dynamic phase jitter after passing through microcontroller interrupt responses and logic gates. If these minute deviations are not quantized and recorded, they will directly lead to misalignment of early channel signals on the time axis. Therefore, establishing a hardware-based real-time phase residual measurement system is of decisive significance for achieving nanosecond-level reconstruction between ground and air nodes.

[0038] This invention utilizes a high-performance time-to-digital converter (TDC-GP22) to construct a precise time calibration system. Its core logic lies in capturing the absolute position of a physical action relative to a globally unified time reference. At the hardware connection level, the TDC's start trigger (START) directly receives the raw second pulse signal (GPS PPS) output from the GNSS module, while the stop trigger (STOP) receives a feedback signal reflecting the actual moment the physical action occurred. For the transmitter, this feedback signal is acquired through a high-speed current sensor or zero-point detection circuit, capturing the logic flip at the moment of current cutoff. For the receiver, the ADC's sampling start pulse (CNV) is directly introduced. This connection method avoids the phase jitter of the local GPSDO itself, directly measuring the absolute time and the absolute interval between the physical action.

[0039] Phase residual generated during the measurement process It is the sum of systematic bias and random jitter, and its mathematical model can be expressed as: (7); In the formula, Represents the absolute moment in which a physical action occurs. The ideal moment representing the original second pulse, and This includes measurement noise caused by electromagnetic interference. The TDC chip converts this tiny time difference into a digital count value through its internal high-speed delay chain loop. Both the ground and air ends of this invention use high-quality oven-controlled crystal oscillators (OCXOs) as the reference standard for the TDC, and their short-term stability is typically better than 10. -11Within a single pulse-per-second (PPS) synchronization period, the cumulative phase deviation caused by crystal frequency drift is far less than 100 picoseconds, which is negligible second-order quantity relative to the nanosecond-level synchronization accuracy pursued in this invention. This ensures that the local TDC clocks at both the ground and air ends have extremely high phase coherence within a 1-second measurement window, providing a physical prerequisite for subsequent residual differential alignment.

[0040] The STM32 microcontroller is responsible for reading the TDC measurement results in real time via a high-speed SPI interface at the end of each second measurement cycle. To ensure data real-time performance and integrity, the microcontroller adopts an interrupt-driven mode. Once the TDC completes phase capture and pulls low the INTN interrupt pin, the STM32 immediately enters the service routine to read the 32-bit raw measurement register. Since a single measurement may be affected by occasional disturbances, the main control algorithm performs a rationality check after reading the data, eliminating jumps that clearly violate physical logic. To completely eliminate the impact of system blocking that may occur during cross-sector erasure and writing on the SD card on real-time measurements, the underlying software architecture of this invention employs a DMA and dual-buffer mechanism. After reading the nanosecond-level residual data and electromagnetic field sampling voltage, the main control unit first writes them to the main buffer in memory. When the main buffer is full, the system triggers an interrupt, and the hardware DMA controller seamlessly moves the data from the main buffer to the SD card in the background. Simultaneously, the CPU automatically and seamlessly redirects subsequently acquired data to the secondary buffer. This strict asynchronous read / write isolation design ensures that the highest priority of TDC interrupt reads is not compromised, avoiding data frame loss or timestamp misalignment caused by file system write time.

[0041] In this implementation, the data storage stage employs a "synchronous metadata staking" technique, deeply embedding phase residual information into the data frame format of the electromagnetic sampling signal. Traditional storage methods often only record the current GPS time (year, month, day, second), and this coarse-grained time stamp cannot support nanosecond-level inversion requirements. This invention designs a custom structured data frame, with each frame containing a frame header, GPS absolute time, latitude and longitude information, altitude prediction value, and, most importantly, the TDC phase residual. For high-frequency sampling data, the system typically inserts a synchronization frame at the beginning of each second, recording the precise offset of the starting sampling point of that second relative to the original PPS.

[0042] The data packet structure logic stored on the SD card can be abstracted into the following mapping relationship: (8); In the formula, Representing the A standardized synchronous data frame written to the storage medium; This represents the original numerical sequence of the induced secondary field voltage acquired and buffered by the ADC during this sampling period; This represents the absolute UTC time label obtained through GNSS message parsing. This represents the picosecond-level phase residual value quantized in real time by the TDC chip; This represents the altitude information and its rate of change component recorded synchronously with the data in this frame.

[0043] Through this encapsulation method, the SD card is no longer merely a recording medium for electrical signals, but becomes a physical ledger carrying complete spatiotemporal information. Each line of written data clearly indicates its accurate coordinates on the absolute time axis. To address the electromagnetic interference that may be generated during high-speed writing of the SD card, this invention incorporates impedance matching design for the SDIO interface and adds a metal shield to the TDC-sensitive analog reference terminal to prevent write pulse crosstalk from affecting the accuracy of phase measurements.

[0044] After the actual field operation is completed, the phase residual data stored on the SD card will play a crucial role in the background processing. Although the GPSDOs carried by the ground station and the air station are stable, their local PPS phase offset relative to the original GPS PPS is independent and random. By recording this absolute residual, the processing software can perform nanosecond-level time shift operations. Assuming the ground station measures the turn-off residual as... The air receiver measured the initial residual of the sampling as follows: The initial phase difference between the two sets of data is then the algebraic difference between them. This method cleverly utilizes the subtraction principle to cancel out all common errors, including local clock skew, microcontroller interrupt delay, and optocoupler response time. Since the OCXO's ultra-high stability for a short period has been ensured through hardware selection, the frequency asynchrony between ground and air nodes will not produce a cumulative offset sufficient to compromise alignment accuracy within a single second. This is achieved by using their independent absolute residuals... Performing algebraic subtraction essentially forces the two physically isolated local time coordinate systems to be projected back onto the globally unified time reference line defined by GPSPPS, thereby achieving true phase coherence reconstruction of ground-to-space nodes.

[0045] The calibrated effective time axis alignment model is shown below: (9); In the formula, This represents a fixed link delay that has been experimentally pre-calibrated, while This achieves ideal time coordinates after physical alignment. This post-processing-based calibration mechanism gives the system strong robustness, ensuring data consistency even in complex drone flight environments.

[0046] The closed-loop operation of the phase residual measurement and storage system represents a substantial shift in the synchronization mechanism from physical pulse alignment to digital residual compensation. In actual operations, this invention no longer simply pursues zero latency on the physical link, but instead utilizes TDC hardware to acquire picosecond-level measurements and associates them in real-time with the storage layer as key synchronization parameters, fundamentally solving the problem of uncontrolled offsets caused by spatial separation of ground and air nodes in semi-airborne geophysical exploration. This approach improves the resolution of transient electromagnetic early trace signal inversion while providing a standardized time annotation scheme for multi-node distributed acquisition tasks. Through this scheme, this invention can maintain extremely high temporal resolution in dynamic flight environments, thereby capturing weak geological anomaly signals that are easily masked by inaccurate ground-air synchronization. This design architecture, where the hardware layer is responsible for physical quantification measurement and the software layer is responsible for data posterior alignment, establishes a technical solution for distributed time-frequency alignment, providing a reliable data foundation for subsequent high-precision geophysical inversion.

[0047] In this implementation, timeline reconstruction and data calibration at the acquisition nodes are the final steps in achieving a ground-to-air synchronization system. Its core task is to align electromagnetic signals, distributed across different media, at the nanosecond level using software algorithms. Because the ground transmitter and air receiver are spatially separated, although both rely on GPSDO to maintain their local clocks, the original data exhibits a physical offset on the timeline due to circuit delays and local phase jitter. This approach utilizes phase residual metadata recorded on an SD card to offline calculate and remove asynchronous components, restoring the true timing between transmission and reception. This provides a foundation for high-precision early trace inversion, specifically including: (1) Preliminary reconstruction of the absolute time axis. The data processing software first parses the UTC second tags in the storage media at both the ground and air ends to achieve preliminary pairing of data packets. Then, it retrieves the TDC phase residual value in the metadata area. t res This corrects the nominal sampling time to a physical time with absolute reference significance. For any sampling point, its global absolute time coordinates... T abs The reconstruction model is as follows: (10); In the formula, The time is in whole seconds according to GPS. For serial number, The sampling period is defined by this mapping. This mapping projects sampling points from both the ground and air ends onto the same global standard time axis, resolving the issue of inconsistent time bases across devices.

[0048] (2) Differential alignment calibration between ground and air nodes. Based on absolute time mapping, the algorithm performs zero-point regression of ground and air data by calculating the relative phase deviation between the transmitter and receiver. Since there is a residual between the transmitter current turn-off and the original PPS... The receiver sampling start has a residual relative to the PPS in the same second. The algebraic difference between the two is the relative offset between the nodes. The calculation formula is: (11); The algorithm shifts the receiver's time axis to force its sampling zero point to align with the moment of transmission shutdown. This differential alignment mechanism uses the subtraction principle to cancel out the random phase jitter that exists in the GPSDO systems at both the ground and air ends, ensuring that the secondary field induction curve can be accurately calculated from the moment of transmission shutdown.

[0049] (3) Static link delay compensation. Based on dynamic residual correction, the fixed system deviation caused by the hardware link must be deducted. This deviation mainly originates from signal transmission line delay, opto-isolator response time, and ADC digital filter group delay, and exhibits constant characteristics under a specific hardware architecture. This invention obtains this constant through preliminary calibration and integrates it into the final calibration model: (12); in, This model outputs the precise absolute time after final correction. It integrates dynamic phase residuals and static physical offsets to ensure that the reconstructed time axis not only has internal logical consistency but also conforms to the absolute accuracy of geophysical theoretical models, completely solving the sampling zero-point drift problem caused by hardware response lag.

[0050] (4) Multi-period signal superposition and reliability verification. After completing the time axis reconstruction, this invention uses the calibrated high-precision time series to superimpose multi-period waveforms to improve the signal-to-noise ratio. Benefiting from nanosecond-level alignment accuracy, the superposition process avoids the waveform blurring effect caused by phase misalignment, enabling the sensing curve to maintain extremely high transient resolution. At the same time, the algorithm filters data quality by evaluating the stationarity of the residual sequence. If residual anomalies caused by GPS signal loss are detected, the time label of the time period is predicted and supplemented by combining the aforementioned altitude feedforward data and frequency aging model. This collaborative path of "measure first, then calibrate, then synthesize" achieves a performance breakthrough for the geophysical instrument in complex operating environments and forms a technical closed loop from clock source stabilization to data axis reconstruction.

[0051] In summary, this invention constructs a frequency feedforward compensation system based on altitude prediction, solving the problem of environmental thermal shock caused by frequent takeoffs and landings of UAVs in semi-airborne geophysical exploration operations. This invention extracts the altitude change rate from GNSS messages in real time and establishes a physical mapping of altitude-temperature-frequency using an atmospheric physics model, intervening in control before thermal hysteresis occurs in the isothermal crystal oscillator temperature control system. This shift from responsive feedback to predictive feedforward effectively offsets frequency fluctuations caused by drastic temperature changes in highly dynamic flight environments, ensuring the frequency stability of the system under complex operating conditions.

[0052] This invention achieves hardware-level real-time quantization monitoring of action signal phase residuals. It utilizes a high-resolution time-to-digital converter (TDC) to directly capture the physical deviation of the transmitter current-off instant or the receiver sampling start pulse relative to the GPS absolute second pulse, quantifying various system deviations including hardware circuit response, charge storage effects, and interrupt processing delays. This scheme avoids the influence of local crystal oscillator phase jitter, transforming the originally uncontrollable random time offset into a deterministic residual value with picosecond-level accuracy, providing crucial physical basis for subsequent nanosecond-level data reconstruction.

[0053] This invention establishes a data calibration path based on "synchronization metadata staking" and algebraic difference. By deeply embedding the measured phase residuals and altitude prediction information into the data frames of the electromagnetic sampling signal, complete encapsulation of spatiotemporal information is achieved. In post-processing, this invention utilizes the absolute phase residuals recorded at both the ground and air ends for algebraic subtraction, and cancels heterogeneous synchronization errors between ground and air nodes through a differential alignment mechanism. This technology enables phase coherence reconstruction of ground and air nodes under a unified global standard time axis, significantly improving the inversion resolution of early weak semi-airborne transient electromagnetic signals.

[0054] Figure 2 A semi-aircraft transient electromagnetic time synchronization system is shown, comprising: The altitude feedforward unit 201 is configured to: acquire altitude data of the target operating area and the raw second pulse signal output by the global navigation satellite system, calculate the altitude change rate based on the altitude data, and generate feedforward compensation voltage components in combination with the altitude-temperature-frequency physical mapping model; The phase feedback unit 202 is configured to: acquire the phase difference measurement result between the local second pulse and the original second pulse, generate a feedback control quantity based on the phase difference measurement result, linearly weight and fuse the feedforward compensation voltage component and the feedback control quantity, and output the adjusted digital control word. The residual quantization unit 203 is configured to quantize the phase residual between the physical action time of the transmitter and the sampling time of the receiver relative to the original second pulse, based on the clock frequency corresponding to the adjusted digital control word. The data reconstruction unit 204 is configured to: embed the phase residual as synchronization metadata into the data frame of the electromagnetic sampling signal, and perform time axis reconstruction and differential alignment on the sampling data of the ground-to-air node based on the synchronization metadata.

[0055] It is understood that the aforementioned units can be individually or entirely merged into one or more other units, or some of the units can be further divided into multiple functionally smaller units. This achieves the same operation without affecting the technical effects of the embodiments of the present invention. The aforementioned units are based on logical functional division. In practical applications, the function of one unit can be implemented by multiple units, or the function of multiple units can be implemented by one unit. In other embodiments of the present invention, the system may also include other units. In practical applications, these functions can also be implemented with the assistance of other units, and can be implemented collaboratively by multiple units.

[0056] According to another embodiment of the present invention, the system of this embodiment can be constructed by running a computer program (including program code) capable of performing the steps involved in the corresponding method of the present invention on a general-purpose computing device, such as a computer, which includes processing elements and storage elements such as a central processing unit (CPU), random access memory (RAM), and read-only memory (ROM). The computer program can be recorded on, for example, a computer-readable recording medium, loaded into the aforementioned computing device through the computer-readable recording medium, and run therein.

[0057] Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed in this invention can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can implement the described functions using different methods for each specific application, but such implementations should not be considered beyond the scope of this invention.

[0058] In the above embodiments, implementation can be achieved, in whole or in part, through software, hardware, firmware, or any combination thereof. When implemented in software, it can be implemented, in whole or in part, as a computer program product. A computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, all or part of the flow or function according to the embodiments of the present invention is generated. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions can be stored in or transmitted through a computer-readable storage medium. The computer instructions can be transmitted from one website, computer, server, or data center to another website, computer, server, or data center via wired (e.g., coaxial cable, fiber optic cable, digital cable) or wireless (e.g., infrared, wireless, microwave, etc.). The computer-readable storage medium can be any available medium that a computer can access or a data processing device such as a server or data center that integrates one or more available media. The available medium can be a magnetic medium (e.g., floppy disk, hard disk, magnetic tape), an optical medium (e.g., DVD), or a semiconductor medium (e.g., solid-state drive), etc.

[0059] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A semi-airborne transient electromagnetic time synchronization method, characterized in that, Includes the following processes: Acquire altitude data of the target work area and raw second pulse signals output by the global navigation satellite system, calculate the altitude change rate based on the altitude data, and generate feedforward compensation voltage components by combining the altitude-temperature-frequency physical mapping model; The phase difference measurement result between the local second pulse and the original second pulse is obtained, and a feedback control quantity is generated based on the phase difference measurement result. The feedforward compensation voltage component and the feedback control quantity are linearly weighted and fused to output the adjusted digital control word. Based on the clock frequency corresponding to the adjusted digital control word, the phase residuals of the physical action time of the transmitter and the sampling time of the receiver relative to the original second pulse are quantized. The phase residual is embedded as synchronization metadata into the data frame of the electromagnetic sampling signal, and the sampling data of the ground-to-air node is reconstructed and differentially aligned based on the synchronization metadata. The altitude change rate is calculated based on altitude data, and a feedforward compensation voltage component is generated by combining the altitude-temperature-frequency physical mapping model, including: The absolute altitude is extracted from the Global Navigation Satellite System (GNSS) messages and fused with the high-frequency relative altitude change rate output by the barometric pressure sensor to obtain the filtered altitude. Calculate the difference between the current filtered height and the previous filtered height to obtain the height change rate; Based on the product relationship between the altitude change rate and the feedforward control gain, the predicted frequency drift caused by altitude change is calculated. The expected frequency drift prediction is converted into a feedforward compensation voltage component and then smoothed by a first-order low-pass filter. The process of establishing the altitude-temperature-frequency physical mapping model includes: Based on the vertical temperature lapse rate under standard atmospheric conditions, a linear relationship model between external ambient temperature and altitude is constructed. Based on the temperature sensitivity coefficient of a specific crystal oscillator, a linear relationship model between frequency deviation and temperature change is constructed. By coupling the linear relationship model between external ambient temperature and altitude and the linear relationship model between frequency deviation and temperature change, the transfer function from the rate of change of altitude to frequency drift is obtained.

2. The semi-airborne transient electromagnetic time synchronization method as described in claim 1, characterized in that, Acquire the phase difference measurement result between the local second pulse and the original second pulse, and generate feedback control quantity based on the phase difference measurement result, including: Receive the phase difference count value output by the high-resolution time-to-digital converter. The phase difference count value represents the time interval between the rising edge of the local second pulse generated by the local temperature-controlled crystal oscillator frequency division and the original second pulse. A Kalman filter is used to estimate the state of the phase difference count values, and the trends of frequency offset and aging rate are extracted. The filtered phase deviation is input into a gain-adaptive proportional-integral-derivative (PID) regulator to calculate the feedback control quantity.

3. The semi-airborne transient electromagnetic time synchronization method as described in claim 1, characterized in that, The feedforward compensation voltage component and the feedback control quantity are linearly weighted and fused, including: In timekeeping logic mode, when the global navigation satellite system signal is lost, the system stops receiving feedback control inputs and maintains output solely by relying on the aging slope and temperature drift compensation characteristics learned before the loss of lock. In the acquisition logic mode, when the global navigation satellite system signal is locked, the feedforward compensation voltage component is superimposed with the feedback control quantity; the final regulated digital control word is output to the digital-to-analog converter to drive the voltage control terminal of the temperature-controlled crystal oscillator.

4. The semi-airborne transient electromagnetic time synchronization method as described in claim 1, characterized in that, The phase residuals between the physical action time of the quantized transmitter and the sampling time of the receiver relative to the original second pulse include: At the transmitting end, the logic flip signal output by the current zero-point detection circuit is obtained to capture the absolute moment of current turn-off; At the receiving end, the sampling start pulse of the analog-to-digital converter is acquired; The absolute interval between the physical action times of the transmitter and receiver and the original second pulse is recorded to obtain the picosecond-level phase residual.

5. The semi-airborne transient electromagnetic time synchronization method as described in claim 1, characterized in that, Embedding the phase residual as synchronization metadata into the data frame of the electromagnetic sampling signal includes: Construct a structured data frame that includes a frame header, global navigation satellite system absolute time, latitude and longitude information, altitude information, phase residual, and electromagnetic field sampling voltage sequence; A synchronization frame is inserted at the beginning of each second to record the precise offset of the starting sampling point of that second relative to the original second pulse; using a double buffer mechanism and direct memory access technology, the synchronization data frame containing the phase residual is written to the storage medium.

6. The semi-airborne transient electromagnetic time synchronization method as described in claim 1, characterized in that, Based on synchronized metadata, the sampled data from ground-to-air nodes undergoes time-axis reconstruction and differential alignment, including: The Global Navigation Satellite System second tags in the storage media of the ground transmitter and the air receiver are analyzed to achieve initial pairing of data packets; Retrieve the transmitter phase residual and receiver phase residual from the metadata area, calculate the algebraic difference between the two, and obtain the relative phase deviation between nodes; The receiver time axis is shifted by relative phase deviation to force the sampling zero point to align with the moment of transmission shutdown.

7. The semi-airborne transient electromagnetic time synchronization method as described in claim 1, characterized in that, Based on synchronized metadata, time-axis reconstruction and differential alignment of sampled data from ground-to-air nodes are performed, which also includes: Obtain the fixed system deviations generated by the hardware link, which include signal transmission line delay, opto-isolator response time, and analog-to-digital converter digital filter group delay. The offset of the final calibration model is obtained by superimposing the relative phase deviation with the fixed system deviation. Based on the final calibration model, the sampling points at both the ground and air ends are projected onto the global standard time axis.

8. A semi-aircraft transient electromagnetic time synchronization system, characterized in that, The semi-airborne transient electromagnetic time synchronization method according to any one of claims 1-7 includes: The altitude feedforward unit is configured to: acquire altitude data of the target operating area and the raw second pulse signal output by the global navigation satellite system, calculate the altitude change rate based on the altitude data, and generate feedforward compensation voltage components in combination with the altitude-temperature-frequency physical mapping model; The phase feedback unit is configured to: acquire the phase difference measurement result between the local second pulse and the original second pulse, generate a feedback control quantity based on the phase difference measurement result, linearly weight and fuse the feedforward compensation voltage component and the feedback control quantity, and output the adjusted digital control word. The residual quantization unit is configured to quantize the phase residual between the physical action time of the transmitter and the sampling time of the receiver relative to the original second pulse, based on the clock frequency corresponding to the adjusted digital control word. The data reconstruction unit is configured to: embed the phase residual as synchronization metadata into the data frame of the electromagnetic sampling signal, and perform time axis reconstruction and differential alignment on the sampling data of the ground-to-air node based on the synchronization metadata.

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