Unmanned aerial vehicle-mounted in-situ water vapor measurement method and system based on infrared absorption spectrum

Abstract

The invention discloses an unmanned aerial vehicle-mounted in-situ water vapor measurement method and system based on an infrared absorption spectrum, and the method comprises the steps: S1, carrying out the calibration of a laboratory before flight, and building a concentration inversion model; s2, starting an in-situ infrared water vapor sensing module, cooperatively working in combination with a differential positioning module, a temperature and pressure sensor and an electronics module, and acquiring data along a cruise path of the unmanned aerial vehicle; and S3, carrying out real-time water vapor concentration inversion and data fusion. The method is different from the traditional detection means that air chamber atmosphere sampling, detection and exhaust processes are adopted, in-situ detection is directly carried out on the atmosphere in the target area, the measurement efficiency is greatly improved, meanwhile, the measurement deviation caused by air chamber residual gas introduction is avoided, and the engineering property of the system is enhanced.

Description

Technical Field

[0001] This invention relates to the field of environmental detection, specifically to an unmanned aerial vehicle (UAV) in-situ water vapor measurement method and system based on infrared absorption spectroscopy. Background Technology

[0002] Water vapor, as a major component of the atmosphere, has significant implications for weather forecasting, environmental monitoring, and industrial production due to its spatial distribution and concentration variations. Existing water vapor measurement technologies mainly include the dew point method, Fourier transform infrared spectroscopy (FTIR), and tunable diode laser absorption spectroscopy (TDLAS). Among these, TDLAS technology is widely used for in-situ measurements due to its fast response, low cost, small size, and high selectivity.

[0003] In wide-area horizontal cruise or vertical profile measurements, the atmospheric environment typically exhibits significant spatial variations, with temperature and pressure varying considerably with altitude and geographical location. Based on spectral absorption mechanisms, the linewidth and intensity of water vapor molecule absorption lines are sensitive to ambient temperature and pressure. Traditional methods rely solely on regional average meteorological data or standard atmospheric model parameters for inversion, neglecting local environmental fluctuations and introducing measurement errors into the results. By introducing temperature and pressure sensors to acquire in-situ measured temperature and pressure data and incorporating real-time environmental parameter compensation algorithms, interference from environmental fluctuations in water vapor detection can be corrected. This improves the accuracy and reliability of water vapor concentration inversion in complex environments with low temperatures, low pressures, or drastic temperature and pressure changes.

[0004] Existing water vapor detection equipment outputs data with a single dimension, typically recording only gas concentration values ​​that change over time, lacking correlation with high-precision geospatial coordinates and meteorological environmental parameters. A three-dimensional water vapor concentration model established by combining multi-source data from a three-dimensional positioning module, a meteorological module, and a water vapor detection module enhances the application value of the detection data in meteorological research and environmental monitoring. Therefore, there is an urgent need for a structurally stable, real-time-enabled, three-dimensionally-positionable, and adaptable in-situ water vapor measurement system that can be mounted on unmanned aerial vehicles (UAVs) for wide-area patrols. Summary of the Invention

[0005] The purpose of this invention is to provide an unmanned aerial vehicle (UAV) in-situ water vapor measurement method and system based on infrared absorption spectroscopy, so as to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for in-situ water vapor measurement on an unmanned aerial vehicle (UAV) based on infrared absorption spectroscopy, comprising: S1, Pre-flight laboratory calibration to establish a concentration inversion model; S2, activate the in-situ infrared water vapor sensing module, which works in conjunction with the differential positioning module, temperature and pressure sensor, and electronics module to collect data along the UAV's cruise path; S3 performs real-time water vapor concentration inversion and data fusion.

[0007] Further, S1 includes: In the insertion calibration unit, a humidity generator is used to sequentially introduce multiple sets of water vapor standard gases with known concentrations into the direct air chamber. During the air circulation process, the direct air chamber is adjusted to meet the standard air pressure P0 and room temperature T0. The near-infrared spectral signal amplitude corresponding to each set of standard concentration water vapor is collected, a calibration function of signal amplitude-water vapor concentration is established, and a concentration inversion model is generated.

[0008] Further, S2 includes: The in-situ infrared water vapor sensing module is activated, and the electronics module generates a drive signal to drive the laser, stabilizing the laser's emission center wavelength at the target wavelength and performing a small-range wavelength scan. Hardware-triggered synchronous initialization: After the UAV-borne in-situ water vapor measurement system is powered on, the clock signal generated by the differential positioning module is used as the hardware trigger source to synchronously start the acquisition of three-dimensional coordinate data of the differential positioning module, the acquisition of temperature and pressure data of the temperature and pressure sensor, and the concentration measurement of the water vapor sensor, ensuring that the three types of data, coordinate, temperature and pressure and concentration, are strictly aligned on the time axis. Path planning and autonomous cruise: The UAV cruise path is set through ground station software, and the spatial sampling density, flight altitude and cruise speed are set. The UAV is equipped with an on-board in-situ water vapor measurement system and performs flight missions according to the set path. Open optical path absorption detection: During flight, the laser emitted by the laser enters the open measurement chamber and undergoes characteristic absorption with the in-situ water vapor along the UAV's flight path. The attenuated laser signal is focused by a lens and received by the detector, converted into an electrical signal, and transmitted to the electronics module.

[0009] Furthermore, in S2, Based on tunable diode laser absorption spectroscopy, a WMS-2f / 1f calibration-free normalized detection method is employed. A high-frequency sinusoidal modulation signal is superimposed on a low-frequency sawtooth wave scanning signal to drive the laser to scan the water vapor absorption peak. The instantaneous frequency v(t) of the laser is expressed as: (1) Where v0 is the center frequency of the laser, v m ω is the frequency modulation depth, ω is the angular frequency of the sinusoidal modulation, and t is time; According to the Lambert-Beer law, after the laser light passes through the water vapor region to be measured and is absorbed, the relationship between the transmitted light intensity I(t) and the initial light intensity I0(t) is as follows: (2) Where exp is the exponential function, C is the mole fraction of the water vapor to be measured, L is the effective optical path length, and α is the spectral absorption coefficient; Under weak absorption conditions, the exponential term can be approximated as a linear term, i.e. Expanding equation (2) using Fourier series, the harmonic components of the transmitted light intensity can be expressed as: (3) Wherein, the k-th order Fourier expansion coefficients H k (v o ,v m ) is represented as: (4) Where S(T) is the water vapor absorption line intensity, P is the total pressure, and Φ is the linear function; The amplitude of the second harmonic is normalized using the amplitude of the first harmonic, and the expression is derived as follows: (5) Where f is the frequency, S 1f S is the signal amplitude of the first harmonic. 2f S represents the signal amplitude of the second harmonic. 2f / 1f To normalize the first harmonic amplitude to the second harmonic amplitude, where i1 is the normalized linear modulation coefficient of the laser intensity, the following scaling function is derived: (6) During the pre-flight calibration phase, standard calibration parameters η are established at standard pressure P0 and room temperature T0 for laboratory calibration.

[0010] Further, S3 includes: S3.1, Concentration Inversion: The concentration inversion model generated in S1 is invoked, and combined with the real-time temperature and pressure data transmitted from the temperature and pressure sensors, the normalized amplitude signal S is processed. 2f / 1f Perform temperature and pressure compensation corrections to calculate the actual water vapor concentration at the current location; S3.2, Data fusion and storage: The calculated water vapor concentration value, the three-dimensional coordinate data output by the differential positioning module, and the temperature and pressure data collected by the temperature and pressure sensor are fused and packaged according to a unified timestamp to obtain fused data. Based on the fused data, a three-dimensional distribution profile of water vapor concentration in the area to be measured is drawn.

[0011] Furthermore, in S3.1, The temperature and pressure sensor collects temperature and pressure data along the flight path in real time. The data is used to correct the concentration inversion model under standard pressure P0 and room temperature T0 conditions during the calibration phase, eliminating the error in the calculation of actual concentration caused by changes in ambient temperature and pressure during UAV flight. The absorption line intensity S(T) is expressed as: (7) Q(T) is the molecular partition function, expressed using a polynomial fitting: (8) Where c2 is the second radiation constant, σ represents the low-state transition energy, σ is the center wavenumber of the spectral line, and a, b, c, and d are the fitting coefficients. The water vapor absorption line intensity is dynamically corrected by substituting the real-time collected temperature data into formula (7) to calculate the actual water vapor absorption line intensity under the current flight environment. Combining formulas (6), (7), and (8), the formula for calculating the concentration after temperature and pressure correction is as follows: (9) Using engineering approximations to define the line strength ratio term This can be simplified to a quadratic polynomial in terms of temperature difference, which can then be fitted and replaced. (10) Among them, C meas C0 is the corrected water vapor concentration, and P is the uncorrected water vapor concentration substituted into the concentration inversion model. meas T represents the actual pressure measured by the temperature and pressure sensor. meas The actual temperature is measured by the temperature and pressure sensor. The coefficients γ and β are regression parameters extracted by performing a quadratic polynomial fitting on the nonlinear curve of the theoretical line intensity ratio calculated based on the HITRAN database within the working temperature range.

[0012] Furthermore, including: An in-situ infrared water vapor sensing module, which measures in-situ water vapor concentration based on tunable diode laser absorption spectroscopy, includes an open measurement chamber and laser emitting and receiving units disposed at both ends of the open measurement chamber. A differential positioning module, which is used for time synchronization and recording three-dimensional coordinate data of the flight path; A temperature and pressure sensor, which is used to collect temperature and pressure data of the measurement environment in real time; The electronics module is used to generate laser drive signals, acquire data from differential positioning modules and temperature and pressure sensors, invert water vapor concentration, and realize synchronous storage of multi-source data.

[0013] Furthermore, the laser emitting unit includes a laser and a collimator, and the laser receiving unit includes a lens and a detector, wherein the laser, collimator, lens, and detector are arranged coaxially in sequence; The in-situ infrared water vapor sensing module is configured such that: the beam emitted by the laser is collimated into a parallel beam by a collimator, and the parallel beam is injected into the open measurement chamber to measure water vapor. After passing through the in-situ water vapor to be measured and undergoing characteristic absorption, the parallel beam is focused to the laser receiving unit. The laser receiving unit converts the received light intensity signal into an electrical signal and transmits it to the electronics module.

[0014] Compared with the prior art, the beneficial effects of the present invention are:

[0015] 1) By adopting an open measurement chamber, unlike traditional detection methods that use a gas chamber for atmospheric sampling, detection, and exhaust, the system directly performs in-situ detection of the target area's atmosphere, significantly improving measurement efficiency. At the same time, it avoids measurement deviations introduced by residual gas in the gas chamber, enhancing the system's engineering feasibility.

[0016] 2) The entire in-situ infrared sensing module adopts a coaxial design, eliminating the need for optical path calibration, thus saving the cumbersome optical path calibration steps of traditional gas detection methods and simplifying the system debugging process.

[0017] 3) Based on wavelength modulation spectroscopy under the branch of tunable diode laser absorption spectroscopy, combined with the normalization of the second harmonic relative to the first harmonic, common-mode noise such as laser current fluctuation and detector gain drift is effectively suppressed, improving the signal-to-noise ratio of water vapor concentration measurement and realizing high-precision water vapor measurement.

[0018] 4) The in-situ infrared sensing module is mounted on the UAV to realize in-situ detection of water vapor concentration during motorized cruise. It can measure water vapor concentration in a wide area of ​​complex and remote areas such as mountains and wetlands, breaking through the spatial limitations of ground fixed-point observation.

[0019] 5) To address the issue of pressure and temperature fluctuations in the atmospheric environment, this paper combines multi-source data fusion and temperature and pressure compensation algorithms to correct the environmental adaptation defects of traditional concentration inversion models under standard laboratory conditions, thereby reducing measurement errors under different environments.

[0020] 6) After system integration, the weight of the entire drone is only 1.5kg, achieving lightweight and miniaturized structural optimization, and adapting to the load limitations of small vehicles. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of an unmanned aerial vehicle (UAV) in-situ water vapor measurement system based on infrared absorption spectroscopy, according to the present invention.

[0022] Figure 2 This is a schematic diagram of the cross-sectional structure of an unmanned aerial vehicle (UAV) in-situ water vapor measurement system based on infrared absorption spectroscopy, according to the present invention.

[0023] Figure 3 This is an interactive diagram of an unmanned aerial vehicle (UAV) in-situ water vapor measurement system based on infrared absorption spectroscopy, according to the present invention.

[0024] Figure 4 This is a flowchart of the signal processing of an unmanned aerial vehicle (UAV) in-situ water vapor measurement system based on infrared absorption spectroscopy, according to the present invention.

[0025] Figure 5 This is a flowchart of an unmanned aerial vehicle (UAV) in-situ water vapor measurement method based on infrared absorption spectroscopy, according to the present invention.

[0026] Figure 6 This is a simulated absorbance curve of the absorption spectrum of a water vapor target in an UAV-borne in-situ water vapor measurement system based on infrared absorption spectroscopy, according to the present invention.

[0027] Figure 7 This is a time-domain characteristic curve of laser driving current, output wavelength and optical power in an unmanned aerial vehicle (UAV) in-situ water vapor measurement system based on infrared absorption spectroscopy.

[0028] Figure 8 This is a time-domain waveform of the original photoelectric signal received by the detector in an unmanned aerial vehicle (UAV) in-situ water vapor measurement system based on infrared absorption spectroscopy.

[0029] Figure 9 This is a waveform diagram of the first and second harmonic signals extracted after phase-locked demodulation in an unmanned aerial vehicle (UAV) in-situ water vapor measurement system based on infrared absorption spectroscopy.

[0030] Figure 10 This is a graph showing the normalized ratio of the second harmonic to the first harmonic in an unmanned aerial vehicle (UAV) in-situ water vapor measurement system based on infrared absorption spectroscopy, calculated using a calibration-free method.

[0031] In the diagram: 1. Protective housing; 2. Streamlined lightweight windshield; 3. Laser; 4. Collimator; 5. Open measurement chamber; 6. Lens; 7. Detector; 8. Electronic module bracket; 9. Electronic module; 10. Differential positioning module; 11. Battery; 12. Temperature and pressure sensor. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Please see Figures 1-4This invention provides an in-situ water vapor measurement system for unmanned aerial vehicles (UAVs) based on infrared absorption spectroscopy. The system includes a protective shell 1, inside which are housed an electronics module bracket 8, an electronics module 9, a differential positioning module 10, and a battery 11. The electronics module bracket 8 covers the differential positioning module 10 and the battery 11, and the electronics module 9 is located on the upper part of the electronics module bracket 8. A streamlined lightweight windshield 2 is located at the lower end of the protective shell 1. The streamlined lightweight windshield 2 has an open measurement chamber 5 with front and rear openings. A laser emitting unit and a laser receiving unit are respectively located at both ends of the open measurement chamber 5. The laser emitting unit, the open measurement chamber 5, and the laser receiving unit constitute an in-situ infrared water vapor sensing module. The laser emitting unit includes a laser 3 and a collimator 4, and the laser receiving unit includes a lens 6 and a detector 7. The laser 3, collimator 4, lens 6, and detector 7 are arranged coaxially in sequence, and there is sufficient space between the collimator 4 and the lens 6 for the insertion of an insertion-type calibration unit. A temperature and pressure sensor 12 is also installed at the bottom center of the protective shell 1. The upper part of the temperature and pressure sensor 12 remains inside the cavity of the protective shell 1, and the probe of the lower part of the temperature and pressure sensor 12 extends into the open measurement chamber 5. The temperature and pressure sensor 12 is located above and between the laser emitting unit and the laser receiving unit. The signal connection relationship of each module is as follows: Figure 3 and Figure 4 As shown.

[0034] Continue reading Figure 1 and Figure 2 The UAV-borne in-situ water vapor measurement system adopts a two-layer layout. The upper layer houses the differential positioning module 10, temperature and pressure sensor 12, electronics module 9, and battery 11, all encapsulated within a protective shell 1. This shell 1 forms a physical protective barrier, preventing rain, dust, and sandstorms during flight operations from corroding the internal precision electronic components. The lower layer houses the in-situ infrared water vapor sensing module, which is housed within a streamlined, lightweight windshield 2, reducing turbulent disturbances from the UAV's flight airflow to the open measurement chamber. The entire system is connected to the UAV via a mounting mechanism.

[0035] Electronic module 9 is electrically connected to in-situ infrared water vapor sensing module, differential positioning module 10, temperature and pressure sensor 12 and battery 11 respectively, serving as the control center of the system;

[0036] The in-situ infrared water vapor sensing module measures in-situ water vapor concentration using tunable diode laser absorption spectroscopy. It comprises a laser emitting unit, an open measurement chamber 5, and a laser receiving unit. The module adopts a coaxial architecture design, with the laser emitting unit, open measurement chamber 5, and laser receiving unit arranged sequentially along the optical axis. The laser emitting unit includes a laser 3 and a collimator 4. A parallel beam is emitted through the collimator 4 and enters the 10cm long open measurement chamber 5 to measure water vapor. The mechanical structure of the open measurement chamber 5 supports the insertion and removal of an insert-type calibration unit. After passing through the in-situ water vapor to be measured and undergoing characteristic absorption, the parallel beam is focused onto the laser receiving unit. The laser receiving unit includes a lens 6 and a detector 7, which converts the received light intensity signal into an electrical signal and transmits it to the electronics module 9.

[0037] The laser emitting unit utilizes a butterfly-packaged, narrow-linewidth distributed feedback laser with single-mode fiber coupling. The laser's center wavelength is set at 1367.9 nm to cover the near-infrared characteristic absorption peak of water vapor molecules at 1367.86 nm. Figure 6 As shown, under conditions of 3% water vapor concentration and an effective optical path of 10 cm, the absorbance of this absorption line is 0.198. Choosing this wavelength band compensates for the insufficient absorbance caused by the short 10 cm optical path in this system, ensuring a high signal-to-noise ratio in the measurement, while also avoiding signal saturation in high humidity environments. Simultaneously, this type of laser 3 has advantages such as narrow linewidth, no mode hopping, and small size, and wavelength tuning is achieved through coordinated temperature and current adjustment. In this example, the peak wavelength temperature drift coefficient is approximately 0.1 nm / ℃, and the current drift coefficient is approximately 0.02 nm / mA. Under conventional driving current, the output power can reach approximately 12 mW, and the wavelength can cover the range of 1366.8 nm to 1370.4 nm. Figure 7 As shown. Laser 3 is driven by the laser drive circuit of electronics module 9, with a temperature control accuracy of ±0.001℃ and current noise RMS <1μA. Electronics module 9 generates a sweeping, superimposed high-frequency drive signal. In this example, a 10Hz low-frequency sawtooth wave is superimposed with a 20kHz high-frequency sine wave, which is input to laser 3 through electronics module 9 to stabilize the output wavelength near the water vapor target absorption peak of 1367.86nm and perform periodic scanning.

[0038] In this embodiment, the fiber optic output of laser 3 is connected to collimator 4. The light source emitted by laser 3 is collimated by collimator 4 to form an approximately parallel laser beam that enters a 10cm long open measurement chamber 5. In this example, collimator 4 uses a lens with a diameter of 8mm and a focal length of 5.95mm. In order to reduce spherical aberration and achieve better collimation, the collimating lens adopts an aspherical design, controlling the divergence angle of the laser output within 4mrad and accurately transmitting it to detector 7 along the optical axis.

[0039] In the open measurement chamber 5, the relative positions of the laser emitting unit and the laser receiving unit are fixed by four support rods distributed parallel to the optical axis (not shown in the figure), forming an open gas absorption optical path with a total length of 10cm. The open, perforated structure on all sides allows ambient air to quickly enter the optical path area through free diffusion or airflow, enabling real-time measurement of water vapor concentration.

[0040] The laser receiving unit comprises a detector 7 and a lens 6. An indium gallium arsenide (IGaAs) photodiode is used as the detector 7, with a wavelength response range covering 900nm-1700nm, meeting the system's wavelength requirements. The laser beam, absorbed by water vapor in the air, reaches the lens 6 and is focused onto the photosensitive surface of the detector 7. The detector 7 converts the light intensity signal into a current signal, which is then converted into a voltage signal by a transimpedance amplifier. Figure 8 As shown, the data is uploaded to the electronics module 9 for processing. In this example, detector 7 has a high responsivity ≥0.8A / W and a wide bandwidth DC-150MHz, with an effective photosensitive area of ​​approximately 0.2mm². 2 With a noise equivalent power of 12.6 pW / Hz^1 / 2, it can capture weak absorption signals. Lens 6 is placed in front of detector 7 to focus the laser beam onto the photosensitive surface of detector 7. Lens 6 is a 7.200 mm diameter, 11.0 mm focal length, and 0.3 numerical aperture aspherical lens with a 1000 nm-1700 nm wide-band antireflection coating.

[0041] The differential positioning module 10 outputs the three-dimensional coordinate data of the UAV's flight path in real time. This system adopts a hardware-triggered synchronization mechanism. When the differential positioning module 10 starts positioning, it generates a stable system clock signal as a synchronization reference. This clock signal is connected to the temperature and pressure sensor 12 and the electronics module 9 respectively, so that all sensor data are strictly aligned on the time axis.

[0042] The temperature and pressure sensor 12 collects temperature and pressure data of the flight path environment in real time. The data is used to correct the concentration inversion model and eliminate the influence of environmental temperature and pressure changes on concentration calculation.

[0043] Electronics module 9 is used to generate laser drive signals, acquire data from various sensors, invert water vapor concentration, and realize synchronous storage of multi-source data. Electronics module 9 has a built-in laser drive circuit, which is used to input a sawtooth wave frequency sweep superimposed with a high-frequency sine wave drive signal to the laser 3, driving the laser 3 to perform periodic wavelength scanning and modulation near the target absorption peak of 1367.86nm.

[0044] The core of the electronics module is an FPGA circuit board, which contains a digital demodulation algorithm used to demodulate the acquired photoelectric signals and extract the first and second harmonic components, such as... Figure 9 As shown, the normalized ratio of the first harmonic to the second harmonic is calculated as follows: Figure 10 As shown, interference from light intensity fluctuations and laser power drift on the measurement is eliminated. The FPGA circuit board calls a preset concentration inversion model to convert the normalized ratio of the first and second harmonics obtained from real-time demodulation into water vapor concentration data. The concentration inversion model is based on a pre-flight insertion calibration unit, and establishes a function mapping relationship by fitting the normalized ratio of the first and second harmonics measured by the system to the standard concentration values ​​using the least squares method under multiple sets of standard water vapor concentration environments.

[0045] The digital demodulation algorithm, implemented using a digital lock-in amplifier, primarily comprises four stages: reference signal generation, quadrature mixing, low-pass filtering, and amplitude extraction. Sine and cosine waves, strictly synchronized with the laser modulation frequency, are used as quadrature reference signals. A digital multiplier multiplies the acquired detection signal with both the in-phase and quadrature reference signals, down-converting the measured harmonic components to the DC domain. A high-order digital low-pass filter removes harmonic terms and high-frequency noise, extracting the in-phase component X and the quadrature component Y. Finally, square root calculation is used... The amplitudes of the first and second harmonics are calculated.

[0046] The electronics module includes a temperature and pressure compensation algorithm. This algorithm uses the temperature and pressure data collected in real time by the temperature and pressure sensor to correct the concentration inversion model and improve the measurement accuracy of water vapor concentration.

[0047] The electronics module, based on a hardware triggering mechanism, associates the concentration data retrieved by the in-situ water vapor sensing module, the temperature and pressure data from the temperature and pressure sensor, and the three-dimensional coordinate data from the differential positioning module with a unified timestamp, and stores them to a large-capacity hard drive.

[0048] The battery provides independent power to each unit of the system.

[0049] This system works in conjunction with an insertion calibration unit, which is used to establish a concentration inversion model before flight. This unit mainly includes a humidity generator and a direct-flow chamber, which can be inserted into the open measurement chamber 5. The direct-flow chamber is made of corrosion-resistant stainless steel, and its internal effective optical path length is consistent with the 10cm optical path length of the open measurement chamber. Temperature and pressure can be measured inside the chamber, and a heating element is wrapped around its exterior. During laboratory calibration, this direct-flow chamber is inserted into the coaxial optical path, occupying the original position of the open measurement chamber. A series of standard water vapor concentrations are generated using the humidity generator and introduced into the direct-flow chamber. The system records the normalized ratio of the first and second harmonics at each concentration. The least squares method is used to fit multiple sets of standard water vapor concentrations to the signal amplitude, establishing a concentration inversion model between the measured signal and the water vapor concentration, which is then written into the electronics module. During actual flight measurements, the electronics module 9 uses this built-in concentration inversion model to invert the detected amplitude signal into an absolute water vapor concentration value in real time.

[0050] The data flow in this system is centered around Electronics Module 9, and the data flow between units is realized through various communication protocols, as follows:

[0051] The differential positioning module 10 sends synchronization trigger pulses to the electronics module 9 and the temperature and pressure sensor 12 via the IF_Sync interface, enabling the entire system to operate under a unified time reference. Simultaneously, the differential positioning module 10 transmits real-time three-dimensional coordinate data unidirectionally to the electronics module 9 via the IF_Pos interface.

[0052] After receiving the synchronization trigger signal, the temperature and pressure sensor 12 detects the current environment and transmits the collected temperature and pressure data to the electronics module 9 via the IF_Env interface for subsequent environmental compensation in gas concentration inversion. The electronics module 9, according to preset timing logic, sends commands to the laser 3 driver board via the IF_Ctrl interface. This control signal contains modulated waveform data of a sawtooth wave superimposed on a sine wave, driving the laser 3 to emit a laser signal of a specific wavelength. After being collimated by the collimator 4, the laser beam propagates along a predetermined optical path in the open measurement chamber. The optical signal, after being absorbed by the gas, is captured by the detector 7 and converted into an electrical signal, which is then transmitted back to the electronics module 9 via the IF_Spec interface. The electronics module 9 performs data alignment and preprocessing within the FPGA, fusing the received spectral waveform data, three-dimensional coordinate data, and temperature and pressure data from multiple sources to generate a comprehensive dataset. Finally, this multi-source fused dataset is transferred from the electronics module 9 to the hard disk via the IF_Store interface for storage and subsequent algorithm analysis.

[0053] Information regarding the transmission direction, interface type, and corresponding data content of each functional interface involved in the above data interaction can be found in the following table, Data Definition Table for UAV-borne In-situ Water Vapor Measurement System Based on Infrared Absorption Spectroscopy:

[0054] Table 1. Data Definition Table for UAV-borne In-situ Water Vapor Measurement System Based on Infrared Absorption Spectroscopy

[0055] Please see Figure 5 The present invention also provides a method for in-situ water vapor measurement on a UAV based on infrared absorption spectroscopy, which is implemented using the UAV-borne in-situ water vapor measurement device described above, including:

[0056] S1, Pre-flight laboratory calibration, establishing a concentration inversion model, including:

[0057] In the insertion calibration unit, a humidity generator sequentially introduces multiple sets of water vapor standard gases of known concentrations into the direct-flow chamber. During the ventilation process, the direct-flow chamber is adjusted to meet the standard gas pressure P0 and room temperature T0. The near-infrared spectral absorption signal intensity corresponding to each set of standard water vapor concentrations is collected, and a concentration inversion model of signal amplitude versus water vapor concentration is established through data fitting. To ensure the reliability of the model, at least six water vapor standard gases with different concentration gradients are selected, and at least 600 repeated measurements are performed on each set of standard gases, with the average value taken as the calibration data.

[0058] S2 activates the in-situ infrared moisture sensing module, which, in conjunction with the differential positioning module, temperature and pressure sensor, electronics module, and battery, works to collect data along the drone's cruise path. This includes:

[0059] The in-situ infrared water vapor sensing module is activated, and the electronics module generates a drive signal to drive the laser, stabilizing the laser's emission center wavelength at the target wavelength, and performing a small-range wavelength scan. The target wavelength is preferably 1367.86nm.

[0060] Hardware-triggered synchronous initialization: After the UAV-borne in-situ water vapor measurement system is powered on, the clock signal generated by the differential positioning module is used as the hardware trigger source to synchronously start the acquisition of three-dimensional coordinate data of the differential positioning module, the acquisition of temperature and pressure data of the temperature and pressure sensor, and the concentration measurement of the water vapor sensor, ensuring that the three types of data—coordinate, temperature and pressure, and concentration—are strictly aligned on the time axis.

[0061] Path planning and autonomous cruise: The UAV cruise path is set through ground station software, and the spatial sampling density, flight altitude and cruise speed are set. The UAV is equipped with an on-board in-situ water vapor measurement system and performs flight missions according to the set path.

[0062] Open optical path absorption detection: During flight, the laser emitted by the laser enters the open measurement chamber and undergoes characteristic absorption with the in-situ water vapor along the UAV's flight path. The attenuated laser signal is focused by a lens and received by the detector, converted into an electrical signal, and transmitted to the electronics module.

[0063] Among them, the WMS-2f / 1f calibration-free normalized detection method is employed based on tunable diode laser absorption spectroscopy. The WMS-2f / 1f calibration-free normalized detection method is a key technology in wavelength modulation spectroscopy (WMS) used to improve the stability and accuracy of gas concentration measurements and reduce reliance on traditional calibration. It is widely used in tunable diode laser absorption spectroscopy (TDLAS) systems, especially in scenarios requiring long-term stable operation such as environmental monitoring and industrial process control.

[0064] A high-frequency sinusoidal modulation signal is superimposed on a low-frequency sawtooth wave scanning signal to drive the laser to scan the water vapor absorption peak. The instantaneous frequency v(t) of the laser is expressed as:

[0065] (1)

[0066] Where v0 is the center frequency of the laser, v m ω is the frequency modulation depth, ω is the angular frequency of the sinusoidal modulation, and t is time.

[0067] According to the Lambert-Beer law, after the laser light passes through the water vapor region to be measured and is absorbed, the relationship between the transmitted light intensity I(t) and the initial light intensity I0(t) is as follows:

[0068] (2)

[0069] Where exp is an exponential function, C is the mole fraction of the water vapor to be measured, L is the effective optical path length, and α is the spectral absorption coefficient.

[0070] Under weak absorption conditions, the exponential term can be approximated as a linear term, i.e. Expanding equation (2) using Fourier series, the harmonic components of the transmitted light intensity can be expressed as:

[0071] (3)

[0072] Wherein, the k-th order Fourier expansion coefficients H k (v o ,v m ) is represented as:

[0073] (4)

[0074] Where S(T) is the water vapor absorption line intensity, P is the total pressure, and Φ is a linear function.

[0075] In actual measurements conducted by unmanned aerial vehicles (UAVs), the output light intensity of lasers often fluctuates due to factors such as temperature drift, particle scattering, and lens contamination. To eliminate such non-absorbent interference, this invention normalizes the second harmonic amplitude using the first harmonic amplitude. The expressions for the first and second harmonic signals are derived as follows:

[0076] (5)

[0077] Where f is the frequency, S 1f S is the signal amplitude of the first harmonic. 2f S represents the signal amplitude of the second harmonic. 2f / 1f As a normalization process of the first harmonic amplitude to the second harmonic amplitude, where i1 is the normalized linear modulation coefficient of the laser intensity, the following scaling function can be derived:

[0078] (6)

[0079] During the pre-flight calibration phase, standard calibration parameters η are established at standard pressure P0 and room temperature T0 for laboratory calibration.

[0080] S3 performs real-time water vapor concentration inversion and data fusion. This includes:

[0081] S3.1, Concentration Inversion: The concentration inversion model generated in S1 is invoked, and combined with the real-time temperature and pressure data transmitted from the temperature and pressure sensors, the normalized amplitude signal S is processed. 2f / 1f Temperature and pressure compensation corrections are performed to calculate the water vapor concentration value at the current location in real time.

[0082] The temperature and pressure compensation correction algorithm is as follows:

[0083] The temperature and pressure sensor collects temperature and pressure data along the flight path in real time. The data is used to correct the concentration inversion model under standard pressure P0 and room temperature T0 conditions during the calibration phase, eliminating the error in the calculation of actual concentration caused by changes in ambient temperature and pressure during UAV flight.

[0084] The absorption line intensity S(T) is expressed as:

[0085] (7)

[0086] Q(T) is the molecular partition function, which can usually be represented by a polynomial fitting:

[0087] (8)

[0088] Where c2 is the second radiation constant, σ represents the low-state transition energy, σ is the center wavenumber of the spectral line, and a, b, c, and d are the fitting coefficients.

[0089] The water vapor absorption line intensity is dynamically corrected by substituting the real-time collected temperature data into formula (7) to calculate the actual water vapor absorption line intensity under the current flight environment.

[0090] Combining formulas (6), (7), and (8), the formula for calculating the concentration after temperature and pressure correction is as follows:

[0091] (9)

[0092] Using engineering approximations to define the line strength ratio term This can be simplified to a quadratic polynomial in terms of temperature difference, which can then be fitted and replaced.

[0093] (10)

[0094] Among them, C measC0 is the corrected water vapor concentration, and P is the uncorrected water vapor concentration substituted into the concentration inversion model. meas T represents the actual pressure measured by the temperature and pressure sensor. meas The actual temperature is measured by the temperature and pressure sensor. The coefficients γ and β are regression parameters extracted by performing a quadratic polynomial fitting on the nonlinear curve of the theoretical line intensity ratio calculated based on the HITRAN database within the working temperature range.

[0095] S3.2, Data fusion and storage: The calculated water vapor concentration value, the three-dimensional coordinate data output by the differential positioning module, and the temperature and pressure data collected by the temperature and pressure sensor are fused and packaged according to a unified timestamp to obtain fused data. Based on the fused data, a three-dimensional distribution profile of water vapor concentration in the area to be measured is drawn.

[0096] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. An unmanned airborne in-situ water vapor measurement method based on infrared absorption spectrum, characterized in that, The application relates to a water vapor concentration measurement system for unmanned aerial vehicles. S1, pre-flight laboratory calibration, concentration inversion model is established; S2, start the in-situ infrared water vapor sensing module, combine the differential positioning module, the temperature and pressure sensor and the electronic module to work together to collect data along the unmanned aerial vehicle cruising path; S3, real-time water vapor concentration inversion and data fusion are carried out.

2. The unmanned airborne in-situ water vapor measurement method based on infrared absorption spectrum according to claim 1, characterized in that, The S1 comprises the following steps: In the plug-in calibration unit, a humidity generator is used to sequentially introduce a plurality of groups of water vapor standard gases with known concentrations into a straight-through air chamber, in the process of air introduction, the straight-through air chamber is adjusted to meet the standard air pressure P0 and the chamber temperature T0, the near-infrared spectrum signal amplitude corresponding to each group of standard concentration water vapor is collected, a calibration function of the signal amplitude-water vapor concentration is established, and a concentration inversion model is generated.

3. The unmanned airborne in-situ water vapor measurement method based on infrared absorption spectrum according to claim 1, characterized in that, The S2 comprises the following steps: The in-situ infrared water vapor sensing module is started, and an electronic module generates a driving signal to drive a laser, so that the laser emits a central wavelength which is stable at a target wavelength and performs a small-range wavelength scanning; Hardware trigger synchronization initialization: after the unmanned aerial vehicle-mounted in-situ water vapor measurement system is powered on, a clock signal generated by the differential positioning module is used as a hardware trigger source to synchronously start the collection of three-dimensional coordinate data of the differential positioning module, the collection of temperature and pressure data of the temperature and pressure sensor and the concentration measurement of the water vapor sensor, so that the coordinate-temperature and pressure-concentration three types of data are strictly aligned on a time axis; Path planning and autonomous cruising: the unmanned aerial vehicle cruising path is set through a ground station software, the spatial sampling density, the flight height and the cruising speed are set, the unmanned aerial vehicle is provided with the unmanned aerial vehicle-mounted in-situ water vapor measurement system, and the unmanned aerial vehicle executes a flight task according to the set path; Open optical path absorption detection: in the flight process, the laser emitted by the laser emits into an open measurement chamber, and the laser is absorbed by the in-situ water vapor on the flight path of the unmanned aerial vehicle, the attenuated laser signal is focused by a lens and is received by a detector, is converted into an electric signal and is transmitted to an electronic module.

4. The unmanned airborne in-situ water vapor measurement method based on infrared absorption spectrum according to claim 3, characterized in that, In the S2, Based on the tunable diode laser absorption spectrum method, a WMS-2f / 1f non-calibration normalization detection method is adopted; a high-frequency sinusoidal modulation signal is superimposed on a low-frequency sawtooth wave scanning signal to drive the laser to scan the water vapor absorption peak; the instantaneous frequency v(t) of the laser is represented as: （1） where v0is the center frequency of the laser, v m is the frequency modulation depth, ω is the angular frequency of the sinusoidal modulation, and t is time. According to the Lambert-Beer law, the relationship between the transmitted light intensity I(t) and the initial light intensity I0(t) after the laser is absorbed by the water vapor to be measured is: （2） Wherein, exp is an exponential function, C is the molar fraction of the water vapor to be measured, L is the effective optical path length, and alpha is the spectral absorption coefficient; Under the condition of weak absorption, the exponential term can be approximated as a linear term, i.e. The Fourier series expansion of equation (2) gives the expression of the harmonic components of the transmitted light intensity as （3） where kth Fourier expansion coefficient H k (v o ,v m ) is expressed as: （4） Wherein, S(T) is the water vapor absorption line strength, P is the total pressure, and Phi is a linear function; The first harmonic amplitude is used for normalization processing of the second harmonic amplitude, and the expression is derived as follows: （5） where f is the frequency, S 1f is the signal amplitude of the first harmonic, S 2f is the signal amplitude of the second harmonic, S 2f / 1f is the normalized processing of the first harmonic amplitude to the second harmonic amplitude, and i1is the normalized linear modulation coefficient of the laser intensity, and the following scaling function is derived: （6） In the calibration stage before flight, the standard calibration parameters eta are established under the standard pressure P0 and the room temperature T0 in the laboratory calibration stage.

5. The unmanned airborne in-situ water vapor measurement method based on infrared absorption spectrum according to claim 4, characterized in that, The S3 comprises the following steps: S3.1, concentration inversion, call the concentration inversion model generated by S1, combine the temperature and pressure data returned by the temperature and pressure sensor in real time, and correct the normalized amplitude value signal S 2f / 1f Perform temperature and pressure compensation correction to calculate the actual water vapor concentration value at the current position; S3.2, data fusion storage, the calculated water vapor concentration value, the three-dimensional coordinate data output by the differential positioning module and the temperature and pressure data collected by the temperature and pressure sensor are fused and packaged according to a unified time stamp to obtain fusion data, and a three-dimensional distribution profile graph of the water vapor concentration of the measured region is drawn according to the fusion data.

6. The unmanned airborne in-situ water vapor measurement method based on infrared absorption spectrum according to claim 5, characterized in that, In the S3.1, The temperature and pressure sensor collects temperature and pressure data of the flight path in real time, and the data is used to correct the concentration inversion model under the conditions of standard pressure P0 and room temperature T0 in the calibration stage, so as to eliminate the error of actual concentration calculation caused by the change of environmental temperature and pressure during the flight of the unmanned aerial vehicle; The absorption line strength S(T) is represented as: （7） Q(T) is a molecular partition function, which is represented by a polynomial fitting: （8） where c2 is the second radiation constant, is the low-state transition energy, σ is the central wavenumber of the line, and a, b, c, d are fitting coefficients; The real-time collected temperature data is substituted into formula (7) to dynamically correct the water vapor absorption line strength, and the actual water vapor absorption line strength under the current flight environment is calculated; The combination of formula (6), (7) and (8) gives the calculation formula of the concentration after correction of temperature and pressure as follows: （9） The linear intensity ratio term is simplified by engineering approximation to a quadratic polynomial in terms of temperature difference for fitting substitution: （10） where C meas is the corrected water vapor concentration, C0 is the uncorrected water vapor concentration, P meas is the actual pressure measured by the temperature and pressure sensor, T meas is the actual temperature measured by the temperature and pressure sensor, and γ and β are regression parameters extracted from a second-order polynomial fitting of the nonlinear curve of the theoretical line intensity ratio based on the HITRAN database within the working temperature range.

7. An unmanned airborne in-situ water vapor measurement system based on infrared absorption spectroscopy, characterized in that, It includes: The in-situ infrared water vapor sensing module measures the in-situ water vapor concentration based on the tunable diode laser absorption spectroscopy method, including an open measurement chamber and a laser emitting unit and a laser receiving unit arranged at both ends of the open measurement chamber; The differential positioning module is used for time synchronization and recording of three-dimensional coordinate data of the flight path; The temperature and pressure sensor is used for real-time collection of temperature and pressure data of the measurement environment; The electronics module is used for generating laser driving signals, collecting data of the differential positioning module and the temperature and pressure sensor, water vapor concentration inversion, and realizing synchronous storage of multi-source data.

8. The unmanned airborne in-situ water vapor measurement system based on infrared absorption spectroscopy according to claim 7, characterized in that, The laser emitting unit includes a laser and a collimator, and the laser receiving unit includes a lens and a detector, and the laser, collimator, lens and detector are coaxially arranged in sequence; The in-situ infrared water vapor sensing module is configured such that the light beam emitted by the laser passes through the collimator to become a parallel light beam, the parallel light beam enters the open measurement chamber to measure the water vapor, and the parallel light beam is focused on the laser receiving unit after passing through the in-situ water vapor to be measured and being absorbed, the laser receiving unit converts the received light intensity signal into an electrical signal and transmits it to the electronics module.

Images