Patent Publication Number: US-2023146441-A1

Title: Drone for measuring data representative of amounts of at least two gases present in the atmosphere away from the ground and associated method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a U.S. National Phase Application under 35 U.S.C. § 371 of International Patent Application No. PCT/EP2021/057765 filed Mar. 25, 2021, which claims priority of French Patent Application No. 20 03027 filed Mar. 27, 2020. The entire contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a drone for measuring data representative of amounts of at least two gases present in the atmosphere away from the ground, comprising:
         a chassis;   at least one propelling device able to allow the chassis to move through the atmosphere, away from the ground;   at least one sensor for measuring the representative data, said at least one sensor being borne by the chassis;   a control system for controlling the sensor for measuring the representative data, said control system being borne by the chassis.       

     BACKGROUND 
     The gases to be measured are especially greenhouse gases such as methane or carbon dioxide. 
     Preoccupations regarding protection of the environment have contributed to reinforcement of legislation on polluting emissions, especially in Europe. 
     Thus, industrial units, such as those present in the petroleum or chemical industry, must adapt to increasingly demanding environmental constraints. 
     In particular, greenhouse gases are emitted during operations of extracting, transporting, refining and storing hydrocarbons. These emissions are tracked by operators and are regularly subject to reduction measures. 
     It is in particular necessary to characterize the sources of these greenhouse gases and the amounts of greenhouse gases emitted by these sources, with a view to ensuring that they are controlled and to reporting progress made. 
     However, the techniques used to identify sources of greenhouse gases and quantify diffuse and short-lived emissions are still not entirely satisfactory. 
     Specifically, these emissions are very difficult to measure, because they are often unchanneled, and potentially located close to basins or lakes or inaccessible places, for example at height or at the centre of the unit in question. 
     To obtain enough measurements, it is therefore necessary to make a high number of gas-measurement passes over the installation. 
     To do so, it is known to use aeroplanes that fly at low altitude and that are equipped with sensors for measuring greenhouse gases. These aeroplanes make many back-and-fourth trips facing the installation with a view to taking the measurement. 
     However, such aeroplanes have the major disadvantage of also emitting greenhouse gases. Their operating cost is also very high and restrictions may exist on flying over certain installations with aeroplanes. 
     To mitigate this problem, it is possible to use drones, even though drones are not entirely satisfactory in the context of these measurements. 
     Specifically, existing drones have by nature a fairly limited range. Furthermore, their payload is low, this limiting the number of pieces of equipment located on-board, in particular with a view to carrying out multiple measurements. 
     SUMMARY 
     One aim of the invention is therefore to provide a measuring drone that has a sufficient range to perform campaigns of detection of gases the emissions of which are diffuse and short-lived, while having a measurement capacity sufficient to perform the desired analyses. 
     To this end, one subject of the invention is a drone such as mentioned above, characterized in that the sensor for measuring the representative data is able to measure data representative of amounts of at least two gases present in the atmosphere, the sensor for measuring the representative data comprising at least one measurement cell that is open to the atmosphere, and, for the or for each measurement cell, at least a first laser source able to inject, into the measurement cell, a first laser beam at a first wavelength characteristic of a first gas to be detected and a second laser source able to inject, into the measurement cell, a second laser beam at a second wavelength characteristic of a second gas to be detected, the sensor for measuring the representative data comprising a detector common to the two laser sources, said detector being able to detect a first measurement signal originating from the measurement cell and resulting from injection of the first laser beam into the measurement cell and a second measurement signal originating from the measurement cell and resulting from injection of the second laser beam into the measurement cell. 
     The drone according to the invention may comprise one or more of the following features, alone or in any technically possible combination:
         the control system is able to implement sequential and successive injections of the first laser beam into the measurement cell, then of the second laser beam into the measurement cell, without injection of the second laser beam into the measurement cell, when the first laser beam is injected into the measurement cell, and without injection of the first laser beam into the measurement cell, when the second laser beam is injected into the measurement cell, respectively;   the control system is able to selectively and sequentially activate the first laser source and the second laser source to implement the sequential and successive injections;   the measurement cell comprises two mirrors located facing and away from each other and defining therebetween a measurement cavity, and two holders bearing the two mirrors, respectively, the laser sources and the detector being joined to be holders, away from the measurement cavity;   the first laser source and the second laser source are able to inject, into the measurement cavity, a laser beam of width larger than 1 mm, and especially comprised between 3 mm and 6 mm;   at least one element among the first laser source, the second laser source and the detector is equipped with a metal heat-exchange plate that is swept by an airflow generated by the propelling device when the propelling device is activated;   the control system comprises a casing and at least one heat exchanger comprising at least one metal heat-exchange plate, and preferably a series of metal plates, the or each metal heat-exchange plate protruding from the casing and being swept by an airflow generated by the propelling device when the propelling device is activated;   it comprises a temperature-measuring sensor and pressure-measuring sensor placed in the measuring cell;   it comprises an altitude-measuring sensor borne by the chassis;   the chassis comprises a plurality of members forming an apertured framework, a first region of the chassis holding the control system, and a second region of the chassis, which second region is located away from the first region of the chassis, holding the measurement cell, the members advantageously being made of polymer, and especially of polyetheretherketone;   it comprises dampers mounted between the chassis and the measurement cell, the dampers especially being formed from spring wire;   it comprises a system for transmitting data, said system being borne by the chassis, the representative data detected by the detector being able to be transmitted by the transmitting system;   it comprises a memory for storing representative data collected by the detector, and an on-board computing unit located in the chassis and able to process the representative data collected by the detector at any given time, with a view to computing amounts of at least two gases at various times, the data-transmitting system being able to transmit the amount values computed by the computing unit;   it has a total mass lower than 10 kg, and especially lower than 8 kg;   the measurement cavity is configured to reflect multiple times the laser beams injected by the first source and by the second source;   the laser component of each of the first laser source and of the second laser source consists of a laser diode;   the measurement cell is configured to operate through direct absorption of laser light in the measurement cavity, on contact with the gases the amount of which is to be measured.       

     Another subject of the invention is a method for measuring data representative of amounts of at least two gases present in the atmosphere away from the ground, comprising:
         flying a drone such as defined above through the atmosphere away from the ground;   injecting, using the first laser source, into the measurement cell, a first laser beam at a first wavelength representative of a first gas;   detecting, using the detector common to the two laser sources, a first measurement signal originating from the measurement cell and resulting from the first laser beam injected into the measurement cell;   injecting, using the second laser source, into the measurement cell, a second laser beam at a second wavelength representative of a second gas to be detected;   detecting, using the detector common to the two laser sources, a second signal measured in the measurement cell and resulting from the second laser beam injected into the measurement cell.       

     The measuring method according to the invention may comprise the following feature:
         the control system implements sequential and successive injections of the first laser beam into the measurement cell, then of the second laser beam into the measurement cell, without injection of the second laser beam into the measurement cell, when the first laser beam is injected into the measurement cell, and without injection of the first laser beam into the measurement cell, when the second laser beam is injected into the measurement cell, respectively.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood on reading the following description, which is given merely by way of example, with reference to the appended drawings, in which: 
         FIG.  1    is a perspective view of a first drone according to the invention; 
         FIG.  2    is a view from above of the chassis of the drone bearing a measuring sensor and a control system for controlling the measuring sensor; 
         FIG.  3    is a front view of the measuring sensor illustrated in  FIG.  2   ; 
         FIG.  4    is a view of the measurement cell, during injection of a first beam; 
         FIG.  5    is a view analogous to  FIG.  4   , during injection of a second beam; 
         FIG.  6    is a view of the signal detected by the detector, successively during injection of the first beam, then during injection of the second beam. 
     
    
    
     DETAILED DESCRIPTION 
     A first drone  10  according to the invention is illustrated in  FIGS.  1  to  3   . The drone  10  is especially able to measure representative data in order to be able to compute the amounts of at least two gases present in the atmosphere through which the drone  10  is moving. 
     The representative data are for example measured facing an industrial installation, such as a petroleum-related installation, in particular an installation for extracting, transporting, refining, processing or storing hydrocarbons. 
     The gases the amount of which is measured are preferably methane and carbon dioxide. 
     In variants, other gases are measurable, such as aromatic gases (especially benzene or even butadiene), ethane, or carbon monoxide. More generally, the measured amount is that of a set of volatile organic compounds (or VOCs) with a view to determining a footprint of these compounds. 
     A gas is measurable provided that it possesses a defined spectral signature, for example in the infrared (especially for wavelengths comprised between 700 nm and 2 μm) or in the ultraviolet (especially for wavelengths between 10 nm and 380 nm). 
     The drone  10  is able to move through the atmosphere above and around the installation with a view to taking, at various points in the atmosphere above and around the installation, measurements of data representative of the amounts of at least two gases. 
     As illustrated in  FIG.  1   , the drone  10  comprises a chassis  12 , and a propelling assembly  14 , which is able to allow the chassis  12  to take off away from the ground and it to move by flying through the atmosphere above the ground. 
     The drone  10  further comprises a measuring assembly  16 , a control system  18  for controlling the measuring assembly  16 , and, advantageously, a transmitting system  20 . 
     The chassis  12  is here formed of an apertured framework, formed from members  22 . In the example shown in  FIG.  2   , the framework is of rectangular shape. It has members  22  along the sides of a rectangle, and members  22  along the diagonals of the rectangle. 
     The members  22  are for example made of polymer, in order to decrease the weight of the drone  10 . 
     The selected polymer is preferably a polymer taking solid form. 
     The polymer is for example selected from polyetheretherketone, poly(acrylonitrile butadiene styrene), poly(lactic acid), and poly(acrylonitrile styrene acrylate). 
     As illustrated in  FIG.  2   , the members  22  of the framework define a first region  24  for holding the control system  18 , and a second region  26  for holding the measuring assembly  16 , the latter region being laterally offset with respect to the first region  24 . 
     With reference to  FIG.  1   , the propelling assembly  14  comprises a plurality of propelling devices  28 , which here are propellers driven to rotate by a motor. 
     The propelling assembly  14  further comprises a power source  30 , here formed by a battery, and a system  32  for locating and for controlling the movement of the drone  10  through the atmosphere. 
     In this example, the drone  10  is a multi-rotor rotary-wing drone. It does not have any fixed wings, its lift being generated by the propelling assembly  14 . 
     The drone  10  is for example a rotary-wing quadcopter drone, and especially a DJI M200 as sold by DJI. 
     The propelling assembly  14  here comprises a plurality of propellers that rotate about substantially vertical axes. By “substantially vertical”, what is generally meant is that the axes of rotation of the propellers are inclined by less than 30° with respect to the vertical. 
     When the motors of the propellers are supplied with electrical power by the battery, the propellers are driven to rotate about their axis, generating a downward airflow, which is able to sweep in part the chassis  12  in the first region  24  and in the second region  26 . 
     The locating and controlling system  32  comprises a position sensor, especially a GPS and/or an inertial measurement unit. It further comprises a controlling unit, which is able to control the movement of the drone  10  along a path recorded before the flight and loaded into the system  32 , or remotely and manually via a remote control. 
     The drone  10  is thus able to automatically follow a predefined path, or, alternatively, to be controlled manually by an operator. 
     The measuring assembly  16  comprises a sensor  40  for measuring the data representative of the amounts of at least two gases, which is mounted on the chassis  12  advantageously via dampers  42 . It further comprises temperature- and pressure-measuring sensors  44 ,  46  and, advantageously, an altitude sensor  48 . 
     With reference to  FIGS.  2  to  5   , the sensor  40  for measuring the representative data comprises a measurement cell  50  that is open to the atmosphere, a first laser source  52 , which is able to detect a first gas, a second laser source  54 , which is intended to detect a second gas, and a common detector  56  that is intended to receive signals allowing the first gas and second gas to be detected, the signals being generated by the first source  52  and by the second source  54 , respectively. 
     The sensor  40  further comprises heat-exchange plates  58  mounted on each source  52 ,  54  and on the detector  56 , respectively. 
     The measurement cell  50  is here a single cell for measuring in the same volume data representative of the amounts of first gas and of second gas. 
     The measurement cell  50  comprises two facing holders  60 A,  60 B, and linking bars  62  that join the holders  60 A,  60 B. The measurement cell  50  further comprises facing mirrors  64 A,  64 B, which are borne by the holders  60 A,  60 B, respectively, the mirrors  64 A,  64 B defining therebetween a measurement cavity  66 . 
     In this example, the holders  60 A,  60 B are mounted parallel to each other, perpendicular to a longitudinal axis A-A′ of the measurement cavity  66 . The axis A-A′ is preferably horizontal when the drone  10  is resting on a planar horizontal holder. 
     The holders  60 A,  60 B here have a prismatic shape and a polygonal, and preferably square, exterior outline. 
     The linking bars  62  set the distance between the holders  60 A,  60 B. In this example, the linking bars  62  extend between the vertices of the polygon defining the outline of the holders  60 A,  60 B. Said linking bars extend parallel to one another, thereby defining intermediate passing spaces. 
     The measurement cavity  66  is therefore open in at least one direction, and preferably in at least two directions, between the facing holders  60 A,  60 B and between the linking bars  62 . 
     The length of the measurement cavity  66 , measured between the holders  60 A,  60 B, is for example smaller than 50 cm and especially comprised between 5 cm and 30 cm. 
     The length of the measurement cavity  66  is tailored to the range of quantities expected for the gas to be measured. For example, the measurement cavity  66  is longer if the gas is in trace amounts and/or if the response that it possesses at the measured wavelength is weak. 
     In contrast, the measurement cavity  66  is shorter if the gas to be measured is present in a relatively large amount or if its response at the measured wavelength is strong. 
     The mirrors  64 A,  64 B are each mounted on one holder  60 A,  60 B, respectively, with a view to being placed facing each other. The mirrors  64 A,  64 B are concave, with their concavities facing each other. 
     A first holder  60 A and a first mirror  64 A comprise at least two holes  68 ,  70  in order to allow a first beam originating from the first laser source  52  and a second beam originating from the second laser source  54  to be injected, respectively. 
     The second mirror  64 B facing the first mirror  64 A, and the second holder  60 B comprise a signal-extraction hole  72 , in order to allow the detector to receive a signal from the measurement cavity  66 . 
     The first laser source  52  and the second laser source  54  are mounted on one side of the first holder  60 A, outside the measurement cavity  66 , on either side of the longitudinal axis A-A′ of the cavity. 
     Each source  52 ,  54  comprises a laser component  74  and a temperature -controlling element  76 , a thermoelectric element for example. 
     The laser component  74  of the first source  52  is for example able to emit a first laser beam centred on a first wavelength λ 1 . The laser component  74  of the second source  54  is able to emit a second laser beam centred on a second wavelength λ 2  different from the wavelength λ 1 . 
     The wavelengths λ 1 , λ 2  are preferably separated, advantageously by at least 5 nm, and especially by at least 100 nm. 
     For example, to detect methane, the first source  52  is able to emit a first laser beam centred on a wavelength λ 1  comprised between 3230 nm and 3250 nm, and especially between 3238 nm and 3242 nm. To detect carbon dioxide, the second source  54  is for example able to emit a second laser beam centred on a wavelength λ 2  comprised between 1990 nm and 2020 nm, and especially between 2000 nm and 2005 nm. 
     More generally, the wavelength associated with a target module is selected depending on the spectral signature of each target module molecule and of any interfering molecules. The wavelength chosen depends on the measurement environment (pressure, temperature, concentration of target and interfering molecules, etc.). 
     The temperature-controlling element  76  is able to stabilize the temperature of the sources  52 ,  54 . 
     In the example shown in the figures, the heat-exchange plates  58  are mounted behind the first laser source  52 , the second laser source  54  and the detector  56 , in thermal contact with the temperature-controlling elements  76 . 
     The heat-exchange plates  58  are made of metal, of aluminium for example. They protrude with respect to the sources  52 ,  54 , with a view to being swept by the airflow generated by the propelling devices  28  during the rotation of the propellers. 
     Thus, the heat transferred by the temperature-controlling element  76  is removed using the heat-exchange plates  58 , without it being necessary to install an additional fan for controlling the temperature of the sources  52 ,  54  or of the detector  56 . This decreases the weight of the drone  10 . 
     The detector  56  is common to the first laser source  52  and to the second laser source  54 . It is able to detect the intensity of a signal extracted from the measurement cavity  66  at wavelengths including the wavelength λ 1  of the beam emitted by the first laser source  52  and the wavelength λ 2  of the beam emitted by the second laser source  54 . 
     Thus, the measured intensity may be related to the incident intensity using the Beer-Lambert law, which is: 
         I=I   0 exp( L.N.K ) 
     where I is the measured intensity, I 0  is the incident intensity, L is the length of the optical path travelled through the measurement cell  50 , N is the number of molecules of the studied gas on the path and K is the absorption coefficient of this gas. 
     Use of a detector  56  common to the two laser sources  52 ,  54  decreases the number of components present in the measuring sensor  40 , this substantially decreasing the weight of the sensor  40  and allowing other sensors and/or instruments to be incorporated into the drone  10 , or its mass to be decreased. 
     The common detector  56  comprises a single detecting device that is sensitive both to the wavelength λ 1  of the beam emitted by the first laser source  52  and to the wavelength λ 2  of the beam emitted by the second laser source  54 . 
     The common detector  56  is formed of a single component, for example one sold by Judson (http://www.teledynejudson.com/), Vigo (https://vigo.com.pl/en/products-vigo/) or Hamamatsu (https://www.hamamatsu.com). 
     The sensor  40  for measuring the representative data advantageously employs a single detector  56  to measure the intensities resulting from the signal output by the measurement cavity  56  for each of the laser sources  52 ,  54 . 
     The dampers  42 , when they are present, comprise spring wires  80  connecting the chassis  12  to each of the holders  60 A,  60 B. These spring wires  80  are able to partially absorb the vibrations of the propelling assembly  14  and of the movement of the drone  10  through the air. 
     The temperature-measuring sensor  44  is placed between the facing holders  60 A,  60 B. The sensor  44  is for example a thermistor, or a thermocouple, able to measure an electrical resistance of a metal element that varies as a function of temperature. 
     The pressure-measuring sensor  46  for example comprises a pressure-measuring tube that opens into the measurement cavity  66 . 
     The presence of a temperature-measuring sensor  44  and of a pressure-measuring sensor  46  directly within the measurement cell  50 , and preferably in the measurement cavity  66 , increases the reliability of the collected data, in particular on account of the low concentration of the gases to be measured in the measurement cavity  66 . 
     The altitude sensor  48 , when it is present, comprises an altimeter, which is for example equipped with a laser pointing towards the ground so as to measure the height at which the drone  10  is located. 
     The control system  18  comprises a unit  90  for supplying electrical power to each of these sources  52 ,  54  selectively, a unit  92  for collecting data measured by the detector  56  and at least one heat exchanger  94 , which is able to remove the heat generated by the units  90 ,  92  without specific fan-assistance. These units are housed in a casing  96 . 
     The unit  90  for supplying electrical power is able to selectively and successively supply power to the first laser source  52  and to the second laser source  54  with a view to obtaining a first phase of illumination of the measurement cavity  66  exclusively by the first laser source  52 , without illumination by another laser source such as the second laser source  54 , then a second phase of illumination of the measurement cavity  66  exclusively by the second laser source  54 , without illumination by another laser source, and in particular by the first laser source  52 . 
     Thus, successive phases of measurement of data representative of the first amount of a first gas, and of the second amount of a second gas, may be carried out in the same measurement cavity  66  of the measurement cell  50 , these data being selectively collected by the same detector  56 , without interference between the obtained signals. 
     These representative data form spectra of light intensity as a function of wavelength, such spectra being schematically shown in  FIG.  6   . 
     The unit  90  for supplying electrical power is for example connected to the power source  30  of the propelling device  28 , or possesses its own power source. 
     The data-collecting unit  92  comprises at least one memory, able to store the spectra of light intensity as a function of wavelength measured at various times by the detector  56 . 
     The data are for example stored at a frequency higher than 10 Hz, and in particular comprised between 1 Hz and 100 Hz. The stored spectra preferably comprise a number of points higher than 256, and for example are comprised between 256 and 4096 points. 
     Thus, a very good resolution is obtained as regards determination of the intensity of the peaks measured in the measurement cell  50  as a function of wavelength, this allowing the amounts of the two gases to be deduced therefrom, even if these amounts are very small. 
     The data-collecting unit  92  is connected to the transmitting system  20  with a view to allowing data to be exported to a receiving station on the ground, during the flight of the drone, at a frequency that may be lower than the acquisition frequency, and for example comprised between 1 Hz and 5 Hz. 
     The heat exchanger  94  makes thermal contact with each of the power-supplying unit  90  and data-collecting unit  92 . It is able to remove the heat produced by these units  90 ,  92  to plates  98  that protrude out of the casing  96  containing the units  90 ,  92 . 
     The plates  98  are able to be swept by the airflow generated by the propelling devices  28 , with a view to removal of the heat produced by the units  90 ,  92 . Thus, no fan is required in the casing  96  to cool the units  90 ,  92 , this decreasing the weight and electrical power consumption of the drone  10 . 
     The transmitting system  20  comprises a transmitter, able to transmit data to a ground station, these data for example being the data collected by the unit  92  or a fraction of these data. 
     A method for measuring the amounts of at least two gases present in the atmosphere, preferably facing an industrial installation, will now be described. 
     Initially, the drone  10  is made to take flight. The propelling devices  28  are activated by the locating and controlling system  32  in order to allow the drone  10  to take off, and it to move towards the zone in which the measurements must be taken. 
     The propellers of the propelling assembly  14  generate a lifting force. The locating and controlling system  32  controls the movement of the drone  10 , either under the effect of manual remote control, or by following an automatic program loaded into the system  32 . 
     During the movement of the drone  10 , measurements are taken. To this end, the sensor  40  for measuring representative data, the temperature-measuring sensor  44 , the pressure-measuring sensor  46  and optionally the altitude sensor  48  when it is present, are activated. 
     Measurements are taken by the various sensors  40 ,  44 ,  46 ,  48  during the movement of the drone  10 , without having to immobilize the drone  10 . To this end, the unit  90  for supplying electrical power selectively and successively supplies power to the first laser source  52 , then to the second laser source  54 . 
     In each phase of activation of the first laser source  52 , the laser component  74  of the second laser source  54  is deactivated. The laser component  74  of the first laser source  52  emits a first laser beam at the wavelength λ 1 , which is injected via the injection hole  68  into the measurement cavity  66 . 
     As indicated above, the thickness of the first laser beam is larger than 1 mm, and especially comprised between 3 mm and 6 nm. This makes it possible to avoid measurement artefacts that might otherwise be created by particles in suspension in the measurement cavity  66 . 
     In order to increase the length of the optical path L, the first laser beam is successively reflected by the mirrors  64 A,  64 B and makes back-and-forth trips through the measurement cavity  66 . 
     As illustrated in  FIG.  6   , by curve (a), a first signal, which results from the first beam emitted by the first source  52 , is collected through the sampling hole  72 . 
     This first signal is detected by the detector  56 , and the data detected by the detector  56  are sent to the data-collecting unit  92  with a view to being stored. 
     Next, in each phase of activation of the second laser source  54 , the laser component  74  of the first laser source  52  is deactivated. The laser component  74  of the second source  54  emits a second laser beam at the wavelength λ 2 , which is different from the wavelength λ 1 . This laser beam is introduced, via the injection hole  70 , into the measurement cavity  66 . 
     As above, the thickness of the second laser beam is larger than 1 mm, and especially comprised between 3 mm and 6 mm. 
     In order to increase the length of the optical path L, the second laser beam is successively reflected by the mirrors  64 A,  64 B and makes back-and-forth trips through the measurement cavity  66 . 
     As illustrated in  FIG.  6   , by curve (b), a second signal, which results from the second beam thus emitted by the second source  54 , is collected through the sampling hole  72 . 
     This second signal is detected by the same detector  56  that detected the first signal, and the data detected by the detector  56  are sent to the data-collecting unit  92  with a view to being stored. 
     Since the measurements are taken successively in the first phase and in the second phase, using the same detector  56 , they allow light intensities, at two wavelengths λ 1 , λ 2 , that are representative of the amount of a first gas and of the amount of a second gas, respectively, to be determined. 
     When the drone  10  has finished its mission and returns to the ground, the spectra measured by the detector  56  and stored in the memory of the data-collecting unit  92  are transmitted by the transmitting system  20  to a monitoring station. 
     In the monitoring station, these spectra are stored in association with data on the geographical position of the drone  10  (which data were measured by the locating and controlling system  32 ), with the temperature and pressure measured by the sensors  44 ,  46 , optionally with the altitude measured by the altitude sensor  48  and with the time at which each measurement was taken. 
     The drone  10  according to the invention is therefore particularly compact and light. It nevertheless allows precise and reliable data to be obtained, allowing at least two amounts of two gases present in the atmosphere to be deduced, in difficult environments, and for example in the vicinity of industrial installations, by virtue of the presence of a single detector  56  associated with at least two different laser sources  52 ,  54 . 
     It is thus possible to measure, in the same measurement cell  50 , almost simultaneously, data representative of the amounts of each of the two gases, selectively and practically. 
     In one variant, the drone  10  comprises an on-board computing unit  100  in the chassis  12 . The computing unit  100  is able to process the data collected by the detector  56  at any given time, and in particular the light-intensity spectra measured at any given time, with a view to computing amounts of at least two gases at various times, on the basis of the representative data collected by the detector  56  and of a prior calibration. 
     The data-transmitting system  20  is then able to transmit the amount values computed by the computing unit  100 , instead of the spectra of light-intensity data, this decreasing the amount of data to be transmitted in real time and allowing more measurements of amounts of the two gases to be obtained in real time. 
     In another variant, the drone  10  comprises a plurality of measurement cells  50 , of structure analogous to the measurement cell  50  described above, each being dedicated to detection of at least two different gases. 
     In the example that has just been described, each laser component of the first laser source  52  and second laser source  54  is for example a laser diode. A laser diode is an optoelectronic component that is produced based on semiconductors. It emits coherent monochromatic light. 
     It is for example formed of a semiconductor junction, which possesses three features: an n-type confinement layer, an active region and a p-type confinement layer. The diode is for example a distributed-feedback laser diode. 
     As indicated above, the measurement cell  50  employs direct absorption of laser light in the measurement cavity  66 , on contact with the gases the amount of which is to be measured. It is therefore a question of a measurement cell  50  for performing direct laser absorption spectroscopy. 
     The measurement cavity  66  allows the injected laser beams from the first source  52  or from the second source  54  to be reflected multiple times, with a view to increasing the length of the optical path. The measurement cell is thus a multi-pass spectroscopy cell, or Herriott cell.