INTRODUCTION

LIDAR (Light Detection And Ranging) is an optical device for measuring the distance to an object and/or for studying the dependence of the scattering coefficient or reflection of the medium along the sounding path (Measures, 1984). A pulsed or modulated laser is used as a transmitter in the devices, and a sensitive detector is used in the receiving part with the ability to record the temporal shape of the reflected signal. The delay time and the amplitude of the probing radiation reflected from the obstacle or scattered by the medium is useful information. More advanced lidar systems use multiple sensing wavelengths and sophisticated multichannel optical receivers to distinguish between the spectral and polarization properties of the reflected radiation. This provides additional information about the density, gaseous composition of the atmosphere, wind profile, temperature distribution, and other physical properties of the medium along the sounding path.

Lidars have been used twice to explore Mars. The Mars Orbiter Laser Altimeter (MOLA) on the Mars Global Surveyor (MGS; NASA, 1996–2006) (Zuber, 1992) consisted of a 1064-nm Nd:YAG laser and a 50‑cm receiving telescope with an avalanche photodiode as detector. Based on the MOLA data, the global altimetry of Mars was performed (Smith et al., 1998; 1999), in particular, it was found for the first time that the northern hemisphere of Mars is lower than the southern one relative to the equatorial plane. The receiving part of MOLA was not designed to study backscattering by the atmosphere; nevertheless, it was possible to obtain data on clouds and haze (Ivanov and Muhlemann, 1998).

The surface atmospheric lidar, which was part of the meteorological complex of the Phoenix lander (NASA, 2007–2008), also operated on a pulsed Nd:YAG laser at two wavelengths (1064 and 532 nm). The receiving part included a telescope with an entrance pupil of 10 cm and two detectors: an avalanche photodiode for the 1064-nm channel and a photomultiplier for 532 nm. Depending on the aerosol content, this instrument made it possible to probe the atmosphere up to altitudes of 15 km (Whiteway et al., 2008). Using the Phoenix lidar, the polar atmosphere of Mars was studied for five months during the northern summer season. The landing site was at 68.22° north latitude. During this period (Ls 97°–150°), the sublimation of the northern polar cap developed and faded, and in the experiment, in addition to dust, it was possible to observe the features of the atmospheric water cycle, the formation of condensation clouds, precipitation, etc. (Whiteway et al., 2009). Experimental data on the planetary boundary layer, dust, and condensation clouds continue to be widely used (see, for example, Hinson et al., 2022; Tamppari and Lemmon, 2020; Moores et al., 2011; Daerden et al., 2010; 2015).

In this paper we present a miniature lidar based on a semiconductor pulsed laser for solving atmospheric problems from the stationary landing platform ExoMars-2022 (Vago et al., 2015a). The prototype of the device was a lidar (Arumov et al., 1999; Bukharin et al., 1998; Linkin et al., 2004), developed at the Space Research Institute (IKI) of the Russian Academy of Sciences under the supervision of V.M. Linkin for the NASA Mars Polar Lander project (MPL or Mars Surveyor 98 Lander). The basis of this development, in turn, was a portable lidar for monitoring the environment (Pershin et al., 1992; 1993; Pershin, 1995). The MPL landing in 1999 in the southern polar region of Mars ended in failure. For the next version of the NASA Phoenix polar platform, the Agence Spatiale Canadienne (ASC) meteorological complex, which includes a larger lidar, was chosen (Whiteway et al., 2008). The Russian device was excluded.

The lidar device is part of the meteorological complex (MTC) of the ExoMars-2022 landing platform and has the abbreviation ASU MTK. The flight model of the device was manufactured, tested and calibrated in the IKI RAS and has been at the spacecraft manufacturer since November 2019. The article considers the scientific tasks of the experiment, the program of measurements on the surface of Mars and describes in detail the components of the equipment and the features of the device.

SCIENTIFIC TASKS

The main scientific tasks solved with the help of lidar are as follows: measuring the optical properties of the atmospheric aerosol of Mars, the density of the cloud layer, detecting and detailing the fine structure of cloud layers and haze of the surface layer of the atmosphere. An instrument of this kind is supposed to be used in the equatorial latitudes of Mars (Vago et al., 2015b) for the first time. Lidar promises to provide especially valuable data for the study of the planetary boundary layer (Petrosyan et al., 2011; Read et al., 2017). The structure and dynamics of profiles of both dust and condensation aerosols in the near-surface layer will provide unique information about this still poorly studied region of the Martian atmosphere (Daerden et al., 2010; Dickinson et al., 2010, 2011). To understand the water and dust cycles of the Martian atmosphere, it is important to study the structure of clouds on different time and space scales and observe the diurnal and seasonal dynamics of atmospheric aerosol (see, e.g., Daerden et al., 2015; Moores et al., 2011). Long-term regular observations may shed light on the processes in the atmosphere during the passage of the front of a dust storm. Regular measurements during the day under various seasons and weather conditions will make it possible to elucidate the regularities of the cycle of water vapor and mineral aerosol in the near-surface planetary boundary layer. Active sounding will significantly expand the altitude range of the Martian atmosphere being studied.

The main measurable quantity is the atmospheric backscatter coefficient. Its profile is measured at a distance of 10 m to 1.5 km during the day or 10 km at night with a spatial resolution of 7.5 or 15 m. Measurements of the backscatter coefficient make it possible, subject to additional information, to refine such aerosol properties as the size, composition, shape, and concentration of particles (Davy et al., 2009; Komguem et al., 2013).

In addition, in the passive mode of measuring the background illumination, the lidar can operate as a precision photometer. In this case, quasi-continuous measurements of solar radiation scattered by the Martian atmosphere are made in a narrow spectral range and in a narrow solid angle (2 mrad). The frequency of such measurements can be set up to hundreds of hertz, which will make it possible to detect intensity fluctuations, as a rule, associated with fast nonstationary processes in the atmosphere. This option, based on the statistical analysis of the received signal and in combination with pressure measurements (also with a high time resolution), may make it possible to determine the parameters of daytime atmospheric turbulence and find out its relationship with dust activity (Kurgansky, 2018; Mason and Smith, 2021; Spiga, 2021).

Of great interest is the comparison of lidar data with data from other instruments of the meteorological complex and the landing platform as a whole, primarily with measurements of pressure, temperature, wind speed and direction. Features of the formation of condensation clouds and turbulence parameters can be traced by comparing lidar data with temperature profiles from the FAST thermal infrared Fourier spectrometer on board the landing platform. Of particular interest is the connection of lidar data with instruments that continuously measure the optical transparency of the Martian atmosphere (ODS, SIS) (Toledo et al. 2016; Arruego et al. 2017). With their help, it will be possible to track the temporal dynamics of cloudiness during the day, and, probably, to detect links with vertical wind flows. Data from the instruments of the dust complex (PC) of the lander (Zakharov et al., 2022), in particular, the MicroMED nephelometer (Scaccabarozzi et al., 2018), will make it possible to supplement the lidar data in terms of the properties of Martian dust particles.

PERFORMANCE CHARACTERISTICS AND GENERAL STRUCTURE

The lidar is implemented according to the bistatic optical scheme, i.e., its transmitting and receiving paths are separated. The development is based on the use of a relatively low-power laser emitter with a high probing pulse repetition rate and a photodetector operating in the photon counting mode. The probability of registering a weak backscattering signal from a single probing pulse is low. An increase in the recorded level of such a signal is achieved by accumulating single quantum events up to obtaining the required signal-to-noise ratio. The total duration of signal accumulation can be from several seconds to ten minutes at a repetition rate of probing pulses up to several kilohertz. The main characteristics of the lidar are given in Table 1; its general view and block diagram are shown in Figs. 1 and 2.

Table 1. Main characteristics of the lidar
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Fig. 1.
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General view of the lidar device.

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Fig. 2.
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Block diagram of the lidar.

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The lidar, the aerosol sensor unit of the meteorological complex (ASU MTK), is part of the ExoMars-22 landing platform. The device is assembled as a monoblock. The power supply (5 V), reception of commands, and transmission of scientific and service information are implemented through the upper atmosphere sensor block of the meteorological complex (UASU MTK). The UASU block is connected to the systems of the landing platform through a unit for collecting scientific information and controlling scientific equipment. The device includes the following main components: a receiving–transmitting path module, an optical–mechanical module with electronics (with transmitting and receiving objectives), a control module, a thermal stabilization module, a power supply module, and laser and photodiode modules. All nodes are controlled by the control module. These modules are detailed below.

OPTICAL MODULE

For the maximum efficiency of the lidar, the spot (cross-section) of the laser beam must completely fall into the field of view of the receiving lens, starting from a certain distance from the device. In other words, the angle of view of the lens must be greater than or equal to the divergence angle of the laser beam. At the same time, the smaller the angle of view of the receiving lens and the larger the size of the receiving aperture, the higher the signal-to-noise ratio when the device is operating in conditions of external, background illumination.

The lens module has transmitting and receiving optical paths. The optical scheme of the node is shown in Fig. 3.

Fig. 3.
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Optical layout of the lidar: (1) laser, (2) laser emitter module, (3) transmitter and receiver lenses, (4) receiving channel aperture 20 × 190 μm, (5) interference filter, (6) protective window and cut-off light filter made of colored glass, (7) photodiode module, and (8) photodiode.

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The optical unit is assembled in a single housing, which ensures high temporal and temperature stability of the optical characteristics of the module. The direction of the optical axes O1 and O2 coincides with an accuracy no worse than 0.1 mrad. The design of the optical assembly provides for mutual adjustment of the transmitting and receiving lenses in order to ensure the alignment of the lenses with an accuracy of ±0.1 mrad in two mutually orthogonal directions. The diameters of the receiving and transmitting objective, together with the structural elements, are 30 mm for the transmitting objective and 26 mm for the receiving one. The laser radiation is collected by the transmitting lens into a parallel beam with a divergence of about 2 mrad. Due to the fact that the laser diode has a rectangular radiation zone of 10 × 160 μm (the spot in the far zone has the form of a strongly elongated ellipse), a slit diaphragm with similar dimensions is installed in the receiving path close to the focal plane. This provides the best signal-to-noise ratio of the received radiation. For the same purpose, a narrow-band interference filter is installed in the receiving path, which cuts off the background light entering the receiving path in the rest of the spectral range.

LASER MODULE

The laser module includes a semiconductor laser diode in a housing (hereinafter abbreviated laser), laser thermal stabilization systems, a current driver, a charge circuit and a laser capacitor discharge key. The task of the node upon arrival of the start pulse (Start) is to generate one laser pulse with a power of up to 50 W. The laser parameters are given in Table 2. A block diagram of the node is shown in Fig. 4.

Table 2.   Laser parameters at 22°C
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Fig. 4.
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Block diagram of the laser module.

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Stabilization of the laser wavelength is necessary to match the position of the emission line with the bandwidth of the interference filter. The wavelength is determined by the temperature of the laser. The diode temperature is stabilized by a thermostat based on an external Peltier element and a temperature sensor. The temperature reference value is selected taking into account the characteristics of the laser and the operating temperature range of the instrument. The working temperature lies in the range from plus 15°С to plus 20°С and is stabilized with an accuracy of ±0.01°С. The thermostat works for both cooling and heating. The current through the Peltier element is controlled by a pulse-width modulation (PWM) chip. The maximum current is 1.5 A, which is sufficient to stabilize the laser temperature in the operating temperature range.

To start the laser diode, a pump capacitor is used, which is discharged through the laser through a key based on an avalanche transistor. The current driver generates an avalanche transistor turn-on pulse based on the START laser trigger pulse. In order for the avalanche transistor to enter breakdown mode, a current of 200 mA must be reached. At the moment of discharge, the current through the laser reaches 20 A and a light pulse is emitted with a pulse power of up to 50 W and a duration of 15 ns. During the pause between the emitted pulses, the laser discharge capacitor is charged through a resistor from a 160 V power source with a current of ~5 mA. The selected parameters provide a laser trigger frequency of 2 kHz.

PHOTODIODE MODULE

Avalanche photodiodes operating in the photon counting mode make it possible to effectively record weak pulsed signals without the use of precision amplifiers and high-speed analog-to-digital converters (ADCs). They allow you to work at the level of extremely low radiation fluxes and give a signal in digital form. Intensity measurement is achieved by accumulating counts with repeated repetition of probing pulses from pulsed lasers capable of operating at frequencies up to tens of kilohertz (in our case, 2 kHz). A large number of events that increase the statistical accuracy can significantly reduce the requirements for parameter instability, jitter of the fronts of electronic timing circuits.

The module includes an avalanche photodetector, which is located in the focal plane of the receiving lens, a thermostat on the built-in Peltier and a temperature sensor, a trigger for turning on the time window of the photodetector in the photon counting mode, and a single photon reception signal shaper. A silicon avalanche photodiode operating in the single photon counting mode SPAD SAP500T6 (Single-Photon Avalanche Diode) is used as a photodetector. Avalanche diodes have a sealed housing and a built-in Peltier cooler. Characteristics of the avalanche diode are given in Table 3, and the block diagram of the node is in Fig. 5.

Table 3.   Avalanche diode characteristics at 22°C
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Fig. 5.
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Block diagram of the photodetector module.

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The photodiode thermostat consists of a built-in Peltier and a temperature sensor and is similar to a laser thermostat. The thermal control module maintains the temperature of the photodiode at 15°C with an accuracy of 0.5°C.

The blocking voltage of the photodiode is set to about 135 V, 2 V above the avalanche threshold. For each photodiode, the response threshold is selected individually when setting up the device. The blocking voltage is maintained with an accuracy no worse than ±20 mV with a load current of 0.5 mA. The photodiode is driven by a time window trigger. In the initial state, the photodiode is energized below the avalanche threshold by 1.3 V (avalanche extinction phase). The diode is switched on to the photon counting mode when the “GATE” signal arrives at the trigger. The trigger generates a boost pulse, setting the operating voltage above the avalanche breakdown threshold.

The duration of the GATE signal can take two values, 50 and 100 ns, during which time the light travels 15 and 30 m, respectively, which corresponds to a spatial resolution of 7.5 and 15 m. If at least one photon arrives at the photodiode during the GATE time window, an avalanche current will occur, upon the appearance of which the trigger switches to the avalanche suppression mode with a short delay. In this case, a photon arrival signal is generated and transmitted to the control module. If no photon hits the photodetector, the photodetector is switched to blanking mode at the end of the GATE signal.

CONTROL MODULE

The module is intended for control and monitoring of all modules of the device, provides for the collection and processing of accumulated information with its subsequent transfer to the MTK control unit. The module includes a time to fly counter assembled on an FPGA and a microcontroller. The time to fly counter is designed to start the cyclogram of the modules operation and form the time delays and duration of the reception window with subsequent summation of the received signals for each window as a function of the range distribution.

The time to fly counter also includes a time-to-code converter. The parameters of event discretization by time and level can be changed by commands from the microcontroller. The received histograms are read by the controller into the device memory, where they are processed, packed into frames and transferred to the MTK control unit. Technical characteristics of the time to fly counter are presented in Table 4, and its block diagram is in Fig. 6.

Table 4.   Specifications of the time-interval meter
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Fig. 6.
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Block diagram of the time to fly counter: ADC—digital-to-analog converter; MDC—module of discrete comparison; FPGA—time interval meter; M1, M2—memory blocks; Σ1—preadder; Σ2—main adder; microcontroller; RS422—computer port; and EEPROM—reprogrammable memory.

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The microcontroller controls the FPGA via the SPI port. A frequency of 100 MHz is supplied from the crystal oscillator of the microcontroller to the FPGA, on the basis of which the frequency grids necessary for the operation of the discrete comparison module, laser module, and photodiode module are formed. On the “Start” command, a cyclogram is started from the microcontroller, according to which 64 Start pulses are fed to the laser with a given frequency. In the pause between laser pulses, the synthesizer generates four sequences of 256 GATE pulses, shifted in time relative to each other by 25 ns, and which are fed to the photodiode. The duration of GATE is 50 ns. The time delay between pulses is 100 ns, respectively. The number of these pulses can be changed by command from the microcontroller from 256 to 4096. For each GATE pulse, memory cells are allocated in the FPGA memory with a summation depth from 256 bits to 64 Kbps. If a STOP pulse arrives during the GATE time window, it is added to the appropriate FPGA memory location. The depth of the memory cells is set by command from the microcontroller. A time window duration of 50 ns corresponds to a spatial length of 7.5 m. Accordingly, each memory cell accumulates photons reflected by the medium in the field of view of the device from a volume (truncated cone) 7.5 m long.

After the completion of hardware accumulation, the histogram is read into RAM for subsequent software data processing. The size of the histogram data and, accordingly, the transfer time to the RAM depend on the ratio of the value of the set time window to the value of the histogram channel.

The board also has an analog-to-digital converter and a multiplexer for interrogating control signals with subsequent recording in memory. From the received data, the microcontroller generates packets for transmission through the RS-422 port. The packets are read in response to requests from the UASU MTK control unit.

THERMAL CONTROL MODULE

The thermal control module, or thermal stabilization, consists of two identical channels, each of which includes a bridge circuit with amplifiers and a Peltier control circuit based on pulse-width modulation of the current. The block diagram of the module is shown in Fig. 7.

Fig. 7.
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Block diagram of the thermal stabilization module.

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The temperature sensor of the laser or photodiode is included in one arm of the bridge circuit. There is an adjustable divider in the other leg of the bridge. By adjusting this leg, the voltage corresponding to the selected stabilization temperature is set. The bridge is powered by a highly stable voltage of 4 V (REF). The signal difference between the two dividers is amplified and fed to the Peltier control circuit. Depending on the temperature, the bridge is unbalanced and a negative or positive signal appears at the amplifier outputs, which is converted into a PWM Peltier control current. Also, the difference signal enters the control module as a control parameter.

POWER MODULE

The power module generates the necessary voltage levels for all instrument systems from the 5 V power rail. The module includes converters for 160, 3.3, 2.5 V, a 4-V reference voltage, as well as a 4.3-V linear regulator and a 135-V high-stability voltage regulator. A block diagram of the module is shown in Fig. 8.

Fig. 8.
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Block diagram of the power supply module of the device.

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The low-voltage 4.3-V converter is assembled on a linear stabilizer; the reference voltage on a highly stable stabilizer. For an output voltage of 160 V, a DC/DC converter is used. The output voltage of 135 V (adjustable from 120 to 150 V) is obtained by pulling down 160 V. High stability and regulation of the buck converter are provided by the DAC and amplifiers included in the return circuit. The voltage level is set by command from the controller.

INSTRUMENT OPERATION PROGRAM ON THE SURFACE OF MARS

The device has three standard operating modes:

(1) Day mode.

(2) Night mode.

(3) Passive mode.

The operation of the device is tied to the general cyclogram of the operation of the meteorological complex of the ExoMars-2022 lander. The control is carried out from the UASU MTK unit, which forms the time diagram of the lidar inclusions in the daytime and at night. The parameters of the period and duration of the basic scenario of the device operation are given in Tables 5, 6, and 7. If necessary, the time sequence of switching on can be changed by commands from the Earth; similarly, all parameters of the device operating modes can be changed. The device is switched on for the first time after the parachute is fired during the landing of the device on the surface of Mars, which continues until the moment it touches the surface. The frequency of inclusions at the initial stage is associated with limited possibilities for transmitting information from the lander to the Earth. Therefore, during the descent, the device is switched on after the parachute is released and operates until landing with the parameters of the day mode (see Table 5). In the future, when operating on the surface, the device operates according to the time cyclogram with the parameters given in Tables 5–7.

Table 5.   Mode 1, day
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Table 6.   Mode 2, night
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Table 7.   Mode 3, Photometric (Passive)
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Each mode has the following parameters: range, number of laser activations during measurement, number of starts (strobes) of the photodetector for accumulation in the range cell. The day mode includes obtaining an aerosol distribution profile at a distance of up to 1500 m, and the measurement range is limited by the background illumination noise in the receiving channel. The night mode includes obtaining an aerosol distribution profile at a distance of up to 2000 m and detecting a possible cloud layer up to 10 km and is limited by the inherent noise of the photodetector. The passive mode is activated each time before the active mode, three measurements with a duration of 1 s in succession.

The ability to correct parameters and time sequences will make it possible to adaptively correct the operation of the instrument based on the first data obtained from the surface of Mars.

INSTRUMENT CALIBRATION

The main lidar equation for calculating the energy of the received signal by the lidar photodetector has the form (Measures, 1984):

$$P\left( {\lambda ,R} \right) = \,\,~{{E}_{{\text{L}}}}\xi (\lambda )T\left( R \right)K\left( R \right)\frac{{{{A}_{0}}}}{{{{R}^{2}}}}\beta \left( R \right)\frac{{c{{T}_{{\text{g}}}}}}{2},$$
(1)

where ЕL is the energy of the laser pulse; G(R) is the normalized function of overlapping the FOV and the laser beam (geometric form factor); A0 is the area of intrinsic pupils; Tg is the photodetector response time, strobe duration; c is the speed of light; K is the transmission of the optical path of the receiver; T(R) is the transmittance along the sounding path; and β(R) is the 180° backscatter coefficient (βπ).

For a lidar operating in the photon counting mode, Eq. (1) can be written as a dependence of the number of photons arriving at the photodetector:

$$P\left( r \right) = \frac{{{{E}_{l}}}}{E}KG\left( R \right)T\left( R \right)\frac{{{{A}_{0}}}}{{{{R}^{2}}}}\beta \left( R \right)\frac{{c{{T}_{{\text{g}}}}}}{2},$$
(2)

where Е = hν is the energy of one quantum.

For a lidar, the directly measured quantity is the number of registered photocounts Ni during N tests (switching on the recording strobe). The relationship between the number Ni and the number of photons Pi (P(R) is replaced for the ith cell corresponding to the range R) is determined by the formula:

$${{P}_{i}} = - \frac{1}{\eta }\ln \left( {1 - \frac{{{{N}_{i}}}}{N}} \right),$$
(3)

where \(~\eta ~\) is the quantum efficiency.

It should be understood that Pi is a statistical quantity that determines the energy, expressed in units of photons, recorded by a photodetector, and may be less than unity. Moreover, for optimal operation, the value Piη should be less than unity to prevent saturation of the photodetector.

Note that for a lidar operating in a medium without selective absorption, as well as in a weakly scattering medium, where absorption by aerosol particles is not taken into account, the transmission coefficient T(R) = 1.

The purpose of lidar calibration is to obtain a correspondence between the measured number of photocounts and the value of the back reflection coefficient of the medium β in the form of a certain coefficient. The value of such a coefficient can be obtained both by calculation, by computer simulation under substituting the passport values of the elements, and by full-scale calibration.

EXPERIMENTAL LIDAR CALIBRATION

At the first stage of the calibrations, the form factor was measured along the lidar sounding path and the overlap function G(R) was measured. The calibration consisted in measuring the magnitude of the lidar signal during the scattering of the probing pulse by the surface of the topographic target, depending on the distance to the target. The measurements were carried out on a 100-m path in the IKI RAS building. To avoid saturation, attenuators (neutral optical filters) were placed in front of the lidar.

The normalized experimental dependence is shown in Fig. 9 together with a similar dependence determined by numerical simulation. The simulation was carried out according to the measurement data of the intensity distribution in the cross section of the laser beam and the measured sensitivity over the FOV of the receiving objective. It should be noted that the experimental dependence is in good agreement with the numerical simulation data. The experimental curve does not cover the entire operating range of the lidar. Therefore, it is assumed that appropriate approximation or numerical simulation data will be used to process lidar data.

Fig. 9.
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Normalized experimental dependence of the lidar overlap function G(R).

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Dependence G(R) is necessary to correct the signal in the near zone in the range from 10 to 30 m and to confirm the accuracy of alignment of the laser beam and the receiver FOV.

Diffuse-reflecting targets with a known albedo value were used to obtain an experimental calibration coefficient of the relationship between the backscattering coefficient and the measured signal. The measurements were carried out on a horizontal track in the IKI RAS building. The targets were installed in the far zone of the sounding path at a distance of 40–70 m from the lidar. To prevent saturation of the measured signal, neutral attenuating optical filters with certified absorption coefficients were installed in front of the lidar. The filter aperture overlapped simultaneously the transmitting and receiving channels in order to compensate the influence of the filter surfaces non-flatness on the mutual orientation of the optical axes of the lidar channels. For a diffusely reflecting target, the reflection coefficient along the normal was calculated in the approximation of the Lambert law. The values of such coefficients for the different targets used were in the range from 0.2 to 0.3 for the operating wavelength of the lidar. The measurements carried out made it possible to determine the conversion factor of units of the ADC to units of the reflection coefficient of a solid target that simulated the backscattering coefficient of the medium. In the course of measurements for an attenuation coefficient of 2 × 10–6 at a distance of 50 m, the detectivity of the lidar was 4 × 10–7 1/m/sr when the measured signal exceeded the level of intrinsic noise by 3σ (in the absence of background illumination and the minimum time of signal accumulation).

CONFIRMATION OF THE FUNCTIONALITY OF THE DEVICE ON REAL TRACKS

To test the operation of the device in real conditions, measurements of urban aerosol were carried out on an inclined path in the presence of weak cloudiness. An example of the measured aerosol distribution for a clean atmosphere is shown in Fig. 10.

Fig. 10.
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Aerosol distributions on an inclined path (70° from the horizon) under conditions of a clear atmosphere and light, variable cloudiness.

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To estimate the value of the urban aerosol back reflection coefficient, the readings of the weather station at Vnukovo Airport during the measurement period from 18:00 to 20:00 on October 15, 2019, were used as the nearest measurement point and the data from the website www.accuweather.com. Data on the height of the cloud layer boundary, cloud cover thickness, and meteorological visibility range (MVR) were used. For the Vnukovo meteorological station, the readings according to the MVR are more than 10 km, the height of the cloud layer is 650 m, the cloudiness is 6 points, and according to the website https://www.accuweather.com/, the MVR indication is 52 km.

For numerical estimation of the backscattering coefficient in the near zone, the MVR value equal to 52 km was used, which corresponds to the scattering coefficient βsc = 7.8 × 10–5 m–1 (McCartney, 1979). The coefficient βsc is an integral value for all scattering angles, and the calculation of the backscatter coefficient measured by a lidar is possible only with accurate data on the physical properties of the aerosol. To confirm the functionality of the device, one can use an estimate of the relationship between the backscattering coefficient βπ and the MVR (or scattering coefficient) for coastal atmospheric hazes given in the literature (Kaloshin et al., 1994). Accordingly, the measurements showed that the developed lidar is capable of detecting aerosol with a reflection coefficient βπ ≈ 2 × 10–5 m–1 sr–1 at a distance of at least 200 m under conditions of weak background illumination.

Also, to confirm the functionality of the device, measurements were taken to the lower cloud cover with a decrease in the slope of the path. The maximum range of the sloping path was limited by urban areas and allowed measuring the dynamics and structure of the cloud layer at a distance of at least 1200 m.

The experimental results confirmed the requirements for the device for its intended purpose. Unfortunately, due to severe time and financial constraints during the development of the device, it was not possible to carry out cross-calibration with lidars of other developers and with reference aerosols in the aerosol chambers of NPO Typhoon.

CONCLUSIONS

A miniature lidar based on a semiconductor pulsed laser, developed for the landing platform of the European Space Agency (ESA) and Roscosmos ExoMars-2022, has been manufactured and passed all the necessary test tests. Its high characteristics are confirmed by laboratory and full-scale calibrations. After the completion of the joint ESA-Roscosmos program, the device can be used in the national Mars exploration program using the landing platform. Its low mass (1 kg, compared to 6 kg of the Phoenix lidar) increases the chances of the device being installed on other research facilities on the surface of Mars, increasing the amount of information about the boundary layer and the planet’s meteorology. Such a lidar also has the prospect of being used from a balloon platform to study the cloud layer of Venus.