Laser location of space objects. The use of lasers in military affairs

Annotation

Introduction

Chapter 1. Study of the characteristics of the rangefinder-altimeter analogue DL-5

1.1 Rangefinder range. Energy calculation

1.1.1 Calculation methodology

1.1.2 Calculation results in monopulse mode

1.1.3 Energy calculation in storage mode

1.2 Calculation of range measurement accuracy

1.2.1 Range measurement accuracy in monopulse mode

1.2.2 Range measurement accuracy in accumulation mode

Chapter 2. Processing of location information

2.1 Methods for processing location information

2.1.1 Methods for increasing the accuracy of time fixation of the received signal

2.1.2 Incoherent accumulation method

2.1.3 Optimal method for determining speed in terms of accuracy and noise immunity

2.2 Working in the near field and methods for reducing the minimum measurable range

3.1 Radiation divergence corrector using a cylindrical lens

3.2 Optical combiner based on birefringent elements

Chapter 4. Experimental testing of technical proposals for upgrading the DL-5 altimeter

4.1 Experimental results

4.1.1 Transmission channel energy measurement results

4.1.2 Result of visualization of the shape of light spots

4.1.3 Results of using an optical design with a birefringent crystal

4.1.4 Transmission channel layout results

4.1.5 Results of measuring the power at the output of the optical unit

4.2 Design and technological part

4.2.1 Description of the design of the DL-5 laser altimeter

4.2.2 Technological features of the construction of the DL-5 laser altimeter

Chapter 5. Life safety

5.1. Dangerous and harmful factors when operating laser systems

5.2 Laser hazard classes

5.3 Methods and means of protection against laser radiation

5.4 Calculation of laser safety of laser altimeter DL-5

Chapter 6. Ecological part

6.1 Electromagnetic pollution environment

6.2 Impact of low power EMF on biological objects

6.3 Foreign and Russian experience in standardizing electromagnetic fields

Chapter 7. Economic part

7.1 Calculation of the cost of a prototype of the DL-5M altimeter

7.2 Calculation of the cost of the DL-5M altimeter in mass production

Conclusion

References

Annotation

Laser altimeters have become an integral part of the on-board equipment of unmanned aerial vehicles. Their widespread implementation is due to a range of tasks to support flights using satellite images, determining the coordinates of observed objects, monitoring the underlying surface, and measuring the rate of descent when landing an unmanned aerial vehicle.

The thesis presents theoretical and experimental studies of the best domestic laser altimeter DL-5 based on a semiconductor laser, proposes methods and techniques for increasing range measurement ranges, increasing measurement accuracy, as well as measuring speed when landing a UAV.

The scientific and experimental results obtained became the basis for the creation of a new generation laser altimeter.

Introduction

Modern methods and technologies for laser ranging of underlying surface objects.

The development of pulsed laser ranging at the present stage is marked by a wide functional diversity: rangefinders, altimeters, lidars, 3D registration systems, etc. This diversity depends on the consumer market and on the solid-state and semiconductor lasers used.

Laser ranging is the field of optoelectronics that deals with determining the location of various objects using electromagnetic waves optical range emitted by lasers. Objects of laser ranging can be: military and civilian equipment, industrial and military structures, components of the underlying surface - ravines, forests, reservoirs, etc. Laser sensing is integral part the latest methods and geoinformatics and digital photogrammetry technologies.

The first location pulse solid-state rangefinders were based on neodymium garnet (YAG Nd3+,) and neodymium potassium gadolinium tungstate (KGV Nd 3+, - safe for vision). They have large dimensions and weight, so portable rangefinders are made using semiconductor lasers

Review of the use of pulsed rangefinders based on semiconductor lasers for sensing ground objects.

The requirements for a laser diode with (radiation hazardous to vision) or with 0 differ significantly from the requirements for a solid-state laser of a monopulse rangefinder for the following reasons:

1) a semiconductor pulsed laser emits into a corner; behaves like a diffuse emitter with dimensions ( dimensions p-n transition) at and; due to the optics of the transmitting channel, the divergence of the probing radiation is obtained (for solid-state ones 0.5 mrad), providing 50% of the power emitted by the laser;

2) fundamental difference- a semiconductor pulsed laser has several orders of magnitude lower radiation energy and coherence length. With an output radiation energy of 10-2 J, a solid-state pulse laser provides measurement to a large-sized target at a distance of 10,000 m, and a semiconductor laser with an energy of 10-6 J allows measurement of a range only up to 100 m.

Therefore, to increase the measured range in rangefinders with semiconductor lasers, it is necessary to use the incoherent accumulation method - multiple target probing. Incoherent accumulation allows you to “increase” the equivalent signal energy by a factor. N is the number of soundings in a series (accumulation volume). The accumulation method will be discussed in detail in Chapter 2.

Let us give, for example, the use of a DL-1 pulse rangefinder based on a semiconductor laser with a radiation wavelength of 905 nm for a ground-based environmental reconnaissance complex.

The DL-1 rangefinder is used as part of a ground-based environmental reconnaissance complex designed to monitor the state of the environment in the area of ​​industrial facilities (Fig. 1B). The environmental reconnaissance complex includes the IR-FSR “Climate” passive spectroradiometer, which provides measurements of parameters from the location of the complex to the controlled object.

The IR-FSR receiving channel is aimed at the area of ​​the polluting emission, and the DL-1 is aimed directly at the wall of the building.

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Figure 1. Ground-based environmental reconnaissance complex

A complex of environmental control similar in composition (Fig. 2B) can be deployed as part of a customs post at a port terminal to provide remote monitoring of ships moving in the direction of the port: determining the degree of danger of the cargo they transport and making a decision to stop the vessel at a safe distance, in the event detection of potential danger from the cargo it transports for the port complex. The environmental control complex can be located permanently at the entrance to the port. The DL-1 rangefinder provides measurement of the distance to the vessel and the speed of its approach. In addition, as in the previous version, the complex can be deployed on a mobile carrier (vehicle), this will make it possible to quickly analyze the potential danger from the cargo of ships carrying out loading and unloading operations at the berth wall along the entire port water area.

The IR-FSR receiving channel is aimed at the area of ​​space above the deck of the ship, while the DL-1 is aimed directly at the hull or superstructure of the ship.

The stationary location of the environmental control complex is shown in Figure 2.

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Figure 2. Complex for monitoring surface objects

Figure 3B shows the LIND-27 laser altimeter (Developed by Polyus Research Institute), which was installed on the MI-8 helicopter and was intended to work as part of a radiation monitoring measuring complex when assessing the background radiation above the Chernobyl nuclear power plant.

Problems of laser altimetry. Altimeters

Laser altimeters have become an integral part of the on-board equipment of aircraft, helicopters and unmanned aerial vehicles (UAVs). Their widespread introduction is due to a range of problems, the solution of which became possible thanks to laser ranging technology. These tasks can be divided into the following main groups:

Laser navigation means for an aircraft measuring slant range (altitude) and speed as a relative increment of range per unit time;

Optical-electronic means of special aircraft for viewing space, detecting targets, identifying them, determining coordinates and target designation for targeting ground or airborne weapons;

Complexes for geophysical research, etc.

This range of applications determines the differences in the design and characteristics of laser altimeters.

In terms of composition and principle of operation, laser altimeters do not differ significantly from laser rangefinders designed for operation on ground-based horizontal routes. However, laser altimeters have differences and features associated with their installation on board an aircraft.

Laser altimeters:

They do not have their own sight, guidance is carried out according to information from special systems vision or according to the flight program of the course processor;

They do not have working control bodies; their operation is controlled from the central console;

They do not include a display, which is located on the central console;

They have a developed interface for two-way communication with the central processor.

The working field of the altimeter moves in the picture plane relative to the underlying surface at an aircraft speed of 30-400, which imposes a requirement on the speed of the altimeter. Figure 3 shows the block diagram of the rangefinder-altimeter.

The rangefinder-altimeter works on the principle of measuring the travel time of a probing laser pulse to a reflecting object and back.

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Figure 3. Block diagram of the rangefinder-altimeter

where is the distance to the object, is the speed of light (Fig. 4).

Figure 4. The principle of measuring distance with a laser pulse rangefinder: 1- rangefinder; 2- pulse of transmitter radiation; 3- pulse of reflected radiation; 4- goal; 5-starting light pulse; 6- stop pulse; 7- pulses of the reference (clock) frequency generator; R - measured range, m; R=cT/2=nc/2f; c - speed of light, m/s; T is the time of propagation of laser radiation to the object and back, s; T=nt=n/f; n is the number of pulses of the reference frequency generator of the time interval meter (TIM); t - period of oscillations of the IVI reference frequency, s

The probing pulse triggers the time interval meter (TIM), implemented as part of the decision device, and, with the help of optics that forms a given radiation pattern, arrives at the object. The radiation reflected by the object is focused using receiving optics onto the photosensitive element of the photodetector device (PDE). A standard electrical pulse is generated at the output of the FPU, stopping the IVI counting circuit. Information about the measured range is taken from the IVI output. The operation of the rangefinder-altimeter units is ensured by a power supply and control unit that generates required voltages and synchronizing signals.

Information processing is carried out in the decision device. The launch of the IVI (start) in our case is carried out according to a combined scheme - part of the radiation energy is allocated to the FPU receiver. Launching the IVI (start) according to a separate scheme requires the addition of a start-pulse generation circuit with a separate receiver to the rangefinder-altimeter.

With a combined scheme, the probing pulse and the pulse reflected by the target (object) pass through one channel. Thanks to this, some systematic errors are compensated and maximum measurement accuracy is ensured.

3. Review of the use of pulsed rangefinders-altimeters (analogues) based on semiconductor lasers for monitoring objects on the underlying surface

At the Polyus Research Institute, among the developed, implemented and mass-produced pulse rangefinders-altimeters for monitoring objects on the underlying surface, LD-1 and LD-5 can be distinguished.

The main comparative technical characteristics of the DL-1 and DL-5 rangefinder-altimeters are given in Table 1.

Table 1. Comparative technical characteristics of rangefinders-altimeters DL-1 and DL-5

The complex consists of a laser analyzer, an autonomous navigation system, an altimeter, a television camera, a system for transmitting video images and measured data to a ground point.

In terms of purpose and principle of operation, the DL-5 rangefinder is similar to the DL-1 device, but thanks to the transition to a more modern element base and information processing principles, it surpasses its analogue in the main parameters (Table 1) - maximum measurable range, dimensions and weight. This allowed the DL-5 to be used in the navigation systems of an unmanned aerial vehicle.

The use of the DL-5 altimeter when installed on the Rakurs UAV (Fig. 8B), take-off weight 27 kg, developed by OJSC NIITP, made it possible to measure the topography of the underlying surface to ensure that the resulting images from the on-board television camera are linked to satellite images of the flight mission and additionally provide information to the navigation complex about the glide path in the mode automatic landing UAV.

Laser altimeter DL-5 provides:

Determining the distance to the underlying surface;

Linking the moment of height measurement to the center of the television frame with the image of the underlying surface;

Automatic transfer of measured distances to an external device.

The disadvantages of the DL-5, based on the requirements for unmanned aerial vehicles, include:

Inability to measure vertical speed with the required accuracy when landing a UAV;

Enough great value lower controlled height (2 m) and low accuracy of its measurement when landing a UAV (0.5 m);

Limited value of the maximum measured range (1000 m) and accuracy (2 m) when probing remote objects on the underlying surface.

Thus, the modernization of the studied rangefinder-altimeter DL-5, aimed at eliminating the above shortcomings, is very relevant.

Therefore, we can formulate the goal thesis and research objectives.

Purpose of the work

Conducting complex theoretical and experimental studies, as well as performing calculations, circuitry and design solutions aimed at improving the basic technical characteristics of rangefinders-altimeters: expanding the range of the measured range and increasing the accuracy of measurements; ensuring vertical velocity measurement with high accuracy as part of an unmanned aerial vehicle for monitoring objects on the underlying surface.

Research objectives

1. Comparative studies of existing pulse rangefinders-altimeters based on semiconductor lasers to improve their technical characteristics and the need to measure speed when landing an aircraft.

2. Analysis of methods for processing location information.

3. Research on ways to optimally construct a pulse rangefinder-altimeter with improved basic technical characteristics.

4. Experimental studies rangefinder-altimeter with improved technical characteristics.

Thus, for the effective use of pulsed rangefinders-altimeters based on semiconductor lasers (in systems of unmanned aerial vehicles for monitoring objects on the underlying surface), their modification is necessary, namely:

Increased maximum measuring range (> 1000 m) and accuracy (< 2 м);

Reducing the minimum measurable range (< 2 м) при повышении точности измерения (< 0,5 м) для обеспечения посадки БПЛА.

Possibility of measuring the vertical component of speed with the accuracy of its measurement.

altimeter semiconductor laser signal

Chapter 1. Study of the characteristics of the rangefinder-altimeter analogue DL-5

The optical design of the DL-5 laser altimeter is shown in Figure 1.1.

Figure 1.1 Schematic optical diagram of the rangefinder-altimeter DL-5

1. Laser diode SPL PL90-3 from OSRAM

2. Lens

3. Light filter

Assessing the energy level of the DL-5 pulse laser rangefinder-altimeter necessary to ensure maximum range measurement (Table 1.B) is the first step in studying its characteristics and searching for methods for their possible improvement: expanding the range measurement range (increasing max range and decreasing min range); increasing accuracy when measuring range, measuring vertical speed when landing a UAV.

Improving the characteristics of the DL-5 must be carried out without changing the weight and dimensions and without reducing the requirements for external interfering factors.

1.1 Rangefinder range. Energy calculation

The range of measured ranges is the main characteristic of a rangefinder (altimeter), which determines the possibilities of its use. The range of measured ranges is provided by: 1) hardware limitations (shadow zone, capacity of the time interval meter, probing frequency, etc.) 2) energy potential of the rangefinder, determined by the energy characteristics of the optical-electronic elements of the receiving-transmitting path, the design characteristics of the optical system. The actual range measured by the device to a given target under certain conditions and with known probabilistic detection characteristics is called the range.

1.1.1 Calculation methodology

The range of 1000 m specified for the analogue is ensured subject to the inequality determined by the laser ranging equation, provided that the fields of the emitter and receiver are matched:

Emin< Eпр = EoКD2прао/4R2, (1.1)

where Emin is the minimum signal energy received with a given probability, provided by the sensitivity of the photodetector (real sensitivity);

Epr - signal energy arriving at the working platform sensitive element FPU;

Eo is the energy of the probing signal;

K = - coefficient of energy overlap of the probing beam by the target (radiation utilization coefficient);

(x,y) - spatial distribution of the target brightness coefficient;

(x,y) - radiation pattern of the output probing beam;

Dpr - diameter of the receiving lens;

a = e-2R - atmospheric transmittance along the path;

Attenuation index;

o is the transmittance of the optics of the rangefinder receiving channel;

R - range to target.

The attenuation index is related to the meteorological visibility range V, km, by the well-known empirical expression:

where is the working wavelength, µm;

The initial data for calculating Epr are given in Table 1.1

Table 1.1 Initial data for calculating the range of an analogue laser rangefinder (DL-5)

1.1. 2 Calculation results in monopulse mode

Calculation range of the ranging system was carried out for the received initial data (equation optical location 1.1 and table 1.1) are shown in tables 1.2 and 1.3.

Table 1.2. Energy calculation results for a height of 1000 m

As can be seen from the above calculation results, at the maximum range to the target, even a large-sized target is not able to create a signal on the photodetector sufficient for its operation, and there is a deficit of received energy = Epr/Emin. For a given target with a diameter of 5 m at a distance to it R = 1000 m, the energy deficit is = 20.

Table 1.3. Energy calculation results for a height of 200 m

According to the data given, at an intermediate height of 200 m at favorable conditions the possibility of measurements in monopulse mode is provided.

1.1. 3 Energy calculation in storage mode

The rangefinder range is determined by its energy potential, determined mainly by the energy of the probing signal, the sensitivity of the receiver and the diameter of the receiving lens. For a given energy potential of the rangefinder, the magnitude of the Epr signal on the sensitive area of ​​the photodetector is determined, as follows from location equation 1.1, by the parameters Eo and D2, which have a limit due to restrictions on the weight and size characteristics of the rangefinder. The sensitivity of the receiving channel Emin is limited by the noise of the receiver and the input stage of the amplifier, which are determined by the physical nature of signal conversion in the photoreceiving path and also have a theoretical limit, below which it is impossible to reduce Emin in principle. The ratio Epr/Emin, called the signal-to-noise ratio, determines the range of the rangefinder and, as shown above, with a monopulse measurement mode and given design limitations, it does not provide the ability to measure a range of 1000 m for a given target under given meteorological conditions.

There is a method for increasing the range of a rangefinder without increasing its energy potential. The essence of this method lies in N-fold repetition of measurements and statistical processing of the results obtained, which makes it possible, with optimal implementation of this method, to increase the effective value of the signal-to-noise ratio up to times.

The energy deficit indicated in Table 1.2 can be compensated by a similar method, so that the condition / = 1 is satisfied, from which the accumulation volume N required to measure a range of 1000 m with the same energy potential of the rangefinder is determined by the relation N = 2 = 202 = 400.

With a sounding frequency of 8000 1/s, the range measurement time will be 400/8000 = 0.05 s, which allows measurements to be carried out with a specified information update period of 0.1 s.

To compensate for the energy deficit when working on targets with a smaller reflective surface, the measurement time can be increased to 0.1 s, while the accumulation volume is N = 800, and the maximum possible energy deficit = ~ 28, which allows measurements to be carried out on the specified targets.

Consequently, the energy assessment of the DL-5 rangefinder showed:

The energy potential of the rangefinder in monopulse mode provides range measurement in a range of up to 200 m, and in accumulation mode it provides measurement of a maximum range of up to 1000 m;

To increase the maximum range measurement beyond 1000 m, additional methods to increase the energy potential of the rangefinder must be explored.

1.2 Calculation of range measurement accuracy

1.2.1 Range measurement accuracy in monopulse mode

In the considered ranging system, a combined launch scheme is used, in which most error components are compensated. Of the uncompensated sources of error, the following have the greatest impact.

Discreteness of the RIVI time interval meter.

To ensure standard tasks, it is usually sufficient that the data sampling error does not exceed 5 m. Most laser rangefinders are built with such discretization. However, there are a number of tasks that require significantly greater accuracy. These primarily include:

The need to measure target speed;

Using rangefinder data to determine the absolute coordinates of objects using information from satellite coordinate determination systems.

Determination of the target profile (underlying surface) along the flight path of the aircraft;

Determination of the spatial extent of the target;

Ensuring the safe landing of the aircraft.

In this regard, the discreteness of RIVI in modern monopulse ranging systems usually does not exceed 1 m. In systems with accumulation, the necessary accuracy can be ensured by averaging data during the accumulation process. The DL-5 rangefinder uses a clock frequency of 25 MHz, which corresponds to a resolution of 6 m in each individual measurement.

The probability density distribution w(r) of the random error r caused by this component has a rectangular shape with a synchronized start and triangular shape, when the IVI clock pulses are not tied to the start moment (Fig. 1.2).

Figure 1.2 Probability density distribution of the range measurement error component r due to the discreteness of the IVI during asynchronous start

In this case:

w (r) = 1/(R)2r + 1/R at r< 0,

1/(R)2r - 1/R for r > 0. (1.2)

The variance of this error

DIVI = r2w (r) dr = R2/6,

And its mean square value

IVI = = 0.408 R = 2.448 (1.3)

Instability of the threshold device operation when recording received pulses at the leading edge.

Figure 1.3 Instability of threshold device operation

The mechanism of instability of the temporary fixation of the received signal is clear from Figure 1.3, where R1 is the response delay of the threshold device at the maximum signal amplitude S(r), and R2 at the minimum signal.

The minimum excess of the signal over the threshold is set by the required signal/threshold ratio, determined by the required probability of a reliable measurement. The maximum excess of the signal over the threshold is determined by the dynamic range of the received signals.

When the leading edge has a sine-square shape, it is described by the expression.

S(r) = Sin2 (r/4rmax)

where rmax = ctmax/2;

c is the speed of light;

tmax - duration of the front at levels 0-1.

From this expression it is possible to determine R1 and R2 with a known rise time tmax and the above-mentioned limit values ​​of the signal/threshold ratio.

So, with a front duration of 100 ns, which corresponds to rmax = 15 m, R1 = 0.1 m, and R2 = 8.4 m, i.e. the maximum response delay spread is 8.4 - 0.1 = 8.3 m.

At short and medium ranges, the minimum excess of the signal amplitude above the threshold is usually 100 times or more.

Then R2< 4 rmax arcSin()/, что для приведенного примера составляет 1 м. Угол arcSin(х) измеряется в радианах.

Obviously, this value depends on the range of measured ranges and is determined by the energy potential of the rangefinder in this range.

The value of the root mean square error fr can be related to the maximum spread of the response delay by the known relation

fr = (R2 - R1)/6 = m (1.4)

1.2.2 Range measurement accuracy in accumulation mode

WITH The statistical spread of measurement results during averaging decreases with increasing volume of statistical data. Average variance

where D is the variance of the result of one measurement, and N is the number of measurements in the series. Accordingly, the standard deviation of the averaged measurement

Thus, to increase the accuracy in the accumulation mode with N measurements, it is necessary to form an estimate of the measured range

Ri is the result of the i-th measurement;

i is the serial number of the measurement.

The root mean square error of such an estimate, due to the discreteness of the time interval meter, with the above accumulation volume N = 800, will be

N = 0.408 R/ = 0.408 6/ = 0.08 m.

The specified measurement accuracy at the specified clock frequency of the time interval meter is ensured. Thus, the resulting root mean square measurement error of 0.08 m allows us to consider that in the accumulation mode the DL-5 has a significant margin in range measurement accuracy (see Table 1B).

Thus, energy potential The rangefinder in monopulse mode provides measurement of an intermediate height of 200 m. At a distance to the target of 1000 m, the energy deficit is 20 times.

Operating the rangefinder in accumulation mode compensates for the energy deficit, which allows you to measure a maximum range of up to 1000 m.

Calculation of the accuracy of range measurement in the accumulation mode showed that its energy potential provides a root-mean-square measurement error of 0.08 m, which is significantly lower than the norm specified in the technical specifications agreed with the customer DL-5: 0.5 m for measurements in the range 2-200 m and 2 m for the range 200-1000 m.

Chapter 2. Processing of location information

2.1 Methods for processing location information

Selection of targets and interference

The most important task of the rangefinder is to determine the range to the selected target in conditions of the interfering influence of internal noise and foreign objects located in the target range. Such objects are atmospheric inhomogeneities, which are most pronounced at ranges of 20-200 m (backscatter interference), vegetation, terrain folds, structural elements, etc.

Figure 2.1 shows a diagram of the location path with the most common interference and the corresponding signals at the input and output of the threshold device. When vertically probing the underlying surface from an aircraft, the target interference environment remains fundamentally the same, although the nature of the interference and their relative influence may differ somewhat.

To combat these interferences, use various schemes selection. The most commonly used:

Limitation of the minimum measured range (gating);

Selecting a target by its ordinal position (first, second, last target);

Selection of signals by their shape; this method is most effective for combating extended interference, mainly backscatter interference;

amplitude selection (temporary automatic gain or threshold adjustment).

Figure 2.1 Locating route, locating signals and their selection. Targets selected in selection modes are marked: first, second and last target

Accumulation method

The accumulation method assumes:

Multiple repetition of measurements;

Accumulation and storage of location information in range channels corresponding to the serial number and duration of the clock pulse;

Correlation or other processing of an array of accumulated data in order to isolate the signal reflected by the target;

Time reference of the selected signal to the clock sequence of timing pulses.

2.1.1 Methods for increasing the accuracy of time fixation of the received signal

In Chapter 1 of this work A method for fixing the temporary position of a pulse reflected by a target along its front is considered. As shown in the considered example, with a pulse duration of 100 ns, the spread of the moment of temporary fixation in the entire amplitude dynamic range can be ~ 8 m. Unlike the sampling error of the measured interval, this error component does not reset to zero during accumulation, since signals in one series arrive approximately equal amplitude, and the timing error is systematic rather than random for a given measurement.

This drawback is eliminated by binding to the maximum signal. and fixing the derivative at zero.

Figure 2.2 Method of fixing the maximum signal: S1(t) - signal; t1 - time reference point corresponding to the signal maximum

Figure 2.3 Method of fixing the derivative at zero: a) S1(t) - signal at the input of the fixation circuit; S1? (t) - signal at the input of the NK null comparator; t1 - result of timing; b) a differentiating link in the structure of the receiving path with a timing device - DZ. In this case, the time constant of the DZ, and 0 is much less than the duration S1(t).

Figure 2.4 Zero crossing method: a) S1(t) - signal at the DS input; S1?(t) - signal at the input of the NK null comparator; tm - maximum position. t1 is the result of the timing. b) timing device with a differentiating element DS and a null comparator

The maximum fixation method (Fig. 2.2) represents perfect solution the maximum represents a limit in the region of infinitesimal approximations that are not practically feasible. This remark is also true with respect to the derivative method (Fig. 2.3), in which the maximum of the signal is noted at the time instant corresponding to the zero of its derivative. In practice, the zero crossing method is widely used (Fig. 2.4), which is a “deviation” from the zero derivative method in that the “differentiation” of the signal is carried out by passing it through a differentiating link (differential chain) with a non-zero time constant, and also by the fact that the differentiated the signal is compared in general case with a non-zero comparator response threshold.

This results in a maximum fix error.
tm = t1 - tm. Usually this error does not exceed 2-5 ns, however, with significant overloads of the receiving path, the signal shape is greatly distorted and this error can increase significantly. To eliminate this drawback, automatic gain control of the received signal is introduced.

Methods for increasing the accuracy of temporary fixation of an array of accumulated information

The accumulation method provides not only energy gains, but also increased measurement accuracy. Thanks to this, it is possible and desirable to set the duration of the probing pulse several times longer than the duration of the IVI sampling period. According to the known technical solution, the time reference of the accumulated data array is carried out as a projection onto the time axis of the intersection point of the tangents to the front and rear “fronts” of the accumulated array (Fig. 2.5).

The analysis showed the insufficient efficiency of such methods for processing accumulation results. Firstly, as can be seen from Figure 2.5, the “fronts” of the array cannot be accurately interpreted and the position of the tangents to them is ambiguously established. Secondly, the shape of the array envelope depends significantly on the signal magnitude. As a result, the timing using this method has a significant scatter.

Figure 2.5 Method of timing the accumulated array using the tangent method with signal-to-noise ratio = 1

These shortcomings are eliminated by the method of timing the data array by determining the position of its first initial moment (center of gravity), calculated by the expression:

Tз = ((j-p) + ) T , (2.1)

Where j is the number of the time disc in which the accumulated amount is maximum;

K(a) - accumulated amount in the (a)th discrete;

k(a) - weight coefficient of the (a)th discrete; if the position of the signal is unknown a priori, we can take k(a) = 1;

m = tfr/T - number of discretes corresponding to the duration of the leading edge of the laser pulse;

tfr is the duration of the leading edge of the laser pulse;

q = ti/T - number of discretes corresponding to the pulse duration;

ti is the duration of the laser pulse;

p - correction number characterizing the signal timing point;

T is the duration of the discrete.

This method maintains high timing accuracy not only in the linear range of the input signal, but also under significant overloads.

2.1.2 Incoherent accumulation method

The accumulation problem is formulated as follows: d The range of measured ranges DR is divided into m equal intervals Dr = DR/m; all intervals are considered statistically independent and are considered as range channels where the processing (accumulation) of location information is carried out; it is believed that the measured signal is in one of these channels (j-th channel). To obtain the measurement result, N range soundings are carried out. At the receiver output there is a mixture of signal with amplitude S and noise with effective value y. During i-th sensing, analog information from the receiver output is converted into digital information by single-level threshold quantization (STC) or multi-level threshold quantization (MLT) of the signal.

The OPK is called binary: the i-th signal of the j-th range channel is assigned the value kij=0 if

where Uj0 is the analog quantization threshold, or kij=1 if Sij>Uj0. These values ​​are summed (accumulated) in each j-th channel during each of the N soundings, forming the sums

Kj= kji (i=1…N)

If Kj>Kj0 is the threshold level, then it is decided that the range to the target is determined j-th channel range and is equal to:

where R0 is the beginning of the range of measured ranges.

Computer modeling of the receiving path with accumulation

Was developed computer model receiving path with accumulation. The model uses the Monte Carlo method and is based on MATLAB 7.0 software. At the output of the linear path there is a random process representing the sum of the signal and noise. One such implementation is shown in Figure 2.6. The signal is characterized by a relative amplitude S, specified in levels effective value noise y and representing the signal-to-noise ratio. The program parameter A is related to S by the ratio A = 1.85 S. In the figure, S = 1. Figures 2.7 and 2.8 show the results of computer simulation of a two-threshold storage device under the above conditions and the number of accumulation cycles (accumulation volume) N = 200. The index below shows the position the center of gravity of the resulting arrays.

Figure 2.6 Implementation of a random signal + noise process at the input of a two-level threshold device. Threshold levels +0.5 and -0.5 are shown by dotted lines. Signal to noise ratio S = 1

Drive simulation results

Figure 2.7 Realization of accumulation results with accumulation volume N = 200 and signal-to-noise ratio at input S = 1. Calculated range R = 205 m. Measurement result R* = 204.8 m.

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Figure 2.8 Implementation of accumulation results with an accumulation volume N = 200 and signal-to-noise ratio at the input S = 10. Calculated range R = 5 m. Measurement result R* = 5.0 m

The data in Figure 2.7 were obtained for the signal-to-noise ratio at the drive input S/N = 1, and the results in Figure 2.8 were obtained for S/N =10. With a further increase in S/N, the estimate of the time position of the signal shifts slightly to the left towards the original value. As can be seen, with such a construction of the storage device and an algorithm for estimating the temporal position of the signal at the center of gravity of the accumulated array, the spread of range measurement results in the unlimited amplitude range of signals does not exceed 20% of the value of the IVI sample. For the example considered, this corresponds to 0.2 m, which is a systematic error that can be eliminated at short distances by introducing a correction.

Despite such a small spread in range estimates during accumulation, there are ways to further reduce it. This is possible due to the introduction of a correction depending on the number of overflowed storage cells or the sum of accumulated amounts in cells adjacent to the center of gravity of the accumulated array. Then the error in range estimation can be reduced to 10% of the discrete value or less.

2.1.3 Optimal method for determining speed in terms of accuracy and noise immunity

Optimal speed measurement algorithm

If a number of range measurements are available, a procedure for measuring the target's speed can be proposed by determining the coefficient xy of the regression line y = xy x + b (Fig. 2.9).

Figure 2.9 Determination of speed as a regression coefficient pxy of a series of measurements y(x)

In this case, the dispersion of the estimate рху is minimal if it is optimized using the method least squares. In the general case, for arbitrary moments of time of measuring ranges and the volume of a series of measurements n, the velocity estimate, optimal in the sense of least squares, is determined by the expression valid for values ​​of V* from 0 to 5 m/s and above.

In particular, for equally spaced samples Ri with a period DT:

or, after simplifications,

In this case, the root-mean-square error of speed estimation is:

where is the root-mean-square error of range measurement in each of the measurements.

In particular:

Table 2.1 shows the calculation results for several accumulation modes.

Table 2.1 Calculation results of the velocity measurement error V at R ~ 0.41 R=2.4 m

Note Calculations of V were carried out according to formula (2.7)

The choice of the optimal accumulation mode depends on the aircraft’s flight mission, altitude and piloting mode.

It must be noted that in speed determination procedures, all measurements must be reliable. Any false range reading or missed measurement (= 0) will result in a gross distortion of the speed measurement result. Therefore, when developing a calculation algorithm, measures must be taken to eliminate unreliable range measurements, for example, by eliminating measurements that differ from the average speed estimate for each range by an amount greater than 3.

Consequently, the speed measurement algorithm that is optimal in terms of standard deviation provides the ability to measure speed within specified limits from 0 m/s to 5 m/s and above. The speed measurement error can be reduced to acceptable values ​​by increasing the accumulation time to 0.5-1 s; in this case, the frequency of updating speed data can be the same as in the height measurement mode - for this, the speed calculation algorithm must provide for a shift in the accumulation interval with each specified update period, a given error of 0.2 m/s is ensured with an accumulation time T = 1 With.

2. 2 Near-field work and methods for reducing the minimum measurable range

Hardware function and shadow zone

With increased requirements for the minimum measurable range of a laser rangefinder, the problem arises of forming its hardware function (geometric factor) in such a way that the length of the shadow zone does not exceed the specified minimum range. The diagram for the formation of a typical hardware function of a laser rangefinder with separated transmitting and receiving channels is shown in Figure 2.10.

The hardware function A(R) characterizes the degree of overlap of the fields of the transmitting and receiving channels and varies in the near zone of the range from 0 to 1.

In the shadow zone, A(R) = 0, so range measurements in this zone are impossible. Typically, when constructing a rangefinder according to the above scheme, the shadow zone of the rangefinder R0 is 2-20 m, depending on the mutual configuration and optical characteristics emitting and receiving channels.

The value of R1 has virtually no effect on the rangefinder characteristics in the near zone, and R0 determines the minimum measured range, which cannot be less than this value. To reduce the minimum measured distance with the DL-5 altimeter to 0.5 m, it is enough to outer surface To the lens of the transmitting channel, on the side of the mandrel, glue a plate of milk glass type MC21 measuring 7x3x0.3 mm.

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Figure 2.10 Scheme of formation of the hardware function: Di - diameter of the exit pupil of the emitting channel; Dп - diameter of the entrance pupil of the receiving channel; B is the distance between the axes of the emitting and receiving channels (base); R0 is the far boundary of the near (shadow) zone, where the fields of view of the emitting and receiving channels begin to combine; R1 is the near boundary of the far zone, in which there is complete overlap of the fields of view of the emitting and receiving channels; - angle of the receiving channel field of view; - angular divergence of the output beam of the emitting channel

Features of the rangefinder in the near zone

Requirements for the minimum measurable range and measurement accuracy are contradictory. The first of these requirements forces us to reduce the shadow zone of the rangefinder, and the second forces us to reduce the level of overload of the receiving path with reflected signals, which places opposite demands on the hardware function.

An additional factor that negatively affects near-field accuracy is the different mode structure of laser radiation in the near and far zones. These differences are aggravated by the influence of partial overlap of the fields of the emitting and receiving channels in the near zone. As a result, in the near zone the hardware function selects some modes and suppresses others. The difference in the temporal position of the radiation components corresponding to these modes can reach 0.1-1 ns, which corresponds to a range measurement error of 0.01 - 0.2 m.

Thus, to reduce the minimum measurable range< 2 м необходимо принять меры по сокращению теневой зоны аппаратной функции и устранению влияния модовой структуры излучения лазера.

Chapter 3. Proposals for the optimal construction of a pulse altimeter using a semiconductor laser

Methods for increasing the energy of the probing signal

Currently, several directions have been outlined for increasing the energy of the probing radiation of rangefinders through the use of a radiation divergence corrector made using a cylindrical lens and by combining radiation beams from several lasers using special optical combiners. Thanks to this and the simultaneous use of highly sensitive receivers, effective methods accumulation, interference selection tools and signal timing algorithms, it was possible to increase the range of rangefinders to 2-3, and in some cases up to 10 km.

3.1 Radiation divergence corrector using cylindrical lens

In the sample of the DL-5 altimeter under study, a laser diode SPLPL90-3 is used, the size of the luminescent body is 200x10 µm. Three emitting junctions fit into a size of 10 µm.

The far-field characteristics of the laser diode used are shown in Figure 3.1.

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Figure 3.1. Radiation divergence of pulsed diode SPL PL90-3

Only part of the laser diode power is transmitted to the underlying surface, lying inside the cone with an angle at the apex AND equal to:

And = 2arctg(D/2Fob)

Where: D=18mm - Lens luminous diameter.

Fob =65mm - Focal length lens.

For our case AND? 160

From Figure 3.1A it is clear that in the plane parallel to the p-n junction almost all the energy is taken, and in the plane perpendicular (Figure 3.1B) approximately at a level of 0.8. The measured relative energy value in this coal is approximately 30% of total energy radiation. At the same time, the size of the glow body in the plane perpendicular to the p-n junction is h+=10 µm and the geometric divergence of the rangefinder radiation in this plane is equal to:

2g = h+/Fob = 0.15x10-3rad

The size of the glow body is parallel to the plane is h=200 µm and, accordingly, the divergence of radiation in this plane is equal to:

2g =h///Fob = 3x10-3rad

This shows that an increase in radiation power can be obtained by increasing the divergence of radiation in a plane perpendicular to plane rn transition.

The radiation correction scheme with a cylindrical lens is illustrated in Figure 3.2.

Figure 3.2 Correction of radiation from a pulsed diode with a cylindrical microlens: n0 = 1 - refractive index of air; n > 1 - refractive index of the lens material; r is the radius of curvature of the microlens; D - distance from the glow body to the center of curvature

Parameter D is determined by the design of the laser diode and equal to the distance from the glow body to the output end of the diode body. The average statistical value of this parameter for the SPL PL90-3 laser is 0.285mm within a batch of 50 pcs.

N - reduced size of the glow body in the plane perpendicular p-n transition;

h is the size of the glow body;

In a plane perpendicular p-n junction the image is shifted by the amount L, and in parallel by the amount L1. As a result of this setting for the output lens, the light source becomes astigmatic.

The value S = L1+L is the astigmatism of the light source.

And+ is the angle at which light energy is absorbed in a plane perpendicular to the pn junction.

For given values:

A cylindrical lens has the following parameters:

n=1.62, r=0.5 (lens radius)

The calculation gave the following values:

S = L1+L=0.62mm. Astigmatism of the light source.

Divergence in perpendicular p-n plane transition is determined by the expression 2g+ ? H/Fob + S*D/(Fob)2

For the obtained value of astigmatism of the light source, the divergence in the perpendicular plane of the p-n junction will be I+ = 410.

Correction of radiation by a cylindrical microlens allows you to absorb energy in a plane perpendicular to the plane of the p-n junction at approximately a level of 0.2 versus a level of 0.8 without correction.

3.2 Optical th adder on birefringent elements

The radiation beams of two semiconductor lasers are polarized and combined using an optical combiner, the optical combiner is made in the form of a birefringent plane-parallel plate, the laser emitters are located on the side of one of its faces so that their optical axes are parallel, and the polarization planes of the laser radiation are mutually perpendicular. The thickness h of the birefringent plate is determined by the formula:

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It can be carried out using incoherent (searchlight) and coherent (laser) optical signals.

Spotlight location

Used during the first and second world wars. The reflected visible signals were observed visually. The searchlights provided greater radiation energy, but its incoherence reduced the possibilities of angular concentration. Infrared (IR) spotlights are used in modern night vision systems containing converters of IR received images into visible ones.

Laser ranging

Appeared in the early 60s as a result of the creation of sources of optical coherent laser radiation. Laser ranging has a number of important features.

Firstly, the coherence and short wavelength of laser radiation made it possible to obtain narrow radiation patterns (from units to tens of arc seconds) even with small emitter sizes (a few decimeters). With a radiation divergence equal to one arc second (in this case 1 "~ 5x10 - 6 rad), the transverse size of the irradiated area at a distance of 200 km is 1 m, which allows separate observation individual elements goals.

Secondly, the temporal and spatial coherence of laser radiation ensures frequency stability at a high spectral power density. The latter, as well as the highly targeted nature of laser radiation, determine the high noise immunity of laser ranging devices from the effects of natural radiation sources.

Thirdly, a high oscillation frequency leads to large Doppler frequency shifts during mutual movements of the target and the locator. This provides high accuracy in measuring the radial velocity of target elements, but requires expanding the bandwidth of receiving devices.

Fourthly, the propagation of optical waves in gaseous and liquid media is accompanied by significant scattering. This leads to atmospheric backscatter interference at the input of the receiving device and is, in addition, a unmasking factor.

Block diagram and design features of a laser locator.

The main element of the transmitting device is a laser. The spectral line of radiation of the laser working fluid determines the carrier frequency of the locator.

Lasers are used in modern location:
a) on carbon dioxide CO 2;
b) on neodymium ions;
c) on a ruby;
d) on copper vapor, etc.

Gas CO 2 lasers have high average output powers (up to tens of kilowatts), high monochromaticity (spectral width of several kilohertz), high efficiency (up to 20%), operate in both continuous and pulsed modes, and are compact. Solid-state neodymium and ruby ​​lasers are used mainly in pulsed mode (repetition frequency 0.1...100 Hz); the energy of their radiation per pulse is up to units of joules; efficiency unit percent. Copper vapor lasers provide high repetition rates (up to tens of kilohertz) with an average power of up to 100 W.

The required distribution of the probing (laser) radiation flux in space is provided by a forming optical system (FOS). It may include a system of uncontrolled mirrors (3), lenses and controlled deflectors (D), which ensure the movement of the beam. Laser signals reflected from targets are concentrated by a receiving telescope (RT) on photodetectors. The combination of transmitting and receiving systems of laser locators, unlike radars, is rarely used due to overloads of photoreceiving devices and an increase in the level of interference. Both the transmitting and receiving optical systems of promising laser locators are currently implemented in an adaptive version to compensate for distortions of signal wavefronts in the atmosphere and environments of laser generators.

In contrast to radars, laser locator photodetectors practically do not use signal amplification at the carrier frequency. This complicates the design and makes it difficult to view the space. Only direct amplification of video signals is used, and with heterodyne reception - intermediate frequency radio signals. Video frequency amplification is used primarily in the visible and ultraviolet (UV) ranges. For this range, there are low-noise receivers with an external photoelectric effect (i.e., with the knocking out of electrons by optical radiation quanta from the photocathode). Radio frequency amplification is used in the IR range, in which the external photoelectric effect is not realized due to insufficient energy of the radiation quantum, but heterodyne reception reduces the significance of the internal photoelectric effect noise.

Features of heterodyne reception. A laser local oscillator and a mixer in the form of a translucent mirror or beam splitting prism are introduced into the photodetector device. In this case, in the case of mutual coherence of the radiation of the laser local oscillator and the transmitting device, coherent processing of the received signal is possible. Therefore, heterodyne reception is used not only to suppress internal noise in the IR range, but also to extract information from the phase structure of the received field in the visible and UV range.

Features of interferometric reception. At the input of the photodetector, the fields from two or more spatially separated points (regions) of the receiving aperture plane are summed up. Based on the result of the interference of the fields, their mutual coherence and phase relationships are determined.

Based on a set of measurements at different spacing of receiving points, the spatial distribution of the amplitude and phase of the received field can be reconstructed. Interferometric reception is used in the absence of a local oscillator to extract information from the phase structure of the received field, as well as to increase angular resolution and synthesize the aperture.

Application areas of laser locators:

• measurement of range and angular coordinates of moving targets of ships, aircraft, artificial earth satellites, etc. (laser rangefinders, locators such as MCMS, PAIS, etc.);
• high-precision measurements of target movement speeds and flows of liquids and gases (laser Doppler speed meters and anemometers);
• obtaining non-coordinate information about targets: surface parameters (roughness, curvature), vibration parameters and movement around the center of mass, images, etc. (multifunctional laser locators such as KA-98, Lotaws, etc.);
• high-precision guidance of weapon systems (laser locators for target illumination, space surveillance and target distribution);
• ensuring docking of spacecraft, landing of aircraft, navigation (laser navigation systems); f) elements of technical vision in automatic and robotic systems (range measurement systems, image formation, target selection and recognition, etc.);
• diagnostics of parameters and measurement of variations in environmental characteristics, including the atmosphere, as well as monitoring its pollution by products of human economic activity (lidars such as DIAL, etc.; Lidar - LIght Detection And Ranging - light detection and ranging).

Semi-active optical ranging

Uses the phenomenon of secondary radiation (reflection) by targets of optical waves from a source of natural intense primary radiation. Most often this source is the Sun. Semi-active location devices based on this principle are called optical-electronic stations. Semi-active optical location means also include biological visual systems. Neglecting the factor of using secondary radiation, optical-electronic stations are often classified as means of passive optical location.

Passive optical ranging

Uses its own optical radiation from heated areas of the target surface or ionized formations in its vicinity. It is known that the maximum radiation of a completely black body at temperature T (Kelvin) occurs at a wavelength of ~ 2898/T µm. The wavelength at which the maximum emission from real targets occurs is usually in the infrared region of the spectrum (only at T ~4000 K does the maximum coincide with the red region, and at T ~5000 K does it coincide with the yellow region of the visible spectrum). Passive optical location devices therefore usually operate in the near-infrared range. Similar means include IR direction finders, thermal imagers, thermal homing heads, passive night vision devices, etc. They play important role in missile attack warning and missile defense systems.

General features of optical location

Determined by the frequency range used. The high directivity of the probing radiation and the narrow fields of view of the receiving channels significantly limit the capabilities of optical location devices to survey space. Therefore, the search and detection of a target by optical location means is carried out in most cases using external target designation, for which they are interfaced with radar systems. In the process of receiving weak signals, quantum nature electromagnetic waves. Quantum signal noise limits the sensitivity of an ideal optical receiver in the absence of interference at the energy level of even a single photon. In the optical range, it is easier to obtain non-coordinate information about the target, its size, shape, orientation, etc. Upon receipt, the polarization and photometric characteristics of scattered radiation are used and the target image is recorded. Obtaining non-coordinate information is often the main task of optical location aids. Creating intentional interference for optical location is possible, but more difficult than for radar.

Angular resolution is the most important characteristic of any telescopic system. Optics states that this resolution is uniquely related to the wavelength at which the observation is made and to the diameter of the telescope's entrance aperture. As you know, large diameters are a big problem. It is unlikely that a telescope larger than this will ever be built.
One of the ways to significantly increase resolution is the method of synthesizing large and ultra-large apertures, used in radio astronomy and radar. In the millimeter range, the largest aperture - 14 km - is promised to be formed by 66 antennas of the ALMA project in Chile.

The transfer of aperture synthesis methods to the optical region, where wavelengths are several orders of magnitude shorter than those of radars, is associated with the development of laser heterodyning technology.

1.Physical basis of image formation.

It will not be a mistake to say that the image in any optical device is formed by the diffraction of light at the input aperture, and nothing else. Let's look at the image of the object from the center of the aperture. The angular distribution of brightness of the image of an infinitely distant point source of light (as, indeed, of any other) will be the same for a lens and a pinhole camera of equal diameter. The difference between a lens and a pinhole is only that the lens transfers the image formed by its aperture from infinity to its focal plane. Or, to put it another way, it produces phase transformation input flat wave front to spherically convergent. For a distant point source and a circular aperture, the image is the well-known Airy ring pattern.


Angular size The Airy disk can, in principle, be reduced and seem to increase the resolution (according to the Rayleigh criterion) if the aperture is apertured in a special way. There is a radial transmission distribution such that the central disk can theoretically be made arbitrarily small. However, in this case, the light energy is redistributed among the rings and the contrast of the complex image drops to zero.

From a mathematical point of view, the procedure for forming a diffraction image is reduced to a two-dimensional Fourier transform of the input light field (in the scalar approximation, the field is described complex function coordinates and time). Any image recorded by an eye, screen, matrix or other quadratic-intensity receiver is nothing more than a two-dimensional amplitude spectrum of a light field emitted by an object, limited by an aperture. It's easy to get the same Airy picture if you take square matrix from identical complex numbers (simulating a flat wave front from a distant point), “cut” a round “aperture” out of it, zeroing the edges, and perform a Fourier transform of the entire matrix.

In short, if you somehow record the field (synthesize the aperture) for a sufficiently large area without loss of amplitude and phase information, then to obtain an image you can do without the giant mirrors of modern telescopes and megapixel matrices, simply by calculating the Fourier transform of the resulting data array.

2. Satellite location and super-resolution.

We will observe a stabilized object moving across the line of sight, illuminated by a continuous coherent laser source. The radiation reflected from it is recorded by a heterodyne photodetector with a small aperture. Recording a signal during time t is equivalent to implementing a one-dimensional aperture of length vt, where v is the tangential velocity of the object. It is easy to evaluate the potential resolution of such a method. Let's look at a near-Earth satellite in upper elongation, flying at an altitude of 500 km at a speed of 8 km/sec. In 0.1 seconds of signal recording, we obtain a “one-dimensional telescope” measuring 800 meters, theoretically capable of viewing satellite details in the visible range that are a fraction of a millimeter in size. Not bad for such a distance.

Of course, the reflected signal at such distances is weakened by many orders of magnitude. However, heterodyne reception (coherent mixing with the reference radiation) largely compensates for this attenuation. After all, as is known, the output photocurrent of the receiver in this case is proportional to the product of the amplitudes of the reference radiation and the incoming signal. We will increase the share of reference radiation and thereby amplify the entire signal.

You can look from the other side. The spectrum of the recorded signal from the photodetector is a set of Doppler components, each of which is the sum of contributions from all points of the object that have the same radial velocity. The one-dimensional distribution of reflective points on an object determines the frequency distribution of spectral lines. The resulting spectrum is essentially a one-dimensional “image” of the object along the “Doppler shift” coordinate. Two points of our satellite, located at a distance of 1 mm from each other in a plane perpendicular to the line of sight, have a difference in radial velocities of the order of 0.01-0.02 mm/sec. (The ratio of this difference to the speed of the satellite is equal to the ratio of the distance between points to the distance to the satellite). The difference in Doppler frequencies of these points for a visible wavelength of 0.5 μm will be (f=2V/λ) of the order of 100 Hz. The spectrum (Doppler image) from an entire microsatellite, say 10 cm in size, will fall within the 10 kHz range. Quite a measurable quantity.

You can also look from a third side. This technology is nothing more than recording a hologram, i.e. interference pattern that occurs when the reference and signal fields are mixed. It contains amplitude and phase information sufficient to reconstruct full image object.

Thus, by illuminating a satellite with a laser, recording the reflected signal and mixing it with a reference beam from the same laser, we obtain a photocurrent at the photodetector, the dependence of which on time reflects the structure of the light field along the “one-dimensional aperture”, the length of which, as already mentioned, can be determined big enough.

The two-dimensional aperture is, of course, much better and more informative. Let us arrange several photodetectors evenly across the motion of the satellite and thus write down the reflected field on the area vt*L, where L is the distance between the outermost photodetectors, which in principle is not limited by anything. For example, the same 800 meters. Thus, we synthesize the aperture of a “two-dimensional telescope” measuring 800*800 meters. Resolution along the transverse coordinate (L) will depend on the number of photodetectors and the distance between them, and along the other, “temporal” coordinate (vt) - on the bandwidth of the laser radiation and the frequency of digitization of the signal from the photodetector.

So, we have a recorded light field over a very large area and we can do whatever we want with it. For example, getting a two-dimensional image of very small objects at a very large distance without any telescopes. Or you can restore three-dimensional structure object by digitally refocusing in range.

Of course, the real three-dimensional configuration of reflecting points on an object does not always coincide with their “Doppler” radial velocity distribution. There will be a coincidence if these points are in the same plane. But in the general case, a lot of useful information can be extracted from the “Doppler image”.

3. What happened before.

The American DARPA some time ago financed a program, the essence of which was to implement such technology. It was supposed to locate objects on the ground (tanks, for example) from a flying aircraft with ultra-high resolution; some encouraging data were obtained. However, this program was either closed or classified in 2007 and nothing has been heard about it since then. Something was also done in Russia. Here you can see a picture obtained at a wavelength of 10.6 microns.

4. Difficulties in technical implementation at a wavelength of 1.5 microns.

After mature reflection, I decided not to write anything here. Too many problems.

5. Some primary results.

So far, it has been difficult to “see” from a distance of 300 meters the details of a flat, diffusely reflective metal object measuring 6 by 3 mm. It was a piece of some kind of printed circuit board, here is a photo:


The object rotated around an axis perpendicular to the line of sight, and the reflected signal was recorded approximately at the moment of maximum reflection (flare). The laser spot illuminating the object was about 2 cm in size. Only 4 photodetectors were used, spaced 0.5 meters apart. The size of the synthesized aperture is estimated to be 0.5 m by 10 m.
Actually, just in case, the recorded signals themselves (on the left) and their spectra (on the right) in relative units:


From the previous photo of the object, Photoshop has selected only the illuminated and reflective areas of interest to us that we want to see:


Image reconstructed by 2D Fourier transform from 4 signals and scaled for comparison:


This picture actually consists of only 4 rows (and about 300 columns), the vertical resolution of the image is, accordingly, about 0.5 mm, but the dark corner and both round holes seem to be visible. The horizontal resolution is 0.2 mm, this is the width of the conductive tracks on the board, all five of them are visible. (A regular telescope would need to be two meters in diameter to see them in the near-infrared).

In truth, the resolution obtained is still far from the theoretical limit, so it would be nice to bring this technology to fruition. The devil, as we know, is in the details, and there are a lot of details here.

Thank you for your attention.

Laser ranging

Laser ranging in foreign press refers to the field of optoelectronics, which deals with detecting and determining the location of various objects using electromagnetic waves of the optical range emitted by lasers. Tanks, ships, missiles, satellites, industrial and military structures can become objects of laser ranging. In principle, laser ranging is carried out using the active method.

Laser ranging, as well as radar, is based on three main properties of electromagnetic waves:

1. The ability to be reflected from objects. The target and the background on which it is located reflect the radiation incident on them differently.

Laser radiation is reflected from all objects: metallic and non-metallic, from forests, arable land, and water. Moreover, it is reflected from any objects whose dimensions are smaller than the wavelength, better than radio waves. This is well known from the basic principle of reflection, which states that the shorter the wavelength, the better it is reflected. The power of the reflected radiation in this case is inversely proportional to the wavelength to the fourth power. A laser locator fundamentally has a greater detection ability than a radar - the shorter the wave, the higher it is. That is why, as radar developed, there was a tendency to move from long waves to shorter ones. However, the production of radio frequency generators emitting ultra-short radio waves became more and more difficult, and then completely reached a dead end. The creation of lasers opened up new perspectives in location technology.

2. Ability to spread in a straight line. The use of a narrowly directed laser beam, which scans the space, allows you to determine the direction to the object (target bearing). This direction is found by the location of the axis of the optical system that generates the laser radiation. The narrower the beam, the more accurately the bearing can be determined.

Simple calculations show that in order to obtain a directivity coefficient of about 1.5, when using radio waves in the centimeter range, you need to have an antenna with a diameter of about 10 m. It is difficult to install such an antenna on a tank, much less on an aircraft. It is bulky and non-transportable. You need to use shorter waves.

The angular angle of a laser beam produced using a solid-state active substance is known to be only 1.0.1.5 degrees and without additional optical systems.

Consequently, the dimensions of a laser locator can be significantly smaller than a similar radar. The use of small-sized optical systems will make it possible to narrow the laser beam to several arc minutes, if the need arises.

3. The ability of laser radiation to propagate from constant speed makes it possible to determine the distance to an object. Thus, with the pulse ranging method, the following relationship is used: L = ct/2, where L is the distance to the object, c is the speed of radiation propagation, t is the time it takes for the pulse to travel to the target and back.

Consideration of this relationship shows that the potential accuracy of range measurement is determined by the accuracy of measuring the time it takes for the energy pulse to travel to the object and back. It is quite clear that the shorter the impulse, the better.

What parameters are used to characterize a locator? What are his passport details? Let's look at some of them.

First of all, the coverage area. It is understood as the region of space in which observation is carried out. Its boundaries are determined by the maximum and minimum operating ranges and viewing limits in elevation and azimuth. These dimensions are determined by the purpose of the military laser locator.

Another parameter is review time. It refers to the time during which the laser beam produces a single survey of a given volume of space.

The next locator parameter is the determined coordinates.

They depend on the purpose of the locator. If it is intended to determine the location of ground and underwater objects, then it is enough to measure two coordinates: range and azimuth. When observing aerial objects, three coordinates are needed. These coordinates should be determined with a given accuracy, which depends on systematic and random errors. We will use such a concept as resolution. Resolution means the ability to separately determine the coordinates of closely located targets.

Each coordinate has its own resolution. In addition, such a characteristic as noise immunity is used. This is the ability of a laser locator to operate in conditions of natural and artificial interference. And a very important characteristic of a locator is reliability. This is the property of a locator to maintain its characteristics within established limits under given operating conditions.

The scientists used a ruby ​​laser that generated giant pulses with a duration of 5 10“8 s. To send laser pulses to the Moon and subsequently receive pulses reflected by the lunar surface, an optical telescope from the Crimean Observatory with a main mirror diameter of 260 cm was used. In 1969, American astronauts landed on the surface of the Moon from Apollo 11, and in 1970 on The Soviet spacecraft Lunokhod-1, controlled from the Earth, descended onto the lunar surface. The astronauts and the lunar rover delivered special reflective reflectors to the Moon. A reflector, or, otherwise, a corner reflector is designed to return the light beam incident on it back in a direction strictly parallel to the original direction of the beam. This ability is possessed, for example, by a corner formed by three flat mirrors oriented at right angles to each other. Using the reflection of short laser pulses sent from the Earth from corner reflectors located on the lunar surface, scientists were able to determine the distance from the Earth to the Moon (more precisely, from the mirror of an Earth telescope to the lunar reflector) with an error not exceeding several tens of centimeters. To imagine how high such accuracy is, we must remember that the Moon is located at a distance of 380,000 km from

The laser reflector installed on the lunar surface is a square with a side length of 45 cm, consisting of 100 individual corner reflectors. It is possible to change the orientation of the square plane - taking into account the location of the reflector on the lunar surface
Earth. The range measurement error of 40 cm is 109 times less than the specified distance!
But why measure the distance to the Moon with such great accuracy? Is this really being done just out of “sporting interest”? Of course not. Such measurements are performed not in order to more accurately determine the distance from the earth's telescope to the lunar reflector, but in order to more accurately determine changes in this distance over a certain period of time, for example, over a week, a month, a year. By studying graphs that describe changes in distance over time, scientists obtain information to answer a number of questions of great scientific importance: how is mass distributed in the interior of the Moon? At what speed do they approach or diverge? earth's continents? How does the position of the Earth's magnetic poles change over time?
That’s why there are several dozen laser-location systems for space purposes in the world.
readings. They locate the Moon, as well as artificial Earth satellites for geodetic purposes. As an example, we will indicate a laser location system Physical Institute named after P. N. Lebedev of the USSR Academy of Sciences, intended for locating the Moon. The ruby ​​laser generates giant light pulses with a duration of 10“8 s and an energy of the order of 0.1 J. The pulses pass through quantum amplifier, after which their energy increases to 3 J. Then the light pulses hit the 260-cm telescope mirror and are sent to the Moon. The error in measuring the distance to the Moon is in in this case 90 cm. By reducing the pulse duration to * 10“ 9 s, the error is reduced to 25 cm. As another example, we note a laser location system Space Center in the USA, intended for locating artificial Earth satellites. It uses a pulsed ruby ​​laser that generates pulses with a duration of 4 * 10 "9 s and an energy of 0.25 J. The distance measurement error is 8 cm.
Simplified optical diagram of the laser-location system of the Physical Institute of the USSR Academy of Sciences: 7 - ruby ​​laser, 2 - quantum light amplifier, 3 - main telescope mirror with a diameter of 260 cm

Laser locators are installed not only on earth's surface, but also on aircraft. Let's imagine that two spaceships are approaching each other and are about to dock automatically. It is necessary to accurately control the relative position of the ships and accurately measure the distance between them. To do this, a laser locator is installed on one of the ships. As an example, consider a locator based on a CO2 laser, generating a regular sequence of light pulses with a repetition rate of 50 kHz. The laser beam is scanned line by line (similar to an electron beam in a television tube) within a solid angle of 5 x 5°; the viewing time of the beam for this sector of space is 10 s. The laser locator searches for and identifies the docking vehicle in a specified sector of space, continuously measures its angular coordinates and range, and ensures precise maneuvering - right up to the moment of docking. All operations of the locator are controlled by the on-board computer.
Laser locators are used today both in astronautics and in aviation. In particular, they can serve as precise height meters. Note that the laser altimeter was used on the Apollo spacecraft to map the surface of the Moon.
The main purpose of laser locators is the same as radars: detection and identification of objects distant from the observer, tracking the movement of these objects, obtaining information about the nature of objects and their movement. As in radar, optical ranging uses radiation pulses reflected by the object to detect an object and obtain information about it. At the same time, optical location has a number of advantages over radar. A laser locator allows you to more accurately determine the coordinates and speed of an object. Moreover, it makes it possible to identify the size of an object, its shape, and orientation in space. A video image of the object can be observed on the laser radar screen.
The advantages of laser ranging are associated with the sharp directionality of laser beams, high frequency of optical radiation, and exceptionally short duration of light pulses. Indeed, the rest- 66
With a directed beam, you can literally “feel” an object, “view” different parts of its surface. High frequency optical radiation allows you to more accurately measure the speed of an object. Let us recall that if an object moves towards the observer (from the observer), then the light pulse reflected by it will no longer have the original frequency, but a higher (lower) frequency. This is the Doppler effect, well known both in optics and acoustics; this effect is the basis of the laser anemometers discussed earlier. The change in the frequency of the reflected pulse (Doppler frequency shift) is proportional to the speed of the object (more precisely, the projection of the speed on the direction from the observer to the object) and the frequency of the radiation. The higher the radiation frequency, the greater the Doppler frequency shift measured by the location equipment and, therefore, the more accurately the object’s speed can be determined. Finally, we note the importance of using sufficiently short radiation pulses in location. After all, the distance to an object measured using a locator is proportional to the time interval from the sending of the probing pulse to the reception of the reflected pulse. The shorter the pulse itself, the more accurately this period of time can be determined, and therefore the distance to the object. It is not without reason that in space laser ranging, light pulses with a duration of about 10“8 s or less are used. Let us recall that with a pulse duration of 10"8 s the error in locating the Moon was 90 cm, and with a pulse duration of 2 10_9 s the error decreased to 25 cm.
However, optical location systems also have disadvantages. Of course, it is quite convenient to “inspect” an object using a narrow, highly focused laser beam. However, it is not so easy to detect an object using such a beam; The viewing time of the controlled area of ​​space turns out to be relatively long in this case. Therefore, optical location systems are often used in combination with radar systems. The latter provide a quick overview of the space, quick target detection, and optical systems then measure the parameters of the detected target and track the target. In addition, when propagating optical radiation
When transmitting through the natural environment - the atmosphere or water - problems arise associated with the influence of the environment on the light beam. Firstly, the light is partially absorbed in the medium. Secondly, as the radiation propagates along the path, a continuously increasing distortion of the wave front of the light beam occurs due to atmospheric turbulence, as well as light scattering on particles of the medium. All this limits the range of action of ground-based and underwater optical location systems and makes their operation dependent on the state of the environment and, in particular, on weather conditions.