Radar sensing and resolution. Radar method for studying peat and sapropel deposits. Gas analytical methods for monitoring air samples and vehicles based on them
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Livshits M. Resolution of measuring instruments // Quantum. - 2002. - No. 3. - P. 35-36.
By special agreement with the editorial board and editors of the journal "Kvant"
Everyone knows that a microscope is needed in order, for example, to count the number of microbes on a stage, a telescope - to count the stars in the sky, a radar - to determine the number of aircraft in the sky and the distances to them.
In this article we will talk about the most important property of physical devices - their resolution, i.e. the magnitude of the smallest details of measurement objects distinguished during the measurement process. It is the resolution that is the main characteristic of the quality of the meter used (even more important than the accuracy of measurements). For example, its quality depends not only on the magnification of a microscope. If the microscope device does not provide separate perception of sufficiently small details of the object, then the resulting image will not improve even with a significant increase in magnification. We will only get a larger, but equally fuzzy picture of the object in question. In addition, the measurement errors themselves can only be determined after resolution, i.e. after selecting this part of the object from others.
We will show what physical properties of remote (non-contact) meters directly affect the resolution obtained when using them and what methods can be used to improve the resolution of such devices.
First, let's give a quantitative assessment. The finer details of objects can be identified by a given device during the measurement process, the better (higher) its resolution. For different instruments, there are different definitions and different formulas for quantifying resolving power depending on the purposes and methods: for example, whether the resolution of the details of an object (microscope, binoculars, telescope) or individual lines in the emission spectrum (prism, diffraction grating and other spectral devices) is assessed ), whether independence of observation and measurement of coordinates of several targets is used (radar, sonar, animal echolocator), etc. However, the generally accepted basis for quantitative assessment of resolution is the Rayleigh criterion, originally established for the case of separate observation of two point light sources (resolution of double stars). Its generalization, which allows this criterion to be used in a variety of cases, is carried out as follows.
Let the input effect on the measuring device consist of two peaks separated by an interval Δ x; in this case, at the output of the device from each peak a “response” is obtained in the form of a more spread-out X a burst of finite width, characterizing the properties of the device and called the hardware function (Fig. 1). Then the Rayleigh resolution is called the minimum interval Δ x min between the effects of two peaks, at which the total response still has the form of a double-humped curve (Fig. 2, a). If we reduce Δ x, the top of the total burst is flattened and the bursts merge into one (Fig. 2, b).
What parameters of the waves used in remote sensors determine the resolution? It turns out that this parameter is the degree of coherence of the waves (the Latin word “coherent” means “connected”).
First, let us remember the coherence of oscillations. Oscillations are called coherent if the phase differences and amplitude ratios of the oscillations remain constant throughout the observation time. In the simplest case, two sinusoidal oscillations \(~A \cos (\omega t + \alpha)\) and \(~B \cos (\omega t + \beta)\) are coherent, where A, IN, α And β - constant values. Since wave processes are determined by oscillations at all points in space where these waves exist, a necessary condition for the coherence of waves is the coherence of oscillations occurring at each given point of the wave during the observation time.
A more general and concise definition of wave incoherence is that beams of light or other waves will be incoherent if the phase difference between the oscillations at all points in space where these waves coexist changes repeatedly and irregularly during the observation time.
Now we will try to establish a connection between the resolution of the meter and the degree of wave coherence. This can be done most clearly using the example of radar - a method of determining the location of objects using radio waves.
Let us briefly recall the operating principle of a pulse radar station (radar). Figure 3 shows a block diagram of the radar. Here 1 - transmitter, 2 - antenna switch, 3 - antenna, 4 - antenna radiation pattern, 5 - receiver, 6 - indicator. The radar transmitter, using a narrowly directed antenna, periodically irradiates space with short-term trains of radio waves (the so-called probing, i.e., “probing” pulses). By rotating the antenna (or other methods), the direction of radiation of radio waves is changed and, thereby, sequential probing of a larger or smaller sector of space is carried out (or a circular view). Pulses reflected from various targets arrive (usually through the same antenna) to the radar receiver. In this case, the determination of the angular coordinates of targets is based on the use of the antenna radiation and reception patterns. Ranging D produced by measuring the delay time t zap of arrival of the pulse reflected from the target relative to the moment of emission of the probing pulse:
\(~D = \frac(c t_(zap))(2)\) ,
Where c- speed of light. The two in the denominator appears because the delay time is the sum of the time it takes for the probing pulse to reach the target and the same time for the reflected pulse to reach the radar.
The angular resolution of a radar is the smallest angle difference Δ α between directions at two targets located at the same range, at which the reflected pulses from them are observed separately. It is easy to see that this corresponds to the simplest case of spatial incoherence: those targets are resolved (by angle) that cannot be simultaneously hit by the “illuminating” radar radiation, since the directions on them differ by the width of the antenna radiation pattern (Fig. 4).
The range resolution of a radar is the smallest distance δ r between two targets located in the same direction, in which they are observed separately. In the so-called classical radars, a sinusoidal wave train of constant amplitude was used as a probing pulse. This is explained, in particular, by the fact that such a train is easy to create: it is enough to briefly apply a constant high voltage to a high-frequency generator (for example, a magnetron). The uniformity of the train structure leads to the fact that the waves reflected from different targets will have the same frequency (if they move towards the radar at the same speed or if the Doppler effect can be neglected), within the mutual overlap of the reflected pulses they will be coherent, and the targets will be completely separated it won't work. Pulses reflected from two targets will be incoherent only when they do not coincide in time of arrival at the radar receiver and therefore do not overlap on the indicator screen (Fig. 5).
Thus, the range resolution of these radars is
\(~\delta r = \frac(c \tau)(2)\) ,
Where τ - pulse duration. We can say that in the radar under consideration, the incoherence of reflected signals coming from different targets appears in its simplest form: as the absence of their coincidence in time.
As can be seen from the last formula, to increase range resolution it is necessary to reduce the pulse duration τ . But this inevitably leads to a corresponding expansion of the frequency band. The fact is that, on the one hand, there is a fundamental relationship between the duration τ signal (for example, a broken sinusoid) and width Δ ν its spectrum (on the frequency scale), in which the main pulse energy is concentrated:
\(~\Delta \nu \approx \frac(1)(\tau)\) .
On the other hand, it is quite clear that the target detection range is determined by the energy of the probing and, therefore, returning pulse. This means that when the pulse is shortened, the transmitter power has to be increased accordingly, which is not an easy task.
In search of a way out of this situation, radars have taken the path of increasing the pulse bandwidth without changing its duration: by moving from a sinusoidal to a more complicated internal structure of the probing pulse. This is how radars with linear frequency modulated (chirp) probing pulses appeared (Fig. 6). In this case, it turns out that the relationship between the duration and width of the signal will no longer hold true for the pulse duration τ imp , and for coherence time τ kog:
\(~\tau_(kog) \approx \frac(1)(\Delta \nu)\) , where \(~\Delta \nu >> \frac(1)(\tau_(imp))\).
True, for this purpose an additional special filter is introduced in the radar receiver, with the help of which the received pulse is compressed to a duration τ s = τ kog. Now the pulses on the radar screen will be separated at a much smaller distance between the Targets than was the case when using a sinusoidal pulse:
\(~\delta r = \frac(c \tau_s)(2)<< \frac{c \tau_{imp}}{2}\) ,
This confirms the inextricable connection between the resolution of a remote meter and the degree of wave coherence: to increase (improve) the resolution of the meter, it is necessary to worsen the coherence of the waves used.
It is interesting to note that in living nature development in this direction has gone even further. For example, along with bats, whose echolocators also use chirp probing pulses, there are so-called “whispering” bats that use even more broadband noise pulses, i.e. high-frequency pulses modulated by “white” noise. They detect targets at significantly lower radiation powers, while also providing better protection for their locators from interference, especially from mutual interference that occurs when large groups of these bats simultaneously hunt insects.
The invention relates to the field of radar sensing using single ultra-wideband (UWB) pulse signals and can be used when probing several nearby objects, for example layers of asphalt pavement. The method consists in emitting an N-lobe probing radio pulse, continuously receiving the reflected signal, integrating it N-1 times in a selected time window, detecting and evaluating signals from the objects of study. The achieved technical result of the invention is to increase the resolution accuracy of UWB sensing. 6 ill.
The invention relates to the field of radar sensing using ultra-wideband (UWB) pulse signals with duration T and can be used when probing several objects, the distance between which L is comparable to сT, where c is the speed of light in the medium, i.e. in conditions where signals reflected from several objects of study overlap each other. This problem arises, for example, when probing subsurface soil layers, in particular multilayer asphalt road surfaces.
It is known, p. 24, that any signal S(t) that can be emitted by an antenna must satisfy the condition: including a single multi-lobe UWB radar sounding signal.
When using UWB radar sensing of several nearby research objects, the problem arises of resolving signals received from one and another object. This problem is aggravated by the presence of interference, imperfect transmitting and receiving equipment and many other factors.
The traditional way of pre-processing a radar signal reflected from an object of study is its detection - the selection of a low-frequency function - the amplitude (complex) envelope of the radio pulse. When working with UWB signals, the amplitude envelope of the UWB signal obtained using the Hilbert transform does not always correctly reflect the features of its shape p.17. In this case, the potentially high resolution of UWB signals is not realized.
Known Patent RU 2141674 - a method of ultra-wideband radar sensing, which consists in emitting a pulse with one antenna, receiving this pulse with another - a remote antenna, the received pulse is delayed, re-radiated and received by an antenna located at the site of the primary radiation. This method allows the signals received from the antenna and from the surrounding structural elements to be separated in time. With this method, the resolution problem is solved by temporal separation of the reflected signals.
The disadvantage of this method is the limited scope of application due to the fact that the possibility of artificial separation in time of reflected signals from several objects of study rarely arises.
The closest to the claimed method is that they emit an N-lobe probing radio pulse, continuously receive the reflected signal in a selected time window, detect and evaluate signals from the objects of study. To solve the resolution problem, determine:
Direct transmission signal from the emitting to the receiving antenna (when probing open space), which is subtracted from the received signal during subsequent probing of the environment;
Total reflection signal when probing a metal sheet, which is used to calibrate subsequent probings.
The forward signal is subtracted from the signal received from the research objects. The closest response is then detected one by one and, taking into account the attenuation of the known total reflection signal, it is subtracted from the received signal. Thus, it is theoretically possible to resolve received signals.
The disadvantage of this method is low accuracy. Firstly, a signal passing through the medium changes the frequency spectrum, and therefore not only the amplitude, but also its shape. As a result, it turns out to be inappropriate to use the total reflection signal as a calibration signal. Secondly, the recursive nature of processing, in which each new object is discovered based on the results of the detection of the previous one, leads to the accumulation of errors.
The problem solved by this invention is to increase the resolution of UWB sensing reflected from nearby objects, and therefore to obtain more and better quality information from radar sensing.
To solve the problem posed in a method for increasing the resolution of ultra-wideband radar sensing, which consists in emitting an N-lobe probing radio pulse, continuously receiving the reflected signal in a selected time window, detecting and evaluating signals from objects of study, integrating the reflected signal in a selected time window N -1 time, and use the results of integration to detect and evaluate signals from objects of study.
A significant difference between the proposed method and the prototype is that when probing with an N-lobe radio pulse, the reflected signal is integrated in the selected time window N-1 times.
The prototype uses the operation of subtracting known responses from the received signal.
The use of N-1 multiple integration, a linear method for converting received signals, allows you to convert their multi-lobe time structure into a single-lobe one. Figure 1 shows that a three-lobe radio pulse after a single probing becomes two-lobe, and after the second integration - single-lobe. If such a pulse could be emitted by an antenna, then the task of resolving nearby objects would be greatly simplified. Integrating the received signal for a linear system is equivalent to integrating the input signal. Thus, integrating the output signal greatly simplifies the resolution of nearby objects.
The inventive method is illustrated by the following graphic materials.
Figure 1 - results of sequential integration of a three-lobe signal.
Figure 2 - partial signals reflected from three objects.
Figure 3 - total signal reflected from three objects.
Figure 4 is the result of a single integration of the reflected signal.
Figure 5 is the result of double integration of the reflected signal.
Let us consider the possibility of implementing the proposed method.
For radar sounding, single radio pulses with a small number of time lobes N=2-5 can be used, for example, a three-lobe pulse S(t), shown in Fig.1. Such signals have a UWB spectrum. Their processing is possible in the frequency or time domain. In both cases, it is necessary to detect signals reflected from objects of study, evaluate their amplitude, polarity, temporal position and other parameters. Such soundings are used, for example, in the study of road surface layers. In this case, the objects of study are the boundaries of the coating layers, which reflect the probing signal and have different dielectric constants ε. Depending on the ratio of the dielectric constants ε of the media, the reflected signals can have different polarities.
If the objects of study (road surface layers) are located close to each other, then the reflected signals overlap each other. Figure 2 shows partial signals S 3i (t), (i=1, 2, 3), reflected from three different layers. Each of them has its own amplitude and shape. Signal S 32 (t) has reverse polarity. The total reflected signal S 3 (t)=S 31 (t)+S 32 (t)+S 33 (t), Fig. 3, is of little use for analysis. To solve the resolution problem, it is possible to reduce the duration of the probing signal S(t), but this will lead to an unjustified increase in development costs or to technical impracticability.
Single integration of the signal reflected from objects Fig.4 does not solve the resolution problem, but re-integration
Fig.5 allows us to fairly accurately estimate the temporal position, polarity and amplitude of the reflected signals. This assessment can be obtained visually or using a computer.
Note that with the help of the proposed linear transformation, restoration of the ratio of the amplitudes of partial signals and the distance between them is possible even in the case when the signals are delayed relative to each other for a time less than the duration of the period of the central harmonic of the signal spectrum, i.e. in conditions of realizing potential range resolution.
Thus, the proposed method allows UWB radar sensing to detect objects of study, approaching the potential resolution.
Let us consider the possibility of practical implementation of the proposed method. Figure 6 shows a diagram of a device that implements the proposed method, where:
1. UWB signal generator.
2. Transmitting antenna.
3. Receiving antenna.
4. Multilayer medium under study.
5. Stroboscopic receiver.
6. Controlled delay line.
7. Analog-to-digital converter (ADC).
8. Computer.
The signal from the computer 8 triggers the UWB signal generator 1, which is emitted by the antenna 2. The UWB signal reflected from the multilayer medium 4 under study enters the antenna 3. The delay line 6, controlled by the computer 8, triggers the stroboscopic receiver 5, which selects one instantaneous amplitude of the reflected signal. Analog-to-digital converter 7 converts this value into a code that is read by computer 8. The startup frequency of generator 1 can be tens of kilohertz, which does not require high speed ADC 7. The delay value 6 sets the reception window and the position of the reference point in it. By repeating the measurements many times, you can average the values of this sample of the reflected signal, and by changing the delay value, you can obtain the entire implementation of the reflected signal in the selected time window accurate to the scale-time transformation. Thus, as a result of repeated probing, the instantaneous amplitudes of the reflected signal in the reception window are stored in the memory of the computer 8. Integration of the obtained digital samples is carried out by sequential summation of the samples, and multiple integration is carried out by sequential application of this procedure. In Figs. 1-5, the abscissa axis shows the sample numbers of the UWB signal. The obtained integration results can be processed visually by the operator or by known processing methods in a computer 8.
Thus, the proposed method is technically feasible and makes it possible to increase the resolution of ultra-wideband radar sensing.
List of used literature
1. Astanin L.Yu., Kostylev A.A. Fundamentals of ultra-wideband radar measurements. - M.: Radio and Communications, 1989. - 192 p.: ill.
2. Patent RU 2141674.
3. Patent FR 2626666.
4. Theoretical foundations of radar / Ed. V.E. Dulevich. - M.: Sov. radio, 1978. - 608 p.
A method for increasing the resolution of ultra-wideband radar sensing, which consists in emitting an N-lobe probing radio pulse, where N = 2, 3, 4, 5..., continuously receiving reflected signals in a selected time window, detecting signals from objects of study, measuring and evaluate the parameters of the signals reflected from the objects of study, characterized in that the probing of the object of study with an N-lobe radio pulse is carried out repeatedly; when receiving reflected signals, a controllable delay value sets the reception window with the ability to obtain the entire implementation of the reflected signal in the selected time window and the position of the reference point in It integrates the received samples of the reflected signal in the selected time window of reception N-1 times, converting the N-lobe temporal structure of the signal into a single-lobe one, providing resolution of nearby objects of study, and uses the integration results to detect objects of study, measure and evaluate the parameters of signals from objects of study.
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The invention relates to radio engineering and can be used in passive radio monitoring systems for identifying, direction finding and determining the location of ground and air objects by the emissions of their UHF transmitters when using one receiving station.
Brief description and examples of application of the method
The method of georadar subsurface sensing (in generally accepted terminology is georadar; in English literature this method is called “Ground Penetrating Radar” or GPR.) is based on the study of the propagation of electromagnetic waves in a medium. The idea of the method is to emit pulses of electromagnetic waves and record signals reflected from the interfaces between layers of the probed medium that have a difference in dielectric constant. . Such interfaces in the studied environments are, for example, contact between dry and moisture-saturated soils (groundwater level), contacts between rocks of different lithological composition, between rock and material of an artificial structure, between frozen and thawed soils, between bedrock and loose rocks, etc. d. (the diagram of the formation of the wave pattern is shown in Fig.).
Scheme of the formation of a diffracted electromagnetic wave from a pipe located at a depth H and a wave reflected from the interface between media with different dielectric constants: depth (a.) and time (b.) sections.
All problems solved with the help of ground penetrating radar can be divided into two large groups with research methods, processing methods, types of displaying research objects in the field of electromagnetic waves and presenting results characteristic of each group. The first group includes geological, hydrogeological and geotechnical tasks, such as mapping:
- bedrock surfaces under loose sediments;
- groundwater levels and boundaries between layers with varying degrees of water saturation;
- sand, clay, peat, etc.;
- frozen soils;
- determination of the thickness of the water layer and mapping of subbottom sediments;
- thickness of ice and snow.
The second group of tasks includes the search for local objects, inspection of engineering structures, violation of the normal situation, for example:
- search for underground cavities;
- inspection of bridges and road surfaces;
- mapping of communications (pipelines and cables);
- inspection of concrete structures;
- saline soils;
- sections of the section with disturbed natural soil occurrence - reclaimed land, backfilled excavations.
That. Currently, GPR is widely used in research at relatively shallow depths of target objects (0.2 - 15 meters), with the exception of the study of glaciers and frozen rocks, in which, due to high resistance, the depth increases.
Georadar is a digital, portable geophysical device carried by one operator, designed to solve a wide range of geotechnical, geological, environmental, engineering and other problems where there is a need for operational monitoring of the environment, obtaining soil sections that do not require drilling or excavation. During sounding, the operator receives information in real time on the display in the form of a radar profile (called a radargram). At the same time, the data is recorded on the computer’s hard drive for further use (processing, printing, interpretation, etc.).
A set of replaceable antenna modules provides the ability to probe over a wide frequency range (16 - 2000 MHz). The use of a particular antenna system is determined by the problem being solved during sounding. Increasing the probing frequency leads to improved resolution; but at the same time the attenuation of the electromagnetic wave in the medium increases, which leads to a decrease in the probing depth; conversely, by reducing the frequency, you can increase the probing depth, but you will have to pay for this by deteriorating the resolution. In addition, as the frequency decreases, the initial insensitivity zone (the so-called dead zone) of the georadar increases.
Below is a table of the dependence of resolution, dead zone and probing depth depending on the antenna used. It is assumed that soil with a relative dielectric constant of 4 and a specific attenuation of 1-2 dB/meter is being probed. By depth we mean the depth of detection of a flat boundary with a reflection coefficient of 1. It should be borne in mind that these data are very approximate, they strongly depend on the parameters of the probed medium.
Parameter | Center frequency | ||||||
2 GHz | 900 MHz | 500 MHz | 300 MHz | 150 MHz | 75 MHz | 38 MHz | |
Resolution, m | 0.06 — 0.1 | 0.2 | 0.5 | 1.0 | 1.0 | 2.0 | 4.0 |
Dead zone, m | 0.08 | 0.1-0.2 | 0.25-0.5 | 0.5-1.0 | 1.0 | 2.0 | 4.0 |
Depth, m | 1.5-2 | 3-5 | 7-10 | 10-15 | 7-10 | 10-15 | 15-30 |
Modern GPRs are designed to work in hard-to-reach areas with unfavorable climates and can be used at any time of the year (GPR operating temperature -20...+40°C).
Below are examples of applying the method to solve some (very few) problems.
Discovery of three metal pipes buried in the ground to a depth of 1 - 1.5 meters. Each pipe gives a trajectory signal in the form of a hyperbola, the vertex of which corresponds to its location. Sounding frequency 900 MHz. Sounding location - near Daugavpils, Latvia. | |
Discovery of a karst cavity in limestone under a layer of loam. The cavity (circled) is visible on the left side of the profile in the form of alternating stripes. Loam is shown at the top as a continuous signal. Probing frequency 300 MHz. The sounding site is the shore of the Dead Sea, Israel. | |
Probing a brick wall. In the middle of the profile, the signal from the metal cabinet built into the wall is clearly visible. Probing frequency 2 GHz. Sounding location: Riga, Latvia. | |
Profiling a lake from the bottom of a plastic boat. A 500 MHz shielded antenna was used. Metal objects are very clearly visible in the silt (indicated in the figure as MO). | |
This profile was obtained by probing the wall of a salt mine drift. Signals in the form of many hyperbolas from the neighboring drift are clearly visible. The distance between the drifts is approximately 7.5 meters. Probing frequency 500 MHz. Sounding location: Mirny, Russia. |
30 /11
2018
Application of laser scanning in building information modeling
Modern tasks arising in the design, construction, and operation of buildings and structures require the presentation of data in three-dimensional space, which with high accuracy and completeness describes the relative position of parts of buildings, structures, the situation and the topography.
The invention relates to the field of radar sensing using single ultra-wideband (UWB) pulse signals and can be used when probing several nearby objects, for example layers of asphalt pavement. The method consists in emitting an N-lobe probing radio pulse, continuously receiving the reflected signal, integrating it N-1 times in a selected time window, detecting and evaluating signals from the objects of study. The achieved technical result of the invention is to increase the resolution accuracy of UWB sensing. 6 ill.
Drawings for RF patent 2348945
The invention relates to the field of radar sensing using ultra-wideband (UWB) pulse signals with duration T and can be used when probing several objects, the distance between which L is comparable to сT, where c is the speed of light in the medium, i.e. in conditions where signals reflected from several objects of study overlap each other. This problem arises, for example, when probing subsurface soil layers, in particular multilayer asphalt road surfaces.
It is known, p.24, that any signal S(t) that can be emitted by an antenna must satisfy the condition: including a single multi-lobe UWB radar sounding signal.
When using UWB radar sensing of several nearby research objects, the problem arises of resolving signals received from one and another object. This problem is aggravated by the presence of interference, imperfect transmitting and receiving equipment and many other factors.
The traditional way of pre-processing a radar signal reflected from an object of study is its detection - the selection of a low-frequency function - the amplitude (complex) envelope of the radio pulse. When working with UWB signals, the amplitude envelope of the UWB signal obtained using the Hilbert transform does not always correctly reflect the features of its shape p.17. In this case, the potentially high resolution of UWB signals is not realized.
3. Patent FR 2626666.
4. Theoretical foundations of radar / Ed. V.E. Dulevich. - M.: Sov. radio, 1978. - 608 p.
CLAIM
A method for increasing the resolution of ultra-wideband radar sensing, which consists in emitting an N-lobe probing radio pulse, where N = 2, 3, 4, 5..., continuously receiving reflected signals in a selected time window, detecting signals from objects of study, measuring and evaluate the parameters of the signals reflected from the objects of study, characterized in that the probing of the object of study with an N-lobe radio pulse is carried out repeatedly; when receiving reflected signals, a controllable delay value sets the reception window with the ability to obtain the entire implementation of the reflected signal in the selected time window and the position of the reference point in It integrates the received samples of the reflected signal in the selected time window of reception N-1 times, converting the N-lobe temporal structure of the signal into a single-lobe one, providing resolution of nearby objects of study, and uses the integration results to detect objects of study, measure and evaluate the parameters of signals from objects of study.