A dynamic absolute distance measurement method based on sinusoidal frequency modulation interference

Abstract

The application relates to a dynamic absolute distance measurement method based on sinusoidal frequency modulation interference and relates to the field of laser radar phase demodulation technology. The application is aimed at solving the problems of low precision and limited measurement range of the existing absolute distance measurement method. The dynamic absolute distance measurement method based on sinusoidal frequency modulation interference comprises the following steps: inputting light signals emitted by a sweep frequency light source into a first coupler, so that the light signals are divided into two light signals; inputting the two light signals into a measurement interferometer and an auxiliary interferometer respectively; compensating the asymmetry of output signals by using an ellipse fitting algorithm; extracting the phases of the measurement interferometer and the auxiliary interferometer signals with high precision by using an inverse tangent and unwrapping operation; filtering the phase of the measurement interferometer; directly calculating the relative displacement of a target by using the low-frequency part; calculating the envelope of the signal by using the high-frequency part; calculating the envelope of the phase of the auxiliary interferometer; and finally calculating the absolute distance of the target by taking the ratio of the two envelopes and eliminating the influence of modulation bandwidth fluctuation.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of laser radar phase demodulation. BACKGROUND

[0002] The linear frequency modulation continuous wave laser radar has a large frequency modulation bandwidth, and can obtain high range resolution and ranging accuracy. However, the linear frequency modulation process is complex, and the serious nonlinearity will bring great trouble to the measurement process. The sinusoidal frequency modulation interferometry has the advantages of simple modulation, no measurement blind area, and small nonlinearity, and is increasingly prominent in the field of large-size workpiece three-dimensional topography measurement. The technical core is frequency scanning interferometry (FSI), and the basic principle is to measure the intermediate frequency beat signal formed by the interference of the emitted signal and the echo signal through a photoelectric detector, and then extract the frequency domain features to solve the absolute distance. However, the beat signal of sinusoidal frequency modulation is a zero-crossing frequency wideband signal, and the beat frequency is no longer a constant value, so the distance information of the target cannot be directly obtained by spectrum analysis. In order to perform high-precision absolute distance measurement, the above problems must be corrected, otherwise the measurement accuracy will be seriously affected.

[0003] Many scholars have studied and discussed the problem of sinusoidal frequency modulation measurement. Jesse Zheng calculated the constant beat frequency by averaging the frequency of the beat signal, and realized the measurement of absolute distance, but the measurement accuracy is low and cannot meet the requirements of high-precision absolute distance measurement. Shichun Zhang et al. converted the sinusoidal frequency modulation into linear frequency modulation with more serious nonlinearity, and proposed a method based on optical frequency resampling to correct the nonlinearity, successfully converted the beat signal into a single frequency signal, and completed the measurement of target distance. However, this equal optical frequency resampling technology requires an auxiliary interferometer constructed by a long optical fiber with a known length, and the frequency of the generated beat signal must be at least twice the frequency of the beat signal of the measurement interferometer, which increases the requirement for the sampling rate of the data acquisition card and limits the ranging range. In addition, the long optical fiber also causes problems such as dispersion mismatch, temperature drift, poor noise resistance, and poor repeatability.

[0004] The phase generation carrier technology is currently commonly used for vibration information measurement. The phase carrier signal is generated by frequency modulation of the laser, and the measured signal is shifted to the sideband of the carrier, so that the measured signal and the low-frequency noise are separated in the frequency spectrum. Finally, the measured signal is calculated through signal processing. This method has the advantages of high sensitivity, strong anti-interference ability, and strong real-time performance. However, in the demodulation system, the size of the carrier frequency is limited by the performance of the modulation device and the subsequent circuit, and cannot be unlimitedly improved. Therefore, the dynamic range of this demodulation method is limited, and its measurement accuracy is easily affected by the power fluctuation of the light source, the change of the phase modulation depth, and the amplitude drift of the mixed signal. SUMMARY

[0005] The present application is to solve the problem of low precision and limited measurement range in the prior art absolute distance measurement method, and provides a dynamic absolute distance measurement method based on sinusoidal frequency modulation interference.

[0006] The dynamic absolute distance measurement method based on sinusoidal frequency modulation interference comprises the following steps:

[0007] The light signal emitted by the swept source 1 is input to the first coupler 2, and the first coupler 2 divides the input light signal into two beams. The two light signals output by the first coupler 2 are input to the measurement interferometer and the auxiliary interferometer, respectively.

[0008] The elliptical fitting algorithm is used to compensate for the asymmetry of the measurement interference signal E m (t) output by the measurement interferometer and the asymmetry of the auxiliary interference signal E f (t) output by the auxiliary interferometer.

[0009] The arctangent and envelope extraction operation are used to extract the phase of the measurement interference signal E m (t) and the phase of the auxiliary interference signal E f (t).

[0010] The high-frequency term in the equation is high-pass filtered to obtain a high-pass filtering result The low-frequency term in the equation is low-pass filtered to obtain a low-pass filtering result The low-pass filtering result is used to obtain the amplitude ΔR(t) of the measured target according to the following formula:

[0011] Where c is the speed of light, and ω0 is the light frequency of the light signal emitted by the swept source 1.

[0012]

[0013] Where c is the speed of light, and ω0 is the light frequency of the light signal emitted by the swept source 1.

[0014] The envelopes of E and E are extracted, respectively, to obtain the envelope Z of E m and the envelope Z of E f ,

[0015] The ratio of Z m and Z f is obtained to obtain the absolute distance R m of the measured target.

[0016] ​​​Further, the above-mentioned measuring interferometer comprises a second coupler 3, a circulator 4, a 3x3 coupler 6 for measurement and three measuring photodetectors 7,

[0017] The one beam of light signal outputted by the first coupler 2 is incident to the second coupler 3, and the second coupler 3 divides the inputted light signal into two beams, one of which is inputted to the 3x3 coupler 6 for measurement, and the other is focused to the measured target through the circulator 4, the light signal reflected by the measured target is inputted to the 3x3 coupler 6 for measurement through the circulator 4, and the 3x3 coupler 6 for measurement divides the inputted light signal into three beams which are respectively inputted to the light-sensitive surfaces of the three measuring photodetectors 7, and the signals outputted by the three measuring photodetectors 7 are collectively used as the measuring interference signal E of the measuring interferometer m (t).

[0018] Further, the above-mentioned auxiliary interferometer comprises a third coupler 8, a 3x3 coupler 9 for auxiliary measurement and three auxiliary photodetectors 10,

[0019] The other beam of light signal outputted by the first coupler 2 is incident to the third coupler 8, and the third coupler 8 divides the inputted light signal into two beams which are then inputted to the 3x3 coupler 9 for auxiliary measurement, there is an optical path difference between the two beams of light signal between the third coupler 8 and the 3x3 coupler 9 for auxiliary measurement, the 3x3 coupler 9 for auxiliary measurement divides the inputted light signal into three beams which are respectively inputted to the light-sensitive surfaces of the three auxiliary photodetectors 10, and the signals outputted by the three auxiliary photodetectors 10 are collectively used as the auxiliary interference signal E of the auxiliary interferometer f (t).

[0020] Further, the signals E m1 (t) outputted by the three measuring photodetectors 7, m2 (t) and the signals E m3 (t) outputted by the three auxiliary photodetectors 10 are respectively expressed as follows:

[0021]

[0022]

[0023]

[0024] wherein H m1 , H m2 and H m3 are the direct current amounts of the signals outputted by the three measuring photodetectors 7 respectively, D m1 , D m2 and D m3 are the amplitude values of the cross current amounts of the signals outputted by the three measuring photodetectors 7 respectively, is the phase shift amount of the measuring interference signal;

[0025] The signals E f1 (t), E f2 (t) and E f3 (t) are expressed as follows, respectively:

[0026]

[0027]

[0028]

[0029] where H f1 , H f2 and H f3 are the direct current components of the signals outputted by the three auxiliary photodetectors 10, D f1 , D f2 and D f3 are the amplitude values of the cross current components of the signals outputted by the three auxiliary photodetectors 10, and is the phase shift of the auxiliary interference signal.

[0030] Further, the phase m of the measurement interference signal E (t) is extracted by the following formula:

[0031]

[0032] The phase f of the auxiliary interference signal E (t) is extracted by the following formula:

[0033]

[0034] Further, in the expression of the phase m of the measurement interference signal E (t), the parameters H m1 , H m2 , D m1 , D m2 and are obtained by means of an ellipse fitting, and the fitting ellipse has the following quadratic curve equation:

[0035] α 2 +Bβ 2 +Cαβ+Dα+Fβ+G=0,

[0036] Then, we have:

[0037]

[0038]

[0039]

[0040] wherein α and β are the output signals of any two of the three measurement photodetectors 7, B is the coefficient of the quadratic term of β, C is the coefficient of the αβ term, D is the coefficient of the α term, F is the coefficient of the β term, and G is the constant term of the elliptic quadratic equation.

[0041] Further, the above auxiliary interference signal E f (t) has the expression wherein the parameter H f1 , H f2 , D f1 , D f2 , and are obtained by means of an elliptic fit, the fitting ellipse having the following quadratic equation:

[0042] δ 2 + Hε 2 + Iδε + Jδ + Kε + L = 0,

[0043] then it has the expression

[0044]

[0045]

[0046]

[0047] wherein δ and ε are the output signals of any two of the three auxiliary photodetectors 10, H is the coefficient of the quadratic term of ε, I is the coefficient of the δε term, J is the coefficient of the δ term, K is the coefficient of the ε term, and L is the constant term of the elliptic quadratic equation.

[0048] Further, the above phase and the phase have the following expressions, respectively:

[0049]

[0050]

[0051] wherein Δω is the modulation bandwidth, R f is the arm length difference of the auxiliary interferometer, ω m is the modulation frequency, and t is time.

[0052] Further, the above high-pass filtered result of the high frequency term in has the expression

[0053]

[0054] ​ Low pass filter result of middle-low frequency term The expression is:

[0055]

[0056] Further, the above The envelope Z of m And The envelope Z of f The expressions are as follows, respectively:

[0057]

[0058]

[0059] Z is compared with Z m And Z f The absolute distance R of the measured target is obtained: m :

[0060]

[0061] The application first obtains the output signals of the measurement interferometer and the auxiliary interferometer through a 3*3 coupler, compensates the asymmetry of the output signals by means of an ellipse fitting algorithm. Then, the phase of the measurement interferometer and the auxiliary interferometer signal is extracted with high precision by means of arctangent and unwrapping operation, then the phase of the measurement interferometer is filtered, the low frequency part can directly calculate the relative displacement of the target, the high frequency part needs to calculate the envelope of the signal first, and then the envelope of the auxiliary interferometer phase, finally the two envelopes are compared, that is, the influence of the modulation bandwidth fluctuation can be eliminated, and the absolute distance of the target is calculated. After the above processing, the dynamic absolute distance measurement of the sinusoidal frequency modulation interference can be realized. Compared with the existing method, the application has the advantages of simple structure, no measurement blind area, small nonlinear influence, can realize dynamic target real-time trajectory tracking, can realize online traceability and the like. BRIEF DESCRIPTION OF DRAWINGS

[0062] Fig. 1 It is an optical path schematic diagram for dynamic absolute distance measurement based on sinusoidal frequency modulation interference;

[0063] Fig. 2 It is an output signal asymmetry compensation result based on ellipse fitting (signal-to-noise ratio 20dB), (a) indicates before compensation, (b) indicates after compensation;

[0064] Fig. 3 It is a phase demodulation error diagram, (a) indicates the auxiliary interferometer, (b) indicates the measurement interferometer;

[0065] Fig. 4 It is a distance curve diagram of the target, (a) indicates a vibration trajectory, (b) indicates a motion trajectory. DETAILED DESCRIPTION

[0066] The technical solutions in the embodiments of the present application will be clearly and completely described with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application. It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0067] DETAILED DESCRIPTION Figs. 1 to 4 The present embodiment is specifically described. The dynamic absolute distance measurement method based on sinusoidal frequency modulation interference in the present embodiment is specifically as follows:

[0068] The present embodiment is based on a measurement system. The measurement system comprises a swept-frequency light source 1, a first coupler 2, a measurement interferometer and an auxiliary interferometer. The measurement interferometer comprises a second coupler 3, a circulator 4, a 3x3 coupler 6 for measurement and three measurement photodetectors 7. The auxiliary interferometer comprises a third coupler 8, a 3x3 coupler 9 for auxiliary measurement and three auxiliary photodetectors 10.

[0069] The light signal emitted by the swept-frequency light source 1 is input to the first coupler 2. The first coupler 2 divides the input light signal into two light signals, and the two light signals are input to the measurement interferometer and the auxiliary interferometer, respectively.

[0070] In the measurement interferometer, one of the light signals output by the first coupler 2 is incident on the second coupler 3. The second coupler 3 divides the input light signal into two light signals, one of which is input to the 3x3 coupler 6 for measurement, and the other of which is focused to the measured target 11 through the circulator 4 and the focusing lens 5. The light signal reflected by the measured target returns along the original path and is input to the 3x3 coupler 6 for measurement through the circulator 4. The 3x3 coupler 6 for measurement divides the input light signal into three light signals, which are input to the light-sensitive surfaces of the three measurement photodetectors 7, respectively. The signals output by the three measurement photodetectors 7 are collectively used as the measurement interference signal E(t) of the measurement interferometer. m The expressions of the signals E(t), E(t), E(t) and E(t) output by the three measurement photodetectors 7 are as follows: m1 m2 m3

[0071]

[0072]

[0073] ​​​

[0074] wherein H m1 , H m2 , H m3 are the direct current amplitudes of the output signals of the three measuring photodetectors 7, D m1 , D m2 , D m3 are the quadrature current amplitudes of the output signals of the three measuring photodetectors 7, is the phase shift of the measuring interference signal.

[0075] In the auxiliary interferometer, the other beam of light signals output by the first coupler 2 is incident to a third coupler 8, which divides the input light signals into two beams and inputs them to an auxiliary 3x3 coupler 9. There is an optical path difference between the two beams of light signals between the third coupler 8 and the auxiliary 3x3 coupler 9. The auxiliary 3x3 coupler 9 divides the input light signals into three beams and inputs them to the light-sensitive surfaces of three auxiliary photodetectors 10. The signals output by the three auxiliary photodetectors 10 together serve as the auxiliary interference signal E f (t) of the auxiliary interferometer. The expressions of the signals E f1 (t), E f2 (t) and E f3 (t) output by the three auxiliary photodetectors 10 are as follows:

[0076]

[0077]

[0078]

[0079] wherein H f1 , H f2 , H f3 are the direct current amplitudes of the output signals of the three auxiliary photodetectors 10, D f1 , D f2 , D f3 are the quadrature current amplitudes of the output signals of the three auxiliary photodetectors 10, is the phase shift of the auxiliary interference signal.

[0080] This embodiment is to show that the 3x3 coupler outputs three signals with fixed phase difference. Therefore, three photodetectors are used in both the measuring interferometer and the auxiliary interferometer. However, only two of the three signals are needed in the subsequent correction, which is as follows:

[0081] After the measuring interference signal E m (t) and the auxiliary interference signal E f (t) are measured by the measuring system, the following operations are performed:

[0082] The measurement interference signal E m (t) of the asymmetry of the auxiliary interference signal E f (t) of the asymmetry.

[0083] Specifically, since the output of the 3x3 coupler presents asymmetry, the traditional method cannot be used to demodulate the phase, and the ellipse fitting correction is selected in the embodiment. Taking the measurement interferometer as an example, two output signals of the 3x3 coupler are selected, and the two output signals should fall on the ideal fitting ellipse, so there is the following quadratic curve equation:

[0084] α 2 +Bβ 2 +Cαβ+Dα+Fβ+G=0,

[0085] δ 2 +Hε 2 +Iδε+Jδ+Kε+L=0,

[0086] α and β are the output signals of any two of the three measurement photodetectors 7, B is the quadratic term coefficient of β, C is the αβ term coefficient, D is the α term coefficient, F is the β term coefficient, and G is the constant term of the ellipse quadratic curve equation;

[0087] δ and ε are the output signals of any two of the three auxiliary photodetectors 10, H is the quadratic term coefficient of ε, I is the δε term coefficient, J is the δ term coefficient, K is the ε term coefficient, and L is the constant term of the ellipse quadratic curve equation.

[0088] The expression of each parameter in the two output signals and the corresponding relationship of each coefficient in the ellipse fitting equation are as follows:

[0089]

[0090]

[0091]

[0092]

[0093]

[0094]

[0095] The phase of the measurement interference signal E m (t) is extracted by using the arctangent and envelope operation f (t) is extracted by using the arctangent and envelope operation The phase of the auxiliary interference signal E f (t) is extracted by using the arctangent and envelope operation Specifically,

[0096] The measurement interference signal E is extracted by the following formula m The phase of (t)

[0097]

[0098] The auxiliary interference signal E is extracted by the following formula f The phase of (t)

[0099]

[0100] Since:

[0101] The phase And the expression of the phase are respectively as follows:

[0102]

[0103]

[0104] Where Δω is the modulation bandwidth, R f is the arm length difference of the auxiliary interferometer, ω m is the modulation frequency, and t is time.

[0105] The low-frequency term in the phase is low-pass filtered to obtain a low-pass filtered result of the low-frequency term in the phase The expression is:

[0106]

[0107] The low-pass filtered result is used to obtain the amplitude ΔR(t) of the measured target according to the following formula:

[0108]

[0109] Where c is the speed of light, and ω0 is the optical frequency of the light signal emitted by the swept light source 1.

[0110] The high-frequency term in the phase is high-pass filtered to obtain a high-pass filtered result of the high-frequency term in the phase The expression is:

[0111]

[0112] The envelopes of and are respectively obtained by envelope demodulation, and the envelope Z of is obtained.m and the envelope Z of f , the envelope Z of m and the envelope Z of f The expressions are as follows, respectively:

[0113]

[0114]

[0115] Z m and Z f are compared to obtain the absolute distance R of the measured target m :

[0116]

[0117] Since filtering with a filter will change the envelope, in the high-pass filtering and low-pass filtering processes described in the embodiments, a filtering method of Fourier transform-interference frequency zeroing-inverse Fourier transform signal reconstruction is used. This method can avoid changes in the signal envelope (amplitude) during filtering.

[0118] While the application has been described with reference to particular embodiments, it will be understood that the examples are merely illustrative of the principles and applications of the application. It will be understood that various modifications can be made to the illustrative embodiments, and other arrangements can be devised without departing from the spirit and scope of the application as defined by the appended claims. It will be understood that the features described with reference to separate embodiments can be used in combination with each other. It will be understood that features described in relation to one embodiment can be used in other embodiments described.

Claims

1. A method of dynamic absolute distance measurement based on sinusoidal frequency modulation interferometry, characterized in that, Specifically, The light signal emitted by the swept source (1) is input to the first coupler (2), the first coupler (2) divides the input light signal into two light signals, and the two light signals are input to the measurement interferometer and the auxiliary interferometer respectively; The measurement interference signal E output by the measurement interferometer is compensated using an ellipse fitting algorithm m the asymmetry of the auxiliary interference signal E f (t) and the asymmetry of the auxiliary interference signal E The phase of the measurement interference signal E m (t) is extracted using the arctangent and unenvelope operation The phase of the auxiliary interference signal E f (t) is extracted using the arctangent and unenvelope operation high pass filtering the high frequency terms in the phase results in a high pass filtered result low pass filtering the low frequency terms in the phase results in a low pass filtered result Utilizing the low pass filtering result The amplitude ΔR(t) of the measured object is obtained according to the following formula: Wherein, c is the speed of light, ω0 is the light frequency of the light signal emitted by the swept source (1); respectively, and the envelopes Z and are obtained, respectively, by envelope detection of m and f , respectively,​​ Z m and Z f to obtain the absolute distance R m of the measured target.

2. A method of dynamic absolute distance measurement based on sinusoidal frequency modulation interference according to claim 1, characterized in that, The measurement interferometer comprises a second coupler (3), a circulator (4), a 3×3 coupler (6) for measurement and three measurement photodetectors (7), The light signal output by the first coupler (2) is incident on a second coupler (3), which divides the input light signal into two beams, one of which is input to a 3x3 coupler (6) for measurement, and the other of which is focused by a circulator (4) on a target to be measured. The light signal reflected by the target to be measured is input to the 3x3 coupler (6) for measurement by the circulator (4). The 3x3 coupler (6) for measurement divides the input light signal into three beams, which are input to the light-sensitive surfaces of three measuring photodetectors (7), respectively. The signals output by the three measuring photodetectors (7) are collectively used as a measurement interference signal E of the measurement interferometer m (t).

3. A method of dynamic absolute distance measurement based on sinusoidal frequency modulation interference according to claim 2, characterized in that, The auxiliary interferometer comprises a third coupler (8), a 3×3 coupler (9) for auxiliary measurement and three auxiliary photodetectors (10), The other beam of light signals outputted by the first coupler (2) is incident to a third coupler (8), the third coupler (8) divides the inputted light signals into two beams and inputs them to an auxiliary 3x3 coupler (9), there is an optical path difference between the third coupler (8) and the auxiliary 3x3 coupler (9), the auxiliary 3x3 coupler (9) divides the inputted light signals into three beams and inputs them to the light sensitive surfaces of three auxiliary photodetectors (10) respectively, the signals outputted by the three auxiliary photodetectors (10) are collectively used as the auxiliary interference signal E of the auxiliary interferometer f (t).

4. A method of dynamic absolute distance measurement based on sinusoidal frequency modulation interference according to claim 3, characterized in that, The signals E m1 (t), E m2 (t) and E m3 (t) are expressed as follows, respectively: where H m1 , H m2 , H m3 are the direct current components of the output signals of the three measuring photodetectors (7), m1 , D m2 , D m3 are the amplitude values of the alternating current components of the output signals of the three measuring photodetectors (7), is the phase shift of the measuring interference signal. The signals E f1 (t), E f2 (t) and E f3 (t) are expressed as follows, respectively: where H f1 , H f2 , H f3 are the direct current amplitudes of the output signals of the three auxiliary photodetectors (10) respectively, f1 , D f2 , D f3 are the alternating current amplitudes of the output signals of the three auxiliary photodetectors (10) respectively, is the phase shift of the auxiliary interference signal.

5. A method of dynamic absolute distance measurement based on sinusoidal frequency modulation interference according to claim 4, characterized in that, The measured interference signal E is extracted by the equation m the phase of (t) The auxiliary interference signal E is extracted by the equation f the phase of (t) 6. A method of dynamic absolute distance measurement based on sinusoidal frequency modulation interference according to claim 5, characterized in that, The measured interference signal E m The phase of (t) In the expression of, the parameters H m1 , H m2 , D m1 , D m2 and are obtained by means of an ellipse fitting, the fitting ellipse having the following quadratic curve equation: a 2 + Bβ 2 + Cαβ + Dα + Fβ + G = 0, Then, Wherein, α and β are the output signals of any two of the three measurement photodetectors (7) respectively, B is the quadratic term coefficient of β, C is the αβ term coefficient, D is the α term coefficient, F is the β term coefficient, and G is the constant term of the elliptic quadratic curve equation.

7. A method of dynamic absolute distance measurement based on sinusoidal frequency modulation interference according to claim 5, characterized in that, said auxiliary interference signal E f the phase of (t) In the expression of, the parameters H f1 , H f2 , D f1 , D f2 , and are obtained by means of an ellipse fitting, the fitting ellipse being present by the following quadratic curve equation: delta 2 +Hepsilon 2 +I delta + J delta + K epsilon + L = 0, Then, Wherein, δ and ε are the output signals of any two of the three auxiliary photodetectors (10) respectively, H is the quadratic term coefficient of ε, I is the δε term coefficient, J is the δ term coefficient, K is the ε term coefficient, and L is the constant term of the elliptic quadratic curve equation.

8. A dynamic absolute distance measurement method based on sinusoidal frequency modulation interference according to any one of claims 1 to 7, characterized in that, Phase with phase are as follows: where Δω is the modulation bandwidth, R f is the arm length difference of the auxiliary interferometer, ω m is the modulation frequency, and t is time.

9. A method of dynamic absolute distance measurement based on sinusoidal frequency modulation interference according to claim 8, characterized in that, high pass filtered result of the mid-high frequency term is expressed as Low pass filtered result of mid-low frequency term The expression is:

10. A method of dynamic absolute distance measurement based on sinusoidal frequency modulation interference according to claim 8, characterized in that, the envelope Z of m and the envelope Z of f the expressions are as follows, respectively: Z m and Z f to obtain the absolute distance R m of the measured target.

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