WO1988009480A1

WO1988009480A1 - Process and device for compensating for the influence of turbulences in measurements

Info

Publication number: WO1988009480A1
Application number: PCT/CH1988/000093
Authority: WO
Inventors: Bernhard Gaechter; André HUISER
Original assignee: Wild Heerbrugg Ag
Priority date: 1987-05-19
Filing date: 1988-05-19
Publication date: 1988-12-01
Also published as: DE3730093C2; CH674080A5; DE3730093A1

Abstract

In order to acquire and compensate for turbulence effects when determining the direction between a target and a measurement station, a wave radiation with at least two selected frequencies is emitted. The incidence angles of both selected waves are measured practically at the same time at the measurement station. The difference between both measured angular values is then calculated, and on the basis of that difference the angular deviation between the apparent and the real direction of the target is calculated by means of a given algorithm. With this process, short-term non-homogeneity variations of the refractive index of a dispersable medium can also be taken into account when determining the direction of a target. Very precise and corrected angular measurements can thus be carried out very quickly, for instance in a matter of seconds, even in the presence of strong turbulences of the medium along the measurement section extending between the measurement point and the target.

Description

Method and device for compensating turbulence influences in surveying

The invention relates to a method for compensating for inhomogeneities in the refractive index of a dispersive medium when determining the direction of a target point by evaluating the propagation behavior of wave radiation emanating from the target point, according to the preamble of patent claim 1 of the procedure.

In surveying, with increasing precision of the instruments and with the higher demands on the measuring accuracy, turbulence of the medium to be penetrated by the measuring beam becomes noticeable. This manifests itself, for example, in the fact that a point light source is only observed at a certain distance as a flickering light spot. Such a smeared light spot cannot be resolved further using conventional methods, since the true position of the image of the punctiform light source is subject to statistical temporal fluctuations which are based on inhomogeneities in the density of the penetrated medium and thus on fluctuations in the refractive index along the measurement path. Investigations into these phenomena were originally stimulated by observing the stars and the irregularities that occur as a result of observation by the earth's atmosphere. To reduce the resulting measurement errors in optical observations, only methods are known for the compensation of quasi-static large-area inhomogeneities, the so-called refraction compensation. When carrying out these methods, correspondingly long observation times have to be accepted.

In the meantime it has been shown that methods for refraction compensation are no longer sufficient for the correction in high-precision measurements. Rather, the effects of turbulence in the penetrated medium become noticeable as a predominant disruptive factor. The turbulences are particularly unpleasant, e.g. in the case of high-precision optical direction determinations which are carried out with the aid of a directional beam, e.g. a laser beam emanating from the target point. In contrast to refraction, however, turbulences are not long-term effects, but, depending on the flow conditions in the medium, short-term effects, which can decisively change the refractive behavior along the measuring path within minutes or seconds.

This basic knowledge leads to the object of the present invention, which consists in developing a method and a device for detecting and compensating for such turbulence effects when determining the direction, so that even variable inhomogeneities for a short time in the refractive index of a dispersive medium when determining the direction of a target point by evaluating the propagation behavior of a wave radiation emanating from the target point.

This object is achieved according to the invention by the features defined in claims 1 and 5.

The measures according to the invention offer significant advantages in several respects: Even in the event of strong turbulence in the medium along the measuring path between the measuring point and the target point, e.g. perform very precise corrected angle measurements within seconds. The effects of turbulence are recorded briefly and also simultaneously with the actual measurement and processed to a corrected measured value. The reproducible accuracy of the measured values is a few hundred nanoradians when the measuring time of a few seconds is selected.

Contrary to previous assumptions from known methods for refraction compensation, it is crucial for the optimal compensation of turbulence influences to choose the aperture of the receiver optics as small as possible. This means that there is space for a corresponding additional device in a conventional theodolite, as a result of which its measuring accuracy can be considerably improved, in particular when there are large temperature differences and when a certain air flow occurs. The invention is explained in more detail below on the basis of preferred exemplary embodiments with the aid of the drawings.

Show it:

1 shows the schematic representation of the measuring principle,

2 shows the influence of the dispersion during the measurement,

3 shows the variance of the angle difference (curve A) corrected by the method according to the invention in comparison with the conventionally obtained result (curve B),

Fig. 4 shows the example of a transmitter with a laser diode for generating two transmission frequencies, and

Fig. 5 shows a receiver circuit for the invention

Evaluation of optical signals using the multiplex method.

The principle of the present invention is first explained with reference to FIG. 1. There is a direct geometric connection D between a transmitter 1 and a receiver 2. Light rays emanating from the transmitter 1 are deflected along the transmission path as a result of changes in refractive index, so that they are in the receiver at an angle β with respect to the straight line D arrive. Without a correction, the transmitter 1 would be a virtual transmitter l ^r appear on the connection D ', which can lead to incorrect measurements, in particular in the case of angle measurements and measurement methods based thereon.

For long-term compensation of the angular error β caused by the refraction, the dispersion effect is used, by means of which two different frequencies B and R are deflected to different extents along the transmission path. The difference in the angle of incidence caused by the dispersion is delta β. It has now been shown that, due to the dispersion, the angle difference delta β is, in a first approximation, directly proportional to the angle error β originating from the changes in the refractive index. These basic facts are valid within certain limits for all refractive media, ie for air as well as for water or other fluids, as well as for solids, with regard to the propagation of any waves.

Previous methods for compensating the refraction angle β have been based on long-term observations of the angle deviations. Turbulence influences due to time-varying unequal distributions in the density and thus in the refractive index of the medium penetrated by the wave radiation were not taken into account.

2 now shows the snapshot for the curvature of radiation axes of different frequencies, taking into account the turbulence. It must be taken into account here that the turbulence due to flow influences or different heat radiation along the transmission can change route depending on time. Accordingly, the course of the axes of B and R in FIG. 2 and thus the angle of incidence at the receiver would also change.

The method described in more detail below takes this knowledge into account in that the angle deviations delta β caused by the dispersion are measured in very short time intervals as a series of individual measurements and are averaged over a period of seconds or minutes. The required correction for the actual measured value is then obtained from the mean value obtained. The time intervals mentioned for determining the correction values are matched to the actual changes in the refraction behavior of the transmission medium.

If the actual angle measurements and the correction measurements are carried out practically at the same time, those inaccuracies in the measurement result which are attributable to the "smearing effect" due to the earlier long-term integration or averaging in error compensation are eliminated. The measurement accuracy can be easily improved by an order of magnitude. As will be made plausible later, the residual error decreases significantly faster with the measurement time than is the case with the conventional compensation method.

The theoretical background of the invention is briefly discussed below, insofar as it contributes to an understanding of the relationships. If one considers a wave that is flat for Z = 0 and propagates in the Z direction, this wave is distorted by inhomogeneities in the atmosphere. It is known that this distorted wave is approximately given in the plane z = R> 0 by

exp [ik ₀ int dz n (x, y, z)] (Rytov approximation),

where k is the wavenumber of the light used and n (x, y, z) is the refractive index of the medium (e.g. air)

The integral ₀ int ^H dz n (x, y, z) can be done without approximation; partially develop into a Taylor series:

int dz n (x, y, z) = c + ax + by + ^ (x, y),

where j2f (x, y) is the residual term. Assuming that k ^ (x, y) is small against 1, then it holds

expfik ₀ int ^R dz n (x, y, z) J = exp |] ik (c-ι-ax + by) _ l + iko (x, y)

The term exp [ik (c + ax + by)] represents a plane wave propagating in the direction (a, b, l). The angles a and b are only dependent on the wavelength of the light via the refractive index n, such as this is necessary for the turbulence compensation.

For the method to work well, it is important that the interference term k ^. (X, y) is as small as possible. The smaller the section of the wavefront under consideration, the more the disturbance becomes smaller. This has the remarkable consequence that the aperture of the receiver has to be as small as possible, which is quite contrary to the previous view of the relevant experts. So far it has been assumed that a large aperture should be used with a view to averaging the turbulence influences as well as possible. The above result shows that this traditional view has to be put into perspective as soon as the turbulence influences on the measurement accuracy are reduced.

The larger the turbulence structures, or the smaller the spatial frequency of the variations in n (x, y, z), the smaller the interference term k ^ f (x, y) in relation to the angles a and b. These large turbulence structures therefore cause smaller errors than the smaller structures. At the same time, large structures cause the errors that change the slowest and are therefore the worst to be averaged using conventional methods. Since the new method compensates precisely for these errors, the error in the present method decreases with the measuring time much more quickly than in conventional methods.

3 shows the typical regularity in the course of the error variance as a function of the measurement time, specifically curve A according to the invention and curve B according to the prior art.

In order to keep the requirements for the stability of the receiver on a mini ium, it is advisable to use only one common transmission channel for the different ones Frequencies to use. The frequencies can then be identified, for example, by different intensity modulation for the discrete frequencies used.

FIG. 4 shows the principle of obtaining two transmission frequencies from a single laser diode by frequency doubling. The beam emanating from a laser diode 1, e.g. is in the red range, preferably around 430 nm, is split in a partially transparent mirror 7A. The deflected beam passes through a non-linear crystal 8, e.g. a potassium niobate or lithium niobate crystal. In this nonlinear crystal, the frequency of the light wave is doubled, in the example to 860 nm. This frequency lies in the blue range and is designated B in FIG. 4, in contrast to the original frequency, which is in red -Range and is designated with R. To adjust the intensity of the two transmission beams, an intensity filter 9 is provided in at least one of the beam paths.

For better identification of the two transmission beams for evaluation in the receiver, a rotating chopper disk 10 is provided, which carries out a different intensity modulation for both frequencies. This is achieved by slits S1 and S2 of different widths on the circumference of the chopper disk 10.

5 shows one of many possible exemplary embodiments for a receiver circuit for detecting the described angular deviations. As in Fig. 2 schematically is indicated, the light rays R and B emanating from the transmitter 1 of different frequencies are collected jointly by a receiver optics 5 and directed onto the surface of a position-sensitive detector 6. In the example, the different frequencies, as mentioned above, are red and blue light beams which emanate from at least one laser diode in the transmitter. The detector is, for example, a dual photodiode, which has two adjacent light-sensitive areas E and F.

Correspondingly, an evaluation circuit on the receiver side has inputs E and F for the output signals of the photodiode regions and a reference input Ref. The input signals E and F are each fed to a sample and hold circuit 11 or 12. Controlled by a logic and time control unit 13, the outputs of these two circuits are sampled by a multiplex circuit 14, amplified in an amplifier 15 and fed to registers 17 via an analog / digital converter 16, where the signals for further processing in can be temporarily stored in a computer 18.

Angular differences delta ß are obtained from the position signals at the inputs E and F. In the computer, these values are related to other measured values in the sense of a correction in accordance with a predetermined program, so that corrected measured values can be output directly at the computer output, or these values are stored for later processing. In a practical example, two light frequencies of approximately 430 nm and approximately 860 nm were transmitted through an air atmosphere of approximately 24 degrees Celsius. The aperture of the receiver optics was 36 mm in one case and 46 mm in the second case. To demonstrate the error variance, which decreases sharply with the measurement time, the individual measured values were averaged after 1 see and after 10 see. The results are as follows:

1st aperture = 36 mm: slightly cloudy; Wind of 2.5 m / s uncorrected corrected T - 1 lake 1.58 x 10 ^"11 wheel ² 1.74 x 10 ^-11 wheel ² T = 10 lake 8.68 x 10 ^{" 12} wheel ^¬ 2.77 x 10 ^-12 bike ²

sunny; Wind of 2.0 m / s uncorrected corrected T = 1 see 4.69 x 10 ~ ¹² rad ² 5.34 x 10 ^"12 rad ² T = 10 see 2.70 x 10 ^{~ 12} rad ² 5.59 x 10 ^"13 bike ²

2nd aperture - 46 mm: sunny; Wind of 2.0 m / s uncorrected corrected T = 1 see 1.63 x 10 ^"11 rad ² 1.40 x 10 ^{" 11} rad ²

T = 10 see 7.52 x 10 ^-12 rad ² 1.95 x 10 ^{~ 12} rad ²

Claims

PATENT CLAIMS

1. Method for compensating for inhomogeneities in the refractive index of a dispersive medium when determining the direction of a target point by evaluating the propagation behavior of a wave radiation emitted between the target point and a measuring station; characterized in that at least two selected frequencies are emitted and that their angles of incidence are measured practically simultaneously at the measuring station, that the difference between the two measured angle values is formed and that from this difference the angle deviation between the apparent and the actual direction of the Target point is calculated according to a specific algorithm, the angular deviation being linked as a correction value with the measured angles of incidence for calculating the true directional value.

2. The method according to claim 1, characterized in that a series of individual measurements is carried out, the results of which are averaged over a period of seconds in the final measurement result.

3. The method according to claim 1, characterized in that the transmitted measuring waves of different frequencies are modulated with different intensities and that these modulation differences are evaluated to identify the different frequencies in the receiver.

4. The method according to claim 1, characterized in that a measurement pulse is sent from the target point and that at the measuring station at least two discrete frequencies are filtered out as measurement frequencies for further processing.

5. Measuring device for performing the method according to claim 1, with a measuring station and a target point, means for emitting light beams with at least two different frequencies to the measuring station being provided at the target point, characterized in that in the measuring station a common one Receiving channel for the light beams of different frequencies is provided.

6. Measuring device according to claim 5, characterized in that the at least optical elements (5) within the receiving channel, which influence the dispersion of the beams received from the target point, are common to the different frequencies.

7. Measuring device according to claim 5, characterized in that a position-sensitive optoelectric converter device (6) is provided for detecting the light beams of different frequency, the outputs of which are connected to a multiplex signal processing device for separate processing of light signals of different frequency.

8. Measuring device according to claim 5, characterized in that a lens (5) with a minimal aperture is provided in the receiving channel.

9. Measuring device according to claim 5, characterized in that the means for emitting light beams at the target point consist of a laser diode (1) of a first frequency and that part of the laser beam is masked into a second beam path via a beam splitter (7A), in which a frequency multiplier (8) is arranged to generate a second frequency.

10. Measuring device according to claim 9, characterized in that both beams (B, R) pass through means (10) for different intensity modulation.