WO2002042792A1

WO2002042792A1 - System and method for measuring water depth

Info

Publication number: WO2002042792A1
Application number: PCT/SE2001/002515
Authority: WO
Inventors: Rolf ENGSTRÖM; Andreas Axelsson
Original assignee: Airborne Hydrography Ab
Priority date: 2000-11-21
Filing date: 2001-11-13
Publication date: 2002-05-30
Also published as: AU2002215279A1; SE0004259D0; SE0004259L

Abstract

The present invention relates to a method and an airborne system for measuring the depth of an ocean or lake. Laser radiation emitting means (2) are arranged to emit laser radiation so that a first part of the radiation is reflected from the water surface and a second part of the radiation penetrates through the water surface and is reflected from the bottom. Receiving means (3) are arranged so as to receive said reflected radiation, and processing means (4) are arranged so as to process the received radiation parts in order to determine a time difference between the reception of the surface- and bottom-reflected radiation and, from this time difference, calculate the water depth. Means (5) are arranged so as to choose a value from among a set of at least two selectable power density or corresponding values of the laser radiation at a specified distance from the laser, and the system is adjusted for the selected value.

Description

System and method for measuring wate.r depth

TECHNICAL FIELD

The invention concerns a system and a method for measuring distance by laser means. The invention is especially intended for laser bathymetry.

START OF THE ART

A number of different techniques are available to choose among for making hydrographic measurements such as water-depth measurements, depending on the particular application. The technique that formerly dominated is known as multibeam bathymetry. This technique is based on sending multiple parallel "sound beams" down into the water and then recording them. The measurements are thus made using the same principles as in echo sounding. In recent years a new technique known as laser bathymetry has been developed in which the measurements are made using laser light from a flying platform rather than sound from a vessel-based platform. These two techniques complement rather than compete with one another, since laser bathymetry is best suited to shallow water near land, while multibeam measurements are more effective in deep water.

Laser bathymetry systems can be mounted in both helicopters and aircraft, and may include a laser that emits pulsed radiation in the infrared spectrum while simultaneously emitting pulsed radiation in the visible spectrum, preferably green light. The infrared radiation is reflected from the water surface, while a substantial portion of the visible light penetrates down into the water and is reflected from the bottom. The system further contains a receiver arranged so as to receive a portion of the reflected radiation and record the intensity of the received radiation. A calculating unit connected to the receiver calculates the time difference between the reception of the radiation reflected from the water surface and the radiation reflected from the bottom, whereupon the water depth is calculated as half the time distance multiplied by the speed of light, with correction for the angle of incidence of the radiation relative to the water surface. Using the laser bathymetry system it is possible to quickly measure depths over a relatively large area, since a helicopter can typically fly at 30 - 130 knots at a height of 200 - 500 meters above the surface, thereby allowing the laser beam to sweep an area of 100 - 200 meters transverse to the direction of flight. Water depths of some twenty meters can typically be measured using the laser bathymetry system. The measurable depth is limited by how much reflected laser radiation the laser bathymetry system receiver receives relative to the received ambient noise, which consists primarily of solar radiation. Because the radiation is damped exponentially in the water, due to absoi tion and scattering, only a very small part of the light striking the surface will return to the receiver. To attain optimum range, the following factors may be taken into account. First, the measurements can be made when the water quality is optimal; second, the receiver can be designed so that it provides the highest possible sensitivity; third, refined algorithms can be used in the receiver to detect small light reflections in the background light noise; fourth, the flights can be made at low altitude and, fifth, the laser power can be high and the beam divergence low, thereby ensuring a high laser energy density at the water surface.

With respect to the fifth factor, there are a number of laws and restrictions that determine how lasers may be used with a view to preventing personal injuries, i.e. mainly damage to the skin and eyes. Because personnel engaged in laser bathymetry do not come into such close proximity to the laser radiation that skin injuries can occur, the primary concern is with eye safety.

IEC 60825 is the international standard for classifying lasers and laser safety. The standard has a number of local variants, EN 60825 for Europe and SS-EN 60825 for Sweden. The standard specifies the highest permissible energy density and power density per square meter to ensure the safety of the naked eye. The laser can also be treated as a point source. In such cases the provisions of the Swedish State Radiation Protection Institute's regulations regarding lasers, SSI FS 1993:1 or later versions, set forth maximum permissible exposures for various wavelengths, expressed in joules/m . During normal laser use, the laser power is adjusted so that the energy density and power density are kept within the permissible values. The system contains a built-in safety function that monitors to ensure that the permissible values are not exceeded. If these values are exceeded, the laser is turned off automatically. A person located on the shore of a lake or ocean who is using binoculars is exposed to substantially higher laser energy and power levels than with just the naked eye, since the energy density received by the eye increases by the square of the binocular magnification power. A laser bathymetry system that is safe to the naked eye would thus not be safe for eyes using binoculars. To address this problem, the laser bathymetry system operator manually turns off the laser whenever the airplane/helicopter passes over areas where the presence of people who might be using binoculars is suspected. No depth measurements are made during these temporary shutdowns, thus creating areas in which no depth measurements are performed. In lightly trafficked open waters such unmeasured areas in which supplemental measurements are necessary will be small, and few in number. On the other hand, many unmeasured areas in which the laser must be turned off will occur in areas of populated land or dense boat traffic. It may be impossible, or at least extremely time-consuming, to complete the measurements for many such unmeasured areas.

DESCRIPTION OF THE INVENTION

One purpose of the invention is to provide, in comparison with the prior art, a better way to make laser bathymetry measurements over areas in which it may be assumed that people are present, at least in certain locations, and that takes the aforementioned problems into account.

This is achieved by means of a laser bathymetry system according to claim or 1 and 2. The system is arranged so as to continuously determine an energy density, power density or corresponding value of a laser in a cross-section of the laser beam at a specified distance from a laser. The system is further arranged so as to compare the determined power density with a selected threshold value. The threshold value is chosen so that the permissible energy and/or power density values for the laser wavelength(s) used, as specified in the applicable standards, will not be exceeded. The threshold value is preferably chosen with a certain margin. The system is also arranged so as to indicate whether the threshold value has been exceeded, e.g. in that the laser is turned off when this occurs. The system is characterized in that it includes means for selecting one from among a set of at least two selectable threshold values as the selected threshold value, and means for adjusting the system for the threshold value selected. One of the threshold values is chosen to ensure the safety of the naked eye, while the second threshold value ensures the safety of an eye equipped with binoculars. The threshold values can be selected either manually or automatically on the basis of preset criteria. Given that, in the context of distance measuring, the possibilities of receiving and detecting a reflected laser beam degrade dramatically with increasing beam divergence, it is primarily the output power of the laser that is adjusted when a new threshold value is selected. This may be achieved either by inducing the laser to adjust the pumping of the laser medium or, for the lower threshold value, by diverting a portion of the laser beam.

The above described laser bathymetry system is installed in an aircraft such as an aiφlane or a helicopter. The accordance with one embodiment of the invention the laser is arranged so as to emit laser radiation at two frequencies, wherein the radiation at the first frequency is mainly reflected from the water surface, while the radiation at the second frequency mainly penetrates through the water surface and is reflected from the bottom. In an alternative embodiment one single laser radiation frequency is used, the frequency being chosen so that one part of the radiation is reflected in the ocean or lake surface and the other part of the radiation penetrates the water surface and is reflected in the bottom of the lake or ocean.

The laser bathymetry system is equipped with means for receiving said surface- and bottom- reflected radiation, and means for processing the received radiation to determine a time difference between the reception of the radiation reflected from the surface and the radiation reflected from the bottom. These processing means then calculate the water depth on the basis of the time difference. The system is adjustable for a selected power density or corresponding value of the laser beam at a specified distance from the laser, e.g. the point of reflection at the water surface. The system is characterized in that it includes means for selecting one from among a set of at least two selectable threshold values as the selected value, and means for adjusting the system for the selected threshold value. In one embodiment the system is further arranged so as to continuously determine the power density or corresponding value of the laser beam at the specified distance from the laser in order to confirm that the laser radiation is not exceeding the current selected value.

In one particular embodiment in which the number of values in the set is precisely two, one of the values in the set is predefined as the normal setting, and the selecting means are arranged in such a way that, upon actuation, the second value will be selected for a predetermined length of time, after which the means will automatically resume the normal setting.

The invention further includes a method for laser bathymetry measurements according to claim .

The invention further includes methods for measuring the depth of an ocean or lake in accordance with claim 7 or 8.

Using the system and method according to the invention, there will be fewer and smaller areas in which no measurements can be made due to the risk that individuals equipped with binoculars may be present in the vicinity of said areas. A lower laser power will instead be used for such areas, so that eye safety is ensured, even, if binoculars are used. The lower laser power entails a reduction of the maximum measurement depth. Because people are often present in areas near shore, which are seldom especially deep, full bottom coverage can often be achieved, even with the lower laser energy level.

DESCRIPTION OF FIGURES

The invention is described below with the help of exemplary embodiments, and with reference to the attached drawing, in which:

Figure 1 shows an example of a laser bathymetry system according to the invention.

Figure 2 shows an alternative example of a laser bathymetry system according to the invention.

Figure 3 shows a diagram that illustrates the received radiation in the system in Figure 1, including a receiver. PREFERRED EMBODIMENTS

In Figure 1 , reference number 1 indicates a helicopter- or aiφlane-based laser bathymetry system for measuring water depths in oceans, lakes, rivers or other watercourses. The system 1 contains a laser 2. Aiming devices (not shown) are placed in front of the laser 2 to aim the laser beam at the water surface at a selected angle. The aiming devices consist of, e g. mirrors that are rotatable in at least one direction and positioned in the beam path of the laser. The laser beam is caused, by means of the aiming devices, to sweep over an area transverse to the direction of helicopter flight. The laser emits monochromatic pulsed radiation at wavelengths within the infrared spectrum while simultaneously emitting monochromatic pulsed radiation within the visible spectrum, preferably green light. The infrared radiation is reflected from the water surface, while a significant portion of the green light penetrates down into the water and is reflected from the bottom. Alternatively, one single laser frequency is used, the frequency being chosen such that one part of the radiation is reflected in the water surface and another part of the radiation penetrates the water surface and is reflected in the bottom. The system 1 includes a receiver 3 arranged so as to receive the reflected radiation and record its intensity.

Figure 3 shows the thus recorded pulse response with two intensity peaks. The first peak represents the reflection from the water surface, while the second peak represents the reflection from the bottom. A calculating unit 4 connected to the receiver 3 calculates the time difference between the intensity peaks, whereupon the water depth is calculated as half the time difference multiplied by the speed of light, with correction for the angle of incidence of the beam relative to the water surface. The way in which an algorithm could be implemented to perform the foregoing calculation will be obvious to one skilled in the art. The radiation reflected from the surface is damped insignificantly in the course of its laser/water surface/receiver path, while the laser radiation reflected from the bottom will be damped considerably, as shown in Figure 3.

The system 1 includes a switch 5 in the form of, .g. a conventional manually operable switch that is switchable between two setting positions. The switch controls primarily the laser power from the system 1. In another embodiment (not shown), the beam divergence of the laser beam is also controlled. In the first setting position, the power is selected for the given beam divergence to achieve the maximum permissible exposure for the wavelength used, as specified in joules per square meter in accordance with applicable regulations. The Swedish State Radiation Protection

Institute's regulations regarding lasers, SSI FS 1993:1 or later versions, are applicable in Sweden.

The power level that is permissible for use at the first setting position thus depends on the divergence of the laser beam, since said divergence determines the surface area covered by the pulse at the water surface. Divergence levels of from 2- 15 mRad are relevant in a laser bathymetry context.

In a low-energy setting that is obtained by switching the switch 5 to its second setting position, the radiation level is such that eye safety is maintained, including for binocular-aided eyes. At this setting the maximum exposure must be below the maximum permissible exposure for the naked eye divided by the square of the binocular magnification power. Because it is undesirable to increase the divergence of the laser beam, the only remaining alternative is to lower the laser power so that the maximum permissible exposure (expressed in joules/m ) is appropriate for the binocular-aided eye. Conventional "consumer binoculars" normally offer magnifications of lOx or less. Taking this into account, the power level in one embodiment is 100 times lower in the second setting position than in the first.

In Figure 1, the switch 5 controls a beam splitter 6 in such a way that, when the switch 5 is set to its second, low-energy position, the beam splitter 6 is kept in the beam path in front of the laser and, when the switch 5 is set to its first, high-energy position, the beam splitter 6 is kept outside the beam path. The low-energy setting, in which the beam splitter 6 is kept in the beam path in front of the laser 2, is illustrated in Figure 1. A mechanical device of conventional type (not shown) supports the beam splitter 6 and is arranged so as to move the beam splitter 6 into/out of the beam path when the switch 5 is moved from one setting position to the other. The beam splitter 6 is designed to split the incident light into two components, so that the preponderance of the power (99% in the example above) is damped by a beam damper (not shown) in the low- energy setting. In its simplest form, the beam splitter consists of a conventional semitransparent mirror, whose properties are such that it reflects 99% of the radiation while allowing 1% to pass through.

The alternative embodiment shown in Figure 2, which comprises the aforedescribed laser 2, the receiver 3, the calculating unit 4 and the switch 5, is based on the linear polarization of the radiation from the laser. A variable wave plate 7 is arranged in the beam path in front of the laser 2. The wave plate consists of, e.g. a birefractive crystal capable of changing the polarization of incident light. The switch 5 has an operative connection with the wave plate 7 via a control unit 8. The control unit is arranged so that, when the switch is set to its first, high-power position, it controls the wave plate so that the polarization of the linearly polarized light remains unchanged after its passage through the wave plate 7. The linearly polarized light striking the wave plate thus leaves the wave plate in the form of linearly polarized light of the same intensity as the incident light. The control unit is further arranged so that, when the switch 5 is set to its second, low-power position, it controls the shift in the wave plate 7 so that the linearly polarized light leaves the wave plate in the form of circularly polarized or elliptical light. After the wave plate 7 there is in arranged in the beam path, and in the direction of emission, a polarization splitter 9 that is designed so that a first polarization direction, which is coincident with that of the linearly polarized beam in the high-power setting, passes unaffected through the splitter 9, while a second polarization direction, which is perpendicular to the first, is diverted. The thus diverted beam is then removed in that, e.g. it is allowed to strike an absorbent material (not shown). Control of the wave plate 7 thus enables control of the proportion of the beam that will be polarized in the second polarization direction so as to thereby enable control of the final degree of disengagement from full power in the first setting position to low-power in the second setting position.

In yet another embodiment (not shown), the switch 5 is directly operatively connected with the laser 2 to control it to pump the laser medium more when the switch is switched to its high- power setting, and to pump the laser medium less when the switch is switched to its low-power setting.

During normal use of the laser, the laser power is adjusted so that the energy density and power density are kept within the permissible values. Incorporated into the system is a safety function that monitors to ensure that the permissible values are not exceeded. The laser is shut off automatically if these values are exceeded. The safety function is designed in such a way that the calculating unit 4 is arranged so as to measure not only the depth, but also the altitude of the airplane/helicopter above the water. This is accomplished by simple means in that the calculating unit measures the time that elapses until an emitted infrared pulse of radiation reflected from the water surface is received, correcting the measured value for the angle of incidence of the beam relative to the water surface. The pulse energy of the laser is also measured. Using the information about the distance to the water surface and the pulse energy, the calculating unit is arranged, given that the beam divergence is known, so as to calculate the energy density at the water surface. The system shuts off the laser if the calculated energy density value exceeds a predefined threshold value in the calculating unit 4. This threshold value is naturally determined by the standards noted above (e.g. the Swedish State Radiation Protection Institute's regulations regarding lasers, SSI FS 1993:1) and by the power setting in which the system is operating (high- power/low-power). Thus, there is predefined in the calculating unit 4 one threshold value for the infrared beam and another for the visible light in the high-power setting, as well as one threshold value for the infrared beam and another for the visible light in the low-power setting.

It should be pointed out here that these threshold values are based on direct exposure. However, approximating the power density to which the eye is exposed in connection with direct emission as the power density that the beam possesses at the water surface does provide a good approximation.

In the example described above, the switch 5 is manually switchable between two setting positions. The switch 5 can alternatively have a normal setting and be arranged so that, when actuated, it deviates from its normal setting for a predetermined length of time before resuming the normal setting. A number of such manually switchable switches is currently available on the market. In one embodiment the high-power setting constitutes the normal setting, while the low- power setting constitutes the deviant setting. The reverse situation applies in an alternative embodiment. In the embodiment in Figure 2, the switch 5 has a third setting position, AUTO (not shown). In this setting the control unit 8 is arranged so as to receive control information from the calculating unit 4. The control information consists of measured depth information from the laser bathymetry system 1. In the AUTO setting, the control unit 8 is configured to compare the measured depth to a preselected depth value that is predefined in the calculating unit 4. When the measured depth exceeds the preselected depth, the control unit is arranged so as to send to the wave plate 7 a control signal that is equivalent to the high-power setting signal, and to send a control signal equivalent to the low-power setting signal to the wave plate 7 when the depth falls below the preselected value. This provides a way of ensuring that no more laser power is used than is necessary.

In an alternative embodiment (not shown), a position-indicating device such as a GPS receiver is arranged in connection with the laser bathymetry system. A memory is operatively connected to the control unit. The memory consists of, e.g. a table of coordinate intervals, preferably in two dimensions, within which intervals measurements are to be made in the high-power setting, and a table of coordinate intervals within which measurements are to be made in the low-power setting. The control unit in this embodiment is arranged so as to receive, in the AUTO setting, position signals from the GPS receiver, and to compare the received position with the interval specified in the tables in the memory, and to select the high-power/low-power setting based on the table in which the current position is found. In one embodiment this interval is represented as a zone on a map image. In yet another example the distance to land is determined based on the current position and a map image, whereupon the control unit selects the high-power setting if the distance exceeds a predetermined distance, and selects the low-power setting if the distance is less than the predetermined distance.

The invention is of course not limited to the aforedescribed preferred embodiments, and a multiplicity of possible modifications will be obvious to one skilled in the art, without deviating from the basic concept of the invention as defined in the accompanying claims.

Claims

CLA S

1. An airborne system for measuring the depth of an ocean or lake and comprising: laser radiation emitting means (2) arranged to emit laser radiation so that a first part of the radiation is reflected from the water surface and a second part of the radiation penetrates through the water surface and is reflected from the bottom; means (3) arranged so as to receive said reflected radiation, and means (4) arranged so as to process the received radiation parts in order to determine a time difference between the reception of the surface- and bottom- reflected radiation and, from this time difference, calculate the water depth, wherein the system is adjustable for a selected power density or corresponding value of the laser radiation at a specified distance from the laser, characterized in that means (5) are arranged so as to choose a value from among a set of at least two selectable power density or corresponding values, and in that means (6, 7, 9) are arranged so as to adjust the system for the selected value.

2. An airborne system for measuring the depth of an ocean or lake and comprising: laser radiating means (2) arranged to emit laser radiation at two frequencies, whereof the radiation at the first frequency is mainly reflected from the water surface and the radiation at the second frequency mainly penetrates through the water surface and is reflected from the bottom; means (3) arranged so as to receive said reflected radiation, and means (4) arranged so as to process the received radiation in order to determine a time difference between the reception of the surface- and bottom-reflected radiation and, from this time difference, calculate the water depth, wherein the system is adjustable for a selected power density or corresponding value of the laser beam at a specified distance from the laser, characterized in that means (5) are arranged so as to choose a value from among a set of at least two selectable power density or corresponding values, and in that means (6, 7, 9) are arranged so as to adjust the system for the selected value.

3. A system according to claim 1 or 2, characterized in that the system is arranged so as to continuously determine the power density or corresponding value of the laser radiation at a specified distance from the laser in order to confirm that the laser radiation is not exceeded the current selected value.

4. A system according to claim 1 or 2, wherein the number of values in the set is precisely two, characterized in that one of the values in the set is predefined as the normal value, and in that the selecting means (5) are arranged, when actuated, to select the other value for a predetermined length of time before subsequently resuming the normal setting.

5. A system according to claim 1 or 2, characterized in that it contains information about the terrain in the form of, e.g. a map image stored in a memory, and means for receiving position-indicating information, and in that selecting means (5) are arranged so as to select values from the set of power density values based on the current position on the map.

6. A system according to claim 2, characterized in that there is predefined one threshold value for the infrared radiation and another for the visible light in a selectable high-power setting, as well as one threshold value for the infrared radiation and another for the visible light in a selectable low-power setting.

7. A method for measuring the depth of an ocean or lake, according to which: laser radiation is emitted towards the lake or ocean surface so that a first part of the radiation is reflected from the water surface and a second part of the radiation penetrates through the water surface and is reflected from the bottom, wherein the laser is adjustable for a selected power density or corresponding value of the laser radiation at a specified distance form the laser; the reflected radiation is received and, the received radiation is processed to determine a time difference between the receptions of the surface- and bottom-reflected beams, and the depth of the water is calculated from said time difference, characterized in that one of a set of at least two selectable power density or corresponding values is chosen as the selected value, and in that the laser is adjusted for the selected value.

8. A method for measuring the depth of an ocean or lake, according to which: laser radiation is emitted at two frequencies, wherein the frequencies are chosen so that the radiation at the first frequency is mainly reflected from the water surface and the radiation at the second frequency mainly penetrates through the water surface and is reflected from the bottom, wherein the laser is adjustable for a selected power density or corresponding value of the laser radiation at a specified distance form the laser; the reflected radiation is received and, the received radiation is processed to determine a time difference between the receptions of the surface- and bottom-reflected beams, and the depth of the water is calculated from said time difference, characterized in that one of a set of at least two selectable power density or corresponding values is chosen as the selected value, and in that the laser is adjusted for the selected value.

9. An airborne system for measuring the depth of an ocean or lake and comprising: laser radiation emitting means arranged to emit laser radiation so that a first part of the radiation is reflected from the water surface and a second part of the radiation penetrates through the water surface and is reflected from the bottom; means arranged so as to receive said reflected radiation, and means arranged so as to process the received radiation parts in order to determine a time difference between the reception of the surface- and bottom-reflected radiation and, from this time difference, calculate the water depth, means arranged so as to choose a value from among a set of at least two selectable power density or corresponding values of the laser radiation at a specified distance from the laser and means arranged so as to adjust the system for the selected value.