A METHOD FOR THE MEASUREMENT OF RELATIVELY SMALL WATER DEPTHS, ESPECIALLY IN THE VICINITY OF SEA COASTS AND LAKE SHORES

Abstract: A method for measuring relatively small water depths is described. Measuring equipment for transmission having a direction in a small angle alpha to the perpendicular of the water depth and for receiving laser pulses of lambda = about 0.5 mu m is used. A first reflection from the water surface is obtained and a second reflection from the bottom. By means of signal processing the signals corresponding to the reflections the water depth is calculated. In particular, the method is characterized in that the signal processing is made in such a way that the function signal-amplitude/time is scanned and a first portion of this function is approximated by a substantially straight first line, after which the function is further scanned to pass through a minimum of its first derivative (inflection point) to a second increasing portion of the function located before a maximum point, which is approximated by a substantially straight second line, whereafter the time interval between the lines that is related to said distance is used for determining it.

Description: A method for the measurement of relatively small water depths, especially in the vicinity of sea coasts and lake shores. The present invention relates to a method of measuring a relatively small distance between a water surface and a bottom located beneath, i.e. the water depth, comprising that a measuring equipment, provided with a transmitter and a receiver for laser light, preferably having a wavelength λ of about 0.5 μm, is made to issue, above the water surface, short pulses in a direction having a relatively small angle a to the perpendicular of said water surface and to receive a first reflection from the water surface and a second reflection from the bottom, after which the measuring equipment calculates, by signal processing the signals corresponding to the reflections, said distance, preferably comprising a correction for the height of waves on the water surface. Such methods are known and are commonly carried out by having the measuring equipment transported by aircraft, usually a helicopter at a height of some hundred meters. The position of the aircraft is usually determined by means of the GPS and in combination with an inertia platform data are obtained that are combined with said determination of the distance, i.e. the water depth, so that the water depth for the respective coordinates is obtained. The transmitter is usually made to scan according to a predetermined pattern so that the map determination is made rapidly and efficiently. Presupposing the water depth not to be too small, methods of this kind work well. Since the issued pulses usually have a length of some nanoseconds, it means, that in such cases where the propagation time interval of light travelling from the water surface and back thereto is of the same order of magnitude as the pulse length, problems arise in determining the water depth. The object of the present invention is thus to provide a method of the kind mentioned in the introduction, so that also small distances, i.e. water depths, can be determined with a good precision. According to the invention such a method is characterized by making the signal processing in such a way, that the function signal amplitude/ time is scanned and that a first increasing function portion is approximated by a substantially straight first line, after which the function is further scanned through a minimum of its first derivative (point of inflection) up to a second increasing function portion located before a maximum, which is approximated by a substantially straight second line, whereafter the time interval between the lines, that has a relation to said distance, is used for determining it. In an advantageous embodiment of the method the time interval between the lines is used at half the maximum signal amplitude height in excess of the noise level for determining said distance. The invention will be described in greater detail with reference to the accompanying drawings in which Fig. 1 shows a diagram of signal amplitude/time as measured above a water surface having a relatively large water depth, and Fig. 2 shows the corresponding diagram as measured above a water surface having a relatively little water depth. The figures 1 and 2 thus both show diagrams of the signal amplitude as functions of time. In Fig. 1 a first amplitude maximum is referenced by 1, corresponding to a reflection against a water surface. The reflection against the bottom, which in this case is located at a relatively large water depth, is distinguished by a second amplitude maximum referenced 2. In this case the two maximum points are distinct and the time interval therebetween, denoted by 3, can be measured with a good precision and is equal to the time interval/2 x the velocity c/n of light in water, where c is equal to the light velocity in air and n equals the refractive index of water relatively to that of air. A second measurement situation is illustrated in Fig. 2. Here the water depth is small. The function signal amplitude/time here has a first increasing function portion 4, which continues into an inflection point 5 (i.e. a minimum of the first derivative), after that a second increasing function portion 6, followed by a maximum 2 of the function. The first function portion 4 is approximated by a substantially straight first line 8 and the second function portion 6 is approximated by a second, substantially straight line 9. The time interval 10 between these lines is measured, in this case at half the maximum amplitude height 11 in excess of the noise level, and the distance between the water surface and the bottom, i.e. the water depth, is determined in the way indicated above. In order to make the method work, some criteria must be fulfilled. It must be ascertained that the measurement is actually made over water, what can be made by means of a IR-measurement, that differentiates between ground and water. Further, the function signal amplitude/ time must have such a shape that has been described. In other cases measurement is made according to Fig. 1.

Claims: Claims 1. A method of measuring a relatively small distance between a water surface and a bottom located beneath, i.e. the water depth, comprising that a measuring equipment, provided with a transmitter and a receiver of laser light, preferably having a wavelength λ of about 0.5 μm, is made to issue, above the water surface, short pulses in a direction having a relatively small angle α to the perpendicular of said water surface and to receive a first reflection from the water surface and a second reflection from the bottom, whereafter the measuring equipment by means of signal processing of the signals corresponding to the reflections calculates said distance, preferably including a correction for the height of waves on the water surface, characterized in that the signal processing is made in such a way that the function signal- amplitude/time is scanned and a first increasing portion of the function is approximated by a substantially straight first line, after which the function is further scanned to pass through a minimum of its first derivative (point of inflection) to a second increasing portion of the function before a maximum, which is approximated by a substantially straight second line, whereafter the time interval between the lines that is related to said distance is used for determining it. 2. A method according to claim 1, characterized in that the time interval between the lines at half the maximum signal amplitude height above the noise level is used for determining said distance.

Inventors: JOHANSSON KENNETH, JOHANSSON, KENNETH
Applicants: JOHANSSON KENNETH, SAAB DYNAMICS AKTIEBOLAG, JOHANSSON, KENNETH
Application year: 1996
Main classification: G01S17
IPC codes: G01S7, G01S17