Hydroacoustic Doppler logs measuring current velocity. Relative lags

The principle of operation of the hydroacoustic Doppler log is based on the Doppler effect, according to which, with the relative movement of the source or receiver of sound waves, the frequency of the received oscillations changes in relation to the emitted ones, and this change, called the Doppler shift, is proportional to the speed of the specified relative movement.

When using a Doppler hydroacoustic log, both the emitter and the vibration receiver are located on the ship. Consider the process of formation of the Doppler frequency shift, which occurs in this case

The point O, which is the receiver in the case under consideration, is fixed. So based on the results. It can be written that

At the point, the sound beam is reflected without changing the frequency, and then goes to the receiver. Therefore, the O point can be considered as a stationary source emitting waves with a frequency . The frequency in the receiver can be determined taking into account the fact that we now have:

The expression shows that, in principle, the dependence of fd on the ship's speed is non-linear. This is one of the main disadvantages of a single-beam log.

Absolute error in determining the Doppler frequency shift

can be found using the formula

More indicative is the relative error

The dependence of the change in the frequency of oscillations or the wavelength perceived by the observer, on the speed of the source of oscillations and the observer when moving relative to each other, is called the Doppler effect .

The Doppler effect for sound waves can be observed directly. It manifests itself in an increase in the tone of the sound when the source of the sound and the observer approach, and, accordingly, in a decrease in the tone of the sound when they move away.

The principle of operation of a hydroacoustic log based on the Doppler effect and used to measure the speed of a ship relative to the ground (bottom) is as follows.

An antenna is installed in the bottom of the vessel, which acts as a transmitter and receiver of ultrasonic vibrations. In the direction of the bottom, ultrasonic waves with a frequency f 0 are emitted in the form of a narrow beam at an angle Ө to the horizon plane. For simplicity, we assume that the ship's trim angle is zero, the ship's velocity vector coincides with the course, and there are no vertical movements of the ship.

The wavelength of ultrasonic vibrations λ in water radiated from a moving vessel, λ = W/ f 0 where W- the resulting speed of the radiated wave moving away from the ship in the direction of the sound beam.

speed W is determined by the speed of sound with and the projection of the velocity vector Vc ship to the direction of radiation:

W=c - VcCOS Ө1 . Then λ= (c - VcCOS Ө)/ f 0

Due to the unevenness of the bottom relief, the sound wave is scattered in all directions, including in the direction of the antenna. Thus, an echo signal with a wavelength λ will be received from the bottom,

Echo signal approach speed W′ =c + VcCOS Ө

As a result, the frequency of the received oscillations, taking into account the previous equations, can be represented as f p = f 0 (1+(2VcCOS Ө)/c)

The difference between the frequencies of the echo signal that came to the antenna from the bottom and the emitted signal will be the equation of the single-beam Doppler lag (Doppler shift).

f d \u003d f p - f 0 \u003d 2f 0 VcCOS Ө / c

The practical implementation of a single-beam Doppler lag is associated with a number of difficulties, the main of which are the nonlinearity of the dependence f d on V c , angle change Ө

when heeling, trimming and pitching, the influence of the vertical component of the ship's speed on the measured signal. The working depths of Doppler logs are within 200 - 300 m. The error caused by the change in the speed of sound in sea water can reach 4%, therefore, in most log designs, measures have been taken to compensate or take into account the error. Correction is performed manually or automatically according to two parameters: water temperature and salinity. The accuracy of readings of Doppler lags is quite high even at angles of heel, trim, roll, not exceeding 2 - 3%. The total error is 0.1 - 3%.

14.Double-beam and multi-beam Doppler logs.

An effective way to eliminate the non-linear relationship between frequency offset and ship speed is to use dual beam antenna system, the so-called "Janus" scheme (Fig. 8.4). According to this scheme, acoustic signals are emitted along the diametrical plane of the vessel towards the bow and stern at the same angle Θо. The frequency of the signal received by the nose beam f2n can be determined by the expression f2н = fo*(1+2Vx*cos Θо/c + 2V²x*cos² Θо/c +…).-Formula 1). For the signal received on the stern beam, we obtain a similar expression, replacing Vx*cos Θо by - Vx*cos Θо. As a result, we get: f2k \u003d fo * (1-2Vx * cos Θo / c + 1- 2V²x * cos² Θo / c + ...).-formula(2). We find the Doppler frequency shift as the difference between the frequencies of the signals received by the bow and stern beams: fd = f2n- f2k .-formula (3). Substituting in (3) the values ​​f2н and f2к in accordance with expressions (1) and (2), we obtain the true value of the Doppler frequency shift fd= (fo*4* Vx cos Θo)/s . -formula (4), where c is the speed of signal propagation in water. Let us find the relative errors δfd (which is determined by the ratio Δfd/fdl, where fdl is the lag Doppler frequency shift) and δVx (δVx= ΔVx/Vx). The final result looks like: δfd = Δfd/fdl = δVx= ΔVx/Vx = (V²x / s²)* cos² Θo.- formula (5). So, when using the Janus scheme in the hydroacoustic Doppler log, a linear relationship is provided with a high degree of accuracy between the Doppler frequency shift obtained as the difference between the signals received by the bow stern beams and the speed of the vessel. Two-Beam Doppler Lag Equation Vx \u003d (fd * C * sec Θo) / 4 * fo -formula(6), or Vx= fd/ Kv, where Кv=(4* fo* cos Θо)/с - speed sensitivity coefficient of the lag. Kv characterizes the magnitude of the increase in the Doppler frequency shift with an increase in speed by 1 knot. Other things being equal, it is more profitable to have a large value of the Kv coefficient, since the accuracy of velocity measurement (with the same value of instrumental errors) will be higher.

hydroacoustic log

hydroacoustic log

absolute log, working on the principle of an echo sounder. Provides sufficient accuracy at depths not exceeding 300 m. There are Doppler and correlation hydroacoustic logs. The action of Doppler hydroacoustic logs is based on a change in the frequency of the received signal caused by the movement of the ship relative to the bottom, correlation hydroacoustic logs - on a comparison of the bottom topography record obtained by two receivers (with one emitter) located under the bottom in the diametral plane at some distance from each other. The speed is determined by the time between obtaining similar relief records.

Edwart. Explanatory Naval Dictionary, 2010

See what "Hydroacoustic log" is in other dictionaries:

hydroacoustic log- GAL Log, based on the use of the laws of propagation of acoustic waves in water. [… Technical Translator's Handbook

HYDROACOUSTIC LOG- hydroacoustic station for determining the speed of the vessel relative to the seabed and the drift angle of the vessel. The hydroacoustic log is also called the absolute log. There are 2 types of Hydroacoustic Logs: Doppler and Correlation. The principle... ... Marine encyclopedic reference book

hydroacoustic log- 70. Hydroacoustic log GAL E. Acoustic log Log based on the use of the laws of propagation of acoustic waves in water Source: GOST 21063 81: Ship navigation equipment. Terms and definitions original ...

Correlation hydroacoustic log- 71a. Correlation hydroacoustic log Correlation HAL Hydroacoustic log based on the use of correlation analysis in the processing of hydroacoustic signals Source: GOST 21063 81: Ship navigation equipment. ... ... Dictionary-reference book of terms of normative and technical documentation

Currently, induction, hydrodynamic and radio Doppler logs are used on ships of the marine transport fleet, which measure the speed relative to the water.

Induction lags. Their action is based on the property of electromagnetic induction. According to this property, when a conductor moves in a magnetic field, e is induced in the conductor. d.s., proportional to the speed of its movement.

With the help of a special magnet, a magnetic field is created under the bottom of the vessel. The volume of water under the bottom, which is affected by the magnetic field of the lag, can be considered as a set of elementary conductors of electric current, in which e. d.s.: the value of such e. d.s. allows you to judge the speed of movement of the vessel.

The induction log, regardless of the design solution of its nodes, includes:

electromagnet, current-collecting contacts (electrodes) for picking up a signal induced in water; a measuring device for measuring the signal at the electrodes and converting it into speed; corrective device that eliminates the methodological error of the measured speed; a calculating device for generating the distance traveled by the vessel; a broadcasting device for transmitting data on speed and distance traveled to repeaters and ship automation.

The induction logs IEL-2 and IEL-2M operated on ships of the marine fleet are built according to the same scheme:

they measure only the longitudinal component of the relative velocity; there are no parts protruding beyond the hull. The entire measuring and counting part of the logs IEL-2 and IEL-2M is made on semiconductor elements with the maximum use of integrated circuits. The block-functional principle of construction provides quick troubleshooting and their elimination by replacing individual nodes (boards) without subsequent adjustment of the lag. The IEL-2M lag is a modernization of the IEL-2 lag. Currently, only the IEL-2M log is being mass-produced. The IEL-2 lag was discontinued in 1980. The IEL-2M lag can be installed on all sea vessels, including icebreakers and hydrofoils.

The operating instructions are as follows. With the fouling of the ship's hull, the logs IEL-2 and IEL-2M begin to give underestimated readings. At the same time, checking the “working zero”, the zero of the measuring circuit and the scale does not show any changes. To eliminate the error due to fouling of the hull, it is necessary to set a new scale. The value of the new scale:

where M is the originally set scale;

Vl is the observed speed along the log;

Vi - the actual speed of the vessel relative to the bottom at the time of observation.

After calculating a new scale, it is necessary to switch the lag to the scaling mode (set the switch of the type of operation in the device 6 to the “Scale” position) and use the “Coarse scale” and “Fine scale” potentiometers to set a new scale value. After that, return the lag to working mode. Record the new scale value in the log form and on the map in the device 6. The new scale can be set both on the move and when the vessel is at the berth and at anchor.

The IEL-2 and IEL-2M lag circuits include a filter that averages their readings. Therefore, when the ship speed changes, the log fixes this change with some delay. The filters have two time constants, set at the request of the navigator with a special toggle switch. It is recommended to use the first constant when sailing near the coast and in a calm state of the sea, the second constant - when sailing in the open sea and in heavy seas.

Hydrodynamic lags. The principle of operation is based on measuring the hydrodynamic pressure created by the velocity pressure of the oncoming water flow when the ship is moving.

The correction of the hydrodynamic lag is, as a rule, unstable. The main reasons for its changes during navigation are ship drift, trim, fouling of the hull, pitching and changes in the density of sea water with a change in the navigation area.

Practice shows that the greatest error in the measurement of speed is caused by the ship's drift. At large drift angles, the error can reach 3-4%. From a change in trim and fouling of the hull, the error does not exceed 1-2%. When using a stem receiving device, the error from fouling of the ship's hull does not occur at all.

Errors from drift, trim and hull fouling are systematic. Therefore, being determined from observations, they can be taken into account in the future when calculating.

The error of the lag due to pitching is periodic. When developing the distance traveled, this error is integrated and, in the case of symmetrical pitching, vanishes.

The error (in %) of the lag from the change in the density of sea water with a change in the navigation area can be calculated by the formula

where Dr is the change in the density of sea water;

r is the density of water in the navigation area. The highest value that Dv can reach is 1.0-1.5%. When sailing in one basin (Baltic, Black, Caspian Seas), this error does not exceed 0.5%.

2. Absolute lags.

Absolute logs are logs that measure the speed of the vessel relative to the ground. The currently developed absolute logs are hydroacoustic and are divided into Doppler and correlation logs.

Hydroacoustic Doppler logs (GDL). The principle of operation of the GDL is to measure the Doppler frequency shift of the high-frequency hydroacoustic signal sent from the vessel and reflected from the bottom surface.

The resulting information is the longitudinal and transverse components of the ground speed. GDL allows you to measure them with an error of up to 0.1%. The resolution of high-precision GDL is 0.01-0.02 knots.

To measure only the longitudinal component of the ground speed, the GDL must have a two-beam antenna A 1 (beams 1 and 3 in Fig. 4.1). To measure the pitch and roll components, the antenna must be four-beam, Beams 2 and 4 are used in this case to measure the transverse component of the ground speed. Based on the measured longitudinal and transverse components of the ground speed, the hydroacoustic Doppler log allows you to determine the vector of the ground speed of the vessel at each moment of time and the drift of the vessel under the influence of wind and current.

When installing an additional two-beam antenna A 2 (see Fig. 4.1), the GDL allows you to control the movement of the bow and stern relative to the ground, which makes it easier to control a large-capacity vessel when sailing through channels, in narrow places and when performing mooring operations.

Most of the existing GDLs provide absolute velocity measurement at depths under the keel up to 200-300 m. At greater depths, the log stops working or switches to the relative velocity measurement mode, i.e., it starts working from a certain water layer as a relative log.

GDL antennas do not protrude beyond the ship's hull. To ensure their replacement without docking the vessel, they are installed in clinkets.

Piezoceramic elements are used as electroacoustic transducers in Doppler log antennas.

Sources of GDL error can be: Doppler frequency measurement error; change in the speed of sound in sea water; changing the angles of inclination of the antenna beams; the presence of a vertical component of the ship's speed. The total error for these reasons for modern lags does not exceed 0.5%.

correlation lags. The principle of operation of the hydroacoustic correlation log (HCR) is to measure the time shift between the acoustic signal reflected from the ground, received by antennas spaced along the ship's hull (Fig. 4.2). The signal U 2 (t) received by the rear receiving antenna repeats the shape of the signal U 1 (t) received by the front antenna with a time shift t equal to:

where l is the distance between the antennas;

V is the ship's speed.

The time shift is determined by correlation processing of the received signals. For this purpose, a variable time delay is introduced into the signal path of the front antenna, the cross-correlation function of the envelope signals of the diversity antennas is calculated, and its maximum values ​​are monitored.

At depths up to 200 m, the GKL measures the speed relative to the ground and at the same time indicates the depth under the keel. At great depths, it automatically switches to work relative to water.

The advantages of GKL in relation to GDL are the independence of indications from the speed of sound propagation in water and more reliable operation on pitching.