what is positioning

7/28/2019 What is Positioning

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What is Positioning?According to Bowditch (2002), positioning is defined as The process of determining,

at a particular point in time, the precise physical location of the craft, vehicle, personor site. The position determination can vary in quality (degree of certainty as to its

accuracy), in relativity (positioning relative to any number of reference frames), andpoint reference (versus a line of position that is amathematical position referenced

along a given line, circle, or sphere).A position can be derived from any number of means, including deduced (also

termed dead reckoning), resolved (resolving bearing referenced to known fixed ormoving objects also termed geodeticwhen resolved relative to known earth-fixed

objects), estimated (also termedSWAG normally claimed by helmsman immediatelybefore striking submerged rocks/objects), and claimed (or announced I claim this

island in the name of King George)

All positioning is a simple matter of referencing a position relative to some other

known position. From the earliest mariners, navigation was performed through lineof sight with the coast. As explorers ventured further from sight of land, navigation

with reference to the stars became common. Navigation with reference to the NorthStar for determination of latitude was the earliest version of celestial navigation.

Accurate determination of ones latitude could be gained by measuring the angularheight of the North Star above the visible horizon.

The determination of position upon a known line of latitude formed a rudimentary

line of position in that a known position is resolved. In order to gain a higherpositional resolution, a second line of position and then a third (and so forth) will berequired to intersect the lines of position and resolve for two- and three-dimensional

accuracy. This theory works for celestial navigation, GPS positioning, and (of course)acoustic positioning

Acoustic Positioning ATechnological Development

The need for acoustic positioning became apparent with the loss, then difficulty inlocating, the atomic attack submarine USS Thresher, which sank in 8400 feet (2560

meters) of sea water in 1963, as well as a nuclear bomb lost at sea off the coast ofSpain in 1966. The US Navy possessed the manned submersible capability to dive to

the depth of the wreck site. But precision underwater navigation, through any othermeans but visual, was impossible with the available technology.

In the 1970s, as the search for hydrocarbons migrated into deeper water, the need

for repeatable high-accuracy bottom positioning became necessary to place drillstringinto the exact position referenced earlier through seismic instrumentation. Sinceradio frequency waves penetrate just a few wavelengths into water, some other form

of precision navigation technology needed development. Thus was born acousticpositioning technology.

Today, water conditions in ports and harbors, as well as littoral areas around theworld, are such that visual navigation below the surface is either difficult or impossible

due to low visibility conditions. The need for underwater acoustic positioningremains high.

Basics of Acoustic PositioningThe basic underwater speaker is a transducer. This device changes electrical energy

into mechanical energy to generate a sound pulse in water. For transducers used inunderwater positioning, the typical transducer produces an omnidirectional sound

beam capable of being picked up by other transducers in all directions from the signalsource.

Acoustic positioning is a basic sound propagation and triangulation problem. Thetechnology itself is simple, but the inherent physical errors require understanding and

consideration in order to gain an accurate positional resolution.As discussed above, water density is affected by water temperature, pressure,

and salinity. This density also directly affects the speed of sound transmission inwater. If an accurate round-trip time/speed can be calculated, the distance to a vehicle

from a reference point can be ascertained. Therefore, the simple formula RT =D(ratetime= distance) can be used. The time function is easily measurable. The rate

question is dependent upon the medium through which the sound travels. The speedof sound (or sonic speed) through various media is listed in Table 4.2.

As shown in Table 4.2, pure water and sea water have different sound propagationspeeds. For underwater port security tasks, varying degrees of water temperature

and salinity conditions will be experienced. The industry-accepted default value forsound speed in water is 4921 feet/second (or 1500 meters per second). If the extreme

speed of pure water (4724 ft/s or 1440 m/s) to the median (4921 ft/s or 1500 m/s) is


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IntroductionAcoustic sound transmission represents the basic techniques for underwater navigation,

telemetry, echo sounder, and sonar systems. Common for these systems are theuse of underwater pressure wave signals that propagate with a speed of approximately

1500 m/s through the water (Figure 4.1).When the pressure wave hits the sea bottom, or another object, a reflected signal

is transmitted back and is detected by the system. The reflected signal containsinformation about the nature of the reflected object.

For a navigation and telemetry system, the communication is based upon an activeexchange of acoustic signals between two or more intelligent units.

Transmission of underwater signals is influenced by a number of physical limitations,which together limit the range, accuracy, and reliability of a navigation or

telemetry system.The factors described in this section include:

Transmitted power Transmission loss

Transducer configuration Directivity and bandwidth of receiver

Environmental noise81

Requirements of positive signal-to-noise ratio for reliable signal detection Ray bending and reflected signals.

The signal-to-noise ratio obtained can be calculated by the sonar equation.

Sound Propagation4.1.2.1 Pressure

A basic unit in underwater acoustics is pressure, measured in Pa (micropascal) or

bar. The Pa (Pascal) is nowthe international standard. It belongs to theMKSsystem,

where 1Pa=106 newton/m2. The bar belongs to the CGS system.

1bar= 105

Pa

0dB re 1bar= 100 dB re 1Pa.The bar is a very small unit so negative decibels will rarely occur, if ever. To convert

from bar to Pa, simply add 100 dB.4.1.2.2 Intensity

The sound intensity is defined as the energy passing through a unit area per second.

The intensity is related to pressure by:I =p2/c,where

I =intensity,p=pressure,

=water density, andc=speed of sound in water.

DecibelThe decibel is widely used in acoustic calculations. It provides a convenient way of

handling large numbers and large changes in variables. It also permits quantities to bemultiplied together simply by adding their decibel equivalents. The decibel notation

of intensity I is:10 log I/Io,

where Io

is a reference intensity.The decibel notation of the corresponding pressure is:

10 log (p2/c)/(p2o

/c) = 20 log p2/po,

where po is the reference pressure corresponding to Io.Normally, po is taken to 1Pa, and Io will then be the intensity of a plane wave

with pressure 1Pa

When sound is radiated from a source and propagated in the water, it will be spread indifferent directions. The wave front covers a larger and larger area. For this reason the

sound intensity decreases (with increasing distance from the source). When the distancefrom the source has become much larger than the source dimensions, the source

can be regarded as a point source, and the wave front takes the form as a part of anexpanding sphere. The area increases with the square of the distance (Figure 4.2)

from the source, making the sound intensity decrease with the square of thedistance.

Let I and Iobe the sound intensities in the distances r and ro. Then:Io/I = (r/ro)2.

Expressed in decibels, the geometrical spread loss is:TL1 = 10 log Io/I =20 log r/ro.


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Usually a reference point is taken 1 meter in front of the source. Setting ro =1 meterwe get:

TL1 = 20 log r,where r is measured in meters

Absorption loss

When the sound propagates through the water, part of the energy is absorbed by the

water and converted to heat. For each meter a certain fraction of the energy is lost:

dI = A Idr,

where A is a loss factor. This formula is a differential equation with the solution:

I (r) = [I (ro)/(e Aro)] eAr.I (ro) is the intensity at the distance ro:

TL2 = 10 log I (ro)/I (r) = (r ro),

where =10Alog (e).

Expressed in decibels, the absorption loss is proportional to the distance traveled.

For each meter travelled a certain number of decibels is lost.

If ro is the reference distance 1 meter, and if the range r is much larger than 1 meter,

the absorption loss will approximately be:

TL2 = r,

where is named the absorption coefficient. Figure 4.3 shows absorption loss coefficient

as a function of frequency. The value of depends strongly on the frequency.

It also depends on salinity, temperature, and pressure.Sound

One-way transmission loss

The total transmission loss, which the sound sufferswhen it travels from the transducer

to the target (Figure 4.4), is the sum of the spreading loss and the absorption loss:

TL = 20 log r + r,

where r is measured in meters and is measured in dB/meter

4.1.3 Transducers

4.1.3.1 Construction

A modern transducer is based on piezoelectric ceramic properties, which change

physical shape when an electrical current is introduced (Figure 4.5). The change

in shape, or vibration, causes a pressure wave, and when the transducer receives a

pressure wave, the material transforms the wave into an electrical current. Thus, the

transducer may act as both sound source and receiver.

4.1.3.2 EfficiencyWhen the transducer converts electrical energy to sound energy or vice versa, parts

of the energy is lost in friction and dielectric loss. Typical transducer efficiency is:

50 percent for a ceramic transducer

25 percent for a nickel transducer.

The efficiency is defined as the ratio of power out to power in.

4.1.3.3 Transducer bandwidth

Normally a transducer is resonant. This means that they offer maximum sensitivity

at the frequency they are designed for. Outside this frequency the sensitivity drops.


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Beam pattern

The beam pattern shows the transducer sensitivity in different directions. It has a

main lobe, normally perpendicular to the transducer face. The direction in which the

sensitivity is maximum is called the beam axis. It also has unwanted side lobes and

unwanted back radiation.

An important parameter is the beam width, defined as the angle between the two

3 dB points. As a rough rule of thumb, the beam width is connected with the size of

the transducer by:

= /L,

where:

=beam width in radians

=wavelength

L=linear dimension of the active transducer area (side for a rectangular area,

diameter for a circular).

Acoustic Noise

4.1.4.1 Environmental

Noise from thrusters and propellers from surface vessels is the dominating environmental

noise source. This noise is approximately 40 dB above normal sea noise.

Common for all noise sources is that the noise level drops approximately 10 dB per

decade with increasing frequency.

4.1.4.2 Noise level calculations

The noise level at the system detector is calculated by the following equation:

N = (No 10 log (B) DI),

where:

B =detector bandwidth

DI =directivity of transducer.4.1.4.3 Thruster noise

The noise from the thruster is changing depending on the thruster. On pitch-controlled

thrusters (fixed RPM), the noise level is actually higher when running idle (0 percent

pitch) than running with load. In addition, the impact of the thruster noise is

determined by the direction of the (azimuth) thruster.


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Running a thruster on low RPM and high pitch normally generates less noise thana thruster on high RPM and low pitch. In general, thrusters with variable RPM/fixed

pitch generate less noise than thrusters with fixed RPM/variable pitch

Sound pathsThe velocity of sound is an increasing function of water temperature, pressure, and

salinity.Variations of these parameters produce velocity changes, which in turn causea sound wave to refract or change its direction of propagation. If the velocity gradient

increases, the ray curvature is concave upwards (Figure 4.8). If the velocity gradientis negative, the ray curvature is concave downwards.

The refraction of the sound paths represents the major limitations of a reliableunderwater navigation and telemetry system. The multi-path conditions can vary

significantly depending upon ocean depth, type of bottom, and transducertransducerconfiguration and their respective beam patterns. The multipath transmissions result

in a time and frequency smearing of the received signal as illustrated.There are several ways of attacking this problem. The obvious solution is to eliminate

the multiple arrivals by combining careful signal detection design with the useof a directional transducer beam. A directional receiving beam discriminates against

energy outside of the arrival direction and directional transmit beam project the energy,so that a minimum number of propagation paths are excited.

what is positioning

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