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D – ALTIMETER

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❐ Noise level: Acoustic sounds in sea water due to ships, hydraulics, or other sonar

equipment.

❐ Reflectivity: the attenuation of the transmitted sonar pulse due to the material/

angle of the reflector, (in this case the seabed).

❐ DB: This is the term Decibel which is used to express sound level in relation to a

reference level, usually 1 micro Pascal at 1 metre. This can be negative when

expressing receiver sensitivity or positive if expressing transmitted sound level.

❐ Absorption: The loss due to sea water which increases for higher frequency.

❐ Reverberation: Received signals due to various scatterers of sonar signals such

as sea surface, tiny particles in the sea water and bottom reflections. This can be

heard on old war films as the slowly decaying quivering tonal blast following the

ping of an echo sounder. The altimeter locks on to the first signal and rejects the

following reverb.

❐ PRT100: Platinum Resistance Thermometer which consists of a platinum wire cal-

ibrated for 100 ohms at 0°C and 138.5 ohms at 100°C. Accuracy is ±0.15°C for a

class A device as used in the altimeter.

❐ Pressure sensor: Device for measuring depth in sea water. Consists of a strain

gauge element which converts pressure to an electrical signal.

❐ Switch mode power supply: Circuitry within the altimeter which boosts the supply

voltage to the required level.

❐ TVG: Time Varying Gain. This is applied to the sonar signal to compensate for

range and absorption losses in sea water. The altimeter TVG signature is stored in

the non-volatile memory of the microcontroller.

D.3.1.3 Propagation Loss

The propagation loss describes the weakening of sound between a point 1 metre

from the surface of the transducer and a point at distance from that point in the water

column.

The propagation loss consists of spreading or ranging loss and loss due to attenua-

tion in sea water. The altimeter is designed to normalise these losses by applying a

varying gain, (TVG), to the sonar receiver.

Circuitry within the altimeter rejects near field signals from transducer side lobes to

enable detection of minimum range values.

D.3.1.4 Limitations

The altimeter must not be used alongside instruments operating at or near the same

frequency. The power supply should be DC with good regulation; the altimeter is

designed for worst case power supply electrical noise by the use of analogue filters at

the DC power input, however, noise at or near the Sonar frequency may cause prob-

lems.

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