Both are an electronic variety of tide gauge that measures pressure changes based on a standard Atmospheric Pressure of 1013hPa
1hPa change in pressure = 1cm change in depth
Measures the magnitude & direction of horizontal water movement through Doppler shift.The ADCP measures the Dopples Shift based on the difference between the transmitted frequency (300-1200kHz) & the remaining the remaining frequency that has been reflected off scatters.
The scatters are assumed to move at the same speed of the water.
The assumption is that currents are homogenous within each chosen layer of the depth.
S4 Current Meter
Uses electrodes to measure the magnitude & direction of horizontal water measurement.
The magnitude is measured by water passing through the electromagnetic field created by the electrode and producing a voltage proportional to the velocity.
Direction is measured by the internal flux gate compass.
The S4CM can only measure currents at its location in the water column and is therefore usually placed at the same depth as the draft of vessels within that area.
Error's for ADCP
Velocity Uncertainty - Random & Systematic errors
Measurements near the surface/seafloor
Error's for CM
Mooring Hydrodynamics - Cannot differentiate between water motion & its own motion.
Deployment method for CM
'ONO' - when in an area that is in or near navigable, trawling or fishing grounds.
Standard - Areas free of surface navigation
Deployment method of ADCP
Buoy & frame - used as part of a bottom mounted configuration.
Load cage - used as part of a bottom mounted configuration or wharf mounted.
2 types of underwater release mechanisms used in the RAN