Measuring water depth by radar wave has become widespread since the late 1980s and is now a common technique with a large number of sensors available on the market. For coastal tide stations, the advantage of these ultrafrequency radars is that they have a constant speed (the speed of light) offering height measurements over short distances that are unaffected by environmental conditions. These gauges meet all the accuracy requirements, but they can be tricky to install and calibrate. A structure above water is necessary for the transducer to be referenced vertically. This structure does not need to be sophisticated because the acquisition and processing system can be protected in a shelter at some distance from the measurement location. Today, radar level sensors are commonly used in hydrometry and are becoming the standard for tide stations around the world because of the increased need for accurate measurements.

## Presentation

There are two measuring principles depending on the type of sensor used: TDR Time Domain Reflectometry) and FMCW (Frequency Modulated Continuous Wave). Both are based on the reflection of the radar signal at the air / water interface, but while TDR measures transit time, FMCW measures a difference in frequency.

## How TDR works

In TDR the unit emits low intensity electromagnetic pulses whose transit time between the emission and reception of the reflected pulse is proportional to the distance between the reference point of the unit and the surface of the sea.

## How FMCW works

FMCW sensors do not emit frequency pulses but a linearly frequency modulated continuous signal between two frequencies defined as f1 and f2.

The frequency difference df between the frequency transmitted and the frequency received by the sensor after reflection on the water is directly proportional to the distance d. This frequency differential is converted into a frequency range by Fourier transform. The distance is then calculated from the range. This method has the advantage of providing a more accurate resolution, though this requires greater computing power.

## From measured air draft to water depth

The distance calculated by the radar sensor is converted into a digital signal reduced by the data logger to a water depth referenced to the port datum. The tide gauge is always connected to the port tide station by levelling its reference point in relation to the SHOM tide bench mark and the IGN levelling bench mark near the station.

## International recommendations for radar tide gauges

The radar range finders used by SHOM meet the quality requirements of the GLOSS program (instrument measurement uncertainty less than one centimetre and the essential criteria for an operational data network: robustness, low power consumption and compatibility with digital data loggers.

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References

Last updated: 12/12/2012