The accurate and reliable microwave sensor

SM-140 RangeFinder™

Designed for measurement of:
– Airgap and draught
– Ocean wave profiles and tidal variations
– Water level in dams, rivers, canals & lakes
– Bridge clearance

Due to a low frequency of operation compared to laser sensors, fog, rain and water spray will not cause measurement problems. Small physical dimensions, low weight, and high long term stability are other advantages of these sensors. The sensors are available in different range versions in the range from 3 to 95 meters, with different antenna beam widths.

Bridge Clearance Monitoring

In many ports around the world ships have to pass under one or several bridges either in the port entrance,
between island or when coming up or down major rivers. As the ships have become larger and larger over the
years, especially container- and cruise ships, passing safely under bridges has for many bridges and ships become
a critical factor. However, not only the free sailing clearance is critical, but because of increased ship’s draught
also the under keel clearance must be sufficient.

The free sailing clearance and safe water depth will vary continuously and depend upon factors such as tide,
amount of water in respective rivers and for suspension bridges with the load of passing traffic and the ambient
temperature. When passing underneath is not considered safe, ships will either have to wait or make a detour
costing them several extra hours or even days.

On this background it is very important from a safety aspect and for cost saving reasons to be able to accurately
monitor the bridge sailing clearance and make this figure available to the pilot or master onboard ships
approaching or with scheduled arrivals.

Monitoring air gap on the Gerald Desmond Bridge in Long Beach, California. (Pictures courtesy of W. Krug, NOAA)

The technology

The sensor emits a microwave FM chirp signal and receives the echo from the water surface. The signal propagation delay given by the distance from the antenna to the water surface causes a beat signal in the receiver. By means of advanced signal processing the beat frequency is converted to an accurate distance.

The planar patch antenna results in small physical dimensions and low weight.

The FM chrip is generated by a digitally synthesized frequency sweep oscillator with absolute frequency linearity and high stability. The sensor therefore provides accurate range measurements and high long term stability.

Due to the low frequency of operation (compared to laser sensors) fog, rain and water spray will not cause measurement problems.
The sensor signal processing is performed by a microcontroller. The sensor provides the measured range as well as an averaged range with 1 mm resolution. The averaging time constant may be selected by the user.

The signal output may either be continuous at selected rate, or signal measurements in response to user request.
SM-140 is available in different range versions with different antenna beam width.