Specifications

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5.5 Wave motion sensor: GPS
5.5.1 Wave measurement principle
The GPS principle of wave measurement is explained by analogy. Apart from distance
measurements between satellite and receiver, the so called GPS code phase, some GPS receivers
also provide Doppler measurements. The former are used in GPS positioning, whereas the latter
are indicative of satellite and receiver velocities. Now, we exploit the analogy of Doppler
frequency shifts for sound waves from moving sources. As for a passing car blowing its horn, it
is in principle possible to track the motion of the car by listening to the sound of the horn. The
more the frequency differs from the original frequency of the horn source, the higher the speed
of the car. Integrating over time then yields the motion of the car.
5.5.2 GPS motion sensor
A GPS motion sensor consists of a GPS antenna and a GPS receiver. A patented algorithm will
calculate the motion of the antenna centre.
As a GPS receiver is a complicated piece of electronics, usually malfunction means no
functioning at all. However, should you want to test the GPS motion sensor, the procedure is
very simple. Just leave the DWR-G buoy in a place where it has a clear view of the sky (no
buildings or trees) and set it running. If the resulting north, west and vertical signals remain
within a few centimetres approximately the sensor is alright. It may seem paradoxical to test a
motion sensor by leaving it motionless. The way out of this paradox is that GPS satellites orbit
the earth with velocities of 4 Km/s and any motion of the GPS receiver only forms a minor
contribution. So if a motionless GPS motion sensor correctly produces a no-motion result in a
highly dynamic situation then it works fine.
5.5.3 GPS and atmospheric or marine conditions
In line with the military and strategic intentions of the GPS system, GPS receivers and therefore
the GPS buoy, will continue functioning during rain, snow or hail storms. The only problematic
meteorological situation is when the GPS antenna is covered with a continuous layer of ice.
As for marine conditions, spray will also not impede the normal operation of the buoy.
However, GPS signals do not penetrate through a continuous layer of salt or fresh water. A
breaking wave washing over the GPS antenna will result in data missing.
Apart from meteorological and marine influences, also dirt, paint or metal constructions
added by the user may block the GPS signal. If the GPS antenna has become dirty, clean it with
water and soap and a soft piece of cloth.
5.5.4 GPS antenna and pseudo-motion
To be precise the three-dimensional motion of the GPS antenna is measured and not that of the
centre of the buoy. This implies that pitching and rolling of the buoy will result in measured
(GPS antenna) ‘wave’ motion, even on a perfectly smooth sea surface. This false motion is
addressed as pseudo-motion. Experiments where pitch and roll have been logged, have
demonstrated that the Fourier spectrum of the pseudo-vertical always remains far below the
wave spectrum. In the case of pseudo-horizontal motion, only on the high frequency side, above
0.5 Hz, the pseudo-spectrum may be comparable to the true wave motion. This is consistent
with the fact that pitch and roll will exert a larger effect on the horizontal than on the vertical
pseudo-motion component. Furthermore, the frequency given is consistent with the typical pitch
and roll behaviour displayed when the floating buoy is tilted by hand and is released.
5.5.5 Signal loss and flag
In the example of the passing car, one will lose track of the car if one temporarily closes one’s
ears. Likewise if the GPS receiver temporarily loses signal, due to waves washing over or a
momentarily extreme tilt, the buoy velocity is temporarily unknown. As a result the buoy
motion may contain discontinuities. The good thing is that the GPS receiver knows when signal
loss occurs and will inform the user of this through setting a flag: the least significant bit of the
north.
In practice, breaking waves in the surf zone and areas of strong current must be avoided. Wave
height by itself is not a problem. In other locations signal loss occurs less then 0.1% of the time.