Basic HTML Version
ACT Protocols for Wave Measurement Systems
July 2012
10
Reported Resolution/Accuracy/Range
Wave elevation
Upper
limit
Period
range/res,
Direction res.
Depth of
observation
Resolution/Accurac
y
Freq. Dep.
Ant. Dep.
Freq. Dep.
CODAR
n-a / 7-15%
±0.6s
±5-12
o
d~> 2-10m
WERA
0.1m/10-15%
±0.6s
n-a
d~> 2-10m
When considering the First-5 and their accuracy, it must be kept in mind that there is a difference
between the first coefficient (wave energy) and the other four (directional) coefficients. The
wave energy spectrum is a clear concept, directly related to the variance of the sea surface
fluctuations, easily translatable in terms of an effective wave height. The directional coefficients
however, are merely the first four of an infinite series of coefficients (all of which are
inaccessible to single instruments), and their interpretation as direction and directional spread
inevitably requires theoretical modeling (e.g. a cos
2
s-model or a ME distribution). For example,
when using a unimodal directional model for describing a multimodal wave system, directional
estimators could be off the mark by as much as 90 degrees, although the sensor accuracy in
simulations or under laboratory conditions may be 0.01 degree. Hence this ideal sensor accuracy
in simulations or in the lab must be discerned from instrument accuracy at sea. In the latter case,
the instrument response function, the statistical validity of the model and the sampling variability
are at least as important as the fundamental sensor accuracy.
5.0 Objectives and Focus of Wave Measurement System Verification
It is stated in the IOOS National Wave Plan that “
Continuous testing and evaluation of
operational and pre-operational measurement systems is an essential component of the National
Operational Wave Observation Plan, equal in importance to the deployment of new assets.”
To
have the ability to maintain a continuous testing and evaluation program, the protocols must be
created as “living” documents, meaning they should be revisited at timely intervals and updated
with lessons learned and new information. The results of continuous evaluations will not only be
used to set accuracy requirements for new measurement devices and upgrades prior to
implementation of the IOOS National Wave Plan, they will also allow users to determine the
accuracy of historical data.
Permanent facilities should be created on the west coast (Monterey Bay, CA – deep water) and
the east coast (Duck, NC – shallow water) for continued testing as new technologies develop and
current technologies are upgraded. These facilities will also provide unique guidance for
defining operational requirements for servicing and replacement of sensitive components of the
various platforms. This information will be critical for allocating appropriate resources within
the national network.
The development of these permanent facilities will provide opportunities to evaluate sensor
performance in extreme conditions in the future. These extremes may include extreme winds,
wave heights and/or currents, all of which may impact different aspects of sensors and associated