If you have experience with Spotter or other telemetered wave sensing buoys, you may be familiar with ‘spike’ artifacts in the data. These artifacts are noise - data that does not represent the physical phenomena we’re trying to measure - and can be an annoyance and/or confounder to analysis and use of the data.
The good news is that we have been building a robust understanding of this source of noise. There are a variety of near-term work-arounds and some promising development directions we’re exploring.
The tell-tale signature of a noise spike is an unusual positive outlier in Significant Wave Height coincident with an unusual positive outlier in Peak Wave Period.
The cause of this spike in bulk statistics is a temporary degradation in GPS signal quality and/or significant shift in the GPS satellite constellation viewable by the system.
Plot of surface elevation vs time showing normal low energy conditions interrupted by a spike.
These inject mean offsets into the absolute position and Doppler velocity resolutions in the GPS system, which interact with signal processing filters to produce significant excess energy in the lower frequency portion of the surface motion spectra for on the order of 10 seconds to 10 minutes.
Example wave spectra realization with a low frequency spike (blue) vs surrounding conditions (grey).
When this excess energy exceeds the energy generated by the true signal of interest (surface wave motion), it results in a the occurrence of a noise spike.
Low energy conditions
In general, the lower energy and lower frequency the wave environment is, the more likely spikes are to be observed. This is because the noise leading to the spikes tends to be in lower frequencies of the wave spectra (> 20 second period).
Submersion or over-wash
For moored application, a momentary submergence or over-wash of the Spotter can interfere with the GPS signal and result in spikes. This can be exacerbated in combinations of strong currents and waves. Occasional spikes are not a major concern in moored applications, but an increasing frequency of spikes could be a leading indicator of fouling, entanglement, or other emerging issue with the mooring.
Before cleaning and servicing mooring:
After cleaning and servicing mooring:
Obstruction of Spotter by nearby objects
If your Spotter is deployed near larger structures, these may degrade or shade the GPS signal environment and lead to a higher incidence of spikes.
Obstruction of Spotters by nearby topography
If your Spotter is deployed in a topographically constrained region (such as an alpine lake or a fjord) the sudden unexpected constellation shifts as GPS satellites disappear behind the raised horizon can lead to spikes. These often occur on a diurnal cycle or sub-harmonic of the GPS constellation orbital period.
Plots illustrating spikes occurring at exactly the same time on subsequent days in low energy conditions:
11/05/2021, 00:53:18
11/06/2021, 00:53:18
11/07/2021, 00:53:18