approaches zero and at the frequency below which wave energy is known not 

 to be present. This will be discussed further in the next section. 



The error due to tilt of the accelerometer , mentioned by Dorrestein,^ 

 has been examined theoretically by Tucker.'* Computations were made of 

 the magnitude of the errors introduced into wave measurement by using an 

 accelerometer which sets itself in the "apparent vertical," that is, 

 perpendicular to the local water surface, instead of being stabilized to 

 measure the true vertical acceleration. This applies directly to the 

 SPLASHNIK. Tucker found that the spectrum of the error signal rises 

 steeply at low frequencies but does not seriously affect the main wave 

 components. Figure 6, from Tucker's paper, shows several error spectra 

 superposed on hypothetical wave spectra for three sea states. From these 

 graphs, the following computations were made by Tucker: 



Sea State 5 7 9 



Error in spectral density at 



frequency of maximum energy 0.9% 1.3% 2.8% 



Error in rms wave height 



with high-pass filter 3.9% 1.6% 1.1% 



The errors are seen to be relatively small, about 4 percent in rms wave 

 height in State 5 sea and decreasing for higher sea states. 



CALCULATION OF THE WAVE SPECTRUM 



The SPLASHNIK will measure the apparent vertical acceleration of the 

 environmental water particles such that a particular record may be repre- 

 sented by an integral of the form 



■1 



a(t) = ) cos [ 03 t + e(cD)] n/ a(a)) do [l] 



