A performance test of the airborne wave profiler was conducted at Argus 

 Island Tower, Bermuda several years agoo While the aircraft took profiles of the 

 sea surface along flight tracks directed up and downwind, wave measurements 

 were being taken simultaneously at the Tower with a very accurate resistance- 

 wire wave staff. The performance test began about 16 hours after the passage of 

 a frontal system that had winds of speeds 30-35 knots and direction 280°T associated 

 with it. The effective fetch was approximately 400 nautical miles. In short, 

 measurements were made of what would commonly be described as a "fully developed", 

 stationary and homogeneous wind seoo 



Typical resulting comparisons of the energy spectrum of the sea surface as 

 measured by the aircraft and wave staff are shown in Figure 3. Unfortunately, the 

 comparison plane data was evaluated using only the assumption that all waves were 

 travelling downwind. For wave frequencies higher than the frequency of the spec- 

 tral peak, this is an unrealistic assumption and is one of the reasons for the under- 

 estimates in the mid-frequency range of the spectrum. A more general approach 

 (section 6„0) using spectral spreading factors (section 5„2) that are more in line with 

 observation would probably have yielded a better correspondence. It will be seen 

 that by varying the assumed spreading factor one varies the magnitude of the spectral 

 estimates but leaves the qualitative features unaltered (Section 7ol). 



The comparisons of Figure 3 are considered quite good in the frequency 

 range about the spectral maximum. As was expected, the agreement degraded with 

 increasing wave frequency. While some of this was due to the unrealistic spreading 

 factor, a larger portion was due to the system response used during the test. In an 

 effort to minimize system noise, a relatively large time constant (0„19 sec) was used. 

 In correcting the resulting spectra for instrument response, however, the small amount 

 of noise that did remain combined with the large correction factors for higher fre- 

 quency range „ In the present study, as previously mentioned, a compromise response 

 factor of Ool second was used. The relative instrument responses for the 0ol9 and 

 OolO second time constants are shown in Figure 4. In addition, system modifications 

 were made, before the present experiment, that tangibly reduced the noise level. 

 Unfortunately, as will be seen, the noise level was still high enough to provoke con- 

 siderable uncertainty of relatively high frequency spectral estimates. 



3o2 Aircraft Motions 



Pitching, rolling, and heaving motions of the aircraft introduced unwanted 

 noise to the wave profile. Of these three motions, the heaving motions were the 

 most important due to their relative magnitude. As can be seen from the sample analog 

 trace, vertical displacements approaching 25 meters were recorded. Cancellation of 

 the relatively high frequency vertical m.otion by the doubly integrated accelerometer 

 output appears nearly complete. Some error resulted from the practical limits to which 



