the scale factors of the wave height sensor and the vertical motions sensor can be 

 made equaL This error appears in the recording as a very low amplitude noise of a 

 period identical to the period of vertical motion of the aircraft = From the sample 

 of Figure 2, this period appears to be about 30 seconds and corresponds to a wave 

 length of 2^800 meters, well outside the range of interest » 



While it is informative to investigate the magnitude of the errors introduced 

 by pitching and rolling of the plane, it is more to the point to ask if these errors 

 are, in fact. Introduced at all. The performance tests discussed In the previous 

 section would seem to indicate that the plane motions do not significantly affect 

 the results. To investigate this question further, time histories of pitch and roll 

 angles were obtained under turbulent atmospheric conditions, conditions quite simi- 

 lar to those under which the data presented In this paper were taken. These esti- 

 mates of pitch and roll angles came from the dip angle indicator of a fixed airborne 

 magnetometer* . If the plane heading is either magnetic east or west, the variation 

 of dip angle, as seen by the fixed magnetometer. Is essentially a measure of the rol- 

 ling motion of the aircraft. If the plane is heading either magnetic north or sourth, 

 the variation in dip angle Is a measure of the pitching motion. Two typical power 

 spectra of these time series were computed (Section 5.1) and are shown in Figure 

 5o Clearly the majority of the energy in these spectra is associated with quite low 

 frequencies. It will turn out that these frequencies are well below those at which 

 common wind generated gravity waves appear to occur. Further, these measurements 

 show that even under very turbulent conditions, mean values of the roll angle are 

 less than one degree (.02 radians). Maximum roll angles are under 3 degrees. These 

 same measurements indicate also that pitch angles are usually much less than roll 

 angles. It can be shown that even with roll or pitch angles of 3 degrees, the error 

 in apparent wave height is less than .7 meters. 



Unfortunately, it was not possible to monitor the pitch and roll angles of the 

 aircraft during this experiment. In flight, however, automatic control, through the 

 auto-pilot system, corrected all rotational motions greater than 0.25 degrees of 

 angle, which is the minimum level of sensitivity of Its servo mechanisms. Although 

 the lag time of the entire control system would necessarily result in angles greater than 

 0.25 degrees, it seems quite reasonable to assume that these angles were certainly 

 less than 3 degrees. 



*These data were taken in aircraft Identical to the one used in this experiment. The 

 assistance of Mr. Ron Lorentzen, U.S. Naval Oceanographic Office, in obtaining 

 this data is gratefully acknowledged. 



11 



