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M(M-1)/2. This is the number of coefficient pairs N that one can 

 develop. It is suggested that one does not develop all N pairs, but 

 stops at least two pairs sooner so that there are more equations than 

 unknowns. The coefficients may then be fitted by least squares 

 analysis . 



As an example, four pressure gauges positioned at the axes of 

 an irregular three-faced pyramid could potentially develop six 

 coefficient pairs (or the first thirteen directional Fourier 

 coefficients) . It could more effectively develop four coefficient 

 pairs (or the first nine coefficients) using least squares analysis 

 to fit the coefficients. Signals from four pressure gauges 

 positioned at the corners of a square, such as in the Scripps Sxy 

 gauge (Seymour , 1978) , develop only four non-ambiguous 

 cross-correlations and thusly could generate the first four 

 coefficient pairs (or the first nine coefficients). There are two 

 other possible (ambiguous) correlations that can be utilized to fit 

 the first nine coefficients by least squares analysis — just as the 

 pyramid-shaped array does by excluding the development of the last 

 two coefficient pairs. 



Since the DPG utilizes differential pressure gauges, the 

 relations for the cross-spectral densities differ from Equation F.l, 

 The cross- spectral density for two colinear differential pressure 

 signals can be shown to be: 



