Specifications

49
Table 5.7.3. Translation equations for spectral parameters in spectrum file.
Parameter Equation Remarks
Frequency f
n
= 0.025 Hz + nΔf
f
n
= 0.11 Hz + (n−16)Δf
Δf = 0.005 Hz, n = 0…15
Δf = 0.01 Hz, n = 16…63
Direction D = D 360º/256 0º = north, 90º = east
Relative power
spectral density
RPSD = exp(−RPSD/200) max 1, multiply by power
density in system file
Spread S = 0.4476 (S + S
LSB
/4) degrees
m
2
m
2
= (M
2
+ M
2LSB
/4−128)/128 centred Fourier coefficient (cosine)
n
2
n
2
= (N
2
+ N
2LSB
/4−128)/128 centred Fourier coefficient (sine)
Check factor (reciprocal
wave ellipticity)
K = K/100 K = 1/ε,
K = 1 for waves in deep water
5.7.1.3 System file
All system file words of 16 blocks are combined into one system file. A complete system file
consists of 16 words from 16 consecutive blocks. It takes 16×18 = 288 vectors to acquire one
system file. Table 5.7.4 (next page) shows the significance of the system file data. Bits in the
12-bits word are numbered 0 through 11 with bit 11 MSB and bit 0 LSB. LSB and MSB
subscripts of Long, Lat and Inc mean least/most significant bits.
5.7.1.4 Timing
During half an hour 8 spectra of a 200 s data interval each are collected and averaged. At the
end of the half-hour over which the calculations are executed, all (directional) spectral
parameters are available. The transmission of one spectrum file takes 225 s. During the next half
hour the spectrum file is transmitted 8 times for redundancy. Table 5.7.5 gives an overview of
the timing of vectors, blocks and files.
Table 5.7.5. Timing at all levels of data.
Unit Size Time
Vector 64 bits 0.78125 s (1/1.28 Hz)
Block 18 vectors 14.0625 s
File 16 blocks (288 vectors) 225 s
Full cycle 8 repeated files 30 min