Specifications

48
5.7.1.2 Spectrum file or full wave spectrum
One level further up the cyclic data contained within 18 vectors forms one block. The cyclical
data is organized as shown in Table 5.7.2. One block provides information on spectral
parameters at 4 frequencies. The parameters are:
• frequency f
• relative power spectral density RPSD
• K check factor (reciprocal of the wave ellipticity)
• mean direction from D
• direction spread S
• centred Fourier coefficients (m
2
and n
2
), see Equations (5.6.22) and (5.6.23).
Table 5.7.2. Organization of cyclic data in a block of 18 vectors.
16 blocks constitute a spectrum file or full wave spectrum.
The vector number on the left is not transmitted.
No Cyclic data word (16 bits)
1 sync word (hex 7FFF)
2 system file word number
4 bits
system file word
12 bits
3 S
LSB
(n)
2 bits
frequency index n
6 bits
direction D(n)
8 bits
4 M
2LSB
(n)
2 bits
N
2LSB
(n)
2 bits
rel. power spectral density RPSD(n)
12 bits
5 spread S(n)
8 bits
M
2
(n)
8 bits
6 N
2
(n)
8 bits
check factor K(n)
8 bits
7-10 n+1, same as n
11-14 n+2, same as n
15-18 n+3, same as n
The sync word is repeated every 18 vectors. Its pattern is not likely found elsewhere. S
LSB
,
M
2LSB
and N
2LSB
represent the two LSB’s of spread, M
2
and N
2
, respectively. For non-
directional buoys (WR-SG) the check factor, direction, spread and centred Fourier coefficients
are not meaningful. 16 blocks of 16 vectors (omitting the sync word and system file word
number and the system file word itself) make up a complete spectrum file or a full wave
spectrum ranging over 64 frequency values. However, the spectral parameters in the cyclic data
must be transformed first.
Table 5.7.3 explains how the various cyclic data fields translate to the respective spectral
parameters. The intermediate point of n = 15 is an exception with a frequency interval running
from 0.0975 to 0.105 Hz and with a Δf of 0.0075 Hz.