Specifications

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5.7.1.5 Compressed wave spectrum
The wave spectrum computed internally by the buoy covers 128 frequency bands and is
condensed to 64 frequency bands occupying 512 bytes (spectrum file, full wave spectrum). To
save bits while maintaining a large range of values an exponential representation of numbers is
used. Furthermore, the initial 64 frequency bands of the full spectrum are reduced to 13 bands.
Finally, a fixed distribution of the power spectral density (PSD) in a fixed number of frequency
bands is predefined. This does away with transmission of the normalized or relative PSD. Only
the frequency band limits or band widths need to be transmitted. The compressed wave
spectrum consists of 16 words of 16 bits and is detailed in Table 5.7.6. Values must be
transformed according to Table 5.7.7.
Table 5.7.6. Compressed wave spectrum data format
Word
number
Compressed
wave spectrum
word
Fraction of
total variance or
integrated PSD
Significance
0 bits 15-13
bits 12-7
bits 6-0
1/512
battery status number
frequency upper limit of first band F
(lower limit is 0.025 Hz)
total variance or integrated PSD M0
1 bits 15-10
bits 9-4
bits 3-0
1/256 frequency band width of next band ΔF
direction from D
directional spread S
2 1/128 see word 1
3 1/64 see word 1
4 1/32 see word 1
5 1/16 see word 1
6-11 1/8 see word 1
12 1/16 see word 1
13 1/32 see word 1
14 bits 15-10
bits 9-4
bit 3
bits 2-0
17/512 direction from D for remaining band (from
upper limit of previous band to 0.635 Hz)
mean time between zero up-crossings Tz
reserved
absolute vertical acceleration offset
units 0.32 m/s
2
*
15 bits 15-13
bits 12-10
bits 9-0
idem absolute x-acceleration offset*
idem absolute y-acceleration offset*
water temperature
*only on DWR-MkIII, reserved on DWR-G