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###  Gravity Data Processing on R/V Melville  ###
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Description by S.M.Smith, SIO/GDC, January 1995, mlvl62


DATA LOGGING:
------------
Gravity data are collected from a Bell AeroSpace Gravitymeter, Model
BGM3.  No analogue paper record is made.

Time in Unix seconds, milliseconds and meter counts are logged every
second into realtime logfile 'GRAVinpf' by program 'grav_log'.  Once
each day the file is timestamped with the date and this GRAVinpf.[date]
file is transferred to the offline machine for postprocessing.  See
'GRAVinpf.file.description'.

NEAR REALTIME DISPLAY (on mlvl61):
---------------------------------
Unfiltered measured gravity (in mgal) is displayed in the "Lab-Display'
window on the u/w watch terminal, updated at approximately 1 minute
intervals.  A scrolling display of measured gravity, free-air anomaly
and Eotvos correction profiles can also be displayed by executing program
'grvchart' in a window on the u/w watch terminal.


DATA PROCESSING:
---------------
The GRAVinpf file is input to program 'grav_filter' which converts
counts to milligals, applies a gaussian filter with a 3 minute width
and writes the data at 1 minute intervals to file 'grav_uwts.[date]'.
(See grav_uwts.file.description). This file is passed through program
'uwchek' which checks the file for time gaps and flags consecutive
values whose difference exceeds the range specified.  

The grav_uwts.[date] file is then concatenated with the previous
grav_uwts files into a cumulative file, 'grav_uwts.[CRUISE]', which is
merged by program 'uwmerg' with navigation, depth and magnetics into a
binary file, 'uwmrg.[CRUISE]' (see 'uw_merge.description').  During
merging, the International Gravity Formula 1967 and Eotvos correction
are applied to calculate free-air anomaly. Correction for drift of the
gravitymeter is applied after the cruise using gravity ties made in
port at the beginning and end of the leg. (During the leg, zero meter
drift is assumed until the end-leg tie is available.)


#end