MAG/NAV/M1 - SEANAV system end-of-life issues begin

When initially purchased, it was not realized that the SEANAV Fiber Optic Gyros (FOG) have 
a predictable life time before degradation and failure. In October 2012, the FOG 
end-of-life issues started to appear.  The problem was not diagnosed immediately.  

When the internal FOG lasers go below a selected intensity the SEANAV enters a "NO GO" 
mode and data ceases.  When this failure cause was finally identified we learned it was 
possible to delay the failure by adjusting internal laser thresholds somewhat, not affecting 
data quality.  

The specified data outages are due to these NO GO periods and data resumed when laser 
thresholds were further lowered.  Ultimate failure was however reached on Nov 1, 2012 at 
16:24 UTC.

GPS data from the SEANAV was still available and data records with date, time, latitude, 
longitude, northvelocity (m/s), eastvelocity (m/s) were collected into a new data stream 
named magnavgpsM1.a1.  These data do not contain any ship attitude data such as 
roll/pitch/yaw.  

Note that ship attitude data is also collected from motion sensors associated with other 
MAGIC instruments.  These alternate/surrogate motion data sources might be useful 
depending on the particular need.  Contact the SEANAV mentor for discussion.

• Ship surge acceleration along ship length, positive forward(surge_acceleration)
• Yaw angular rate, clockwise bow rotation positive(yaw_angular_rate)
• Ship sway velocity, starboard-to-port displacement, positive to port(sway_velocity)
• base time in epoch(base_time)
• Heave directional velocity, up positive(heave_velocity)
• Ship sway, starboard-to-port displacement, positive to port(sway)
• Roll angle reported by tilt table, positive is port side up(roll)
• North Latitude(lat)
• Ship sway acceleration starboard-to-port, positive to port(sway_acceleration)
• Seanav monitor value as true/false conditions(seanav_monitor)
• Seanav system mode(seanav_mode)
• -180.0 - +180.0(alt)
• time_in_seconds_since_volume_start(time)
• Time offset from base_time(time_offset)
• Heave directional acceleration, up positive(heave_acceleration)
• Roll angular rate, positive is port side up(roll_angular_rate)
• Free running cycle counter(cycle_count)
• Ship yaw, axis vertical, positive is clockwise rotation(yaw)
• Ship surge velocity along ship length, positive forward(surge_velocity)
• Ship surge displacement along ship length, positive forward(surge)
• lon(lon)
• Pitch angular rate, bow up positive(pitch_angular_rate)
• Pitch, bow up positive(pitch)
• Heave: up positive(heave)