Precise Timing and TrueHeave in Multibeam Acquisition and Processing

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Precise Timing and TrueHeave in Multibeam
Acquisition and Processing

• Doug Lockhart, Thales GeoSolutions (Pacific) Inc
• Dushan Arumugam, Thales GeoSolutions (Pacific) Inc

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Objective Enable precise timing and TrueHeave

• Timing overview Why timing is important
• Precise timing
• Selecting an epoch
• Supported Sub-Systems
• System topology
• Modified patch test procedures
• TrueHeave acquisition and processing
• Timing and TrueHeave economic benefits

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Why is Timing Important?

• Multibeam Bathymetry is created from a number of
  data elements
• Sounder ranges and angles
• Position
• Pitch, Roll, Heave
• Heading
• Data elements are merged using time as a common
  index
• Timing errors and variable latencies will result
  in a miss match or irregular merge of the data
  elements
• The bad merge will produce a sounding calculation
  that is both horizontally and vertically
  inaccurate

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Precise TimingSelecting an Epoch

• Time stamp data when it is created, not when it
  is logged.
• Select a standard epoch.
• PC millisecond timers define their own epoch and
  are not standard
• Use a single clock/epoch to time stamp all data.
• Position, attitude, and heading are time stamped
  in the POSMV on the UTC epoch
• TrueHeave data can be time stamped by the POSMV
  on the UTC or GPS time

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Supported Sub-Systems

• Precise Timing and TrueHeave have been
  implemented on the following systems by TGPI
• Reson SeaBat
• Applanix POS/MV
• TritonElics ISIS (XTF)
• Caris HIPS
• Other implementations are possible.

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System Topology
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XTF Packets were created to hold the new time
stamps
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Precise Timing TrueHeave Acquisition

• Logged in XTF by TEI ISIS
• Position, Heading and Attitude data time stamped
  in POS/MV on UTC epoch
• Bathy data Time Stamped in SeaBat using POSMV UTC
  serial string
• Logged by POS/MV Controller
• TrueHeave data time stamped within the POS/MV on
  UTC epoch
• TrueHeave data is logged throughout the survey
  day, independent of line changes

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Precise Timing TrueHeave Processing

• XTF files are converted into HDCS format by Caris
• Options in the XTF converter let the user
  selected the XTF packets containing the new timing

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Modified Patch Test Procedures

• Acquisition
• Line pairs for Pitch, Roll and Yaw are required
• Pitch, Roll and Yaw lines are processed as usual
• Navigation Latency lines are not required
• Navigation latency can be determined from any
  single line
• Processing
• Pitch, Roll and Yaw are processed as usual
• Navigation latency is determined by examining
  roll timing error
• Roll timing error is negated and applied to the
  ping time, accounting for all navigation and
  attitude latencies

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Roll Timing Error in Caris Subset Mode
(simulated data)
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Roll Timing Error in Caris Subset Mode (real data)
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Roll Timing Error Removed
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Multiple Lines with Roll Timing Error
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Multiple Lines with Roll Timing Error Removed
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TrueHeave What is it?

• Without interrupting the flow of the real time
  heave
• Unfiltered heave data is stored in the POS/MV
  memory
• After a few minutes, a zero phase filter is
  passed over the stored heave data
• The delayed heave value and real time heave value
  are output together over the Ethernet

Heave values
Output Times
Valid Times
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TrueHeave Processing

• Caris allows the user to browse and select raw
  POS/MV files containing TrueHeave data.
• The heave record in the HDCS format can be
  replaced with the TrueHeave or real time heave
  record.

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Real Time Heave v. TrueHeave
Meters
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Real Time Heave (3x)
TrueHeave (3x)
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Real Time HeaveTrueHeave
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How TrueHeave Affects Run-In Time
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Data Samples Alaska SurveysCastle Bay
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Sun Illuminated Bathymetry3x Vertical
Exaggeration3m bins, 4m 50m depths
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Sun Illuminated Bathymetry3x Vertical
Exaggeration3m bins, 4m 50m depths
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Precise Timing Benefits

• Reducing timing errors results in more room in
  the error budget for other errors such as Tides
  SVP
• Mechanically induced artifacts are easier to
  diagnose when timing errors are small
• Increased operational weather window. No survey
  time was lost due to excessive vessel motion in
  Alaska this summer.

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TrueHeave Benefits

• Reduced heave component in error budget,
  particularly during long period swell
• Shorter turn times, line changes, and easier
  shoreline surveys
• Realized savings from a single survey
• Prince William Sound
• Estimate turn times for real time heave filter -
  10 min
• Actual turn time - 4-5 min
• Lines - 1293
• Total turn time - 107.75 hrs (at 5 min) 4.5
  days
• Time saved on turns 4.5 days

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Acknowledgments

• NOAA Our existing large Alaska survey contract
  allows us to invest internally on RD efforts
  like this that ultimately benefit our operations,
  NOAA and the hydrographic community.
• Applanix Rob Corcoran implemented TrueHeave in
  the POS/MV and provided valuable technical
  assistance
• TritonElics Inc Caris Software providers made
  quick updates to their software to support the
  new timing and TrueHeave