Logging attitude data in Isis

Bathymetry data, either from a Multibeam Sonar (e.g. Reson, R2Sonic) or from an Interferometric Sonar (e.g. Benthos C3D, SwathPlus) MUST include attitude (Roll, Pitch, Heave, Heading) and navigation data to accurately map the shape and position of the seabed.

In particular the attitude data must be collected and stored at a high data rate in order that each "ping" of acoustic data can be accurately associated with the Roll, Pitch, Heave, and Heading values that existed at the moment the acoustic pulse is detected at the transducer face.

It follows that not only must there be an accurate time reference (source time-tag) for each ping of sonar data and each attitude update but there must be enough attitude updates to closely bracket any particular sonar ping in order that the processing software can interpolate between those values to compute accurate attitude information for each beam/swath/sample.

Isis has two modes that can be used for logging attitude data:

• Low Speed - used for Side Scan sonars that include a motion sensor. In this mode the Attitude data are only written into the XTF ping header at the time of the ping.
• High Speed - used in Bathymetry logging.  In this mode the Attitude information is streamed to the XTF file continuously as an Attitude "packet" at a much higher rate than the sonar ping interval - in this mode every update generated by the motion sensor (IMU) is written to the file with an accurate time tag.

There are three places to check in Isis - it depends of course on exactly how the data are being received in Isis which of these dialog boxes are applicable:

Attitude data from Serial COM port data:

In the typical example we have:

• Heave: {h}
• Pitch: 8
• Roll: 9

The Save as High Speed Data check box MUST be checked.

Custom Attitude Data from a UDP port:

Again a typical custom string is decoded with {h}89, checking the Attitude box will write the data as "High Speed" data.

NOTE: All the other options in this dialog (NMEA0183, TSS1, F180) will automatically choose the correct High Speed option.

POSMV "POSRAW" (packet 102)

Packet 102 contains all necessary information including Pitch, Roll, Heave, Heading and Position and has a special button in the Serial Port setup, in this case the data are automatically logged as High Speed data,  however you should make sure that Disable Data packet logging is NOT CHECKED!

FINAL CHECK DURING ACQUISITION:

When actually writing an XTF file in real time Isis displays the current number of each type of packet in the Switch Dialog, these displays are anly active when actually writing an XTF file to disc.

Not all packet types may be updated depending on which sensors are connected, however as a minimum, for acceptable Bathymetry data you should see the Store Bathy and Store Attitude(3) values increasing.