Calibration

= Seeking Contributors =

This section is under development. If you have expertise in calibration (patch testing), please reach out to the admins at [mailto:omcadmin@ccom.unh.edu omcadmin@ccom.unh.edu] to become a contributor today!

= Overview =

[IN DEVELOPMENT]

Geometric calibration (patch testing) is a process of identifying and correcting residual (small) biases in the swath data. This is performed only after all sensor offsets have been installed, surveyed, and configured to manufacturers' standards, including antenna calibration for the navigation system.

= Pre-Patch Test =

Several steps must be completed before patch testing:


 * 1) A full review of system geometry and sensor configurations
 * 2) Calibration of the navigation system's GNSS antenna baseline
 * 3) Applanix POS MV GAMS procedure (at sea; 10-60 minutes)
 * 4) Seapath GNSS Calibration Wizard (dockside; 2 hrs; see manual)

Applanix has put together an installation and survey story that illustrates several important points to consider for any navigation system.

= Scope =

It is important to consider what it is that we are really aligning through this process. Assuming all linear offsets are correct, the process help to identify position latency and the net effects of angular biases that exist between the navigation/attitude system and the echosounder (as observed in the swath data). It does not account for attitude latency. There are different approaches for how to attribute these results, which may depend on the sensor manufacturer's expectations for how these are applied.

Even with highly accurate offset surveys, patch tests typically reveal small (&lt;0.5 deg) angular biases. However, the process may reveal larger issues (e.g., latency or linear offset errors) which must be addressed before a meaningful calibration can be completed. Patch test results larger than 1 degree strongly indicate the need for a review of system geometry and configurations, and possibly the need for a more accurate sensor offset survey.

= Approaches =

A comprehensive foundation for multibeam calibration is presented in Godin (1998).

There are several concise resources for modern patch test methods:


 * 1) QPS: How to calibrate a multibeam echosounder (patch test)
 * 2) [References to be added]

Assumptions
The approach presented here is typically used by the MAC for deepwater systems with large TX and RX arrays.

This implies a few assumptions about the mapping system reference frame and sensor configuration:


 * 1) The mapping system reference frame is aligned with the vessel axes, with a user-selected origin (e.g., granite block)
 * 2) All sensor survey results meet manufacturer specifications for uncertainty
 * 3) All sensor offsets are reported in the mapping system reference frame
 * 4) All sensor offsets are configured in their respective software using a consistent reference frame:
 * 5) TX and RX array X, Y, Z, roll, pitch, heading
 * 6) Motion sensor X, Y, Z, roll, pitch, heading
 * 7) GNSS antenna X, Y, Z
 * 8) Translation of sensor output is acceptable with careful review
 * 9) For instance, a Seapath Monitoring Point must match the navigation input location expected in SIS

Interpretations
Under these assumptions, the patch test results are interpreted as follows:


 * 1) Results represent the residual misalignment between the echosounder arrays and attitude source
 * 2) Results are attributed to the motion sensor because:
 * 3) TX and RX array installation angle biases cannot be resolved individually from the patch test data
 * 4) Arrays typically provide longer survey baselines (and smaller angular uncertainties) than motion sensor housings
 * 5) Results are applied in the multibeam software rather than the navigation software because:
 * 6) Results are determined from multibeam data
 * 7) Results do not necessarily apply to other systems receiving attitude data
 * 8) This distinguishes patch test results separately from array angles in the multibeam configuration
 * 9) This maintains consistent motion sensor configuration based on the survey report

Exceptions
Note there are cases in which the motion sensor offsets are known more confidently than the array offsets (e.g., retractable keel systems where raising/lowering is not repeatable to &lt;0.01 deg). In these cases, patch test results might be attributed to array misalignment in the mapping frame, summed with the nominal offset survey results, and applied in the multibeam software. This may be a case-by-case consideration of the confidence in each sensor's offsets.

= Planning =

Add topics on pre-patch test review, site selection, and settings.

= Data collection =

Acquiring patch test lines

= Processing =

Notes on processing in Qimera, SIS, Caris, etc.

= Application =

Applying patch test values in SIS

= Documentation =

How to document the process and results