Although Inertial Navigation Systems (INS) are expensive instruments, their use is considered in hydrographic sensor suites for various reasons. An INS is more than an advanced compass and motion sensor, it is also an insurance against incorrect or missing position information. This is especially true for underwater applications where inertial navigation has the ability to make acoustic positioning unnecessary. Based on an extrapolation of the surface position from satellite positioning, the INS updates the vessel's position, motions and velocity at a high resolution in the absence of new external positions. Depending on the quality of the sensors, the underwater operation will continue until positioning becomes inaccurate.

Therefore, the decrease of unaided position accuracy is the main criterion for the selection of products. One product has been selected per manufacturer. It is apparent from the table that
a lot of accuracy information is unavailable.
Reasons for this include confidentiality, but also the difficulty in specifying uncertainties that are neither static nor linearly degrading with time or latitude. If a manufacturer has not given any position accuracy, the product selection is based on the accuracy of other output parameters. As
a final criterion, the product that was indicated as most suitable for underwater use was selected. A complete product survey is available online.

For reasons of brevity, several relevant aspects re not given. For instance, no information is presented on acoustic aiding and EM-log aiding. These aspects would, however, not change the general impression of a product.

Leendert Dorst, contributing editor, Hydro International