Page 473 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

is a 50 percent probability of being located even though the actual error figure is an ellipse.” That is, the radius of a circle of equivalent probability when the probability is specified as 50%. The CEP mea- surement is useful in navigation, weapons delivery computations, and search grid designations.
In order to get around this inherent positional drift in INS systems, the survey system is usually augmented by some sort of aiding device. For surface applications, an aiding device could be a global positioning system (GPS). Omega, or Loran. For subsea applications, an aiding device could be USBL, LBL, or DVL. No matter which aiding device is chosen, the purpose is the same: to decrease or even remove inherent drift in INS deduced position.
In order to remove drift in position, a nondrift position-aiding device is needed. This could be a GPS or a USBL system. If a velocity-aiding device like a DVL is used, it will only be possible to decrease the drift in position. The drift is then primarily depending on the DVL velocity accuracy. It is, of course, also possible to aid an INS system with multiple aiding devices, thus gaining redun- dancy and improved performance.
INS devices are becoming more accurate while being packed in continually smaller units (newer systems are based upon MEMS technology). INS is widely used in subsea vehicle applications as an integral part of the survey package. A size comparison of a FOG-based INS system and a MEMS-based INS system can be seen in Figure 17.13.
FIGURE 17.13
A size comparison between a FOG-based INS system (TOGS 2) (left) and a MEMS-based INS system (MiniSense 3) (right).
(Courtesy CDL.)
17.4 Inertial navigation systems 467