While the position fixing techniques employed in offshore drilling and exploration are in many ways similar to those used by the shipping industry—and, indeed, many systems are available to both—there are, nevertheless, several important differences. Accuracy and repeatability are two, in particular, which stand out. It is difficult to state in precise terms what accuracy is acceptable in an offshore survey because the customer is often obliged to accept the best of what he is offered but something of the order of 100 metres is likely to be the target. This is very much better than is required by the average ship out of sight of land or, for that matter, is obtainable in many cases with the instruments available. Repeatability means little to most navigators who rarely have any requirement to return exactly to any position denned in the past. The surveyor, on the other hand, has a continuing need to do this, running lines across his survey grid for check purposes, or at a later date when carrying out another survey or positioning a drilling rig.