, ▁Roll s - Ro y ce ▁pl c , ▁Pra xis ▁Autom ation ▁Technology . ▁The ▁applications ▁and ▁avail ability ▁depends ▁on ▁the ▁type ▁of ▁work ▁and ▁water ▁depth . ▁The ▁most ▁common ▁Position ▁reference / Me as uring ▁systems ▁/ Equ ip ment ▁( PR S / P ME ) ▁are : ▁ ▁D G PS , ▁D iffer ential ▁G PS . ▁The ▁position ▁obtained ▁by ▁G PS ▁is ▁not ▁accurate ▁enough ▁for ▁use ▁by ▁D P . ▁The ▁position ▁is ▁improved ▁by ▁use ▁of ▁a ▁fixed ▁ground - based ▁reference ▁station ▁( d iffer ential ▁station ) ▁that ▁comp ares ▁the ▁G PS ▁position ▁to ▁the ▁known ▁position ▁of ▁the ▁station . ▁The ▁correction ▁is ▁sent ▁to ▁the ▁D G PS ▁receiver ▁by ▁long ▁wave ▁radio ▁frequency . ▁For ▁use ▁in ▁D P ▁an ▁even ▁higher ▁accuracy ▁and ▁reli ability ▁is ▁needed . ▁Comp an ies ▁such ▁as ▁Ver ip os , ▁F ug ro ▁or ▁C - Nav ▁supply ▁differential ▁signals ▁via ▁satellite , ▁en abling ▁the ▁combination ▁of ▁several ▁differential ▁stations . ▁The ▁advantage ▁of ▁D G PS ▁is ▁that ▁it ▁is ▁almost ▁always ▁available . ▁Dis adv antages ▁include ▁de grad ation ▁of ▁the ▁signal ▁by ▁ion os pher ic ▁or ▁atmos pher ic ▁dist urban ces , ▁block age ▁of ▁sat ell ites ▁by ▁c ran es ▁or ▁structures ▁and ▁d eter ior ation ▁of ▁the ▁signal ▁at ▁high ▁alt itudes . ▁There ▁are ▁also ▁systems ▁installed ▁on ▁vessels ▁that ▁use ▁various ▁Aug ment ation