Multiple-stage missiles, with each stage having its own independent propulsion system, are more efficient for longer range missions. ICBMs typically have two or three stages, with powerful liquid-propellant engines or solid-propellant motors to propel the payload toward its target, in addition to a post-boost vehicle (PBV) with a much smaller propulsion system. A post-boost vehicle can be used to improve the RV deployment accuracy for a single-RV missile. For a missile with a MIRV payload, the PBV is used to release reentry vehicles so that they follow different trajectories, allowing them to hit targets that may be separated by over a thousand miles. Photo credit: TommaX, Inc./Military Parade Ltd. Mobile missiles can move frequently to avoid being targeted by hostile forces. A ballistic missile with a high-quality inertial guidance system is capable of delivering a reentry vehicle within a few hundred feet of the target after a flight of over 6,000 miles. For many missiles, accuracy can be greatly improved by utilizing satellite-aided navigation. As more modern guidance technology is proliferated, countries will be able to improve the accuracy and lethality of their missile forces. However, even a missile with an inaccurate guidance system is capable of inflicting massive casualties when armed with a weapon of mass destruction. Therefore, many Third World ballistic missiles, although inaccurate, have the potential to pose a serious threat to urban targets. Many ballistic missiles carry penetration aids to improve the chances of a reentry vehicle penetrating a ballistic missile defense system. Penetration aids are devices that attempt to deceive or jam sensors used to detect and track missiles and RVs. Penetration aids are of increasing importance to countries developing and operating ballistic missiles.