A number of situations can lead to or exacerbate the risk of friendly fire. Difficult terrain and visibility are major factors. Soldiers fighting on unfamiliar ground can become disoriented more easily than on familiar terrain. The direction from which enemy fire comes may not be easy to identify, and poor weather conditions and combat stress may add to the confusion, especially if fire is exchanged. Accurate navigation and fire discipline are vital. In high-risk situations, leaders need to ensure units are properly informed of the location of friendly units and must issue clear, unambiguous orders, but they must also react correctly to responses from soldiers who are capable of using their own judgement. Miscommunication can be deadly. Radios, field telephones, and signalling systems can be used to address the problem, but when these systems are used to co-ordinate multiple forces such as ground troops and aircraft, their breakdown can dramatically increase the risk of friendly fire. When allied troops are operating, the situation is even more complex, especially with language barriers to overcome.