Turbines, like any large structure, can cause blockage or clutter interference with surveillance radars and electromagnetic systems. These problems can be overcome by careful location selections, or by upgrading the software in existing radars (about 80 percent of which date back to the 1950s-80s), or replacing older radars. In one specific study, the Department of Defense (DOD) assessed potential sites for offshore wind farms in Virginia. Of the 25 tracts identified for optimum winds, the report found that 18 are compatible with military needs and rules. Most of the feared offshore interferences – submarines and live munitions practices – occur beyond the ideal area for wind turbines. The DOD calls fossil fuel dependence itself an issue of national security, and views wind energy as an energy alternative that can be compatible with military readiness and homeland security.