The Union of Concerned Scientists presented a criticism of Excalibur in 1984 as part of a major report on the entire SDI concept. They noted that a key problem for all the directed energy weapons is that they work only in space, as the atmosphere quickly disperses the beams. This meant the systems had to intercept the missiles when they were above the majority of the atmosphere. Additionally, all of the systems relied on using infrared tracking of the missiles, as radar tracking could be easily rendered unreliable using a wide variety of countermeasures. Thus, the interception had to take place in the period where the missile motor was still firing. This left only a brief period in which the directed energy weapons could be used.