received. The second stronglink was the MC2935 Trajectory Stronglink Switch of which there were two, one for each channel of the weapon's firing set. Like the MC2969, it mechanically locked if it received the wrong input signals, but unlike the MC2969 it could only be manually unlocked, which could only take place at the factory. Signals for the MC2935 come from either of the two MC4175s or the MC4137 Trajectory Sensing Signal Generators. A third stronglink existed inside the first exclusion region of the weapon and was involved in weapon arming. Details of the system are still classified. B61 Mod 12 As of 2013 the Pentagon saw the B83 nuclear bomb as a "relic of the Cold War," believing that deploying a megaton-yield gravity bomb, the highest level nuclear weapon left in the U.S. inventory, to Europe was "inconceivable" at this point. It can also only be carried by the B-2 bomber, and integrating it onto additional aircraft would be costly. The Mod 12 upgrade is being pursued as a forward-deployed tactical/strategic nuclear weapon to protect NATO and Asian allies since it can be used from dual-capable fighter aircraft, as well as planned to arm the F-35 and B-21 Raider, and its lower yield options make it more flexible with less collateral effects. Recapitalizing the B61 is hoped to lead to the retirement of the B83, resulting in the elimination of the last megaton-yield U.S. bomb and leave the B61-series as the only U.S. gravity nuclear bomb. In 2013, the Pentagon and NNSA stated that if B61 refurbishment did not begin by 2019, components in the existing weapons could begin to fail. In 2013 Tom Collina of the Arms Control Association said that the new development could complicate arms control efforts with Russia. In 2014, Congress slashed funding for the project and called for alternates to be studied. In January 2014, former Air Force Chief of Staff Norton A. Schwartz stated that the Mod 12 nuclear bomb upgrade would have enhanced accuracy and a lower yield with less fallout compared to previous versions of the weapon. Accuracy has not been a guarantee for air-dropped nuclear weapons, so consequently large warheads were needed to effectively impact a target; the Mod 11 nuclear earth-penetrator is accurate to 110–170 meters from the desired detonation location, so it requires a 400-kiloton warhead. The Mod 12 is accurate to 30 meters from a target and