给你讲个小故事,2010年1月9日,中国进行直升式反导试验前2天,美国驻英联邦国家大使馆向所在国的照会 https://wikileaks.org/plusd/cables/10STATE2634_a.html -- Since the national security imperative is not to jeopardize sensitive intelligence "sources and methods," as well as enable collection on this event, Washington has decided not to demarche Beijing prior to this expected flight-test. Furthermore, it is our assessment that any demarche transmitted prior to the launch event will not dissuade China from conducting the SC-19 flight-test. -- U.S. diplomatic and public diplomacy strategies will be shaped by the observables of the event, China's diplomatic/public explanations (if any), and upon our post-test analysis of risks to "sources and methods." 根据国家安全的要求,我们不能危害到情报收集的敏感“来源与方法”,也为了便于对本次反导试验进行情报收集,华盛顿决定不在试验前照会北京。我们认为,提前照会北京也无法阻止SC-19飞行试验的进行。 美国外交反应、公关策略将会由本次反导试验的可观测现象、中国的外交/公开解释(如果有的话)、以及试验后我们对敏感情报“来源与方法”的风险分析来决定。
1月11日反导试验成功后,国务院对驻华大使馆的电文 https://wikileaks.org/plusd/cables/10STATE2126_a.html An SC-19 was used previously as the payload booster for the January 11, 2007, direct-ascent anti-satellite (DA-ASAT) intercept of the Chinese FY-1C weather satellite. Previous SC-19 DA-ASAT flight-tests were conducted in 2005 and 2006. This test is assessed to have furthered both Chinese ASAT and ballistic missile defense (BMD) technologies. (S//NF) Due to the sensitivity of the intelligence that would have to be disclosed to substantiate the U.S. assessment, the U.S. Government in its demarche to the PRC Government will not associate the January 2010 SC-19 intercept flight-test with past SC-19 ASAT flight-tests. 2007年针对中国FY-1C气象卫星的直升式反卫星试验中,使用了一枚SC-19导弹做为拦截器运载火箭,2005年到2006年的直升式反卫星系统飞行试验中,也是使用了SC-19。我们认为本次试验同时提升了中国的反卫星与反导技术。(密级:秘密/不向外国人传播)因为要实证美国评估结论,可能会暴露出美国敏感情报手段,所以美国政府将不在对华照会中,把2010年SC-19反导试验与过去的SC-19反卫试验联系起来。
所以美方如何估算中方导弹数量?当然是公开信息,卫星照片研读,以及“来源与方法”(sources and methods)了。