Born out of the US Army's experience with airborne radio direction-finding (ARDF) in Vietnam, the Guardrail SIGINT (signal intelligence) system had its genesis in a 1971 quick reaction contract that the US National Security Agency awarded to ESL Inc (subsequently a part of TRW and now absorbed into Northrop Grumman Mission Systems) with regard to the development of a prototype stand-off airborne signals collection and surveillance system to be known as Guardrail I/Crosstalk. The system differed from earlier US Army ARDF systems in that it was divided into airborne and ground segments, with the former being the data collection element and the latter processing it. This negated the need for airborne system operators. The Guardrail I/Crosstalk prototype was delivered in time to participate in the US Army's 1971 Reforger exercise, held in what was then West Germany. Here the ground-based control and analysis complex, known as the Integrated Processing Facility (IPF) and housed in three 39ft 5in (12m) long containers, was located at the former Nike surface-to-air missile base at Gruenstadt, its trio of U-21Gs based at Ramstein Air Base, near Kaiserlautern. Alongside its remotely controlled airborne systems payloads Guardrail I/Crosstalk initially incorporated what were termed ground-mobile remote sensors', equipment that was subsequently dropped from the project.
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机译：源自美国陆军在越南进行机载无线电测向（ARDF）的经验，Guardrail SIGINT（信号情报）系统起源于1971年的一项快速反应合同，该合同是美国国家安全局授予ESL Inc（随后的一部分） TRW公司，现在被诺斯罗普·格鲁曼任务系统公司（Northrop Grumman Mission Systems）收购，以开发一种原型防区外机载信号采集和监视系统，该系统被称为Guardrail I / Crosstalk。该系统与早期的美国陆军ARDF系统的不同之处在于，它分为空中和地面部分，前者是数据收集元素，后者则进行处理。这就消除了机载系统操作员的需要。 Guardrail I / Crosstalk原型机及时交付，以参加美国陆军1971年在当时的西德举行的重造演习。在这里，地面控制和分析中心被称为综合处理设施（IPF），位于三个39英尺5英寸（12m）长的集装箱中，位于前耐克地对空导弹基地格伦施塔特（Gruenstadt）。 U-21G飞机位于Kaiserlautern附近的Ramstein空军基地。除了其远程控制的机载系统有效载荷外，Guardrail I / Crosstalk最初还集成了所谓的地面移动远程传感器，这些设备后来被从该项目中删除。
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