pulse to permanently shut down communications, only 58 of the originally planned 240 GWEN towers were built. In 1994 a defense appropriations bill banned new towers from being built, and shortly after, the GWEN program was cancelled by the Air Force. Operations Command and control messages originating at various military installations were transmitted on the 225 to 400 MHz band and received by a network of unmanned relay stations, called "Relay Nodes", dispersed throughout the contiguous 48 states. The Relay Nodes would re-transmit these command and control messages to each other, and to Strategic Air Command operating locations and launch control centers using low frequencies in the 150-175 kHz range in order to take advantage of ground-hugging radio propagation similar to commercial AM radio stations. Distance between the Relay Nodes were approximately 150–200 miles, determined by the ground wave transmission range. During initial operations, the Relay Nodes would receive and relay brief test messages every 20 minutes. The system had built-in redundancy, using packet switching techniques for reconstruction of connectivity if system damage occurred. Problems Early in its lifetime, electrical interference problems caused by GWEN system operation began to surface. Since the stations were using LF, the chosen frequency was within 1 kHz of the operating frequency of nearby electrical carrier current systems. With GWEN handling constant voice, teletype and other data traffic, it caused interference to the power companies diagnostic two kilohertz side carrier tone. When the side carrier tone disappeared due to interference from GWEN, the power grid would interpret that as a system fault. Site layout The overall area of a GWEN Relay Node was approximately , approximately × 700 feet. It was surrounded on the perimeter by locked, chain-link fences topped with barbed wire. Typical site features included: A main Longwave transmitting tower (generally between tall A radial network of underground wires forming a large ground plane to serve as a reflecting surface for radio waves Three electronic equipment shelters; two located near the perimeter of the site, and one at the base of the tower containing an antenna-tuning unit (ATU) UHF and LF receive antennas mounted on either a 10 ft. mast, 30 ft. light pole, or 60–150 ft. tower. A diesel backup generator, with a two-chambered fuel tank having a capacity of The main GWEN antenna operated intermittently in the LF band at 150 to 175 kilohertz (kHz) (below the bottom of the AM