AN/URC-117 Ground Wave Emergency Network

The Ground Wave Emergency Network (GWEN) was a command and control communications system intended for use by the United States government to facilitate military communications before, during and after a nuclear war. Specifically, GWEN was constructed to survive the effects of a high-altitude nuclear explosion generated electromagnetic pulse (HEMP) to ensure that the United States President or his survivors could issue a launch order to strategic nuclear bombers.

Overview
GWEN was part of the Strategic Modernization Program designed to upgrade the nation's strategic communication system, thereby strengthening the value of nuclear deterrence. The GWEN communication system, established in the late 1980s, was designed to transmit critical Emergency Action Messages (EAM) to United States nuclear forces. EMP can produce a sudden power surge over a widespread area, that could overload unprotected electronic equipment and render it inoperable. In addition, EMP would interfere with radio transmissions that use the ionosphere for propagation. GWEN would use a ground-hugging wave for propagation, being unaffected by the EMP.

The network was conceived as an array of approximately 300 radio transceivers distributed across the continental USA which operated in the Low frequency (LF) radio band. Later revised for 126 towers, plans again changed to include 56 radio towers linking 38 terminals; it was later expanded to 96 towers linking 49 terminals. Final network towers numbered 58.

History
Analysis showed that low frequency (150-190 kilohertz) radio transmissions were largely unaffected by high altitude EMP, and the Air Force Weapons Laboratory (Kirtland Air Force Base) tested a small scale 'groundwave' transmission system in 1978-1982. Based on the groundwave concept's promise, USAF Headquarters issued a draft Program Management Directive (PMD) for a "Proliferated Groundwave Communications System (PGCS)" on 25 August 1981. The name of this proposed network system was changed from PGCS to Groundwave Emergency Network in February 1982 The Air Force placed a tentative initial operating capability for GWEN by January 1992.

Operations
The network had three types of stations: input/output stations (I/Os), receive-only stations (ROs), and relay nodes (RNs).

I/O stations could send and receive messages. ROs only received messages transmitted through I/Os. Dispersed and unmanned RNs, would provide continuous relay links between I/Os and ROs. The I/Os and ROs would reside at locations with strategic military forces, and the RNs would be scattered throughout the country on government or privately leased land. Distance between the relay nodes were determined by the ground wave transmission wavelength, at intervals of approximately 150–200 miles.

Originating stations would send ultra high frequency (UHF) signals by broadcast towers for line-of-site receipt at Relay Nodes (RNs). The RNs would retransmit received messages via LF signals for ultimate reception by receive-only (RO) terminals at existing military communication buildings.

During initial operations, the towers 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 occurs.

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, interference was noticed by local power companies on a diagnostic two kilohertz side carrier tone – if the carrier disappeared, the power grid would interpret that as a system fault.

GWEN transmitter sites
GWEN transmitting antennas consisted of a concrete foundation, 2 ft above grade, a 3 ft insulator, a 290 ft steel tower, and 4 ft lightning rods, all enclosed by a 42 ft × 47 ft, 8 ft chain link fence topped with barbed wire. The tower itself would be supported by 15 guy wires attached to the ground at six anchor points. Surrounding the tower, and attached to it at the top and anchored in the ground by concrete blocks would be 12 top-loading elements (TLEs). The purpose of the TLEs is to improve antenna efficiency. Anchors for the TLEs and guy wires were within the site boundaries.

The GWEN transmitter sites include:
 * A 299 ft   broadcast antenna tower
 * A large ground plane, designed for ground conductivity conditions at the site
 * An antenna tuning unit enclosure at the base of the tower
 * Two equipment shelters
 * Electronic racks that will accept the DGPS equipment
 * All utilities that are required for operation of the DGPS broadcast site
 * Air conditioning and environmental controls
 * Back-up power generators
 * Above ground fuel storage tanks
 * Security enclosures with intrusion alarms

Relay Node
The overall site area of a relay node was approximately 11 acre, approximately 700 ft × 700 feet.

Typical site features include a
 * Longwave transmitting tower (generally between 290 and 299 ft tall)
 * Diesel backup generator, with a two chambered fuel tank having a capacity of 1020 USgal
 * 8 ft × 14 ft × 8-foot antenna-tuning unit (ATU) at the center of the site
 * Radio processor

Electronic equipment was housed in three shelters. Two of the shelters were located inside the fenced area at the perimeter of the property, and the third at the base of the tower. The equipment area and the tower base were surrounded by locked, 8 ft chain-link fences topped with barbed wire.

In addition, each node had a UHF antenna and an LF receive antenna on a ten-foot mast located inside the equipment area. The main GWEN antenna operated intermittently in the LF band at 150 to 175 kilohertz (kHz) (below the bottom of the AM broadcast band at 530 kHz). The peak broadcasting power was from 2,000 to 3,000 watts. The UHF antenna operated at 20 watts, between 225 and 400 megahertz (MHz).

Controversies
Public perception of the GWEN system was extremely negative. Its surmised purposes ranged from communication nodes for government mind control to giving the Soviet Union additional nuclear targets.

Nuclear targets
Citizen public protest groups stood up in Massachusetts, Oregon, Pennsylvania, and California to band together to fight construction of GWEN towers in their areas. The groups believed that the presence of a GWEN node would increase the community's "strategic worth" in the eyes of the Soviet Union. Responding to the protest groups, the Air Force repeatedly downplayed the importance of the towers, stating they were not worth that kind of attention by the Soviet Union.

Health effects
Conspiracy theorists posed the idea that GWEN towers were means by which government mind control messages could be sent. GWEN's construction became a hot political issue when former Defense Secretary James Schlesinger criticized Governor Michael S. Dukakis for opposing the project.

Termination
Amid controversy and world geopolitical changes, GWEN's value diminished greatly in the post-Cold War environment, in addition to its existence being rendered moot by the sustained effectiveness of predecessor and follow-on systems (Survivable Low Frequency Communication System and Minimum Essential Emergency Communication Network respectively). As early as 1990, legislative measures were enacted to terminate the program.

In 1994, new construction of GWEN towers were banned after a defense appropriations bill eliminated any funding for the towers for one year.

A few months later, the United States Air Force announced that they would terminate the construction contract to build the remaining 25 towers, except for monies used to dismantle the system.

Since fiscal year 1998-1999, the GWEN system has been replaced by Milstar SCAMP terminals and GWEN Operations and Maintenance funding has been terminated.

Disposition
The United States Coast Guard began outfitting a number of GWEN sites to house the National Differential GPS system. Existing equipment fit the needs of the NDGPS

Photo gallery
 File: Typical GWEN relay node.PNG|Typical GWEN relay node File: GWEN antenna tuning unit shelter and security fence.PNG|Antenna shelter & security fence File: GWEN equipment area.PNG|GWEN equipment area File:Essex, CA GWEN site 1.jpg|Essex, CA site. File:GWEN Essex CA updated GPD diff gear 1.jpg|Trimble differential GPS gear at Essex, CA site File:Essex CA, GWEN rf rad warning sign.jpg|RF radiation warning sign at Essex, CA 