AN/URC-117 Ground Wave Emergency Network Operations and History

The AN/URC-117 Ground Wave Emergency Network has a rich history that spans decades. Developed in the 1960s, this network was designed to provide emergency communications during times of crisis.

The URC-117 was a significant improvement over its predecessors, offering greater reliability and security. Its use of ground wave propagation allowed for communication over long distances without the need for line-of-sight.

This network played a crucial role in supporting military operations, including the Vietnam War.

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The Ground Wave Emergency Network (GWEN) has a fascinating history and operational setup. The GWEN system was part of the Strategic Modernization Program aimed at upgrading the nation's strategic communication system.

It was designed to transmit critical Emergency Action Messages (EAM) to United States nuclear forces. The system used a network of approximately 240 radiotransceivers distributed across the continental USA, operating in the Low frequency (LF) radio band.

The GWEN network was conceived to use a ground-hugging wave for propagation and be unaffected by electromagnetic pulse (EMP). However, only 58 of the planned 240 towers were built due to doubts about the EMP threat.

The network had three types of stations: input/output stations (I/Os), receive-only stations (ROs), and relay nodes (RNs). The I/O stations could send and receive messages, while ROs only received messages transmitted through I/Os. RNs provided continuous relay links between I/Os and ROs.

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The Ground Wave Emergency Network (GWEN) was part of a Strategic Modernization Program designed to upgrade the nation's strategic communication system, thereby strengthening the value of nuclear deterrence.

GWEN was established in the late 1980s to transmit critical Emergency Action Messages (EAM) to United States nuclear forces. The system was designed to use a ground-hugging wave for propagation and operate in the Low frequency (LF) radio band.

The network was conceived as an array of approximately 240 radiotransceivers distributed across the continental USA.

The Air Force tested a small scale 'groundwave' transmission system in 1978-1982, which showed promise and led to the development of the GWEN system.

The first major phase of construction, called the "thin line", began in 1985.

The Air Force placed a tentative initial operating capability for GWEN by January 1992.

Only 58 of the originally planned 240 GWEN towers were built due to doubts about the threat of electromagnetic pulse to permanently shut down communications.

The GWEN program was cancelled by the Air Force in 1994, shortly after a defense appropriations bill banned new towers from being built.

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The Operations of the GWEN Network were quite fascinating. The network had three types of stations: input/output stations (I/Os), receive-only stations (ROs), and relay nodes (RNs). Each type of station played a crucial role in the network's functionality.

I/O stations could send and receive messages, while ROs only received messages transmitted through I/Os. The relay nodes, on the other hand, would provide continuous relay links between I/Os and ROs.

The distance between relay nodes was determined by the ground wave transmission wavelength, at intervals of approximately 150-200 miles. This allowed for efficient communication between stations.

Here's a breakdown of the different types of stations and their roles:

The relay nodes were scattered throughout the country on government or privately leased land. They were unmanned and dispersed throughout the contiguous 48 states. The relay nodes would re-transmit received messages via LF signals for ultimate reception by receive-only 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 occurred. This ensured that the network remained operational even in the event of damage.

The AN/URC-117 Ground Wave Emergency Network (GWEN) Relay Node sites were massive, covering an area of approximately 11 acres (4.5 ha), with dimensions of about 700 feet (210 m) × 700 feet.

Each site featured a main Longwave transmitting tower, typically standing between 290 and 299 feet (88 and 91 m) tall, which was a significant structure to behold.

The sites were surrounded by locked, 8-foot-high (2.4 m) chain-link fences topped with barbed wire, indicating the high level of security and protection required for these facilities.

A radial network of underground wires formed a large ground plane to serve as a reflecting surface for radio waves, which was a clever design element to enhance the network's performance.

The sites also included three electronic equipment shelters, two near the perimeter and one at the base of the tower, which housed an antenna-tuning unit (ATU) and other critical equipment.

Here's a breakdown of the typical site features:

A GWEN Relay Node site is massive, covering an area of approximately 11 acres (4.5 ha). This is roughly the size of a large football field.

The perimeter of the site is surrounded by a locked, 8-foot-high (2.4 m) chain-link fence topped with barbed wire, which suggests a high level of security.

The main feature of the site is a Longwave transmitting tower, standing between 290 and 299 feet (88 and 91 m) tall. This is an impressive structure that dominates the site.

The site also includes a radial network of underground wires that form a large ground plane to serve as a reflecting surface for radio waves. This is a clever design that helps to improve signal strength.

Three electronic equipment shelters are located on the site, two near the perimeter and one at the base of the tower. These shelters house important equipment, such as an antenna-tuning unit (ATU).

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A diesel backup generator and a two-chambered fuel tank with a capacity of 1,020 US gallons (3,900 L) are also on site. This is a vital system that provides power in case of an outage.

Here's a breakdown of the typical site features:

GWEN LF antennas are designed to radiate a maximum peak power of 3,200 W.

The typical radiated peak and average powers are 2,000 W and 28 W, respectively.

GWEN transmitters are designed to radiate a power of 2 kW, but some can radiate less or more, up to a maximum of 3.2 kW.

The electric field strength decreases with distance due to 1/r spreading loss, but this only applies at distances greater than 0.2 miles.

At longer ranges, diffraction loss reduces electric field strength by less than 10% for distances less than 155 miles.

Absorption loss also affects electric field strength, with a rate of absorption of about 0.1 dB/mile in areas with high conductivity, reducing the field strength by a factor of 10 in 200 miles.

The GWEN signal envelope appears as a train of pulses of approximately 0.557 second with a pulse repetition period of 2 seconds.

The nonlinear process of envelope detection produces a periodic signal with a DC component, as well as the 0.5-Hz fundamental frequency and its harmonics.

The spectral amplitude is related to the radiated signal peak envelope through the proportionality constant At/T, where A is the signal peak envelope, in volts/meter or milligauss.

The GWEN transmitter and antenna are designed to operate at a radiated power of 2 kW, but some antennas can radiate as much as 3.2 kW.

The electric field strength of the GWEN signal depends on the distance from the transmitter, with a 1/r spreading loss in the far field, but this doesn't apply in the near field, where distances less than 0.2 mile are affected by a different geometric dependence.

The earth's curvature causes a diffraction loss, which reduces the electric field strength by less than 1% for ranges less than about 75 miles, and by less than 10% for ranges less than about 155 miles.

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At 300 miles, the diffraction loss can reduce the electric field strength by about 35%.

The surface of the earth also causes an absorption loss, which can reduce the electric field strength by an additional factor of 3 in about 95 miles and by a factor of 10 in 200 miles, depending on the earth's surface conductivity.

Electrical interference problems plagued the GWEN system early on.

The chosen frequency of the GWEN system was within 1 kHz of the operating frequency of nearby electrical carrier current systems, causing interference.

Local power companies noticed the interference on a diagnostic two kilohertz side carrier tone that they used to detect system faults.

The power grid would interpret the disappearance of this tone as a system fault, which caused issues.

The United States Coast Guard began outfitting GWEN sites to house the National Differential GPS system.

This was a good use of existing equipment, as it fit the needs of the NDGPS.

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