Socorro L-3I and AUTOVON Site

For my five years of college, I lived in the small town of Socorro, New Mexico. I now live only about an hour away and continue to be closely involved in that town, and have a particular interest in its history. Considering that Socorro is a rather small town in a rather rural part of New Mexico, it's a nice coincidence that it serves a rather important role in a corner of Cold War history. First, let's cover some background.

Effective nuclear deterrence required a fast and reliable communications network that could be used for command and control of nuclear weapons. Further, this communications mechanism needs to be “hardened,” that is, able to survive an enemy attack. In order to develop such a communications network, the government naturally reached out to the country's foremost experts in long-range communications, AT&T.

For AT&T, this request came at the same time as another major advancement in long-range communications: the L-carrier. “Carrier” in the AT&T network was used to refer to an encoding standard used to carry multiple phone conversations (multiplexing), and the L-carrier multiplexed phone conversations onto a coaxial cable, similar to the kind you use for cable television. AT&T, though, always aimed for maximum capacity, and so they both multiplexed phone conversations onto a coaxial cable and bundled multiple coaxial cables into a single large cable suitable for direct burial in the ground. This whole arrangement was called the L-carrier, and generally constituted a multi-core cable (that is, mutliple coaxial cables bundled into one large cable) along with supporting electrical and other power connections that could carry several hundred phone conversations simultaneously. L-carrier was used to build some of the first high-capacity transcontinental telephone trunks. L-carrier went through various generations, numbered L-1, L-2, L-3 etc.

On top of the L-carrier, AT&T built a system called AUTOVON. AUTOVON, or the Automatic Voice Network, was essentially a separate telephone network similar to the civilian one we all use but specifically dedicated to military use (AUTOVON was a precursor to the more modern defense switched network [DSN] and other military telephone systems). AUTOVON needed to be able to survive a potential nuclear attack on the United States, and so AUTOVON calls were carried over a special variant of the L-carrier called L-3I - this name meant L-carrier, third generation, and the “I” was for “Improved.” “Improved,” in this context, meant hardened against attack.

In order to provide all this infrastructure to the military, AT&T had to run L-3I cables across the country and then connect them up to a separate set of telephone switches dedicated to routing AUTOVON calls. For reasons to be discussed later, one of these L-3I cables passed directly through Socorro, and one of these AUTOVON telephone switches was installed in Socorro, or technically in Luis Lopez, in a site along the I-25 frontage road still easily recognized by its large microwave horn antennas on a short tower.

Anyone familiar with the area would wonder why Socorro was selected to house both a primary L-3I cable station and an AUTOVON switch - it is not particularly close to any military sites, with Albuquerque, Alamogordo, or Roswell all being more obvious choices as each had an air force base at the time. This mystery is resolved to some extent by thinking about the theory of nuclear survivability: it is best to be very far away from a nuclear target. AUTOVON specifications generally required that AUTOVON switches be located at least 20 miles from potential military targets, and Socorro is ideal in that it is fairly centrally located between multiple military installations but still quite far away from all of them.

The site located just south of Socorro along I-25, technically called the “Luis Lopez” or “Socorro #2” site depending on which map you refer to (“Socorro #1” refers to a separate microwave site some distance north of Socorro), served multiple roles. First, and in an ongoing civilian capacity, it served as a terminal station in the AT&T Long Lines microwave relay system for long distance phone calls. Long distance phone calls were routed up and down the Rio Grande via point-to-point microwave links.

Socorro #1, north of Socorro, actually relayed directly to San Antonio south of Socorro, but also had a separate route to Socorro #2 which connected calls to the immediate Socorro area. It was something like a railroad siding, handling only local calls in and out of the area and not through traffic. I am having a hard time determining the exact date, but this went into place sometime in the late 1950s, and the microwave antennas still at the site today were installed for this purpose.

Pictured below is the microwave tower, showing the large horn antennas, the rigid waveguides descending down from them, and an HF antenna on the tower as well (the vertical pole with helical elements) which I am not certain of the purpose of. The microwave horn antennas here are pointed at the Socorro #1 site north of town.

Socorro #2 was constructed as a microwave Long Lines site sometime between 1952 and 1960, and likely closer to the late end of that time range. It is unclear when it ceased to serve in this capacity, but probably not until the first fiber optic cables were laid along the Rio Grande - civilian calls were often carried on L-3I cables but they would likely not have used L-3I as the sole transport into the area.

In 1963 or 1964, Socorro was selected as an L-3I Main Station, where electricity was injected on the L-3I cable and calls were multiplexed and demultiplexed on the cable. Socorro was also selected as a regional phase synchronization master for the L-carrier. To meet L-3I hardening standards, this entailed building an underground vault of several thousand square feet with provisions to survive a nuclear strike five miles away. Likely at the same time, Socorro was selected to serve as an AUTOVON switch, which also required hardening.

The several thousand square foot vault would have had about three staff members at any given time, and multiple shifts served the site to provide 24/7 operations coverage. Access was via one of the buildings at ground level that appears to be a garage but has vents around the roofline, this building is of a standard design and can be used to recognize other L-3I underground sites across the country.

Pictured below is the nuclear detonation detector device (NUDET), a small metal capsule on top of a concrete pedestal. Unfortunately not a great photo as the fence prevents getting close. This is a more advanced type of NUDET device that works by detecting sudden bursts of gamma rays electrically, it triggers blast-resistant dampers to immediately close on all air vents both to prevent damage from the overpressure and to keep out radioactive fallout. Many of these NUDET devices also reported back to a Department of Defense system to map nuclear detonations in the event of an attack, it is unclear though if all L-3I vaults were so connected.

Pictured below, on the left, is the vent building and underground access point. This is a common design for underground L-3I/AUTOVON sites. Certain elements can be used to recognize these sites across the country, including the vents around the roofline and the loudspeaker located near the door, which would have originally been accompanied by a CCTV camera (advanced and expensive at the time) to verify visitors. On the fence to the right, notice the warning sign. It reads:

Willful or malicious destruction of or injury to communications facilities used or intended for use for military or civil defense functions is a violation of the laws of the United States. Violations are punishable by imprisonment for up to 10 years or a fine of up to $10,000 or both.

This sign is a sure indicator of a cold-war era communications facility and was widely applied to L-3I or AUTOVON related AT&T sites.

Below is the exhaust vent structure, which consists of steel grates with a short concrete retaining wall around them. The concrete wall helps both to keep out dust and deflect shockwaves. The pipes sticking up from the vent would carry exhaust from the generators.

Many of these sites were far from towns and often equipped with independent, redundant water wells which were entirely buried. The wells are marked with posts on the surface in case they need to be excavated for repairs.

To serve as an AUTOVON switch, a 1ESS electronic telephone switch was installed at the vault in Socorro. This switch handled long-distance (tandem) routing of calls between the west coast and east side of the country via the east-west L-3I cable. It likely also served local calls to military installations in the area (Kirtland and Holloman AFBs were certainly on the AUTOVON network) although this has not been easy to confirm.

The 1ESS switch was a large electromechanical switch controlled by a central electronic computer. It was a very sophisticated piece of equipment at the time but would require on-site operators to remain reliable. The Socorro AUTOVON switch was one of not very many in the country and would have served a fairly large area, comprising almost the entirety of New Mexico. In case of a nuclear attack, contingency plans were in place for the Socorro site to take network management orders from a central site in Dranesville, VA. Should the connection to Dranesville be lost, Lamar, Colorado would serve as an alternate central control for the region. In an extreme situation, the Socorro operations staff would be on their own to maintain service to wherever they could still connect to.

I have not been able to locate any photos or floorplans for the Socorro site, or of AUTOVON switch sites in general. However, the 1ESS switch was quite large. It seems to be typical for the vault to contain about 3,000 square feet, most of which was occupied by the 1ESS and L-3I equipment, which were mounted in rows of cramped relay racks.

AUTOVON and L-3I are closely intertwined: they were rolled out at about the same time and AUTOVON calls were carried primarily on L-3I cables. L-3I cables consisted of eight coaxial pairs housed in one large cable, and each coaxial pair could carry 1,860 multiplexed telephone calls. In order to achieve this capacity, L-3I required an active repeater every 4 miles along the cable, and in some cases more frequent repeater stations were added later on to allow for higher capacity on the existing cable.

These repeater stations were powered by electrical lines incorporated into the cable. The power on these lines was provided by the L-3I main stations. Larger main stations also provided switching and multiplexing and demultiplexing for local traffic. Some main stations were selected for the special purpose of serving as synchronization masters: the analogue L-3I circuitry required that the carrier frequency on each cable be very well synchronized, so each set of L-3I multiplexing equipment used a reference frequency obtained from a “Regional Frequency Source” station which was synchronized to other regional frequency sources. Socorro was one such station.

The Socorro site lay in particular on a segment of east-west cable running roughly from Kansas City to Los Angeles. In New Mexico, this ran roughly through Tucumcari, Santa Rosa, Socorro, Datil, and Quemado. It is generally fairly easy to follow the cable in aerial images, both because the surprisingly wide scar from trenching is often still visible and because of the distinctive repeater stations at regular intervals.

Repeater stations consisted of an underground vault which contained the actual repeaters, placed directly under a metal shed at surface level which housed electrical switchgear and other accessories. Many repeater stations had no AC power supply, the lights, ventilation blower, and any power tools would be temporarily powered by a generator brought by the technician when needed. Some repeater stations were connected to utility power later on, often to power a cathodic protection system. The following photos are of the first repeater station east of Socorro - this cable segment was abandoned by 1997 and the repeater sites have been left to decay.

White signs were placed near roads to direct AT&T technicians when they needed to access repeater stations for service and repairs. Generally repeater stations were placed next to existing roads for ease of access, but when necessary (as in this case) a new service road was graded.

The faded letters on the sign indicate the identification of the repeater station, the letter “L” seems to refer to the Socorro main station and “1E” indicates the first repeater east. A second sign mounted just out of frame reads “L2E” as this same service road was used to access two repeaters along the line.

The repeater site consists of a metal shed surrounded by a simple barbed wire fence. The square fenced area surrounding each repeater station was included in the original right of way easements for the L-3I routes, as wide spots in the generally 30 foot wide ROW. The one “no trespassing” sign at this site was unfortunately far too sun faded to be legible, but it likely contained the same lengthy notice found at the main station. Note that this repeater station was upgraded with utility AC power, although the cable to the pole has since been cut.

At the time, AT&T rights of way were marked at regular intervals by poles with identification numbers and a small yellow sign indicating the dimensions and relative position of the right of way, as well as the actual position of the cable within. This was of obvious use to anyone digging in the area, but I suspect it was also done as the cables passed through areas that may not have been thoroughly surveyed and mapped for property boundaries at the time.

The shed is almost empty, its main purpose is to cover the hatch leading to the underground vault that houses the actual repeater. This hatch appears to have been sealed and the vault is likely to be flooded anyway. The vault would have been about ten feet square and contained the vacuum tube repeater equipment mounted to one wall.

The shed does contain various switchgear, primarily related to the AC power supply for lights, ventilation, and tools. This power would originally have been provided by a generator connected to a socket on the exterior of the shed, and later by a direct utility connection at this site. This equipment has since been stripped, presumably by metal thieves.

The safety issues with underground utility vaults are not to be forgotten, and this delightfully intact sign included instructions for ventilating the vault using the provided blower before descending to do work.

Finally, the cable right of way is often marked by signs like these. Generally, the wording “Transcontinental cable in this area” seems to indicate a coaxial cable - twisted pair lines are simply labeled “telephone cable” while fiber optic lines are marked by signs that explicitly say “light guide” (an early AT&T term) or “fiber optic.” Note the telephone number on this sign, which is from well before southern New Mexico area code reassignment and lists a Socorro phone number (exchange 835) under the 505 area code. This is rarely seen, as when the state of New Mexico had only one area code it was unusual to specify the area code at all when writing phone numbers.

The L-3I cable route has mostly been abandoned, as seen above. However, the segment extending West from Socorro through at least Magdalena appears to have been reused as a route for a fiber optic cable, likely for local telephone and internet service to the towns along highway 60. This can be identified by the substantially newer equipment shacks, including air conditioning, placed on the foundation pads of the L-3I repeaters. Another clear indicator is the right of way warning signs, which have been replaced with signs that explicitly warn of a fiber optic cable.

AT&T formally applied to abandon most of the right of way in 1997. Because large segments of this cable pass through public land, an extensive environmental impact statement was prepared on various plans to retrieve the cable or abandon it in place. Ultimately, most of it was abandoned in place with minimal remediation, although some sections in sensitive wildlife areas have been cleaned up. Larger sections of real estate, such as the L-3I main stations (of which there are several in New Mexico although Socorro is the largest) are generally now in CenturyLink ownership and are still used for fiber optic equipment, storage, and other purposes.

Several other telephone lines extend from the Socorro main station for service to local areas. These cables were probably twisted-pair cables with loading coils for long distance service.

Several of these cable routes are prominently labeled with letters, although I am not certain of the meaning of these. One runs North to Socorro, presumably to the central office near the town plaza, and another runs east, perhaps for local phone service to Luis Lopez.

At this point it should be mentioned that it is possible that the Socorro AUTOVON equipment also served an interesting alternate purpose, serving as one node of the ECHO FOX communications system that allowed phone calls to be routed to Air Force One (or other aircraft carrying the President) via AUTOVON connection to UHF radios installed at some L-3I sites. Some sources list Socorro as one of the sites so equipped, but others place the region's ECHO FOX radio equipment in Clines Corners. It may be that the radio was moved at some point, but it may well also be that some of these sources are simply wrong, as they were written back when the ECHO FOX system was still classified and so there was necessarily some degree of speculation.

The UHF antenna mounted on the microwave tower at Socorro does not appear to be of the exact same type as one at another confirmed ECHO FOX site, but this isn't a particularly confident conclusion.

I have still been unable to answer several questions I would like to answer. For example, many microwave Long Lines stations include a small satellite antenna mounted on the ground near the microwave antenna. These appear to be more modern additions but I have not been able to determine the purpose - perhaps as a backup telemetry link in case of trouble with the microwave equipment?

There are also some gaps in the timeline. I am not certain when the Socorro L-3I main station was turned up or turned down, and I am not sure when the microwave site was constructed. It appears to have been a later addition to the microwave relay chain running up and down the Rio Grande, both because of its label as “Socorro #2” and because it is connected as a leaf node to Socorro #1 instead of serving as a true relay point.

In any case, it has been a long time since the Socorro main station has had normal business hours. Socorro's odd significance to Cold War military communications is already mostly forgotten and will only become more obscure with time - but it's just one more example of the fascinating infrastructure hiding in plain sight all around us.