USCG Pequot - Indicator Loops in Action

The Pequot's Indicator Loops in Action

This page provides more detail about the role of the U.S. Coast Guard Cable ship Pequot during World War II. The Pequot laid anti-submarine defences known as Indicator Loops in strategic harbors along the east coast of the US. This webpage provides examples of ships and "signatures" (paper charts) generated by the loops as each ship passed overhead. For this - with the help of Marty Dwyer - we have accessed WWII era indicator loop charts stored at the Waltham Massachusetts National Archives and Records Administration (NARA) Northeast Region repository. For specific information on how to read loop charts and details on the equipment and physics associated with this technology see our Reading a Loop Signature Page. Our main page for the USCG Pequot provides extra details about the ship, its crew, its purpose as well as links to our other Pequot pages. For general information about indicator loops see Loops around the World.

The US Coast Guard Pequot. During WWII this cable ship laid top secret Indicator Loop cables to protect harbors from German U-boats. Her mission ranged from the ports of Virginia up to Argentia, Newfoundland. (Calamaio family).

THE LOOP SETUP

Once the indicator loops installed by the Pequot to protect US harbors were functional the officers and specialists who constantly monitored this harbor defense technology routinely saw how the loops picked up and recorded the passage of US warships and merchant craft across the loops cables. The experience gained by loop technicians recording and analyzing the loop signature voltages generated by friendly US and Allied ships enabled them to keep a better eye out for German U-boats or other unidentified enemy ships.

Not only do these charts provide a record of US ship movements but they clearly illustrate how different size ships, traveling at different speeds over the indicator loops produced similar, yet different, electronic signatures. The presence of large surface craft was very clear while the signature of smaller ships and submarines was more subtle.

For illustration purposes we’ve provided a photo of the ships who generated these loop signatures and by using the “Time 1st Impulse” and “Time Last Impulse” on the charts we’ve calculated how long it took these ships to cross the three legs of the indicator loop when coming in and going out of port. You need to picture the chart paper moving downwards past the pen.

USS Chicopee

The oiler USS Chicopee (AO-34) was a workhorse of WWII and served as a supply tanker to support fleet and Atlantic convoy operations before being transferred to the Pacific theater in 1944. The ship’s missions took her to Iceland, Newfoundland, Gibraltar, Northern Ireland, Casablanca, and ultimately Tokyo Bay. (photoship.co.uk)

USS Chicopee Inbound Crossed the loop in 5 minutes. Here we can see a full record of the ship’s passage from first to last impulse readings.

USS Chicopee Outbound

Crossed the loop in 5 minutes

Here we see that when the Juneau steamed out of harbor faster and crossed the loop a minute quicker than it did coming in the ship’s signature on the indicator loop chart was markedly different.

Even though the USS Massachusetts crossed over the loop cables in half the time going out as it did coming into harbor the sheer steel mass of this heavy battlewagon produced a very dense signature when crossing all legs of the loop installation.

Compared to a large ship like the USS Massachusetts (above) we see the very thin signature of a submarine crossing the legs of an indicator loop within an identical 2 minute time span. These short thin loop signatures which could signal the presence of a German U-boat are what everyone remained watchful for.

We can clearly see the difference in signature density when comparing the faster outbound readings with the slower readings when the Corry came into harbor.

The density of the loop signature is seen in the Kilauea’s slower outbound passage over the indicator loop cables.


USS Corry Inbound Crossed the loop within 2 minutes	USS Corry Outbound Crossed the loop within 1 minute