Government pruned electronic ears designed to hunt littoral ships

By Marcus Boone · August 9, 2026

Foreign vessels are tracked by their invisible radiation via the AN/ULR-21, a system meant to hunt in shallow waters before budget cuts pruned its reach in 1998.

The AN/ULR-21 does not look for ships by watching them; it listens for the invisible radiation they bleed into the air. According to a program description hosted by the FAS Intelligence Resource Program, this system—codenamed CLASSIC TROLL—was designed to serve as the electronic ears for the New Attack Submarine (NSSN). It is an Electronic Support Measures (ESM) suite, a tool of passive surveillance that allows a vessel to remain hidden while mapping the electronic fingerprints of everything around it.

In the quiet of the deep, the AN/ULR-21 provides the crew with detection, identification, and direction-finding for radar and communication signals. These signals emanate from ships, aircraft, and other submarines. The goal is simple: knowing who is there, where they are, and what they are broadcasting, all without tipping off the target that they are being watched. The FAS mirror notes that the system was developed to support operations not just in the open ocean, but in the "complex littoral signals environment."

Syracuse, New York

The hardware for this electronic dragnet was developed by Lockheed Martin in Syracuse, New York. The suite consists of signal receivers, displays, and what the record calls "advanced processing and analysis equipment." This is the point where the raw physics of electromagnetic waves are converted into actionable intelligence. For the operator, the AN/ULR-21 is a tool for self-protection and situational awareness, but the document is clear that its utility extends beyond the survival of a single hull. It is used for "intelligence-gathering for battle group support."

This transforms the attack submarine from a solitary predator into a forward-deployed sensor node. The information gathered by the AN/ULR-21 is not kept secret within the submarine; it is disseminated to the wider battle group via the ship’s combat control system and communications equipment. When a submarine slides into the littoral zone—the shallow waters near a coastline—the electronic noise becomes a cacophony. By designing a system specifically for these environments, the government ensured its assets could sift through the clutter of civilian and foreign coastal signals to find the specific pulse of a military radar.

"Minimally manned ESM capability"

The program description emphasizes that the NSSN ESM system provides a "minimally manned ESM capability." In the language of military procurement, this is often a euphemism for the shift toward automation and the reduction of human oversight in the loop. The system is tasked with several critical functions: detection, signal acquisition, identification, localization, and providing threat warnings to the combat system. It handles the heavy lifting of signal analysis, leaving the human operators to react to the alerts the system generates.

To get these signals, the AN/ULR-21 requires a physical interface with the surface. The record states the system requires an antenna that can be positioned on either the photonics mast or the ESM mast. This is part of a larger effort to reduce the submarine's profile. The NSSN ESM and the Integrated Electronics Mast (IEM) programs were described as "companion programs" that, when merged, would create a fully integrated submarine ESM suite. The objective was a seamless transition between the depths and the surface, ensuring the submarine could gather intelligence with the smallest possible physical footprint.

October 1994

The project, which was formerly known as ASTECS (Advanced Submarine Tactical ESM Combat System), entered the Engineering and Manufacturing Development (EMD) phase in October 1994. This was the period when the conceptual designs were supposed to become physical realities. However, the trajectory of the program was altered by the realities of the federal budget. The FAS record notes that the program was "rescoped" following funding reductions in the FY 1998 Program Objective Memorandum (POM 98).

Rescoping is a polite term for cutting features, delaying timelines, or reducing the number of units produced. When the money vanished in 1998, the ambitions for CLASSIC TROLL had to be pruned. Yet, the desire for this capability remained high enough that the government didn't just want it for the new submarines. The program description indicates that the NSSN ESM system was being considered for an "eventual backfit" into older, existing fleets.

Specifically, the government looked at installing the AN/ULR-21 into Seawolf (SSN-21) and Los Angeles (SSN-688/6881)-class submarines. This would have effectively upgraded the eyes and ears of the legacy fleet, bringing the predatory capabilities of the NSSN to vessels that were already patrolling the world's oceans. The record leaves it unclear whether these backfits were ever fully realized across the specified classes or if the POM 98 cuts permanently limited the reach of the CLASSIC TROLL.