GPM 300 Acoustic Modem

- December 14, 2025

The GPM 300 Acoustic Modem represents a breakthrough in underwater communications, delivering unprecedented reliability in a compact, power-efficient package. Designed for both military and scientific missions, this system ensures secure and dependable transmission of data and voice across vast underwater distances, even in the harshest hydro-acoustic environments where conventional modems fail.

Photo Credit: L3HARRIS

The GPM 300 acoustic modem has already proven its superiority in extreme conditions. It was integrated into Triton’s Hadal Exploration System, the world’s first manned submersible commercially certified for repeatable exploration to the deepest point in the ocean. This achievement underscores the modem’s ability to function flawlessly under crushing pressures and in complex acoustic environments.

In 2019, the Hadal system was deployed during the Five Deeps Expedition, a historic mission to dive to the deepest point in each of the world’s five oceans. The expedition culminated in a world record-breaking dive to 10,927 meters in the Marianas Trench, the deepest known point on Earth. Throughout this mission, the GPM 300 modem provided reliable communications, enabling command and control functions that were critical to mission success.

The modem’s legacy extends further back to 2012, when it was used in James Cameron’s Deepsea Challenge Expedition. During this mission, the GPM 300 enabled real-time communications and even facilitated a record-breaking “tweet” from the deepest point of the ocean. These achievements highlight the modem’s unmatched reliability and its ability to support both scientific exploration and military-grade operations.

Minimal Interference in Harsh Underwater Environments

One of the defining features of the GPM 300 is its ability to maintain clear communication in environments plagued by multipath interference, reverberation, and noise. The modem employs third-generation MASQ multichannel direct sequence spread spectrum (DSSS) technology, which ensures high reliability even in complex acoustic channels. This advanced system is robust against inter-symbol interference, a common challenge in underwater communication.

To further enhance performance, the GPM 300 offers continuous channel equalisation, compensating for channel fading and multipath interference in real time. This ensures that data and voice transmissions remain stable and intelligible, even when platforms are moving at speed. With doppler tolerance of +/-15 knots and continuous doppler correction, the modem supports communication between fast-moving naval assets such as submarines, unmanned underwater vehicles (UUVs), and surface ships.

Compatibility with NATO Standards and Allied Systems

Interoperability is a critical requirement in modern defense operations. The network-enabled GPM 300 is fully compatible with the L3Harris ELAC UT 3000 underwater telephone and supports voice communication compliant with NATO STANAG 1074. This ensures seamless integration into allied naval communication networks, allowing joint forces to operate with confidence in multinational missions. For NATO and allied navies, this compatibility represents a significant advantage in coalition operations where secure and reliable subsea communication is essential.

Key Features of the GPM 300 Acoustic Modem

The GPM 300 is engineered with advanced features that make it a leader in subsea communication technology:

Third-generation MASQ multichannel direct sequence spread spectrum
Advanced forward error correction with error rates less than 1x10⁻⁴
Supports remote configuration settings for operational flexibility
Ultra-reliable data rates up to 1000 baud
Voice communications up to 25 km
Digital communication up to 45 km
Low probability of intercept for secure military operations

These features combine to deliver a modem that is not only powerful but also secure, ensuring that sensitive military communications remain protected from interception.

Military Applications of the GPM 300 Acoustic Modem

For naval forces, the GPM 300 offers a wide range of applications. Submarines can use the modem to maintain secure communication links while submerged, reducing the need to surface and risk detection. Unmanned underwater vehicles (UUVs) can be remotely commanded and controlled over long distances, enabling reconnaissance, mine countermeasure operations, and surveillance missions. Surface ships can coordinate with submerged assets, ensuring seamless integration of maritime forces.

The modem’s low probability of intercept is particularly valuable in military operations, where stealth and security are paramount. By minimizing the risk of detection, the GPM 300 allows naval forces to maintain operational secrecy while conducting missions in contested waters.

Options

The GPM 300 acoustic modem is supported by a range of specialized options that enhance its operational flexibility and mission readiness. Voice interface units with microphone and headset provide clear communication channels for operators, ensuring seamless coordination in demanding underwater missions.

Housing configurations are available to meet different depth requirements, including a 5000-meter aluminum option weighing 12 kilograms in air and 7 kilograms in water, and a 12,000-meter titanium option weighing 18 kilograms in air and 13 kilograms in water. These rugged housings guarantee durability under extreme pressure conditions, making the modem suitable for both shallow and deep-sea deployments.

A protective cage assembly for the transducer and connector ensures resilience against physical damage, while integrated pressure and temperature sensors provide critical environmental data. This combination of protection and monitoring capability strengthens the modem’s reliability in unpredictable subsea environments.

Precision timing is delivered through the L3Harris high-precision timing reference unit, offering one-way time-of-flight measurement with 1 millisecond accuracy and stability better than 1 nanosecond per second. Such accuracy is vital for synchronized operations, ranging, and secure communication in military and scientific missions.

The enhanced network access communication layer allows messages to be relayed beyond the standard 25 to 45 kilometer range, enabling communication around obstructions and extending operational reach. This feature is particularly valuable in contested or cluttered underwater environments where direct line-of-sight communication is compromised.

Transponder mode and range pulse distance measurement provide additional tactical advantages, allowing operators to determine precise distances and maintain situational awareness. Private communication channels with message encoding further secure transmissions, ensuring sensitive military data remains protected from interception.

For operations requiring higher speed, the modem can be factory-configured with doppler tolerance up to +/- 80 knots, though this increases power consumption. This option supports communication between fast-moving platforms such as submarines, unmanned underwater vehicles, and surface vessels.

Custom solutions are available for extended communication ranges, including a high-power 2 kW ceramic transducer rated at 194 dB µPa that operates with a standard power amplifier. This capability ensures robust connectivity across vast underwater distances, supporting missions that demand long-range coordination.

Battery pack options include a 200 Wh rechargeable unit and a 500 Wh alkaline battery, providing flexibility for different mission durations and logistical requirements. These power solutions ensure sustained operation during extended deployments without compromising performance.