America's most powerful surface combatants, the Arleigh Burke class destroyer and the Ticonderoga class cruiser, are widely understood to be highly advanced floating radar systems, with their powerful SPY-1 radars and Aegis Combat System taking center stage when it comes to providing targeting information and tactical situational awareness to the ship's crew. The vessels' AN/SLQ-32, and in some cases their AN/SLQ-59 electronic warfare suites also provide situational information and soft-kill defenses against marauding cruise missiles and more. Beyond that, the list of sensor systems on these ships is long and impressive, but one critical and highly flexible system remains widely unacknowledged even though it sits prominently above the bridges of these ships and those of the Coast Guard's National Security Cutters—the Mark 20 Electro-Optical Sensor System (EOSS).

The Mark 20, and its similar-looking Mark 46 progenitor—which was introduced to the fleet in 1993—have a very science-fiction-like appearance. Movies like Short Circuit or Wall-E may come to mind when looking at the turreted sensor system, with its rectangular appendages and big apertures. But beyond its intriguing looks, the system gives the ship it is attached to some very important capabilities. So much so that, frankly, it deserves more attention than it gets.

L3Harris Mk20 EOSS

With this in mind, The War Zone reached out directly to the manufacturer of the system, L3Harris, to find out more about it and the exact role it plays on multi-billion dollar fighting ships. Here is that exchange: What components make up the Mk 20 EOSS?

The Mk20 Mod 1 and the Electro-Optic Sighting Systems that precede it (Mk20 Mod0 and Mk46) include an “above deck” unit with visual and thermal imaging sensors, a laser range finder, stabilization system and shock isolation unit, and a “below deck unit” that contains the control, tracking, power supplies and interface to display and control stations.

Kollmorgen/public domain An info sheet on the legacy Mark 46 EOSS.

What does it do primarily and what does it do in a secondary capacity?

The primary purpose of the system is to provide highly accurate targeting information to the ship’s gun weapon system. In many instances, the ship’s radar may make an initial detection of a potential threat, and the Mk20 is then cued to visually confirm the target, and provide precise range and position for gun engagement. The goal is first shot hit accuracy. Of course, sailors are quite creative and immediately recognize the Mk20’s excellent performance as a situational awareness and navigation aid. We have heard of crews using the Mk20 to visually investigate suspicious vessels, and help navigate through congested waters. The operators find the thermal imaging capability and resolution particularly useful in nighttime operations.

USCG USCGC Hamilton with its EOSS atop its bridge.

How does it accomplish those functions? Can you provide some scenarios that showcase how it would be used?

We feel the Mk20 is an essential element of the ship’s weapon system and overall situation awareness capabilities. We can share three general scenarios that rely upon the excellent range, resolution, and accuracy of the Mk20: surface action – gun engagement: in a busy sea-lane, the ship’s radar detects a potential hostile craft approaching. The contact is passed to the Mk20. The Mk20 rapidly slews to the radar cue, detects and initiates track on the hostile craft. Thermal and visual images are used in combination to verify the hostile intent of the approaching craft and the laser range finder is used to provide a highly precise range for gunfire control calculations. Confirmed hostile and having passed precise track data to the weapon system, the main deck gun engages the target. The Mk20 is then used to verify target damage and assess the need for further engagement. The crew visually detects a potential threat using the Mk20 in as a situation awareness, long-range imaging system. The Mk20 passes the contact to the radar through the ship’s combat system to initiate a radar track for prioritization and potential future engagement (this occasionally happens in congested waters where the radar operator may have some difficulty distinguishing among multiple radar contacts) Ships transit at night and without use of radar. The Mk20 is used to detect surface traffic, allowing the crew to plot a course for safe passage. The laser range finder is used to verify ships positions by accurately ranging on known landmarks (lighthouses, etc.).

What can it see and how far away can it see and in what conditions?

We are excited to see the new Mk20 Mod1 go to sea this year. It will provide a new, wider field of view with better resolution than the legacy systems. This enables detection and tracking of small targets in even higher seas and at night. As you can imagine, we are not able to share specific range of performance figures. But we can say that the intent is to be able to detect and track small targets beyond the range of the deck gun in most conditions.

Author's note: The Mark 45 five-inch gun system used on U.S. Navy cruisers and destroyers has a range of nearly 15 miles.

USN EOSS seen mounted on the port bridge wing of the Ticonderoga class cruiser USS Chancellorsville.

Can you provide some sample videos of it in use?

I am sorry to say we do not have any interesting videos to share – the customer typically classifies data coming from the ship.

Where did this type of system come from and how has it evolved into its latest incarnation?

The EOSS evolved from the critical need to accurately, visually verify targets prior to engagement by the Aegis combat system. From this rather simple problem and solution, the EOSS has evolved to include the latest sensor technologies to arrive at its current Mk20 Mod1 configuration. This new system is less than half the weight of prior systems, enabling installation on smaller surface combatants like LCS. All of the EOSS systems L3Harris provides meet the most stringent environmental qualifications including shock – the systems maintain boresight accuracy in the event the ship is attacked and hit.

USN Mark 20 EOSS seen on its perch atop the bridge of an Arleigh Burke class destroyer. Note the non-lethal LRADs on the bridge wings as well.

What is unique about the latest variant and what types of ships beyond U.S. destroyers and cruisers could it end up on?

We’ve already talked about this new system being much lighter than prior versions and with higher resolution than prior systems. But we’ve also made it much more modular. It is now possible to replace the sensor units on the above deck unit while underway. This means that, for the first time, ships are able to deploy with spares allowing replacement of visual, thermal or laser range finder modules, eliminating the need to replace the entire unit with a crane from pier-side. Not only does this new capability improve the readiness of the fleet, but now the sparing and logistics training to maintain these systems around the world just became much less burdensome.

Future plans for the system? What about incorporating it into distributed aperture systems for enhanced situational awareness?

That is an excellent idea and one that, as you may imagine, we are not able to fully address here. But it is fair to say that systems like the Mk20 Mod1 that provide highly accurate real-time targeting data without giving away the ship’s position, is a key component of how we see L3Harris enabling Distributed Maritime Operations.