🛡️ Japan's first Aegis destroyer "Kongō" was commissioned in 1993— Now, after 30 years of service, this cornerstone of Japan's air defense is finally approaching a generational transition. Two next-generation radars are competing for the successor ships: RTX's (formerly Raytheon) SPY-6 and Lockheed Martin's SPY-7. Interoperability with the US Navy or compatibility with existing systems? The selection of these "eyes" will shape Japan's air defense for decades.
Over 30 Years in Service — What Is the Kongō-Class?
The Japan Maritime Self-Defense Force's (JMSDF) Aegis destroyers form the backbone of Japan's air defense and missile defense capabilities. As of 2026, the JMSDF operates eight Aegis-equipped destroyers: four Kongō-class, two Atago-class, and two Maya-class vessels.
The Kongō-class represents the oldest among them. The lead ship "Kongō" was commissioned in 1993, while the fourth and final ship "Chōkai" entered service in 1998. All four vessels have now accumulated approximately 30 years of operational history. Given the JMSDF's standard 40-year service life for naval vessels, the Kongō-class is expected to begin phased retirement within the next decade.
The Kongō-class currently operates with Aegis Baseline 5.3, while the newer Atago and Maya classes are equipped with Baseline 9, creating a capability gap regarding advanced missile threats. With limited potential for further upgrades, complete replacement has become the most practical option.
FY2025 Budget Launches Serious Successor Study
Japan's Fiscal Year 2025 defense budget includes ¥3.3 billion (approximately $22 million) for "Aegis Ship Research," specifically for technical investigations related to the "decommissioning of Kongō-class Aegis destroyers and successor ship planning."
What makes this particularly significant is that this selection could affect up to 10 ships' worth of radar procurement. Beyond replacing the four Kongō-class vessels, the JMSDF is also considering expanding its Aegis fleet from eight to ten ships. Once successor ship specifications are finalized, additional vessels would likely follow the same design, representing a major business opportunity for radar manufacturers.
Two Next-Generation Radar Candidates
Two cutting-edge radar systems are under consideration for the Kongō-class successors: SPY-6 and SPY-7.
SPY-6 (AN/SPY-6), developed by RTX Corporation (formerly Raytheon Technologies), uses Radar Modular Assemblies (RMAs)—approximately 60cm cubic modules that combine to form the complete radar array. The US Navy has adopted SPY-6 for its Flight III Arleigh Burke-class destroyers, with USS Jack H. Lucas becoming the first operational ship in 2023. Plans call for installation across 65 ships of seven different classes by 2033.
SPY-7 (AN/SPY-7), developed by Lockheed Martin, utilizes Sub-Array Suites (SAS) based on technology from the Long Range Discrimination Radar (LRDR) deployed at Clear Space Force Station in Alaska. As a Lockheed Martin product like the legacy SPY-1, it offers native compatibility with existing Aegis systems.
Both radars dramatically outperform the current SPY-1D in detection range. While SPY-1D is credited with approximately 500km range, SPY-6 achieves roughly three times that distance (approximately 1,500km), and SPY-7 delivers about 3.3 times the range (approximately 1,650km). SPY-7 is also said to expand search coverage during ballistic missile defense operations by approximately 20 times.
The Modular Revolution
A common characteristic of both radars is their modular architecture. By combining small radar units into larger arrays, the system can be scaled according to ship size and operational requirements.
This design also delivers significant maintenance advantages. Traditional SPY-1D systems could suffer severe capability degradation from even partial antenna damage, but modular systems allow damaged units to be replaced individually from behind the array. This substantially improves combat survivability.
Interoperability with the US Navy Is Key
The central debate in the SPY-6 versus SPY-7 selection revolves around interoperability with the US Navy.
The US Navy has standardized on SPY-6 for its next-generation radar needs, deploying it across Flight III Arleigh Burke destroyers, Constellation-class frigates, and Gerald R. Ford-class aircraft carriers. If Japan adopts SPY-6, system commonality would facilitate seamless data sharing and mutual support during combined operations.
However, SPY-7 presents its own advantages. Japan's two Aegis System Equipped Vessels (ASEV), currently under construction and scheduled for delivery in FY2027 and FY2028, will carry SPY-7. Adopting SPY-7 for Kongō successors would unify Japan's domestic radar infrastructure. Furthermore, since SPY-7 comes from the same manufacturer as SPY-1, it requires no new interfaces for Aegis system integration.
The Massive Aegis System Equipped Vessel
The ASEV, set to enter service from 2027, will be enormous: 12,000 tons standard displacement, 190 meters in length, and 25 meters in beam. This substantially exceeds the US Navy's Flight III Arleigh Burke-class (approximately 9,800 tons) and will become the largest Western surface combatant excluding aircraft carriers.
The ASEV originated as a maritime adaptation of SPY-7 radars originally procured for the land-based Aegis Ashore missile defense system. When the ground installation plan was abandoned, the already-ordered SPY-7 systems were repurposed for ship-based deployment.
With 128 Vertical Launch System (VLS) cells—the world's largest complement—the vessels will carry SM-3, SM-6, Tomahawk missiles, and more. The two-ship program carries a price tag of approximately ¥1.9 trillion ($12.6 billion).
Germany's Choice May Influence Japan
Interestingly, Germany's naval decisions may influence Japan's selection. Germany is proceeding with plans to equip its Type F-127 frigates with SPY-6.
If Germany formally commits to SPY-6, it could strengthen the radar's position as the "Western standard" from a NATO interoperability perspective. Japan might need to reconsider the implications of continuing independently with SPY-7.
Future Outlook — Two Possible Scenarios
Two main scenarios exist for the Kongō-class successor radar selection:
Scenario 1: SPY-6 Adoption Prioritize interoperability with the US Navy by selecting SPY-6. Consider SPY-6(V)4 backfits for existing Atago and Maya classes to unify the fleet's radar systems.
Scenario 2: SPY-7 Continuation Leverage the SPY-7 operational infrastructure being built for ASEV by equipping successor ships with the same radar. Prioritize efficiency in domestic maintenance and support systems.
Either way, the next one to two years represent a critical window for manufacturer lobbying. Both RTX and Lockheed Martin are actively promoting their respective systems' advantages.
Conclusion
More than three decades have passed since Japan's first Aegis destroyer entered service, and the transition to the next generation is becoming reality. Ballistic missile threats from North Korea, China's maritime expansion—Japan's security environment grows increasingly severe.
The radar selection for successor ships will determine Japan's air defense capabilities for the next 20 to 30 years. Interoperability with the US Navy versus efficiency in domestic operations—both technical and political dimensions warrant close attention.
Various opinions exist in Japan regarding the Aegis successor issue. What discussions are happening in your country about naval modernization and air defense system selection? Please share your thoughts in the comments.
References
Reactions in Japan
So the retirement of the Kongō-class is coming into view. Japan's air defense philosophy will be tested by whether we choose SPY-6 or 7. Going with the US Navy is the orthodox approach, but we've already committed to SPY-7 for ASEV...
Over 3x detection range is incredible. 1500km means you can see Shanghai from Okinawa. I'm genuinely happy we'll be able to detect North Korean missile launches earlier.
¥1.9 trillion for 2 ASEVs, and now 4-6 more successor ships... How much will this country's defense budget balloon? Population decline means less tax revenue, are we really okay?
I served on Kongō, and it was cutting-edge back then. Now it's considered aging after 30 years. But SPY-1D served us well. Looking forward to the next generation.
Don't underestimate interoperability. If the US Navy standardizes on SPY-6, Japan should follow. If we go Galapagos, we won't be able to coordinate in emergencies.
RMA, SAS, 'modular' sounds fancy but it just means easier to repair. But in combat, that can mean life or death. Unglamorous but super important.
SPY-7 is based on LRDR technology, specialized for ballistic missile detection. Given Japan's threat environment, I think SPY-7 is more suitable.
No no, SPY-6 is the only choice. Being adopted on 65 US Navy ships means faster bug fixes and updates. SPY-7's smaller user base is a support concern.
Wonder if MHI or JMU will build the Kongō successors. Good if they can leverage ASEV experience, but do we have capacity to build multiple ships in parallel?
With 10 ships, we can cover both the Sea of Japan and Pacific simultaneously. Can watch China and North Korea at the same time. Expanding Aegis fleet is the right call.
Honestly I don't care which one, just decide fast. The longer the decision takes, the more construction delays, and we'll be scrambling when Kongō hits its limit.
More military expansion. A radar that can monitor 1500km away is nothing but a threat from neighboring countries' perspective. Questioning investments that only raise military tensions.
Longer detection range means more early warning time. Which means more time to evacuate. I don't understand the logic of people calling this a threat.
Germany adopting SPY-6 advances NATO standardization. Japan continuing with SPY-7 in isolation is risky. Should read the Western trend.
Radar debate matters, but crew shortage is more serious. 10 ships mean nothing without sailors. Automation and crew reduction must proceed simultaneously.
I went to see the 1993 commissioning ceremony. I was moved thinking 'Japan finally has Aegis ships.' I'll definitely go see the next-gen ship commissioning too.
Why not differentiate? SPY-6 for successors, keep SPY-7 for ASEV. Different missions, so having both shouldn't be a problem.
Operating 2 radar types is a logistics nightmare. Parts and training all separate. Field perspective says unify, but politically difficult I suppose.
Former US Navy Aegis CIC officer here. SPY-6 is a great system, but don't underestimate SPY-7. Japan's choice is worth watching. From an interoperability standpoint I'd recommend SPY-6, but for BMD focus, SPY-7 makes sense too.
As an Australian, Japan's naval modernization is very welcome. As a Quad member, stronger Japanese Aegis capabilities contribute to regional stability. Our Hobart-class uses SPY-1D too, and we're watching future upgrade paths closely.
Interesting article as a German. Our F-127 frigates plan to adopt SPY-6. Japan's choice will indicate the direction of Western standardization. NATO and quasi-allied equipment unification makes sense.
Korean Navy's KDDX program has similar debates. US interoperability matters, but developing indigenous tech is also valuable. Japan's decision will be a reference for us.
From Taiwan's perspective, Japan's Aegis enhancement is welcome. As a counter to China's Type 055 destroyers, improved Western air defense capabilities serve as regional deterrence.
Royal Navy's Type 45 destroyers also face SAMPSON radar limitations. Every nation struggles with next-gen radar transitions. We can learn a lot from Japan's choice.
From Vietnam's standpoint, Japan's naval buildup could help maintain South China Sea balance. We welcome major regional powers modernizing their military responsibly.
Italian Navy uses EMPAR radar on ORIZZONTE-class, but we're considering next-gen transitions too. Modular design is revolutionary for maintenance. Japan's decision may influence Europe too.
Swedish defense industry worker here. We have SAAB radar tech too, but choosing integrated systems like Aegis directly affects national strategy. Interested in Japan's selection process.
Indian Navy is indigenously developing MF-STAR radar, but comparison with US systems is always debated. Balancing alliance relations and autonomy is a difficult issue.
Spanish Navy's F-110 frigates will adopt SPY-7(V)2. We chose Lockheed Martin like Japan for cost efficiency and Aegis compatibility. Though each country's circumstances differ.
Canada's Canadian Surface Combatant (CSC) will also adopt SPY-7. As Japan, Spain, and Canada adopt SPY-7, economies of scale might reduce costs.
As an American, honestly I'd prefer allies choose SPY-6. But Japan has its own circumstances. What matters is ultimately building a system where we can fight together.
Japanese living in the US. Living near a Navy base, I've seen Flight III with SPY-6. Those massive radar panels are impressive. Looking forward to Japan's ships too.
From a French perspective, how much Japan depends on American systems is interesting. We value strategic autonomy, but Japan seems to prioritize alliance.
New Zealand is small so we can't have large fleets, but we're interested in regional security. Watching how Japan's choice affects Pacific-wide stability.