India's deployment of INS Udaygiri, INS Shakti, and INS Kavaratti for a passage exercise with the Royal Thai Navy is less about bilateral goodwill than about projecting anti-submarine warfare capability into the Andaman Sea — the corridor through which Chinese submarines must transit to reach the Indian Ocean, according to defence analysts tracking PLA Navy movements.
The 5W+H: Who, What, When, Where, Why, How
- Who: The Indian Navy's Eastern Fleet — INS Udaygiri (stealth frigate), INS Shakti (fleet tanker), and INS Kavaratti (anti-submarine warfare corvette) — alongside the Royal Thai Navy.
- What: A Passage Exercise (PASSEX) conducted in the Andaman Sea, featuring coordinated manoeuvres, communication drills, and anti-submarine warfare interoperability.
- When: June 2025, during the Indian Navy's broader Eastern Fleet deployment in the Indo-Pacific.
- Where: The Andaman Sea, the strategic waterway connecting the Strait of Malacca to the Bay of Bengal.
- Why: To deepen India-Thailand maritime interoperability and, according to defence analysts, to demonstrate anti-submarine surveillance capability in a corridor increasingly frequented by PLA Navy submarines.
- How: By deploying a ship combination optimised for detecting and tracking submarines — the Kavaratti-class corvette's hull-mounted and towed-array sonar paired with the Udaygiri's stealth profile and multi-role sensor suite — India signalled sustained ASW readiness in the Andaman corridor.
Read the ship manifest before you read the press release. When the Indian Navy announces that INS Udaygiri, INS Shakti, and INS Kavaratti have conducted a Passage Exercise with the Royal Thai Navy in the Andaman Sea, the natural instinct is to file it under routine diplomacy — two friendly navies waving flags, exchanging pennants, and sailing home. But the Indian Navy does not send an anti-submarine warfare corvette to a handshake drill. The ship selection is the story.
INS Kavaratti is a Kamorta-class corvette built for one primary mission: hunting submarines. It carries a hull-mounted sonar, a towed-array sonar system, and rocket-launched torpedoes — the toolkit you deploy when you want to find something quiet lurking beneath the surface. INS Udaygiri, a Project 17A stealth frigate commissioned into the Eastern Fleet, is among the most advanced surface combatants India has ever built — reduced radar cross-section, integrated combat management systems, and a sensor suite designed to operate in contested waters without giving itself away. INS Shakti, the fleet tanker, is the logistics backbone that extends operational endurance far beyond home waters. Together, the three ships form not a ceremonial flotilla but a self-sustaining anti-submarine task group.
And the location — the Andaman Sea — is not incidental. It is the funnel.
The Andaman Funnel: Why Geography Is the Real Story
Every Chinese submarine heading from the South China Sea into the Indian Ocean must pass through one of the narrow straits feeding into the Andaman Sea — principally the Strait of Malacca or the deeper channels near the Nicobar Islands. According to reports tracked by the Indian defence establishment and corroborated by open-source intelligence analyses cited in Indian Express and The Hindu over recent years, PLA Navy submarine deployments into the Indian Ocean have increased steadily since 2017, with nuclear-powered attack submarines and conventional boats making regular sorties ostensibly for anti-piracy patrols but practically for operational familiarisation of the Indian Ocean's acoustic environment.
India's Andaman and Nicobar Command — the military's only tri-service theatre command — sits squarely astride these chokepoints. But a static island garrison, however well-armed, cannot project the message that a live, at-sea anti-submarine task group can. When INS Kavaratti's towed-array sonar is streaming in the Andaman Sea during a coordinated exercise with a partner navy, the signal is acoustic in more ways than one: any submarine in the vicinity knows it is being listened to.
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Political Pulse
The talk in South Block corridors, according to defence sources who spoke to multiple outlets, is that the PASSEX with Thailand is one node in a much larger 'Act East' mosaic that New Delhi has been assembling with quiet urgency. The pattern is unmistakable: India conducted the Tasman Saber exercise with Australia, deepened MALABAR with the US and Japan, ran bilateral drills with Vietnam and the Philippines, and now pairs its most capable ASW assets with Thailand — effectively ringing the approaches to the Malacca Strait with interoperable partnerships.
The whisper among naval strategists, as India Herald's read of the broader pattern suggests, is that this is not coalition-building in the traditional alliance sense — India remains studiously non-aligned in rhetoric. Instead, it is what one retired vice-admiral described to a leading defence journal as "sonar diplomacy": building a shared acoustic picture of the eastern Indian Ocean, one PASSEX at a time, so that when a Chinese submarine transits, multiple navies have practiced tracking it together even if no formal intelligence-sharing pact exists on paper. The plausible deniability is the point — Bangkok gets to tell Beijing the drill was routine; New Delhi gets to tell Parliament it was bilateral goodwill. The submarines, however, know better.
Thailand's calculus is equally telling. Bangkok has historically walked a careful tightrope between Washington and Beijing, purchasing Chinese submarines (a deal for the S26T that has faced repeated delays, as reported by Reuters) while simultaneously deepening exercises with India and the US. By drilling with an Indian ASW corvette, the Royal Thai Navy gains exposure to detection capabilities it may need against the very submarines it once tried to buy — an irony not lost on analysts in the region.
By the Numbers
3 — Indian warships deployed: INS Udaygiri (stealth frigate), INS Shakti (fleet tanker), INS Kavaratti (ASW corvette).
4 — Kamorta-class corvettes in the Indian Navy's fleet, all optimised for anti-submarine warfare in littoral waters, according to the Indian Navy's official fleet data.
~8-10 — Estimated PLA Navy submarine deployments into the Indian Ocean annually since 2020, per open-source tracking cited in defence journals including India Today's defence wing and analyses published by the Belfer Center.
1,200+ km — Approximate length of the Andaman and Nicobar island chain, India's natural barrier and surveillance post across the eastern Indian Ocean approaches.
The Ship Selection Tells You the Threat Assessment
Navies do not deploy randomly. The composition of a task group is a coded message about what threat the sending navy considers most salient. India could have sent a destroyer and a patrol vessel — a combination that signals surface presence and flag-showing. Instead, it chose a stealth frigate and a dedicated ASW corvette, supported by a fleet tanker for extended endurance. This is the combination you choose when your concern is not piracy, not smuggling, and not surface engagement — it is the combination you choose when your concern is what is moving underwater.
INS Udaygiri's Project 17A design incorporates a combat management system capable of integrating data from the Kavaratti's sonar arrays, creating a fused undersea picture that a single ship cannot achieve alone. The two ships practising coordinated manoeuvres with the Royal Thai Navy means three navies developing interoperability in the one domain — anti-submarine warfare — that China's Indian Ocean strategy most depends on concealing.
Defence commentators, writing in publications including The Hindu's defence analysis pages, have noted that India's ASW capability has been the most quietly upgraded dimension of naval modernisation in the past five years: new P-8I Poseidon maritime patrol aircraft, MH-60R Seahawk helicopters with dipping sonar, and the Kamorta-class corvettes together form a layered detection net. The PASSEX with Thailand tests whether that net can be extended to include a partner navy's assets — and whether the Andaman Sea's acoustic conditions, which differ from the Arabian Sea's, can be mapped jointly.
What Comes Next — The Forward Read
India Herald's assessment of where this trajectory leads is clear: watch for the institutionalisation of ASW-specific exercises with ASEAN navies. If New Delhi follows the MALABAR playbook — which evolved from a bilateral US-India drill into a quadrilateral anti-submarine exercise over two decades — the Thailand PASSEX could be the seed of a dedicated Andaman Sea ASW forum within the next two to three years. The first marker will be whether the next iteration of this exercise includes a submarine element (India deploying one of its own Kalvari-class boats as a target for joint tracking), which would escalate the interoperability from surface coordination to genuine undersea warfare rehearsal.
The second signal to watch: whether India invites Thailand to observe or participate in the next MILAN exercise hosted at the Andaman and Nicobar Command, and whether the exercise brief explicitly includes an ASW module. According to Indian Navy statements from previous MILAN iterations, the exercise has gradually expanded from search-and-rescue to full-spectrum naval operations — adding a Thai-interoperable ASW dimension would complete the circuit.
The larger question this PASSEX forces — the one neither New Delhi nor Bangkok will answer on the record — is whether India is constructing something that looks less like bilateral goodwill and more like a distributed sonar network across the eastern Indian Ocean, with each PASSEX, each MALABAR, each MILAN adding another listening node. If so, the Andaman Sea is not a venue for the drill. It is the point of the drill.
Three ships. One corvette built to listen. And a sea that China's submarines must cross to reach the ocean India considers its own. The handshake is real. The sonar was already on.
By the Numbers
- PLA Navy submarine deployments into the Indian Ocean estimated at 8-10 annually since 2020, per open-source defence tracking
- 4 Kamorta-class ASW corvettes in the Indian Navy fleet, all optimised for submarine detection in littoral waters
- The Andaman and Nicobar island chain stretches over 1,200 km, forming India's natural surveillance barrier across eastern Indian Ocean approaches
Key Takeaways
- India's deployment of INS Kavaratti — a dedicated anti-submarine warfare corvette — for a routine-looking PASSEX with Thailand signals that the real strategic concern is Chinese submarine transits through the Andaman Sea, not surface-level diplomacy.
- The ship selection of a stealth frigate (INS Udaygiri) paired with an ASW corvette and a fleet tanker constitutes a self-sustaining submarine-hunting task group, not a flag-showing flotilla.
- The Andaman Sea is the geographic funnel through which PLA Navy submarines must pass to enter the Indian Ocean — making every ASW exercise here a direct strategic message to Beijing.
- India is building a pattern of ASW-centric exercises across the eastern Indian Ocean (MALABAR, Tasman Saber, and now this PASSEX) that collectively resemble a distributed sonar network, one bilateral drill at a time.
- Watch for the next escalation marker: whether India deploys a Kalvari-class submarine as a tracking target in future Thailand exercises, which would signal a shift from surface coordination to genuine undersea warfare rehearsal.
Frequently Asked Questions
Why did India send INS Kavaratti to the Thailand PASSEX?
INS Kavaratti is a Kamorta-class corvette built specifically for anti-submarine warfare, equipped with hull-mounted sonar, towed-array sonar, and rocket-launched torpedoes. Its deployment signals that the Indian Navy's primary concern in the Andaman Sea is tracking submarine activity — widely assessed to mean Chinese PLA Navy submarines transiting toward the Indian Ocean.
What is a PASSEX and why does it matter strategically?
A Passage Exercise (PASSEX) is a coordinated naval drill conducted when ships of two navies cross paths at sea. While often framed as routine, the ship composition and location reveal strategic intent. This PASSEX in the Andaman Sea with ASW-optimised ships amounts to a rehearsal for tracking submarines in the corridor China must use to access the Indian Ocean.
How does this drill fit into India's Act East policy?
The PASSEX with Thailand is one element in a broader pattern of ASW-focused exercises India is building with Indo-Pacific partners — including MALABAR with the US, Japan, and Australia, and bilateral drills with Vietnam and the Philippines. Together, these exercises are creating interoperable submarine-detection capabilities across the eastern Indian Ocean approaches, consistent with India's Act East maritime strategy.
Has China's submarine presence in the Indian Ocean increased?
According to open-source defence tracking cited in Indian defence journals, PLA Navy submarine deployments into the Indian Ocean have risen to an estimated 8-10 per year since 2020, including nuclear-powered attack submarines making sorties through the Strait of Malacca and Andaman Sea corridors.
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