Dr. Siông-Ui Frederick Tsiam*

On June 17, 2024, a violent confrontation at Second Thomas Shoal left a Philippine Navy sailor severely injured. Yet, while global attention fixed on the visceral images of machetes and water cannons, a quieter, more insidious contest was unfolding in the ether. Philippine crews reported screens going dark, radios filling with static, and GPS coordinates drifting aimlessly. This episode illustrates an emerging pattern in China’s approach: physical clashes at the surface increasingly coincide with systematic attempts to blind and distort the battlespace in the electromagnetic spectrum.

Since 2012, Beijing has aggressively challenged the Philippines’ Exclusive Economic Zone （EEZ）. More recent dynamics, however, reveal a critical evolution in strategy: China has graduated from relying primarily on “kinetic deterrence” to integrating what this article terms “spectrum siege.” By layering electromagnetic spectrum （EMS）suppression over physical coercion, Beijing seeks to degrade. Manila’s ability to monitor and publicise maritime incidents and thereby incrementally expand its de facto control over key contested waters in the South China Sea.

Manila’s “naming and shaming” strategy relies on a digital lifeline: the ability to promptly document and broadcast footage of Chinese coercive behaviour to the world. Beijing’s counteroffensive is designed to strain this link. By electronically isolating Philippine vessels, China aims to erode operational transparency at the point of contact, limiting Manila’s capacity to produce timely, verifiable evidence of frontline realities.

In recent years, Western defence communities and think tanks have paid growing attention to how China and Russia employ electromagnetic spectrum operations （EMSO）and spectrum manipulation as tools of nonkinetic coercion in the grey zone.[1] Building on joint doctrine that frames EMSO as military actions to exploit, attack, protect, and manage the electromagnetic environment,[2] a wide range of analyses now examine how adversaries use jamming, spoofing, and GNSS denial to shape operational behaviour short of war. However, much of this literature focuses on cataloguing technical capabilities and documenting threat trends, rather than on how these activities interact with maritime transparency mechanisms and alliance commitments in specific theatres such as the South China Sea.

Against this backdrop, the present article introduces two operational concepts—“spectrum siege” and “transparency denial”—to analyse China’s evolving grey zone strategy in the South China Sea. Rather than claiming to discover an entirely new form of warfare, the article builds on existing EMSO and grey zone scholarship and adapts it to the specific problem of how Beijing degrades Manila’s ability to observe, document, and publicise coercive behaviour at sea. Whereas traditional A2/AD analysis emphasises missile salvos and kinetic exclusion, spectrum siege highlights the cumulative effect of routine jamming, spoofing, and interference in making navigational and informational conditions so uncertain that access becomes prohibitively risky even in “peacetime.” Transparency denial, in turn, captures the deliberate use of these EMSO tools to obstruct, manipulate, or delay the information flows that underpin the Philippines’ “naming and shaming” strategy and allied decisionmaking.[3]

The article also adapts the notion of a “kill web” to describe a distributed network of PLA, China Coast Guard, and maritime militia platforms that share ISR, EW, and targeting functions via overlapping sensors and communications links. Over time, persistent EMS activity by this network creates what the article calls an “electromagnetic maritime baseline”: a background level of surveillance and interference that normalises exclusionary control below the threshold of armed conflict. This framing builds on PLA concepts of “informationised” and “intelligentised” warfare by highlighting how cyber-electromagnetic operations are embedded in day-to-day law enforcement and grey zone practices at sea.

China’s electronic warfare capabilities are not merely theoretical; they are increasingly visible in operational settings. During the regionally held “Talisman Sabre” exercises, China’s Type 815G electronic surveillance ships were observed operating in the vicinity, with analysts assessing that they were likely collecting electromagnetic signatures of allied vessels.[4] Since the Philippine Navy and Coast Guard operate platforms largely sourced from the United States and Japan, many of their key signatures are plausibly already catalogued. Compared with its allies, the Philippines has relatively limited electronic protection （EP）capabilities, making its frontline units particularly vulnerable to China’s emerging EMS operations.

First is navigation denial. The Asia Maritime Transparency Initiative （AMTI）has documented that China has deployed fixed and mobile radar and electronic warfare systems on its artificial islands, including Mischief Reef and Fiery Cross Reef.[5] Opensource modelling and interference protection criteria suggest that, from a location such as Mischief Reef, a tacticallevel jamming source on the order of tens to lowhundreds of watts could be sufficient to disrupt civiliangrade GPS/GNSS receivers on Philippine vessels operating roughly 30 kilometres away at Second Thomas Shoal. Mounted on elevated infrastructure and combined with directional antennas, such systems can create localised “cones of silence” in which L1 and L2 signals fall below usable thresholds for nonhardened navigation equipment.[6]

A mobile tactical jammer mounted on a CSK181 chassis, or comparable platforms, could be used to execute this form of localised denial, even before considering the higherpower systems that can be supported by the islands’ militarised power grids. The construction of high towers and other elevated structures on these features appears designed, among other purposes, to mitigate lineofsight limitations and extend the effective range of such systems. Philippine Coast Guard statements and media reporting indicate that local vessels have experienced not only suspected jamming but also apparent AIS spoofing in these waters.[7]

Second is tactical isolation and digital fabrication. Beyond jamming, Beijing increasingly targets tactical communications in the AIS （161.975/162.025 MHz）and voice （around 156 MHz）bands. Jamming VHF voice channels—such as Channel 16 used for distress calls—is technically straightforward and has been reported in multiple encounters near Second Thomas Shoal.[8] Because AIS packets are broadcast in the clear on standardised VHF channels, even relatively lowpower transmitters can inject large volumes of fabricated messages into the local traffic picture, a vulnerability already documented in technical studies of AIS spoofing and system resilience.[9]

This combination of jamming and spoofing can generate several tactical effects. First, it disrupts maritime awareness. By generating “ghost ships,” an actor can create a false picture of overwhelming numbers to psychologically pressure Philippine commanders, or conversely, clutter the display to mask the presence of its own vessels and delay tactical responses.[10]

Second, it enables digital lawfare. AIS manipulation makes it technically possible to fabricate digital traces showing Philippine vessels “intruding” into restricted waters or performing dangerous manoeuvres, providing a putative legal pretext for “law enforcement” actions. Finally, it creates navigational lures. When combined with GPS spoofing, manipulated AIS data can mislead Philippine crews into believing they are in safe waters when they are in fact drifting toward shoals or into tactically disadvantageous positions, especially under conditions of poor visibility or operational stress.

Third is strategic silencing. Perhaps most critically, China appears to be targeting the commercial satellite uplinks （Inmarsat, VSAT, etc）that Philippine crews rely on to upload imagery and video evidence. Unlike hardened military systems with frequencyhopping capabilities, commercial terminals are vulnerable to relatively simple forms of broadspectrum noise. By raising the bit error rate just enough to intermittently sever these connections, an adversary can ensure that, while the physical clash occurs in real time, the flow of verifiable information is delayed — or, in some cases, lost entirely.

Targeting commercial Lband or Ku/Kaband links is technically less demanding than degrading hardened military systems. Commercial terminals typically lack the electronic countercountermeasures（ECCM）— such as frequency hopping —found in military gear. A coast guard cutter or auxiliary vessel emitting broadspectrum noise can raise the bit error rate sufficiently to disrupt throughput on nearby civilian SATCOM terminals.[11] Given that China has already demonstrated the ability to interfere with militarygrade GPS signals on US RQ4 drones,[12] disrupting commercial SATCOM terminals on lightly protected civilian or coast guard vessels is well within its demonstrated technical reach and is likely to be significantly easier from an operational standpoint.

China’s emerging positional advantage in the South China Sea rests on a theatre-wide “kill web” that integrates military and non-military assets into a progressively denser network of sensors and emitters.

Island fortresses. AMTI reporting indicates that China has successively installed clusters of fixed monopole antennas at multiple locations, notably Subi, Mischief, and Fiery Cross Reefs. In addition to large concrete pads, commercial imagery suggests the presence of vehiclemounted sensor systems potentially configured for specific spectrum bands. Whether employed primarily for ISR or for selected suppression roles, this infrastructure provides essential “backend computational” support and contributes to widearea coverage for surveillance and electromagnetic effects.

Militarised coast guard. The China Coast Guard is no longer just a lawenforcement agency. A notable example is the transfer of Type 056 corvettes from the PLA Navy（PLAN）to the CCG. While missile launchers were removed, these vessels retained their SR64 air/surface search radars and associated firecontrol systems, as well as bridgemounted antennas assessed to support electronic warfare and signals intelligence functions. Such platforms can, in principle, detect and interfere with commercial drone frequencies and some military radars. The 12,000ton Zhaotou-class cutters also serve as major SIGINT and commandandcontrol nodes within this network.

Source: “Sabina Shoal: A Dangerous New Flashpoint is Fast Emerging in the South China Sea,” CNN, https://edition.cnn.com/2024/08/27/asia/china-philippines-new-flashpoint-sabina-shoal-intl-hnk.

Maritime militia. Tasked with challenging peripheral waters and asserting presence, these ostensibly civilian fleets can contribute to “distributed jamming” and “spectrum saturation.” Drawing on opensource analyses, Chinese maritime militia units have been reported to employ highpower VHF and HF transceivers and modified radar reflectors, enabling them to clutter radar images, mask specific frequency bands, and complicate target discrimination. By altering hull structures or adding reflectors to increase radar crosssection（RCS）, smaller fishing vessels can appear as larger ships on an opponent’s sensors, potentially misleading tactical judgment and slowing decisionmaking.[13]

Air and space layer. The final layer is multidimensional support from air and space. Leveraging extended runways on major reefs, PLAN and PLAAF specialmission aircraft（including Y8/Y9 electronic warfare variants）reportedly maintain more regular rotations over the southern theatre. These platforms carry highpower airborne EW systems that can conduct standoff jamming and serve as “aerial data relays,” passing frontline data back to rear command centres and thereby shortening the kill chain. Above them, the BeiDou Navigation Satellite System （BDS）provides a unique shortmessage communication function, offering Chinese vessels a resilient, encrypted backup link even in heavily contested electromagnetic environments.

Since assuming power, Xi Jinping has identified “building a maritime power”[14] as a critical element in realising the “Chinese Dream”[15] and the “Great Rejuvenation of the Chinese Nation”. Beyond large-scale aircraft carrier construction, Beijing has actively pursued anti-access/area-denial（A2/AD）capabilities. While in the northern seas this architecture is heavily missilecentric, in the South China Sea it increasingly relies on nonlethal but potent electronic warfare and related tools as key means of shaping access and imposing costs.

Modern sea control is less about fleetonfleet battles and more about establishing exclusionary zones that influence behaviour. China’s emerging “spectrum blockade” concept seeks to make navigation and communication in certain areas sufficiently risky and uncertain that commercial and government actors selflimit their presence. Much like the Red Sea crisis forced shipping to reroute due to prohibitive insurance costs, China’s nonlethal EW activities can, under certain conditions, create a “highrisk area” that in practice constrains the Philippines’ operational freedom without a formally declared blockade.[16]

Since 2023, a series of incidents — including the use of militarygrade lasers, such as CCG 5205 temporarily blinding[17] Philippine crew members,[18] and reported AIS spoofing events that generated confusion over vessel locations, [19]— have highlighted a pattern of coercive actions in the electromagnetic and optical domains. In the same period, Philippine reporting has pointed to increased cyber and electronic attacks against public vessels, disrupting rotation and resupply（RORE） missions,[20] while navy officials have stated that their ships have faced “electronic interference” in these waters for the past three to four years.[21]

Beijing now explicitly frames the “cyber and electromagnetic domains” as extensions of national sovereignty, crucial for maintaining national security and global influence. This conceptualisation has translated into the use of cyber infiltration and electronic jamming as tools of sovereign defence, deterrence, and ideological competition.[22] Combined with persistent activities around disputed features, these practices are contributing to a zone of exclusionary maritime control that falls below the traditional thresholds of open war yet steadily alters the operational status quo.

China’s emerging spectrumcentric strategy presents a significant challenge to the US–Philippines Mutual Defense Treaty（MDT）. International practice and expert restatements such as the Tallinn Manual 2.0 generally apply an “effectsbased” test to determine whether a cyber or electronic operation constitutes a “use of force.” Actions that result in death, injury, or physical damage may qualify as an “armed attack.”[23] China’s operations in the South China Sea appear calibrated to stay just below this threshold: its EW activities are designed to produce “functional paralysis”—temporary loss of navigation and communications—rather than direct physical destruction, while physical harm is typically inflicted through “cold weapons” such as water cannons and ramming. This pattern creates a legal grey zone in which the conditions for invoking Article IV of the MDT remain contested, granting Beijing considerable room for manoeuvre and strategic initiative.

Even though Washington has repeatedly stated that armed attacks on Philippine public vessels would trigger treaty obligations, definitional ambiguity over what constitutes an “armed attack” in the cyberelectromagnetic domain complicates timely and credible responses. An overly expansive interpretation risks drawing the United States into a legal and political quagmire over rules of engagement（ROE）, while an overly narrow one risks eroding alliance credibility. China’s longterm pattern of “lowintensity, highfrequency” operations thus serves two purposes: it reduces the likelihood of a clearcut treaty trigger while gradually establishing an “electromagnetic maritime baseline” of exclusionary control that normalises higher levels of risk for Philippine forces.

The South China Sea increasingly functions as a testing ground. The tactics employed against the Philippines today—AIS spoofing, GPS denial, and SATCOM jamming—could, in a future contingency, be adapted and scaled for use against Taiwan. By expanding these pointdefence tactics into a widerarea “electromagnetic quarantine,” Beijing could seek to disrupt the navigation and communications of energy tankers and cargo ships bound for Taiwan and nearby transshipment hubs.

In a worstcase scenario, sustained interference of this kind could create an “electromagnetic black hole” in critical shipping lanes, forcing ships to abandon routine reliance on digital navigation and revert to more rudimentary methods. More importantly, persistent uncertainty about navigational safety and communications reliability could prompt marine insurers and shipping lines to reassess coverage and routing, gradually constraining Taiwan’s economic lifelines through market mechanisms. Without declaring a formal blockade or firing a missile, China could, in theory, use spectrum suppression combined with commercial risk calculations to marginalise a globally interconnected island economy. This section is forwardlooking and extrapolates from current patterns rather than describing an already operational campaign.

Recent naval developments — from Japan’s Mogami-class to Australia’s Hunter-class frigates — signal a wider shift: in highend maritime competition, spectrum control increasingly rivals, and in some missions outweighs, sheer displacement. The likely victor in future naval warfare will be the side that best optimises SWaPC （Size, Weight, Power, and Cost）to generate resilient sensing, communications, and electronic warfare power across contested bands.

Taiwan, as a critical node in the First Island Chain, is well placed to champion a regional “Electromagnetic Spectrum Cooperation Mechanism” built around three pillars.

Common Spectrum Operating Picture（CSOP）. Taiwan, the United States, South Korea, Japan, the Philippines, New Zealand, and Australia should move toward integrating selected spectrummonitoring data. When a Taiwanese frigate detects a distinctive jamming waveform from a Chinese militia vessel in northern waters, that information should be shared nearrealtime via encrypted datalinks with a Philippine patrol ship or a Japanese destroyer. Such triangulation and crossverification would allow allies to distinguish deliberate jamming from technical malfunction and to expose spoofing attempts quickly, sharply reducing the effectiveness of China’s informationasymmetry tactics.

Shared Electronic Order of Battle（EOB）. Allies should jointly build a library of acoustic and electronic signatures — “fingerprints” — with particular emphasis on Chinese paramilitary vessels operating under civilian cover. Once a vessel’s electronic fingerprint is positively identified by one ally, it can be flagged as a threat or highrisk contact across the entire network, raising the political and operational costs of China’s camouflage and grey zone operations.

Resilient PNT. Given the growing normalisation of GPS denial and spoofing in the South China Sea, regional states need to standardise and diversify alternative positioning, navigation, and timing（PNT）solutions. Beyond strengthening inertial navigation systems（INS）, they should accelerate testing and fielding of lowEarthorbit PNT（LEOPNT）and signalofopportunity（SoOP）approaches, so that commercial and government vessels can maintain safe operations even under sustained Chinese electromagnetic pressure.

The window to secure the electromagnetic spectrum is closing. If Washington and its allies cannot credibly ensure that a Philippine sailor can trust his navigation solution or call for help under electronic duress, they risk losing the contest for the South China Sea without a single missile being fired.