Passive sensor triggered weapons have been in use for a considerable time by the military. They have been in form of Bangalore Torpedo, anti tank mines or anti personnel mines on land and as ground or moored mines at sea. Passive sensors have been extensively used on land for electronic support measures and at sea for detection of ships by submarines. One of the largest chains of passive sensors in the WWII era was Sound Surveillance System or SOSUS. It was a chain of hydrophone sensors located at various places in the Atlantic and Pacific Ocean. The main aim was locating Soviet submarines transiting the Greenland, Iceland, United Kingdom gap (GIUK gap). With developments in stealth technologies, other elements have been added to it such as the Surveillance Towed Array Sensor System (SURTASS), and it has become part of the Integrated Undersea Surveillance System (IUSS).
One of the important weapons of the cold war era, that lay dormant until activated, was the anti submarine encapsulated torpedo MK 60 CAPTOR. It was a deep-water mine which could be laid by aircrafts, ships or submarines. The mine could distinguish between surface ships and submarines as well as between friendly and enemy submarines based on their acoustic signatures. It would thereafter launch the MK 46 torpedo, which would then acquire and attack the enemy submarine. Both Russia and China had also developed similar mines.
With the rapid advances in sensor technologies, it is now feasible to expect robustness, high quality, and reliability in commercially produced sensors. The sensors today are produced using novel signal processing methods, provide very high speeds, and utilize low cost electronic components. Similarly, two main developments in manufacture of chips, which have acted as a catalyst in exponential improvements in computing technologies, include, firstly, coupling of traditional electronics with optical components using Ge Laser to obviate usage of wiring in chips. The ongoing work at MIT’s Microphotonics center utilizes a series of subterranean tunnels instead of buried fiber cables for transmission of the laser. This would achieve at least 100 times faster speeds than current systems. Secondly, the use of Mermisters or resistive random access memory (ReRAM) chips. These are 1000 times faster and can store twice as much data as flash memory chips. The main advantage is that ReRAM does not lose contents once power is switched off.  Further, they can be used in logic computations, implying thereby that both memory and computation functions can be carried out on the same chip.
Interestingly, Russia, China, and Iran have taken active interest in passive radar technologies. As per reports of a Rossiyskaya Gazeta’s online affiliate, in February 2015, Moskva-1 (developed by KRET) is a passive radar system, which would enable Russian troops to detect and identify airborne targets as far as 240 miles away without disclosing their location. It is understood that this could also be supplied to Iran.
The USAF had also released a request for information RFI RFI-PKS-0001-2012 for development of a Phased Array Antenna in respect of its Passive RF Sensing program. This involves development of analog and digital beam forming techniques for wideband phased array radar antennas that can operate over a 10:1 bandwidth. The US Army too has evinced interest in such systems that lie in wait submerged at sea and could be launched at an opportune time.
The Defense Advanced Research Projects Agency (DARPA) has undertaken a project titled Upward Falling Payload (UFP) in which it is envisaged that drones would be made to lie in wait at concealed locations on the sea floor, for prolonged periods before being launched to the surface and into the air.
As per DARPA, “Nearly 50 percent of the world’s oceans are deeper than 4 km, which provides vast areas for concealment and storage. Concealment provided by the sea also provides the opportunity to engage remote assets that may have been dormant and undetected for long periods, while its vastness allows simultaneous operation across great distances. Getting close to objects without warning, and instantiating distributed systems without delay, are key attributes of UFP capability.” The DARPA UFP program in its study phase, looked at long-range communications, deep-ocean high-pressure containment, and payload launch. It is understood that one of the firms that participated in the first phase was Sparton Electronics of De Leon Springs, Florida; this firm had worked to develop conceptual designs of a system with the potential to launch a plethora of non-lethal weapons like electronic warfare jammers, blinding lasers, and distracting light strobes upon surfacing.
The second phase would be development of proto types. The sub systems of the UFP program include; the pressure tolerant container or riser which would hold the payload for prolonged periods; the communication package, which would trigger the encapsulated payload to be launched to the surface, and the payload, which should be able to execute its function after it, is made to surface. To achieve the above aims the technologies that DARPA is looking at include, long endurance reliable electro-mechanical systems, very small sensors, small-unmanned systems, long-range underwater communications, navigation technologies etc. Phase 3 would be demonstrations of the systems at sea.
Once developed the UFP would provide pre-deployed sensors or non-lethal weapons in open seas. These could be used by the US Forces for surprise deployment in times of international conflicts across the globe.
The author is not aware of any such futuristic research initiatives in respect of Indian Armed Forces by the Defence Research and Development Organisation in India.