Signals intelligence collection, ship-based
Ships can provide mobile, yet longer-endurance platforms for collecting signals intelligence (SIGINT), without the limited antennas feasible on submarines, the short endurance of aircraft, or the cost and range issues of satellite-based platforms. Their major disadvantages are their low speed and lack of stealth.
Ad hoc installations were placed on US warships in the 1940 on. Modern ship installations generally involve intercept stations in mobile vans, which can be put onto the deck of a warship, although some nations, such as Russia and Spain, use essentially unarmed modified fishing vessels. There is a high level of interoperability among NATO vessels, using the Joint Tactical Information Distribution System (JTIDS). While not all ships have sufficiently secure areas for all-source (i.e., including SIGINT) intelligence sensors, commanders with access to all-source information can distribute appropriate parts to units under their command.
China: Ship Platforms
China operates at least 10 AGI-type (i.e., modeled after fUSSR intelligence collection "trawler") vessels.
Denmark: Ship Platforms
Denmark can field one containerised SIGINT/ELINT component, to be fitted in its FLYVEFISKEN class patrol-crafts 
France: Ship Platforms
France has operated several generations of SIGINT ships, but is moving to its first purpose-built vessel as the third generation. The first was a German cargo ship built in 1958, converted in 1976-7, and decommissioned in May 1999. The first, a German cargo ship built in 1958 by a shipyard in Bremen, was transformed in France into an electronic eavesdropping ship between 1976 and 1977. Decommissioned in May 1999, the next generation was a former supply ship used since 1988 by the Nuclear Experiments Department for the Pacific Tests Centre (CEP), the Bougainville . For its new mission, it was equipped with SIGINT sensors and a Syracuse II satellite communication system, and has been operating since July 1999. It carried out significant missions in the Indian Ocean following the 9/11/2001 attacks.
On 14 January 2002, the French Ministry of Defense launched a new purpose-built "Intelligence Gathering Auxiliary" ship project called MINREM, and will be named the Dupuy-de-Lôme. scheduled to go into operation in 2005, to replace Bougainville. Thales is providing the electronics, and Compagnie Nationale de Navigation is building the ship, to requirements defined by the Military Intelligence Directorate (DRM). with a planned 30 year lifetime. Thales is assigning overall systems and COMINT to its Thales Communication division, while Thales Systèmes Aéroportés will do the ELINT.
Germany: Ship Platforms
The German Navy operates the Oste class fleet service ships which are purpose built SIGINT and ELINT reconnaissance ships. Also other Navy vessels, such as the Bremen class frigates, Brandenburg class frigates, Sachsen class frigates and Braunschweig class corvettes are equipped with extensive SIGINT/ELINT gear.
Russia: Ship Platforms
Before and after the breakup of the USSR, the Russian Navy operated a large number of AGI (Auxiliary General Intelligence) intelligence collection "trawlers".
Spain: Ship Platforms
Spain has been reported to have acquired an ex-East German AGI, which it may operate in cooperation with its SIGINT aircraft . The vessel concerned is the 1,900 ton renamed Alerta, In East German service, she had extensive antennas and a large radome. Based in Cartagena, the SIGINT work is reportedly by two Israeli companies and a Spanish firm. A different source says that the SIGINT equipment is Russian. A Saturn 35 satellite antenna has been, according to Spanish sources, added.
Sweden: Ship Platforms
Sweden operates the HMS Orion and plans to rebuild the HMS Karlskrona as a SIGINT ship.
United Kingdom: Ship Platforms
UK Type 42 and 45 destroyers carry radar and communications intercept receivers for tactical ESM. They also can receive information from NATO and national sensors, via JTIDS.
While it is experimental, see passive covert radar, a MASINT technique, for a system by which a task group of ships, with their radar transmitters off (i.e., under EMCON), to get images of aircraft from reflections either from the other side's radars, or strong television or radar transmitters.
United States: Ship Platforms
After two international incidents, US doctrine is to conduct ship-based SIGINT missions with warships, which can protect themselves as the Pueblo and Liberty could not. The Gulf of Tonkin incident, in 1964, involved two-destroyer DESOTO patrols equipped with intercept vans, backed up with carrier air patrols. Why this level of protection was not available in 1967 is difficult to understand. One exception, the purpose-built SIGINT auxiliary, the ARL-24 Sphinx, generally stayed off the Nicaraguan coast.
Current USN warships carry some version of the AN/SLQ-32 electronic warfare system, which has ESM capabilities.
In addition to the AN/SLQ-32, Arleigh Burke class destroyers are in the process of evaluating an open-architecture Integrated Radar/Optical Sighting and Surveillance System (IROS3) and Ship Protection system, currently including an AN/SPS-73 radar, an electro-optical/infrared sensor, acoustic sensors and spotlights, coupled with remotely controlled machine guns. 
Standardized USN systems go beyond simple direction finding and into COMINT. The AN/SLR-25 is a passive cryptologic exploitation system principally for tactical use, but that can make contributions to higher levels of intelligence. The SLR-25(V)1 Advanced Cryptologic Carry-on Exploitation System (ACCES) is a portable version of the SLR-25(V)2 SSEE (Ship Signal Exploitation Equipment) without dedicated SIGINT spaces. Coupled with an AN/SSQ-120 Transportable Radio Direction-Finding system, the ACCES provides a complete SIGINT collection system. The AN/SSQ-120 has HF, VHF, and UHF antennas and direction-finding logic .
The area in which the SIGINT systems are operated is called the Ship Signal Exploitation Space, and is a sensitive compartment information facility (SCIF) also used for the ship's compartmented security communications.
More capable than the AN/SLR-25 with AN/SSQ-120 is the AN/SSQ-137 Ship Signal Exploitation System, an open-architecture system for command & control as well as intelligence.
- Ball, Desmond (March 1999), Signals Intelligence in China
- Flyvefisken-klassen PG/MHC/MLC (Standard Flex 300)
- Alain Duhamel (November 2002), "A New Sigint Vessel for France", The French AOC's Newsletter. Retrieved on 2007-10-18
- Shcherbakov, Aleksey (March 22,1999), Major Loss of Intelligence Gathering Capability. Retrieved on 2007-10-08
- HJH (March 2005), "Recycled AGI", ENIGMA 2000 Newsletter (no. Issue 27). Retrieved on 2007-10-08
- Joris Janssen Lok (2007-11-20), Sweden's Next Spy Ship, Aviation Week. Retrieved on 2008-04-09
- Navy ISR, January 2006. Retrieved on 2007-10-08
- Transportable Radio Direction Finding (TRDF) System (AN/SSQ-120), March 22,1999. Retrieved on 2007-10-08