The development of the surface-to-air missile after the Second World War conferred anti-aircraft warfare (AAW) to the frigate mission, in the form of the "guided missile frigate". In the USN these vessels were called "Ocean Escorts" and designated "DE" or "DEG" until 1975 - a holdover from the World War II Destroyer Escort or DE. Other navies maintained the use of the term "frigate".
From the 1950s to the 1970s, the USN commissioned ships classed as guided missile frigates which were actually AAW cruisers built on destroyer-style hulls. Some of these ships - the Bainbridge-, Truxtun-, California- and Virginia- classes - were nuclear-powered. These were larger than any previous frigates and the use of the term frigate here is much more analogous to its original use. All such ships were reclassified as guided missile cruisers (CG / CGN) or, in the case of the smaller Farragut-class, as guided missile destroyers (DDG) in 1975. The last of these particular frigates were struck from the Naval Vessel Register in the 1990s.
Nearly all modern frigates are equipped with some form of offensive or defensive missiles, and as such are rated as guided missile frigates (FFG). Improvements in surface-to-air missiles (like the Eurosam Aster 15) has meant that the modern frigate can increasingly be used as a fleet defence platform, negating the need for such specialised AAW frigates, and form the core of many modern navies.
Modern frigates are only related to earlier frigates by name. The term "frigate" passed out of use in the mid-19th century and was readopted during World War II by the British Royal Navy to describe a new type of anti-submarine escort vessel that was larger than a corvette, but smaller than a destroyer. The frigate was introduced to remedy some of the shortcomings inherent in the corvette design, namely limited armament, a hull form not suited to open ocean work, a single shaft which limited speed and manoeuverability, and a lack of range. The frigate was designed and built to the same mercantile construction standards (scantlings) as the corvette - allowing manufacture by yards unused to warship construction. The first frigates of the River class (1941) were essentially two sets of corvette machinery in one larger hull, armed with the latest Hedgehog anti-submarine weapon. The frigate possessed less offensive firepower and speed than a destroyer, but such qualities were not requisite in anti-submarine warfare (for instance, ASDIC sets did not operate effectively at speeds of over 20 knots). Rather, the frigate was an austere and weatherly vessel suitable for mass-construction and fitted with the latest innovations in anti-submarine warfare. As the frigate was intended purely for convoy duties, and not to deploy with the fleet, it had limited range and speed.
It was not until the Royal Navy's Bay class of 1944 that a frigate design was produced for fleet use (although it still suffered from limited speed). These frigates were similar to the United States Navy's (USN) destroyer escorts (DE), although the latter had greater speed and offensive armament to better suit it to fleet deployments. American DEs serving in the British Royal Navy were rated as frigates, and British-influenced Tacoma class frigates serving in the USN were classed as patrol frigates (PF). One of the most successful post-1945 designs was the British Leander class frigate, which was used by several navies.
Anti-submarine warfare frigates
At the opposite end of the spectrum, some frigates are specialised for anti-submarine warfare (ASW). Increasing submarine speeds towards the end of the Second World War (see German Type XXI submarine) meant that the margin of speed superiority of frigate over submarine was greatly reduced. The frigate could therefore no longer be a relatively slow vessel powered by mercantile machinery, and as such postwar frigate construction was of fast vessels, such as the Whitby class. Such ships carry improved sonar equipment, such as the variable depth sonar or towed array, and specialised weapons such as torpedoes, ahead-throwing weapons such as Limbo and missile-carried anti-submarine torpedoes like ASROC or Ikara. They can retain defensive and offensive capabilities by the carriage of surface-to-air and to-surface missiles (such as Sea Sparrow or Exocet, respectively). The Royal Navy's original Type 22 frigate is an example of such a specialised ASW frigate.
Especially for ASW, most modern frigates have a landing deck and hangar aft to operate helicopters. This negates the need for the frigate to close unknown sub-surface contacts it has detected, and thus risking attack and is especially pertinent as modern submarines are often nuclear powered and faster than surface warships. The helicopter is utilised for this purpose instead, allowing the parent ship to stand off at a safe distance. For this tasking the helicopter is equipped with sensors such as sonobuoys, wire-mounted dipping sonar and magnetic anomaly detectors, to identify possible threats and combat confirmed targets with torpedoes or depth-charges. With their onboard radar, helicopters can also be used to reconnoitre targets over-the-horizon and, if equipped with anti-ship missiles such as Penguin or Sea Skua, to engage in anti-surface warfare as well. The helicopter is also invaluable for search and rescue operation and has largely replaced the use of small boats or the jackstay rig for such duties as transferring personnel, mail and cargo between ships or to shore. With helicopters, these tasks can be accomplished faster and less dangerously, and without the need for the frigate to deviate from its course.