H2X

H2X radar ("Mickey set", AN/APS-15, "BTO" or "Bomb Through Overcast" radar) was an American development of the British H2S radar, the first ground mapping radar to be used in combat. It differed from the original H2S mainly in its 10 GHz operating frequency which was in the X band rather than H2S' 3 GHz S band emissions. This gave H2X higher resolution than H2S, which had problems over large cities, which appeared as a single mass on the H2S display.

It was used by the USAAF during World War II as a navigation system for daylight overcast and nighttime operations. It was introduced as an improvement of the earlier H2S set, which had been supplied to the US to aid in the war effort. While the RAF Bomber Command utilized ground mapping radar as an aid to night area bombing, the primary use by the USAAF was as a fallback, to allow cities to be bombed even when hidden by cloud cover, an issue that had dogged their policy of precision daylight bombing since the start of the war, especially in cloud-prone Europe. With H2X, a city could be located and a general area targeted, night or day, cloud cover or no, with equal accuracy. H2X used a shorter 3 cm "centimetric" wavelength (10 GHz frequency) than the H2S, giving a higher angular resolution and thus a sharper picture, which allowed much finer details to be discerned, aiding in target identification. H2S subsequently also adopted 3 cm in the Mark III version entering operational service on November 18, 1943, for “Battle of Berlin”).

H2X is not known to have ever been spotted by the German Naxos radar detector, due to that receiving device's specific purpose being to spot the original British H2S equipment's lower frequency, 3 GHz emissions.

The first H2X-equipped B-17's arrived in England in early February 1944, and were first used in combat later that month. Those missions where bombing was done by H2X were called "Pathfinder missions" and the crews were called "Pathfinder crews", after RAF practice of using highly trained Pathfinder crews to go in before the main bomber stream and identify and mark the target with flares. American practice used their Pathfinder crews as lead bombers, with radar equipped aircraft being followed by formations of radar-less bombers, which would all drop their loads when the lead bomber did. The ventral hemispherical radome for the H2X's rotating dish antenna replaced the ball turret on B-17 Flying Fortress Pathfinders, with the electronics cabinets for the "Mickey set" being installed in the radio room just aft of the bomb bay.

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