H2X, eventually designated as the AN/APS-15,
was an American
ground scanning radar system used for
blind bombing during
World War II
World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
. It was developed at the
MIT Radiation Laboratory under direction of Dr.
George E. Valley Jr. to replace the less accurate British
H2S radar
H2S was the first airborne radar system, airborne, Airborne ground surveillance, ground scanning radar system. It was developed for the Royal Air Force's RAF Bomber Command, Bomber Command during World War II to identify targets on the ground f ...
, the first ground mapping radar to be used in combat.
H2X was also known as the "Mickey set"
and "BTO" for "bombing through the overcast" radar.
H2X differed from the original H2S primarily in its 3-cm wavelength
X band rather than H2S' 10-cm
S band. This shorter wavelength gave H2X higher resolution than H2S, allowing it to provide usable images over large cities which appeared as a single blob on the H2S display. The
Royal Air Force
The Royal Air Force (RAF) is the Air force, air and space force of the United Kingdom, British Overseas Territories and Crown Dependencies. It was formed towards the end of the World War I, First World War on 1 April 1918, on the merger of t ...
(RAF) initially considered using H2X as well, but would instead develop their own X band system, the H2S Mk. III. The RAF system entered service in late 1943, before the first use of H2X in early 1944.
The desire for even higher resolution, enough to image individual docks and bridges, led to a number of variations on the H2X system, as well as the more advanced
AN/APQ-7 "Eagle" system. All of these were replaced in the post-war era with systems customized for the
jet powered strategic bomber
A strategic bomber is a medium- to long-range Penetrator (aircraft), penetration bomber aircraft designed to drop large amounts of air-to-ground weaponry onto a distant target for the purposes of debilitating the enemy's capacity to wage war. Unl ...
s that entered service.
Usage

H2X was used by the
USAAF
The United States Army Air Forces (USAAF or AAF) was the major land-based aerial warfare service component of the United States Army and ''de facto'' aerial warfare service branch of the United States during and immediately after World War II ...
during
World War II
World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
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
The United States Army Air Forces (USAAF or AAF) was the major land-based aerial warfare service component of the United States Army and ''de facto'' aerial warfare service branch of the United States during and immediately after World War II ...
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
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), often described as being equivalent to one event (or Cycle per second, cycle) per second. The hertz is an SI derived unit whose formal expression in ter ...
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 18 November 1943, for "
Battle of Berlin").
H2X is not known to have ever been spotted by the German FuG 350
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.
Pathfinder missions

The first H2X-equipped
B-17's arrived in England in early October 1943, and were first used in combat on 3 November 1943 when the USAAF VIII Bomber Command attacked the port of Wilhelmshaven. Those missions where bombing was done by H2X were called "Pathfinder missions" and the crews were called "Pathfinder crews", after the 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. The system was used extensively by The 91st Bomb Group in 1945 with occasional excellent but generally inconsistent results.
The H2X on later
B-24 Liberators also replaced the ball turret, being made retractable as the ball turret was for landing on the Liberator. The operators panel was installed on the
flight deck behind the co-pilot (where the radio operator's normal position was). In combat areas the Mickey operator directed the pilot on headings to be taken, and on the bomb run directed the airplane in coordination with the
bombardier. The first use of Mickey was against
Ploiești
Ploiești ( , , ), formerly spelled Ploești, is a Municipiu, city and county seat in Prahova County, Romania. Part of the historical region of Muntenia, it is located north of Bucharest.
The area of Ploiești is around , and it borders the Ble ...
on 5 April 1944.
Radar mapping of Germany
Due to the absence of radar maps, in late April 1944 six PR Mk.XVI
de Havilland Mosquito
The de Havilland DH.98 Mosquito is a British twin-engined, multirole combat aircraft, introduced during the World War II, Second World War. Unusual in that its airframe was constructed mostly of wood, it was nicknamed the "Wooden Wonder", or " ...
aircraft in the 482nd Bomb Group were equipped with H2X equipment. The idea was to produce photographs of the radar screen during flights over Germany allowing easy interpretation of these radar images in later bombing runs. Three aircraft were subsequently lost in training, and the project was discontinued. A further twelve PR Mk.XVI Mosquitos of the 25th Bomb Group (Reconnaissance) of the
Eighth Air Force were fitted with H2X and beginning in May 1944, flew radar mapping night missions until February 1945.
The sets tended to overload the Mosquito's electrical system and occasionally exploded. Mickey-equipped Mosquitos had the highest loss, abort, and mission failure rates of any version of the otherwise successful Mosquito reconnaissance aircraft, and were severely curtailed after 19 February 1945. Three were lost to enemy action and one was shot down by
friendly fire from a Ninth Air Force
P-47
The Republic P-47 Thunderbolt is a World War II-era fighter aircraft produced by the American company Republic Aviation from 1941 through 1945. It was a successful high-altitude fighter, and it also served as the foremost American fighter-bombe ...
. In Europe several
P-38 fighters were also converted to carrying H2X radar in the nose, along with an operator/navigator in a cramped compartment in the nose behind the radar dish, provided with small side windows and an access/exit hatch in the floor (much like the earlier P-38 "Droop Snoot" bomber-leader variants, but with a radome instead of a glazed nose). These missions were to obtain radar maps of German targets but plans to produce the variant in quantity never materialized.
B-29 equipment
In the
Pacific theater,
B-29
The Boeing B-29 Superfortress is a retired American four-engined Propeller (aeronautics), propeller-driven heavy bomber, designed by Boeing and flown primarily by the United States during World War II and the Korean War. Named in allusion to ...
’s were equipped with the improved H2X radar called the
AN/APQ-13, a ground scanning radar developed by Bell, Western Electric, and MIT. The radome was carried on the aircraft belly between the bomb bays and was partially retractable. The radar operated at a frequency of 9,375 ± 45 megahertz and used a
superheterodyne receiver. The radar was used for high altitude area bombing, search and navigation. Computation for bombing could be performed by an impact predictor. A range unit permitted a high degree of accuracy in locating beacons.
Post war usage
Post-war
A post-war or postwar period is the interval immediately following the end of a war. The term usually refers to a varying period of time after World War II, which ended in 1945. A post-war period can become an interwar period or interbellum, ...
, the
AN/APQ-13 became the first military radar converted to a domestic peacetime application as a storm warning radar. About thirty systems were converted and installed on military bases. It was replaced by the
AN/CPS-9 system in 1949.
See also
*
AN/APQ-7, an advanced
X-band
The X band is the designation for a band of frequency, frequencies in the microwave radio region of the electromagnetic spectrum. In some cases, such as in communication engineering, the frequency range of the X band is set at approximately 7.0� ...
radar used briefly at the end of WWII
Lists
*
List of World War II electronic warfare equipment
*
List of radars
*
List of military electronics of the United States
References
Bibliography
*Freeman, Roger A. ''The Mighty Eighth War Diary'' (1990). page 240
*{{cite book , last1=Watson , first1=Raymond C. , title=Radar Origins Worldwide: History of its Evolution in 13 Nations through World War II , date=2009 , publisher=Trafford Publishing , location=n.p. , isbn=978-1-4269-2111-7
*Fine, Norman (2019). Blind Bombing: How Microwave Radar brought the Allies to D-Day and Victory in World War II. Nebraska: Potomac Books/University of Nebraska Press. ISBN 978-1640-12279-6.
Aircraft radars
X band radar
World War II radars
World War II American electronics
Military radars of the United States
Military equipment introduced from 1940 to 1944
Military electronics of the United States