Japan developed naval radar to a far greater extent than her Axis partners, because her fleets spent much more time in contact with enemy fleets and enemy aircraft. Unlike the Germans, the Japanese pursued both metric-wave and microwave technology. They did not develop the powerful magnetrons which made British and US microwave radar fully effective (output was typically half a kW rather than the hundreds of kW of the Western sets). Also, because the first radars they saw (in Germany) were air-search sets, the Japanese were inclined to begin with that type of radar rather than, as in the German navy, to limit themselves to fire-control sets. The first two installations were on board the battleships Ise and Hyuga, prior to Midway. Meanwhile US and British metric army radars fell into Japanese hands at Singapore and in the Philippines.
The first major operational sets were a metric air-search radar (Type 2 Mk 2 Mod 1) and a microwave surface-search set (Mk 2 Mod 2, a developmental designation). Mk 2 Mod 1 had a mattress antenna. On a carrier it was typically free-standing; on a battleship or cruiser it was on the foretop. Range on an aircraft was 70 to 100km (38 to 54nm). Because there was no Japanese PPI scope, the surface-search set was also treated as a fire-control rangefinder.
After the battle of the Philippine Sea, the Naval General Staff ordered all surviving ships equipped with both an air-search set (Type 3 Mk 1 Mod 3) and a surface-search set (Mk 2 Mod 3, with increased power [10 kW]). Type 3 Mk 1 was a new set, development of which was completed only in February 1944, based on a land-based radar. The great deficiency was a complete lack of anti-aircraft fire-control radar. There were also airborne radars, including sea-search sets. Unfortunately they were heavy. Thus during the battle of the Philippine Sea Japanese torpedo bombers equipped with Mk 6 radars were ordered to remove them: they could not lift both the radar and a torpedo.
The pre-war Imperial Japanese Navy was determined to maintain radio silence, which to the higher staff included radar silence. That view stopped radar development for a time before the war, when it was first proposed, and the ban on emissions (until ships were under fire) was not completely lifted until the spring of 1944, just before the battle of the Philippine Sea. By that time ships had been lost under circumstances suggesting that they would have survived had they had air warning. The battleship Musashi had been damaged by a surprise attack while steaming from Japan to Palau in the latter part of February 1944. The cruiser Atago had given a practical demonstration of the value of radar. During repairs after Guadalcanal, the Communications Officer of First Fleet convinced his superiors to mount an early Mk 2 Mod 2 microwave surface-search set aboard the cruiser. The radar was credited with the survival of seven cruisers after the battle of Empress Augusta Bay in November 1943.
In the 1930s, like the US Navy, the Imperial Japanese Navy considered its carriers both powerful and vulnerable; the question was how to use them most effectively before they could be destroyed. The initial answer for both navies was dispersal. Thus a November 1936 Staff College study called for dispersal of the carriers so that they could envelop an enemy force. The largest carriers would steam alone, the smaller ones in dispersed formations so that they could combine to provide sufficient striking power. The paper emphasised the need for greater range than the enemy’s, a continuing theme later on.36 The experience of war in China seems to have convinced the Imperial Navy that only by massing could it realise the offensive power of its carriers. The 1939–40 fleet exercises employed co-ordinated air group attacks. Since it was essential not to reveal the positions of the carriers, the Japanese saw little point in using radio to co-ordinate dispersed ships. They had to be within visual range if their air groups were to work together. The compromise solution reached in 1940 was for the carriers of each division to concentrate, but the divisions to disperse so as to envelop the enemy.
The Japanese Navy was the first in the world to concentrate all the aircraft of several carriers into an integrated Air Fleet. By May 1941 it had a multi-carrier operational doctrine. By this time, at least as conveyed to the contemporary Royal Italian Navy by a high-level Japanese mission, the Imperial Navy considered the enemy carriers its prime target, since once they were sunk the enemy’s naval force would be deprived of essential air services: search, attack, and fighter defence. Once the carriers were gone, the enemy fleet would lose about half its fighting potential. Conversely, great attention had to be paid to safe-guarding the Japanese carriers. The Japanese seem to have envisaged a two-phase battle, the carriers first destroying the enemy carriers, and the main body (surface force) then engaging the enemy’s surface force, the unstated assumption being that carrier aircraft probably could not sink the enemy’s battleships. That was a logical assumption at the time, given the large number of torpedoes a modern battleship could absorb and the ineffectiveness of dive-bombing armoured decks.
Each Japanese task force (a phrase used in the 1941 Italian notes) would have attached to it a carrier division of up to three carriers. At this stage there was no expectation of unifying the air groups. Instead, the idea was to specialise – to place all the fighters on one carrier, the dive bombers on a second, and the torpedo bombers on a third. It was understood that specialisation might be dangerous, but until the two fleets engaged the only real danger was submarine attack, which was considered minimal. Apparently Japanese doctrine also allowed for mixing different types on each carrier, but that was considered an inferior solution. All-fighter carriers would be responsible for protection of non-fighter carriers, but carriers with mixed air groups would be at the direct disposal of a task force commander both to protect the main body and for use during the surface battle and subsequent pursuit.
The carriers had to be separated from the main body – the surface force – so that they were unlikely to fall victim to enemy light or heavy ships. Prevailing wind, for launch or recovery, would determine where the carriers might be placed. In the simplest formation, the carriers were about ten miles ahead of the main body, with attached destroyers for submarine protection and cruisers to protect them from enemy light forces. Once combat was imminent, the carriers would move so that the Japanese main body was between them and the enemy, beyond gun and torpedo range (a 1941 diagram showed the carriers 20–25nm off the track of the Japanese main body, with 20nm a bare minimum).
The emphasis on fighter defence of both the carriers and the main body suggests that by the spring of 1941 the Imperial Japanese Navy had limited faith in its anti-aircraft guns – which was much the position of the US Navy at the time. However, by 1941 the Imperial Japanese Navy was building Akizuki class destroyers which may have been intended specifically to provide anti-aircraft support to carriers. The provision of the destroyers may have reflected fear that it would be difficult to spot high-performance attackers in time. Carriers, like capital ships, were well armed with anti-aircraft guns. Overall the Japanese seem to have had much the same view of carrier survivability as the US Navy: carriers were eggshells armed with hammers. Fighters were the only real defensive option, but attackers would probably succeed. That is certainly what happened in the 1942 carrier battles.
It is not at all clear that the Japanese had thought through the requirements of fleet air defence. The standard carrier fighter complement was eighteen aircraft, nine of which were expected to accompany its strike aircraft. Without reliable voice radio, and without radar, fighter direction could not even be imagined. The standard fighter formation was the three-plane shotai. In theory a carrier would maintain a shotai continuously aloft (with two-hour endurance), keep another on deck alert and a third at a lesser readiness. If an attack developed, the two reserve units would be launched to supplement the one aloft. Individual pilots had assigned sectors. In theory, further aircraft could be vectored to a threatened sector, but little attention went to direction of any kind.
Carriers were grouped together as an integrated force (mobile aircraft force) for the first time in the June 1940 manoeuvres, and on 1 April 1941 two carrier divisions were formed into the 1st Air Fleet as a unified entity. By the time of Pearl Harbor six carrier air groups had been integrated together. In such a force each carrier had its own mixed air group. That presented a new operational problem, as each carrier would have to launch and recover aircraft more frequently, usually manoeuvring into and out of the wind. Like the contemporary US Navy, the Imperial Japanese Navy seems to have concluded that carriers should be well separated to this end. Separation would also make it more difficult for an enemy to find and attack all of the carriers. Separation in turn made it impossible to provide the destroyer screens previously envisaged. Instead, each carrier was assigned two plane guard destroyers. Carrier survival would be based on manoeuvre (evasion) and dispersal (an enemy strike should not find all of them together).
Given their First World War experience working alongside the Royal Navy, it seems likely that the Japanese adopted the British emphasis on radio silence for security. On that basis it is not clear how well air operations from separated carriers could be conducted. It would be much simpler to concentrate a pre-arranged strike than fleet air defence. Fighter defence seems to have been conducted by a strong CAP, without any form of fighter direction. It does appear that the Japanese hoped for early warning provided by lookouts on board dispersed surface ships.
Given limited faith in fleet anti-aircraft firepower, ships could manoeuvre freely to evade attack, even though that would ruin fire-control solutions. The Japanese adopted a standard circular evasive manoeuvre, familiar from photographs of Japanese ships under attack, from Shoho at the Coral Sea onwards, had the important virtue that it might frustrate dive bombing, as an attacker would find it difficult at best to compensate for a radically varying aim point and with the changing effect of wind as the ship moved. At least one US officer espoused exactly this manoeuvre in 1942. Note that if all the ships in a formation began to circle at high speed, the formation would break up. That was probably acceptable given that there was apparently little interest in mutual support other than by integrating the air groups of carriers working together. It is not clear whether the circular manoeuvre was adopted pre-war or during the war.
Thus the Akizuki class, armed with four twin 10cm/60, and specifically intended for anti-aircraft screening, seemed to contradict evolving Japanese carrier thinking. The first six were inserted into the naval programme as modified in December 1938, under the designation ‘direct protection’ destroyers. As sketched they would displace 2600 tons, with a speed of 34kts, armed with four twin 10cm, four 25mm and a bank of four torpedo tubes. The light torpedo battery testified to their unconventional mission. They were described as close protection for carrier divisions. The projected Maru 5 and Maru 6 programmes planned in 1941 included a new class of anti-aircraft cruisers, to displace about 5000 tons and to be armed with six twin 10cm guns. However, they were not included in the programme as projected in the spring of 1941. Instead, it included sixteen improved Akizukis (2900 tons, design F-53). The other sixteen destroyers in the programme were of more traditional (Shimakaze) type (a repeat pre-war type was eventually built).
The Japanese distinguished three types of operation. One was the massed strike against a land target, as at Pearl Harbor. In that operation the carriers occupied a box about 8km on a side, the distance providing sufficient sea room for the carriers. The six ships which struck Pearl Harbor were in two widely-separated staggered columns, with screening ships around the box. Much the same formation was adopted for Midway, the only major difference being that there was columns of two rather than three. In the Indian Ocean operation (March/April 1942), the major units were in line ahead, the head of the column being a bent screen of destroyers with a cruiser on either flank. This formation would make sense (as in British operations in the Mediterranean) if the object was to clear a corridor of submarines.
A second type of operation was a fleet vs. fleet battle, as at the Coral Sea. Carriers operated loosely co-ordinated, each having only plane guards in company. The surface force (the main body) would be nearby but not too close. Probably the Japanese had learned from pre-war exercises that it was much easier to spot a large surface force from the air, and that carriers should not be so close that any scout spotting the surface force would also see them. That was much the lesson the US Navy had learned pre-war.
At Santa Cruz battleships and cruisers formed a vanguard formation, with the carriers about 100km astern. This vanguard was largely line abreast, which suggests it was a scouting formation, except that it had a pair of Kongos in line ahead at its centre. The four carriers were in loose formation, the carrier Shokaku leading her sister Zuikaku by about 8000m, with Zuiho 8000m to one side and Junyo 100 miles away (originally she was to have had Hiyo with her, but that ship had to retire due to an engine fire). Each carrier had two plane-guard destroyers as her only escorts. Compared to the Eastern Solomons, the carriers were considerably further astern of the vanguard. This combination of Main Body and Advance Force was used again at the Philippine Sea in June 1944. A third type of operation was direct support of a surface force, a single carrier being more or less integrated with the surface force. That was the case of Shoho at the Coral Sea, with cruisers in company.
In August 1943 Combined Fleet issued a memorandum explaining that for future operations it was necessary to have a standard set of operating procedures and doctrines. At just about the same time the US Navy was developing its own set of standardised procedures under the designation PAC-10 (or USF-10A). Both navies had to deal with the disintegration of the pre-war fleet. In the US case, the flood of new construction (and new air groups) could be absorbed only in standardised forms, so that each new ship or squadron knew where it fitted in. In the Japanese case, the pre-war navy understood enough that limited orders sufficed. Enough of that fleet was destroyed, particularly in the Solomons, to bring up officers who had not absorbed enough standard procedures pre-war. Like their US counterparts, they needed standardised procedures laid out explicitly. Probably the doctrines involved were not too different from those of the past, but the officers of the past did not need such explicitness. At about the same time the Japanese introduced circular screening formations for carriers, mainly for protection against submarines (almost no Japanese destroyers had area anti-aircraft capability). No other surface combatants were involved, and multicarrier formations were not envisaged; this was not contemporary US circular-screen practice.
A May 1943 paper on strike force tactics drew the lesson of the 1942 carrier battles: ‘the secret … is to divert and restrain the enemy on one side, and then to attack suddenly from the flank. This discovery was a product of chance in successive battles. We must deliberately develop such situations and, advancing, destroy the enemy on the field of battle.’ That changed the likely role of the heavy surface group from a hammer against the enemy’s surface ships (and damaged carriers) to a means of diverting his attention from the carriers, which were now to be the hammer. The May paper advocated forming a diversionary force comprising a battleship division and decoy carriers. The latter could be useful only if they were used aggressively, hence had to be fast. A light cruiser, for example, might be camouflaged to look like a carrier. Official doctrine promulgated in March 1944 went further. The Advance Force of the past (the heavy surface force) was renamed the Diversionary Attack Force in line with current thinking. This logic was inverted at Leyte Gulf, the carriers, almost bare of aircraft, acting to divert attention from the heavy attack force approaching invasion shipping.
The new doctrine emphasised air attack against the enemy’s carriers, not only as a means of reducing the enemy’s overall strength prior to a surface battle, but as the main part of a battle. In the face of enemy aircraft, the fleet would retire quickly and reorganise. That made sense, since the Japanese could expect to outreach the Americans, striking first to (they hoped) destroy US carriers and their aircraft. Only if the US Navy achieved surprise (due to a failure of Japanese scouting) or managed to wipe out the Japanese air strike force (as happened at the Philippine Sea) would the Japanese carriers see US carrier aircraft. Alternative fleet dispositions were a concentration of all three carrier divisions, a concentration of two with a third carrier division at a distance, and three separate carrier divisions. Since relatively few surface ships operated with each carrier or carrier division, the Japanese did not feel any pressure to consolidate the carriers into US-style formations.