Lone Sentry: Antiaircraft Measures, Japan (U.S. WWII Intelligence Bulletin, November 1943)

1. GENERAL

The following notes on Japanese antiaircraft measures were obtained from U.S. observers and from enemy documents.

Observers report that, until recent months, the antiaircraft defenses on Japanese naval ships have been more accurate and more concentrated than the land-based antiaircraft fire. This distinction is no longer true, the observers say, because of improvements in the quantity and quality of land-based weapons.

The nature of antiaircraft fire over enemy targets can frequently be predicted by evaluating certain factors concerning the target. For example, if the target has tactical or economic importance but is far distant and has not been visited in recent months, all types of antiaircraft guns may be encountered. However, they usually are not numerous and the crews are poorly trained. A less important but remote target frequently has only medium and light antiaircraft guns. A new target under development and not previously attacked may have no antiaircraft defense. The number and caliber of guns at frequently bombed targets will, as a rule, be continually increased. Apparently the enemy feels that such targets are strategically important to us because we bomb them often.

2. AT SEA

a. Weapons

The size of Japanese sea-borne antiaircraft weapons generally is in proportion to the size of the ships. Merchant ships of 5,000 or more tons frequently carry heavy guns, while those between 3,000 and 5,000 tons usually carry medium guns. As a rule, smaller merchant vessels are armed only with light weapons, but torpedo boats and even smaller vessels usually mount medium guns, while heavy guns are always found on destroyers and larger naval vessels.

In arranging antiaircraft weapons on ships, the Japanese concentrate the guns at the bow and stern in order to obtain effective vertical (or near vertical) fire. Antiaircraft machine guns and pom-pom guns are generally placed on the top bridge or near the bow and stern. Light machine guns and rifles are placed around the front, back, and sides of transports in order to "cover" dead space caused by equipment. Sandbags are used to secure the tripods of the light antiaircraft weapons, and also to protect personnel from hostile fire and sea waves.

b. Tactics

Japanese ships usually execute sharp evasive movements, with frequent changes in course, when attacked by aircraft. These tactics, the Japanese admit, lessen the accuracy of antiaircraft fire.

The following notes were extracted from translations of Japanese documents dealing with antiaircraft defenses.

Air sentries and soldiers will report airplanes according to the direction of the clock, using such expressions as "2 o'clock direction."

Open fire against hostile planes as soon as they come within effective range. Fire heavily, concentrating on the most threatening targets, and seek to break up the hostile plan of attack before it can be executed.

In firing at aircraft with rifles and light machine guns, it is essential to have a good position. Rest your body and your left elbow on the gunwale and keep alert. Fire when the ship is at the top or at the bottom of a wave. Get the bearing of the hostile planes, align your sights, and then use following fire. If the hostile aircraft are over 2,000 feet high, and if their cross-country speed is small, you will find it profitable to fire on fixed lines.

The following are the most important points in antiaircraft firing:

a. Do your best to judge the height of aircraft with your measuring instruments, and to judge their course and speed with your naked eye.

b. Because of the pitching and rolling of the ship, the height-finder generally is accurate only to within 4 to 5 degrees. If the pitching and rolling is considerable, special adjustments are made, or corrections are made when the boat is level.

c. When attacked by a dive bomber, wait until the plane pulls out of its dive and then try to shoot it down.

d. Against a torpedo bomber, it is necessary to open fire quickly and try to shoot it down at a range of more than 1,000 yards.

Since the torpedo bombers fly low over the water as they come in to attack, fire shrapnel at them with field artillery and mountain artillery guns which are distributed aboard ship. A rapid rate of fire should be used.

To prevent hostile planes from strafing at low altitudes, small balloons should be raised quickly.

3. WHILE LANDING

The protection of landing coasts against air attacks is the responsibility not only of the divisional antiaircraft dispositions, but also of each individual unit commander. In accordance with this policy, the first-line units must form detachments for antiaircraft protection while landing, and must prepare for air raids immediately after landing. These detachments and the unloading units must dig the necessary protective trenches near the shore.

During landing operations, antiaircraft boats must be properly disposed, and part of the antiaircraft artillery company must be landed immediately for shore protection.

When hostile planes dominate the skies, try to use the weapons attached to the large and the small motor landing boats.

Boats which are not being used should be hidden immediately in the shade along the edge of the water, or camouflaged with materials similar to the surroundings.

If boats in use are attacked, the formation and the direction of travel should be changed immediately.

To reduce damage from hostile bombs, transport ships should be anchored in an irregular formation. Anchoring in a straight line would limit the field of fire of machine guns. As a precaution against bomb damages, ships should be anchored some distance apart.

4. ON LAND

According to observers, Japanese land-based antiaircraft guns usually open fire while United Nations planes are still out of range, and continue firing until our aircraft are well away from the target. In most cases the fire is of the barrage type, although a good deal of aimed fire has been encountered.

In one Southwest Pacific area, Japanese searchlights were slow in picking up planes—perhaps because the planes approached in a glide. However, after picking the planes, the searchlight crews held them well.

One crew observed two or three banks of searchlights. Each bank consisted of 12 searchlights, and each acted as a unit. In the meantime, 15 or 20 other searchlights operated individually. Individual searchlights were spaced at intervals of not more than 400 yards along the shore of the defended area.