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Headquarters AAF Intelligence Summary 45-10 (30 May 1945): 18-25.

  • Theater of Operations--Pacific
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A Study in Jap Flak Defense

Tokyo is the capital of the Japanese Empire. As such it is the hub of the war wheel which the Japanese started rolling toward us on 7 December 1941. This city, one of the largest in the world, is the general headquarters of the Japanese armed forces, and is in addition the headquarters for the great monopolies which have dominated Japanese industry and commerce. It is the communications center of the Empire, in terms of military operations, government, and equipment manufacture.

The combined population of Tokyo, Kawasaki, and Yokohama is about 8,000,000 or 10% of the total population of Japan proper. The Tokyo population density of 31,000 per square mile is three times that of Berlin. In three congested wards (Asakusa, Honjo, and Kanda), the density is over 100,000 per square mile.

The Tokyo area contains major industrial plants of all types. There is almost no war industry which is not represented by its factories. Major aircraft production units, iron and steel mills, arms and munitions factories, and many large plants of all types make Tokyo one of the densest industrial concentrations in the world.

Tokyo’s vulnerability to fire has been conclusively demonstrated by AAF [Army Air Force] B-29s and by the U.S. Navy. High density of population resulting in almost unbelievable congestion, combined with flimsy construction, makes some sections of the city giant tinder boxes, requiring little more than a spark to turn them into bonfires.

Thus, Tokyo is a juicy target for air attack. We know it, and the Japs know it. That the Japs know it is demonstrated by the fact that they have given the city the largest known AA [antiaircraft] gun defense in the Japanese Empire. About 500 heavy guns protect the area formed by the cities of Tokyo, Kawasaki, and Yokohama. This is as many guns as ever protected the German capital of Berlin.

But mere numbers of guns don’t mean much to the Jap unless he uses them to good advantage, and they don’t mean much to us until we set them down in their proper places and study their layout. It may not be out of place to quote the old saying, “It isn’t what you have, but what you do with it, that counts.” We may have, for example, a jig-saw puzzle with 500 pieces, but we won’t see the picture they form until we put the puzzle together. So let’s try to fit these 500 guns into their proper places and interpret the picture they form.

First of all, let’s remember that the 500 pieces of our puzzle are from photographs up to 11 March 1945. Since that time certain parts of Tokyo have been given careful attention by AAF B-29s and U.S. Navy carrier-based aircraft. This means that right now the puzzle probably isn’t exactly the same as it was on 11 March. Some areas formerly considered important by the Japs may be no longer considered worthy of defense. Some targets may have been moved to other sections of the city, or even to other cities. However, here we are discussing broad trends in Jap flak tactics, and the Tokyo defenses today are in principle undoubtedly the same as they were on 11 March.

In attempting to analyze a defense of the size of the Tokyo defenses we are confronted with a highly complex and difficult situation. For example, in Hq. AAF Intelligence Summary No. 45-4 we discussed the AA gun defenses of the city of Nagoya. In that discussion we showed how the Japs had apparently included only a few key industrial units, chiefly aircraft plants, in their defended area. We went on to show how the AA gun defenses of Nagoya appeared to be keyed to the relatively small area formed by these few key industries. At Nagoya the problem of analysis was a relatively simple one, since the apparent method of deployment of the defenses was relatively simple, and since we had a much smaller defense to consider.

With the Tokyo defenses, on the other hand, the problem is far more complex, and presents many more confusing issues. Each of these issues points to a different solution. Here we are concerned, not with one or two target systems such as aircraft production and arsenals, but with many; not with a relatively small, compact defended area, but with a large, sprawling one which contains vital industries of many different types, all important to the continuation of the war by the Japs.

In addition, with a city the size and importance of Tokyo, the problem of analyzing the hundreds of war industries and of determining which ones are important and which are unimportant becomes far more complex. We never learn all we wish to know about the enemy’s production. We can proceed to a certain point, but there are always gaps in our knowledge of his industries which we can never completely fill. Similarly, in this analysis we will get just so far. Beyond that we cannot go.

Because the problem is so complex, perhaps we should approach it indirectly. In other words, let’s see what is indicated by the disposition of the AA gun defenses, and then try to fit this in with what we know of Tokyo’s industries. In order to do this we must make some basic assumptions.

The first and most important assumption we will make is that the Japs know how to lay out a flak defense. There may be room for argument on this point, but in Hq. AAF Intelligence Summary No. 45-4 we showed how at Nagoya they seemed to have applied certain basic principles of antiaircraft artillery defense which have been developed in our own service. Let’s assume that they knew and applied these same principles when they laid out the defense of Tokyo. From this the rest of our assumptions will develop.


Figure 10 - AA Gun Defenses, Tokyo, Japan.
Figure 10 - AA Gun Defenses, Tokyo, Japan. From photos to 11 March 1945. Figure shows scale in yards, with legend for gun heavy battery, occupied; gun heavy battery, empty; gun heavy battery, occupied; gun heavy battery, empty; gun heavy battery, 2 positions; gross battery, consisting of 2, 3, 4, or 5 6-gun batteries, as indicated; fire control radar; theoretical defended area; area bomb release line altitude: 30,000 ft.; 75 degree line; and grid lines are world polyconic grid, at each 5000-yard interval.
Legend for Figure10, Expanded.
Legend for Figure10, Expanded. AA Gun Defenses, Tokyo, Japan. From photos to 11 March 1945. Figure shows scale in yards, with legend for gun heavy battery, occupied; gun heavy battery, empty; gun heavy battery, occupied; gun heavy battery, empty; gun heavy battery, 2 positions; gross battery, consisting of 2, 3, 4, or 5 6-gun batteries, as indicated; fire control radar; theoretical defended area; area bomb release line altitude: 30,000 ft.; 75 degree line; and grid lines are world polyconic grid, at each 5000-yard interval.

The Tokyo defenses are shown in Fig. 10, as interpreted by Photographic Intelligence Center, Navy Yard, Washington, D. C. From the appearance of the Tokyo defenses in Fig. 10, it looks as though the Japs have set up a coordinated area defense. The gun batteries are disposed to include and give protection to the


entire city of Tokyo, with parts of Kawasaki and Yokohama included. We are therefore looking, not for just a handful industrial units which the Japs are trying to protect, but for a large, over-all defended area which should include all industrial targets which the Japs consider worthy of flak defense.

The shape of the defenses should give us some idea of the shape of this defended area. Let’s look at Fig. 10. At first glance all we see is a confused mass of flak batteries apparently deployed indiscriminately over the entire Tokyo area. However, upon closer inspection we see that the area is surrounded by an outer ring of flak positions consisting of more than one battery. These multiple-battery positions, or “grossbatteries,” as we have come to call them, appear to form a rather definite pattern. Examination of these grossbatteries on photos of the Tokyo area (the biggest appears on page 12 of Hq. AAF Intelligence Summary No. 45-7) will show that they are elaborately constructed.

These grossbatteries didn’t just happen. We can give the Japs credit for doing some rather extensive thinking before they set them down where they are. Therefore, since they were deployed as they appear in Fig. 19 for a purpose, the area they include should give us some idea of the shape of the Tokyo defended area. It should tell us the approximate boundaries of the area which the Japs consider important. Therefore, let’s reconstruct this area based upon the location of the outer-ring of grossbatteries.

In planning the Tokyo defenses the Japs undoubtedly gave some thought to the type of air attack to which the area would be subjected. Since the beginning of B-29 operations by XX Bomber Command in June 1944, the Japs have had ample opportunity to watch these aircraft in action, and to determine the tactics employed by the B-29 in attacks against vital Japanese targets. Based upon these tactics, let’s assume that the Japs have deployed the Tokyo defenses to be most effective against aircraft attacking at 30,000 feet altitude and at 300 mph ground speed. All of the flak batteries on the defenses should then lie within a bomb release line constructed for these conditions. This is well-established doctrine in our own antiaircraft artillery, and is in accordance with flak tactics employed by the Germans. The principle is based upon the assumption that attacking aircraft will take evasive action before entering the bombing run, and immediately after bombs away. Therefore, if aircraft are to be engaged at all, the guns must be so located that they can fire at the aircraft while on the bombing run, where they must fly straight and level for a considerable period of time. Furthermore, bringing fire to bear during the bombing run interferes with the difficult bomb sighting operations and operates to destroy bombing accuracy. Hence, if AA gun batteries are sited on or near the bomb release line, bomb-laden aircraft can be engaged for the greatest possible portion of this rectilinear flight, and the maximum effectiveness of the defense can be realized.

Carrying this reasoning one step further, a consideration of the maximum elevations of antiaircraft guns, and of their reduced rates of fire as these maximum elevations are approached, has led to the establishment in our own AA of the so-called “75-degree line” as the ideal location for AA gun batteries. This line connects all points about the defended area from which a line of sight at an angle of 75˚ with the horizontal will intersect the bomb release line at the expected altitude of attack and for the expected ground speed of attack. In other words, this line lies inside the bomb release line. If gun batteries are sited on this line, and open fire at directly approaching formations at maximum range, they will have reached maximum elevation and must cease firing just as each formation reaches the bomb release line and begins evasive action.

Let us therefore assume that the Jap officers responsible for the flak defense of Tokyo have attempted to locate the outer ring of grossbatteries on the 75˚ line about the Tokyo defended area. If this is true (and we assume it is) we can then construct the 75˚ line through these grossbatteries and from it we will get the shape of the defended area which we think they are protecting. The results of such a process are shown in Fig. 10. The solid line through the outer ring of grossbatteries is our 75˚ line, while the shaded area is our theoretical defended area. We have also constructed a bomb release line for aircraft attacking at 30,000 feet altitude and at 300 mph ground speed, and this is shown by the outer dotted line in Fig. 10.

We now have a theoretical Tokyo defended area, represented by the shaded polygon in Fig. 10. Now let’s see what the actual defended area looks like. Our problem now becomes highly complex. We do not know by any means all we would like to know about Tokyo’s industries. Certain vitally important units and areas have been definitely identified. Some of these have already been subjected to unrelenting air attack. There are, however, other units and other areas which have not yet been completely analyzed, and there must be still others about which we know nothing. In spite of these difficulties, however, we can make a reasonable estimate of the actual Tokyo defended area.

We have already said that the city represents a tremendous concentration of vital war industry. It is far beyond the scope of this discussion to analyze in detail each target. In fact, it is unnecessary, since we are looking for an over-all defended area, rather than certain defended points. We must, however, have some idea of the industrial concentrations which will determine the boundaries of the actual defended area. Fig. 11 shows a map of the Tokyo metropolitan area, including the cities of Kawasaki and Yokohama. The principal industrial concentrations with which we shall be concerned are shown in northern (A) and northeastern (B) Tokyo, along the west side of Tokyo harbor (C), along the west side of Tokyo bay (D), and along the Kawasaki and Yokohama waterfronts (E). In addition,


Figure 11-Tokyo Metropolitan Area.
Figure 11-Tokyo Metropolitan Area. Denotes approximate boundaries of principal industrial concentrations and actual defended area.


there is the important Musashino concentration (F), a critical point in the boundary of the Tokyo industrial area.

Based upon these known concentrations of industry, we can construct the area which is most likely to be defended by the Japs. This area takes the form of the shaded polygon in Fig. 11.

We now have two polygons. One is the theoretical Tokyo defended area which we have derived from the disposition of the defenses in Fig. 10, while the other is the actual defended area which we have constructed in Fig. 11 on the basis of our knowledge of the city of Tokyo. Now let us place one upon the other and see if they coincide. We have done this in Fig. 12, and we see that there is close agreement.

Part of the slight disagreement between our theoretical and actual polygons in Fig. 12 may be explained by the fact that we based our theoretical polygon on the assumption that the Japs had been able to locate all the grossbatteries in the outer ring exactly on the 75˚ line. In practice this is seldom possible. A perfect plan for AA gun defense may be drawn up on paper, only to find that some batteries fall in the middle of a large body of water, while others are located in inaccessible terrain. Therefore, we are almost always faced with a compromise between theory and practice. The problem in the Tokyo area is serious because Tokyo is a waterfront city, and because much of her industry is concentrated along the waterfront. This problem has been partially solved by the emplacement of flak batteries on built-up land out in the Bay, but there are sectors where the batteries cannot be located a sufficient distance out from the defended area.

Figure 12 - Shows theoretical area and actual defended area.
Figure 12 - Shows theoretical area and actual defended area.

We find real disagreement between our real and theoretical polygons on the Kawasaki and Yokahama waterfronts (Sector "A," Fig. 12). The boundary of our theoretical polygon in this sector is some 4,400 yards inland, whereas the boundary of the actual defended area runs along the shore line. If we were to refer to a theoretically perfect defense for this sector, we would find that some of our batteries were located in Tokyo Bay. In this case we would either have to locate our batteries on barges anchored the required distance out from the shore, or else, as the Japanese have done, emplace them as close to the shore as possible.

Figure 13 - Shows bomb release line and 75 degree line.; 75 degree line.
Figure 13 - Shows bomb release line and 75 degree line.; 75 degree line.

The limitations imposed by water area upon effectiveness of a flak defense are again revealed by the fact that there is a rather large “hole” in the Tokyo defense along the southeastern side of both the theoretical and actual defended areas. Again, the ideal solution would be to emplace the batteries in this sector on barges and anchor them out in the Bay where they belong. But the Japs have again accepted the second best solution, which is to emplace them on built-up land as far out from the actual targets in the defended area as possible. Here they have also strengthened the weak sector by the addition of more fire power. The grossbattery in this sector is the largest known to exist in any Japanese flak defense. It consists of five six-gun batteries, or 30 guns in all. One of these batteries was empty at the time of the photography upon which Fig. 10 was based, but it has probably become active by now.


We now have the over-all picture of the entire area defense of Tokyo. We see that in general it consists of an outer ring of grossbatteries surrounding the entire city, with individual batteries spotted throughout the area to give added protection in the interior. The dispositions of the batteries fit, in general, our conception of the Tokyo defended area, and the Japs appear to be protecting what we assume they would protect.

But our problem doesn't stop here. Within the overall defense there appear to be three individual defense systems protecting point targets or smaller target areas. For example, batteries W, J, K, and AA through 00 appear to form such an individual defense system. Let's lift this system out of the main Tokyo defense and look at it separately, as in Fig. 13. It is deployed about the Tokyo arsenal area in the northern part of the city. In Fig. 13, connecting the individual targets within the arsenal area, we form the boundary of our individual defended area (shaded polygon). Drawing a bomb release line for 30,000 feet altitude and 300 mph ground speed, we see that the batteries in the system conform, in general, to the shape of the defended area. If we construct our 75° line we see that most of the batteries are on or near this line. Thus we have the Tokyo arsenal area being given special consideration with the over-all Tokyo defended area.

The other two individual systems are not so easy to explain. The first of them is the rough rectangle formed by grossbatteries V, W, X and Y, in the eastern section

Figure 14 - 75 shows degree and bomb release lines.
Figure 14 - 75 shows degree and bomb release lines.

of Tokyo. Let us take these out of Fig. 10 and look at them separately, as in Fig. 14. Applying the same indirect method we applied to the over-all defense, we are able to conclude that these four grossbatteries are deployed to protect some vital target located in the center of the rectangle. If, as in Fig. 14, we construct a 75˚ line as a circle whose center is this hypothetical point objective, we see that all four grossbatteries are either on or very close to it. The trouble is that as far as we know there is nothing known to be at this location which warrants such special attention. There are industrial units in this area, but none of them is considered worthy of a special little flak defense all its own, such as we have in Fig. 14. The apparent importance of this point objective is further indicated by the fact that three of the four grossbatteries in this little defense are radar-controlled. It seems reasonable to assume that the Japs don’t waste fire-control radar on unimportant targets. Perhaps the answer is that, in analyzing a defense, what we think is important is not half so vital as what the Japs themselves regard as important and worth defending.

Figure 15 - Shows 75 degree and bomb release lines.
Figure 15 - Shows 75 degree and bomb release lines.

A similar problem, and on a somewhat larger scale, is presented by the ring formed by batteries M through U, near the Musashino concentration in western Tokyo. We see that each of these batteries is equipped with fire-control radar, and that they are about evenly spaced


in an almost perfect ellipse. This appears to be more than an accident, so let’s lift this little ring of batteries out of Fig.10 and look at it more closely, as in Fig. 15. Again we don’t know what is in the middle of this ring of batteries, so let’s employ the indirect method and see if we can determine the apparent shape of the small area they are protecting. Again assuming that the Japs apply the basic principles of AA gun defense as demonstrated at Nagoya, we find that the apparent defended area takes the form shown by the shaded polygon in Fig. 15. Again, the puzzling fact is that, in our opinion, there is nothing in this area which is worthy of such special treatment. And yet here we have an almost perfect little defense of nine batteries, all radar-controlled. The only conclusion we can draw is that there is some target in the middle of the ring which the Japs consider important, but whose importance we have not yet determined.

The Tokyo defenses shown in Fig. 10 are vaguely reminiscent of the German AA gun defense of Ploesti as it existed during the early part of 1944. This defense appeared as Fig. 10 of the Preliminary Report, “Flak Intelligence from Ploesti,” published by AC/AS, Intelligence on 2 December 1944. The Ploesti refinery area was surrounded by an outer ring of grossbatteries located on or near the area bomb release line. The outer ring was supplemented by individual batteries deployed toward the center of the area.

Figure 16 - Flak Clock-Tokyo Japan, Target: Center of Gravity, Theoretical Defended Area.
Figure 16 - Flak Clock-Tokyo Japan, Target: Center of Gravity, Theoretical Defended Area. Shows relative effectiveness. Altitude of attack: 30,000 ft., and TAS of attack: 300 MPH, wind: none, unlimited bombing run data based on capabilities of Japanese Navy 120 mm dp gun
Figure 17 - Flak Clock-Tokyo, Japan, Target: Center of Gravity, Theoretical Defended Area.
Figure 17 - Flak Clock-Tokyo, Japan, Target: Center of Gravity, Theoretical Defended Area. Shows relative effectiveness. Altitude of attack: 30,000 ft., and TAS of attack: 300 MPH, wind: 100 mph from 270 degrees, unlimited bombing run data based on capabilities of Japanese Navy 120 mm dp gun.

At the time of the occupation of the Ploesti area, in August 1944, it was found that the outer ring of grossbatteries still existed, but that the individual batteries in the center had been moved out to the outer ring, and formed into additional grossbatteries. This same tendency has appeared on recent photographs of the Tokyo defenses. The outer ring of grossbatteries is still being built. In addition, many of the individual batteries in the center of the Tokyo defended area appear to be in the process of being moved out into the outer ring.

Now let’s test this Tokyo defense for balance. Fig. 16 is a flak clock showing relative effectiveness numbers along twelve headings of attack, using the center of gravity of the actual Tokyo defended area as a target. The analysis has been made for aircraft attacking at 30,000 feet altitude, and at 300 mph ground speed, with no wind. We see, under these conditions, that the defenses have been fairly well balanced. In addition, comparison of Fig. 15 with Fig. 10 shows that the strong points and the weak points are just about where we would expect them to be. For example, in Fig. 16 we have a strong sector on heading 90˚. We see from Fig. 10 that there is a dense concentration of grossbatteries in this sector. Again, the defense appears to be strong on a heading of 0˚. Again looking at Fig. 10 we see that there are several grossbatteries close together in the southern sector. We also see that on a heading of about 330˚, the defense is relatively weak. Inspection of Fig. 10 shows


us that it is in this sector, the southwestern side of the defended area, that the limitations imposed by the large water of Tokyo Bay make it impossible to put the batteries far enough out from the defended area to bring the defensive strength up to the required value. This has caused the “hole” in the defense on heading of 330˚.

Now let’s again analyze the defense for the same conditions of attack, with the exception of the fact that this time we shall assume a 100-mph wind blowing from 270˚. We see the results in Fig. 17. The defense has been thrown entirely out of balance by the effect of the high wind blowing from the west, and it is now extremely strong in the east and south, while being extremely weak in the west. This unbalance of antiaircraft defenses in a direction away from that which the wind is blowing is caused by the fact that the aircraft bombing downwind are travelling at high ground speeds, thereby reducing both the accuracy of AA fire and the time of engagement. Aircraft attacking upwind, on the other hand, are moving at slow ground speed, and firing accuracy and the time of engagement are both increased.

Through this confusing picture, however, we have seen that there does appear to be a plan. That is what is important. If our air operations were being conducted against Japanese targets whose aircraft defenses were haphazard our problem would be very much simplified. That, unfortunately, is not the case. The Jap knows how to lay out his defenses. We have seen him demonstrate that at Nagoya and here, at Tokyo. And upon occasion he has shown that he knows how to use them. We can’t forget missions like the XX Bomber Command B-29 attack on the Singapore Naval Base on 2 March 1945 (see Hq. AAF Intelligence Summary No. 45-8). In opposing this attack the Japanese defenses were extremely well-conducted, engaging formations attacking at altitudes between 19,000 and 25,000 feet with intense and accurate heavy AA fire. Throughout the engagement the flak defenses exhibited a high standard of skill, accuracy, fire discipline and fire direction. It was no small-time flak defense that did this.

The main trouble that faces the Jap at the moment is his equipment. He has good directors, and he has some excellent AA guns. But at present he hasn’t enough of them. Nevertheless, we must not forget that he knows this just as well as we do, and that he is leaving no stone unturned in his concerted effort to improve the quality and quanitity of his antiaircraft artillery.

One fact we know to be true. If right now the Jap cannot match the German in the quality of his flak, that doesn’t necessarily mean that he cannot improve. Already he has made great strides. The obvious answer to that is for us to take full advantage of the benefits of flak intelligence and flak analysis, and thereby make it just as hard for him to shoot effectively at us as we possibly can. It doesn’t cost us much, and it may cost him everything.



Published: Thu Sep 14 12:26:44 EDT 2017