364. Letter From the Deputy Secretary of Defense (Gilpatric) to the Chairman of the Senate Committee on Armed Services (Russell)0

Dear Mr. Chairman: Set forth below is the Top Secret annex to my letter to you of this date.1

[Page 882]

In addition to the assurances and standards concerning Safeguard (a),2 which I set out in my unclassified letter, the following can be said about existing plans to maintain a vigorous underground test program:3

The fiscal year 1964 underground test program was [less than 1 line of source text not declassified] tests. (The monies to fund this testing program are provided for in the Department of Defense and Atomic Energy Commission appropriation bills, at present awaiting action by the Congress.) This figure of [less than 1 line of source text not declassified] somewhat low in view of the addition of the requirements which flow from the new restrictions on atmospheric tests to the requirements that already exist for the underground program. Although it is the quality rather than the number of tests which is of primary importance, the requirements for testing in the immediate future will probably average [less than 1 line of source text not declassified] tests per month. As a result of changing needs, added experience, and improved techniques, this estimate may change in either direction.

Actions are now being taken preparatory to augmenting the current (Niblick) underground test program and future programs to include additional underground tests of higher yield as well as more elaborate and complex effects tests. [2-1/2 lines of source text not declassified] The Niblick underground program will also be continued in the area of nuclear weapons research, and in the development of [1 line of source text not declassified].

The AEC, in conjunction with the DOD, will develop new techniques for effects testing underground. Current techniques for warhead vulnerability testing will be improved. Continuous study will seek to develop methods of conducting meaningful effects tests beyond those associated with the warhead itself. In addition, component and precursor tests directed at strengthening our readiness to resume atmospheric testing on short notice will be carried out.

With respect to Safeguard (d), I state in my unclassified letter that the intelligence community has increased its activities and will continue to increase its activities to cope with the new conditions under the treaty. I state also that the Administration has under consideration proposals by [Page 883] which our present Atomic Energy Detection System (AEDS) resources can be augmented to enhance our capabilities. The proposals now being reviewed, placed in the context of present and programmed capabilities of the AEDS, can be summarized as follows:

Meaning of “Threshold.” It is necessary to realize, in the first instance, that there are no sharp “thresholds” marking the limits of the capability to detect and identify nuclear events. A country might make the judgment that it would consider a detection and identification system to be good if it could identify events with 3/4 probability. This would give a potential violator a 1/4 chance of getting by with a violation and a 3/4 chance of getting caught. Whether a potential violator would be deterred by a 3/4 chance of getting caught requires a separate judgment, which depends on many factors—political, military, etc.—and would involve an assessment of the incentives he has to cheat, as well as the risks attendant to discovery of the violation. If, for a given set of conditions, a shot could be identified as such with a 3/4 probability, then a shot would have to be of substantially lower yield to have a 3/4 probability of evading identification. One could call the latter the “threshold of evasion,” and it is clearly substantially lower than the former, the “threshold of identification.”

The estimated thresholds given in this letter, except where clearly labeled otherwise, are “thresholds of identification.” That is, they reflect a requirement for a high probability of identification. For a high probability of evasion, all thresholds would be substantially reduced.

Tests in the Atmosphere, at High Altitude, and in Space. The AEDS has a current capability of detecting and identifying nuclear explosions between the surface and 10 km altitude with a 60-90% probability for 5 KT within the USSR and China and for 200-500 KT in the least favorable remote areas of the world. At other altitude ranges, by the end of FY 64 the programmed AEDS will include essentially all of the facilities needed to detect and identify with that same confidence nuclear tests of the yields indicated in columns (2) and (4) of the table which appears on the next page.4

As can be seen from the table, the threshold of the programmed system for detection and identification of nuclear tests at various altitudes varies from as small as 1 KT to as large as 100 KT at some altitudes over the USSR and China. And, since the programmed AEDS has been oriented primarily toward detection of tests in the USSR and China, its programmed capabilities for tests from the low atmosphere to as high as 10,000 km in remote areas of the Southern Hemisphere show thresholds [Page 884] as large as a megaton or more for high confidence of detection and identification.

The system will be improved as necessary, taking into account the considerable difficulties for the USSR and China in successfully carrying out useful clandestine tests in remote areas. The improvement can be made by programming additional ground-based detection stations, utilizing a variety of geophysical detection methods with which AEDS has had experience. The AEDS can be augmented to produce the detection and identification thresholds indicated under columns (3) and (5) of the table on page 4. With such detection and identification thresholds applicable, tests throughout the altitude ranges and geophysical areas indicated would have to be conducted at yields in the very low kiloton range in order to have high confidence in escaping detection.

Also, as indicated by Secretary McNamara in his testimony before the Senate Foreign Relations Committee on August 13, it is within the capability of US technology to supplement this ground-based detection network by satellite-based sensors. No decision has been made with respect to satellites, and their possible contribution is not reflected in the table. Such satellites would detect radiations from nuclear tests in the atmosphere and space. The combined system would force a violator to even lower yields to escape detection between the surface of the earth and 1 million km and, with the extreme sensitivity considered possible with satellite-based detectors, would force him to test at 10s or even 100s of millions of kilometers from the earth to avoid detection by the combined system. A simplified system of satellites, if found to be necessary, could provide a substantial enhancement to the identification capability of the ground-based system. A satellite designed to provide information essential to such a system is scheduled for launch in September or October.


The seismic element of the AEDS can detect underwater tests of about 0.5 KT in the territorial waters and seas adjacent to the USSR and China, and of about 0.5-10 KT in ocean areas remote from the USSR and China. This capability could be improved by installation of a hydroacoustic system. This system would provide capability for detecting tests as low as a small fraction of 1 KT throughout most of the open oceans of the world down to depths of about 3,000 feet. The addition of a water sampling capability would permit identification of nuclear tests in the oceans through identifying radioactive debris obtained by sampling the waters.


The existing seismic component of the AEDS has a detection threshold for fully tamped underground explosions in the USSR of about 1 KT [Page 885] for explosions in hard rock such as granite, with the threshold yield increasing in less consolidated rocks until it rises to about 10-20 KT in dry alluvium. The system can determine the location of the explosion site within 5-50 kilometers in the USSR. The lower number corresponds to those areas where previous data have provided a calibration. The capability of the seismic system to distinguish between underground nuclear tests and many earthquakes is limited. During a period when all tests but those underground are banned, the likelihood is that the USSR will accelerate its underground testing program. It is important therefore that the AEDS be capable of providing improved intelligence on the number and yields of Soviet tests being conducted underground. Since these tests would be permitted under the Treaty, it is reasonable to assume that, for reasons of economy, these tests would take place at the known Soviet testing sites. Such tests would be reported by the AEDS as having occurred at a particular known test site.

Study of the problem indicates that the seismic portion of the AEDS can be augmented to improve the detection threshold by about a factor of two and to improve the precision of epicenter location. Improvement in this direction will be made. A proposal for accomplishing it is to accelerate the change-over from existing equipment to improved equipment, to install three additional high-class stations, to incorporate one existing research station with the AEDS, and to construct one new station with somewhat simpler instrumentation.

I hope that the details in this letter, together with the assurances and standards in the unclassified letter, provide you with the information you desired.


Roswell Gilpatric
  1. Source: Kennedy Library, National Security Files, Departments and Agencies Series, ACDA, Disarmament, Test Ban and the U.S. Military. Top Secret.
  2. For text of the letter, see Nuclear Test Ban Treaty: Hearings Before the Committee on Foreign Relations, pp. 977-981.
  3. For enumeration of the safeguards, see ibid., pp. 274-275.
  4. In his unclassified letter, Gilpatric stated that the “standards established governing the type and magnitude of tests to be conducted will not be more restrictive than the spirit of the treaty limitations.” In Major Smith’s memorandum of the White House daily staff meeting on August 21, Bundy was reported as saying, with regard to a proposed definition of what constituted an underground test, that “we should keep our position [on the definition of an underground test] indefinite because we do not know what the needs for testing will be 10 years from now. He said the success of treaties depended on their ‘artful interpretation’ and made it clear that we will leave as many things as open-ended as we can.” (National Defense University, Taylor Papers, T-204-69, Chairman’s Staff Group August 1963)
  5. For text of the table, entitled “Programmed and Augmented Capabilities of AEDS to Detect and Identify Nuclear Tests with 60-90% Probability,” see the Supplement.