Jim Klotz, SYSOP                                 Dale Goudie, Information Director
 Computer UFO Network                       UFO Reporting and Information Service

May 5, 1993


The controversial "Bentwaters Case" (1980) has long been of interest;  especially so because the "Halt Memo" which documents the anomalous events to some  degree. These events have been the subject of Radio and television programs,  magazine and newspaper articles, books, and much research, discussion and  speculation.

It was with interest that we read Out of the Blue, by Jenny Randles, (Global Communications, New Brunswick, New Jersey, 1991.) We found it interesting that located in the vicinity of the infamous Orford Ness lighthouse, (which some say is what was seen in Rendlesham Forest those December nights in 1980), is an apparently secret research installation. One of the projects mentioned by Ms. Randles, located at Orford Ness, was "Cobra Mist", officially an advanced "over the horizon" radar..." (1)

We filed Freedom of Information Act requests for information on Cobra Mist with several U.S. agencies, and after some delay, received some heavily censored documents and one large complete document. The one large item is an article from a classified research journal, the "JDR" (Journal of Defense Research). The Defense Advanced Research Projects Agency (DARPA) was the declassification authority for this document. Judging by the MAR 23, 1993 declassification date, we may very well be the first to obtain this material. The "automatic" declassification downgrade date is given as Dec. 31, 1991, but apparently the document had not been reviewed until our request. The documents and FOIA letters received are reproduced in this file.

Indeed, from the released documents, COBRA MIST was an OTH radar operated from the late 1960's until June 30, 1973; a costly but apparently ineffective system. Confirming the 'understanding of the locals' as Ms. Randles reports, the COBRA MIST radar was "connected with nuclear missile test launches" (2) as part of the stated mission was to oversee Soviet and Soviet Bloc Countries' aircraft and missile operations. However, no connection whatever to any operations similar to "Star Wars" is revealed by the released documents. (3)

Whether or not COBRA MIST is connected with the "fireball" and other unusual phenomena Ms. Randles reports as occurring in the vicinity of Orford Ness is not revealed by the released documents. However "antenna arcing" and "corona" are mentioned. Both of these are electrical effects which occur at high power levels and reveal the strength of the electrical fields surrounding the radar antenna which itself was physically massive. As a speculation, we suppose it is remotely possible that SOME sightings or other effects MIGHT be attributed to the intense radio frequency electrical field present when the radar was being operated at high power levels. Note that, according to the documents, the design power level of the radar was never realized.

For those interested in obtaining further information about COBRA MIST, the documents contain many references which may be obtainable. There are also references to other OTH radar systems.

Some notes: Some changes were necessary to accurately reproduce the documents.  Other letters have been substituted for Greek letters used in formulae and the various fonts and type sizes are not represented, although pagination, indentation and general appearance are retained.

Text Conventions:

        A row of ========== indicates a page break.

        A double row of   =======  indicates a page break between two documents.

        Deleted or blacked out material is enclosed with { }.

Footnotes in the JDR article The Enigma of the AN/FPS-95 open a new window with Part 4 of  the Cobra Mist presentation (References) bookmarked close to the reference number.  Close this new window afrer each footnote or you will end up with a bunch of them open.

Footnotes to Introduction:

1. Out of the Blue_, by Jenny Randles, Global Communications, New Brunswick, New Jersey,
    1991., pp. 182.

2. Ibid., pp. 182.

3. Ibid., pp. 183.



         System 441A, known as COBRA MIST, was initiated to acquire, install, and test the AN/FPS-95 OTH Radar Set in an operational over-seas environment at Orfordness, England, (See ESD History for FY 1972, p. 172 ff.).

         System turnover was scheduled for 1 July 1972. However, due to several test delays the turnover was rescheduled for 1 January 1973.  In the interim, it was decided to combine the Design Verification System Test (DVST) and Initial Operational Test & Evaluation (IOT&E) in order to expedite the test program.

In the combined DVST and IOT&E during the summer and fall of 1972, a severe noise problem was encountered, which resulted in a reduction in detection Capability. The range related noise had a median value of -65 decibels; in addition, there were noise modulation sidebands 10 to 30 hertz from the carrier with a median value of -60 decibels.


      In January 1973, a joint United States/United Kingdom (US/UK) Scientific Assessment Committee (SAC) was formed to analyze the problem.   The committee was comprised of:

Dr. M. Balser

Xonics Corporation


Prof. E. D. R. Shearman, V. C.

Birmingham University


Dr. J. Aarons

Air Force Cambridge Research Labs


Dr. E. N. Bromley

Radio & Space Research  Station


Dr. C. M. Crain

Rand Corporation


Dr. T. Croft

Stanford University


F. A. Kingsley

UK Ministry of Defense


Dr. B. Roberts

Royal Radar Establishment


Wing Commander D. R. J. Evans

Royal Air Force, Orfordness


          The purpose of the SAC was to:

     1.   Investigate the fundamental causes of the degradation in performance in the COBRA MIST OTH RADAR.

     2.   Recommend measures to overcome system deficiencies.

     3.   Assess the potential radar performance.

           The SAC met during the period February-May 1973. In May, the committee issued a final report which contained 10 recommendations, with the following four listed as high priority:

     1.  Implement a more flexible and effective set of processing and display programs and procedures.

     2.  Develop and install a pulse compression system with a pulse length of the order of 100 microseconds.

     3.  Further study, both experimentally and theoretically, the characteristics of excess noise, with the aim of minimizing its effect on radar operations.

     4.  Provide for strong technical management to carry out the proposed program which must include further tech-


nique developments in addition to evaluating operational effectiveness.

         The SAC said that given the proposed system modifications and Studies, the system would undoubtedly be greatly enhanced in performance and potential. The committee added that following the modifications, the system should undergo a series of operational feasibility tests for one year, the results to be the basis for any future decisions on COBRA MIST.

          The findings, results, and recommendations of the committee were briefed to the U.S. Secretary of the Air Force (SAF) and the United Kingdom Ministry of Defense (UK/MOD) in May 1973. The result of these briefings was a joint US/UK decision, based on an economic analysis of the requirements, to terminate operations at Orfordness.  The decision was publicly announced in London by the Ministry of Defense on 29 Jun 1973.



                       441A, known as COBRA MIST, is a program to acquire, install, and test the AN/FPS-95 OTH Radar Set in an operational overseas environment. The 441A contractor is RCA; the 441A site is Orfordness, England.

                       The missions of the 441A AN/FPS-95, an over-the-horizon (OTH) backscatter radar, are to detect and track aircraft; detect missile and earth satellite vehicle launchings; fulfill current and critical intelligence requirements; and to provide a research and development (R&D) testbed for determining optimum backscatter techniques for other operational missions.

     By 10 July 1971 the 18th (and last) antenna string was installed. Phasing at high power began on 17 July.

     The Rome Air Development Center (RADC) aircraft conducted antenna pattern measurement tests 5-17 August, with the radar operating 10 decibels down. All 18 strings were tested at all ranges, including the side lobes and back lobes. The aircraft long range tests were completed early in September 1971.

The upper catenary nuplaglass rod between mast #38 and string #18 failed on 23 September, and the resultant shock wave caused most of mast 38 and most of mast 39 to crack off at the 120-foot level.  The initial assessment indicated the failed rod had previously been rejected by RCA because of a deep transverse cut, and


the unit should not have been installed. String 18 was electrically sound, and RCA elected to repair the masts rather than replace them.

     A string 13 upper catenary nuplaglass break which had existed for several weeks was repaired by 13 December. The repair was facilitated by the use of a 144-foot extension ladder fire apparatus which RCA purchased under fortuitous circumstances. An Argentine organization had ordered two of these trucks, and subsequently was unable to consummate the purchase.

     System Test (Category II) was delayed until 30 August 1971 because of problems with a PDP-computer and the failure of two transmit/receive (T/R) switches.

     By 3 September it appeared that problems in integrating all end items into a workable system would delay system test another two or three weeks. The problems did not appear major when considered individually, but the system did not perform in the sense that meaningful data could be retrieved at the displays. Most of the problems appeared to be in the receiver and signal processing areas, since it was known that the transmitters were working and the antenna was radiating at expected power levels. 

     RCA provided four modification kits for the screen regulators, and system functional tests began on 29 September, except for Reliability and Maintainability (R&M) test. R&M, which requires operation 24 hours a day, was delayed until 15 October because repair work on the damaged masts had to be done during the daylight hours.

     On 9 February 1972 the radar was accepted from the contractor.


Design Verification System Test (DVST)

     DVST began on 10 February 1972. Because of a power shortage caused by the strike of British coal miners, the test was suspended on 14 February and resumed on 1 March.

     In the DVST, a "noise" problem appeared, and as of 30 June 1972 on-site activity was concentrated on a task force investigation of noise sources and effects, which were considered to be a limiting factor in system performance capability.

     Among the prerequisites for the DVST were the XEROX SIGMA V computer and the Signal Analysis Subsystem (SAS). 

     The SIGMA V, leased from XEROX, arrived on-site on 20 July 1971.  Subsequently, it was determined to be economically advantageous, over the life of the system, to buy the computer. Funds for the purchase were released in February 1972.

     The SAS, purchased under a contract placed by the Army Security Agency with Interstate Electric Corporation (IEC), was installed during the first week of January 1972.

     Originally, the Naval Research Laboratory (NRL) was to be responsible for the integration of the SIGMA V/SAS. When NRL said it would perform only that portion of the integration concerned with software, the integration was made a responsibility of IEC.

Tube Aging Rack

    On 19 July 1971 RCA submitted a fixed-price proposal for the tube aging rack which included a quote for $800,000 more than the original proposal, submitted on a cost-plus-fixed-fee basis. (Aging: allowing a permanent magnet, capacitor, meter, or other device to


remain in storage for a period of time, sometimes with voltage applied, until the characteristics of the device become essentially constant.)

     After a study to determine the most advantageous approach, the Program Director favored a fixed-price procurement, on the basis that RCA would probably try to recover some of the costs connected with a deficiency correction of the 4641 tube, under the guise of development of the tube aging rack.

     By 13 August 1971 RCA's proposal on a fixed-price basis was $2.6 million, and the Program Office directed that this be negotiated as a fixed-price Supplemental agreement (SA) rather than cost-plus-fixed fee as desired by RCA.

     The Defense Contract Administration Services Agency (DCASA) rejected the RCA proposal, and directed resubmission on the basis that the $800,000 increase in pricing for a fixed-price contract was not justified.

     Negotiations were completed in November 1971 at a definitized figure of $2,446,000. 

     The tube aging rack was operational on 30 June 1972.


RCA Studies

          A contract was placed with RCA on 12 July 1971 for a technical evaluation and cost trade-off study on the inclusion of a second beam capability and the addition of antenna strings to increase the geographical coverage of 441A. Three volumes on the technical aspects of various configurations and a draft volume on the cost trade-off were received in December 1971. The final version of the cost trade-off volume was received in February 1972.

          RCA was also given a contract for a study of Automated Data Analysis and Presentation (ADAPT) techniques. The ADAPT study is intended to determine methods of meeting real-time operational requirements through the automation of AN/FPS-95 output. A Government Working Group established for ADAPT held its first meeting on 25 May 1972.

RCA Proposal for Equitable Adjustment

    The ESD History for Fiscal Year 1971 (PP. 149-150) noted the RCA proposal for "equitable adjustment" of the 441A contract because of delays in the program.

    On 16 July 1971 RCA submitted a revised cost proposal in the amount of $6.3 million (up from $5.56 million).

    In November 1971 final agreement was reached on a settlement at $3.05 million.


Cobra Mist

        (1) Based on guidance by Hq USAF, the one year Design Verification System Tests planned for this system were terminated after six months on 10 August 1972 and Initial Operational Test and Evaluation (IOT&E) of the system was begun with the Using Command, USAFE as specified by AFR 80-14 in an effort to accelerate the system operational date to January 1973.

        (2) The testing revealed that the system was plagued by excessive noise of then undetermined origin which prevented the system from meeting its operational performance requirements. Various on-site and prime contractor efforts to alleviate the problem were unsuccessful and resulted on 29 December 1972 in direction by Hq USAF to terminate the IOT&E and to establish a joint US/UK blue-ribbon scientific team of OTH experts to determine the cause of the noise problem and recommend appropriate fixes. As a result, a Scientific Advisory Committee (SAC) was established which conducted a series of special tests at the site to isolate the noise source(s).  The committee published its findings in a final report on 1 May 1973 indicating that the noise problem could be over come by certain system modifications and that the system could be brought into an operational state with certain limitations on its expected capabilities.

        (3) On 19 June 1973, the UK was advised that the US has decided to close down the site and deactivate the system on 30 June 1973 based on the SAC findings and other considerations.



AFHRA/CC                                                                                            8 March 1993
600 Chennault Circle
Maxwell AFB AL  36112-6424                                                               RSQ 93/O-138.dg

Dear Mr. Goudie
[ Address deleted by CUFON ]

This is in reply to your Freedom of Information Act request of 16 February 1993. Your letter was received by the Agency on 22 February 1993 and it was assigned FOIA case Nr. 93-33. We have attached what releasable information we hold on "Project Cobra Mist" as found in the history of the 81st Tactical Fighter Wing. Other documents that we've located on your topic are not within our purview to release. These documents have been forwarded to the appropriate agencies for review, and they will respond directly to you with the results of their declassification determination.



WARREN A. TREST                                         1 Atch
senior Historian                                                    History, 81TFW, Vol 1, pp.
                                                                            29-30, Apr - Jun 73 (Declas)


{   BLACKED OUT       }              RETURN TO:                         Apr-Jun 1973
                                                The Albert F. Simpson                    Vol 1
     DECLASSIFIED               Historical Research Center
     DOD Dir. 5200.30              Maxwell AFB, AL 36112
     By AFSHRC
     Date: 25 FEB 1993

1 APRIL - 30 JUNE 1973




{FORMERLY RESTRICTED                      }                     24 JUN 1982
{Unauthorized                             }
{is       and criminal                    }
{RESTRICTED DATA in foreign dissemination }
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{S                                        }             3-8299-4
{EXEMPTION                                }              902116
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{Not Releasable to Foreign Nationals}

{SECRET}                                     RCS:HAF-CHO (AR)7101

     DOD Dir. 5200.30              COPY   1   OF   5   COPIES
     By AFSHRC
     Date: 25 FEB 1993



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  (U)(added) In June, a decision was made by the US Air Force in Conjunction with the UK Ministry of Defence to terminate the Cobra Mist test program. The Public announcement was made on 29 June. It stated:  8

       Under 1967 agreements the USAF, in cooperation with UK authorities has conducted a joint program of long range radio propagation research at RAF Orfordness, Suffolk The program has been completed. Accordingly,




it has been decided that the USAF will not renew the contract for the operation and maintenance of the equipment as from 30 June 1973, all activities associated with the program will be phased out as soon as possible thereafter.

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FASTC/CC                                                                                           20 April 1993

Mr Dale Goudie
[ Address deleted by CUFON ]

Dear Mr Goudie

This is in reference to your 16 Feb 93 Freedom of Information Act request for documents that would show the purpose, goals, and operations of Project Cobra Mist.

A line-by-line declassification review determined that portions of the record you seek are releasable under the Act. However, some parts are exempt from disclosure under United States Code, Title 5, Section 552(b) (1) since the unauthorized disclosure of such information could reasonably be expected to cause damage to national security and is properly and currently classified in the interest of national defense, as specifically authorized under the criteria established by Executive order 12356 and implemented by regulation.

Some portions of the information were removed because they did not pertain to the subject matter of your request.

Should you decide that an appeal to this decision is necessary, you must write to the Secretary of the Air Force within 60 calendar days from the date of this letter. Include in your appeal any reasons for reconsideration you wish to present, and attach a copy of this letter. The appeal should be forwarded to;

             Secretary of the Air Force
             Thru: HQ FASTC/IMOR (FOIA)
             41l5 Hebble Creek Road Suite 14
             Wright-Patterson AFB, OH 45433-5618


/s/ James E. Miller                                                       1 Atch
James E. Miller, JR., Colonel, USAF                           Releasable Documents


"Freedom Through Vigilance"


{SECRET}              315

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        COBRA MIST was cancelled in June 73.  Hardware has been           
dismantled. 509

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         504 - 510.  _Ibid._




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   COBRA MIST: (S,{                                                      }

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Cobra Mist (U):

(U) COBRA MIST ceased operations as of 1730Z 29 Jun 73.  FTD was queried as to its need of COBRA MIST site equipment and data tapes.  FTD requested that the SIGMA-5. CSP-30, IEC 284, and PDP-9 processing/analysis equipment and any data tapes of recorded events on certain dates and times be furnished to FTD. 573

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         571 - 573  Ibid.



WASHINGTON, D.C. 20301-1400

PUBLIC AFFAIRS                                                                            14 APR 1993

                                                                                                            Ref :  93-F-0584

       Dear Mr.  Goudie:
       [ Address deleted by CUFON ]

     This responds to your February 15, 1993, Freedom of Information Act (FOIA) request related to "Project Cobra Mist" which was referred to this Directorate by the Defense Nuclear Agency (DNA). We received the referral from DNA on March 5, 1993. Our March 12, 1993, interim response and our March 25, 993, letter refer.

     The Advanced Research Projects Agency (ARPA) has provided the enclosed document, "The Enigma of the AN/FPS-95 OTH Radar," as responsive to your request. All fees associated with processing this request have been waived in this instance.


                                        /s/ W.M. McDonald
                                         W.M. McDonald
                                         Freedom of Information
                                           and Security Review

       As Stated


{The article appearing below is classified SECRET NOFORN}  UNCLASSIFIED

{Classified by DD 254, 10-1-78, Contract}
{19628-79-C-0001 and 4140 Security Guide}
{dated April 1975.}
{Exempt from general declassification}
{schedule of Executive Order 11652}
{Exemption category 3.}
Declassify on Dec. 31, 1991

PER Director, ARPA, S&IO/TIO     

The Enigma of the AN/FPS-95 OTH Radar (U)

E.N. Fowle, E.L. Key, R.I. Millar, and R.H. Sear
the MITRE Corporation, Bedford, Mass.

(Received May 22, 1979)

(U) {(S)} Cobra Mist, the AN/FPS-95 over-the-horizon (OTH) radar built on the English North Sea Coast in the late 1960's to overlook air and missile activity in Eastern Europe and the western areas of the USSR, was the most powerful and sophisticated radar of its kind up to that time. The design, which emulated Naval Research Laboratory's Madre over-the-horizon radar, incorporated rather coarse spatial resolution and relied upon ultralinear, wide dynamic range components and complex signal processing in attempting to achieve the extreme subclutter visibility (scv) of 80 to 90 dB needed to separate target returns from the strong ground clutter - a goal well beyond the 60-odd decibel subclutter visibility previously  achieved. The detection performance of the radar was spoiled, however, because the actual subclutter visibility achieved was only 60 to 70 dB, the limitation being due to a noise with approximately flat amplitude-versus-doppler frequency, which appeared in all range bins containing ground clutter and aircraft returns. Experiments performed at the site failed to uncover the source of the noise, either in the equipment or in the propagation medium. Other experimental results imply that the noise was associated with returns from land areas and not from sea surfaces; the possibility of electronic countermeasures was not ruled out.  Because the source of the noise was not located and corrected, the radar program was terminated in June 1973 and the equipment removed from the site. The cause of the noise is unknown to this day.


This is as strange a maze as e'er men trod; And there is in this business
more than nature Was ever conduct of: some oracle Must rectify our knowledge.
                                                    Shakespeare    (The Tempest)

(U) {(S)}  This paper recounts the story of Cobra Mist, the AN/FPS-95 over-the-horizon radar built in England on the North Sea Coast in the late 1960's and operated there until mid-1973, when the program was discontinued.

(U) {(S)} As many will remember, the AN/FPS-95 was the largest, most powerful, and most sophisticated OTH (over-the-horizon) radar of its time; and the OTH community as a whole had high hopes that in performance and capability Cobra Mist would set new standards for the OTH radar art.  Quite the opposite happened, however. The radar was plagued from the beginning by difficulties, and although the problems within the equipment it self - which were never very serious - were tracked down and corrected, a residual problem, apparently in the external environment, seriously impaired the detection performance of the radar and led ultimately to the discontinuance of the program. The source of the difficulty that caused Cobra Mist's demise was never found. At the conclusion of the program a rather extensive set of reports on the program (1) were prepared for the U.S. Air Force, but these were not widely distributed. Consequently, the community did not benefit fully from the AN/FPS-95 experience.

(U) {(S)} The authors of this paper were all in some way intimately associated with the AN/FPS-95, both in its initial operational phases and in the final phase when an all-out, though time-limited, attempt was made to locate and correct the critical difficulty.  The point of this paper is to give an account of the final phase: to list the

(U) {The markings NOFORN and NF used in this paper have been derived from the source material.}

{SECRET NOFORN}                         UNCLASSIFIED               JDR 289



possible causes of the radar's poor performance and to describe the evidence for and against those causes as gathered through measurements and tests conducted at the site.

 (U) {(S)} The paper begins with a brief history of the AN/FPS-95 program, its origins, the design basis, factors that led to the choice of a site in England, and the essential features of the difficulties that led to program termination. Following that is a careful description of the radar system itself, equipment components and all. Next is a discussion of the radar's capabilities and limitations, both those that were expected and those that were actually observed; the nature of the principal difficulty, the so-called "clutter-related noise," is then described. The last three sections treat, in turn, some possible causes for the noise: in the radar equipment itself, in the external environment in general, and in postulated countermeasure activities. The evidence is reviewed and weighed, and inferences are drawn in the summary and conclusions section at the end of the text.

(U) An appendix contains a detailed account of one experiment considered by the author to be possibly quite significant.


By the early 1960's, the promise that early experimenters had seen for long range, high-frequency radar apparently had been realized by the Madre OTH radar of the Naval Research Laboratory on Chesapeake Bay, when Naval Research Laboratory personnel reported detection of aircraft targets at ranges of 1,500 to 2,000 nmi  (2) and of missiles in early launch phase from Cape Canaveral. (3) Plans then were made by the U.S. Air Force to incorporate this new technology into a radar sensor to be located in Turkey which, looking over the Soviet Union, Would gather intelligence data on Soviet missile and air activities, at that time the cause of much official concern.

(U) {(S)} In 1964, the U.S. Air Force solicited bids on the contract definition phase of the over-the-horizon radar in Turkey. The award was given to RCA on the basis of a design approach that closely paralleled the design of the Madre radar.  In 1965, following contract definition, bids were solicited for an operational radar in Turkey, but a hiatus developed when a site in Turkey was not made available to the United States. A search for a site then was made in other countries, and after some time and negotiation, the British offered a site in Suffolk, on the coast of the North Sea near the town of Orford. The Air Force accepted this offer, and the program proceeded.

(U) {(S)} In 1966, the Air Force again solicited bids for an operational OTH radar to overlook air and missile activities in the Soviet Union and Eastern Europe, this time from England. Program management was assigned to the Electronic Systems Division of the Air Force Systems Command at Hanscom Air Force Base, Mass. Engineering responsibility before this had been assigned to the Rome Air Development Center, Griffiss Air Force Base, N.Y.; scientific support was to be furnished by the Naval Research Laboratory. Toward the end of 1966, a contract to build the radar was awarded to RCA Corp., Moorestown, N.J. The radar had by that time come to be called the AN/FPS-95 with code name Cobra Mist. In the United Kingdom, the fact that the system to be located on Orford Ness was a radar was classified at the security level of  "Secret."

(U) {(S}) Construction of the radar, whose design was still heavily influenced by the Naval Research Laboratory's Madre radar, began in mid-1967.  Possessing capabilities previously unrealized in either experimental or operational OTH radar, the AN/FPS-95, it soon became clear, would have to be operated initially by a crew of scientist caliber, both to verify the design concepts and to develop procedures for the ultimate RAF-USAF operational crew to use in illumination of desired regions, detection and tracking of aircraft and missile targets, extraction of signatures, and so on. In 1969, plans for the scientific program, which was called the Design Verification System Testing (DVST), were drawn by the Cobra Mist Working Group, (4) which included representatives from most of the U.S. and U.K. over-the-horizon radar groups; the DVST program was to have a duration of one year. In early 1970, The MITRE Corp., Bedford, Mass. branch and the Naval Research Laboratory were chosen to conduct the DVST program, and later in mid-1970 the Air Force formed the DVST Technical Advisory Committee to assist in the technical direction of the program.

290 JDR                                 UNCLASSIFIED                                {SECRET NOFORN}



Figure 1. Aerial View of AN/FPS-95. (USAF Photo.) (Figure unclassified.)

When detailed experimental plans were complete in mid-1971, groups from MITRE and the Naval Research Laboratory moved to the site, which by then had assumed the form shown in the aerial view of Fig. 1.

(U) {(S)} Technical difficulties with the system delayed both acceptance of the radar by the Air Force and the commencement of the DVST program until February 1972. From the beginning, the DVST program was hampered by problems, the most serious being the appearance of a mysterious noise, which occurred in all Doppler filters corresponding to range intervals in which returns from the earth's surface (that is, "clutter" returns) were received. The range intervals containing the clutter return also contained the returns from the missile and aircraft targets the radar was to observe. The level of this "clutter-related noise" was high enough to impair seriously the capability of the radar to detect aircraft and missile targets, and as time went on, activities at the site shifted more and more from DVST to efforts to locate the source of the noise and to eliminate it.

(U) {(S)} The DVST Technical Committee viewed the noise problem with increasing alarm and, in the report to the Air Force which followed its meeting in November 1972,  (5)  the Committee recommended that top priority be given to solving the noise problem, that control of operations at the site be shifted from the Air Force to a civilian scientific director, and that the latter mount a coordinated, systematic program to isolate and identify the source of the noise. The Air Force on Dec. 27, 1972 moved to put the recommendations into effect.

(U) {(S)} DVST Program was suspended, a scientific director was recruited from Stanford Research Institute and a committee, called the Scientific Assessment Committee (SAC), was appointed with U.S. and U.K. members. The U.S. members previously had no direct involvement with the Cobra Mist radar. This committee took a fresh look at the system, system performance, and noise data and structured a series of basic experiments (6) to determine the source of the noise. These experiments were conducted in the period from January to May in 1973, and in the ensuing appraisal it was found that the source of the noise

{SECRET NOFORN}                              UNCLASSIFIED                               JDR 291



Figure 2. The geographical coverage of the AN/FPS-95 radar. {(Figure classified Secret)} (U)

had not been conclusively located. The Scientific Assessment Committee submitted its report (7) in May 1973 and the Cobra Mist radar program was terminated abruptly on June 30, 1973. Afterward, the radar was dismantled, and the components were removed from the site.

(U) {(S)} So ended a program that had occupied the efforts of hundreds of people for an interval of several years and had cost the United States, by various estimates, between $100 million and $150 million. The principal product was an enigma which has not been resolved to this day.

292 JDR                                    UNCLASSIFIED                        {SECRET NOFORN}




(U) {(S-NF)} The AN/FPS-95 over-the-horizon back scatter radar (8) was located at Orford Ness on the east coast of England. By beam steering, the radar was designed to make observations within a 91-deg azimuth sector extending from 19.5 to 110.5 deg clockwise from true north. The maximum range, assuming one-hop propagation via the ionosphere F-layer, was approximately 2,000 nmi, but the equipment would permit the observation of suitable, more distant targets using multihop propagation modes. A minimum range of approximately 500 nmi was set by the lower radar frequency limit and the upper elevation limit of the radar beams. Figure 2 shows the nominal coverage of the radar using single-hop propagation modes. The operating frequency range extended from 6 to 40 MHz.

(U) The radar employed the pulse-Doppler method to detect the radar signals from moving targets against the much larger return from the earth's surface. The waveforms used for search and tracking tasks took the form of radio frequency pulses, with durations selectable from 250 to 3,000 microseconds and pulse repetition frequencies (PRF) from 40 to 160 pulses/sec. Received pulse-trains of selectable lengths were processed in a frequency analyzer, which in effect provided a contiguous set of bandpass filters that were approximately "matched" in the radar sense for targets with constant Doppler frequencies and also for targets with linear Doppler rates of change (constant accelerations). An oblique ionospheric sounder mode of operation was also available, wherein the earth surface backscatter returns could be displayed as functions of radar frequency and propagation time delay.

(U) To achieve sufficient signal-to-noise ratios against the predicted noise background, the radar was capable of very high transmitted power output. A peak power of 10 MW and an average power of 600 kW were originally specified, although these figures were not achieved in practice. Such high powers were incorporated in the design to compensate for the relatively low antenna gain of approximately 25 dB. 

(U) {(S)} Both ionospheric propagation limitations and the scarcity of clear HF operating frequencies impose severe limitations on the design bandwidths of OTH radar signals and therefore on the attainable range resolution. This fact, coupled with the broad (7 deg) beamwidth of the AN/FPS-95, resulted in a very large radar resolution cell and, consequently, a large earth-surface radar backscatter power. To accommodate such large signals without causing unacceptably high inter-modulation and cross-modulation effects, a radar receiver with the very large linear dynamic range of 140 dB was provided, together with signal processing equipment of commensurate capabilities. A simplified block diagram of the system is shown in Fig. 3, and the major parameters are summarized in Table 1.

(U) Following are brief descriptions of the major elements of the AN/FPS-95 radar.


(U) {(S)} The antenna consisted of 18 log-periodic antenna strings, which radiated like spokes in a wheel from a central "hub." Figure 4 is a close-up photograph of one such string. Each string was 2,200 ft in length and carried both horizontal and vertical radiating dipoles. The strings were separated by 7 deg in angle, and they thus occupied a 119-deg sector of a circle. The complete antenna was located over a wire-mesh ground screen, which extended beyond the strings in the propagation direction.

(U) To form a beam, six adjacent strings were connected, by means of a beam-switching matrix situated underground at the hub of the antenna, to the transmit or receive beam-forming networks in the main building. The pointing direction of the beam was controlled solely by selecting the appropriate set of six adjacent strings from among the 18 available. According to the frequency of operation, a specific small section of each log-periodic string became resonant. Thus, at high frequencies the active portion would be close to the antenna hub, and it would move out toward the larger dipole elements as the frequency was lowered. While the linear extent of the active area extending across all six strings thus increased as the frequency was lowered, the net effect was to produce a beam whose angular dimensions and, hence, gain were almost independent of frequency. A simple way to view the action of the antenna is to regard it as a six-element broadside array, which moved around within the physical boundary of

{SECRET NOFORN}                            UNCLASSIFIED                                     JDR 293



Figure 3. System block diagram. (Figure unclassified.)

the antenna structure in response to frequency changes and to the choice of strings. 

(U) An important point to consider is that only a small fraction of the total physical aperture of the complete antenna was devoted at any one time to the task of beam formation. To shape the beam, it was necessary to ensure that the correct phase relationships were preserved between each of the six active strings. Thus, during transmission, phase shifts were introduced in the outer four of the six strings to compensate for the arc-shaped configuration of the radiating elements and thereby produce an approximately planar wavefront. Each string was driven on transmission by a separate high-power transmitter. On reception, beam forming networks offered both in-phase addition to yield the "sum" antenna beam shape, similar to the transmission beam, and the appropriate phase shifts to yield a monopulse "difference" beam pattern for use in estimation of the target's azimuth angle.

(U) The antenna design parameters are listed in Table 2. A limited set of antenna-pattern measurements performed at the site revealed significant variations from the design values as a function of beam position and operating frequency. These variations were most pronounced in the elevation beamwidths, sidelobes, and beam-pointing directions.


(U) The transmitter operated in the frequency range of 6 to 40 Mhz. Although the design called for peak powers of 10 MW and average powers of 600 kW, in practice the peek powers achieved were approximately 3.5 MW.

(U) The power was generated in six separate linear-distributed amplifiers, one of which is shown in Fig. 5. The output from each unit was fed to a separate antenna string. The power could be varied over a 20-dB range by adjusting the exciter drive level, and harmonic frequencies were filtered from the output by means of four sets of switchable low-pass filters.

(U) The exciter furnished three generic types of amplitude-modulated CW pulse shapes as follows:

294 JDR                                 UNCLASSIFIED                                {SECRET NOFORN}



TABLE 1. AN/FPS-95 Parameters. {(Table classified Secret)} (U)

     Type Log-Periodic Array
     Frequency Range 6-40 MHz
     Polarization Vertical or Horizontal
     Number of Beam Positions 13
     Azimuth Coverage 91 deg.
     Azimuth Beamwidth (3 dB) 7 deg.
     Elevation Beamwidths (3 dB)
         Vertical Polarization 2 deg. to 10 deg.
         Horizontal Polarization 9 deg. to 30 deg.
     Gain (Vertical Polarization)
25 dB
         First -13 dB
         Second -18 dB
-20 dB
     Type Linear Distributed Amplifier
     Frequency Range 6 to 40 MHZ
     Power Output
        Peak 3.5 MW
        Average 300 kW
     Pulse Shapes Cosine-Squared, Flattened
   Cosine-Squared, Sin Mx/Sin x
 Pulse Repetition Rates 10*, 40, 53.33, 80, 160 p/s
 Pulse Widths
250 to 3,000 microsec, 6,000 microsec *
 Receiver/Signal Processor
     Type Analog and Digital
     RF Bandwidth 5 kHz
     Dynamic Range 140 dB
     Noise Figure 7 to 14 dB (Frequency Dependent)
     Analog/Digital Converter 18 Bit
     Clutter Filtering 100 dB
     Doppler Range 3 Hz to PRF/2
     Acceleration Range 20 g
     Integration Times 0.3125, 1.25, 2.5, 5, 10, 20 s

*(U) For special nonoperational use.

1. (U) Truncated cos2: This is a cos2 envelope modulation, which is truncated at the 10-percent
    voltage envelope points.

2. (U) Flattened cos2: This is a flat-topped pulse with truncated cos2 leading and trailing edges.

3. (U) Sin Mx/Sin x: This pulse was used for the oblique ionospheric sounder mode of radar
    operation. The pulse was formed by the superposition of 10 carrier pulses, each of 200-microsec
    duration, with frequency separations of 100 kHZ.

(U) The major transmitter parameters are shown in Table 3.


(U) The receiver consisted of monopulse sum and difference channels to match the sum and difference outputs of the antenna beam-forming net-

{SECRET NOFORN}                              UNCLASSIFIED                                  JDR 295



Figure 4. One of 18 log-periodic strings that formed the AN/FPS-95 radar antenna. (Figure unclassified.)

works. Each channel contained a band-switched receiver with a very large linear dynamic range (140 dB).  The receiver outputs were converted to baseband frequencies by in-phase and quadrature mixers and were then converted to a digital form by means of analog-to-digital (A/D) converters.

(U) Following the analog-to-digital converters, the digital signals were time weighted to reduce the  ground clutter Doppler sidelobes, digitally filtered

TABLE 2. Antenna design parameters. {(Table unclassified.)}



6-40 Mhz

Gain (vertical Polarization)

25 dB

Azimuth Bandwidth (3dB)

7 degrees

Azimuth Coverage (13 beam positions

91 degrees.

Elevation Bandwidths (3 dB)

    Vertical Polarization

2 deg. to 10 deg.

    Horizontal Polarization

9 deg. to 30 deg.


-13 dB 1st sidelobe


-18 dB 2nd Sidelobe


-20 dB Other Lobes


Figure 5. One of six transmitter linear amplifiers. (Courtesy RCA Corp.) (Figure unclassified.)

to remove the ground clutter, and then stored by range cell in preparation for analysis by the velocity and acceleration processors. The processing was achieved by converting the stored signal back to an analog form and then playing them back, greatly speeded up in time, with appropriate frequency translations through filters that were matched to the reconstituted pulse sequences. By these means, the entire range of Doppler shifts and acceleration profiles could be sequentially accommodated during a period shorter than that of the original radar pulse train being processed. Meanwhile, new signals were being received and stored. The durations of the pulse trains thus processed (integration times) were selectable over a range from 0.3125 to 20 sec. There was also a facility for recording the raw signals on magnetic tape at the output of the analog-to-digital con-

TABLE 3. Transmitter parameters (Table unclassified.)


 Frequency range

6 to 40 MHz

    Power Output

3.5 MW Peak


300 kW Average

    Pulse Shapes



Flattened Cos2


Sin Mx/sin x

    Pulse Widths

250 to 3000 microsec. 6000 microsec. *


10*, 60, 53.33, 80, 160



* For special nonoperational use.

296 JDR                               UNCLASSIFIED                              {SECRET NOFORN}



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