Cobra Mist (Orfordness).

Its objectives :- Detect and track aircraft. Detect missile and satellite launchings. Fulfil intelligence requirements. Determine techniques for other missions.

  The system was scheduled for operation in July 1972, however, due to noise problems detected during system tests, this was rescheduled for January 1973.

  An Assessment Committee was formed to analyse these system problems and in May 1973 the committee issued its final report, proposing modifications which would have greatly improve performance. The committee also stated that following modification, the system should undergo tests for at least a year.
  Having studied the committee findings a joint US/UK decision was made, based on economics, to terminate operations at Orford. This decision was announced by the Ministry of Defence in June 1973.

Over the Horizon Radar.

COBRA MIST.

  The AN/FPS-95 over-the-horizon back scatter radar was located at Orford Ness on the east coast of England. By beam steering, the radar was designed to make observations within a 91^o azimuth sector extending from 19.5^o to 110.5^o clockwise from true north. The maximum range, assuming one-hop propagation via the ionosphere F-layer, was approximately 2,300mls, but the equipment would permit the observation of suitable, more distant targets using multi-hop propagation modes. A minimum range of approximately 575mls was set by the lower radar frequency limit and the upper elevation limit of the radar beams. The operating frequency range extended from 6 to 40MHz.

  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 3,000µseconds and pulse repetition frequencies (PRF) from 40 to 160 pulses/sec. Received pulse-trains of selectable lengths were processed in a frequency analyser, which in effect provided a contiguous set of band pass 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.

  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 10MW and an average power of 600kW were originally specified. Such high powers were incorporated in the design to compensate for the relatively low antenna gain.

TRANSMITTER-EXCITER

The power was generated in six separate linear-distributed amplifiers. The output from each unit was fed to a separate antenna string. The power could be varied by adjusting the exciter drive level, and harmonic frequencies were filtered from the output by means of four sets of switchable low-pass filters.
The exciter furnished three generic types of amplitude-modulated CW pulse shapes.

RECEIVER-SIGNAL & DATA PROCESSOR

The receiver consisted of mono-pulse sum and difference channels to match the sum and difference outputs of the antenna beam-forming net-works. Each channel contained a band-switched receiver with a very large linear dynamic range.  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.
Following the analog-to-digital converters, the digital signals were time weighted to reduce the ground clutter Doppler side lobes, digitally filtered 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.
There was also a facility for recording the raw signals on magnetic tape at the output of the analog-to-digital converter.

ANTENNA

The antenna consisted of 18 log-periodic antenna strings, which radiated like spokes in a wheel from a central "hub." Each string was 2,200ft in length and carried both horizontal and vertical radiating dipoles. The strings were separated by 7^o in angle, and they thus occupied a 119^o sector of a circle. The complete antenna was located over a wire-mesh ground screen, which extended beyond the strings in the propagation direction.

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

Note:- An Anti-Ballistics Missile agreement was signed between the USSR and USA in 1972, this agreement limited the operation of certain radar systems. These radar systems were only allowed to operate from within the territories of the USA and USSR.