Array of 25 Yagi antennas used to pinpoint the direction of radio signals received from "forward scatter" transmission (page 136).

of system capabilities and limitations. Standards and methods of measurement are developed for radio systems to fulfill the needs of federal agencies and industry involved in radio telecommunication operation and regulation. Studies of information theory and coding, modulation, and antenna design are directed toward improvement of the reliability of systems and to the efficient utilization of the radio frequency spectrum.

Low and Very Low Frequency Systems (30-300 kc/s). Theoretical computations of the propagation of ELF and ULF electromagnetic waves have been carried out to provide a solution for the propagation of spherical waves in and about a spherical earth of finite conductivity. A flexible theoretical computing model for the lower ionosphere was developed for VLF-LF-MF and HF wave propagation. The model is a multi-slab electron-ion plasma with superposed magnetic induction of arbitrary direction. Computations have been made of the effects of various types of disturbances on waves propagated via the lower ionosphere. Graphs, curves, and charts are being prepared to assist in practical system studies in the ELF, VLF and LF region.

Analysis of transient propagation of LF radio waves has been conducted to assist in evaluating the accuracy of pulse navigation systems. Techniques for transforming from the frequency domain to the time domain for analysis of linear systems were further developed. A method was devised and tested for measuring effective ground conductivity and long paths by comparison of recorded complex spectra of atmospheric (lightning discharge) waveforms. The nature and occurrence of atmospherics, as well as modulation and receiving techniques, are studied to improve methods for elimination or reduction of noise effects in VLF/LF systems.

High-Frequency Systems. Experimental studies of high-frequency ionospheric radio propagation over Arctic paths were completed for the Air Force and the Navy. Results were obtained on the attenuation, or transmis

sion loss, of radio signals as a function of frequency over paths subject to severe ionospheric disturbances. Galactic noise (VHF riometer) absorption measurements taken at vertical incidences near the path midpoint were compared with the oblique path transmission loss to determine useful relationships between galactic noise absorption and oblique high-frequency radio signal attenuation. The dependence of transmission loss on the geographical location of terminals and the angle of signal arrival was investigated for paths transiting the Arctic. The short term pulse-to-pulse and amplitude perturbations during ionospheric disturbances were investigated to determine modulation limitations for HF arctic circuits.

A study was completed showing the effect of the radiation angle upon highfrequency transmission loss for long-range transmission.

Previous experiments have suggested that certain high-frequency radio waves may be propagated by ducting along the magnetic field lines of the earth through the exosphere. The experimental program is being expanded by higher pulse powers and a continuous wave radar technique for more detailed study of propagation of backscatter echoes. Frequency shifting will be used to obtain the wavelength dependence of this propagation mode. The statistics of the occurrence of this mode of propagation will be examined, including the change in path transmission loss.

An electronic computer program for computing the path Maximum Usable Frequency and Optimum Traffic Frequency has been completed for the Navy. The program utilizes for ionospheric input data the numerical mapping technique developed by another NBS laboratory. With this program it is only necessary to know the path coordinates, month, and solar index to determine the usable frequencies as a function of time of day. The program, which can be readily changed to accommodate any particular system, is being extended to cover Lowest Useful Frequency computations.

A study is being conducted for the Navy to determine the applicability of electronic computers to real time computation of the optimum operating frequency for any HF circuit. All known factors influencing the performance of HF radio circuits are being examined to determine their predictability and usefulness for the computer techniques.

Comprehensive high-frequency propagation studies on behalf of the Advanced Research Projects Agency are being undertaken to measure phase and path-length changes, and group path time delays. This program is directed toward determining the short-term behavior of the natural ionosphere in relation to limitations of nuclear detection at long ranges. Both short and long term variations will be measured to determine sporadic and cyclic effects. A study was undertaken to determine instrumentation requirements for observations of the fine structure of the ionosphere by the observation of the amplitude, phase and polarization of both backscattered and forward propagated signals.

Planning has begun for a new high-power ionospheric radar research facility for HF and VHF studies, to be located at CRPL near Boulder Laboratories.

The U.S. Air Force has made available components of a Ballistic Missile Early Warning System radar transmitter which will serve as the power supply and nucleus of the 5-million-watt research radar.

Very High Frequency Systems. A long-term project on the study of signals scattered from the D region was continued. Signals were recorded at 30, 40, 50, 74, and 108 Mc/s through September, and at 30 and 50 Mc/s following completion of the frequency-dependence program at that time. It was determined that signals at the lower VHF frequencies suffered longer fade durations. Characteristic depths of fade are dependent upon prevailing transmission loss and are greater under weak signal conditions. The power spectrum of the received signal increases with carrier frequency and antenna beamwidth. Average signal-envelope fading is greatest near midnight and lowest near noon.

Antenna Research.

An electronically scanned antenna capable of high scan rates was expanded to 25 elements, giving a 12° beamwidth. Observations of path direction variation of ionospheric scattered signals were demonstrated. This technique of antenna steering is expected to be valuable in observing the direction of arrival of radio signals arriving from the great circle path.

The principles of the electronic scan-demonstrated by the successful operation of the seven-element array-were further developed. The array was increased in size to 25 Yagi elements so that the overall width is now approxi

Part of a single antenna, consisting of 18,000 dipoles and covering 22 acres, built near Lima, Peru. The antenna will be used to probe the ionosphere, exosphere, interplanetary medium, and sun by means of radar (page 140).

mately 800 feet (33.6 wavelength). The width of the main beam was reduced to between 1.5 and 1.7 degrees, giving a rather fine-grain resolution between the components of signals arriving from different directions.

The equipment was operated successfully in order to collect data and to develop techniques for recording, interpreting and displaying the data. On June 16th an observation was recorded of signals arriving simultaneously from two directions via sporadic-E reflections. It is believed to have been the first such observation on record.

Studies and experimental measurements are being conducted to provide an improved antenna capable of receiving multiple steerable beams for any direction of transmission or reception. Concentric circular arrays of vertical monopoles using phasing and amplitude tapering are being studied. Measured results approaching theoretical expectations have been obtained on an array of one center element, an inner ring of ten elements, and an outer ring of 20 elements operated at 90 Mc/s. Construction of a 20 Mc/s model is underway.

A comprehensive study is being conducted on techniques and methods of measuring complex fields. Waves of arbitrary polarization and with multiple propagation paths and directions of arrival are being considered. The purpose is to develop a procedure for determining the response of any antenna (of known response pattern) to a complex field. Methods are being developed and tested for measuring amplitude, direction of arrival, polarization, and relative time phase of several multipath components of a complex field. Field-strength meters are being examined for their adequacy for both cw and pulse measurements.

Modulation Research. The chief obstacles to obtaining reliable signal transmission through the ionosphere are atmospheric noise, time-variant transmission loss, and multi-path propagation. One of the principal aims of the modulation research program is to characterize the time-variant, dispersive nature of ionospheric channels and the noise limitations at the receiver. In order to study the effect of channel distortions on signals it is also necessary to study the nature of the "communication source function" or the input modulation to the channel.

During the past year a survey of results obtained in many laboratories throughout the world on statistics of human speech as a modulating signal envelope has been made and supplemented by laboratory measurements. The effects of pre-emphasis of high audiofrequencies and clipping and filtering of speech signals has been studied. In channel characterization, significant advancements were made in the observations of pulse-to-pulse phase stability and pulse-amplitude fading over high-frequency auroral paths. Using pulses of 1 to 20 milliseconds duration, phase perturbations between successive pulses were analyzed over the path from Barrow, Alaska, to Boulder, Colo. Interpretation of the results was aided by concurrent sweepfrequency ionospheric soundings and oblique incidence measurements over the same path. It was found that the phase variations on this path mainly represented rapid movements of ionospheric irregularities rather than interference effects between separate modes of propagation.

A new program has been initiated in information theory and coding for radio channels. New requirements for great communication capacity and reliability, with corresponding demands to reduce congestion in the radio spectrum, suggest a trend toward digital transmission in the design of communication systems. This trend is fostered by developments in digital computers and the theoretical tools of information theory and coding. To approach the accuracy and efficiency of transmission indicated by Shannon's Theorem, digital transmission is essential. In this program a preliminary study and report has been made of the applicability of error-correcting codes in radio circuits as compared with existing error-detection automatic-repetition systems such as the ARQ.

Further improvements were made in techniques for noise reduction and multiple frequency shift digital transmission in the VLF band for defense applications.

Navigation and Timing Systems. A year ago, feasibility was established for time synchronization of clocks separated by up to 1,500 miles, to an accuracy of one microsecond, using clocks associated with a low-frequency Loran C navigation system. The 100 kc/s ground wave pulse is used. Time synchronization obtained by this means is approximately 1,000 times more precise than that obtainable by using high-frequency radio techniques such as WWV radio signals, and probably 10 to 50 times better than obtainable with very-low-frequency transmissions. The study was carried out for the U.S. Air Force, and the synchronization was demonstrated on the Atlantic Missile Range for application to launching and tracking problems. Further studies have been carried out on possible extension of the distance range by use of sky-wave signals.

The work on the Loran C clock provided background for design and construction of instrumentation for an atomic time accumulator, for use with the NBS national primary standard of frequency. Times derived from astronomical sources are subject to errors of the order of 1 millisecond for any given observation. A clock operating from the best available frequency source is capable of measuring time intervals to better than 1 microsecond as related to that frequency source. This program will provide a means for the Bureau to maintain a time scale based on the period of an atomic transition. Several such times scales are being maintained internationally, and their comparison is of scientific importance. in view of international consideration of redefining the second in terms of an atomic transition. A means is also provided for the Bureau to check various time signals against an atomic source and to publish corrections of these time signals as they relate to the atomic time source.

The basic concept of this instrumentation is first, a number of pulse dividers to provide redundancy and allow for checks against each other; second, battery standby power to provide for uninterrupted service if primary power is interrupted; and third, a means for reading out or checking the dividers one against another. The objective of the entire instrumentation