Cw Propagation Research Articles

Multipath Interference of Broad-Band Signals in RBR Propagation

Propagation experiments with fixed source and receivers in the Florida Straits have been characterized by large temporal fluctuations in transmission loss for both CW and sequences of pulsed sinusoidal signals. Multipath interference can result in fading of received signals of as much as 50 dB for CW propagation at 420 Hz and as much as 20 dB for pulses of 8 cycles in duration. These deep fading events occur at random intervals when the slowly varying sound-velocity structure is such that signals arriving from various paths interfere destructively, a condition that may persist for several minutes. For the types of signals used (BW = 100 Hz at 420 Hz) interference effects are responsible for the largest fluctuations in transmission loss. A model of refracted bottom reflected (RBR) propagation in an ideal ocean having horizontal bottom and two layers of negative constant-velocity gradient is used to examine multipath interference. The conditions for occurrence of fades are determined by varying model parameters. For pulse signals, the magnitude of transmission loss for a particular fade depends upon average characteristics of the sound-velocity structure, while the condition for occurrence of a fade is sensitive to small variations about these averages. Results from the model are used to interpret recent experimental results from project MIMI.

2. Rocket Measurements

A considerable amount of data on the electron density profile of Es layers has been accumulated starting with Seddon's experiments using his CW propagation technique. More recent data have been presented at this conference. The motivation for recent observations has resulted from the wind‐shear theory. It should be pointed out that the determination of winds by the vapor‐trail technique is at present limited to night and twilight observations. Now, Es in midlatitudes, where most of these observations are being taken, is primarily a daytime phenomenon, and the basic statistical information on Es has been derived from ground‐based radio techniques. Thus the rocket data is unfortunately biased by the nighttime observations.

Electromagnetic Modeling Studies of Lithospheric Propagation

The propagation of radio waves through the lithosphere between buried transmitting and receiving sites is a communication technique of great interest at the moment due to the physical hardness or invulnerability of such a communication system to nuclear blasts. Theoretical studies and laboratory experiments were performed to investigate the propagation of electromagnetic waves in a low-conductivity lithospheric duct (granitic basement layer). The real earth situation was approximated by a low conductivity layer with uniform properties located between two regions of high conductivity (the upper crust of the earth and the earth's mantle). Models were also used to investigate the effects of geological discontinuities and inhomogeneities on the electric field strength. Different models were constructed using scaling factors of the order of 105. Aluminum plates were used in the first model to represent the mantle and the upper crust. The low conductivity layer was modeled by a salt water solution. In a second model the upper crust was modeled by a carbon sheet instead of an aluminum sheet. The conductivity of the carbon sheet accurately modeled the value of conductivity assumed for the upper crust of the earth. Excellent agreement was obtained between theoretical and experimental results for CW propagation. This agreement demonstrates the validity of the laboratory modeling techniques and lends confidence to the experimental results obtained for the effects of inhomogeneities in the duct.

Experimental evidence for a low ion-transition altitude in the upper nighttime ionosphere

A Scout rocket fired from Wallops Island, Virginia, on March 29, 1962, at 02h 27m AM local time was instrumented for measuring ionospheric electron densities by means of a two-frequency CW propagation beacon and ion densities by means of a positive ion trap retarding potential experiment. Although vehicle performance was good (6000-km apogee) instrumentation difficulties encountered during fourth stage burning severely limited the scientific mission of the payload. However, electron density values were obtained at altitudes up to 750 km and an ion density profile up to 2800 km. The ion profile is the principle subject of this paper.

RF impedance probe measurements of ionospheric electron densities

Ionospheric electron density profiles obtained by the cw propagation technique and by rf impedance probe techniques

Rocket measurments of the upper lonosphere by a radio propagation technique

Presented at International Convention on Radio Techniques and Space Research, Oxford, England, July 5-9, 1961. High-altitude measurements of the electron density distribution were performed well above the F2 peak of the ionosphere by using the two-frequency rocket-borne propagation experiment of Seddon. An example of a measurement of the electron density profile up to 620 km by means of the CW propagatlon technique is presented, including the inferred scale height and temperature of the upper ionosphere. For the anticipated measurements up to oae earth radius by the CW propagation experiment, the variation of the ionosphere below a vehicle must always be considered in order to arrive at reliable local electron density data. A correction for this time variation can be made by using recorded information on the ordinary and extraordinary propagation modes at two harmonically related frequencies. this correction procedure is briefly outlined. (auth)

Rocket measurement of a daytime electron-density profile up to 620 kilometers

On April 27, 1961, a four-stage research rocket was fired from Wallops Island, Virginia, to measure the ionospheric electron-density distribution by means of Seddon's [1953] CW propagation technique. This experimental technique is based upon the measurement of the dispersive Doppler effect at two harmonically related frequencies—in this instance, f = 12.267 Mc/s and 6 f = 73.6 Mc/s. The actually measured quantities in the CW propagation experiment are the beat frequencies due to the difference between the received high- and low-frequency signals, the latter multiplied by the factor 6 at the ground, for the two magnetoionic components. These beat frequencies can be expressed by where c is the velocity of light in vacuo, n(h) is the refractive index at the high frequency (73.6 Mc/s), n(l) that at the low frequency (12.267 Mc/s), and the subscripts o and x refer to the ordinary and extraordinary modes, respectively, and ṙ is the rocket velocity in the ray direction.