Skip Zone Research Articles

Tests of Long-Range Ocean Data Telemetry Using Frequency-Agile HF Packet-switching Protocols

Experiences with two prototype telemetry systems developed for potential use with moored or drifting ocean instruments are described. The systems transfer data and commands between remote and base stations using direct high-frequency (HF) ionospheric radio propagation (shortwave radio) without intervening relay stations or satellites. The strategy exploits recent developments in digital packet-switching technology, which is readily available and can be inexpensively applied to oceanic problems. The potential advantages of packet methods over satellite methods include low cost, autonomy, two-way exchanges with the remote stations, and typical data rates of 1–10 bit s−1 averaged over several days. Coverage is effectively global but intermittent. Disadvantages mostly result from the interference and skip zones that characterize HF propagation. The tests described here took place near the time of a sunspot minimum; the utility of HF packet telemetry will be greatly increased when the sunspot cycle is near its maximum.

Field interference structure fluctuations near the boundary of the skip zone

Measurements of signal trajectory parameters near the boundary of the skip zone made with a highly directional antenna are used to construct a model profile of ionospheric F-layer electron concentration. Statistical characteristics of fluctuations in the interference structure of the field near the caustic are calculated with consideration of random inhomogeneities in electron concentration. The uncertainty in determination of the critical frequency of the ionospheric layer is estimated. Comparison of calculated and experimental data permits determination of ionospheric inhomogeneity parameters.

Determination of horizontal gradients in the ionosphere from measurements of SW parameters in the vicinity of the skip zone

This paper presents results of triangulation measurements of SW radio signals in the vicinity of the skip zone, allowing one to determine the volocity and direction of motion of the caustic over the earth's surface. The observed difference of the direction of motion from the transmitter azimuth is explained on the basis of a model of an inclined ionospheric layer. It is shown that data on the ange of arrival of radio waves can be used to determine the angle /PSI/ characterizing the magnitude of the horizontal gradient in the ionosphere.

Iris Neovascular Tufts-Reply

In Reply. —Dr Kottow has misdirected the emphasis of my clinical study on iris neovascular tufts from the observable pupillary lesions to a nonspecific angiogram finding he calls tuft-like dye leakage. He fails to relate this peripupillary leakage to iris tufts. While I defer to Dr Kottow's expertise on the incidence of iris circumscribed leakage spots in diverse ophthalmic conditions, I do not believe that glomerular microcapillary outgrowths (tufts) occur in these same conditions. Tufts are specific to certain patient groups. The neovascular coils I have described always cause both an early and late persistent focal smokestack extravasation of dye that is profuse. In eyes with tufts, the skip zones on the pupillary border between tufts either do not leak or they fluoresce diffuse Minor iris leakage may occur in some normal persons. However, the study quoted by Dr Kottow never found profuse fluorescein leakage in healthy eyes. Although some

Misadventures and Ramifications

The search continues for an ideal animal model of human idiopathic interstitial pulmonary fibrosis, which has included a broad spectrum of interstitial lung diseases depending at least partly upon stages in the development of the disease. Ten years ago, Carrington 1 Carrington CB Organizing interstitial pneumonia. Definition of the lesion and attempts to devise an experimental model. Yale J Biol Med. 1968; 40: 352 PubMed Google Scholar proposed the following as requirements for an animal model: anatomic changes characterized by a mixed cellular exudate in the interstitium, protein exudate with or without cells in the air spaces, proliferation of lining epithelium, gradual progression to fibrosis with continuing activity and eventual honeycombing, and diffuse distribution with the possibility of skip zones.

Ionospheric Irregularities and Propagation at Frequencies Above the ‘Classical’ M.U.F.

AbstractSweep frequency pulse measurements, carried out elsewhere, have shown that the maximum usable frequency via the F1 and F2 layers may be extended by an anomalous mode of propagation. The extension may be 10–15 per cent above the value calculated from classical theory and is observed only during daytime. It is absent at night. The phenomenon has been ascribed by some authors to scattering by irregularities present in the reflecting layers.Attention is drawn in this connection to the weak short-wave reflections observed at vertical incidence from heights below the normal E layer. These reflections are obtained during day-time and the reflecting layer merges during night-time with the meteoric E layer responsible for V.H.F. forward scatter propagation. It is argued that the extension of the ‘classical’ M.U.F. may be due to scattering produced by the irregularities responsible for the above weak reflections. It is possible that a part of the energy enters the skip zone due to forward scatter during the...

The reception of atmospherics at high frequencies

Observation of the amplitudes of atmospherics received in the high frequency band offers a possible method of assessing the distances of their sources. In a preliminary investigation of the potentialities of the technique, atmospherics have been recorded simultaneously at two frequencies in the upper part of the h.f. band. Reception would be expected at both frequencies from sources within ground-wave range (about 100 km). At rather greater distances, such that the receiving point is in the skip zone for both frequencies, no reception should be possible. There should then be a zone from which there should be reception on the lower frequency only, and at greater distances energy should again be received at both frequencies. The preliminary results conform generally with this pattern, and the boundaries of the various zones are in agreement with the ionospheric data derived from vertical incidence soundings. The sources of the atmospherics observed were located by use of the network of direction finders operated by the Meteorological Office. It is concluded that the technique forms a basis for routine estimation of the distances of storms and merits further examination to determine more precisely the accuracy which can be achieved and to overcome certain practical difficulties in the observations.

Measurements of the Delay and Direction of Arrival of Echoes from Near-by Short-Wave Transmitters

Observations on pulses radiated by a high-power beam transmitter operating in the short-wave range show that when the receiver is located within the skip zone, echoes are observed having delays of from 1 to 50 milliseconds. These echoes are the result of scattering and three different types may be recognized, each arising from a different source. Echoes of the multiple type were found to occur the most frequently and to have many of the characteristics of signals transmitted over long distances. Components were observed from regions up to 4000 miles distant. Direction-of-arrival measurements using steerable arrays operating on the musa principle indicate that these multiple echoes are scattered from regions along the transmitted beam. Vertical angle-of-arrival measurements using a musa receiving system indicate that the surface of the earth may be the source of scattering. Similarities between multiple echoes and southerly deviated waves from European transmitters have been found which indicate that the same phenomena may be responsible for both.

Analysis of the effect of scattering in radio transmission

It has been known for a long time that signals from strong commercial stations can be received in the skip zone. Early investigations with a short-wave Adcock direction-finder showed that the signals within the skip zone are in general characterized by absence of bearing. The present paper is a complete investigation into the causes of such effects. It was found that they are due to momentary irregularities and small clouds in the E region of the ionosphere. These irregularities produce scattered signals. The investigation, first carried out with the Adcock direction-finder, was later made more definite and accurate by using short impulses from highpower stations. The later investigations entirely confirmed the original deductions. The effects of such scattering on long-distance transmission, direction-finding, etc., are discussed. The phenomenon is considered to be a major factor in practically all transmissions.