Ionospheric radiowave propagation effects observed with a large aperture antenna array.

The wavefronts of high frequency (HF) radio waves received after reflection from the ionosphere exhibit both spatial non-linearities and temporal variations which limit the performance of large aperture receiving arrays. The first objective of this investigation was to measure the phase and amplitude of ionospherically propagated signals at several widely spaced antennas in order to relate these parameters to the reflection process. From the amplitudes and phases measured at pairs of spaced antennas, the direction of arrival (DOA) of the signal in both azimuth and elevation was determined. Furthermore. by combining the DOA and reflection height measurements the transmitter location can be estimated from a single receiving site. The second objective of this study was to investigate the ability of the system to determine DOA and transmitter locations correctly. Two seven element antenna arrays were employed with maximum apertures of 1526 m and 294 m respectively. The associated multi-channel receiving and data logging equipment is described together with a pulsed sounding system employed for mode identification. Signals received from several European transmitters exhibited widely differing behaviour and this was interpreted in terms of their modal content. For predominantly single moded signals the observations indicate that the diffracted components normally contribute less than 10% of the received power, moreover the DOA varies in both azimuth and elevation by approximately 1-2° over time periods of several minutes. The use of the smaller array for DF and SSL applications is discussed in detail. In particular, the performance of the system was severly affected by multi-moded propagation. Techniques were developed for recognising periods of single moded propagation, when accurate measurements are to be expected. Good position fixes were obtained when measurements were restricted to these periods provided accurate reflection height information was also available

The introduction of electronic assessment into engineering degree programmes: our experiences and lessons learnt

The Department of Engineering at the University of Leicester has successfully introduced automatically marked electronic assessment methods into the first and second years of its undergraduate degree programmes. This paper provides a description of the system as it is currently implemented and some of the problems encountered in developing the system. The outcomes from the electronic assessments are comparable to those from written examinations and also allow faster feedback

Measurements of the Doppler and multipath spread of HF signals received over a path oriented along the mid-latitude trough

The presence of the midlatitude trough can severely impact on HF radio systems since the electron density depletion within the trough reduces the maximum frequency which can be reflected by the ionosphere along the great circle path. Furthermore, the associated horizontal gradients in the electron density distribution frequently result in propagation well displaced from the great circle path. The signal characteristics associated with this type of propagation have been investigated for a 1400 km link oriented along the midlatitude trough between Sweden and the UK. As anticipated, the observed delay and Doppler spread characteristics are strongly dependent upon time of day and season since the trough is a nighttime feature which occurs predominantly during the winter. In particular, the Doppler spread is often large when great circle propagation has been suppressed and reflections are from the north of the great circle path (i.e., from the poleward wall of the trough or from gradients and/or irregularities associated with the auroral zone)

The effect of solar activity on the Doppler and multipath spread of HF signals received over paths oriented along the mid-latitude trough

Measurements of the Doppler and delay spread associated with HF signals propagating along an oblique (1440 km) path tangential to the midlatitude ionospheric trough are presented for sunspot maximum and minimum. During the day, Doppler spread is independent of solar activity, but for winter and equinoctial nights, it is very much higher at sunspot maximum. The delay spread is also generally higher at sunspot maximum for all seasons and times of day. For sunspot minimum, measurements from a second, longer path (1800 km) are also presented. The observed Doppler and delay spreads are similar for both paths. Finally, a novel method of more accurately deriving the delay spread defined by the International Telecommunication Union (i.e., the largest delay spread including all modes that have a peak power within a user-defined threshold of that of the strongest mode) from Voice of America coverage analysis program (VOACAP) predictions is presented. For the first time, the predicted values are compared with the measured delay spreads and, while there is generally good agreement at sunspot minimum, the agreement at sunspot maximum tends to be poor because the behavior of the high-order ionospheric modes (e.g., 3F2) is not well predicted by VOACAP

Signal strength measurements at frequencies of around 300 MHz over two sea paths in the British Channel Islands

In order to gather information about the occurrence of ducting/superrefraction and signal-fading effects at frequencies around 300 MHz with antenna heights appropriate to intership communications, an experimental investigation has been undertaken with a transmitter and two receivers deployed in the British Channel Islands. Signal strength measurements made over a period of 17 months for a path from Jersey to Guernsey from April 2001 to September 2002 and 8 months of data for a path from Jersey to Alderney from November 2001 to September 2002 have been analyzed. Comparisons have been made between the received signal characteristics and several meteorological parameters such as sea state, weather conditions, and season, and the statistics of the occurrence of enhancements in signal strength due to superrefraction and ducting are presented

Nighttime sporadic E measurements on an oblique path along the midlatitude trough.

Observations of nighttime sporadic E (Es) made within the HF band on a 1400 km path that lies approximately along the midlatitude trough are presented. Although the probability of occurrence of Es (PEs) is generally below that predicted by the ITU-R model, a significant increase in PEs is found when Kp ≥ 6. The signal parameters (azimuth, elevation, and Doppler spreads) also increase for high values of Kp. This behavior is consistent with the character of the propagation changing from midlatitude to auroral as the polar cap expands and the trough moves equatorward with increasing Kp

A comparison of observed and modelled deviations from the great circle direction for a 4490 km HF propagation path along the midlatitude ionospheric trough

Measurements of the direction-of-arrival of signals propagating on a long (4490 km) path along the midlatitude trough show that the azimuth can be deviated by up to 100° from the great circle bearing. In this paper an attempt has been made to model the shift in azimuth through a ray tracing simulation. Two possible mechanisms which lead to changes in azimuth have been investigated: (1) propagation along the density gradients which form the equatorward wall of the trough and (2) side scatter from regions of the sea well to the south of the trough. Of these two mechanisms, sea scatter gives results which are much closer to those observed

The effect of the mid-latitude trough on the direction of arrival and time-of-flight of HF radio signals

The effect of the mid-latitude trough on the direction of arrival and time-of-flight of HF radio signal

Transhorizon radiowave propagation due to evaporation ducting: the effect of tropospheric weather conditions on VHF and UHF radio paths over the sea

A detailed description of evaporation ducts and their relevance to radiowave propagation over the sea has been presented. The constantly changing weather conditions over the sea mean that marine and coastal environments, in particular, are prone to these unusual tropospheric phenomena that facilitate radio waves to have higher signal strengths and to travel longer distances than expected. Therefore, the influence of evaporation ducts on over-sea radiowave propagation needs to be thoroughly investigated. Research in this area will have implications for maritime communication systems used in coastal cellular telephone networks, commercial shipping, naval radar operations and sea-rescue

A comparison between the measured and predicted parameters of HF radio signals propagating along the midlatitude trough and within the polar cap

Prediction of the propagation characteristics of HF signals is an important aspect in the planning and operation of radio systems operating within that frequency band. Various computer codes have been developed by a number of organizations for this purpose. These prediction techniques assume that propagation is along the great circle path and ignore the effects of various large-scale ionospheric structures that can be present in the northerly ionosphere and result in propagation well displaced from the great circle path. This paper reports on a statistical analysis of observations of the direction of arrival and signal strength, and their comparison with VOACAP predictions for four paths, two roughly tangential to the midlatitude trough, one trans-auroral, and one entirely located within the polar cap