High-frequency over-the-horizon radars (OTHR) provide an economical means to detect and track noncooperative air targets over large expanses of land and sea. Due to the dynamic nature of the ionosphere, an OTHR requires a frequency management system where the operating frequencies and launch angles (azimuth and elevation) change periodically to maintain constant target detection. Accurate electron density models are required for the purpose of improving OTHR performance and for system planning and design. This is particularly true in the polar regions where large-scale electron density structures taking the form of patches and arcs of enhanced electron density are the common features of the F-region ionosphere. These structures form tilted reflection surfaces for HF radiowaves and result in propagation well displaced from the great circle direction. The effects of patches on the ray paths have been simulated, and their impact on OTHR frequency management is illustrated.

High frequency (HF) radio wave propagation is sensitive to ionospheric disturbances caused by space weather. Changes in propagation conditions can be directly measured with an HF transmitter/receiver link. This paper presents data from one such link, consisting of a transmitter located in Ottawa, ON, Canada, and a directional receiver located in Alert, NU, Canada. The transmitter emits signals at 6 distinct frequencies between 5.4 and 14.4 MHz on a set schedule that are detected and processed by the receiver. Based on 644 days of archived data, a statistical analysis characterizing HF radio wave propagation conditions over the course of the day and over the year are presented. HF propagation is found to follow the regular diurnal variation in ionospheric density, modulated by seasonal trends. A new method for distinguishing space weather effects from diurnal changes to HF propagation is presented, which utilizes quiet day baselines, similar to riometer quiet day curves. This method allows for easy analysis of the impacts of space weather on HF radio propagation between two points. This method is applied to an auroral absorption event that occurred on 06 June 2016, and is used to successfully distinguish auroral absorption related dropouts from dropouts related to daily ionospheric variation.

High frequency Over-the-Horizon Radar (OTHR) provides an economical means to track non-cooperative air targets over large expanses of land and ocean. Because of dynamic ionospheric conditions in polar regions, any OTHR would necessitate a system where the operating frequencies and elevation angles change periodically to maintain constant detection of targets downrange. In this regard, an accurate electron density model is necessary for the purpose of improving operational OTHR and OTHR planning/design. Observations over recent years have established that large-scale electron density structures are a common feature of the polar cap F-region ionosphere. These structures take the form of convecting patches and arcs of enhanced electron density which form tilted reflection surfaces for HF radiowaves, allowing off-great circle propagation paths to be established. Numerical ray tracing has been employed to simulate the effects of these structures on the ray paths of the radiowaves. The effect of frequency monitoring system for OTHR due to the presence of patches of enhanced electron density within the polar cap ionosphere has been studied in this paper.

With the widespread usage of wireless sensor networks for many monitoring and control applications, a significant number of researchers proposed different types of structures for the WSNs to control the large volume of data that flows through the networks and save sensor node energy to increase the network lifetime. One which can achieve all these requirements is a hierarchical structure which is also called clustering. All previous clustering algorithms that have been proposed use the IEEE 802.15.4 standard which comes with a technique to access the shared communication medium by using the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). On the other hand, many researchers demonstrated that the CSMA/CA does not work well with an increasing number of nodes in the network. Therefore, in this paper a novel distributed method has been presented to classify the individual nodes in clusters without depending completely on CSMA/CA technique. The proposed method classifies the network nodes based on a knowledge database installed in advance inside each node in the network. Hardware nodes have been used to evaluate the performance of the new method. The results show that the proposed clustering algorithm achieved better performance compared with the LEACH algorithm in terms of clustering scalability, but it takes longer time than the LEACH to classify the cluster members.

Neighbour discovery is essential to achieve high reliability and connectivity between network nodes. It provides each node with a list of information about its neighbours and hence the best route to the base station. The Received Signal Strength Indicator (RSSI) is commonly used to evaluate the link quality between pairs of nodes and to estimate the neighbour location. However, very few experimental tests on the effect of the RSSI on neighbour discovery have been reported in the literature. Thus, the main goals of this paper are investigating how to improve the link quality between the network nodes based on the RSSI value and calculating the required time for each node to discover their neighbours experimentally changes with the number of nodes using Waspmote testbed equipped with XBee-Pro S1 802.15.4 radio module. The results indicate that neighbour discovery is reliable for RSSI above -80 dBm, while, the time required to discover neighbours increases with more neighbours, for example, discovering 1 to 5 nodes reliably requires 3 s, whereas 4 s is required to discover 6 to 10 nodes.

The propagation of 5.9‐GHz radio signals and performance of an 802.11p network were measured at three road junctions each having a different density of buildings. The maximum range for which acceptable performance (defined as where the packet delivery ratio was greater than 90%) was dependent on the junction but lies in the range of 45–70 m. While reflections from transient vehicles were often found to have a small positive impact on network performance, this could not be relied upon to provide a reliable improvement in communications. The received signal strength was dependent on the junction type with the strong reflections from buildings located on the opposite side of a T‐junction leading to higher signal strength. Finally, an empirical relationship between the packet delivery ratio and the received signal strength has been established that will allow modelers to link signal strength to network performance for field conditions.

Vehicle to vehicle (V2V) communication plays a key role in the safety of autonomous vehicles. Therefore, practical tests in many environments are essential. Due to the relatively low altitude of antennas on connected vehicles, any change in the environment is expected to affect the performance of wireless systems from vehicle to vehicle (V2V). This study presents measurements of the impact of vehicle size on the Doppler spreads in V2V radio channel at 5.9 GHz. The measurements were conducted at an irregular three-way intersection where one side includes buildings and the other two sides consist of large open areas surrounded by a row of trees and a metal fence. Vehicles were classified into three groups: small passenger cars (Group A), medium vehicles (Group B), and larger vehicles (Group C). The results demonstrate that the size of the vehicle and the distance between transmitter and receiver are important factors in determining the impact on the V2V link.

Spacecraft communication systems operating at X band are strongly affected when the propagation path passes close to the Sun (which can be characterized by the Sun‐Earth‐Probe, SEP, angle). In this paper, a channel model that can generate a time series of signal amplitude and phase, and hence the signal in‐phase and quadrature values, is presented. For X band, the channel model reproduces the observations well for SEP > 0.8°. While the model has not been experimentally validated at SEP angles below 0.8°, the behavior is consistent with measurements reported in the literature and is therefore likely to be suitable for use, with caution, in that region too.

Some results so far achieved in the framework of the HELIOS (Highly rEliable Links during sOlar conjunctions) Project, founded by the European Space Agency (ESA), are presented. The purpose of the project is the definition of a TT&C communication subsystem architecture (including both ground and space segments, as well as operational methods) being robust to impairments due to superior solar conjunction, especially when the Sun-Earth-Probe angle is below 5 degrees.

HF communications can be difficult in the polar regions since they are strongly influenced by space weather events. Airline communications within the polar regions rely on HF communications and improved nowcasting and forecasting techniques in support of this are now required. Previous work has demonstrated that ray tracing through a realistic, historical ionosphere provides signal coverage in good agreement with measurements. This paper presents an approach to providing a real-time ionospheric model by assimilating TEC measurements and validates it against observations from ionosondes.

Commercial airlines began operations over polar routes in 1999 with a small number of proving flights. By 2014 the number had increased to in excess of 12,000 flights per year, and further increases are expected. For safe operations, the aircraft have to be able to communicate with air traffic control centres at all times. This is achieved by VHF links whilst within range of the widespread network of ground stations, and by HF radio in remote areas such as the Polar regions, the North Atlantic and Pacific where VHF ground infrastructure does not exist. Furthermore, the Russian side of the pole only has HF capability. This has created a demand for improved HF nowcasting and forecasting procedures to support the polar operations, which are the subject of this paper.

The authors have previously reported on the development of an HF propagation model for signals reflected from the northerly regions of the ionosphere, and its validation by comparison with measurements made over a number of paths within the polar cap, crossing the auroral oval, and along the mid-latitude trough. The model incorporates various features (e.g. convecting patches of enhanced plasma density) of the polar ionosphere that are, in particular, responsible for off-great circle propagation and can lead to propagation at times and frequencies not expected from on-great circle propagation alone. Currently, the model drivers include ionosonde measurements and geomagnetic data from a period of several days spanning the time of interest. We have previously only examined the propagation effects on a historical basis, and have achieved good agreement between measurements and simulations.

We have previously reported on a significant new multi-national project to provide improved predictions and forecasts of HF radio propagation for commercial aircraft operating on trans-polar routes. In these regions, there are limited or no VHF air-traffic control facilities and geostationary satellites are below the horizon. Therefore HF radio remains important in maintaining communications with the aircraft at all times. Space weather disturbances can have a range of effects on the ionosphere and hence HF radio propagation - particularly in the polar cap. While severe space weather effects can lead to a total loss of communications (i.e. radio blackout), less intense events can still cause significant disruption. In this paper we will present the effect of a series of M and X class solar flares and a relatively weak CME on HF radio performance from 6 to 13 January 2014. This is an interesting interval from the point of view of HF radio propagation because while the solar effects on the ionosphere are significant, except for an interval of approximately 12 hours duration, they are not so intense as to produce a complete radio blackout on all paths. Observations of the signal-to-noise ratio, direction of arrival, and time of flight of HF radio signals on six paths (one entirely within the polar cap, three trans-auroral, and two sub-auroral) will be presented together with riometer measurements of the ionospheric absorption. Global maps of D-region absorption (D-region absorption prediction, DRAP) inferred from satellite measurements of the solar wind parameters will be compared with the HF and riometer observations. In addition, a ray-tracing model using a realistic background ionosphere and including localised features found in the ionospheric polar cap (e.g. polar patches and arcs) will be used to model the expected and observed HF radio propagation characteristics.

The morphology of the auroral, sub-auroral and mid-latitude trough region of the ionosphere is strongly dependent on the interplanetary magnetic field and the level of geomagnetic activity. Changes in the morphology impact on the characteristics of HF signals propagating through these regions of the ionosphere. In order to develop a better understanding of these effects, a number of experiments have recently been undertaken in which the time of flight and direction of arrival of HF signals have been measured over several paths aligned along the mid-latitude trough. In addition, observations made by the DEMETER satellite of the mid-latitude trough electron density structure, dynamics and wave activity were used in order to investigate the effect of the fine structure of the ionosphere on HF signals. For two types of relatively common night time HF time of flight and azimuth of arrival behaviour (referred to here and elsewhere as ‘Type 1’ and ‘Type 2’ propagation), the signal behaviour is consistent with scatter from irregularities in the auroral region in the one case, and from irregularities present on the floor of the trough in the other.

A dual-frequency GPS Ionospheric Scintillation and TEC Monitor (GISTM) receiver, GSV4004B, has been installed at Alert, Canada (82.48° N, 297.75° E, corresponding to 86.95° N, 155.77° E geomagnetic coordinate) since May 2008 to study the ionospheric variability at a high-latitude location. This GISTM receiver is able to measure the Total Electron Content (TEC), and both the amplitude and phase scintillations. This study focuses on the phase scintillation observed over Alert for 26-month period from June 2008 until July 2010 where it was mostly during the minimum state of solar activity. It is found that at least 93% of the phase scintillations, sf of the observed data were in the range of 0-0.1 rad. The remaining 7% varied from 0.1 to 0.7 rad, where it mostly never exceeded 0.2 rad. σφ <; 0.15 is considered as insignificant. Phase scintillations were found to be increased with the increasing of solar activity where most of the cases of σφ > 0.15 rad were detected in 2010.

Observations of the effect of polar patches (fast moving regions of strong ionization) on the Doppler and time of flight behavior of HF signals propagating in the polar cap ionosphere are presented. The observed patch-induced characteristics of the HF propagation have successfully been reproduced using a simple model of the movement of the patches, thereby allowing the trajectory and velocity of a patch to be estimated. The increased ionization in the patch allows higher frequencies to reflect than those that would normally be expected. A comparison of the fluctuations in the measured TEC data (a proxy for the presence of patches) with the detection of HF signals has provided some evidence for this relationship.

A dual-frequency GPS Ionospheric Scintillation and TEC Monitor (GISTM) receiver, GSV4004B, has been installed at Alert, Canada (82.48° N, 297.75° E, corresponding to 86.95° N, 155.77° E geomagnetic coordinate) since May 2008 to study the ionospheric variability at a high-latitude location. This GISTM receiver is able to measure the Total Electron Content (TEC), and both the amplitude and phase scintillations. This study focuses on the polar patches occurrences observed over Alert for 26-month period from June 2008 until July 2010 where it was mostly during the minimum state of solar activity. It is found that the difference in the occurrences of small patches between months of the same year was about 1-2% (for low intensity patches) and 1-10% (moderate-high intensity). This difference increased to 1-4% (low intensity) and 1-12% (moderate-high intensity) for large patches. UT dependence can be seen where ~80 large and ~300 small patches were identified ~1000 to 2000 UT from June to October in both 2008 and 2009, and they doubled in 2010.

The northerly ionosphere is a dynamic propagation medium that causes HF signals reflected from this region to exhibit delay, Doppler shifts and Doppler spreads that significantly exceed those observed over mid-latitude paths. These Doppler effects are due to large-scale motion of the ionosphere and/ or the turbulent motion of plasma irregularities that are a common feature of the auroral and polar cap ionospheres. From the perspective of communications systems, such large delay and Doppler spreads can result in a significant degradation in system performance (e. g. in achievable data throughput).

Information on delay and Doppler spreading is of importance to designers and operators of digital communication systems within the HF band since modem performance rapidly deteriorates when delay and Doppler spreads exceed system dependent thresholds. The northerly ionosphere is a particularly challenging region in this regard as delay and Doppler spreads often significantly exceed those for similar length paths at midlatitudes. This paper presents results from an extensive set of measurements undertaken over three northerly paths: two subauroral paths aligned along the midlatitude trough (Uppsala to Bruntingthorpe and Nurmijärvi to Bruntingthorpe) and one path (Qaanaaq to Ny‐Ålesund) contained entirely within the polar cap. Measurements span the period from the 2009 solar minimum to July 2012, close to solar maximum and 2001, close to the previous solar maximum.