VHF Propagation Research Articles

Lisa Zurk's contributions to MIT Lincoln Laboratory

Lisa Zurk made numerous technical contributions making a substantive impact on the ISR community at a national level. This talk will highlight some of her many technical contributions, ranging from early work in UHF and VHF propagation in rough, overland, terrain, to her pioneering work in adaptive signal processing applied to underwater acoustics. This talk will also recognize her outstanding leadership, always pushing the boundaries as a researcher and as a leader, laying the groundwork for future ISR decision support and architectures. The talk will close with a summary of her role as a mentor and colleague who always challenged those around her to bring out their best. DISTRIBUTION STATEMENT A. Approved for public release. Distribution is unlimited. [This material is based upon work supported by the Department of the Air Force under Air Force Contract No. FA8702-15-D-0001. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Department of the Air Force.]

Performance Analysis of Propagation in VHF Military Tactical Communication System

The main challenge of military tactical communication systems is the accessibility of relevant information on the particular operating environment required for the determination of the waveform's ideal use. The existing propagation model focuses mainly on broadcasting and commercial wireless communication with a highs transceiver antenna that is not suitable for numerous military tactical communication systems. This paper presents a study of the path loss model related to radio propagation profile within the suburban in Kuala Lumpur. The experimental path loss modeling for VHF propagation was collected from various suburban settings for the 30-88 MHz frequency range. This experiment was highly affected by ecological factors and existing wave propagation effects such as reflection, diffraction, scattering, and Doppler effect. Radio propagation performance is evaluated by collecting received power at the allocated substation and comparing it against existing propagation models. The existing propagation model also will be tuned close to the measurement value by identifying the best path loss exponent to perform a suitable model for a suburban area. Theoretical assessments and analysis of the initial measurement stage for radio propagation show the extensive contribution of radio field from potential obstacles at lower VHF frequencies for both short and medium ranges around there. The explanation indicates the standard radio propagation prediction models that are generally reasonable for the suburban area. From the general error analysis, it is seen that, the performance of the LDPL with adjusting path loss exponent is the suitable model since it has least value of error metrics.

A Statistical Study of Anomalous VHF Propagation Due to the Sporadic‐E Layer in the Air‐Navigation Band

AbstractThe sporadic‐E (Es) layer is an ionospheric layer which appears occasionally near 100‐km heights with extremely high electron density. The Es layer may reflect very high‐frequency radio signals when the incident angle is shallow (Es layer anomalous propagation [EsAP]). It is known that radio signals with frequencies above 100 MHz sometimes reach distant locations, more than 600 km apart from the transmitters, due to EsAP. Since air‐navigation radio channels are allocated on frequencies between 108 and 118 MHz, EsAP may interfere with directly propagating wave (ground wave) signals. However, as the occurrence and strength of EsAP on these frequencies have not been studied well, it is difficult to assess the impact of EsAP on the air navigation. In this paper, we report the statistics of the occurrence and strength of EsAP based on a 3‐year continuous monitoring of very high‐frequency air‐navigation radio signal strength on the ground. The statistics show that strong EsAPs occur frequently in summer. The occurrence pattern of EsAP is generally consistent with ionosonde observations of Es layer: Most Es layers appear during summer, primary peak in daytime, with a second peak in the evening. During an extreme EsAP event, in a particular channel, an EsAP signal was superposed on a ground wave signal which resulted in a fluctuation of ground wave signal by more than ±10 dB. Our statistical results suggest that the Es layer has a potential impact on air‐navigation applications.

Maritime VHF-DSC monitoring with low cost SDR receiver

The main part of the alerting system of the GMDSS is done by automating alerting using Digital Selective Calling on VHF or MF/HF bands. Most of the small craft used to transport migrants use VHF-DSC to alert European's agencies of distress situations, which, in the all-most situations are fake. Because the maximum range of VHF propagation is around 50NM and it's related to the antennas' heights, it is necessary to increase the coverage area by installing many DSC receivers. The paper contains an example of a VHF-DSC receiver using a 20 Euros dongle and free GNU radio software. Instead to buy a maritime radio coast station the authorities can implement this system using low-cost dongle and small computers like Raspberry Pi, which also permit internet connection and IoT.

Orographic role in anomalous VHF propagation on the background of impending earthquakes (EQ)

<p>The role of orographic factors in modifying atmospheric dynamics on the background of an impending earthquake (EQ) that might lead to propagation of a signal beyond the Line Of Sight (LOS) is examined in the paper. The analysis is based on the data of anomalous reception of VHF FM-signal at Gauhati University (GU, 26.15°N, 91.66°E), a station located in the sub Himalayan terrain. The signal reception records show that their anomalous appearance are associated closely with earthquake events of M >5.0, a phenomenon identified as EQ precursor. To identify sources of such reception, the paper presents some analytical approaches involving the terrain factors, impending EQ and atmospheric variabilities. Finally, the contribution of location/topography of the transmitter, receiver and epicenter in modifying the EQ induced atmospheric factors, is brought in to the ambit of analysis for explaining the observed over horizon VHF signal propagation. A special reference is made on the EQ time drop in temperature, increase in humidity and hence modification in Radio Refractive Index (RRI) gradient, in favoring such reception. A model in support of the observation on EQ time lithosphere-atmosphere coupling relevant to widening of radio horizon limit is also proposed.</p>

First results of mapping sporadic E with a passive observing network

Sporadic E (Es) can have dramatic effects on communications in the HF and low VHF range, producing over‐the‐horizon propagation for signals normally restricted to line‐of‐sight, and sometimes blocking F region propagation of signals in the lower HF range. Measuring the E region winds believed to produce Es is difficult, and no practical means of predicting Es occurrence currently exists other than statistical models. We describe a low‐cost observing network based on software‐controlled receivers that continuously watches for Es in near‐real time using oblique HF propagation from existing transmitters. Results from an 11‐day pilot campaign in July 2008 demonstrated that even a limited number of receivers in the network can readily determine the presence and extent of Es patches. These observations indicate that Es often develops quickly over regions of several hundred kilometers rather than gradually drifting across an area. These widespread Es “blooms” have been observed near winter solstice and occasionally at other times of the year; their lifetime depends on the season but can be several hours during the summer. The current network allows the extent of Es in portions of North America to be evaluated: the geographical distribution of Es and bounds on the density of the layer are inferred from its effects on the ionospheric maximum usable frequency (MUF). This study demonstrates quantitatively that Es mapping can provide information about Es layer geographical growth and decay. The observed sudden widespread Es blooms are space weather events that can have significant impact on HF/lower VHF communications and propagation model predictions.

Estimating Reception Coverage Characteristics of AIS

The Automatic Identification System (AIS) is now well established and widely used in commercial shipping. The system originated from a safety mandate but AIS messages have also been shown to be useful from a security situational awareness perspective. In terms of coastal security, AIS messages are often received by land-based receivers positioned along a nation's coastline. The operational range of the receivers is controlled by complex variable VHF propagation characteristics, power of the transmitter, etc. However, certain characteristics of the reception coverage area can be determined from the AIS message receptions themselves. This paper presents an algorithm to compute coverage characteristics using AIS messages. The algorithm is applied to synthetic data with known coverage characteristics, and also real AIS data obtained from the Maritime Safety and Security Information System. Results from the Norwegian, North and Baltic Seas show how the coverage estimate is influenced by the coverage edge, lack of vessel activity, and diversity in the source data.

Precursors of earthquakes in the line-of-sight propagation on VHF band

This paper was intended to find out any relation between anomalous line-of-sight propagations on VHF band and occurrences of earthquakes near the VHF propagation path. The TV broadcasting waves on the VHF band were measured continuously for 504 days, in which we assembled an automatic measurement system. The waves propagated on the line-of-sight from Tokyo Tower to Gunma Univ. in Kiryu were measured on VHF band. In order to distinguish anomalous propagation data from normal data, a statistical process is adopted in such a way that six times deviation from the mean value was adopted as a criterion for distinguishing between the anomalous or normal propagation data. In a time interval between the appearance of anomalous propagation and the occurrence of earthquake was shorter than 48 hours (2 days), the anomalous propagation was additionally recognized as being associated with the earthquake. During the observation period of 504 days, only four anomalous propagations happened and we observed three earthquakes associated with these anomalous propagations. As a result, it is found that anomalous line-of-sight propagations on VHF band might be associated with the earthquakes near the VHF propagation path.

ELECTROMAGNETIC PULSE PROPAGATION OVER NONUNIFORM EARTH SURFACE: NUMERICAL SIMULATION

Parabolic equation method proposed by Leontowich and Fock [1,2] is an efficient simulation approach to VHF propagation over the earth surface. Deep physical analysis and advanced mathematical methods [3,4] turned Leontovich’s PE into a universal tool of diffraction theory. Its applications go far beyond the initial problem circle – e. g. [5-8]. The key role in this development played the decisive turn to straightforward numerical techniques pioneered by Malyuzhinets and Tappert [9,10]. In radio wave propagation, PE was used first to derive explicit analytical formulae for the EM field strength in model environments. A simplification has been reached by introducing the impedance boundary condition (BC) [11]. Taking into account tropospheric refraction ducts required the use of sophisticated asymptotic methods [12]. Further development (almost exclusively towards numerical implementation) was aimed at refined PE modifications [13-15], account for irregular terrain [16], introducing artificial transparent boundaries [17,18] and nonlocal BC to describe rough interfaces [19]. A non-stationary PE counterpart and a finite-difference (FD) scheme for its solution have been proposed by Claerbout and applied to seismic problems [13]. Afterwards, this “time-domain parabolic equation” (TDPE) was used to calculate acoustic propagation in ocean [20]. At the same time, little attempts of using TDPE to simulate EM pulse propagation in realistic environments are known. In this paper, we consider computational aspects of EM pulse propagation along the nonuniform earth surface. For ultrawide-band pulses without carrier, TDPE results directly from the exact wave equation written in a narrow vicinity of the wave front. To solve it by finite differences, we introduce a time-domain analog of the impedance BC and a nonlocal BC of transparency reducing the open computational domain to a strip of finite width. Numerical examples demonstrate the influence of soil conductivity on the received pulse waveform which can be used in remote sensing.

Investigation of Rainfall Effect on Forested Radio Wave Propagation

In this paper, the rainfall is found to have distinct effect on various parts of the propagating components for the VHF radio wave propagation in a tropical forest. By the studies of the measured power delay profiles under different weather conditions, it is observed that the lateral wave is the dominant mode of the VHF propagation and is not affected by the presence of rain as it is similar to free space propagation over the tree tops, while the significant multipath components which propagate through the forest medium is heavily influenced. This is again verified through the estimated delay spread statistics.

VHF radar sea scatter and propagation at grazing angles less than 1°

Measurements of low grazing angle radar sea backscatter and multipath in the frequency range from 30 to 300 MHz, along with the supporting theoretical modeling, have been lacking because of the practical difficulties associated with fielding radars in this band. An ultrawideband (UWB) radar that radiates waveforms encompassing this band has recently been installed at a seaside location near the Naval Research and Development facility, Pt. Loma, San Diego, California. Propagation path loss measurements were performed at 40, 90, 140, and 190 MHz. Broadband clutter measurements were performed over the frequency band 200 to 300 MHz. The theoretical model is based on an integral equation solution for the surface fields. The surface fields are obtained using a new mathematical derivation of the integral equation for the surface fields (Ott, 1992). This new integral equation extends the frequency range of the former algorithm, WAGNER, to the VHF band. The surface fields are then integrated to obtain either the forward scatter (multipath) or the backscatter (clutter).

An investigation of the feasibility of utilizing GPS/TEC "signatures" for near-real time forecasting of auroral-E propagation at high-HF and low-VHF frequencies

VHF propagation on /spl les/5300 km polar paths has been documented during the maximum phase of sunspot cycle 19. Mode analysis on these polar paths has shown that auroral-E ionization (AEI) supported some modes. Electron densities and plasma frequencies which could support AEI modes at frequencies up to 46 MHz have also been measured. Long distance VHF propagation from AEI has also been reported by radio amateurs using frequencies in the 2 m band in a "sidescatter mode". An AEI experiment has been in operation between Wales (Alaska) and Fairbanks (Alaska) where a 75-watt CW transmitter located in Wales transmits the Morse letter "R" every 5 s, and a receiver in Fairbanks detects the 25.5 MHz signal whenever AEI is present near the midpoint of the 960 km path. Another experiment is underway using a GPS total electron content (TEC) receiving station at Fairbanks also using AEI data from the Wales-Fairbanks experiment. From this, the authors examine 58 passes of GPS satellites whose E-layer penetration points lie close to the midpoint of the Wales-Fairbanks path and find that there is a threshold value of TEC above which auroral-E (AE) propagation occurs. They also find that AEI propagation is strongly correlated with large- and medium-scale E-region structures in TEC determined by the GPS measurements. When TEC ionospheric structures are not present near the Wales-Fairbanks path midpoint, no AEI signal is received. The authors tentatively conclude that the occurrence of these specific TEC signatures may be utilized as predictors of AEI forward propagation on paths within and parallel to the auroral oval.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

VHF transequatorial propagation via three dimensional waveguides

The three dimensional model of VHF propagation along ionospheric waveguides or ducts developed by Platt I. G. and Dyson P. L., 1989 [ J. atmos. terr. Phys. 51, 897–910] has been used to study transequatorial propagation (TEP) along equatorial bubble irregularities aligned along the earth's magnetic field. Bubbles with circular and elliptical cross-sections and different apex heights have been used to determine the region illuminated in the conjugate hemisphere by TEP and the power of the signals. The results show that the three dimensional waveguide model can explain many features of TEP, including those not well explained by the simpler waveguide models used previously. The calculations therefore confirm that TEP can be caused by guided propagation along equatorial bubble irregularities aligned along the magnetic field. The results show that a bubble located on the transmitter's longitude can illuminate a wide area of the conjugate hemisphere. A well defined maximum central power region occurs which in most cases is centred at a latitude slightly lower than the conjugate of the transmitter. Bubbles with circular cross-section can illuminate a region more than ten degrees in latitudinal extent and five degrees in longitude. Bubbles which are vertically elongated at their apex illuminate a much narrower region in longitude. TEP between points with relatively large separation in longitude can result from either leaky rays or ducts tilted out of the magnetic meridian plane. This explains some observations of TEP previously thought to be inconsistent with waveguide propagation. The variation of power loss with frequency depends on a number of factors such as bubble shape and height. Co-ordinated experiments involving a transmitter and several receivers in the conjugate hemisphere should enable properties of bubbles to be determined.

VHF propagation modes within ionospheric waveguides

A three dimensional waveguide model of VHF ducting has been developed. The waveguide is aligned along the Earth's magnetic field and has an elliptical cross-section which, based on observations of ionospheric bubble irregularities, is taken to have axes of 10–100 km. Rays propagating to the conjugate hemisphere via the duct undergo relatively few reflections at the duct walls (typically 3–20). Consequently the waveguides are not fully excited and significant power is transmitted by modes which are not fully trapped. A complete analysis of the propagation characteristics of such ducts requires the consideration of trapped and leaky modes, focusing and tunnelling. The cross-section of the guide is defined by specifying the vertical and East-West horizontal axes at the apex of the guide (magnetic equator). Comparison of cases with the same vertical axis shows that, when the vertical axis is larger than the East-West axis, the curvature on the upper surface of the guide is consequently increased and tunnelling becomes more important. On the other hand, when the vertical axis is the smaller, focusing is greater and leaky rays become increasingly important. In some models the region of highest power in the conjugate hemisphere is due almost entirely to rays which have suffered some leakage. Obviously non-trapped modes need to be considered when attempting to explain phenomena such as trans-equatorial propagation at VHF.

VHF propagation characterization during atmospheric disturbances

Abstract The propagation of a television (TV) signal is usually perturbed during periods of atmospheric disturbance such as in thunderstorms. The region under investigation here is situated in the coastal area of the Bay of Bengal and has experienced frequent thunderstorm activities. An investigation to study their effects on VHF signal propagation transmitted from Satkhira in Bangladesh and received at the Indian Statistical Institute, Calcutta, has been undertaken. The depths of the atmospheric disturbances within a 1 km range have been recorded and measured by acoustic sounder (sodar) operated continuously at the receiving station. The inset of the storm is marked by the enhanced signal level with deep fluctuations characteristic during the periods of thunderstorms.

A Waveguide Model for the Evening Type Transequatorial VHF Propagation: Comparison with Experiment for the Euro-African Sector

ABSTRACT A waveguide model is proposed for the evening type transequatorial propagation mode. Field aligned irregularities observed in equatorial ionosphere are assumed to form dielectric waveguides. A local Champan type layer electron density distribution is taken inside the guide. The transequatorial wave propagation is treated by using this model and the propagation theory of dielectric waveguides. The total attenuation is computed between two points located in opposite hemispheres. The analytic form of the results permits easy interpretation of the propagation characteristics. A good agreement is observed between the experimental results and the theoretical calculations for the total attenuation. Comparisons have also been done for the propagation delay and the incoming elevation angle at the reception point.

The Effect of Tree Cover on Air-Ground, VHF Propagation Path Loss

A theory accounting for attenuation caused by tree cover for line-of-sight propagation paths at VHF using vertical polarization is presented. An approach to implementing this theory using a terrainsensitive GTD propagation model is also given. Comparisons of model predictions and airborne measurements show that the approach is valid.

VHF propagation over hilly, forested terrain

Propagation measurements at low altitudes over hilly, forested terrain made with the objective of developing a computer-based propagation model capable of predicting path loss over similar terrain given the terrain profile between transmitter and receiver are reported. The measurements were made at a frequency of 110.6 MHz with the very high frequency omnidirectional range (VOR) station at Gardner, MA, as a transmitter. The received signals were measured at distances between 7 and 15 km by making vertical descents with a helicopter from altitudes of roughly 600 m down to 10 m above ground. Negligible specular ground reflection was observed from this terrain, and no reflection is included in the model. We found good agreement between the measurements and a multiple-diffraction model based on an extension of the Deygout approximation. Use of two knife edges was sufficient to characterize the terrain diffraction. Predictions of the Longley-Rice computer model are included for comparison.

Results of a VHF propagation study

Beyond-line-of-sight ground-wave propagation measurements were made near 30 MHz, 54 MHz, and 148 MHz over two different propagation paths 42 km and 51 km in length. Data were collected over a period of six weeks using a vehicle-mounted vertically polarized transmitting antenna (whip) and fixed position Yagi receiving antennas. Results presented here include <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(S + N)/N</tex> ratios observed at the receiver for a 1-W transmitter, and a comparison between measured path loss and path loss predictions made by the Electromagnetic Compatibility Analysis Center (ECAC) using diffraction model techniques. The measured <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(S + N)/N</tex> ratio (against a 1-W transmitter) was between 5 and 13 dB for the two propagation paths and three test frequencies. Measured path-loss data demonstrated the usefulness of the ECAC model in providing path-loss predictions. ECAC predictions best agreed at the lowest test frequency.

Transequatorial VHF propagation through equatorial plasma bubbles

The support by the nighttime equatorial ionosphere of radio wave propagation at frequencies exceeding 50 MHz is explained in terms of guiding along field‐aligned tubes of depleted ionization resulting from the postsunset creation of equatorial plasma bubbles. By using the observed and theoretical properties of these bubbles, and considering in a numerical fashion a large ensemble of model bubbles, it is possible to reproduce many of the observed characteristics of transequatorial propagation at VHF.