Maximum Usable Frequency Research Articles

A new campaign for oblique-incidence ionospheric sounding over Europe and its data application

New systematic oblique ionospheric radio sounding measurements over Central Europe, concerning the radio links between Inskip (UK, 53.5°N, 2.5°W) and Rome (Italy, 41.8°N, 12.5°E) and between Inskip and Chania (Greece, 35.7°N, 24.0°E), have been performed since November 2003. Different long-term (i.e. monthly median) ionospheric predictions and nowcasting techniques have been applied and compared with the oblique-incidence radio sounding measurements. The MUF (basic maximum usable frequency) measurements observed during the early part of the experiment have been used to compare the performances of different methods. The preliminary analysis has shown good performances for the long-term prediction models, in particular in winter months for ICEPAC (ionospheric communications enhanced profile analysis and circuit) and in equinox/summer months for ASAPS (advanced stand-alone prediction system) and SIRM&LKW (simplified ionospheric regional model & lockwood). The nowcasting methods SIRMUP&LKW (SIRM updating method & lockwood) and ISWIRM (instantaneous space weighted ionospheric regional model) reveal good results for moderate and disturbed geomagnetic conditions when compared with the long-term prediction methods.

Specific behavior of the radioemission wave field near the maximum usable frequency

The character of a change in the wave field in the frequency range near the maximum usable frequency is analyzed for the model of a simple ionospheric layer. The estimates of the characteristic scale of diffraction damping of the wave field in the frequency range, which are in rather good agreement with the experimental data of LFM or chirp sounding of the ionosphere on two mutually perpendicular paths, have been obtained.

Validation of the Utah State University Global Assimilation of Ionospheric Measurements (GAIM) model predictions of the maximum usable frequency for a 3000 km circuit

The accuracy of the Utah State University Global Assimilation of Ionospheric Measurements (GAIM) monthly median values of the maximum usable frequency for an HF communications circuit has been determined for 3000 km circuits centered on Australian ionosonde locations for March and April 2004. The accuracy is discussed in terms of foF2, M(3000)F2, and the product MUF(3000)F2 = foF2 * M(3000)F2. Ground truth is provided by hourly values of foF2 and M(3000)F2 that have been hand‐scaled from ionograms. In general, the accuracy of the GAIM predictions of foF2 exceeds that for M(3000)F2. Given that M(3000)F2 is a function of the height and shape of the F2 subpeak, it follows that these parameters of the GAIM F2 peak need to be improved. The GAIM errors in foF2 are quite large at Learmonth during the day, even though Learmonth is a digital ionospheric sounding systems (DISS) station that provides real‐time data to GAIM. We interpret these errors in terms of limitations of the current GAIM model. Comparisons of the GAIM errors in the monthly median MUF(3000)F2 with those given by the propagation prediction program Advanced Stand Alone Prediction System (ASAPS) show that the GAIM errors are generally somewhat greater than the ASAPS errors, but we consider GAIM's performance to be creditable given that most of the data available for assimilation is GPS total electron content observations. Overall, the daytime GAIM errors are 3 MHz or 10%, while the nighttime errors are 3 MHz or 16%. On average, the GAIM errors are about 3/2 the ASAPS errors during the day and twice the ASAPS errors at night.

The IRI model residual difference and the new method of N(h)-profile determination

The effectiveness of performance of the HF radio communication systems depends in many aspects on the accuracy of description of ionospheric conditions. One of possibilities of such a description is to use the IRI model. In the paper it is shown that discrepancies of the model N(h)-profile shape and the real shape can make a contribution to errors of description of ionospheric conditions by means of the IRI model. A residual difference (RD) remains even after adjusting the IRI model to the real-time parameters of foF2 and hmF2. These differences are estimated for the N(h)-profiles calculated from ionograms by the traditional method POLAN and the new method NeXtYZ. Estimates are given for the influence of these differences on the prediction accuracy of such parameters as the maximum usable frequency and the distance D between transmitters and receivers. An attempt was made to decrease the residual difference along the HF path by means of an analytical function approximating RD.

MOF/MUF behavior over one European path

Many applications of HF propagation via the ionosphere (e.g. bearing-fingers, single station location, emergency systems and so on) demand an accurate description of ionospheric conditions. The International Reference Ionosphere (IRI) model is very often used for such a description. A lot of publications were devoted to the verification and improvement of this model. But most of them were based on data from vertical sounding (VS). Not so many comparisons with oblique sounding results have been reported. Specifically, this concerns parameters of the maximum usable frequency (MUF) and a ground range, D. In this paper errors of MUF and D determination by means of the IRI model are estimated on the example of the MOF data on Inskip-Rome path for December 2003. Adaptation of the model to current conditions by means of VS and total electron content TEC-GPS data is used to reduce these errors.

Adaptation of the ionospheric radio channel parameters to the current conditions using observations of the signals of reference radio stations

A method of adaptation of the ionospheric channel parameters to the current conditions is proposed. The observational data on the temporal behavior of signals of radio stations with known location and set of working frequencies (the so-called reference stations) are used as information describing the current state of the channel. A facility that makes possible to determine by simple means the moments of radio rises and declines on the reference paths has been created. A technique that relates for these instants working frequencies of the stations with maximum usable frequencies of the paths has been developed. By means of an approximate solution of the inverse problem using the method of characteristics for the calculation of radiowave propagation and the monthly average semiempirical model of the ionosphere, the height profile of electron density in a certain spatial region is refined. The efficiency of the proposed method of adaptation of the ionospheric channel parameters using the data of vertical sounding is assessed and the expediency of practical application of such adaptation is shown.

Automatic scaling of F2-layer parameters from ionograms based on the empirical orthogonal function (EOF) analysis of ionospheric electron density

Abstract In this paper we apply a method based on empirical orthogonal function (EOF) analysis to automatically scale the F2-layer parameters obtained from ionograms. For a given location, the F layer’s height profiles of ionospheric electron density are represented as an EOF series with adjustable coefficients derived from the EOF analysis of electron density profiles obtained from the International Reference Ionosphere (IRI-2001) model or measured ionograms. By adjusting the coefficients of the series and combining image matching technique, we were able to construct a calculated trace that approximates as close as possible the observed ionogram F2 trace. The corresponding parameters are then known, including the critical frequency (foF2), peak height (hmF2) and maximum usable frequency [MUF(3000)F2] of the F2 layer. Polarization information is unnecessary; only an amplitude array is essential. Consequently, this method is universal and can be applied to many kinds of ionograms (even the film ionograms after an appropriate conversion of the information to the digital form from the film ionogram). To evaluate the acceptability of the obtained parameters, large numbers of foF2, hmF2 and MUF(3000)F2 values from the manually scaled ionograms at Wuhan (114.4°E, 30.5°N), China are compared with those from automatically scaled ionograms by the Automatic Real Time Ionogram Scaler with True height (ARTIST) and our method. The results indicate that the scaled parameters are acceptable and stable.

Monitoring and Forecasting the Ionosphere Over Europe: The DIAS Project

Knowledge of the state of the upper atmosphere, and in particular its ionospheric part, is very important in several applications affected by space weather, especially the communications and navigation systems that rely on radio transmission. To better classify the ionosphere and forecast its disturbances over Europe, a data collection endeavour called the European Digital Upper Atmosphere Server (DIAS) was initiated in 2004 by a consortium formed around several European ionospheric stations that transmit in real-time ionospheric parameters automatically scaled. The DIAS project is a collaborative venture of eight institutions funded by the European Commission eContent Programme. The project seeks to improve access to digital information collected by public European institutes and to expand its use. The main objective of the DIAS project is to develop a pan-European digital data collection describing the state of the upper atmosphere, based on real-time information and historical data collections provided by most of the operating ionospheric stations in Europe. Various groups of users require data specifying upper atmospheric conditions over Europe for nowcasting and forecasting purposes. The DIAS system is designed to distribute such information. The successful operation of DIAS is based on the effective use of observational data in operational applications through the development of new added-value ionospheric products and services that best fit the needs of the market. DIAS is a unique European system, and its continuous operation will efficiently support radio propagation services with the most reliable information. DIAS began providing services to users in August 2006. The Need for Accurate Ionospheric Products Radio frequency communications and satellite positioning and navigation systems are applications most affected by ionospheric disturbances. Such disturbances can cause drastic and large-scale changes in the usable ranges of high frequency (HF) or below HF bands affecting standard ground-to-ground and submarine communication systems. The characteristics of an ionospheric propagation channel, whether it is HF or transionospheric frequencies, are highly variable on timescales ranging from a few seconds to the 11-year solar cycle. Even during its quietest periods, the Sun produces electromagnetic radiation and solar wind, both of which can affect a variety of geomagnetic and ionospheric phenomena, which in turn affect radio waves propagating through the ionosphere. Hence day-to-day and hour-to-hour changes in propagation channel characteristics can occur.

Daily ionospheric forecasting service (DIFS) III

Abstract. The daily variability of the ionosphere can greatly affect HF or SATCOM communications. HF skywave operators plan frequency schedules months in advance, however, they also require daily knowledge of the ionospheric conditions in order to modify assignments. SATCOM operators also require daily information about the levels of scintillation, which are variations in phase, amplitude, polarisation and angle of arrival that can cause severe degradation of the received signal. Using a number of ionosonde measurements and geomagnetic and solar values, a Daily Ionospheric Forecasting Service (DIFS) has been developed, which provides HF and SATCOM operators with daily forecasts of predicted ionospheric conditions. The system uses in-house algorithms and an externally developed Global Ionospheric Scintillation Model (GISM) to generate HF and SATCOM forecasts. HF forecasts consist of a past summary and a forecast section, primarily displaying observed values and predicted categories for the Maximum Usable Frequency (MUF), as well as an Ionospheric Correction factor (ICF) that can be fed into the ionospheric propagation prediction tool, WinHF. SATCOM forecasts give predictions of global scintillation levels, for the polar, mid and equatorial latitude regions. Thorough analysis was carried out on DIFS and the results conclude that the service gives good accuracy, with user feedback also confirming this, as well.

A ray‐tracing model to account for off–great circle HF propagation over northerly paths

Off–great circle HF propagation effects are a common feature of the northerly ionosphere (i.e., the subauroral trough region, the auroral zone, and the polar cap). In addition to their importance in radiolocation applications where deviations from the great circle path may result in significant triangulation errors, they are also important in two other respects: (1) In systems employing directional antennas pointed along the great circle path, the signal quality may be degraded at times when propagation is via off–great circle propagation modes; and (2) the off–great circle propagation mechanisms may result in propagation at times when the signal frequency exceeds the maximum usable frequency along the great circle path. A ray‐tracing model covering the northerly ionosphere is described in this paper. The results obtained using the model are very reminiscent of the directional characteristics observed in various experimental measurement programs, and consequently, it is believed that the model may be employed to enable the nature of off–great circle propagation effects to be estimated for paths which were not subject to experimental investigation. Although it is not possible to predict individual off–great circle propagation events, it is possible to predict the periods during which large deviations are likely to occur and their magnitudes and directions.

Radar observations of artificial ionospheric turbulence during a magnetic storm

We present the results of radar observations of artificial ionospheric turbulence (AIT) created due to modification of the ionosphere by high-power radio emission from the “Sura” heating facility (Nizhny Novgorod region, Russia). Measurements were carried out in August 18–22, 2003 in the evening time (16:00–20:00 UT) with the use of over-the-horizon chirp HF radars on the Khabarovsk-Rostov-on-Don, Irkutsk-Rostov-on-Don, Inskip (England)-Rostov-on-Don paths, and also on the Moscow-Rostov-on-Don path for which reference signals of the standard-time RVM station were received. It is found that conditions for propagation of HF signals through the upper ionosphere at frequencies exceeding the maximum usable frequency for standard hop propagation through the F region were realized on the Irkutsk-“Sura” path in the presence of the powerful sporadic Es layer. The presence of such signals was revealed at the Rostov-on-Don station by receiving radio waves which escape from the altitudes of the ionospheric F region due to scattering by artificial small-scale magnetic-field-aligned irregularities. We studied the ionospheric effects of a magnetic storm occurring during the experiment by using the measurement data of the Doppler frequency shift of signals scattered by artificial ionospheric turbulence. It is shown that during a magnetic storm, the electric field and the drift velocity of irregularities at the altitudes of the F layer over the “Sura” facility reach values of ∼ 8.6 mV/m and 186 m/s, respectively, which are typical of the high-latitude ionosphere. We consider the relation between quasi-periodic oscillations of the Doppler frequency shift of the scattered signal and propagation of magnetohydrodynamic waves excited during the magnetospheric storm.

On the global behaviour of the day-to-day MUF variation

Using data from more than a hundred ionospheric stations spread worldwide, day-to-day variations with location and time of the conventional maximum usable frequency (MUF) decile values are investigated. New bounds of day-to-day MUF variability are calculated and compared with reference figures currently used by radio users. An hourly-daily validation process shows a clear improvement of spatial bias and model accuracy. A significant dependence on longitude is revealed that might be introduced into future specifications.

Experimental Studies of Variations in the Maximum Usable Frequency on Oblique Sounding Paths

We present the results of experimental studies of variations in the maximum usable frequency (MUF) on the latitudinal (England–Moscow and Khabarovsk–Nizhny Novgorod) and meridional (Cyprus–Moscow) paths. It is found that the quasi-period of MUF variations ranges from 20 min to several hours in spring 2002. The absolute value of the MUF varied in the range from 0.2 to 2 MHz. We show that variations in distance–frequency characteristics of HF signals propagated over the long-distance latitudinal path are strongly affected by quasi-periodic disturbances being the ionospheric response to acoustic-gravity waves excited by the terminator.

Influence of source frequency on impact-echo data quality for testing concrete structures

In impact-echo data, a correct interpretation of the measured frequencies may become a problem when in presence of a multitude of peaks in the spectra. An important evaluation criterion is then the magnitude of the measured peaks. This signal parameter is strongly influenced by the frequency components of the wave generated at the impact point. On-site measurements showed that, in particular cases, the wave centre frequency might have greater importance than the maximum usable frequency of the signal. Values of impact contact time, wave centre and maximum frequency in function of the impact source size were calculated from on-site data and from calibration measurements in the laboratory. The consequences for the planning of the tests and for a correct data interpretation are discussed. It was concluded that the expected measured frequency should be excited with maximum energy in the input signal in order to achieve an optimum quality of the data and ease data interpretation.

Oblique ionogram features associated with off-great circle HF propagation at high and sub-auroral latitudes

Oblique ionograms received over northerly paths are often difficult to interpret, with many features not being of the form observed on the more familiar mid-latitude soundings. In particular, the ionograms contain unusual traces at frequencies close to and above the conventional or basic maximum usable frequency, i.e. above the junction frequency of the high and low angle modes. A series of experiments have been undertaken over a period of several years to investigate off-great-circle propagation mechanisms over high latitude and sub-auroral oval paths. In addition to measurements of the signal direction of arrival, oblique ionograms were also recorded over several of the paths. By these means, the characteristic ionogram signatures associated with off-great-circle propagation effects arising due to the presence of convecting patches of enhanced ionisation or sun-aligned arcs within the polar cap ionosphere, or to the presence of steep gradients to the side of the path within the mid-latitude trough region, have been identified.

Nonlinear forecasts of ƒ<i>o</i>F2: variation of model predictive accuracy over time

Abstract. Space weather effects can strongly influence high-frequency (HF) communications by changing the ionospheric environment through which the radio waves propagate. Since many systems utilize HF communications, the ability to make real-time assessments of propagation conditions is an important part of space weather monitoring systems. In this paper, we present new techniques for measuring high-latitude HF communications link parameters using data from SuperDARN radars. These techniques use ground-scatter returns to define the variation in skip distance with frequency. From these data, the maximum usable frequency (MUF) as a function of range is determined and ionospheric critical frequencies are estimated. These calculations are made in near-real-time and the results are made available on the World Wide Web. F-region critical frequencies calculated using this method show good agreement with ionosonde data.Key words. Ionosphere (active experiments; instruments and techniques) – Radio science (ionospheric propagation)

Determining characteristics of HF communications links using SuperDARN

Abstract. Space weather effects can strongly influence high-frequency (HF) communications by changing the ionospheric environment through which the radio waves propagate. Since many systems utilize HF communications, the ability to make real-time assessments of propagation conditions is an important part of space weather monitoring systems. In this paper, we present new techniques for measuring high-latitude HF communications link parameters using data from SuperDARN radars. These techniques use ground-scatter returns to define the variation in skip distance with frequency. From these data, the maximum usable frequency (MUF) as a function of range is determined and ionospheric critical frequencies are estimated. These calculations are made in near-real-time and the results are made available on the World Wide Web. F-region critical frequencies calculated using this method show good agreement with ionosonde data.Key words. Ionosphere (active experiments; instruments and techniques) – Radio science (ionospheric propagation)

Application of radio tomographic imaging to HF oblique incidence ray tracing

Radio tomography is a technique for generating images of the spatial structure of ionospheric electron density over a wide area. This paper assesses the potential use of radio tomography in HF oblique propagation and ray tracing applications. Synthetic ionograms produced by ray tracing through tomographic images and ionospheric models have been compared with experimental oblique ionograms from six paths lying close to the image plane in the United Kingdom. In particular, study has been made of the effects of various types of input information used to constrain the vertical electron density structure in the tomographic reconstructions. It was found that use of a fine height resolution (5 km) and incorporation of information from one vertical ionosonde in the reconstruction process makes significant improvements to the overall reliability of the tomographic image. As expected, E layer propagation is better defined using a climatological model than by tomography. However, in comparison with three ionospheric models, use of tomographic images can significantly reduce the RMS error in the determination of the F2 layer maximum usable frequency.

HF detecting radar and communication frequency selection system

Real time communication (RTC) frequency selecting system is used to the maximum usable frequency (MUF) between two communication points, then finds the best frequency between 0. 85 MUF and 1. 0 MUF. Determination of electric wave delay is mostly introduced, and of MUF values, the form of frequency-controlling code and relative interface circuits in the frequency selecting system are introduced in detail.

Robust HF data communications at high latitudes

It is well known that the ionosphere can be highly disturbed in the auroral and polar cap regions. However, efforts to quantify ionospheric effects on high-frequency communications have concentrated mainly on lower-latitude regions. The Doppler and multipath sounding network (DAMSON) has been used in an extensive campaign to measure the channel characteristics on four paths in the auroral region of northern Scandinavia. The conditions observed indicate that robust data modems, used when less-robust, higher-rate modems fail, are required up to 90% of the time. A detailed analysis of the propagation parameters indicates that these modems, designed to operate in these regions, should be able to operate when the Doppler spread exceeds 40 Hz and the multipath spread is greater than 11.5 ms. Multiple propagation modes are common, especially between the lowest and maximum usable frequencies. The distribution of the received signal power in these cases is examined, and it is concluded that laboratory simulations used for robust-modem evaluation should include channel models with dominant modes.