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With support from the NASA sounding rocket program, the Air Force Research Laboratory (AFRL) launched two sounding rockets in the Kwajalein Atoll, Marshall Islands in May 2013 known as the Metal Oxide Space Cloud (MOSC) experiment. The rockets released samarium metal vapor at preselected altitudes in the lower F-region that ionized forming a plasma cloud. Data from ALTAIR incoherent scatter radar and high frequency (HF) radio links have been analyzed to understand the impacts of the artificial ionization on radio wave propagation. The HF radio wave ray-tracing toolbox PHaRLAP along with ionospheric models constrained by electron density profiles measured with the ALTAIR radar have been used to successfully model the effects of the cloud on HF propagation. Up to three new propagation paths were created by the artificial plasma injections. Observations and modeling confirm that the small amounts of ionized material injected in the lower-F region resulted in significant changes to the natural HF propagation environment.

Ultrahigh frequency space-based synthetic aperture radar (SAR) can suffer from the degrading effects of a scintillating ionosphere which modulates both the phase and the amplitude of the radar signal. In this paper, we use Global Navigation Satellite System (GNSS) signals to synthesize an L-band SAR point spread function (PSF). The process of transforming the GNSS signal to the equivalent SAR PSF is described. The synthesized PSF is used to explore the possibility of using a phase correction determined from a point target in a SAR image to correct the ionospheric degradation. GNSS data recorded on equatorial Ascension Island during scintillation events are used to test the feasibility of this approach by applying a phase correction to one GNSS receiver from another located along a magnetic east-west baseline. The peak-to-sidelobe ratios of the synthesized L-band SAR point spread functions before and after the correction are compared, and it is shown that this approach improves the L-band PSF over distances of ~3000 m in the ionosphere, corresponding to 6000 m on the ground.

The high-latitude HF channel has been measured and characterized in terms of Doppler spread, delay spread, and signal-to-noise ratio. The performance of data modems has been determined over a comprehensive range of simulated channel conditions. A comparison is made between the channel measurements and the modem characterizations. Thereby the percentage of time that the modem works satisfactorily (availability), during the measured channel conditions, is determined. Two 75 bits/s modems (compliant with STANAG 4415 and STANAG 4285), a 2400 bits/s modem (compliant with STANAG 4285), Morse code, and voice have been selected for analysis. The two 75 bits/s modems show 60–75% higher availability than the 2400 bits/s modem on the measured paths. The 2400 bits/s modem is degraded in performance during a geomagnetic disturbance whereas the 75 bits/s modems maintain approximately the same performance level. The paper also addresses the necessary frequency pool (at a specific time) required for an HF circuit to achieve the best possible communications availability. For the 75 bits/s modems, independent of path length, a one-frequency pool is all that is required during summer and winter when the ionospheric conditions are quiet. A four frequency pool is necessary during disturbed ionospheric conditions. For the 2400 bits/s waveform, three to four frequencies are needed on the 200 km west-east path, and four to six frequencies are required on the north-south 2000 km path. This applies to all seasons and geomagnetic conditions analyzed.

During the total solar eclipse observed in Europe on August 11, 1999, measurements were made of the amplitude and phase of four VLF transmitters in the frequency range 16–24 kHz. Five receiver sites were set up, and significant variations in phase and amplitude are reported for 17 paths, more than any previously during an eclipse. Distances from transmitter to receiver ranged from 90 to 14,510 km, although the majority were 10,000 km. Negative phase changes were observed on most paths, independent of path length. Although there was significant variation from path to path, the typical changes observed were ∼3 dB and ∼50°. The changes observed were modeled using the Long Wave Propagation Capability waveguide code. Maximum eclipse effects occurred when the Wait inverse scale height parameter β was 0.5 km−1 and the effective ionospheric height parameter H′ was 79 km, compared with β=0.43 km−1 and H′=71 km for normal daytime conditions. The resulting changes in modeled amplitude and phase show good agreement with the majority of the observations. The modeling undertaken provides an interpretation of why previous estimates of height change during eclipses have shown such a range of values. A D region gas-chemistry model was compared with electron concentration estimates inferred from the observations made during the solar eclipse. Quiet-day H′ and β parameters were used to define the initial ionospheric profile. The gas-chemistry model was then driven only by eclipse-related solar radiation levels. The calculated electron concentration values at 77 km altitude throughout the period of the solar eclipse show good agreement with the values determined from observations at all times, which suggests that a linear variation in electron production rate with solar ionizing radiation is reasonable. At times of minimum electron concentration the chemical model predicts that the D region profile would be parameterized by the same β and H′ as the LWPC model values, and rocket profiles, during totality and can be considered a validation of the chemical processes defined within the model.

Analytic ray tracing is considerably less time consuming than numerical ray tracing. However, despite the advantage of speed, analytic ray tracing has a major limitation: the difficulty of including the effects on the ray path of horizontal gradients in electron density. In the past this difficulty has been addressed using ionospheric models whereby an effective tilt is introduced by displacing the center of the ionosphere from the center of the Earth. In this paper, ray equations for the ground range, phase path, group path, and divergent power loss are developed for a new two-dimensional analytic ray tracing technique, known as the segmented method for analytic ray tracing (SMART), which is able to deal with horizontal gradients in electron density. Ray tracing using two tilting methods are compared with ray tracing using SMART. Further, these results are compared with a numerical ray tracing program known as homing-in ray tracing (HIRT).

Measurements of Doppler spread, multipath spread, and signal-to-noise ratio have been made on four high-latitude high-frequency (HF) communications paths. The measurement system and analysis techniques are outlined, and an analysis of the data pertinent to the design of robust HF data modems is presented. A summary of the spreads that are exceeded for 5% of time is presented for each path. Doppler spreads range from 2 to 55 Hz, while multipath spreads range from 1 to 11 ms. Physical interpretations of the data are made, and the data are related to the measured performance characteristics of an HF data modem to estimate the modem availability on the paths considered. When there is mode support, availabilities range from 64% to 100% for a signal-to-noise ratio of 0 dB, although the data indicate that the availabilities can generally be increased by optimizing frequency selection.

A computer model to predict the underdense meteor arrival rate over a forward meteor scatter communications link is presented. The model incorporates important effects such as major shower streams, a nonuniform radiant distribution, and antenna polarization coupling. A particularly useful aspect of the model is its capacity to predict the passage of sporadic and shower hotspot regions across the sky and thereby provide directional information to drive an adaptive beam steering system. Directional data from a phased array reception system in the United Kingdom has been used to determine the diurnal arrival distribution of sporadic meteors. This diurnal variation is broadly reproduced by the model and suggests that just two daily changes in the direction of a high-gain beam would offer considerable advantage over a fixed beam system. This paper shows that further improvements in system performance can be achieved with accurate predictions of the time of appearance and location of shower streams. A comparative study between monthly predictions and experimental data from a high-latitude link in Greenland over a year shows reasonable agreement but highlights the need for higher-resolution radiant data.

A new two-dimensional analytic ray tracing method, known as segmented method for analytic ray tracing (SMART), which automatically segments the ionosphere along the ray path is described. Unlike other techniques, SMART is able to accurately ray trace through horizontal gradients which vary with altitude along and in the direction of the ray path. Computer run times are approximately 10 times faster than those of numerical ray tracing packages, and ground range errors are typically less than 2.5%.

We have analyzed the duty cycle, due to ionospheric propagation, of very high frequency sounding signals for both polar cap and auroral paths. We find that at 35 and 45 MHz the propagation is often sustained by sporadic E layers and other nonmeteoric modes rather than by meteor scatter. At the higher frequencies of 65 and 85 MHz we find that the path is generally dominated by meteor scatter modes. These results have important ramifications for frequency reuse and security in meteor burst communications systems and for the development of extended frequency range HF systems (above 30 MHz) with a capability to operate on any available propagation mode. The diurnal, seasonal, and geomagnetic variations of the nonmeteoric duty cycle have been examined. A polar cap path model is presented for the nonmeteoric duty cycle as a function of geomagnetic activity.

The method of passive beam formation using a four-element Butler matrix to improve the signal availability of meteor scatter communication systems is investigated. Signal availability, defined as the integrated time that the signal-to-noise ratio (snr) exceeds some snr threshold, serves as an important indicator of system performance. Butler matrix signal availability is compared with the performance of a single four-element Yagi reference system using ∼6.5 hours of data from a 720 km north-south temperate latitude link. The signal availability improvement factor of the Butler matrix is found to range between 1.6–1.8 over the snr threshold range of 20–30 dB in a 300-Hz bandwidth. Experimental values of the Butler matrix signal availability improvement factor are compared with analytical predictions. The experimental values show an expected snr threshold dependency with a dramatic increase at high snr. A theoretical analysis is developed to describe this increase. The signal availability can be further improved by ∼10–20% in a system employing two four-element Butler matrices with squinted beams so as to illuminate the sky with eight high-gain beams. Space diversity is found to increase the signal availability of a single antenna system by ∼10–15%, but the technique has very little advantage in a system already employing passive beam formation.

This paper examines the seasonal duty cycle variation of meteor and nonmeteor propagation at 45 MHz on two high-latitude links. Software techniques for automatic data processing and analysis of the data are discussed. It is shown that for large portions of the year nonmeteoric propagation, most likely sporadic E, is the dominant propagation mechanism. Consequently, protocols for an operational meteor scatter communication network must be designed to cope with a situation in which close to an entire network may be connected for long periods of time. System duty cycles at 45 MHz in excess of 20% are common during the summer months in the high-latitude region.

We have studied 37-MHz signals received over an 800-km temperate latitude path using 400-W continuous wave transmissions. Signals collected during a 9-day period in February 1990 on two antennas at separations of 5λ, 10λ, and 20λ were analyzed. Three signal categories were identified (overdense, underdense, and not known (NK)) and cross-correlation coefficients between the signals received by the two antennas were calculated for each signal category. No spatial variation, and in particular no decrease, in average cross-correlation coefficient was observed for underdense or NK signals as the antenna spacing was increased from 5λ to 20λ. At each antenna separation the cross-correlation coefficients of these two categories were strongly dependent on time. Overdense signals, however, showed no cross-correlation time dependency at 5λ and 10λ, but there was a strong time dependency at 20λ. Recommendations are made in regard to the optimum antenna spacing for a meteor burst communication system using spaced antenna diversity. Our measurements support the views of Bartholome and Vogt [1968], who suggested that a 4λ spaced antenna diversity configuration contributed to the exceptional signal throughput of the COMET meteor burst communication system.

[1] The spatial decorrelation of V/UHF signals by equatorial ionospheric turbulence is studied using 150 and 400 MHz signals transmitted from low-earth-orbiting beacon satellites. The signals are monitored on a linear array of spaced antennas located on Ascension Island, and processed coherently to determine the cross-correlations of the phases of the received signals. Analyzing signals from the low-inclination satellite C/NOFS has provided an opportunity to investigate the correlation variations in and out of scintillating structures. As a necessary step, the geometrical component of the phase difference between antennas has been accurately removed by adjusting the satellite orbital information using the measured phases. In order to unambiguously measure the spatial phase correlation without any temporal effects, the phase cross-correlation was calculated as the cross-correlation function of time-synchronized signals. As expected, the VHF signals were more affected by scintillation than the UHF signals. When the signal propagated through patches of strong scintillation, the VHF signal became completely uncorrelated over an ionospheric distance of 130 m, while over the same distance the UHF phase correlation decreased to 0.55. The time-synchronized technique limited the spatial variations assessed to east-west distances of ∼300 m. To extend this range, a novel ‘phase reconstruction’ technique was developed to link arrays of samples together. In the absence of scintillation the measured decorrelation distance is ∼10 km at both frequencies, but with increasing scintillation, the decorrelation distance falls to ∼100 m at VHF and 300 m at UHF. A clear relation between the decorrelation distance of the measured phase and S4 is observed and a simple empirical model has been derived.

The potential for meteor burst (MB) spaced antenna diversity is studied using cross-correlation techniques at a temperate latitude, using 37-MHz signals scattered over an 800-km path. Approximately 9 hours of data were analyzed from a 2-day period in February 1990, when the antenna separation was 10λ. The correlation variations with time of signals with durations ≥ 0.75 s, which were categorized as “underdense,” “overdense,” or “not known” (NK), were investigated. The cross-correlation coefficients of signals from underdense and overdense trails are high when correlated over their total signal envelopes, but this is not true when the signal envelope is segmented in time. NK signals are observed to be more uncorrelated than underdense or overdense signals and are, therefore, likely to be the most advantageous to a diversity system. Analysis of data from all three categories combined shows that 40% of signals with duration ≥ 0.75 s have correlation values of less than 0.6 after the first 0.25 s. The correlation-time dependency observed for NK and underdense signals is not identified for the overdense signal category. It is proposed that for underdense and NK signals the correlation-time dependency is due to the vector addition of other weak signal modes. These weak signal modes, however, have little effect on overdense signals which, typically, exhibit higher signal powers in the early stages of the signal envelope.

[1] The Electron Density Assimilative Model (EDAM) has been developed to provide real-time characterization of the ionosphere. A thin-client Web-based ionospheric situational awareness tool, which relies on data from EDAM, has been developed. The tool provides the high-frequency (HF) maximum usable frequency (MUF) and the optimum working frequency (FOT) on the basis of the current ionospheric electron density grid from EDAM. Two validations are reported. First, EDAM has been tested against a set of vertical ionosondes. The results show performance comparable to that of the Utah State University Global Assimilation of Ionospheric Measurements Gauss–Markov Kalman filter model. Second, MUFs estimated from EDAM were compared with those measured on an oblique path from New Zealand to Australia. Although EDAM demonstrates better performance than a monthly median model in some days, over the whole test period EDAM performed no better than the international reference ionosphere. This can be attributed to the lack of input GPS total electron content (TEC) data near the midpoint of the path and a probable TEC bias in the background model.

[1] Irregularities in the electron density of the ionosphere cause phase and amplitude scintillation on transionospheric VHF and UHF radar signals, particularly at lower radio frequencies. The design of radar and other transionospheric systems requires good estimates of the coherence bandwidth (CB) and coherence time (CT) imposed by a turbulent ionosphere. CB and CT measurements of the equatorial ionosphere, made using the Advanced Research Project Agency Long-range Tracking and Identification Radar 158 MHz and 422 MHz phase coherent radar located on Kwajalein (9.4°N, 167.5°E), are presented as a function of the two-way S4 scintillation index at 422 MHz The log linear regression equations are CT = 1.46 exp(−1.40 S4) s at 158 MHz and CT = 2.31 exp(−1.10 S4) s at 422 MHz. CT also varies by a factor of 2–3 depending on the effective scan velocity through the ionosphere, veff. The CT and CB, as a function of S4, have been compared to those from the Trans-Ionospheric Radio Propagation Simulator, a phase screen model. A close agreement is achieved using appropriate values of veff and midrange values of phase spectral index and outer scale. Validation of CB is, however, limited by insufficient radar chirp bandwidth. Formulating the model in terms of the two-way S4 index (an easily measurable parameter) rather than more fundamental phase screen parameters (which are difficult to obtain), improves its utility for the systems engineer. The frequency dependencies (spectral indices) of S4 and of CT are also presented to allow interpolation and some extrapolation of these results to other frequencies.

The design and operation of transionospheric VHF and UHF radars requires knowledge of amplitude and phase scintillation due to ionospheric scattering. Phase coherence is of particular importance where long coherent integration periods and large bandwidths are required. A thin phase screen, parabolic equation based, Trans-Ionospheric Radio Propagation Simulator (TIRPS) is described. Modeled channel scattering functions (CSFs) are compared to experimental VHF and UHF data derived from the Advanced Research Projects Agency Long-range Tracking and Instrumentation Radar on Kwajalein Island (9.4°N, 166.8°E). TIRPS quantitatively reproduces the experimental results, including the quasi-parabolic profile observed in the measured CSFs under strong turbulence conditions. Variations in the simulated CSF with ionospheric phase screen parameters are also presented. Under conditions of high integrated strength of turbulence (CkL), a low phase spectral index (p = 1), indicating relatively dense small-scale irregularities, produces pronounced range spreading. Conversely, when the spectral index is high (p = 4), indicative of strong focusing/defocusing by large-scale irregularities, there is increased Doppler spreading and, when the outer scale of irregularities is large, a greater likelihood of asymmetry of the CSF about the zero Doppler axis.

[1] The ionosphere is a critical element in the design and operation of many military RF systems. While judicious engineering choices, such as transmission frequency, can mitigate many ionospheric effects, the research scientist and engineer are being asked to meet ever more demanding operational requirements. These, in turn, require ever more sophisticated situational awareness, mitigation, and exploitation techniques, which are being variously realized against a backdrop of evolving ground- and space-based instrumentation. The latter, in combination with improved modeling and assimilative techniques, promises a new generation of more capable real-time ionospheric models. Through examples this paper describes the benefits and problems associated with the ionospheric medium. HF communications and space radar systems are discussed as exemplars, and an emphasis is placed on the need to understand the engineering details of those systems if ionospheric research and development is to provide its full benefit.

[1] Space-based synthetic aperture radars can be detrimentally affected by electron density variations in the ionosphere, which cause signal phase variations resulting in image defocusing. In order to evaluate the significance of this problem, this paper determines how often the phase perturbations are large enough to cause defocusing. High cadence GPS TEC signals collected on Ascension Island during sunspot minimum and in Papua New Guinea over half a solar cycle have been analyzed. Both locations lie in the equatorial crest region. For a space-based synthetic radar operating at 435 MHz, the average probability of defocusing varies from zero at quiet times, to around 1% between 18:00 and 24:00 at solar minimum, and to 45% between 18:00 and 24:00 at solar maximum. Corresponding values are given for radars operating at other frequencies. A method is developed to estimate such variations from the large database of S4 measurements.

[1] To the best of our knowledge, we report the first determination of ionospheric distortion, comprising the simultaneous characterization of both multipath and Doppler, on wideband, transionospheric VHF (158 MHz) and UHF (422 MHz) signals. The measurements took place as part of the test phase of the United Kingdom–United States Wideband Ionospheric Distortion Experiment during the evening (∼1000 UT) of 18 January 2005. This characterization has been achieved using the ALTAIR radar at the Ronald Reagan Ballistic Missile Defense Test Site on Kwajalein Atoll (9.395°N, 167.469°E (12.87°N, 237.16°E corrected geomagnetic)) in the Pacific, in conjunction with a low Earth orbiting, constant radar cross-section, passive satellite (calibration sphere). During the period when the two-way S4 index was above ∼0.8 on both channels, the median coherency times were 43 and 96 ms at VHF and UHF, respectively (at 1.5σ). The corresponding median coherency bandwidths were 0.8 and 2.1 MHz.

[1] Data assimilation processes aim to combine measurement data with background models in an optimal way. In anticipation of the availability of global radio occultation (RO) measurements, a computationally practical data assimilation technique for combining RO data with background ionospheric models has been implemented, and simulations have been conducted to assess the utility of the technique. In simulations where tomographic images provide the truth data and the Parameterized Ionospheric Model (PIM) provides the background, a fourfold decrease in the electron density error at 300 km altitude was achieved. A global assimilation simulation has also been conducted using the International Reference Ionosphere as the truth data. For a constellation of eight RO satellites, a factor of four decrease in the vertical total electron content RMS error has been demonstrated. The same simulation also results in a factor of three decrease in the NmF2 RMS error and a halving of the hmF2 RMS error.

The problems of polarization rotation losses, in meteor burst communication systems, are examined using a theoretical model developed for the purpose. The paper takes into account both the polarization changes due to ionospheric Faraday rotation and the rotation of the wave polarization that takes place as a result of scattering from underdense meteor trails. Linearly polarized systems, employing copolar transmitting and receiving antennas, and hybrid systems, employing a linearly polarized transmitting antenna and a circularly polarized receiving antenna, are studied. It is shown that, particularly in linearly polarized systems, polarization rotation may introduce unexpected diurnal performance variations in systems operating at frequencies ∼40 MHz. For the two 40-MHz linearly polarized links investigated in detail the model predicts that, for noon summer solstice conditions during periods with a high solar sunspot number, these losses cause a reduction in data throughput to between 15 and 70% of the value expected had no polarization rotation occurred. By using a cross polarization approach, qualitative experimental confirmation of the predictions is also given.