Tropospheric amplitude scintillations at C-band along satellite up-link

Long-term data for unusually strong tropospheric amplitude scintillations at 6.2 GHz on a satellite up-link are presented. A diurnal maximum in the afternoon and seasonal peaks in spring and autumn are found. The high-level scintillation activities are explained in terms of local tropospheric conditions and propagation theory. A typical power spectrum and distribution are discussed.     

Impact of ionospheric scintillations on LEO satellite DS-CDMA communication system

The impact of ionospheric scintillations is investigated in terms of bit error rate for a multispot beam LEO satellite communication system employing direct sequence-code division multiple access (DS-CDMA) technique. Depending on the satellite orbit parameters and the Earth station location, the effect of time-varying ionospheric scintillation on the system varies. Performance degradation can be substantial in times of severe scintillations.     

Measurements of radio star and satellite scintillations at a subauroral latitude

Observations of two radio stars, Cygnus A and Cassiopeia A, and of two satellites, Cosmos I and Transit 4A, have yielded data on lower and upper atmospheric irregularities. The frequencies studied have included 20 Mc, 40 Mc and 54 Mc for satellite transmissions, and 30 Mc to 3000 Mc for radio star signals. The antennas used have ranged from a dipole to a 150-foot parabola. The irregularities in refractive index in the lower atmosphere produce amplitude fluctuations up to several times average with a fading rate of 3 to 0.5 per minute while ionospheric amplitude fluctuations can increase several decibels, with rates from 1 to 60 per minute. Lower atmosphere scintillation rates increase as the elevation angle increases and generally disappear above5degof elevation. During intense magnetic storms, the ionospheric scintillation rate is a function of wavelength in the UHF range. Ionospheric scintillations are functions of both magnetic conditions and of the subionospheric latitudes of the irregularities; they are observed at zenith at the geomagnetic latitude of the Sagamore Hill Radio Observatory (54deg). Using spaced receivers, heights of single irregularities have ranged from about 100 to 600 km and representative sizes from about 0.1 to 4 km. Irregularities constitute the prime factor affecting the level, phase and angle of arrival of signals propagated through the auroral regions.     

Simultaneous analysis of downlink beacon dynamics and skybrightness temperature. Part II. Extraction of amplitude scintillations

This paper addresses the long-standing problem of separating the tropospheric amplitude scintillations from the dominant trend of atmospheric attenuation in a satellite downlink. Following extensive theoretical and experimental work, it is shown how the use of a radiometer coaxial with the communications beacon receiver constitutes an excellent tool for an optimum separation regardless of the meteorological conditions along the propagation path and avoids the use of the long-traditional high-pass filter approach. The experimental and theoretical work has revealed that the angular resolution of the radiometer together with the dynamics of rain attenuation and tropospheric scintillations determine the success of the extraction. This is because fast fadings require large radiometer antennas in order to resolve the sky temperature fluctuations. The dynamic behavior of rain attenuation has been reanalyzed and adapted for this study with special focus on the Maseng-Bakken (MB) model and the impact of the slant path on attenuation dynamics. The importance of the antenna pattern in the time response of the radiometer is studied in detail and permits to derive the maximum Fourier component observable for a given antenna size. The theoretical work has been verified by means of extensive experimental results obtained using a dual radiometer system and a beacon receiver tracking the ITALSAT 39.5 GHz F40 beacon. Finally, because of its importance and direct relevance to future communication systems benefiting from fade countermeasure strategies, the minimum size of the radiometer antenna for a successful extraction of amplitude scintillations is determined as a function of the elevation angle and carrier frequency     

Estimating leaf area index from satellite data

A method for estimating leaf area index from visible and near infrared measurements of vegetation above a soil background is applied to a Landsat Thematic Mapper data set. Some constants required for the procedure are inferred from the scattergram of data values. The resulting image illustrates variability of leaf area index over an agricultural area. The mixed-pixel case, corresponding to low-resolution data from the NOAA Advanced Very High Resolution Radiometer (AVHRR) is also discussed, and a vegetation index is suggested for both high- and low-resolution data. Consideration of the two types of data leads to the suggestion that a sampled high spatial resolution sensor (50-100 m) be added to the AVHRR in order to permit accurate inference of vegetation conditions over agricultural areas     

Fade slope statistics for three 12-GHz satellite beacon links inBrazil

The fade slope is defined as the rate of increase or decrease of attenuation over time. The possible correlation between the statistics of fade slope values and attenuation levels is investigated. Results show a direct positive correlation between the mean values of fade slope and the attenuation levels. The distributions of fade slope values present a degree of skewness toward the negative values that makes the curves depart from the Gaussian distribution observed by other authors     

The use of complex monopulse techniques to detect scintillations produced by propagation through turbulence

Propagation of radar signals through turbulence produces an erroneous off-axis angle indication in a conventional monopulse radar. Such turbulent propagation also creates a phase-quadrature (imaginary) component of the indicated angle which can be used to detect the presence of these errors in the real angle indication.     

A method for the determination of ionospheric vertical profiles through beacon satellite measurements

A method is described for the determination of the parameters of an assumed ionospheric profile from a given measured impulse response function. In the considered examples the measured data are simulated by ray tracing techniques and the least mean square method is applied iteratively to solve the inverse problem. Measurements errors and horizontal dependence of the ionospheric profile are also considered along with the effect of the choice of the starting values of the assumed profile parameters.     

Modeling and simulation of mobile satellite propagation

Mobile satellite systems are subject to severe fading due to blockage of the line-of-sight (LOS) path by roadside vegetation. A thorough understanding of the fading effects is necessary for the design of a reliable land-mobile satellite system. Analytical and empirical models are presented for predicting fade statistics for vegetative shadowing of mobile satellite terminals. A software simulator for generating simulated fade data is also presented. A physical model relating physical path parameters to propagation model parameters is presented, and results using the model are shown     

Measurement and modelling of the probability density function of amplitude scintillations on an X-band satellite down-link

The experimentally determined probabiliiy density function of the amplitude scintillations on an X-band satellite down-link is shown to depart significantly from the log-normal distribution for scintillations with amplitudes greater than ±0.15 dB, which occur on average for less than 5% of the time. This departure is explained in terms of a Gaussian process with a variable standard deviation, and the impact and use of this model is discussed.     

Melting layer attenuation at 28.6 GHz from simultaneous comstar beacon and polarisation diversity radar data

Attenuation data at 28.6 GHz obtained from measurements of the Comstar beacon show that, for moderate rain, slant path attenuation may significantly exceed that calculated from simultaneous radar reflectivity measurements. Polarisation diversity radar data were used for positive identification of the rain and the melting layer, and for estimating the rain attenuation along the path. These results indicate that the melting layer attenuation is significant.     

Centimeter wave propagation experiments using the beacon signals of CS and BSE satellites

The wave propagation experiments using Japanese geostationary satellites CS (20/30 GHz) and BSE (12/14 GHz) satellites have been performed at the Kashima earth station of the Radio Research Laboratories (RRL). Cumulative rain attenuation and cross-polarization discrimination (XPD) statistics are given for the period of three years at 11.7 GHz (vertical polarization) and for the period of four years at 19.5 GHz (circular polarization). It is shown that the yearly rainfall rate and attenuation distributions are well approximated by log-normal distributions, and the XPD distribution is well approximated by a normal distribution. Monthly and time-of-day variation of the attenuation and XPD distributions are presented. Duration statistics of attenuation and XPD are presented and characterized. Other characteristics in the wave propagation, such as effective path length, frequency dependence of attenuation, and joint statistics of attenuation and XPD are derived and discussed. Rainfall events are classified into three rainfall types, "stratus," "cumulus," and "others" using measurements of the radar reflectivity factor along the satellite-to-earth path, and the dependence of XPD characteristics on the rainfall type is also presented and discussed. Some prediction methods of calculating attenuation and XPD statistics are applied to the data obtained in these experiments and the predicted results are compared with the measured ones. It is found that some corrections are needed when the XPD statistics are predicted from the attenuation statistics using the theoretical relation between XPD and attenuation.     

A satellite data transmission antenna

A retrodirective satellite antenna for a broad bandwidth, satellite-to-ground data transmission link, is described. An information-collecting satellite which transmits its data in the direction of the ground receiver upon command, is considered. The satellite antenna described for this system is an active retrodirective antenna which utilizes the beam-forming characteristics of a phasing matrix. A signal from the ground station is received on a particular beam port of the matrix which corresponds to a particular beam-pointing direction. Control circuitry in the satellite activates a switching matrix and a transmitter so that the data-carrying signal is transmitted in the same direction from which the satellite is interrogated. Other retrodirective techniques in addition to the phasing-matrix antenna are evaluated in terms of their relative merit. The systems considered include the active Van Atta array, the automatic three-dimensional electronic scanning, antenna (ATHESA), the Luneberg lens and various other antenna array techniques.     

An anti-jam packet data satellite link

With the advent of the open systems interconnection philosophy, interworking between satellite and ground-based data networks is rapidly becoming important. X.25 LAPB/HDLC is an important standard in this field. It defines a connection between two data switching nodes. This paper examines how a low capacity (～kb/s) anti-jam (A/J) satellite circuit may carry a X.25 LAPB/HDLC data link. It has been shown that use of an unmodified A/J modem employing convolutional coding gives a particularly poor performance in the presence of jamming. A study of the properties which govern the performance of a packet link has led to some recommendations for a special ‘packet mode’ for anti-jam modems and to some suggested extensions to the data link protocol. These recommendations have been implemented in a simulator and this has revealed a performance far superior to an unmodified system. The recommendations are being implemented in a trials system, developed at the University of Aberdeen, for RSRE Defford.     

Results from the 19- and 28-GHz COMSTAR satellite propagation experiments at Crawford Hill

Strong interactions of radio waves with rain and ice particles in the lower atmosphere significantly affect the performance of earth-space radio links operating at frequencies above 10 GHz. Because of the uncertainties and variabilities in the microstructures of weather phenomena, direct measurements of the effects of rain and ice on radio propagation are needed for the economic design of reliable satellite communications systems. Unique and comprehensive space-earth propagation measurements have been made at Crawford Hill, New Jersey, using the 19- and 28-GHz beacons on the COMSTAR satellites. This paper summarizes these measurements of rain attenuation, rain and ice depolarization, phase and amplitude dispersion, off-path rain-scatter coupling, angle-of-arrival variations, and cloud scintillation. These results provide empirical design data and will serve as checks on theoretical models needed for predicting communication system performance in geographical regions where measurements are not available.     

A microterminal satellite data communication system

An experimental microterminal satellite data communication system known as CODE has been developed by the European Space Agency with support from universities, research organisations and industry. The system, which operates at Ka band, consists of a large number of microterminals with 80 cm diameter antennas which can communicate directly in a mesh network at 64 kbit/s. A central hub station is used for channel allocation and for general monitoring and control of the microterminals. The system supports a wide range of standard application software which can run on standard personal computers and workstations. The design rationale of the system is described together with some of the applications and demonstrations that have been transmitted over the Olympus satellite to date     

Comparison of antenna noise temperature with rain attenuation of a satellite beacon signal at 12 GHz

Simultaneous measurements of antenna noise temperature Ta and satellite beacon attenuation were made to examine the quantitative relation between them, and to determine the `apparent? effective medium temperature which provides an accurate estimate of slant-path attenuation. The correlation between the attenuation and the increase in Ta due to rain is very good, and the effective medium temperature derived from the `apparent? one is reasonably consistent with a theoretical result.     

Inversion of the surface properties of ice sheets from satellite microwave data

Electromagnetic models are used as the basis for a least squares inversion technique to estimate the dry snow zone surface properties of the terrestrial ice sheets from active and passive microwave satellite data. Retrieved parameters include grain size, density, layer thickness, and accumulation rate. The prime motivation is to provide information of direct value to the Cryosat altimeter mission. The derived parameters can be used to convert from elevation change to snow mass change. They can also be used to predict geophysical retracking errors in altimeter data and to estimate the resulting uncertainty in the altimeter elevation measurement. With this technique, snow accumulation rate can also be estimated using passive microwave data. These data can then be compared to historical European Remote Sensing (ERS) satellite altimeter data in order to assess the impact of interannual variability in accumulation rate on the significance of rates of elevation change. The technique is in the preliminary stages of assessment, but is demonstrated using ERS-2 altimeter data in conjunction with spatio-temporally colocated Special Sensor Microwave/Imager (SSM/I) and QuikSCAT data. It is planned to apply the technique ultimately to Cryosat.