Timothy Pratt

Prediction of slant path rain propagation statistics using dual-polarized radar

The authors conducted a year-long experiment in which a dual-polarized S-band radar probed the volume surrounding two 11.45-GHz satellite downlink paths during rain. Accuracy was assessed by comparison to directly measured link attenuation with two 11-GHz beacon receivers 7.3-km apart at an 18.5 degrees elevation angle, one colocated with the radar. Drop size distributions calculated from the radar horizontal reflectivity (Z/sub H/) and differential reflectivity (ZDR) measurements were used to predict 11.45-GHz satellite beacon attenuations. The radar-predicted attenuations and those measured on the radio links agree, both on an event basis and in terms of annual cumulative distributions. >

Coded multiple chirp spread spectrum system and overlay service

An asynchronous spread-spectrum system called coded multiple chirp is proposed, and the possible spread-spectrum overlay over an analog FM-TV is investigated by computer simulation. In the proposed system, multiple single-sloped up and down chirps are encoded by a PN code and decoded by deciphers (pulse compression filters) followed by a digital code correlator. Performance of the proposed system, expressed in probability of bit error and code miss probability, is similar to that of noncoherent FSK using codewords, if sufficient compression gain is used. In the overlay where chirp is used to overlay an FM-TV channel, two chirp signals with data rate up to 25 kb/s could be overlaid in a 36 MHz satellite transponder without significant mutual interference. Performance estimates for a VSAT earth station operating at C-band show that a 2.4 m antenna and 300 mW transmitter could send a 2.4 kb/s signal to a large central earth station over an occupied channel. >

Satellite path diversity reception at 11.4 GHz: direct measurements, radar observations, and model predictions

Two-site diversity gain measurements made with a pair of 11.4 GHz satellite beacon receivers are compared to predictions based on dual-polarized radar reflectivity data and to values calculated from the Hodge model. All three agree closely. Examinations of data for a third path synthesized from radar data alone indicate that baseline orientation has a negligible effect on diversity gain. >

Spread-Spectrum Multiple Access Using Wideband Noncoherent MFSK

Two spread-spectrum multiple access systems which usewideband M-ary frequency shift keying (FSK) (MFSK) as theprimary modulation are presented. A bit error rate performanceanalysis is presented and system throughput is calculated for sample C band and Ku band satellite systems. Sample link analyses areincluded to illustrate power and adjacent satellite interferenceconsiderations in practical multiple access systems.

Dispersion in the 10-30 GHz Frequency Range: Atmospheric Effects and Their Impact on Digital Satellite Communications

Earth-space communications links operating above 10 GHz may be adversely affected by the earths atmosphere and hydrometeors, particularly rain. These propagation phenomena are usually evaluated only for fading and depolarization effects. However, dispersion is an important factor in wide-band digitally modulated satellite links. In this paper we examine the mechanisms responsible for dispersion over bandwidths as great as 500 MHz in the 10-30 GHz range. The amplitude and phase effects of dispersion are quantified for a wide range of propagation conditions. The resulting propagation model is then used to generate estimates of performance degradation in wide-band QPSK systems through the use of computer simulation.

Results of a three-year 11.6 GHz low-angle propagation experiment using the SIRIO satellite

The results of three years of attenuation and depolarization measurements at 11.6 GHz on a 10.7\deg elevation angle downlink from the SHRIO spacecraft are summarized. Cumulative distributions and worst-month statistics for rain rate, attenuation, and cross-polarization discrimination (XPD) are presented. Beacon attenuation and radiometric attenuation are compared, and the statistics of fade durations and of rapid XPD changes are discussed.

Results from the Virginia Tech propagation experiment using the OLYMPUS 12, 20, and 30 GHz-satellite beacons

Virginia Tech has performed a comprehensive propagation experiment using the OLYMPUS satellite beacons at 12, 20, and 30 GHz. Four receive terminals were designed and constructed, one terminal at each frequency plus portable one with 20 and 30 GHz receivers for microscale and scintillation studies. Total power radiometers were included in each terminal to set the clear air reference level for each beacon and to predict path attenuation. Statistical results for one year of data collection have been analyzed. The following studies were also performed: a microdiversity experiment in which two closely spaced 20 GHz receivers were used, a comparison of total power and Dicke switched radiometer measurements, frequency scaling of scintillations, and adaptive power and control algorithm development. Attenuation statistics, attenuation ratios, fade and inter-fade durations, and fade slope for all three frequencies have been examined.<<ETX>>

Instantaneous diversity gain in 10-30 GHz satellite systems

As usually defined, diversity gain is a statistical measure which conveys little or no information about the instantaneous behavior of site-diversity reception in a satellite communications system. A new quantity called instantaneous diversity gain is introduced and some measurements of it from an 11.6 GHz low-angle two-site downlink are presented. It is shown how instantaneous diversity gain is related to system reliability and some results are presented which indicate that designs based on statistical diversity gain will achieve their intended reliability levels.

System Performance of Dual Polarization Satellite Communication Links Affected by Attenuation and Depolarization

New models have been developed to analyze the performance of dual polarization frequency re-use links when affected simultaneously by attenuation and depolarization. The models have been used to determine the baseband signal to noise ratio for analog modulation links using FM and bit error rate for links using digital modulation (PSK). The methods are applicable to links using slant paths to satellites and also to terrestrial links.

On estimating an upper bound to MSE of position error in time-of-arrival position location systems

This paper defines a closed-form expression for an upper bound to MSE of position error in terms of statistics of an observable parameter: the residual. The upper bound estimation is verified through simulations including different geometries of receivers and different magnitudes of errors. The number of receivers needed to measure accuracy is discussed for the proposed bounds. Furthermore, this paper introduces a novel measure of accuracy of a positioning system using a single receiver, given the smallest number of receivers used to locate a transmitter.