Tri T. Ha

Geostationary Satellite Navigation Systems

The concept of position determination using geostationary satellites as an alternative to the global positioning system (GPS) is studied. The advantage of a geostationary system is that only three, or at most four, satellites are required to cover the continental United States. A total of twelve satellites are sufficient for global coverage (excluding polar regions), or eight if only longitude and latitude, but not altitude, are measured. The system involves the determination of the range to either four geostationary satellites or, if the altitude is not measured, three geostationary satellites. The accuracy of the proposed systems are evaluated to obtain the rms error associated with position determination, and the concept for the implementation of measurements required by the systems is presented. The accuracy of the systems are adequate for civilian use in the continental United States; however, there is a degradation in accuracy as the location of the user approaches the equator.

A performance analysis of DS-CDMA and SCPC VSAT networks

Spread-spectrum and single-channel-per-carrier (SCPC) transmission techniques work well in very small aperture terminal (VSAT) networks for multiple-access purposes while allowing the Earth station antennas to remain small. Direct-sequence code-division multiple-access (DS-CDMA) is the simplest spread-spectrum technique to use in a VSAT network since a frequency synthesizer is not required for each terminal. An examination is made of the DS-CDMA and SCPC Ku-band VSAT satellite systems for low-density (64-kb/s or less) communications. A method for improving the standard link analysis of DS-CDMA satellite-switched networks by including certain losses is developed. The performance of 50-channel full mesh and star network architectures is analyzed. The selection of operating conditions producing optimum performance is demonstrated. >

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. >

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.

Optimum noise source impedance determination for GaAs FETs at room and cryogenic temperatures

Abstract An analytical technique is developed to determine the optimum noise source impedance for an extrinsic, or packaged, gallium arsenide field-effect transistor (GaAs FET) using only the small-signal s-parameters and the minimum noise figures available from the manufacturers data sheet. The procedure is then modified to treat the special case of the intrinsic, or chip, GaAs FET as well. The technique is then extended to determine the optimum noise source impedance and minimum noise figure at cryogenic temperature using the room temperature results. The procedure is used to determine the optimum noise source impedance of a Mitsubishi MGF-1412 and a Mitsubishi MGFC-1412 at room and cryogenic temperature. At room temperature and 4 9 GHz, the technique yields an optimum noise source impedance of 28·4 ft -Ω64·1 Ω for the extrinsic device as compared with the measured optimum noise source impedance of 20 Ω + j55Ω. At cryogenic temperature, the optimum noise source impedance obtained is 21·1 Ω + j70·8 Ω as...

Real Time Satellite Position Determination for TDMA Systems

A real time satellite position determination method using a single reference station is presented. The satellite position is determined from these three parameters: the distance between the reference station and the satellite that can be obtained by a single-hop measurement, the elevation angle, and the azimuth angle obtained from the antenna encoder. The error of the transmit frame delay is determined from the position equations, together with other possible errors. The accuracy is about ± 42 symbols for a 90 Mbit/s timedivision multiple access (TDMA) system using an encoder with a ± 0.001 Å resolution.

Circuit switched demand assignment multiple access

Abstract Demand assignment multiple access (DAMA) can lead to efficient utilization of expensive satellite channels although its equipment may be more expensive than equipment for fixed assignment. In many applications, the cost savings from DAMA may be substantial. In this overview fundamental concepts of DAMA suitable for a variety of applications are presented.

Noise models for gallium arsenide field-effect transistors at room and cryogenic temperatures

An analytical technique is developed to determine the optimum noise source impedance for a chip gallium arsenide field-effect transistor (GaAs FET) using only the small-signal s-parameters and the minimum noise figures available from the manufacturers data sheet. The technique is then extended to determine the optimum noise source impedance and minimum noise figure at cryogenic temperatures using the room temperature results. The procedure is used to determine the optimum noise source impedance of a Mitsubishi MGFC-1412 at room and cryogenic temperatures. At room temperature, the technique yields an optimum noise source impedance that is 16% in error as compared with the measured optimum noise source impedance. At cryogenic temperatures, the result is in error by 19% for the real part and by 9% for the imaginary part as compared with the available experimental value, and by 7% for the real part and by 12% for the imaginary part as compared with a more rigorous theoretical result.