Propagation results of aeronautical satellite communicationexperiments using INMARSAT satellite

Aeronautical satellite communication experiments were conducted using a top-mounted high-gain antenna of the phased-array type. Slow fading of 3 to 4 dB in the L-band channel was observed when the wing direction coincides with the satellite direction, in contrast with the small fading observed in the cases where the aircraft wing is out of the beam pointing to the satellite     

Low-cost TDRSS communications for NASA's long duration balloonproject

A transponder and RF ground support equipment that are making tracking and data relay satellite system (TDRSS) communications available to lower budget NASA projects are discussed. The Goddard Space Flight Center (GSFC)/Wallops Flight Facility's long duration balloon project (LDBP) will be the first to take advantage of this TDRSS user hardware for communications through the TDRSS S-band multiple access system during their around-the-world scientific balloon flight campaigns. The transponder is one tenth the cost of the current generation space-qualified transponder, and the TDRSS user RF test set (TURFTS) is less than one half the cost of the currently available transponder test equipment     

CCRS C/X-airborne synthetic aperture radar: An Rand D tool for the ERS-1 time frame

The airborne synthetic-aperture radar (SAR) system developed for the Canada Centre for Remote Sensing (CCRS) is described. It consists of two radars, at C-band and X-band. Each radar incorporates the following features: dual-channel receivers and dual-polarized antennas; a high quality, 7-look, real-time processor; a sensitivity time control for range-dependent gain control; a motion-compensation system for antenna steering in azimuth and elevation; and baseband I and Q signal phase rotation. The system also uses a high-power transmitter with a low-power back-up. The SAR maps to either side of the aircraft, at high or low resolution, at incidence angles which in high resolution span 0° to 80°. Radar operating parameters, data products, key specifications and the motion compensation scheme used are presented. Properties of the real-time imagery are discussed and examples of C-band SAR data in the three operating modes are given     

Experiments on aeronautical satellite communications using ETS-Vsatellite

The world's first aeronautical satellite communication experiments, conducted at L-band frequencies using a commercial aircraft, are described. An airborne antenna with electronically steerable beam and communication equipment was installed in a B-747F freighter flying over transoceanic flight routes. The satellite used in these experiments is the Engineering Test Satellite-Five (ETS-V). During the test period, various experiments, such as antenna pattern measurements, transmission performances, and voice quality evaluation were conducted. As the airborne antenna is the key component for the aeronautical satellite communication system, emphasis has been placed on antenna characteristics. Its performance is found to be closely related to the fading characteristics in low-elevation areas and to transmission error performance     

Management of transponder resources in mobile satellite systems

Allocation of L-band spectrum for land mobile satellite (MSAT) services in Mobile-WARC-87 paves the way for the introduction of such services in the early 1990s. An overview of the proposed network architecture of the Canadian MSAT network is presented. Management of transponder resources, i.e. power, bandwidth, and spot-beam antennas, which poses a set of unique and significant problems in the design and operation of MSAT networks, is addressed. The three-level hierarchical approach used by the Canadian network is described, and the technical requirements for solving the above problems are identified. The two upper levels employ reconfigurable fixed assignment for allocating transponders to antenna spot beams, and transponder power and bandwidth to different services. The lowest level employs demand assignment and packet multiple access protocols for resource sharing among different subscribers. Uncoupling the resource management problems into manageable portions reduces the complexity and enhances the robustness of the network     

Some practical strategies for reducing intermodulation in satellitecommunications

Various heuristic procedures for obtaining practical solutions to the general one-level carrier frequency assignment problem are described. The problem treated is general in the sense that it accommodates the case where L of the N slots may be explicitly designated as prohibited and unavailable for assignment. This problem occurs in satellite transmission with many small carriers accessing the same transponder where, due to multipath and TV interference from crosspolarized transponders of the same satellite and from copolarized transponders of the adjacent satellites, some portions of the bandwidth of the considered transponder cannot be used. To permit comparison with respect to the intermodulation advantage and central-processing-unit time required, the case without prohibited slots is considered. The sequential insertion procedure in which, starting with two carriers at the two end slots, one additional carrier at a time is optimally inserted into one of the unassigned slots is found best when the ratio between the available bandwidth and the total carrier bandwidth is greater than about 125%. All the heuristic procedures produced assignments whose intermodulation advantages are all greater than the bandwidth ratio     

On-board multibeam deployable antennas using Ka,C, andS frequency bands

The configuration, on-ground tests, and design verification results for the multibeam antenna flight model for Engineering Test Satellite-VI (ETS-VI) are described for onboard multibeam antennas with thirteen and five beams for the Ka- and S-bands, respectively. The antennas feature Ka-band frequency reuse in alternate beams through use of a cluster horn; shared use of a reflector for two frequency bands using a novel frequency selective surface (FSS); large, lightweight deployable main reflectors with diameters of 3.5 m and 2.5 m; and a highly accurate antenna-pointing control system consisting of RF sensors and an antenna-pointing mechanism to drive the subreflector     

Frequency diversity in millimeter wave satellite communications

Dual-band frequency diversity seems to be an effective fade countermeasure to rain-induced attenuation in satellite communications above 20 GHz. This method, particularly suited for satellites operating in two frequency bands, typically K_a band and C or K_u band, achieves very low levels of outage probability especially where the rain fades are severe. A theoretical analysis of a frequency diversity system is performed to evaluate the amount of reserve capacity needed to counteract fading in a satellite network. The problems that arise in implementing adaptive fade countermeasures because of the dynamic characteristics of fading are discussed, and the results of the simulations performed using the attenuation time series at 11.6 GHz, measured with the Sirio satellite throughout four years, are presented     

The autocorrelation of m-sequences over nonprime finitefields

Nonbinary m-sequences (maximal length sequences) for spread-spectrum communication systems that have a two-level autocorrelation are presented. The autocorrelation function of an m -sequence over the Galois field of q elements GF(q), where q=p^k, for p a prime and k an integer greater than 1, is developed and shown to be bilevel when the elements of GF(q) are expressed as elements of a vector space over the pth roots of unity     

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     

Polar ionospheric disturbance effects on NNSS satellite positioning

From the analysis of the United States Navy navigation satellite system (NNSS) positioning data of about 10000 passes, it is clearly found that polar ionospheric disturbance affects the positioning. The positioning error increases with increasing geomagnetic disturbance level (local K-index), and the pass number of position fixes decreases by one or two per day when the K-index is large. These effects may arise from the spatial gradients of electron density and/or the radio wave scattering due to well-developed ionospheric irregularities inherent to the disturbed auroral ionosphere     

Periodic ambiguity function of CW signals with perfect periodicautocorrelation

A periodic ambiguity function (PAF) is discussed which describes the response of a correlation receiver to a CW signal modulated by a periodic waveform, when the reference signal in the receiver is constructed from an integral number N, of periods T, of the transmitted signal. The PAF is a generalization of the periodic autocorrelation function, to the case of non-zero Doppler shift. It is shown that the PAF of N periods is obtained by multiplying the PAF of a single period by the universal function sin(Nπν T)/N sin(πνT), where ν is the Doppler shift, to phase-modulated signals which exhibit perfect periodic autocorrelation when there is no Doppler shift. The PAF of these signals exhibits universal cuts along the delay and Doppler axes. These cuts are functions only of t, N and the number M, the modulation bits in one period     

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 ? resolution.     

Geometric accuracy in airborne SAR images

Uncorrected across-track motions of a synthetic aperture radar (SAR) platform can cause both a severe loss of azimuthal positioning accuracy in, and defocusing of, the resultant SAR image. It is shown how the results of an autofocus procedure can be incorporated in the azimuth processing to produce a fully focused image that is geometrically accurate in azimuth. Range positioning accuracy is also discussed, leading to a comprehensive treatment of all aspects of geometric accuracy. The system considered is an X-band SAR     

Cancellation of specular and diffuse jammer multipath using ahybrid adaptive array

It has been demonstrated that specular or diffuse jammer multipath can be canceled to a desired level by using an adaptive array that combines bandwidth partitioning with tapped delay lines. Such hybrid systems are studied. In particular, the author studies an ideal two-element array that uses bandwidth partitioning in both the main and auxiliary channels, with an Mth-order adaptive finite impulse response filter in each subband of the auxiliary. The ability of this system to cancel specular moderately diffuse and diffuse multipath is studied. The combinations of bandwidth partitioning and filter order that can achieve a specified jammer cancellation level are discussed     

Cascaded detector for multiple high-PRF pulse Doppler radars

A postdetection design methodology for a multiple high-pulse-repetition frequency (PRF) pulse Doppler radar has been developed. The postdetection processor consists of an M out of N detector where range and target ambiguities are resolved, followed by a square-law detector which enhances the minimum signal-to-noise (S/N) power-ratio per pulse burst performance. For given probabilities of false alarm and detection, formulas are derived from which the three thresholds associated with the cascaded detector can be found. Fundamental tradeoffs between the minimum S/N required, number of ghosts, and the number of operations (NOPs) that the cascaded detector must perform are identified. It is shown that the NOPs and the number of ghosts increase and the minimum S/N required decreases as the binary M out of N detector passes more detections to the square-law detector     

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.     

Position and force control of coordinated multiple arms

A technique is presented for controlling multiple manipulators which are holding a single object and therefore form a closed kinematic chain. The object, which may or may not be in contact with a rigid environment, is assumed to be held rigidly by n robot end-effectors. The derivation is based on setting up constraint equations which reduce the 6×n degrees of freedom of n manipulators each having six joints. Additional constraint equations are considered when one or more degrees of freedom of the object is reduced due to external constraints. Utilizing the operational space dynamic equations, a decoupling controller is designed to control both the position and the interaction forces of the object with the environment. Simulation results for the control of a pair of two-link manipulators are presented     

GPS code and carrier multipath mitigation using a multiantennasystem

Multipath is a major source of error in high precision Global Positioning System (GPS) static and kinematic differential positioning. Multipath accounts for most of the total error budget in carrier phase measurements in a spacecraft attitude determination system. It is a major concern in reference stations, such as in Local Area Augmentation Systems (LAAS), whereby corrections generated by a reference station, which are based on multipath corrupted measurements, can significantly influence the position accuracy of differential users. Code range, carrier phase, and signal-to-noise (SNR) measurements are all affected by multipath, and the effect is spatially correlated within a small area. In order to estimate and remove code and carrier phase multipath, a system comprising a cluster of five GPS receivers and antennas is used at a reference station location. The spatial correlation of the receiver data, and the known geometry among the antennas, are exploited to estimate multipath for each satellite in each antenna in the system. Generic receiver code and carrier tracking loop discriminator functions are analyzed, and relationships between receiver data, such as code range, carrier phase, and SNR measurements, are formulated and related to various multipath parameters. A Kalman filter is described which uses a combination of the available information from the antennas (receivers) in the multiantenna cluster to estimate various multipath parameters. From the multipath parameters, the code range and carrier phase multipath is estimated and compensated. The technique is first tested on simulated data in a controlled multipath environment. Results are then presented using field data and show a significant reduction in multipath error