An advanced tracking and data relay satellite system (ATDRSS) reference architecture that embodies many of the system features and technological enhancements considered essential is presented and described. The architectural and user service features of the existing TDRSS are reviewed, and certain evolutionary features that will take place by the mid-1990s are discussed. NASA's projected user service requirements for the first decade at the next century, which are the principal drivers for the ATDRSS architecture, are described, including such aspects as the real-time data rates and the quantity of simultaneous services that must be supported. The discussion of the ATDRSS reference architecture covers rationale, technology considerations, and key features of the future ATDRSS user service. A status summary of the ATDRSS program is given.>

Under examination in this paper is the uncoded bit error rate (BER) performance of a satellite communications system in which the modulation scheme is binary PSK, the transponder contains an arbitrary amplitude nonlinearity, and high-level pulsed radio frequency interference (RFI) is present. The RFI may be characterized as either pulsed Gaussian noise or pulsed CW, with the burst duration shorter than that of the information bit and arrivals assumed to follow a Poisson law. Of special significance is the fact that a general approach is presented for direct BER evaluations; this is in contrast to other approaches which may, for example, employ signal-to-noise ratio suppression factors. Computed results are based on arbitrarily specified RFI scenarios and the presence of one of three amplitude nonlinearities-hard limiter, clipper, or blanker. In the context of these three specific nonlinearities, performance curves demonstrate the superiority of an appropriately chosen blanker when the RFI environment is particularly severe. Comparisons between hard limiting and clipping are also made. Additional results examine performance sensitivity to information bit rate, variations in signal power, and the ratio of CW to noise content. With respect to the latter, CW effects are found to be most severe.

Pseudonoise (PN) coding may be incorporated into the signal structure of a communication system for a variety of reasons. Once present, the PN code must be appropriately acquired before it can be tracked by the receiver. Toward this end various acquisition schemes have been proposed. These schemes include combinations of predetection/postdetection processing and code search strategies. With regard to the latter, a uniform search over the entire code uncertainty region would be undertaken in the absence of a priori epoch information. In the presence of such information a more sophisticated scheme, which may generally be termed an "expanding window search," would tend to reduce acquisition time. The present paper focuses on uniform and expanding window search strategies with the goal of analytically developing the PN acquisition time statistics as functions of salient system parameters such as predetection SNR, detection and false alarm probabilities, false alarm penalty time, and a priori information on epoch location. The significance of this analysis is its general applicability to arbitrary postdetection processing schemes-e.g., fixed dwell and sequential detection. Computed performance results account for the above salient parameters, wherein sequential detection is employed in conjunction with uniform and selected expanding window search strategies. Performance curves demonstrate the utility of the analysis and provide intuitive insight into performance sensitivity to search strategy.

The coded bit error rate (BER) performance of a satellite communications system, wherein the satellite repeater contains an arbitrary nonlinearity and the system operates in the presence of pulsed radio frequency interference (RFI), is examined. A major result is an analytic method for determining soft decision statistics of the receiver demodulator output in which pulsed RFI effects are accounted for. It is further demonstrated how this result can be analytically applied to the approximate determination of the BER at the output of the Viterbi decoder when convolutional coding is employed. Computed results specialize the nonlinearity to either a hard limiter or clipper, in conjunction with an arbitrarily specified AM/PM characteristic. Performance curves examine BER sensitivity to RFI duty cycle, form of RFI (CW or noise), and various coding/decoding conditions.

The bit error rate (BER) performance analysis of a data communication system is generally based on the assumption that signal waveforms are ideal and hardware-induced distortion is absent. In a satellite communication system, such distortion arises in the satellite repeater, as well as in the transmitter and receiver portions. NASA, which is in the process of developing its Tracking and Data Relay Satellite System (TDRSS), is very much interested in understanding the impact of numerous hardware constraints, that have been identified, on BER performance. The present paper, which treats one segment of this broad problem area, examines the cumulative impact of nine forms of distortion induced by the repeater on BPSK and QPSK signals. These include frequency offset, filter amplitude and phase ripple, phase noise, spurious phase modulation (PM), AM/AM and AM/PM conversion, incidental AM, and spurious outputs. For the present analysis, the transmitter and receiver are assumed to operate in essentially ideal fashions and thermal noise is introduced at the receiver front end only. Computed results indicate that BPSK and QPSK performances are impacted in substantially different manners, with QPSK generally more sensitive to a given form of distortion. One noteworthy example is the combined impact of the phase noise and spurious PM parameters which affect BPSK only slightly, but lead to very rapid QPSK performance degradation as the parameter values increase. This and the other distortion effects are illustrated via computed parametric performance curves. Results also demonstrate the need to account for interactions among distortion parameters.

Employment of a repeater in a communication system may lead to the introduction of several sources of potential performance degradation. They are hard-limiting, IF filter group delay distrotion and center frequency offset, and additional noise in the links from transmitter-to-repeater and repeater-to-receiver. Specifically, the hardlimiter and IF filter may introduce signal waveform distortion while the former may also introduce signal-to-noise ratio suppression. In this paper the impact of the above on a pseudo-noise (PN) time-of-arrival (TOA) estimation system is considered. The assumed modulation scheme is differential phase shift keying (DPSK) and realistic nonconstant envelope signals are considered; the performance criterion is TOA estimate standard deviation. One surprising result is that the presence ot the limiter tends to degrade performance; this is contrary to corresponding results that have been derived for data detection. It was also found that performance degrades quite gradually with increasing IF filter differential group delay over the signal bandwidth. Finally, although analytical results presented here pertain specifically to the hard-limiter, they may be straightforwardly extended to determine the effects of repeaternth law devices.

Although the effects of a hard-limiting repeater on coherent phase shift keying (CPSK) data transmission systems have been studied, corresponding effects pertaining to differential phase shift keying (DPSK) systems are yet to be examined. The present paper contains the results of a study on DPSK data detection performance when a hard-limiting repeater is part of the overall data link under consideration, and white Gaussian noise is introduced at the repeater and receiver front ends; the extension to the multi-link system, containing cascaded hard-limiting repeaters, is also included. The results obtained were found to be analogous to those of previous CPSK studies. Specifically, the inclusion of the hard-limiter was generally shown to improve probability of error performance when downlink noise is present. A comparison of present results with those of previous ones for corresponding CPSK systems shows that DPSK is degraded noticeably more by downlink noise than its CPSK counterpart.

The present paper considers discrete time analyses of firstand second-order digital phase lock loops. These loops are characterized by the fact that they track the zero crossings of the incoming signal; consequently, the sampling intervals are nonuniform. The firstorder loop is analyzed for phase step and frequency step inputs; mean time to skip cycle is also considered. For phase step input, approximate expressions are obtained for the steady-state phase error probability density and phase error variance, the second of which leads directly to a theoretical prediction of threshold. The second-order loop is analyzed for frequency step input. Approximate expressions for the steady-state phase error probability density, phase error variance, and a theoretical prediction of threshold are obtained. The analyses are confirmed by numerical results and simulation.