Severe Fades Research Articles

Implementation of Advanced Carrier Tracking Algorithm Using Adaptive-Extended Kalman Filter for GNSS Receivers

Use of Global Navigation Satellite Systems (GNSS) receivers for real-time applications has improved significantly all over the world. The main problem with the designed receivers is their failure to function under harsh environmental conditions because of the structured phase-locked loop (PLL) architecture. One of the most critical phenomena that cause signal degradation in GNSS receiver is ionospheric scintillations, which create disturbances in amplitude and phase of the received signal. The problem in signal acquisition and tracking, even in the severe canonical fades (deep amplitude fading correlated with reference to half cycle phase jumps), can be mitigated using robust and adaptive carrier tracking algorithms. The autoregressive exogenous modeling parameters are useful in estimating the amplitude and phase scintillations. The proposed adaptive-extended Kalman filter (AEKF) approach works as an effective carrier tracking algorithm maintaining a balance in dual problems faced by PLL-based receivers, i.e., (estimation versus mitigation) and (dynamics versus noise reduction) tradeoff. The developed AEKF algorithm performed well for synthetic Cornell scintillation monitor data and for Global Positioning System L1 PRN 12 data collected around 21.30 H (local time) on October 24, 2012, in Rio de Janeiro, Brazil, with GNSS Software Navigation Receiver.

Multiplexed synchronization: an electronic warfare countermeasure

The authors present a method for transporting data through very noisy channels such as those encountered in electronic warfare. This method is based on two very important concepts. One of these is multiplexing via Boolean functions and the other is code combining. This technique spreads synchronization information throughout the message. The multiplexed synchronization technique is costly. The amount of traffic that must be sent per message is often quite large. The technique is robust, however, and may be of value for countering severe random fades and the severe and nonstationary interference processes encountered in electronic warfare.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Using Times-Four Carrier Recovery in M-QAM Digital Radio Receivers

We have investigated the performance of the times-four method of carrier recovery in a digital radio link transmitting M -level quadrature amplitude modulation ( M -QAM), where M can be 16, 64, 256, etc.. With the help of a recently built multipath fade simulator, we measured rms phase jitter for various combinations of fade notch frequency and fade depth. This paper reports on the experiment and the associated analysis. The times-four method has the virtues that it does not depend upon accurate data decisions, and it does not interact with the receiver's adaptive equalizer. Our results show that the rms phase jitter using this method, for all but the most severe and improbable fades, could be held to a few degrees. Although the measurements were done only for 16 QAM, the analysis predicts only modest increases for higher numbers of levels. The analysis also predicts, and the measurements verify, that the recovered carrier amplitude shrinks to zero for four distinct combinations of fade notch frequency and fade depth. To our knowledge, this phenomenon has not been reported in the literature heretofore. We show that, from a statistical viewpoint, the damage associated with it (measured in terms of outage time) should be quite small.

Adaptive Control of Satellite EIRP to Reduce Outage Caused by Fading

The output EIRP or the available bandwidth of a satellite in general limits the amount of traffic which can be handled by SATCOM systems. As the frequency congestion drives future SATCOM systems to higher frequencies, the former constraint will be the more important one because of the severe fades caused by precipitation. Rather than using fixed fade margins, the link power can be dynamically allocated from a common pool of satellite EIRP. By judicious, adaptive allocation of the satellite EIRP among the various users, the traffic supported can be maximized while maintaining the link availability. This paper reports techniques and statistical results of four different ways of allocating the EIRP. By using adaptive centralized control of the terminals transmit power and a variable satellite transmit antenna adaptively controlled by ground commands to compensate for link fades, most efficient use of the satellite EIRP is made. The performance of this technique is derived and compared with others of less complexity and that of a system with fixed satellite antenna and transmit power levels.

SS/TDMA Steady-State Synchronization: Analysis and Operation During Deep Signal Fades

An analysis of SS/TDMA steady-state synchronization is presented, in which the need for reliable sync maintenance during periods of deep signal fades, beyond the point of communications outage, is considered. The time required for sync-maintenance measurements, together with impact on guard time requirements, will lead to degradation of SS/TDMA transmission efficiency. The need to ensure a sufficiently low rate of occurence of errors in the syncmaintenance measurements also impacts transmission efficiency. A generalized formulation relating the degree of immunity to signal fading, degradation in transmission efficiency, and rate of occurrence of measurement errors is developed. Numerical results are obtained for a typical satellite communications system of current interest. It is shown that sync-maintenance systems can be designed to operate reliably during severe signal fades, orders of magnitude beyond the point of communication outage, at the expense of minimal degradation of transmission efficiency. For this level of performance it is shown that, with proper choice of parameters, virtually error-free syncmaintenance operation can be achieved. Numerical values of typical design parameters are presented.

Experimental swept-frequency tropospheric scatter link

An experimental swept-frequency tropospheric scatter link, with a path length of 194 miles, has been established in France between Cholet and Corbeville. The transmitter (designed by Compagnie Générale de Télégraphie Sans Fil) sweeps in a nearly linear manner from 3100 to 3600 mc and back to 3100 mc at about 10 cps. The average transmitted power is 250 watts. The receiver and data display (designed by Airborne Instruments Laboratory) sweep in synchronism with the transmitter. The synchronizing signal is derived from the carrier frequency of a broadcast station (Paris-Inter). The receiver is designed to track swept signal amplitudes down to about 4 db above noise level, and after severe fades, to reacquire signals larger than 6 db above noise level in about 1 to 2 milliseconds. Received signal amplitude is presented as a simultaneous function of frequency and time. Preliminary data obtained from the link indicate that privileged transmission frequencies exist at all times in this band at which the transmission level is about 10 db above median. The preliminary data gathered from the link are described, as well as general design features of the system.