A digital-signal- processing algorithm (somewhat arbitrarily) called “SUMPLE” has been devised as a means of aligning the outputs of multiple receiving radio antennas in a large array for the purpose of receiving a weak signal transmitted by a single distant source. As used here, “aligning” signifies adjusting the delays and phases of the outputs from the various antennas so that their relatively weak replicas of the desired signal can be added coherently to increase the signal to-noise ratio (SNR) for improved reception, as though one had a single larger antenna. The method was devised to enhance spacecraft-tracking and telemetry operations in NASA’s Deep Space Network (DSN); the method could also be useful in such other applications as both satellite and terrestrial radio communications and radio astronomy.
Heretofore, most commonly, alignment has been effected by a process that involves correlation of signals in pairs. This approach necessitates the use of a large amount of hardware most notably, the N(N - 1)/2 correlators needed to process signals from all possible pairs of N antennas. Moreover, because the incoming signals typically have low SNRs, the delay and phase adjustments are poorly determined from the pairwise correlations.
SUMPLE also involves correlations, but the correlations are not performed in pairs. Instead, in a partly iterative process, each signal is appropriately weighted and then correlated with a composite signal equal to the sum of the other signals (see Figure 1). One benefit of this approach is that only N correlators are needed; in an array of N>>1 antennas, this results in a significant reduction of the amount of hardware. Another benefit is that once the array achieves coherence, the correlation SNR is N - 1 times that of a pair of antennas.
Two questions about the performance of SUMPLE have been investigated by computational simulation. The first question is that of how SUMPLE performs at the beginning of a signal-processing pass, before coherence is achieved among the antennas. The second is a question of phase wandering: In some other methods of correlation, one antenna is designated the reference antenna and all the other antennas are brought into alignment with it. However, in SUMPLE, all the antennas are aligned to what amounts to a “floating” reference. There is concern as to whether the phase of the floating reference wanders as a function of time, introducing unknown phase instability.
In one simulation, the combining loss as a function of time (equivalently, as a function of the number of iterations) was computed for a 100-antenna array by use of SUMPLE. At the beginning of the simulated reception process, the signal phases were taken to be random, resulting in a very large combining loss. The combining loss was found to decrease to a few tenths of a decibel in about eight iterations and to remain at this level thereafter (see Figure 2). Simulations of many different array configurations yielded essentially the same results.
Answering the question of phase wandering, the simulations did, indeed, show slow phase variations of a few degrees over time intervals of 10 to 20 iterations. However, it was found that this wandering could be prevented by forcing, to zero, the total phase correction obtained by summing the individual corrections over all the antennas. Inasmuch as the phase corrections are meant to bring the antenna signals into alignment with each other, forcing the total phase correction to zero does not pose an obstacle to the achievement of array coherence.
SUMPLE has been tested on an array of 34-m-diameter antennas in the DSN. The results of this test have been found to agree with those of the simulations.
This work was done by David Rogstad of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free online at www.techbriefs.com/tsp under the Electronics/Computers category. The software used in this innovation is available for commercial licensing. Please contact Karina Edmonds of the California Institute of Technology at (818) 393-2827. Refer to NPO-40574.