A microwave aeronautical-telemetry receiver system includes an antenna comprising a seven-element planar array of receiving feed horns centered at the focal point of a paraboloidal dish reflector that is nominally aimed at a single aircraft or at multiple aircraft flying in formation. Through digital processing of the signals received by the seven feed horns, the system implements a method of enhanced cancellation of interference, such that it becomes possible to receive telemetry signals in the same frequency channel simultaneously from either or both of two aircraft at slightly different angular positions within the field of view of the antenna, even in the presence of multipath propagation.
The present system is an advanced version of the system described in “Spatio-Temporal Equalizer for a Receiving-Antenna Feed Array” (NPO-43077), NASA Tech Briefs, Vol. 34, No. 2 (February 2010), page 32. To recapitulate: The radio-frequency telemetry signals received by the seven elements of the array are digitized, converted to complex baseband form, and sent to a spatio-temporal equalizer that consists mostly of a bank of seven adaptive finite-impulse-response (FIR) filters (one for each element in the array) plus a unit that sums the outputs of the filters. The combination of the spatial diversity of the feed-horn array and the temporal diversity of the filter bank affords better multipath-suppression performance than is achievable by means of temporal equalization alone. The FIR filter bank adapts itself in real time to enable reception of telemetry at a low bit error rate, even in the presence of frequency-selective multipath propagation like that commonly found at flight-test ranges.
The combination of the array and the filter bank makes it possible to constructively add multipath incoming signals to the corresponding directly arriving signals, thereby enabling reductions in telemetry bit-error rates. The combination of the array and the filter bank also makes it possible to extract, in real time, pointing information that can be used to identify both the main beam(s) traveling directly from the target aircraft and the beam(s) that reach the antenna after reflection from the ground. Information on the relative amplitudes and phases of the incoming signals, which is indicative of the difference between the antenna pointing direction and the actual directions of the direct and reflected beams, is contained in the adaptive FIR weights. This information is fed to an angle estimator, which generates instantaneous estimates of the difference between the antenna-pointing and target directions. The time series of these estimates is sent to a set of Kalman filters, which perform smoothing and prediction of the time series and extract velocity and acceleration estimates from the time series. The outputs of the Kalman filters are sent to a unit that controls the pointing of the antenna.
For the purposes of the present system, each telemetry signal is assumed to be conveyed by a constant-envelope phase modulation, known among specialists as SOQPSK-TG, that is commonly used on flight-test telemetry ranges. The main distinction between the present and previously reported versions of this system lies in the algorithm governing the adaptation of the FIR filters. In the previously reported version of this system, the filter weights would be adapted by an algorithm, known in the art as the constant-modulus algorithm (CMA), which tends to lock onto the strongest constant-envelope signal while suppressing others.
The algorithm used in present system, denoted the interference-canceling constant-modulus algorithm (IC-MA), is an extended version of the CMA. The IC-CMA goes beyond the CMA by incorporating adaptive interference-canceling and multiple-beam-forming subalgorithms. Unlike the CMA, the IC-CMA does not lock onto a single signal: instead, utilizing adaptive estimates of cross-correlations between signals, it separates two interfering telemetry signals, making it possible to utilize both of them or, if desired, to ignore one of them. In addition, if multiple signals are present and the stronger ones are deliberately suppressed in early stages of IC-CMA processing, then weaker signals can sometimes be recovered.
This work was done by Ryan Mukai and Victor Vilnrotter of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Electronics/Computers category.
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Refer to NPO-44079, volume and number of this NASA Tech Briefs issue, and the page number.