A method of arraying of receiving radio antennas involves utilization of all of the signal information available across a broad spectral band that includes any signal(s) of interest. As used here, "arraying" signifies combining the signals received by multiple antennas at different locations in such a way as to improve reception, as though one had a single larger antenna. Going beyond synthesis of a larger antenna, the present method also provides for extraction of Doppler frequency shifts and differential delays of signals, thereby enabling the generation of information on the ranges and velocities of signal sources. 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 radio astronomy, commercial satellite communications, and radio (including television) broadcasting.
In this method, the signals from the multiple antennas in an array are combined in real time by use of correlation processing, among other techniques, implemented by a combination of analog and digital electronic hardware and software. The signal received at each antenna is characterized by a delay and a Doppler shift that depend on the relative position and motion of the antenna and the spacecraft or other signal source. In order to achieve full-spectrum arraying, it is necessary to alter the signal received by each antenna to make its delay and Doppler shift equal to the delays and Doppler shifts of the similarly altered signals received by the other antennas. The altered signals are then combined coherently to obtain an improved detection of telemetry and navigation data.
In the original DSN application (see figure), the signals received by as many as eight geographically diverse antennas are processed by full-spectrum receivers (FSRs) followed by a full-spectrum combiner (FSC). The analog signal from each antenna is first down-converted to an intermediate-frequency (IF) band centered at 300 MHz. Then in an FSR, the IF signal is subjected to a combination of analog-to-digital (A/D) conversion and frequency down-conversion that yields an in-phase (I) and a quadrature-phase (Q) data stream, each consisting of 8-bit samples at a rate of 16 megasamples per second. The delay and phase of the I and Q streams from each antenna are altered by use of a delay line and a phase rotator. Adjustment is made first by using delay prediction, followed by a feedback measurement of residual delay and phase by the FSC.
In the FSC, cross-correlations of upper and lower sidebands from different antennas (e.g., of the upper sideband received by antenna 1 with the upper sideband received by antenna 2) are computed. The correlations contain information on frequency-dependent and frequency-independent phase offsets related in known ways to differential delays and Doppler shifts. The correlations are processed to generate phase and a delay offset for feedback to each FSR. The I and Q data streams from the FSRs are weighted and summed; the sum signal is then subjected to digital-to-analog (D/A) conversion and frequency up-conversion to obtain the desired enhanced IF signal.
This work was done by Andre Jongeling, Timothy Pham, and David Rogstad of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.