An array of small optical receivers is proposed as an alternative to a single large optical receiver for high-data-rate communications in NASA's Deep Space Network (DSN). Because the telescope for a single receiver capable of satisfying DSN requirements must be greater than 10 m in diameter, the design, building, and testing of the telescope would be very difficult and expensive. The proposed array would utilize commercially available telescopes of 1- m or smaller diameter and, therefore, could be developed and verified with considerably less difficulty and expense.
The essential difference between a single- aperture optical-communications receiver and an optical-array receiver is that a single-aperture receiver focuses all of the light energy it collects onto the surface of an optical detector, whereas an array receiver focuses portions of the total collected energy onto separate detectors, optically detects each fractional energy component, then combines the electrical signal from the array of detector outputs to form the observable, or "decision statistic," used to decode the transmitted data.
A conceptual block diagram identifying the key components of the optical-array receiver suitable for deep-space telemetry reception is shown in the figure. The most conspicuous feature of the receiver is the large number of small- to medium-size telescopes, with individual apertures and number of telescopes selected to make up the desired total collecting area. This array of telescopes is envisioned to be fully computer-controlled via the user interface and prediction-driven to achieve rough pointing and tracking of the desired spacecraft. Fine-pointing and tracking functions then take over to keep each telescope pointed toward the source, despite imperfect pointing predictions, telescope-drive errors, and vibration caused by wind.
The turbulence-degraded image of the laser source in each telescope would be sensed by a focal-plane photodetector array, the outputs of which would then be digitized. The digitized array outputs would be synchronized and combined by field-programmable gate-array circuits that would execute digital-signal-processing algorithms, for both the individual telescopes and the entire array. Symbol detection and decoding operations would then be carried out on the synchronized and combined array signal. Receiver parameters would be controlled adaptively at each telescope to accommodate changing atmospheric conditions, thus optimizing the performance of the optical-array receiver in real time.
This work was done by Victor Vilnrotter, Chi-Wung Lau, Meera Srinivasan, Kenneth Andrews, and Ryan Mukai of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/ Computers category.