A signaling-and-detection scheme has been proposed for use in conjunction with the remote-monitoring concept described in "Beacon Monitoring Would Reduce Interactions With Remote Systems" (NPO-19706), NASA Tech Briefs, Vol. 21, No. 5 (May 1997), page 54. To recapitulate: A remote system (a spacecraft in the original intended application) would contain a semiautonomous computer-based monitoring subsystem that would analyze data acquired by onboard instrumentation and would produce a summary assessment of the status of the system. The summary assessment would be represented by one of four messages. To minimize the demand on communication resources and thereby minimize the cost of operation, the subsystem would ordinarily not transmit full telemetric data to a control station (a ground station in the original application). Instead, the subsystem would transmit one of the four messages as a radio beacon signal. Operators on the ground could decide, partly on the basis of the received beacon message, whether and when full telemetric data were needed. Full telemetric data would be transmitted from the remote system to the control station only on command by operators on the ground.
In the proposed scheme, each of the four beacon messages would be represented by a pair of subcarrier tones with unique frequency spacing. These tones would be phase-modulated onto a carrier signal with a modulation angle of 90°. The tones would be detected noncoherently at the control station. At the low data rate inherent in the four beacon messages, this scheme offers adequate performance at a signal level 10 dB below that needed for a bit-based, binary-phase-shift-keying (BPSK) scheme that is used to transmit full telemetric data. This 10-dB performance advantage could be exploited by reducing the transmitted power at the remote station and/or using a smaller antenna to receive the signal at the control station. The larger antenna needed to receive full telemetry could thus be allocated to other tasks until there was a need to receive full telemetric data from the remote system.
The proposed scheme, and two alternative weak-signal-detection techniques that could be used in the scheme, have been tested in a laboratory, but have not yet been tested in an operational system. In both detection techniques, decision statistics are derived from incoherent sums of power spectra and compared with thresholds. In one technique, the frequency drift of the signal is modeled after evaluation of the power spectra; in the second technique, the frequency model is applied before computation of the power spectra.
This work was done by Miles K. Sue, Robert Kahn, Gabor Lanyi, Victor Vilnrotter, Marvin Simon, Ted Peng, John Caraway, and Bruce Crow of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Electronic Systems category, or circle no. 121on the TSP Order card in this issue to receive a copy by mail ($5 charge).