"Ultra-BOC" (where "BOC" signifies "binary offset carrier") is the name of an improved generic design of microwave signals to be used by a group of spacecraft flying in formation to measure ranges and bearings among themselves and to exchange telemetry needed for these measurements. Ultra-BOC could also be applied on Earth for diverse purposes — for example, measuring relative positions of vehicles on highways for traffic-control purposes and determining the relative alignments of machines operating in mines and of construction machines and structures at construction sites. Ultra-BOC provides for rapid and robust acquisition of signals, even when signal-to-noise ratios are low. The design further provides that each spacecraft or other platform constantly strives to acquire and track the signals from the other platforms while simultaneously transmitting signals that provide full range, bearing, and telemetry service to the other platforms. In Ultra-BOC, unlike in other signal designs that have been considered for the same purposes, it is not necessary to maneuver the spacecraft or other platforms to obtain the data needed for resolving integer-carriercycle phase ambiguities.

A prior design provided for the broadcasting of acquisition signals, followed by rough-clock-synchronization signals, followed by ranging and telemetry signals. In contrast, in Ultra-BOC, the acquisition, ranging, and telemetry signals are always present: Ultra-BOC combines the BOC structure with constant transmission of unmodulated tones (that is, subcarrier signals) as acquisition signals, plus low-rate clock synchronization data, a pseudorandom-noise (PRN) precise ranging code, and telemetry. A unique combination of code-division multiple access and frequency-division multiple access are employed to support simultaneous transmission and reception of these signals by many radio transceivers in the same allocated frequency band while enabling the use of the signals for precise metrology.

The acquisition signals (unmodulated tones) do extra duty by making it possible to increase the precision of range and bearing measurements: The ranging code used in Ultra-BOC is adequate to resolve the ambiguity of a synthesized delay formed by a pair of closely-spaced unmodulated BOC tones. This delay is used to resolve the ambiguity on a more widely spaced pair of tones. This process is continued with increasingly widely spaced tones until either the range and bearing precision requirements are satisfied by use of such pairs of tones or the integercycle ambiguities in the phases of the carrier signals are resolved. The range measurements made in this manner can be more precise than are those that can be made by use of the PRN codes alone, because (1) the delays synthesized from pairs of tones have smaller errors attributable to system noise and (2) multipath-induced errors are the leading errors in ranging by use of PRN and the delays synthesized from pairs of tones are less susceptible to multipath-induced errors.

This work was done by Lawrence Young, Jeffrey Tien, and Jeffrey Srinivasan 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. NPO-40569

This Brief includes a Technical Support Package (TSP).
Signal Design for Improved Ranging Among Multiple Transceivers

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This article first appeared in the December, 2004 issue of NASA Tech Briefs Magazine.

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