"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
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Signal Design for Improved Ranging Among Multiple Transceivers
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Overview
The document titled "Signal Design for Improved Ranging Among Multiple Transceivers" from NASA's Jet Propulsion Laboratory outlines advanced techniques for enhancing communication and ranging capabilities among spacecraft. It emphasizes the importance of signal design in achieving precise measurements and effective data transmission in space missions.
The signal design employs bi-phase shift keying (BPSK) modulation at a center frequency of 2115 MHz, combined with a pseudo-random noise (PRN) code operating at 10 Megachips per second. Additionally, the system incorporates unmodulated BOC tones at ±150 MHz and modulated tones at ±110 MHz, which carry synchronization information. This multi-layered approach allows for robust signal acquisition and processing.
The acquisition process is divided into three phases. In Phase 1, unmodulated tones at ±150 MHz are detected using model tones, requiring only a one-dimensional frequency search. Each spacecraft in the constellation transmits at a unique frequency, facilitating quick searches within a defined range. Phase 2 involves acquiring signals at ±110 MHz once the tones are detected, eliminating the need for further frequency searches. This phase also includes rapid bit synchronization and decoding of low-rate data, which helps approximate the clock offset between the transmitting and receiving spacecraft. Phase 3 focuses on acquiring the remaining high-rate PN code modulation and telemetry data by searching over a small delay range.
The document also discusses the ambiguity resolution process, which is critical for accurate ranging measurements. It outlines a three-step error analysis for ranging code measurements taken from pairs of antennas on a spacecraft. The errors are quantified for each step, with specific calculations provided for ranging errors and bearing angle errors. For instance, the document details how the ambiguity and error for different frequency separations are calculated, leading to precise bearing measurements.
Overall, the document serves as a technical support package, providing insights into the methodologies and calculations necessary for improving ranging accuracy among multiple transceivers in space. It highlights the significance of signal design in enhancing communication reliability and precision in aerospace applications, making it a valuable resource for researchers and engineers in the field.
