A report proposes a method to increase the accuracy of Doppler measurements made at the beginnings and endings of Earth/spacecraft radio occultations. Such measurements can reveal structural details of occulting objects in outer space. Heretofore, one-way measurements have been used, and have been subject to degradation of accuracy by frequency fluctuations of an onboard oscillator.
This work was done by Massimo Tinto of Caltech for NASA's Jet Propulsion Laboratory. To obtain a copy of the report, "Spacecraft radio occultations using multiple Doppler readouts," access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.
NPO-30216
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Uplink/Downlink Spacecraft Radio Occultation Measurements
(reference NPO-30216) is currently available for download from the TSP library.
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Overview
The document presents a report on a novel method for enhancing the accuracy of Doppler measurements during radio occultations between Earth and spacecraft, developed by Massimo Tinto at NASA's Jet Propulsion Laboratory. Traditionally, spacecraft radio occultations have relied on one-way measurements, which are limited by the frequency stability of onboard oscillators. This limitation affects the accuracy of the data collected, particularly in terms of signal-to-noise ratio (SNR) and the reconstruction of physical quantities characterizing the media through which the radio beam propagates.
The proposed method involves equipping spacecraft with a Doppler extractor to perform simultaneous uplink and downlink measurements. By combining these two data sets, the noise from the onboard oscillator can be effectively canceled out, leading to a significant improvement in frequency stability—by two to three orders of magnitude compared to previous methods. This enhancement allows for a more stable phase reference, determined by the atomic clock at the Earth station, which is inherently more stable than space-qualified oscillators.
The improvements in measurement accuracy are expected to yield several scientific benefits. The increased sensitivity will enhance the ability to investigate tenuous target regions, such as planetary rings and atmospheres, while also improving the dynamic range within opaque regions. Furthermore, the method is anticipated to provide greater spatial resolution for mapping both large- and small-scale structures of celestial targets.
The document emphasizes the technical novelty of this approach, highlighting how it addresses the limitations of prior art in radio occultation measurements. By eliminating the phase noise associated with onboard frequency references, the new technique allows for full utilization of the expected improvements in SNR from uplink measurements.
Overall, this advancement in Doppler measurement techniques represents a significant leap forward in the capabilities of radio occultation observations, potentially transforming our understanding of various celestial phenomena. The work was conducted under NASA's sponsorship and reflects ongoing efforts to enhance the precision of space exploration tools and methodologies.
