This software processes the flyby spectra of the Chirp Transform Spectrometer (CTS) of the Microwave Instrument for Rosetta Orbiter (MIRO). The tool corrects the effect of Doppler shift and local-oscillator (LO) frequency shift during the flyby mode of MIRO operations. The frequency correction for CTS flyby spectra is performed and is integrated with multiple spectra into a high signal-to-noise averaged spectrum at the rest-frame RF frequency. This innovation also generates the 8 molecular line spectra by dividing continuous 4,096-channel CTS spectra. The 8 line spectra can then be readily used for scientific investigations.
A spectral line that is at its rest frequency in the frame of the Earth or an asteroid will be observed with a time-varying Doppler shift as seen by MIRO. The frequency shift is toward the higher RF frequencies on approach, and toward lower RF frequencies on departure. The magnitude of the shift depends on the flyby velocity. The result of time-varying Doppler shift is that of an observed spectral line will be seen to move from channel to channel in the CTS spectrometer. The direction (higher or lower frequency) in the spectrometer depends on the spectral line frequency under consideration. In order to analyze the flyby spectra, two steps are required. First, individual spectra must be corrected for the Doppler shift so that individual spectra can be superimposed at the same rest frequency for integration purposes. Second, a correction needs to be applied to the CTS spectra to account for the LO frequency shifts that are applied to asteroid mode.
This work was done by Seungwon Lee of Caltech for NASA’s Jet Propulsion Laboratory. This software is available for commercial licensing. Please contact Daniel Broderick of the California Institute of Technology at
This Brief includes a Technical Support Package (TSP).
Frequency Correction for MIRO Chirp Transformation Spectroscopy Spectrum
(reference NPO-47304) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA's Jet Propulsion Laboratory (JPL) detailing the Frequency Correction for the MIRO Chirp Transformation Spectroscopy Spectrum, specifically in the context of the Steins flyby conducted on September 5, 2008. It outlines the methodology and findings related to the analysis of spectral data collected during this mission.
The primary focus is on the processing of data from the Chirp Transform Spectrometer (CTS), which was used to measure thermal emissions from the asteroid Steins. The analysis involved a total of 191 spectra, out of which 23 spectra containing strong emission signals from Steins were selected for averaging. This selection was crucial as not all spectra exhibited significant signals. The averaging process was conducted over a time frame of 0 to 2 minutes from the closest approach to Steins, allowing for a focused examination of the thermal emission signals.
Figures included in the document illustrate the results of this analysis. Figure 2 presents the averaged spectra, while Figure 3 shows the spectra after the removal of background signals, which were defined as the average signal across all frequency bins for each band. The removal of the background signal was essential to isolate the true thermal emission from Steins, as it helped to eliminate variations caused by changes in the solid angle and thermal intensity during the flyby. The results indicated that the slopes observed in the original spectra were primarily due to these time-dependent changes rather than actual molecular line signals.
Figure 4 provides a time-dependent view of the background signal, further emphasizing the dynamic nature of the thermal emissions during the flyby. The document also includes technical details about the background signal calculation and the implications of the findings for understanding the thermal characteristics of Steins.
Overall, this Technical Support Package serves as a comprehensive resource for understanding the frequency correction techniques applied to MIRO CTS data, the significance of the Steins flyby, and the broader implications for future aerospace research and technology. It highlights the collaborative efforts of JPL and NASA in advancing our knowledge of celestial bodies through innovative instrumentation and data analysis techniques.
