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| Titel |
Implementation of the earth-based planetary radio occultation inversion technique in Shanghai Astronomical Observatory |
| VerfasserIn |
Sujun Zhang, Jinsong Ping, Tingting Han, XiaoFei Mao, Zhenjie Hong |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250051775
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| Zusammenfassung |
| Comparing to the other atmosphere detection techniques, a radio science experiment only
involves the telecommunication subsystems onboard the spacecraft and ground stations, and
does not use any special space borne scientific instruments. By examining the changes in the
frequency, phase, amplitude and polarization of a radio signal propagating from a spacecraft
to a ground station, investigators can study the atmospheric and ionospheric structure of
planets and their satellites, planetary gravitational fields, shapes, masses, planetary rings,
ephemerides of planets, solar corona, magnetic fields, cometary comae, and such
aspects of the theory of general relativity like gravitational waves and gravitational
redshift[1].
The history of radio occultation studies goes back to the early 1960s. The first radio
occultation experiment was implemented in 1965 when Mariner IV flew by Mars. Scientists
used occultation data to determine the features of the Martian atmosphere, and proved that the
Martian atmosphere is predominantly CO2 and that the surface pressure is less than one
percent that of Earth, or an order of magnitude less than what had been previously
believed[2].
In 2007-2008, Shanghai Astronomical Observatory (SHAO) successfully conducted the
radio tracking experiment of the Chang’E-1 (CE-1) satellite. This experience is valuable for
the upcoming joint Russian-Chinese Yinghuo-1(YH-1) & Phobos-Grunt(FGSC) Mars
exploration project, in which the earth-based planetary radio occultation experiment will
be carried out. The newly developed SHAO Earth-based Planetary Occultation
observation Processing System (SPOPS) is especially designed for this project.
The main objective of SPOPS is to retrieve the temperature, pressure, molecular
number density profiles of Martian atmosphere and electron density profiles of the
ionosphere from the earth-based planetary occultation observations. This system
includes five parts: control module, observation arrangement module, observation
pre-processing module, ionosphere and neutral atmosphere inversion modules.
Utilizing the open-loop and closed-loop Doppler residual data of the Mars Express
(MEX) radio occultation experiment provided by the ESA Planetary Science Archive
(PSA) and the NASA Planetary Data System (PDS), the temperature, pressure,
number density profiles of the Martian atmosphere and electron density profiles of the
ionosphere are successfully retrieved by the Abel inversion method. The results
are validated by the released level 04 radio science products of the ESA MaRS
group.
As the MEX Doppler residual data has already been calibrated for effects such as: relative
position and motion of the ground station and spacecraft, tuning of the transmitter, and a
model-based correction for one- or two-way propagation through the Earth’s neutral
atmosphere. We have only to do a zero-reference correction for the Doppler data by
subtracting any Doppler bias that may remain after the gross effects of the trajectory, and
Earth’s atmosphere. This Doppler bias is determined by taking the mean of the Doppler
residuals obtained prior to the start of the profile.
The neutral atmosphere and ionosphere inversion profiles of MEXMRS_0046 on DOY
139, 2004 by SPOPS and the level 04 product of MaRS are compared. The lowest sample of
MEXMRS_0046 is located at (267.350Ë E, 52.523Ë N), with local true solar time 17.025h,
solar zenith angle 69.965Ë , and solar longitude 32.339Ë E. This is an observation in the
spring afternoon of the northern mid-latitude.
In the neutral atmosphere inversion by SPOPS, we assume the Martian atmosphere is
composed of 27% N2, 95.3% CO2 and 16% Ar[3]. Three upper boundary temperatures, 130K,
165Kand 200K are considered at the altitude of 40 km. The inversion results of the S-band
and X-band Doppler residuals by SPOPS are consistent. Compared with the X-band inversion
result, higher noise is observed in S-band, especially in the temperature profiles between 20
~ 40km, which may come from the small structures in the atmosphere, Doppler
quantification error, and the specific data processing method. Although three kinds of
atmospheric boundary conditions are adopted, the retrieved pressure and temperature
profiles converge to the same extent below 10 km above the Martian surface. For the
selected observation MEXMRS_0046, the pressure on the Martian surface is about
627Pa, the temperature is about 227K, and the molecular number density is about
0.2Ã1024/m3.
The retrieved ionosphere profiles by SPOPS generally agree well with the result given by
MaRS, with the same order of density magnitude and a similar profile shape. Both profiles
reflect the basic features of the main peak (M2) and secondary peak (M1) of the Martian
ionosphere. Because of the possible difference in height calculation and baseline correction
methods used, deviations exist in the M2 peak density and peak height of the two
profiles. Our derived M2 peak height is 130.53 km, and the peak electron density is
10.313Ã1010el/m3, while the results of MaRS are 137.933 km (potential height) and
10.891Ã1010el/m3 respectively.
In order to determine whether or not the two-way closed-loop and three-way open-loop
experiments can yield consistent results, we also compare the inversion results of the IFMS
A (X-band), IFMS B (X-band) and IFMS RS (S-band) closed-loop observation
(MEXMRS_0669) on DOY 355, 2005. The inverted neutral atmosphere profiles of IFMS A
(X-band), IFMS B (X-band), and IFMS RS (S-band) are basically consistent. Since the
sample rate of IFMS B and IFMS RS is ten times that of IFMS A, and IFMS RS is in the RS
post-processing mode, the temperature profile of IFMS A is smoother compared to the other
two profiles. The noise in IFMS B and RS profiles may come from the small structures of the
atmosphere, and may also come from some uncertain errors which need further study. It was
shown that the accuracies of the open-loop and closed-loop inversion results are
comparable.
The completion of SPOPS can be directly applied to the occultation observation process
in China’s upcoming YH-1 Mars Exploration Project.
Reference:
Asmar S W, Renzetti N A. The deep space network as an instrument for radio
science research. Pasadena: JPL Publication 80-93,Rev. 1, 1993
Kliore A J, Cain D L, Levy G S et al. Occultation experiment: results of the first
direct measurement of Mars’ atmosphere and ionosphere. Science, 1965.149:
1243-1248
Allen C W. Astrophysical Quantities. 4th ed. London: The Athlone Press, 1999.
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