DATA_SET_DESCRIPTION |
Data set Version 2.0
====================
The Science RIDs from the October 2018 review have been applied
to this ROSINA dataset. The ROSINA team also found corrupted
files in the RTOF, DFMS, and COPS science data V1.0 of this
dataset, these corrupted files have been removed in the V2.0.
The list of these removed files can be found in the
REMOVED_L2_V1_TO_V2.ASC file. Some of the DFMS PDS files were
missing in the L2 V1.0 and were added them into the V2.0.
The COPS science HK order also changed without modifying the
structure: the COPS HK table composed by the 5 last standard
COPS HK blocks are now in time order, this change is reflected
in the latest version of the ROSINA EAICD document. As a result,
V2.0 of this CODMAC L2 dataset has been created.
Data set Overview
=================
Rosetta was at distances between 26.89 and 0 km from the comet
during this time. The COPS HK packet was generated every 4s
during the last hour of the mission.
ROSINA was permanently analysing the coma of the comet with all
three sensors, except during wheel offloadings when the mass
spectrometers DFMS and RTOF were in standby and during orbit
control manoeuvers, when all sensors were off. Explanations
on the data generation of the ROSINA sensors can be found in
the OPERATION_LOGBOOK.ASC
Parameters
==========
DFMS and RTOF data contain neutral gas data as well as ion data.
The data are raw, uncalibrated data. COPS data contains nude
gauge pressure values of the combined spacecraft and cometary
signal.
Calibration
===========
COPS data are calibrated. However, the calibration factor has
been determined for Nitrogen gas, and may not be appropriate for
the the coma composition. For this the data from RTOF or DFMS
have to be taken into account.
Note on the DFMS and RTOF data:
The user of our level 2 data has to make the conversion from
pixel to mass and from ADC counts to ion current himself. In
this way he will be aware that the precision is limited and
depends on many parameters and he will use the data accordingly.
The procedure required to calibrate the DFMS is described in our
User manual, Annex D1 (DOCUMENT directory). ROSINA team are
developed the necessary algorithms, putting emphasis on the
quality control for the algorithm. They are defining quality
flags for the data which will then tell the user how well the
mass scale and the intensity scale have been defined.
For calibrated level 3 data with calibrated mass scale and
intensity we refer the user to the level 3 data.
Processing
==========
The DPU measures the ion current. To calculate the pressure,
the DPU use the following formula:
For the nude gauge:
p(mbar)NG = (Ion_current_fil(A) - Ion_offset_fil(A)) /
(Immission_current_fil(A) - Immission_offset_fil(a)) * S
For the ram gauge:
p(mbar)RG = (Ion_current_mic(A) - Ion_offset_mic(A)) /
(Immission_current_mic(A) - Immission_offset_mic(a)) * S
The Ion offset value is measured in background mode, that is
after switching on and then every 24h with the filament and the
microtips, respectively off. S is the calibration factor, it is
about 30 for the nude gauge and 5 for the ram gauge.
This factor depends of the temperature and the ambient gas.
Flux formula for the Ram gauge:
phi_out = 0.25 * SQRT((8*kB+T_RG) / (pi * mass)) *
(p_RG / (T_RG*kB))
phi_out = gas flux [1/m2*s]
kB = Boltzmann-constant = 1.38e-23 [J/K]
T_RG = ram gauge temp. [K]
mass = 18 amu = 2.9889e-26 [ Kg] (H2o is the default mass)
p_RG = ram pressure [Pa] = 100*[p(mbar)RG]
The pressure values given directly in the COPS housekeeping are
pressures for N2 at 20DegC. In order to derive the density this
pressure value has therefore to be converted to density as
follows:
n (m-3) = p /(kT) with p in Pa; T=293K, k=1.380*10e-23 J/K
e.g. if p=1*10e-10 mbar n=2.4*10e12 m-3 = 2.4*10e6 cm3
In order to get the real density, the relative contribution of
the different species in the coma has to be known, as ionization
cross sections are different for each gas. For more details see
the ROSINA user manual.
Software
========
No software is provided.
Media/Format
============
The standard distribution format for the data is an electronic
volume.
Coordinate System
=================
The geometry items SC_SUN_POSITION_VECTOR,
SC_TARGET_POSITION_VECTOR and SC_TARGET_VELOCITY_VECTOR provided
in the label of the data product are relative to the Earth Mean
Equator and Equinox reference frame of J2000.
SUB_SPACECRAFT_LATITUDE, SUB_SPACECRAFT_LONGITUDE are given in
the Cheops reference frame.
The SPACECRAFT_ALTITUDE gives the distance to the spacecraft
from the target center of mass. These parameters are computed at
time t=START_TIME.
Distances are given in km, angles in degrees.
Cheops Reference Frame
======================
Scholten, F., Preusker, F., Jorda, L, and Hviid, S.,
Reference Frames and Mapping Schemes of Comet 67P/C-G,
RO-C-MULTI-5-67P-SHAPE-V1.0:CHEOPS_REF_FRAME_V1,
NASA Planetary Data System and ESA Planetary Science Archive,
2015.
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