| DATA_SET_DESCRIPTION |
Data Set Overview
=================
The GAS-Instrument on ULYSSES provides angular distributions
of count rates from interstellar helium particles (in the
NG-mode) or from celestial UV-intensities (UV-mode).
Therefore the data are presented in the form of maps rather
than as time series (as for most other ULYSSES experiments).
The intensities are mapped in an ecliptic coordinate system
centered in the spacecraft. In this system, the positions of
the Sun and the planets vary with time and with the position
of the spacecraft along its trajectory, however, the positions
of stars remain fixed. The actual positions of Sun, Earth, and
Jupiter are indicated by yellow, green and white dots,
respectively. The area around the Sun is excluded from
observation to avoid damaging intensities from direct
sunlight.
The intensities obtained in the UV-mode (the housekeeping
parameter HV given in the bottom line of each map, has a
positive value) are color coded in the range 10...1000
counts/s, in the NG-mode (HV has a negative value) in the
range 0.1...10 counts/s.
File Name Conventions:
[Note that the naming convention was modified from that
originally provided to ISO 9660 and DOS compliant 8.3 at the
PDS/PPI node. The description provided below reflects these
changes.]
The maps are provided in three different formats, as indicated
in the extensions of the filenames yddd_NV.ext.
1.) *.ps : postscript format.
2.) *.gif : ready-to-view GIF-format.
3.) *.txt : ASCII-format to allow further computerized
processing. This format is described in
detail in the separate file FORMAT.TXT. These
files are contained in a compressed format in
the *.ZIP-files.
The further information encoded in the filename is
yddd : date of measurement(yy: year,ddd: day of the
year). N : indicator of the telescope used,
1 = narrow field of view (NFOV),
2 = wide field of view (WFOV).
V : Version indicator,
1,2, ... : Running number of measurement
during that day.
C : The map is composited from
individual sub-images, named in
the lower left corner of the map.
Ix : This map contains data from more
than one scan through the covered
area (to improve counting
statistics).
For further information see the separate files
/CATALOG/REF.CAT and GASDATA.FMT.
Interstellar Gas (NG-mode):
Most of these maps are restricted to an area around the
apparent flow direction of the interstellar helium particles.
It is important to note, that the locally observed, apparent
flow direction deviates from the flow direction at large
distances (outside the solar system) for two reasons:
1.) The local flow direction results from a deflection of the
asymptotic direction due to the solar gravity (hyperbolic
orbits).
2.) The observed, apparent flow direction deviates from the
local flow direction due to the aberration effect,
resulting from a combination of the particle velocity
vector and the spacecraft velocity vector. This effect is
significant (of the order of 10 degrees) as the
spacecrafts velocity is of the same order as the particle
velocity (several 10 km/s).
The systematic variation in the flow intensity which can be
observed during over extended periodes in the ULYSSES mission
is a result of the variation of the relative energy of the
particles in the instrument's system. The detection efficiency
of helium particles is strongly energy dependent with a sharp
cut-off at energy around 30 eV (equivalent to a velocity of
about 30 km/s for helium particles). This threshold is only
exceeded when the spacecraft velocity (in magnitude and
direction) adds in a favourable way to the local particle
velocity. This is the case during extended periodes of the
ULYSSES mission, from launch to Jupiter fly-by (Nov. 1990 to
Feb. 1992) and in periods before and after perihelion (July
1994 to July 96 and again in 1999 to 2001).
Due to a residual sensitivity of the instrument to UV-photons,
even in the NG-mode, also the signal from UV-stars is
registered. This has been used to verify the pointing accuracy
of the instrument to be of the order to a few tenths of a
degree.
Neutral particles from Jupiter:
Close to Jupiter, in January to April 1992, the integral
fluxes of low energy neutral particles can be derived from the
observation of their angular distributions.
Record Format
-------------
The files are written in ASCII, free format. Each file
consists of a
- HEADER section (records 1 to 50) and the
- PIXEL-DATA section (records 51 to EOF)
1.) HEADER
records:
1-10: HOUSEKEEPING DATA of date:
1: date of start of measurements, source-filename
2-6: housekeeping-data (command register contents)
7: file generation date
8: ecl. longitude and latitude of Sun, as seen from
ULS
9: ecl. longitude and latitude of Earth, as seen
from ULS
10: ecl. longitude and latitude of Jupiter, as seen
from ULS
11-17: CELESTIAL DATA of date (ecliptic system, sun-centered)
11: date(yy,doy,hh,mm);
direction of spin axis (ecl.long; ecl.lat;(deg.))
12: position + velocity of ULS(ecl.kart.coord.(AU,
Km/s))
13: position + velocity of Earth(ecl.kart.coord.(AU,
Km/s))
14: position + velocity of Jupiter(ecl.kart.coord.
(AU, Km/s))
15-17: transformation matrix, S/C-system -> ecliptic
system
18: source-files for this image
19-20: TBD
21-50: Model parameter (TBD)
2.) PIXELDATA
records 51-end of file(EOF):
each record contains the data for one pixel in the fields:
1: measured counts/s * 100
2: total number of counts in the accumulation
period
3: star flag
0 = O.K.;
1 = pixel contaminated by star light
-1 = not yet determined
4: counts/sec, as calculated along the line of
sight from a model (dummy value = -99.)
5-7: TBD (dummy value = -99.)
8-EOR: 5 pixel coordinates
(each direction is given in longitude and
latitude (deg.) of the ecl.system, ULS
centered)
8: direction of the pixel center (line of
sight)
9-12: 4 directions to the corners of the
pixel,
(9-EOR): (there can be 3 or 5 or more corners, in
case the pixel intersects with the
borders of the coordinate system)
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