PDS_VERSION_ID = PDS3 |
RECORD_TYPE = STREAM
OBJECT = TEXT
PUBLICATION_DATE = " "
NOTE = "Experiment description for the
Jupiter Occultation Experiment conducted
starting in 1995 through 1997 (DOY 342
through DOY 320). Formatted for display
or printing with up to 78 constantwidth
characters per line."
END_OBJECT = TEXT
Jupiter Occultation Measurement Experiment
The Radio Propagation Team (part of the Radio Science Team) conducted the
first Jupiter Occultation Experiment on 95-342 approximately 8h 45m after JOI
(Jupiter Orbit Insertion). Thus, this experiment was the first science data
received on the Earth during the Galileo Tour. This occultation was the
first radio sounding of Jupiter's upper atmosphere and ionosphere since the
Voyager 2 fly-by of Jupiter in July 1979.
In general, an occultation of the Earth occurs when the spacecraft
travels behind a body from the Earth"s point of view (or, from the space-
craft's point of view, the body "occults" the Earth). Ingress occurs when
the spacecraft disappears behind the body, and egress occurs when the
spacecraft reappears on the other side. Right before ingress and right
after egress, the signal travels from the spacecraft to the Earth through
the body's atmosphere (or ionosphere). These are the periods in which
Radio Science is interested.
The atmosphere (or ionosphere) acts like a lens, slightly changing
the signal as it passes through. The signal actually "bends" around the
planet, and a phase change is observed on the Earth. Since the spacecraft
position is known, the amount of refraction caused by the atmosphere (or
ionosphere) can be deduced. The amount of refractivity is plotted verses
atmospheric (ionospheric) dept in a refractivity profile. The current
model of the atmospheric composition (ionospheric density) of the body,
which takes into account the types and behavior of the gases (electrons)
is then altered to account for this refractivity profile. Temperature and
pressure profiles can be developed for the atmosphere from this data along
the ingress and egress radio pathways and electron density profiles can be
developed for the ionosphere along the same pathways.
For the Jupiter Occultation Experiment, the signal was recorded
starting at 25,000 km above the 1-bar pressure level at Jupiter before
ingress and continued to be recorded out to a 25,000-km altltude after
egress. This provided a baseline before the signal began changing due to
the atmosphere. The nominal duration of the Jupiter occultation was 3h 31m
and the baseline (corresponding to 25,000 km) before ingress and after
egress was originally 1h. But, because of uncertainty in the time of
ingress/egress, the baseline was extended to 1h 20m. The entire observation
was 6h 11m in duration and occurred entirely over DSS 63 (Madrid).
The spacecraft attitude was held between 5 and 10 degrees. This
experiment was conducted in a one-way mode (downlink referenced to the USO
on the s/c or TWNC ON) and the DSP was used to record the open-loop data
over DSS 63. Normally, the TMU (telemtry modulation unit) drivers would
have been turned off for the duration of the occultation experiment
(i.e., Mod Index = 0 degrees) but Project Policy #38 did not allow the
telemetry drivers to be completely turned off until the Probe data was
completely retrieved. Therefore, the drivers were turned to their lowest DN
(DN=0 is equivalent to a modulation index of 20 degrees).
This was the first Earth Occultation by Jupiter Experiment in the
Tour and both ingress (-25.7 degrees) and egress (-44.8 degrees) were in the
southern mid-latitude region of the planet. Of the eight Jupiter occultations
in the Tour (including this one), four will be in the southern mid-
latitude region, two in the equatorial region, and two will be in the
northern mid-latitude region. This accumulation of atmospheric profiles is
one of the key elements which improve Galileo radio occultation measurements
over Voyager measurements. Ionospheric measurements of electron number
density will probably be limited to the region above the 800 km altltude
(relative to 1 bar), corresponding to Jupiter's topside ionosphere.