PDS_VERSION_ID = PDS3 |
RECORD_TYPE = STREAM
OBJECT = TEXT
PUBLICATION_DATE = " "
NOTE = "Experiment description for the
Satellite Occultation Experiment conducted
starting in 1996 through 1997 (DOY 354
through DOY 215). Formatted for display
or printing with up to 78 constantwidth
characters per line."
END_OBJECT = TEXT
Satellite Occultation Measurement Experiment
The Radio Propagation Team (part of the Radio Science Team) conducted
occultation experiments during closest approach and during the cruise portion
of the orbit. The occultation experiments were done at Europa, Ganymede,
Callisto, and Io.
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 Satellite Occultation Experiment, the observation begins
at 10,000 km above the Satellite and continues until ingress. The
observation resumes at egress and continues until 10,000 km above the