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NOTE = "Experiment description for the
Galileo Gravitational Wave Experiments
conducted in 1993 (jointly with Ulysses
and Mars Observer from DOY 081 through DOY
102), 1994 (from DOY 118 through DOY 158),
and 1995 (from DOY 140 through 160).
Formatted for display or printing with up to
78 constant-width characters per line."
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The Galileo Gravitational Wave Experiment is an effort to search for
low-frequency gravitational waves generated by massive astrophysical systems.
Gravitational waves--waves of space-time curvature--are transverse, carry
energy and momentum, and propagate from their sources at the speed of light.
The strength of the waves is characterized by the strain amplitude, h, which
measures the fractional change in the separation of test masses and the
fractional change at which separated clocks keep time. In a spacecraft
gravitational wave experiment, the earth and a distant spacecraft act as
separated test masses, with the transponded 2- or 3-way Doppler signal
continuously measuring the relative dimensionless velocity delta-v/c between
the Earth and the spacecraft. The metric perturbation due to the gravity
wave, h, produces a signature in the Doppler time series that is of order h in
delta-f/f0 and is replicated three times in the Doppler time series: once when
the wave "shakes" the Earth, once when the wave shakes the spacecraft
(suitably delayed by a one-way light time) and once when the initial shaking
of the earth is transponded back to the earth a two-way light time later.
This three pulse response is crucial in discrimination of gravitational waves
from a noise background.
The Gravitational Wave Experiment will be most sensitive to waves having
periods ~100-1000 seconds. Waves with these periods are generated by
supermassive astrophysical systems undergoing violent dynamics. Searches will
be made for gravitational waves of differing temporal character: bursts (e.g.,
produced during formation, collision, and coalescence of supermassive black
holes), periodic waves (e.g., produced by black holes orbiting each other) and
stochastic waves ( e.g., produced at the Big Bang). Hybrids in this
classification scheme (e.g. chirp waves from coalescing binaries) are also
possible signals.
During the gravitational wave experiment, care must be taken to
maximize sensitivity. This leads to the following general requirements:
(1) To the extent practical, observations should be done in
the antisolar direction in order to minimize solar wind phase
scintillation noise.
(2) Tracking should be done with the highest radio frequencies
possible, again to minimize solar wind scintillation noise.
(3) Tracking should be done in the two- and three-way coherent
modes.
(4) Stations should be configured for maximum Doppler
stability.
(5) Tracking should be done during the highest elevatio angles
practical at each station.
(6) Data should be taken using both the closed and open loop
receivers with the Doppler sample rate set to be as large as is
practical to minimize aliasing of thermal noise into the digital
band.
(7) Where practical, an independent assessment of station
stability as well as the tropospheric and ionospheric noise
should be done.
(8) The spacecraft should be in quiet, minimum-dynamics modes.
(9) Engineering telemetry from the spacecraft, logs of station
and spacecraft events, etc. should be gathered to create a
master file of "veto signals".
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