4 Apparatus and Principal Noise Sources
The detector consists of the earth and a spacecraft as separated test masses, electromagnetically-tracked
using a precision Doppler system. The ground stations for the Doppler system are the antennas of the
NASA/JPL Deep Space Network (DSN). Figure 3 shows DSS 25, the high-precision tracking
station used in the Cassini gravitational wave observations and other Cassini radio science
investigations.
Figure 4 shows an example of the other part of the Doppler system. This is the Cassini spacecraft
during ground tests. (Reference [68
] gives a popular discussion of the Cassini mission, the spacecraft, and
its instrumentation.) The Doppler system is shown functionally in Figure 5: A precision frequency standard
from the Frequency and Timing Subsystem (FTS) provides the frequency reference to both the transmitter
and receiver chains. On the transmitter side, the so-called exciter produces a near-monochromatic
signal, referenced to the FTS signal but at the desired transmit frequency. This is amplified
by the transmitter (with a closed-loop feedback system around the power amplifier to ensure
frequency stability is not degraded) and routed via waveguide to the transmitter feedhorn in
the basement of the antenna. (To correct for aberration the Ka-band transmit feed horn is
on a table which is articulated in the horizontal plane. This allows the Ka-band transmitted
beam to be pointed correctly relative to the received beam. The X-band feed is common to
both the transmit and receive chains.) In a beam waveguide antenna the transmitted beam is
reflected off of six mirrors within the antenna up to the subreflector (near the prime focus),
then back to the main dish and out to the spacecraft (passing first through the troposphere,
ionosphere, and solar wind). When the signal is received at the spacecraft it is amplified and
phase-coherently re-transmitted to the earth. The received beam bounces off the main reflector
to the subreflector and then, via mirrors and dichroic plates, to the receiver feed horn in the
antenna basement. The received signal is downconverted to an intermediate frequency where it is
digitized. The digital samples are processed to tune out the (very predictable) gross Doppler
shift, and reduce the bandwidth of the samples. For GW operations, the bandwidth of the
pre-detection data is typically reduced to 1 kHz, and those data are recorded to disk along with
the tuning information. The phase of the signal is detected in software and, using the tuning
information, the received sky frequency is reconstructed. This and the known frequency of
the transmitted signal are used to compute the Doppler time series. Removal of the orbital
signature and correction for charged particle and tropospheric scintillation gives Doppler residuals,
which are used in subsequent processing steps to search for GWs (or for other radio science
objectives [30, 124, 74, 22]).
Of course this cannot be done without introducing noise. The following
Sections 4.1 – 4.10 summarize the principal noises, their spectra or Allan
deviations,
and their transfer functions to the two-way Doppler time series.
shows DSS 25, the high-precision tracking
station used in the Cassini gravitational wave observations and other Cassini radio science
investigations.
