SOUNDER OPERATION
The Sounder is
intended to monitor the electron density Ne. The regions of interest
are especially :
• the dense cold equatorial plasma torus (between 2.5 and 7 Rs
of the planet)
• the plasma sheet (between 7 and 17 Rs) and the
outer magnetosphere, including Titan's neutral cloud (plasma source) and
ionosphere
• the Solar Wind density when its plasma frequency is >3.5
kHz (Ne >0.15 cm-3, i.e. up to Jupiter, and up to
Saturn during periods of high solar activity)
Sounder should
be operated on a regular basis (exploratory, mapping purposes), especially in
low Ne regions. Its duty cycle should however be lower than a few %,
to minimize possible disturbances to other observations or instruments.
Measurements are most interesting in the magnetosphere: as the S/C velocity
will be ~ 0.5 Rs/hour near periapsis, there should be 5-10 minutes
on the average between measurements for a spacing of 0.05-0.1 Rs
between measurements. This duty-cycle should be higher near Titan (one
measurement every 2.5 min.?), depending on the efficiency of passive Ne
measurement methods (essentially fuh determinations; sensitivity
will probably lack for quasi-thermal noise measurements). A lower duty-cycle
will be required in the Solar Wind (one measurement every ~20 minutes).
The complete
frequency range should be explored, at least during the first orbits. It could
be eventually reduced thereafter.
The Sounder is
incorporated in the HFR (receiver and preamplifiers). Technical heritage is
from ULYSSES. The sounder emitter produces a pulse (squarewave 24 V
peak-to-peak) at a frequency controlled by the HFR syntheziser on the Dipole
antenna. After eventual amplification through resonances excited at
characteristic frequencies in the ambiant medium, an echo is received on Ez
(or Ex if desired); a sounding sequence consists of m Passive sweeps (used as reference for
the identification of resonances, with the emitter Off) followed by n Active sweeps (with emitter On), such
as PAA (m=1, n=2) or AAA (m=0, n=3). Several active spectra taken
consecutively are useful to separate spurious signals or strong fluctuating
emissions from resonances.
During an
active sweep, emission is immediately followed by reception, frequency by
frequency: at frequency f, a pulse of width dt
and spectrum of width df centered on
f is emitted; 3 successive measurements are performed at each frequency (each
with AGC + auto-correlation, for a maximum dynamic range) within 30-50 msec,
from which 18 useful quantities can be later derived (decay scale of
resonances, etc...). A flag should be sent to other experiments when the
Sounder is active.
There are 90
frequency steps grouped in 5 bands of 18 channels:
Band
Bandwidth Excitation:
Pulse width
(kHz) (or Freq. resolution) (msec)
3.6 - 7.2 200 5
7.2 - 14.4 400 2.5
14.4 - 28.8 800 1.25
28.8 - 57.6 1600 0.625
57.6 - 115.2 3200 0.3125
A complete
sweep lasts for 2.7 to 4.5 sec. The bit-rate corresponds to 6 bytes (3 AGC + 3
auto-correlations) every 30-50 msec, or 960-1600
bps. The data output is thus 540 bytes (4320 bits) per sweep.
Start and stop
frequencies are programmable, so that only part of the band may be explored
(especially after a few orbits). The other programmable parameters include the
Wait times (T1, T2, T3) between excitation,
measurements and next frequency, the Receiving antenna, and the Number of
consecutive sounding cycles and of Passive/Active sweeps in each cycle.
There should
be only ONE type of Operating mode between 2 sounding sequences (i.e. no HFR
setup change), i.e. over a 2.5 - 20 minutes interval.