Rosetta/CONSERT
Technical description
In CONSERT (3D COmet Nucleus Sounding Experiment by Radiowave
Transmittion) narrow pulses of radio waves are transmitted through a cometary
nucleus. By transmitting and receiving the narrow pulses on the Orbiter
and on the Lander the attenuation and the time delay of the radio signal
propagating through the comet are determined.From such measurements the
electrical properties ands spatial structures of the cometary materials, as
listed in 'Science goals', can be derived.
CONSERT works as a tronsponder in the time domain. The apparently
complicated procedure reduces the required accuracy on the clocks on the
Orbiter and Lander and makes it possible to stay within the contraints
on mass and power consumption imposed on the space experiment.
The CONSERT experiment on the Orbiter and on the Lander both consists of
a transmit/receive antenna and a transmitter and receiver = contained
in an E-box.
A periodically coded signal will be transmitted from the orbiter to the
lander. The transmit/receive cycle last about 25 microsec. The signal
propagates through the comet and is received on the lander.
The transmit/receive cycle is repeated during about 200 msec.
The received signal is digitised and accumulated in the lander in order
to increase the signa to noise ratio. Once the accumulation is finished,
the signal is compressed to obtain the time/space resolution corresponding
to 100 nsec (about 20 metres in the comet). After the signal processing on
the lander, which determines the position of the strongest path, the landers
transmits the same code with the delay corresponding to the one of the
strongest path. The transmit/receive dycle last again about 25 microsec.
The cycle is repeated during 200 msec. The signal propagates back to the
orbiter by the same path, because the orbiter does not have time to move
much on this time sacale. The signal is received on the orbiter, accumulated
and stored in the memory in order to be sent to Earth. A ktotal measurement
cycle lasts about 1 sec.
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Antennas
Description of the Antennas on the Orbiter and Lander.
The CONSERT experiment has a transmit/receive antenna on both the orbiter
and on the lander. A fundamental requirement to the antenna is that it must
have a broad main antenna lobe centered on the antenna normal. This to ensure
that the whole comet is 'illuminated' by the main lobe when the antenna normal
is pointing towards the comet. No steering of the antenna is required in order
to explore the whole comet. A further requirement for the antenna is a
bandwidth of 10% of the center frequency. In order not to impose restraints
on the relative orientation of the orbiter and lander antennas it was decided
to use circular polarization. These 'electrical' requirements must now be
considered in view of 'mechanical' limitations on the antenna (especially
limit on the mass), which are different for the orbiter and the lander. This
lead to two different solutions for the antenna on the orbiter and lander.
A dipole has a broad radiation pattern as required. A half-wave dipole has a
large real component of the impedance as is needed for a wide bandwidth
antenna. Two dipoles oriented 90 degrees to each other and feed 90 degrees
out of phase form a circular antenna. The orbiter antenna is formed of two
crossed dipoles and two crossed dipole reflectors. To minimize the interaction
with other experiments on the spacecraft, the dipoles and reflectors are
mounted on a mast that deployed the antenna out to the side of the instrument
panel of the spacecraft
(
radiation diagram).
The linear scale length of the antenna elements is about half a wavelength at
the center frequency of 90 MHz, i.e. about 1.5 m. Clearly such a large antenna
has to be folded during launch and then deployed on command once the
spacecraft is in interplanetary space. The antenna is therefore constructed
of basically 10 linear elements: 2 masts: one to support the dipoles and
reflectors, and one to deploy the antenna away from the spacecraft; the
remaining 8 elements form pairwise the dipoles and the reflectors. In the
folded state the 10 elements are placed parallel to each other, and are
lashed together with a steel wire, which can be cut by a pyro cable cutter.
The elements are connected by springs such that when released the spring
forces deploy the antenna. (Antenna
pictures)
On the lander there are severe restrictions on mass and on the location of the
antenna. In order to ensure a good coupling of the energy radiated by the
antenna into the comet it is desirable to place the antenna as close to the
comet surface as possible, and possible at a height less than 1/10 of a
wavelength. In order not to interfere with possible later movements of the
lander the antenna could not be placed on the feet or legs of the lander.
Using the lander body as ground plane for monopoles, the antenna elements
could be placed on top of the body to yield good gain in the nadir
hemisphere, but that was mechanically not possible, and electrically the
antenna was far from the surface. In the end two monopoles were placed about
1/10 of a wavelength over the surface on the lander body base plate.
A monopole has a broad antenna diagram, and a relative large real component
of the impedance. Two monopoles oriented90 degrees to each other and feed
90 degreesout of phase form a circular antenna. These monopoles form the
lander antenna (
radiation diagram)
The linear scale of the monopoles is a quarter of a wavelength, or 0.8 m.
During launch the monopoles are folded back along the base plate and fixed
in place by the collapsed lander legs. As the legs deploy, spring forces
acting at the foot of the monopoles deploy the antenna. (antenna pictures).
E-box: Electronics
The antenna is connected to TR switch (transmit and receive switch). The Radio
Frequency section: High Fequency amplifier (HF), Band Pass filters, automatic
gain control (AGC), and a mixer with a 120 MHz Local Oscillator. The Wide band
Intermediate Frequency section (WIF) at 30 MHz, feeding in the in-phase and
quadrature demodulators (mixers). Low path filters are provided for both
I and Q channels, in front of the wide Base Band amplifiers (BBAmp).
A high pass section is also present to eliminate DC components. Each
receiver section (HF, WIF, and BB) has a maximum gain of about 30 db.
The in-phase and quadrature signals are convented by two 8-bits along
the digital converters (AD) and are accumulated in the coherent integrator
systems ("CANACCU"). This accumulation is done with the periodicity
of the transmitted signal.
The tuning system tuns the Orbiter Master Oscillator to the Lander
Master Oscillator, with a relative accuracy of 10^-7, and syncronize
the Orbiter Consert timetable and the Lander Consert timetable, with
an accuracy of less than 10 milliseconds. The tuning Phase Locked
Loop (PLL) is controlled by the on-board micro-controller. Once the
necessary tuning is achieved, the Digital to Analog Converter word is
frozen fixing the Master clock frequency, for the entire orbit.
The Transmitted is equipped with a shift register Pseudo-Noise
genrator, frequency multipliers, a phase modulator and a power
amplifier (HFPA). Frequency multipliers are used to generate Local
Oscillators for the Receiver (120 MHz and 30 MHz) and for the
Transmitter (90 MHz).
There are some functional differences between the Lander and the Orbiter
instruments. However we maintained the two designs as close as possible.
During the Tuning Phase, the tuning PLL system is only on the Orbiter,
the reference signal being by the Lander. During transponder operation,
the Lander micro-controller has to perform more complicated operations,
as it has to detect the peak of the correlated signal (position of the
time of arrival).
The two spcaecraft interfaces are physically and functionally different.
On the Lander, our telemetry contains vatious hausekeeping data (instrument
information), a summary of each compressed frame (arrival time for each
measurment point, level of the main peak, level of gain control...), and
a few dumps of the complete accumulated signal,. On the Orbiter, our
telemetry will contain the whole-accumulated signal from each received frame,
plus hausaekeeping data.
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