Rosetta/CONSERT
Principle of measurements
In CONSERT (COMmet Nucleus Sounding Experiment by Radiowave Transmissuin)
narrow pulses of radio waves are transmitted through a cometary nuceaus.
The phase and amplitude of the radio signal are measured after the signal
has passed through the comet and are compared to the theoretical value
for propagation in vacuum. The changes in phase and amplitude reflect
the electrical properties and spatial structures of the cometary materials.
Stated differently the experiment measures the attenuation and the time
delay of a short pulse of radio signal propagating through the comet.
The experiment is operting in the VHF band at 90 MHz. It consists of a
transmitter/receiver on the Orbiter and of a transmitter/receiver on
the Lander. Om each spacecraft the experiment uses a single transmit/receive
antenna. In principle a narrow pulse (100 ns wide or 10 MHz signal
bandwidth) is transmitted from the Orbiter. The pulse propagates through
space, enters the comet nucleus, propagates through the nucleus, and
emerges from the nucleus. At the position of the Lander the signal is
received, and its amplitude and phase (attenuation and time delay) are
detected. As the Orbiter moves in its orbit and as the comet spins, the
relative positions of the Orbiter and Lander is changing with time.
Thus, measurements of a function of time corresponds to measurements
along varying radio waves paths through the comet.
The wanted time delay, tau, can be written as:
tau = tRX - tTX = L/c + r1*sqrt(e)/c
where:
| tTX |
time of transmitting |
| tRX |
time of receiving |
| L |
Orbiter/Comet surface distance |
| r1 |
path length in nucleus |
| e |
mean permittivity |
Thus, the wanted mean permittivity in terms of the measured timd delay
and kwown distances is given by:
e={ c*(tau-L/c)/r1}**2
In order to derive tau with the required accuracy from measurements in a
simple experiment as described would require aq very accurate relative timing
between the clocks on the Orbiter and on the Lander. Such an accuracy can
currently not be to achived within the limits on mass and power consumption
in the space experiment. Thus, the actual experiment is technically more
complicated. The CONSERT experiment involves transmission and receiving
of radio signals on both the Orbiter and on the Lander.
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