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Rosetta/CONSERT: Principle of measurements
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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

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.

top  Top Nielsen, 27-11-2001 drucken   Print−friendly Page
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