CONSERT: Comet Nucleus Sounding Experiment by Radiowave Transmission
CONSERT (Comet Nucleus Sounding Experiment by Radio wave Transmission) is a time domain transponder that operates between one module that will land on the comet surface and another that will orbit the comet. A radio signal is transmitted from the orbiting component of the instrument and passes through the comet nucleus to the component on the comet surface. The signal is received on the lander, where some data is extracted, and then immediately re-transmitted back to the orbiter, where the main experiment data collection occurs. The variations in phase and amplitude that occur as the radio waves pass through different parts of the cometary nucleus will be used to perform tomography of the nucleus and determine the dielectric properties of the nuclear material.
The overall science objective of the CONSERT investigation is to gather information about the geometrical structure and electrical properties of the deep interior of the comet nucleus. Inferences about the composition of the interior of the comet will then be made from the measured electrical properties.
The main scientific objectives are:
- To measure the mean dielectric properties and, through modelling, to set constraints on the cometary composition (like material and porosity)
- To detect large-scale embedded structures (several tens of metres), and stratifications
- To detect small scale irregularities within the comet
CONSERT works as a time domain transponder. The indirect and apparently complicated transponding procedure reduces the required accuracy of the clocks on the Orbiter and Lander, and makes it possible to stay within the constraints on mass and power consumption imposed on the space experiment.
The CONSERT experiment on the orbiter and on the lander both consist of a transmit/receive antenna and a transmitter and receiver contained in a common box.
A 90 MHz radio signal, phase modulated with pseudo-randomly encoded data is transmitted from the orbiter towards the comet. The transmission lasts about 25 microseconds. The signal propagates through the comet nucleus and is received on the lander. The transmission cycle is repeated every 200 milliseconds. The received signal is digitised and accumulated in the lander in order to increase the signal to noise ratio. Once the accumulation is finished, the signal is compressed to obtain a time/space resolution corresponding to 100 nanoseconds, which corresponds to about 20 metres in the comet. After the signal processing on the lander, which determines the position of the strongest path, the lander transmits the same pseudo-random code with a delay corresponding to that of the strongest path. The transmission cycle again lasts about 25 microseconds. The signal propagates back to the orbiter along virtually the same path, since the orbiter does not travel far during the measurement cycle. The signal is received on the orbiter, accumulated and stored in the memory in order to be sent to Earth. A complete measurement cycle lasts about 1 second.
Last Update: 04 July 2014