Time transponder technique and radiowave transmission for comet nucleus sounding

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Technology abstract

CONSERT is a bi-static RADAR developed by IPAG (Planetology and Astrophysics Institute) UGA/CNRS in Grenoble in collaboration with Latmos Université de Versaille/CNRS , France. It is embarked on-board ESA’s mission Rosetta in order to determine the chemical and physical properties of the nucleus of comet 67/P Churyumov-Gerasimenko. The synchronisation of the instruments is achieved without relying on a GPS clock or on heavy ultra-stable oscillators.

The specifications shown above are for the system designed 20 years ago. Today the system can be built with lower power consumption, better clocks and higher transmitted power.

- Nicolas Louée -

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Technology Description

In the framework of the Rosetta mission (2004-2016), the purpose of the CONSERT experiment is to determine the mean dielectric properties of the nucleus of comet 67/P Churyumov-Gerasimenko. This is done by measuring the propagation delay of radiowaves, using the CONSERT instrument (Comet Nucleus Sounding Experiment by Radiowave Transmission). CONSERT is a bi-static RADAR: the transmitter and receiver are separated by a distance. Indeed, the transmitter is placed on the orbiter (Rosetta), and the receiver is placed on the lander (Philae). Because the distance between the transmitter and the receiver varies, they need to be perfectly synchronised.
In order to measure the absolute time difference between the transmitted and received signals with the needed time accuracy, an ultra-stable oscillator on each end of the path would be needed. Due to the strict limit on the total mass available (3kg on the orbiting spacecraft and 2.3kg on Philae) and on the average power consumption (3W on both parts), the use of such oscillators is impossible. 
CONSERT therefore relies on a time-transponder technique: Philae detects the signal coming from Rosetta, and sends back to Rosetta a new signal, synchronized on the arrival time of the coming signal. One can say that Philae works like the reflector of the coming signal. The signal is thus measured in the time reference of Rosetta: this enables to relax the constraints on the stability of clocks.
On Earth, precise synchronisation can be achieved thanks to GPS clocks. The method developed in the framework of CONSERT could therefore be used by mobile bi-static RADARs in conditions where GPS do not work. For instance, this system could be used to image cavities with the help of a quasi-autonomous probe. Oil&Gas and geology are therefore potential non-space application sectors.

Innovations & Advantages

The main innovation of CONSERT is its ability to auto-synchronise, without relying on heavy equipment such as ultra-stable oscillators.

Further Information

See picture for the technical specifications of CONSERT

Current and Potential Domains of Application

The current domain of application is comet / asteroid nucleus sounding. The aim is to get information on the physics, chemistry and distribution of matter within the nucleus.
The potential domains of applications are Oil&Gas and geology, where imaging cavities with the help of a quasi-autonomous probe could be needed, in absence of a GPS signal.