Known by the scientific community as the NDA (standing for Nançay Decameter - or decametric - Array), the instrument was built during the period 1975 et 1977 under the impetus of André Boischot.
The NDA became operational in late 1977, soon after the launch of Voyager, to supplement the observations of the space mission for the exploration of Jupiter and the other giant planets.
The decameter array is a phased array. Relatively modest in size, it is made up of 144 helix-conical antennas, 9 meters high and with a base diameter of 5 meters, and is installed on a surface of about 10 000 square meters.
The NDA does not make images. It was designed to observe systematically radio emission in the frequency range 10-100 MHz. It was for many years among the largest low frequency radiotelescopes in the world.
Its sensitivity has enabled it to make efficient observations of the radio emission of two bodies only : the Sun and Jupiter.
A data base unique in the world
Four decades worth of almost daily observations of the Sun and Jupiter has produced, for these bodies, the longest low frequency radio data base in the world.
Since the angular resolution of the NDA is limited to several degrees, which is insufficient to resolve radio sources in the environment of these two celestial bodies, research is essentially based on dynamic spectro-polarimetry, i.e. the analysis of the spectral content and the polarization of the radio signal received as a function of time.
During its 40 year-long existence, many different spectrographs and spectro-polarimeters have been successively connected to the NDA :
- the low time resolution ones (≥ 1 second / spectrum) enable the accumulation of long observational sequences ;
- the others, with high time resolution (down to ≤1 msec /spectrum, and even digitizing the received the waveform) enable the very detailed time-frequency structure of the emission to be studied.
Since 1990, the data has been acquired in digital form.
The most recent spectrograph was commissioned in 2016. Baptized JunoN (standing for Juno-Nançay), it was developed as as a means to supplement the observations of the Juno mission in polar orbit around Jupiter since 2016.
It enables observations to be made with a resolution of up to 2.6 ms × 3.05 kHz, to measure the full polarization of the waves received and to record the waveform of the signal.
It constitutes an important "ground support" unit for the Juno space mission, and it will also play this role for the upcoming solar missions Solar Orbiter and Solar Probe +.
