The Nançay Decameter Array

The project of building a large decameter radiotelescope in Nançay, with a better sensivitity than the decameter interferometer operating since 1970, was elaborated in the mid-1970s on the reference of the Clark Lake telescope (Aubier et al., 1975). The original ideal was to observe heliospheric radio emissions in stereoscopy from the Earth and NASA spacecraft, along the project named STEREO-MJS 77 (for Mariner-Jupiter-Saturn 1977). The large Nançay Decameter Array (NDA) was therefore build between 1975 and 1977 with support from the CNES french spatial agency, and the NASA project became the Voyager mission. In this frame, the NASA agency even provided a spare model of the Planetary Radio Astronomy (PRA) experiment embarked on both Voyager 1 & 2 so that NDA observations could be carried on with the same receiver performances (Leblanc et al., 1975).

The NDA is made of 144 log-period spiral antenna, 9 m-high and 5 m-wide in diameter, spread across a surface of about 8000 m2. Such antenna are sensitive to circularly polarized waves, and the array sub-divides into two sub-arrays, 72 antenna each, sensitive to left- and right-handed circular polarieation. The antenna are fixed and analogically phased with a system of delay lines, which makes the NDA steerable and capable to track moving sources in the sky. In its final configuration, the gain of each sub-array is 25 dB over 10-120 MHz. The NDA characteristics are detailed in (Boischot et al., 1980) and (Lecacheux, 2000).

A variety of radio receivers of increasing performances have been used through years and, 4 decades after its completion, the NDA is still in operation and continues to acquire high resolution observations in support to space missions such as Juno and Parker Solar Probe, and soon Solar Orbiter and JUICE (Lamy et al., 2017).

The NDA team celebrated the 40 years of the instrument during a scientific day organized in Nançay on 4th Oct. 2017->https://www.obs-nancay.fr/Journee-du-4-Oct-2017-Celebration-des-40-ans-d-existence-du-telescope-a-Nancay.html], which provided the occasion to the variety of researchers having worked on the Nançay decametric observations to meet, in presence of the president of the Observatory of Paris.

References :

– Project of a large collecting array at Nançay, M. Aubier, A. Boischot, G. Daigne, Y. Leblanc, A. Lecacheux, J. De la Noé, B. Moller-Pedersen, C. Rosolen, Proceedings of the Meeting, CESRA-5, Committee of European Solar Radio Astronomers, Florence, 25-27 February, 1975. Edited by C. Chiuderi, M. Landini, and A. Righini., p.110
– L’expérience STERO-MJS 77, Observations des émissions en provenance du soleil et de Jupiter dans la bande 15 MHz à 40.5 MHz, Y. Leblanc et le groupe décamétrique, rapport interne de l’Observatoire de Paris, Novembre 1975.
– A new High Gain, Broadband, Steerable Array to study Jovian Decametric Emission, A. Boischot, C. Rosolen, M.G. Aubier, G. Daigne, F. Leblanc, A. Lecacheux, J. de la Noë and B.M. Pedersen, Icarus, 43, 399-407, 1980.
– The Nançay Decameter Array: A Useful Step Towards Giant, New Generation Radio Telescopes for Long Wavelength Radio Astronomy, A. Lecacheux, Geophysical Monograph Series, 119, 2000.
– 1977–2017: 40 years of decametric observations of Jupiter and the Sun with the Nançay Decameter Array, L. Lamy, P. Zarka, B. Cecconi, L. Klein, S. Masson, L. Denis and A. Coffre, in Planetary Radio Emissions VIII, edited by G. Fischer et al., Austrian Academy of Sciences press, Vienna, 455–466, 2017.