The Netherlands-China Low-Frequency Explorer - NCLE, is a low-frequency radio experiment for the Chinese Chang'e 4 mission went in a Lissajous orbit around the Earth-Moon L2 point in 2018. NCLE is considered a pathfinder mission for a future low-frequency space-based or moon-based radio interferometer which has the detection and tomography of the 21-cm Hydrogen line emission from the Dark Ages period as the principle science objective.
Low-frequency radio astronomy, i.e. below ~30 MHz, can only be done well from space due to the cut-off in the Earth's ionosphere, the man-made RFI and the AKR and QTN noise that make sensitive measurement from ground-based facilities impossible. At the Earth-Moon L2 point NCLE will be outside the Earth's ionosphere and relatively far away from terrestrial interference, which, however, will still be detectable. As the Earth will always be in sight we can measure and quantify this emission for the first time since 50 years and with unprecedented quality. This will allow us not only to study the radio- and plasma physics of the earth-moon system, but also to explore mitigation and calibration techniques for exploring radio emission from the early universe and compare it with measurements made in true lunar far-side locations made by future (Chinese) Lunar Lander missions.
Hence, the objective of the NCLE mission is two-fold. In addition to the characterisation of the lunar radio environment, NCLE allows for unique radio science and astronomy. This includes a wide range of topics, such as constraining the 21-cm line Dark Ages and Cosmic Dawn signal, measuring the auroral radio emission from the large planets in our Solar system, determining the radio background spectrum at the Earth-Moon L2 point, studying the Solar activity and space weather at low frequencies, creation of a new low-frequency map of the radio sky, studying the Earth's ionosphere and its interaction, and the detection of bright pulsars and other radio transient phenomena at very low frequencies. In addition, the access to a previously unexplored frequency regime will undoubtedly lead to new discoveries - NCLE will be the first step towards opening up the virtually unexplored low-frequency domain for astronomy.
The NCLE instrument on the Chang'e 4 relay satellite breaks the ground for the radio experiments on the future Chinese Lunar lander mission and for which an intense collaboration between the Chinese and Dutch teams is foreseen, in particular on joint science collaboration on the above mentioned science objectives, instrument calibration, noise and radio background spectrum characterisation and VLBI. The opportunity of an experiment on the relay satellite with sufficient provision of power and mass, is ideal for flying one of the most advanced and flexible low-frequency radio receiver ever flown. The experiment also opens the possibility to demonstrate the first ever very long baseline interferometry on moon-space baselines.
The NCLE baseline design concept involves 3 co-located, orthogonal, monopole antenna elements, each of ~5 meters in length. The constellation of these 3 active antennas is mounted perpendicular to the upper side of the satellite body. NCLE has its optimal sensitivity in the frequency range between 1 and 80 MHz where the highest priority science signals are expected, but will extend down to the kHz regime, albeit with reduced sensitivity. As the expected signals at the very low frequency regime are predominantly the relatively strong AKT, QTN and RFI noise sources and the relatively strong planetary emission, the chosen sensitivity characteristic is optimised for the chosen science objectives. The analogue signals are digitised in a DSP system on which dedicated science modes are implemented in a flexible software-defined radio system. These modes for instance perform fast fourier transforms (FFTs) to create average radio spectra for which spectral resolution and frequency range can be selected, allow triggering on transient radio events, or allow to retrieve Direction Of Arrival (DOA) information using beam forming techniques or goniopolarimetry techniques. Raw time traces can be stored for ground-based post-processing and VLBI, possibly with a radio instrument on the future Chinese Lunar Lander. Preliminary analysis of the antenna beam pattern shows that via beam forming techniques, the antenna sensitivity can be optimized for a certain direction, for instance for looking in the direction of the Earth, Sun or Jupiter.
Launch Chang’e 4 to the moon with the Dutch NCLE instrument
On Sunday evening May 20 (May 21 in China), a Dutch radio antenna on a Chinese satellite was successfully launched to the other side of the Moon. The Netherlands Chinese Low-Frequency Explorer (NCLE) was built by a team of scientists and engineers from Radboud Radio Lab, ASTRON and ISIS.