The very low flux of ultra-high-energy cosmic-rays (UHECRs) requires detectors with a large effective area and high duty cycle to obtain a statistically relevant sample. Radio detection of extensive air showers (EAS) presents attractive aspects for future giant detectors of high energy cosmic particles, with very low cost per detection unit, easiness of deployment over large areas, and a duty cycle close to 100%. However autonomous detection of EAS -a necessary step towards the realization of this type of ambitious detectors- remains a challenge.
GRANDproto35 aims at demonstrating that radio-detection of air showers can be performed with very good background rejection, high efficiency, and an almost 100% duty cycle. The 35 GRANDproto antennas will perform a full measurement of the detected wave polarization. This makes GRANDproto35 uniquely qualified for the investigation of polarization characteristics of the radio emission from EAS, which may contribute to discriminate them from background signals. In addition, an array of 24 scintillators will allow offline cross-checks of the nature of the selected radio-candidates. We detail here the principle, progress and prospects of GRANDproto35, which serves as a step towards the Giant Radio Array for Neutrino Detection (GRAND) project. GRAND will consist of an array of ∼ 10^5 radio antennas deployed over ∼ 200, 000 km^2 in mountainous sites.