This paper describes the system of the Swiss national metrological institute (IRA-METAS) for the absolute standardisation of radon gas. This method relies on condensing radon under vacuum conditions within a specified cold area using a cryogenerator, and detecting its alpha particles with an ion-implanted silicon detector, through a very accurately defined solid angle. The accuracy of this defined solid angle standardisation technique was corroborated by another primary measurement method involving 4πγ NaI(Tl) integral counting and Monte Carlo efficiency calculations. The 222Rn standard submitted by IRA-METAS to the Système International de Référence (SIR) at the Bureau International des Poids et Mesures (BIPM) has also been found to be consistent with an analogous standard submitted by the German national metrological institute (PTB). IRA-METAS is able to deliver radon standards, with activities ranging from a few kBq to 350 kBq, in NIST-Type ampoules, and glass or steel containers usable for calibrating radon-measuring instruments.