A radio navigation system (100; 400), comprising a plurality of satellite-borne transmitters (110, 114, 118, 122) and at least one ground-based receiver (104), the receiver (104) being adapted to receive radionavigation signals (112, 116, 120, 124; 412, 416, 420, 424) from each of a plurality of the transmitters (110, 114, 118, 122). Each of the transmitters (110, 114, 118, 122) and the receiver (104) being adapted to access a predetermined first key chain, the first key chain comprising a first encryption key (K; Kj) and one or more further encryption keys (K; Kj,1, Kj,2, Kj,3, Kj,4). The system may comprise a first group of transmitters from said plurality of transmitters (110, 114, 118, 122), each transmitter in the first group of transmitters being operable to transmit a first radio navigation signal (112, 116, 120, 124; 412, 416, 420, 424), the first radionavigation signals including, at a given instant or for a given subframe (k, k+1), radionavigation data, a MAC (MAC1, MAC2, MAC3, MAC4), and one of said one or more further encryption keys (K; Kj,1, Kj,2, Kj,3, Kj,4). The MAC code (MAC1, MAC2, MAC3, MAC 4) is unique to each transmitter (110, 114, 118, 122) and is generated using the first encryption key (K; Kj,) and the one of said one or more further encryption keys (K; Kj,1, Kj,2, Kj,3, Kj,4) is transmitted a predetermined time after transmission of the MAC. The receiver (104) is operable, upon receipt of all or part of the first radionavigation signal (112, 116, 120, 124; 412, 416, 420, 424) from one or more of the first group of transmitters (110, 114, 118, 122), to authenticate a first radionavigation signal received from one of the first group of transmitters based on one of said one or more further encryption keys (K; Kj,1, Kj,2, Kj,3, Kj,4) received from that transmitter or from any other transmitter in said first group of transmitters. The same key K may be used for all transmitters in the first group, or each transmitter may use different key from the key chain. The transmitters may be formed into a second group, in addition to the first, with the second group using its own key chain in a similar manner. An advantage is to increase the availability of an authenticated position even in degraded reception environments. Embodiments also relate to the optimal transmission of authentication information—though the use of unpredictable bits interleaved with predictable bits—so as to minimize the signal predictability time and the increase of robustness against replay attacks.