A radio navigation system (100; 400), comprising a plurality of satellite-borne transmitters (110, 114, 118, 122) and at least one ground-based receiver, the receiver 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 being adapted to access a predetermined first key chain, the first key chain comprising a first encryption key (K; K j) and one or more further encryption keys (K; K j,1, K j,2, K j,3, K j,4), the system comprising: 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 message authentication code (MAC1, MAC2, MAC3, MAC4), and a one of said one or more further encryption keys (K; K j,1, K j,2, K j,3, K j,4); wherein the MAC (MAC1, MAC2, MAC3, MAC4) is unique to each transmitter (110, 114, 118, 122) and is generated using said first encryption key (K; K j); wherein the one of said one or more further encryption keys (K; K j,1, K j,2, K j,3, K j,4) is transmitted a predetermined time after transmission of the MAC, and wherein the receiver 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 the MAC received from said one transmitter and on one of said one or more further encryption keys (K; K j,1, K j,2, K j,3, K j,4) received from that transmitter or from any other transmitter in said first group of transmitters. 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 message authentication code (MAC1, MAC2, MAC3, MAC4), and a one of said one or more further encryption keys (K; K j,1, K j,2, K j,3, K j,4); wherein the MAC (MAC1, MAC2, MAC3, MAC4) is unique to each transmitter (110, 114, 118, 122) and is generated using said first encryption key (K; K j); wherein the one of said one or more further encryption keys (K; K j,1, K j,2, K j,3, K j,4) is transmitted a predetermined time after transmission of the MAC, and wherein the receiver 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 the MAC received from said one transmitter and on one of said one or more further encryption keys (K; K j,1, K j,2, K j,3, K j,4) received from that transmitter or from any other transmitter in said first group of transmitters.