A time reference system for generating a time reference from signals produced by a global navigation satellite constellation has a satellite signal receiver to receive and down-convert code-modulated signals from a plurality of satellites and a correlator to track and decode the down-converted signals to provide signals containing partial pseudo-range measurements for respective satellites. A data processing arrangement receives assistance data from an external source and performs data-bit synchronization in which bit edges of a low frequency data bit stream carried by the received satellite signals are identified, to perform a preliminary position-velocity-time solution to provide an approximate time reference, and to perform auto-correlation of pre-selected data sequences in the data stream to resolve time ambiguities thereby to compute a precise time reference signal in weak received signal conditions. The pre-selected data sequences may be the repeated data preamble in the GPS navigation message constituted by the data stream.