A receiver for use with a global navigation satellite system GNSS comprising multiple satellites, wherein each satellite transmits a respective navigation signal containing a spreading code, and wherein the GNSS maintains a time reference which is encoded into the navigation signals, the receiver comprising: a receiver clock; at least one antenna for receiving multiple signals over multiple respective channels, each channel being defined by a transmitting satellite and a receiving antenna at opposing ends of the channel; and at least one correlator for calculating cross-correlation functions between (i) the signals received over the multiple channels and (ii) reference versions of the navigation signals provided by the receiver; wherein the receiver is configured to use the calculated cross-correlation functions to perform a joint estimate of (i) a clock bias of the receiver clock relative to the time reference maintained by the GNSS, and (ii) a composite channel comprising the combined contribution of the multiple channels as a function of time-delay, wherein the clock bias of the receiver clock is estimated as an offset from an approximate clock bias, characterised in that the receiver is configured to, in an indoor environment, estimate the approximate clock bias, prior to performing the joint estimation, by using the calculated cross-correlation functions to determine a maximum of a composite cross-correlation function for the composite channel, and the composite cross-correlation function is defined by the equation: Λ ^ Φ 0, n = ∑ j = 1 N x j H B Φ 0, n − ΔΦ j B H Φ 0, n − ΔΦ j B Φ 0, n − ΔΦ j − 1 B H Φ 0, n − ΔΦ j x j, wherein Λ̂( Φ 0, n) is the composite cross-correlation function for the recevier's state vector Φ 0, n, x j represents the received signal at respective antenna j, and B ( Φ j) is the basis function matrix for the state vector Φ j, which is composed by the basis function vectors for each satellite of the j -th antenna. a receiver clock; at least one antenna for receiving multiple signals over multiple respective channels, each channel being defined by a transmitting satellite and a receiving antenna at opposing ends of the channel; and at least one correlator for calculating cross-correlation functions between (i) the signals received over the multiple channels and (ii) reference versions of the navigation signals provided by the receiver; wherein the receiver is configured to use the calculated cross-correlation functions to perform a joint estimate of (i) a clock bias of the receiver clock relative to the time reference maintained by the GNSS, and (ii) a composite channel comprising the combined contribution of the multiple channels as a function of time-delay, wherein the clock bias of the receiver clock is estimated as an offset from an approximate clock bias, characterised in that the receiver is configured to, in an indoor environment, estimate the approximate clock bias, prior to performing the joint estimation, by using the calculated cross-correlation functions to determine a maximum of a composite cross-correlation function for the composite channel, and the composite cross-correlation function is defined by the equation: Λ ^ Φ 0, n = ∑ j = 1 N x j H B Φ 0, n − ΔΦ j B H Φ 0, n − ΔΦ j B Φ 0, n − ΔΦ j − 1 B H Φ 0, n − ΔΦ j x j, wherein Λ̂( Φ 0, n) is the composite cross-correlation function for the recevier's state vector Φ 0, n, x j represents the received signal at respective antenna j, and B ( Φ j) is the basis function matrix for the state vector Φ j, which is composed by the basis function vectors for each satellite of the j -th antenna.