An apparatus for vector tracking a plurality of satellite signals received by a Global Navigation Satellite System (GNSS) receiver from a plurality of satellites, the apparatus being characterised by: a velocity hypothesis unit for generating a plurality of velocity hypotheses based on a navigation engine output indicative of a current and/or a previous extended velocity solution for the GNSS receiver and based on a predicted change in one or more elements of the extended velocity solution, wherein an extended velocity solution comprises a 3-D velocity and a local clock drift of the GNSS receiver; a hypothesis determiner configured to determine a most likely velocity hypothesis from the plurality of velocity hypotheses based on a plurality of correlation values, and to transfer data related to the most likely velocity hypothesis to the navigation engine of the GNSS receiver for tracking the satellite signals, and a plurality of correlators (406a-406n), the apparatus being configured to, for each velocity hypothesis: obtain a set of first signals, the first signals comprising expected Doppler shifts for that hypothesis for respective ones of said plurality of satellites, such that the sets of first signals together provide a plurality of first signals; obtain a plurality of second signals, the second signals comprising true Doppler shifts derived from respective ones of the plurality of satellite signals; and determine using the plurality of correlators (406a-406n) said plurality of correlation values representing correlations between said plurality of first signals and said plurality of second signals, wherein, the hypothesis determiner is configured to sum the correlation values across all of said plurality of satellite signals relating to each single velocity hypothesis to obtain a set of summations and to determine the most likely velocity hypothesis to be the velocity hypothesis having the largest summation. a velocity hypothesis unit for generating a plurality of velocity hypotheses based on a navigation engine output indicative of a current and/or a previous extended velocity solution for the GNSS receiver and based on a predicted change in one or more elements of the extended velocity solution, wherein an extended velocity solution comprises a 3-D velocity and a local clock drift of the GNSS receiver; a hypothesis determiner configured to determine a most likely velocity hypothesis from the plurality of velocity hypotheses based on a plurality of correlation values, and to transfer data related to the most likely velocity hypothesis to the navigation engine of the GNSS receiver for tracking the satellite signals, and a plurality of correlators (406a-406n), obtain a set of first signals, the first signals comprising expected Doppler shifts for that hypothesis for respective ones of said plurality of satellites, such that the sets of first signals together provide a plurality of first signals; obtain a plurality of second signals, the second signals comprising true Doppler shifts derived from respective ones of the plurality of satellite signals; and determine using the plurality of correlators (406a-406n) said plurality of correlation values representing correlations between said plurality of first signals and said plurality of second signals, wherein, the hypothesis determiner is configured to sum the correlation values across all of said plurality of satellite signals relating to each single velocity hypothesis to obtain a set of summations and to determine the most likely velocity hypothesis to be the velocity hypothesis having the largest summation.