Snapshot positioning without initial information

Snapshot techniques are based on computing a position using only a set of digital signal samples captured over some milliseconds. Existing techniques require, in addition to the satellite ephemerides, a rough knowledge of the position and/or time at which the snapshot was captured. We propose a new method to instantaneously compute a snapshot position and time solution without any reference time or position. The method is based on the addition of a fifth unknown to the instantaneous Doppler equations, which accounts for a time difference between the reference time and the measurement time. Using this new system of equations at different initialization times separated by some hours, time uncertainties of days or weeks can be solved. The algorithm has been implemented in a snapshot GPS software receiver in MATLAB, proving that position accuracies of a few meters with time uncertainties of several weeks can be obtained in a few seconds.