Global navigation satellite systems (GNSSs) remain, by far, the most reliable and widely spread sources of accurate outdoor positioning. With the advent of the new and modernized signals, novel challenges related to GNSS receivers have also been recognized. One of the most important challenges in dealing with modernized GNSS signals is posed by the oscillatory behavior of the autocorrelation of some of those signals, which causes ambiguities in the measurement of the propagation delay. This article is an extensive survey of the solutions in this area proposed during past decades. Not only does it illustrate that a large pool of solutions is available, but it also shows that none of these solutions alone can currently overcome all of the challenges related to GNSS ambiguities. A thorough overview of the problems caused by the ambiguities in the delay estimation and the unambiguous techniques proposed to counteract them is presented. We hope to spark the interest of the signal processing community and to stimulate new advances in this field. Unambiguous methods are classified into three main classes, and we compare the main solutions in terms of complexity and performance to identify the most promising techniques and directions to be followed. We point out that there is an inherent tradeoff between the unambiguous acquisition and unambiguous tracking, and that the receiver stages of acquisition and tracking can be designed in a disjoint manner when dealing with the ambiguities.