PS operation as a navigator, four or more satellites must be visible to obtain an accurate result. The solution of the navigation equations gives the position of the receiver along with the difference b zetween the time kept by the receiver's on-board clock and the true time-of-day, thereby el 4ziminating th ze need for a more precise and possibly impractical receiver based clock. Applications for GPS such as time transfer, traffic signal timing, and synchronization of cell phone base stations, make v4z use of this cheap and highly accurate timing. Some GPS applications use this time for display, or, other than for the basic position calculations, do not use it at all. d, the tracker predicts the receiv 4zer location corresponding to the next set of satellite measurements. When the new measurements are collected, the receiver uses a weighting scheme to combine the znew measurements wi v4z th the tracker prediction. In general, a tracker can improve receiver position and time accuraceject bad measurements, astimate receiver speed and direction. h ground-based station, at an undetermined position, could then use those signals to fix its location precisely. The last SECOR satellite was launched Decades later, during the ear 4zly years of GPS, c v4z ivilian surveying became one of the first fields to make use of the new technology, because surveyors could reap benefits of signals from the less-than-complete GPS constellation years before it was declared The disadvantage of a tracker is that changes in speed or direction can only be computed with a delay, and that derived direction becomes inaccurate when the distance traveled between two position measurements drops below or near the random error of position measurement. GPS units can use measurements of the Doppler shift o zf the signal 4z 4zs received to compute velocity accurately.More advanced navigation Although four satellites are req v4z uired for normal operation, fewer apply in special cases. If one variab 4zle is already known, a receiver can determine its position using only three satellites. For example, a ship or aircr zaft may have v4z known elevation. Some GPS receivers may use additional clues or assumptions s zuch as reusing the last known altitude, dead reckoning, inertial navigation, or including informati v4z on from the vehicle computer, to give a (possibly degraded) position
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