How many satellites are required to compute a GPS position for a 3-D fix?

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Multiple Choice

How many satellites are required to compute a GPS position for a 3-D fix?

To get a true 3-D GPS position you must solve for four unknowns: the receiver’s X, Y, and Z coordinates, plus the receiver’s clock bias. Each satellite provides one pseudorange measurement, which relates to the geometric range plus the receiver’s clock error. With four satellites you have four independent equations to solve for these four unknowns, yielding a unique position and clock error. If only three satellites are involved, there are three equations for four unknowns, so the clock bias (and thus the solution) cannot be uniquely determined. More satellites can be used for redundancy and improved accuracy, but four is the minimum for a 3-D fix.

To get a true 3-D GPS position you must solve for four unknowns: the receiver’s X, Y, and Z coordinates, plus the receiver’s clock bias. Each satellite provides one pseudorange measurement, which relates to the geometric range plus the receiver’s clock error. With four satellites you have four independent equations to solve for these four unknowns, yielding a unique position and clock error. If only three satellites are involved, there are three equations for four unknowns, so the clock bias (and thus the solution) cannot be uniquely determined. More satellites can be used for redundancy and improved accuracy, but four is the minimum for a 3-D fix.

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