For the Global Positioning System (GPS) to work, at any given time there must be at least ____________ (two/three) GPS satellites above the horizon.

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Well in reality you need signals from 4 (four) satellites to get the most accurate position data. Three satellites can give you your position, but accuracy is achieved by using the data from the fourth satellite.

Let's understand how the GPS works. At any time, there are a number of satellites over us. The GPS receiver receives data from a number of satellites. The data contains the exact time at which the signal was sent. By calculating the time difference between signal sent and received, the distance of the receiver from the satellite is calculated. The GPS receiver also knows the location of each satellite at the time signal was sent. So, given the travel time and satellite location, the receiver can calculate the x, y and z coordinates of your location. However, the glitch is that the receiver does not know the time exactly and it will have to use the signal from another satellite to solve for the time (since all satellites have atomic clocks for precise time determination). Thus, to solve for 4 unknowns: x, y, z (for location) and t (for time), we need signals from 4 satellites.

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