Special Topics - GPS (43 of 100) Why Do We Need 4 Satellites?

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We will learn why we need 4 satellites for GPS instead of 3.

Title: Special Topics - GPS (42 of 100) Clock Error (Bias) Revisited

Title: Special Topics - GPS (44 of 100) How to Find the Geometric Range? (R = True Range)

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Thank you, professor. You explained the meaning of time.

張洪鈞

Excellent, as always! For marine GPS, three will suffice as the surface of the sea is effectively at zero elevation, the vertical accuracy is usually about 3x less than the horizontal accuracy.

karhukivi

Here's what I remember reading a long time ago:

You need four satellites because each data from one satellite put you in a sphere around the satellite. By computing the intersections you can narrow the possibilities to a single point. Three satellites intersection places you on two possible points. The last satellite give you the exact location.

schifoso

Here's what the FAA website says:

With information about the ranges to three satellites and the location of the satellite when the signal was sent, the receiver can compute its own three-dimensional position. An atomic clock synchronized to GPS is required in order to compute ranges from these three signals. However, by taking a measurement from a fourth satellite, the receiver avoids the need for an atomic clock. Thus, the receiver uses four satellites to compute latitude, longitude, altitude, and time.

schifoso