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How your phone turns four timestamps into a location.

GPS satellites orbit at about 20,180 km and constantly broadcast their position and the exact time. Your receiver measures how long each signal took and converts that into distances using the speed of light. Three distances would fix a point, but the cheap phone clock is wrong. A fourth satellite supplies the unknown for the clock error. Accuracy: a few metres.

Useful: If your phone sees fewer than four satellites, say between tall buildings, the position jumps around.

Source: en.wikipedia.org

NavigationSatellitenTechnik

Difficulty:

Question on this card

Why does a GPS receiver need the fourth satellite?

  1. For altitude above sea level
  2. To determine its own clock error
  3. As a backup in cloudy weather
  4. For compass heading
Show the answer

Correct: To determine its own clock error

Position has three unknowns; the clock error is the fourth. Four equations, four unknowns.

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