How Your Phone Knows Where You Are
Written by Studio AM.
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Open a map on your phone and a blue dot appears. One way the phone finds that position is GPS. It receives timed signals from satellites in space. Phones can also use nearby phone networks and Wi-Fi.
High above the earth, satellites circle in fixed paths. Each one carries a very accurate clock and sends out a steady signal. The signal says two things: where the satellite was, and the exact time the message left. The signal travels at the speed of light, which is fast but not instant. Your phone uses the travel time of the message to work out a distance.
One distance is not enough. Knowing you are 20,000 kilometres from one satellite places you somewhere on a huge sphere. A second satellite narrows it. With signals from at least four satellites, the phone can work out a position and correct an error in its own clock. GPS is often less reliable indoors. The signals are faint, and thick walls are enough to lose them.
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Questions
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Question 1 of 4
According to the passage, what two things does a satellite's signal say?
The answer is B: Where the satellite was, and the exact time the message left.
The passage lists position and send time as the two pieces of information used here.
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Question 2 of 4
What is the passage mainly explaining?
The answer is C: How a phone works out its position from satellite signals.
Every paragraph builds the same explanation: signals carry a send time, the phone turns the gap into a distance, and several distances fix a position.
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Question 3 of 4
Why could a small clock error affect a GPS position?
The answer is A: The calculated signal travel time would be wrong.
Travel time is used to calculate distance, so a clock error can change the distance and the resulting position.
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Question 4 of 4
In this passage, "faint" means
The answer is D: weak and hard to pick up
The word explains why the dot struggles indoors, alongside thick walls that lose the signals. A weak signal is the reason. Distance alone is not the point, since the satellites are always far away.
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