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GPS can put you across the street when
buildings distort satellite signals.

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Reflected signals take longer routes,
which can shift the calculated position.

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You are standing here.

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But the map places you over there.

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A signal reaching your phone does not
guarantee an accurate location.

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GPS estimates distance by timing radio
signals from satellites.

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The signals include timing information

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and information about the satellites'
positions.

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Your phone compares the signal's timing
with its own clock.

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That timing helps estimate how far the
signal travelled.

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But your phone's clock is not perfectly
synchronized with satellite time.

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The calculation must account for that
timing difference too.

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A standard position calculation normally
needs at least four satellites.

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Together, their measurements help estimate
your position

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and your clock's timing error.

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Tall buildings change which satellite
signals reach your phone.

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Some direct paths disappear behind
buildings.

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Other signals arrive after reflecting from
a surface.

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Here, only the reflected signal reaches
the phone.

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The direct signal is blocked.

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Engineers call this non-line-of-sight
reception.

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The reflected signal travels farther than
the direct route would.

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If that extra journey is unrecognized, the
measured distance becomes too large.

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Imagine the reflection adds one hundred
nanoseconds to the journey.

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That is one ten-millionth of a second.

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It adds about thirty metres, or roughly
one hundred feet.

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That is an error in one satellite distance
measurement.

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It does not mean your map position shifts
by exactly thirty metres.

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Your position comes from combining several
satellite measurements.

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One misleading distance can shift the
result when the calculation accepts it.

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In the simplified example, one distance
estimate becomes too large.

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The calculation shifts the estimated
location to fit the measurements.

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The person has not moved.

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The blue marker has.

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The size and direction of that shift
depend on several things.

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Which signals are affected matters.

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So do the satellite positions and the
receiver's calculations.

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Reflections can also cause trouble when
the direct signal still arrives.

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The direct signal and reflected copies
combine at the receiver.

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That is called multipath interference.

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Buildings can also hide useful satellites.

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The remaining measurements may constrain
some directions poorly.

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That can make the position estimate less
reliable.

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A wrong map location is not always a
satellite signal problem.

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The map itself might contain an incorrect
address or road.

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That is different from calculating your
position incorrectly.

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Your phone may also combine satellite
information with other location clues.

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For example, Android location services can
use GPS and Wi-Fi.

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The displayed blue marker may reflect that
combined estimate.

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Some positioning systems use building maps
to help interpret satellite signals.

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Those maps can help predict blocked or
reflected paths.

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But the benefits depend on the system and
its surroundings.

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Receiving more signals is useful only when
their information is handled well.

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A stronger signal is not proof that it
travelled directly.

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So the phone can receive a signal and
still misplace you.

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Buildings can hide direct signals and send
others along longer routes.

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Those measurements can shift the
calculated position across the street.

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The blue marker is an estimate of your
location.

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It is not a direct observation of where
you stand.