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How GPS Finds You

8th grade nonfiction passage about satellites and navigation: 272 words, five questions, an answer key and vocabulary.

8th grade nonfiction satellites and navigation 272 words

How GPS Finds You

When a phone shows your position as a blue dot on a map, it is using a system of satellites that orbit high above Earth. The system is called GPS, short for Global Positioning System. More than thirty satellites circle the planet, and each one constantly broadcasts a radio signal that includes the exact time the signal was sent.

Your phone does not send anything to the satellites. It only listens. A radio signal travels at the speed of light, so by comparing the time a signal was sent with the time it arrived, the phone can work out how far away that satellite is. The delay is tiny, only a fraction of a second, so the clocks involved must be extremely accurate. Each satellite carries atomic clocks for exactly this reason.

One distance by itself is not enough. A single satellite only tells the phone that it is somewhere on a huge imaginary sphere around that satellite. With a second satellite, the possible locations shrink to a circle where two spheres overlap. A third narrows it to just two points, and one of those is usually far out in space. A fourth signal helps the phone correct its own less precise clock, so it can fix its position and even its height above the ground.

Tall buildings, deep canyons, and tunnels can block or bounce the signals, which is why the blue dot sometimes drifts in a big city. Even so, GPS usually places a phone within a few meters. Delivery drivers, pilots, farmers, and hikers all depend on it, and the same timing signals help keep cell networks in step.

Questions

  1. What does each GPS satellite broadcast? in the text

  2. Does the phone send signals to the satellites? in the text

  3. How does a phone work out its distance from a satellite? in the text

  4. Why must the clocks used in GPS be extremely accurate? think about it

  5. Why might the blue dot drift among tall buildings? think about it

Words to know

satellite
an object that circles a planet in space
orbit
to travel in a curved path around something
signal
a message sent by waves, such as radio
accurate
exactly correct, with very few mistakes
sphere
a perfectly round ball shape
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One passage, three levels

The same topic and facts written three ways, so a mixed class can read and talk about one text. Each group’s sheet shows only a shape; the key says which is which.

● Easier · 250 words · Flesch-Kincaid grade 4.4

A phone can show your spot as a blue dot on a map. It does this with satellites that orbit high above Earth. The system is called GPS, which stands for Global Positioning System. More than thirty satellites circle the planet. Each one sends out a radio signal. The signal holds the exact time it was sent.

Your phone does not send anything to the satellites. It only listens. A radio signal moves at the speed of light. The phone checks when the signal was sent and when it arrived. This tells the phone how far away that satellite is. The delay is tiny, only a fraction of a second. So the clocks must be very exact. Each satellite has atomic clocks for this reason.

One distance is not enough. One satellite only says the phone is somewhere on a huge imaginary sphere around it. A second satellite shrinks the spots to a circle where two spheres overlap. A third leaves just two points, and one is usually far out in space. A fourth signal helps the phone fix its own clock, which is less exact. Then it can find its spot and even its height above the ground.

Tall buildings, deep canyons, and tunnels can block or bounce the signals. That is why the blue dot sometimes drifts in a big city. Still, GPS usually puts a phone within a few meters. Drivers, pilots, farmers, and hikers use it. The same timing signals help keep cell networks in step.

▲ This passage · 272 words · Flesch-Kincaid grade 8.3

When a phone shows your position as a blue dot on a map, it is using a system of satellites that orbit high above Earth. The system is called GPS, short for Global Positioning System. More than thirty satellites circle the planet, and each one constantly broadcasts a radio signal that includes the exact time the signal was sent.

Your phone does not send anything to the satellites. It only listens. A radio signal travels at the speed of light, so by comparing the time a signal was sent with the time it arrived, the phone can work out how far away that satellite is. The delay is tiny, only a fraction of a second, so the clocks involved must be extremely accurate. Each satellite carries atomic clocks for exactly this reason.

One distance by itself is not enough. A single satellite only tells the phone that it is somewhere on a huge imaginary sphere around that satellite. With a second satellite, the possible locations shrink to a circle where two spheres overlap. A third narrows it to just two points, and one of those is usually far out in space. A fourth signal helps the phone correct its own less precise clock, so it can fix its position and even its height above the ground.

Tall buildings, deep canyons, and tunnels can block or bounce the signals, which is why the blue dot sometimes drifts in a big city. Even so, GPS usually places a phone within a few meters. Delivery drivers, pilots, farmers, and hikers all depend on it, and the same timing signals help keep cell networks in step.

■ Harder · 285 words · Flesch-Kincaid grade 10.9

When a phone displays your position as a blue dot on a digital map, it is relying on a constellation of satellites that orbit far above Earth. This arrangement is known as GPS, which is short for Global Positioning System. More than thirty satellites circle the planet, and each one continuously broadcasts a radio signal that contains the precise moment at which that signal was transmitted.

Your phone never transmits anything to the satellites; it merely listens. Because a radio signal travels at the speed of light, the phone can compare the time a signal was sent with the time it was received and thereby calculate how far away that satellite is. The delay is minuscule, only a fraction of a second, so the clocks involved must be extraordinarily accurate. For exactly this reason, each satellite carries atomic clocks.

A single distance, however, is not sufficient. One satellite can only reveal that the phone lies somewhere on an enormous imaginary sphere centered on that satellite. A second satellite narrows the possible locations to a circle where the two spheres overlap. A third reduces the candidates to just two points, one of which is usually located far out in space. A fourth signal allows the phone to correct its own less precise clock, so that it can determine both its position and its height above the ground.

Tall buildings, deep canyons, and tunnels can block or reflect the signals, which explains why the blue dot occasionally wanders in a crowded city. Nevertheless, GPS typically places a phone within a few meters of its true location. Delivery drivers, pilots, farmers, and hikers all depend on it, and the same timing signals also help keep cell networks synchronized.

Download as one PDF

How to use it

  1. Press Read it big (or B) to put the text on the projector, or Full screen (F) to fill the whole screen.
  2. Ask the questions one at a time and use Show answer when the class has decided. A shows or hides every answer.
  3. Print student copy for the class, or Print with answers for yourself.

Ideas for class

  • Before reading, put satellite, orbit, signal, accurate and sphere on the board with their meanings from the Words to know section and have students predict what the text will be about.
  • Read it aloud once while students follow along, then have them reread it alone or with a partner. At a calm 100 words a minute it takes about 3 minutes to read aloud.
  • Use the Read it big button (or B) to put the text on the projector, and Show answer on one question at a time so the class commits to an answer first.
  • Question 4 asks the reader to infer. Ask students to point to the words in the passage that led them to their answer.
  • Ask students to write one question the passage did not answer, then look for the answer in a book or on a classroom screen.
  • Print the student copy for independent work and keep the answer-key copy for yourself.

Related

Questions

What reading level is "How GPS Finds You"?

It was written for 8th grade. It has 272 words and works out to a Flesch-Kincaid grade level of about 8.3. That is a formula's estimate from sentence length and syllables, and it is not a Lexile or a guided reading level, so check it against your students.

What do the questions ask?

There are five. 3 ask for something the text says directly and 2 ask the reader to work something out from clues. Each has an answer in the key, and the answers are written as a teacher would accept them, not as the only possible wording.

How long does it take to read?

About 3 minutes read aloud at 100 words a minute. For a one-minute fluency check, paste it into the fluency timer and use the word count.

Can I print it?

Yes. Print student copy gives the passage, the questions with lines to write on, and the word meanings. Print with answers gives the same page with the answers filled in. Both are made in your browser.

Is this passage from a published book?

No. It was written for Peabody Teach, so there is no book to buy and no login to read it.

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