Why are four satellites needed for a position fix?

To solve location and clock error

What event in 1983 led the US to open GPS to civilians?

The downing of Korean Air Lines Flight 007

GPS Quiz: Satellites, Selective Availability and Civil Use

This gps quiz finds its position from signals in orbit. Play 10 interactive questions on the US Navstar system, the first satellite in 1978, at least 24 satellites about 20,000 km up, why four satellites are needed for a fix, atomic clocks and relativity, the 1983 decision to open GPS to civilians, Selective Availability switched off in 2000, and rival systems like Galileo.

Start GPS Quiz

🎯 Daily Trivia Challenge • 🧠 10 Questions • ⏱️ 150 Seconds

Question 1 / 10 Time: 150s

Owner
Which country built GPS?
Name
What was the system's original program name?
First
When was the first GPS satellite launched?
Count
At least how many satellites does GPS need for global coverage?
Four
Why are four satellites needed for a position fix?
Clock
What kind of clocks do GPS satellites carry?
Rel
Which theory requires corrections to satellite clocks?
Civil
What event in 1983 led the US to open GPS to civilians?
SA
What happened to Selective Availability in 2000?
EU
What is Europe's satellite navigation system called?

Explanations: answer key for all 10 questions

Q1: Which country built GPS?
Answer: The United States
It was a military project.
Q2: What was the system's original program name?
Answer: Navstar
Navstar GPS.
Q3: When was the first GPS satellite launched?
Answer: 1978
Full service came in the 1990s.
Q4: At least how many satellites does GPS need for global coverage?
Answer: 24
Extra satellites act as spares.
Q5: Why are four satellites needed for a position fix?
Answer: To solve location and clock error
Three dimensions plus time.
Q6: What kind of clocks do GPS satellites carry?
Answer: Atomic clocks
Timing must be precise.
Q7: Which theory requires corrections to satellite clocks?
Answer: Relativity
Without them, errors would grow quickly.
Q8: What event in 1983 led the US to open GPS to civilians?
Answer: The downing of Korean Air Lines Flight 007
Better navigation could prevent tragedies.
Q9: What happened to Selective Availability in 2000?
Answer: It was switched off
Civilian accuracy improved.
Q10: What is Europe's satellite navigation system called?
Answer: Galileo
Russia has GLONASS.

Signals From Orbit

GPS, originally the Navstar program, was built by the United States as a military navigation system. The first satellite launched in 1978, and full service arrived in the 1990s. At least 24 satellites, orbiting about 20,000 km up, give global coverage, with extras as spares.

Each satellite carries atomic clocks and broadcasts precise timing signals. A receiver measures how long signals take to arrive, and needs four satellites to solve three dimensions plus its own clock error. Relativity means satellite clocks run at slightly different rates from clocks on Earth, so corrections are built in.

After Korean Air Lines Flight 007 was shot down in 1983 after straying off course, the US opened GPS to civilians. Selective Availability, which deliberately degraded civilian accuracy, was switched off in 2000. Other systems include Europe's Galileo, Russia's GLONASS and China's BeiDou.

What This Quiz Teaches

Science and tech fans will enjoy these questions.

Who built GPS
How many satellites it uses
Why four are needed
Why relativity matters
How civilians gained access

Did You Know?

Phones combine several satellite systems.
Tall buildings can bounce signals.
Farmers use GPS for precision planting.
GPS also provides precise time.
The US Space Force operates GPS.