Where am I?
For thousands of years, travellers found their way using the Sun, stars, landmarks, maps and compasses. Today, a tiny chip in your phone can tell you where you are within a few metres, almost anywhere on Earth. This is GPS, the Global Positioning System.
The history of GPS
| Year | Event |
|---|---|
| 1957 | Soviet Sputnik 1 launches; U.S. scientists track it using the Doppler shift of its radio signal and realize the process could work in reverse to find locations on Earth |
| 1960s | Early U.S. Navy satellite navigation (Transit) |
| 1973 | U.S. Department of Defense begins developing GPS |
| 1978 | First GPS satellite launched |
| 1983 | After a Korean airliner strays off course and is shot down, the U.S. announces GPS will be available for civilian use once complete |
| 1995 | Full system of 24 satellites operational |
| 2000 | The U.S. turns off "selective availability," making civilian GPS about 10 times more accurate |
| Today | About 31 operational GPS satellites |
Other countries have their own systems: GLONASS (Russia), Galileo (European Union) and BeiDou (China). Phones often use several at once. Together, they’re called GNSS (Global Navigation Satellite Systems).
How GPS works
GPS has three parts:
| Segment | What it is |
|---|---|
| Space | About 31 satellites orbiting about 20 200 km above Earth, each circling twice a day |
| Control | Ground stations that track satellites and correct their orbits and clocks |
| User | Your GPS receiver: a phone, car, watch or hand-held device |
Step by step
- Each satellite carries very precise atomic clocks and constantly broadcasts its position and the exact time.
- Your receiver picks up these radio signals.
- It calculates how long each signal took to arrive. Since signals travel at the speed of light (about 300 000 km/s), time tells distance:
distance = speed of light × travel time - Knowing its distance from several satellites, the receiver figures out where it is. This is trilateration.
Trilateration explained
- If you know you’re 100 km from Town A, you could be anywhere on a circle around A.
- If you’re also 150 km from Town B, there are only two points where those circles cross.
- A third distance, from Town C, narrows it to one point.
GPS works the same way in 3-D using spheres around satellites. It needs signals from at least four satellites: three to pinpoint location and a fourth to correct the receiver’s less precise clock.
Why timing matters
Light travels about 30 cm in one billionth of a second. A timing error of just one millionth of a second would put your position off by 300 m! That’s why satellites carry atomic clocks.
GPS even has to correct for Einstein’s relativity: satellite clocks gain about 38 millionths of a second per day compared with clocks on the ground. Without correction, positions would drift by about 10 km a day.
Location in numbers
GPS gives your position as latitude and longitude.
| Place | Latitude | Longitude |
|---|---|---|
| Ottawa, ON | About 45.4° N | About 75.7° W |
| Vancouver, BC | About 49.3° N | About 123.1° W |
| Iqaluit, NU | About 63.7° N | About 68.5° W |
Uses of GPS
| Area | Use |
|---|---|
| Navigation | Car and phone maps, ships, aircraft |
| Emergency services | Locating 911 callers, search and rescue |
| Farming | Self-steering tractors plant and fertilize precisely |
| Science | Tracking animal migration, measuring plate movements of a few millimetres a year |
| Timing | Banks, cell networks and power grids use GPS time signals |
| Recreation | Hiking, geocaching, fitness tracking |
| Delivery and transit | Tracking trucks, buses and packages |
Limitations
- GPS signals are weak and can be blocked by buildings, tunnels, thick trees or mountains.
- Indoors, phones use Wi-Fi and cell towers instead.
- Errors can come from signal bouncing, space weather and poor satellite positions.
- Always carry a paper map and compass in the backcountry in case batteries die.
Common mistakes
- Thinking your phone sends signals to GPS satellites. It only receives them.
- Thinking GPS needs internet. Basic GPS works without it, though maps may need data.
- Thinking three satellites are enough. A fourth is needed for timing.
Try it yourself
- Use a map and string to practise trilateration: draw circles around three towns at given distances and find where they meet.
- Find the latitude and longitude of your school using a map app.
- Try geocaching with an adult: use GPS coordinates to find hidden containers.
Quick recap
- GPS uses about 31 satellites with atomic clocks to find locations.
- Receivers calculate distance from signal travel time.
- Trilateration with at least four satellites pinpoints position.
- GPS must correct for relativity to stay accurate.
- GPS is used for navigation, emergencies, farming, science and timing.