How Does GPS Work? KS3 Computing Guide

What satellites make up GPS?

The GPS constellation is operated by the United States military and consists of at least 24 operational satellites arranged so that at least four are always visible from any point on Earth's surface. The satellites orbit at an altitude of about 20,200 km and complete two orbits every 24 hours.

Each satellite constantly broadcasts a radio signal that includes:

• the satellite's precise position in space at the moment of transmission,
• the exact time the signal was sent (from an atomic clock on board).

Other countries operate their own satellite navigation systems: Russia's GLONASS, the European Union's Galileo, and China's BeiDou. Modern smartphones receive signals from multiple constellations simultaneously to improve accuracy.

How does trilateration work?

A GPS receiver calculates its position using a technique called trilateration (often loosely called triangulation). Here is the principle:

• The receiver receives a signal from Satellite A and notes the travel time. Because radio waves travel at the speed of light (~300,000 km/s), the receiver computes the distance: Distance = Speed × Time. This places the receiver somewhere on a sphere centred on Satellite A.
• Adding Satellite B gives a second sphere. The receiver must be on the circle where both spheres intersect.
• Adding Satellite C narrows the position to two points — usually one is in deep space or underground, so the receiver can discard it.
• A fourth satellite is needed to correct for timing errors in the receiver's clock (receiver clocks are far less accurate than the atomic clocks on satellites).

What is the role of atomic clocks on satellites?

The entire system depends on measuring incredibly small time differences — a nanosecond's error corresponds to 30 cm of positional error. Ordinary quartz clocks drift too much. GPS satellites carry multiple atomic clocks (accurate to about 20–30 nanoseconds per day), and ground stations regularly synchronise them. The receiver's own clock — far cheaper — is effectively corrected using the fourth satellite signal.

How accurate is GPS?

Accuracy is reduced by:

• Atmospheric delay — the ionosphere and troposphere slow signals unpredictably.
• Multipath errors — signals bouncing off buildings before reaching the receiver.
• Satellite geometry — if available satellites are clustered in one part of the sky, position calculations are less precise.

Where is GPS used in everyday technology?

GPS is embedded in a huge range of devices and systems relevant to KS3 computing:

• Smartphones — navigation apps, geotagging photos, location-based services.
• Fitness trackers and smartwatches — recording running routes and distances.
• Logistics — parcel delivery companies track vehicles in real time.
• Aviation and shipping — primary navigation tool for aircraft and container ships.
• Precision agriculture — tractors guided by GPS sow seeds in exact rows.
• Emergency services — ambulances and fire engines dispatched to precise coordinates.
• Internet of Things (IoT) — asset-tracking sensors, wildlife monitoring tags, and smart city infrastructure all rely on GPS coordinates.

How does GPS connect to broader computing topics?

GPS is an excellent example of a system that combines several KS3 and GCSE computing ideas:

• Binary data — satellite signals are digital transmissions encoded in binary.
• Networking — GPS receivers often send their location data over mobile networks to cloud services.
• Algorithms — the trilateration calculation and the route-planning in navigation apps are algorithms.
• Privacy — location data is personal data under GDPR; apps must ask for permission before accessing it.
• Embedded systems — GPS chips are tiny embedded processors that run firmware, not a general operating system.

Frequently asked questions

Does GPS work indoors?

Poorly or not at all. Radio signals from satellites are weakened by walls, roofs, and metal structures. Indoors, smartphones typically fall back to Wi-Fi positioning (comparing visible network names against a database of known hotspot locations) or cell-tower triangulation, which is less accurate but works without a direct sky view.

Is GPS free to use?

The GPS signal itself is broadcast freely by the US government and anyone with a receiver can use it at no charge. However, the devices that contain GPS receivers cost money, and services built on top of GPS — such as Google Maps, TomTom, or fleet-tracking software — are commercial products.

What is the difference between GPS and a sat-nav?

GPS is the underlying satellite system that provides raw position data. A sat-nav (satellite navigation device) is a consumer product — typically an in-car device or a smartphone app — that receives GPS signals, converts them into a position on a digital map, and adds route-planning software. The sat-nav is the application layer; GPS is the infrastructure it depends on.

How does GPS relate to the Internet of Things at KS3?

The Internet of Things (IoT) connects physical objects to the internet. Many IoT devices use GPS as their location sensor: a parcel-tracking tag reports its position every few minutes, a smart dog collar lets owners monitor their pet's location, and smart electricity meters help engineers find which transformer is overloaded. GPS turns a passive object into a location-aware, connected device.

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