The Ultimate GPS Quiz

They used the position of the stars to know their location. They also used landmarks and created detailed maps to help them know their bearings.

美国军方开发和实现了27-satellite GPS network as a military navigation system, but soon opened it up to everybody else.

On average, each GPS satellite weighs about 3,000-4,000 pounds (1,361-1,814 kilos).

GPS卫星每天做两旋转。

The orbit of each GPS satellite is arranged so that, at any given time, there are at least four satellites "visible" from anywhere on Earth.

The mathematical principle is called trilateration. Using this, a GPS receiver locates the four GPS satellites, figures out the distance to each, and uses this information to deduce its own location.

二维trilateration calculates your location on land.

3-D trilateration calculates your location on space.

Radio waves are electromagnetic energy, and thus travel at the speed of light, which is about 186,000 miles per second in a vacuum (300,000 km per second ). GPS receivers receive and analyze high-frequency, low-power radio signals from the GPS satellites.

Every day, at fixed time, the GPS satellite sends out a long, digital pattern, which is called pseudo-random code. The GPS receiver uses the lag between the time when the code is sent and the time it receives the code to determine the distance between itself and the satellite.

All GPS satellites contain an atomic clock, which is extremely accurate. However, it is very expensive, costing around $100,000 apiece.

All GPS receivers contain an ordinary quartz clock, which is constantly reset. The receiver receives the incoming signals from four or more satellites and calculates its own inaccuracy, that is, the difference between the set time of the atomic clocks inside GPS satellites and the far cheaper quartz clocks that all receivers have. This way, GPS receivers are able to enjoy the accuracy of atomic clocks.

The GPS receiver uses an almanac, which is updated constantly by the US Department of Defense. The almanac tells the receiver where every satellite should be at any given time.

It is called Differential GPS (DGPS). DGPS helps correct errors caused by radio signals bouncing of large objects. It also deals with incorrect almanac data (position of GPS satellite).

When GPS receivers have built-in map files in memory, it makes them more user-friendly. This way they are then able to guide your way through a terrain effortlessly by mixing the position data and the locations on the map. Some GPS receivers will also let you download detailed maps into memory or supply detailed maps with plug-in map cartridges.

The process is called initializing. You need to perform receiver initializing only one time, and the GPS will hold this information in its memory. Keep in mind that each GPS model has a different method for initializing, so you should check your owner's manual.

If the GPS signal is weak, or you find that there are fewer than four satellites on the screen, you shouldn't rely on GPS. Instead, you should navigate using a map and compass.

Use the waypoints function. Waypoints are basically points that you can enter into the memory of your GPS for a particular journey. You can designate various stops along the journey as waypoints; for example, motel, gas stations, conveniences, rendezvous points and so on.

Use the go-to function to find out about all the waypoints that will appear on your screen. Select the waypoint you want and the GPS receiver will immediately let you know how far away it is and what direction you need to travel to get there.

Laying a track is an important GPS skill that helps you to not get lost. To lay a track, you use the button on your GPS receiver responsible for dropping track points along the way, thus creating a virtual record of your journey down the trail (also known as a virtual trail). Remember to place your track points as close to each other as possible, which will make it easy for you to follow your way out.