How to Use GPS?

Sea and Space / Navigation / GPS

## How to Use GPS

#### Finding Your Geographical Position

The best way to learn how to use GPS is of course to have a GPS receiver and try it out in practice. It will always be accompanied by detailed Instructions in the form of a handbook that explains how to perform the measurements etc.

If you do not have access to a GPS receiver and just want to know how one is used - here are some general guidelines that will give a first impression.

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GPS receivers come in different sizes, from small, handheld ones to bigger, more complex systems. In most cases, they will have a LCD (Liquid Crystal Display) on which the status of the receiver and the current measurement are displayed. Most of the present models will also have a computer outlet by means of which the device can be connected to a PC with suitable software. In that way, more comprehensive functions are available and a better display with higher resolution is possible on the computer monitor.

#### Acquiring the satellite signals

A GPS receiver will not work inside buildings that shield the high-frequency satellite transmissions. You must therefore first find the right place to stand, preferably in a fairly level area with a full horizon, before switching on the GPS receiver.

Assuming its "knows" its approximate geographical position (you may have to tell it in which country you are), your receiver will establish a list of satellites that are currently above the local horizon. It can do so, because it knows the approximate orbits of the GPS satellites and also the local time from the internal clock. Some devices will display a graphic image of the positions of the satellites above you.

Next it begins to acquire the satellite signals and may tell you when it has a good connection by displaying the signal strength. When signals from three satellites are being received, it may perform a first, rough calculation of the geographical position and inform you about the longitude and latitude. It may also give you a first estimate of the uncertainty (in metres). When more satellites are acquired, it will add the altitude above the sea level.

By prolonging the session, most GPS receivers will be able to improve the accuracy, as the distance measurements become more precise due to averaging of many time delay measurements. After some minutes (if the receiving conditions are good and there are 5 or more satellites in view), the accuracy in all three coordinates (latitude, longitude, altitude) may stabilize at around 15 metres.

#### Following a Route

Once you have established your first position, you may try to move around with your GPS receiver. You will be amazed, how fast it is able to follow you movements and to indicate in which direction you proceed. In this sense, it will act as a super-compass, giving you this direction in degrees.

Many GPS devices allow to store a series the momentary positions, resulting in a graphic display of the route you are following. This can be very useful if you are walking in unknown territory, because it is then possible for you to follow the same route back again to your starting point without getting lost in the wilderness or on the sea.

It is also possible to read the distance to the goal (of course, assuming that you have told it where it is) and, based on the currrent speed with which you move in this direction, it may tell you the estimated remaining time until you reach this goal.

If you want to move to a specific location, you may also pre-select a route from a map, store the coordinates of intermediate points along the chosen route and then move under GPS guidance to your destination.

#### Exercises:

1. Find examples of the use of GPS navigation by explorers in the recent literature. Discuss some of the arctic and antarctic traverses by individuals and small teams, the routes followed and the problems encountered. Do you think that it would have been possible to accomplish such travels without access to GPS?

2. If you live near the sea or at a lake, visit the local sailing club and ask to be shown the GPS receivers in use. Talk to the sailors and learn how they profit from the availability of GPS. Ask them to explain how GPS helps them to navigate (they may tell you how they used to navigate before).

3. If you have access to two handheld GPS receivers, try this game. Two teams, each with one receiver, agree to meet at a certain location (as defined by geographical latitude and longitude) at a given time. They start in different places and move towards the point of rendez-vous. The game is more difficult if the teams have no access to maps of the area.

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