GPS system includes three components. The space component made up of GPS satellites that orbit the earth. These satellites continuously transmit signals which are used by the GPS receiver (the user component) to find out its location. There are also satellite monitoring stations on the earth that control the satellites and transmit the updated information back to the satellites. It is the control component of the GPS system.

The GPS receiver almost does everything. It receives time and position information from the satellites and calculates its location with its computing system involving a bit of geometry and mathematical formulas. Now, the central question is, how does a GPS receiver do it?

Triangulation Method

In the olden days, the navigators used to work out the position of a location using the triangulation method. The basic idea of the triangulation method is that if you know how far you are from three or four reference points, you can compute your current location just by drawing a few curves on a chart and the intersected area triangulated by these curves will roughly tell you where you are. More the reference points, smaller the triangle they will form and higher the accuracy. GPS uses the same technique and use satellites as reference points and calculate their position with the information provided by the satellites themselves.

GPS receivers read two types of information from the signals transmitted from the GPS satellites - Almanac and Ephemeris data. Almanac provides the theoretical positions of the satellites for the GPS receiver so that it can store this information in its memory. Almanac data always keep changing with the movement of satellites on their orbits and the GPS receiver gets it updated when it’s turned on. Ephemeris data provides the actual position data that includes a few corrections to be made when the path of GPS satellites slightly varies every six hours. Both almanac and ephemeris data helps the GPS unit to find out the position of the satellites.

Since, it is not possible for GPS receiver to directly measure its distance from a satellite, it does the same in a round about manner using the simple formula involving time and speed. If the time taken by the signal to reach the receiver is known, multiplying that value with the speed of the signal (which is the speed of light) will give the distance. To measure the time interval, the GPS satellite has an atomic clock on board which gives the time when the signal left. The GPS Receiver, using its clock, finds out the time the signal reached it and simply finds the difference between these time points. It also corrects the time errors that occur due to ionosphere and other sources of errors like GPS receiver clock errors, multi-path signal etc.

Now, the GPS repeats these steps for all the visible satellites, finds the distance and uses the triangulation method to compute its location. To get 3D fix, the GPS unit needs to lock signals from at least four satellites so that it can provide other information like direction of movement, speed and altitude etc.

When you turn on your GPS for the first time, it has to do some preliminary things before showing anything. This is called cold start. It receives basic data like almanac, ephemeris and time data from the satellites and all this takes a few minutes till it comes warm, i.e., it becomes ready to compute the 3D fix of its location.

What is differential GPS?

The standard GPS receivers are capable of calculating the location within a radius of 10 to 15 meters. However, differential GPS receivers (DGPS) are more accurate with an accuracy of less than five meters. Typically, differential GPS system requires a few powerful GPS receivers stationed at fixed locations and collect GPS information from the satellites and transmit the corrected data to other receivers in the same area using beacon transmitters. To use differential GPS, your GPS unit should have differential beacon receiver that is compatible with the beacon transmitter of the DGPS that relays the signals. Within the range of the DGPS coverage, your GPS receiver collects the data and computes navigational information with greater accuracy.

GPS Mapping

Most GPS satnav units can provide the basic information of your location on its screen even without pre-loaded maps. These units just show you where you are with reference to routes or waypoints you have previously recorded. These basis GPS models can show the names of cities and a few important places but not more than that.

With some GPS units, you can upload detailed maps of particular regions from CDs and get them on the screen. These maps can show the finest details of cities like roads, streets, banks, restaurants, petrol filling stations, airports and other points of interests (POI).

Some GPS units also use data cartridge for mapping while others get the map data recorded in their internal memory. Using any of these options, you can choose a specific area and get its map loaded into your GPS receiver.

GPS navigation: How to use GPS?

If you want to use your GPS unit for guiding you to a specific location, you would first create waypoints in its memory that may include your home, office, gas stations, restaurants or any other places you think you will often visit. You can also include new waypoints which you have not visited earlier but you think they are important and useful. Once waypoints were made, you can input the place of destination and your GPS unit will show the direction and whichever route you prefer by connecting the existing waypoints or you can just instruct the GPS to go from one waypoint to another and connect all of them to make your route.

It is not just the direction and route to your destination that GPS provides, you can also know other information like your current position, speed and the time you will take to reach your destination etc.

There is another feature 'track log' that stores all the details of your traveled route and guides you back to the place where you started. For example, Garmin GPS units have TracBack feature which once gets activated, generates the reverse route based on the information of your journey recorded in the memory.

A few points to remember:

• The performance and accuracy of your GPS unit largely depends upon its ability to pick the signals from the GPS satellites. The primary requirement will be a clear view of sky so the antenna of the GPS receiver can pick strong signals.
• GPS signals travel on the line of sight. They can pass through lighter objects like glass, plastic but not through solid structures like buildings, tunnels or thick foliage. In such environmental conditions, it may take longer time for the GPS unit to put the navigation details in 3D format.
• Most GPS units come with multi-channel receivers or circuits, capable of processing of GPS signals from up to 12 GPS satellites at a time.
• Make use of different GPS antennas depending upon geographical location. It can make up when the signals are not strong enough or when it takes too long for your GPS unit to turn warm.