What is a GPS?
GPS or Global Positioning System is a satellite based technology that aid people to navigate their way anywhere on the earth. A must have navigation tool with multiple uses; GPS has become one of the most popular devices in recent times. Anyone can use this amazing gadget free of cost. All you need is a small, handheld GPS receiver to know where you are and find your way to a specified location. It is as simple as that.
What is GPS?
Global Positioning System (GPS) refers to a group of satellites orbiting round the earth and the ground stations that control these satellites. The GPS satellites send low frequency radio waves that carry precise information of time & location to anywhere in the world, round the clock. The GPS receiver gets this information and processes it to find out the exact location of its user.
Components of GPS
There are three components that make up Global Positioning System. They are:
- GPS Satellites
- Ground Monitoring Stations
- GPS Receivers
GPS Satellites
Known as GPS space segment or NAVSTAR satellites, there are as many as 24 satellites that are stationed in geocentric orbits at a distance of 12,000 miles from the earth. Installed by the US Department of Defense, they circle the earth at a speed of 7,000 miles per hour and make two orbits everyday. The high altitude of the satellites and the carefully planned orbital paths of the satellites ensure that the user gets signals from at least five satellites from any location on the earth.
GPS satellite on its orbit
GPS satellites mainly use solar energy. In the event of solar eclipse, there are backup batteries that aid running of the satellites. In addition, GPS satellites also have rocket boosters that guide them to orbit the earth on specified paths.
The US Department of Defense launched the first NAVSTAR satellite in 1978 and completed stationing the full constellation of 24 GPS satellites in 1994. GPS satellites are constantly monitored and maintained and also get replaced every 10 years.
Now, we come to the nucleus of the GPS system. GPS signal.
GPS satellites transmit radio signals in two different frequencies. L1 and L2. The commercial GPS receivers are capable of processing only L1 signals that are transmitted on the frequency of 1575.42 MHz in the UHF band while L2 frequency signals are generally reserved for military use.
GPS signals travel on a straight path and reach the earth. However, they are so low powered that they can be easily affected by mountains, thick foliage, buildings and interferences from devices like mobile phones etc. This could cause position and time errors, or sometimes even complete blocking of the signals. In other words, GPS receivers need open sky for their best performance and they work rather well in outdoors than in areas enclosed by tall buildings and other obstructions.
GPS signals carry three kinds of data that help a GPS receiver to determine its exact location. Each satellite has a pseudorandom code and the signals that it transmits contain this code. The pseudorandom code just tells the GPS receiver which satellite transmitted the signal.
Ephemeris data provides the corrections that have to be made to find out the precise location of a satellite in the sky. This information is transmitted by ground stations via satellites and changes every six hours so a GPS receiver has to update it while calculating its exact location.
Almanac contains the position information of the satellites and constantly stored and updated in the memory of the GPS receivers. It is like cookies that help you to get access the websites much faster. In the same way, almanac data helps GPS receivers to remember the position of satellites at a specific time. That is why, after a period of non-use, a GPS receiver requires some time called warm acquisition time to update almanac before calculating the position. This may take as much as ten minutes for most GPS units.
There are many factors that can alter the course of a GPS signal, affecting the accuracy of GPS receiver. Some of the typical errors that GPS receivers are prone to are as follows:
Multi-path signal error: Sometimes, GPS signal get reflected by objects like tall buildings or mountains so takes much longer time to reach a GPS receiver. This may lead to potential time & position errors.
Tall buildings - a source of multi-path signal error
Atmospheric factors: The atmospheric layers like ionosphere and troposphere slow down the speed of GPS signals and this causes delays and time errors. However, most GPS units can minimize this error by applying the average time delay while they calculate the distance traveled by GPS signals.
Atmospheric factors
GPS receiver clock is not an atomic clock though it is much more precise than what we use to know time so there could be slight time errors meaning GPS cannot calculate the position very precisely.
Satellite Visibility: GPS satellite navigation units need open, unobstructed view from the satellites for greater accuracy. Tall, solid structures like buildings, hills and thick forests highly populated with trees block can affect the quality of GPS signals. As a result, GPS receivers calculate the location with much less accuracy or simply do not work in such conditions. On the contrary, when GPS unit is able to lock signals transmitted from more than three or four satellites, the accuracy is getting better and better.
GPS receivers require clear view of sky for better accuracy
Thick foliage can prevent GPS unit from getting maximum signals
Other errors: For better accuracy, the satellites should be visible from different directions in the sky rather than all signals reaching the Satnav GPS unit from just one or two directions. While calculating 3D view of a location, this geometrical alignment is crucial. Another type of error may occur when locating the satellites. Satellites do not circle the earth in perfect orbits so the resultant errors need to be corrected with the help of ephemeris data.
Ground Monitoring Stations
Ground Monitoring Stations
The Control segment has five ground stations situated in different locations on the earth. These ground stations monitor and control GPS satellites and they continuously update orbital positions and time data received from satellites. The four unmanned stations get GPS data from the satellites and transmit them to the Master control station which identifies the errors from the data and sends back the corrected GPS information to the satellites.
GPS Receiver
This is the Satnav GPS unit that you buy for your navigational purposes. Also known as user segment in GPS system, GPS receiver reads the signals transmitted from GPS satellites and computes its distance from satellites. To find out its exact location, GPS unit needs to know its distance from at least three GPS satellites.
Once GPS receiver is able to get signals from three or more satellites, it can use the triangulation method to find out its current position. The signal from the fourth satellite allows the GPS to determine the altitude.
Now, with the help of preloaded maps, the GPS receiver displays the current location along with other details like time, speed, direction and destination on the LCD screen.
Garmin nuvi Car navigation GPS unit
Modern GPS receivers come with parallel multi-parallel channel designs and each channel or circuit is capable of receiving signals from a specific satellite. There could be as many as 12 parallel channels in a GPS unit so it can receive strong signals at any place and at all times. This type of GPS receivers work well even when the ground conditions are a bit difficult such as thick forest, trees, rocky hills etc.
