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Current GPS – Optimal Satellite Positioning and Structure
Millions of GPS users across the world are dependent on the signals sent by the GNSS network in the sky to achieve certain tasks that include determining their location, navigation, tracking something or someone, and mapping areas, among the many other applications of GPS. At the moment, GPS is divided into two categories: civilian and military. Accordingly, the GNSS send signals at L frequencies: L1 for civilians and L2 for sole military purpose.
L2 is highly encrypted and stronger to increase the accuracy of GPS for military. This provision has been made so that no terrorist organization can use the advanced signals on L2 frequency for destruction. Besides, the US reserves the right to alter or limit the signal strengths or angles for both frequencies if required. This is possible because NAVSTAR, the US owned GNSS is the only space based network that is fully functional.
The current GPS constellation is supported by 31 active satellites revolving around the earth in six different orbits, with a minimum of four satellites in each orbit. The GPS system makes sure that the earth based GPS devices and GPS servers receive signals from five or more satellites round the clock (even though a GPS device can function with signals from four satellites).
To enhance the accuracy of GPS device, the orbits are inclined at 55 degrees each, thereby facilitating different, yet direct (straight) paths for the signals to reach earth-based GPS devices without any hindrances. Still, there are several factors that interfere with the GPS signals to create a GPS blackout or to reduce the accuracy of GPS device. The next section checks out what interferes with a GPS signal.
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Weak Signal GPS – What Interferes with a GPS Signal
The most common factor that interferes with a GPS signal is what is termed "Urban Canyon." The term refers to the high rise concrete buildings and skyscrapers that do not allow the signals to pass through them. These urban structures either totally block the radio signals sent by the satellites or alter their path so that they never reach the GPS servers or GPS device. Even if the signals are able to get through the buildings, they become so weak that it becomes weak for the GPS device's receiver to interpret the signal.
The radio signal has to travel a long distance before it reaches the GPS device. During this journey, it passed through vacuum (in space) and the different layers of Earth's atmosphere. While in space, the radio signals may not face any distraction or may get some interference (flying rocks or condensed dust clouds). The journey becomes rough as soon as it enters Earth's atmosphere. Several factors interfere with a GPS signal: dense clouds, dust particles, natural elements such as mountains, and even man-made flying objects, such as airplanes.
If the GPS devices are being used underwater such as in submarines, the distance becomes too long for the radio signals to retain their strength. The density of ocean water also contributes to the weakening of the radio signals.
In short, there are many things that interfere with a GPS signal even though the current GPS system has been designed in a way to offer ground based receivers with the radio signals in a straight line, as explained in the first section. The next page continues the discussion on weak signal GPS while offering a look at differential GPS as one solution for better accuracy.
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Weak Signal GPS – An Overview
Among other factors that interfere with a GPS signal is the intersection of two or more signal paths. During this clash of signal paths rising from different satellites, the data may change or get corrupt. Though the placement of satellites is done thoughtfully, it is sometimes natural for the signals to intersect, causing damage to the data.
The aim of GPS satellites is to offer a straight path that carries data to the GPS without any radio signal interference. However, it is not always successful. On the contrary, the problem seldom occurs. The intelligent GNSS receivers inside the GPS devices are capable of detecting erroneous data and stop computing until they start receiving correct data.
Finally, you are made to believe that GPS is not for indoor use. It is true that the GPS signal strength is very weak when you are inside any building but this does not mean that you cannot use the device indoors. The workaround is Re-radiation.
As the ground based GPS device receives the signal, it filters the signal to extract data on: the three planes (X, Y, and Z axis), satellites through which it received the signals, position of each satellite, time stamps of each signal, and the possible redundant clock errors at the satellite's end. Based on all this data, the GPS device filters, boosts, and finds the strongest GPS signal before computing it to offer you with the desired information.
In its endeavor to locate the strongest GPS signal, the GPS device may find that the strongest signal is a very weak signal out of which, it has to extract the data sent by the satellites. Still, the GPS is a technology that won't let you down. Among other methods, WAAS (also called SBAS) is one method to recreate accuracy of GPS.
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Weak Signal GPS – Using WAAS to get Proper Accuracy of GPS
Though the Wide Area Augmentation System (WAAS) was originally developed for air-navigation to get proper accuracy of GPS in case of weak signals, it is now a common feature in most of the GPS devices, especially in the portable models. If the GPS device contains WAAS, it has more than one receiver.
The first receiver is stationary, receiving data from a specific satellite. The other one is roving. WAAS uses a network of ground based GPS reference-stations when it detects erroneous or faint GPS signals. The technology uses GPS reference stations, primarily in Hawaii and North America, to determine the variations in GPS signals. These reference stations then send these measurements to the master stations. Currently, there are five active ground-based GPS stations across the world.
Once the master station receives the measurements, it sends back the measurements in the form of signals to the satellites in the GPS system. The satellites take care of the incorrect measurements and send the corrected signals back to Earth based GPS-stations and GPS devices.
Thus, it becomes easy to ascertain the integrity of data while obtaining full data based on the weakest signals. As a GPS device can easily access up to six satellites at a time, the WAAS enabled GPS devices make full use of the facility to compute and present accurate results for weak signal GPS.
Note: It is the radio waves that lose strength over distance and resistance but not the data being carried by the waves. This fact makes it easy for the WAAS technology for presenting results while working with weak signal GPS.