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As shown in Figure 32.2, the GPS receiver is a microprocessor-based (or DSP-based) system along with a multichannel satellite receiver, which receives the signals from NAVSTAR satellites visible at that location. The data sent by each satellite, known as almanac, consists of coded time and location of the satellite. The GPS receiver calculates its own location parameters from a minimum of four such signals. These parameters are calculated once per second and sent to the display unit. Optionally, the location data can be integrated with an application software package such as mapping software.
Figure 32.2: GPS receiver.
Note | The GPS receiver has to carry out very complex calculations to obtain the position parameters using the signals received from the satellites. A digital signal processor (DSP) is generally used in the GPS receiver. |
GPS accuracy: In GPS, errors in measurement can occur due to propagation delays or clock variations. In coarse acquisition, the GPS receiver gives an accuracy variance of less than 100 meters 95% of the time and 300 meters 99.99% of the time. It may happen that your car could be shown to be moving on a lake when you are traveling on a road adjacent to a lake! The U.S. government used to degrade the GPS signals available to the public, intentionally and this was known as selective availability (SA). On 1st May 2000, the U.S. government discontinued SA, and hence the GPS signals are no longer intentionally degraded. This resulted in an improved accuracy of about 20 meters. This measure was taken after demonstrating the capability to deny GPS signals selectively on a regional basis. This was intended to show that the government can ensure that GPS signals are not received in a specific area. If more accurate location data is required, differential GPS (DGPS) technology is used.
An accuracy of about 20 meters can be obtained using the GPS receiver. The accuracy can be improved using differential GPS.
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