Global positioning system
Satellite communication tutorial
Global Positioning System
Global Positioning System (GPS) is a satellite based navigation system. He created the revolution in navigation and locating positions. It is mainly used in positioning, navigation, surveillance and surveying applications.
The main advantages of satellite navigation are real-time positioning and time synchronization. This is why satellite navigation systems have become an integral part of most applications, where mobility is the key parameter.
A complete operational GPS space segment contains twenty-four MEO satellites. These satellites are constellation . The two adjacent constellations are separated by 60 degrees of longitude.
TheEach satellite's orbital period is approximately equal to twelve hours . Therefore, all satellites revolve around the Earth twice a day. At all times, GPS receivers will receive signals from at least four satellites.
GPS codes and services
Each GPS satellite transmits two signals, L 1 and L 2 are of different frequencies. Trilateration is a simple method to find the position (Latitude, Longitude, Altitude) of the GPS receiver. Using this method, the position of an unknown point can be measured from three known points
These are the two types of GPS codes. Rude acquisition code or C / A code Precise code or P code
The signal, L 1 is modulated with a pseudo-random bit sequence of 1.023 Mbps. This code is called coarse acquisition code or code C / A and it is used by the public.
The signal, L 2 is modulated with 10.23 Mbps pseudo-random bit sequence. This code is called precise code or P code and it is used in army positioning systems. Usually, this P code is transmitted in an encrypted format and is called Y code
The P code gives better measurement accuracy compared to the C / A code, since the bit The bit rate of the P code is higher than the bit rate of the C / A code.
These are the two types of services provided by GPS. Precise Positioning Service (PPS) Standard Positioning Service (SPS)
PPS receivers keep followed by both C / A code and P code on two signals, L 1 and L 2 . The Y code is decrypted at the receiver in order to obtain the P code.
SPS receivers only follow the C / A code onthe signal, L 1 .
There is only one-way transmission from satellite to users in the GPS system. Therefore, the individual user does not need the transmitter, but only a GPS receiver . It is mainly used to find the precise location of an object. It performs this task using signals received from satellites.
The block diagram of the GPS receiver is shown in the figure below.
The function of each block present in the GPS receiver is mentioned below.
The receiving antenna receives satellite signals. It is mainly a circularly polarized antenna.
Low noise amplifier (LNA) amplifies the weak signal received
The buck converter converts the signal frequencyreceived as an intermediate frequency (IF) signal.
IF Amplifier amplifies the intermediate frequency (IF) signal.
ADC performs the conversion of the analog signal, which is obtained from the IF amplifier to digital. Suppose the sample and quantize blocks are also present in ADC (Analog to Digital Converter).
DSP (Digital Signal Processor) generates the C / A code.
The microprocessor calculates the position and supplies the synchronization signals in order to control the operation of the other blocks. It sends the useful information to the display unit in order to display it on the screen.