Global Positioning System (GPS) makes use of signals sent by satellites in space and ground stations on Earth to accurately determine their position on Earth. Radio Frequency signals sent from satellites and ground stations are received by the GPS. GPS makes use of these signals to determine its exact position. The GPS itself does not need to transmit any information. The signals received from the satellites and ground stations contain time stamps of the time when the signals were transmitted. By calculating the time difference between the time the signal was transmitted and the time the signal was received, and using the speed of the signal, the distance between the satellites and the GPS can be determined using a simple formula for distance using speed and time. Using information from 3 or more satellites, the exact position of the GPS can be triangulated. GPS system plays very crucial role for all types of security and communication systems used in railways and location based systems.

The interface is with a PIC microcontroller rated at 5V or a low rating which may not match exactly with the GPS modem. Therefore, there is a need to design additional power supply unit – other than the microcontroller power supply unit. Such a designing process includes rectification, filtration and regulation of units to obtain a regulated DC supply from the mains AC supply. Connect the Rx and Tx pins of the GSM modem to the Tx and Rx pins of the microcontroller through a level shifter IC. A DB9 connector is used to connect this modem to the level shifter IC. While using GSM modem or PC interface to see the results (longitude and latitude), there arises a need to use multiplexer IC to interface with the microcontroller. Since the microcontroller has only one set of Rx and Tx pins, both GSM and GPS modem cannot be interfaced at a time.

MAX232 is used as a level shifter IC to convert the RS232 signals of GPS or GSM modem or PC to the TTL signals of the microcontroller. This level shifter IC is then connected between the multiplexer IC and the GSM/GPS modem. Connect the Rx and Tx pins of the GSM modem to the Tx and Rx pins of the microcontroller through a level shifter IC. A DB9 connector is used to connect this modem to the level shifter IC. While using GSM modem or PC interface to see the results (longitude and latitude), there arises a need to use multiplexer IC to interface with the microcontroller. Since the microcontroller has only one set of Rx and Tx pins, both GSM and GPS modem cannot be interfaced at a time.

GPS satellites broadcast beams in two carrier frequencies; L1 (1,575.42 MHz) and L2 (1,227.60 MHz). Beams that can be accessible to the general public are encoded in C/A (Coarse/Acquisition) code, and the beams that can be used only by the US military force are encoded in P (Precise) code.

Ionosphere

The ionosphere is a portion of the upper atmosphere, between the thermosphere and the exosphere. When GPS signals pass through this layer, the propagation velocity of the GPS signal goes slower, hence causing propagation error.

Troposphere

The troposphere is the lowest portion of Earth's atmosphere. Radio reflections caused by dry atmosphere and water vapor within provoke GPS position error.

Multipath propagation

GPS signal is not immune to reflection when it hits on the ground, structures and many others. This phenomenon is called multipath propagation, one of the causes of GPS position errors.

• Road Transport

• Aviation

• Shipping & Rail Transport

• Science

• Security

• Telecommunications