GPS - Global Positioning System Presented By Brindha Narayanan

What is GPS?  The Global Positioning System (GPS usually called GPS receiver) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit.  The GPS is made up of three parts : Satellites orbiting the Earth Control and monitoring stations on Earth The GPS receivers owned by users

How GPS works FOR EXAMPLE: If a person lost somewhere and he gets the information from somebody. First person say that you are in 625 miles from Boise, Idaho Second person says that you are in 690 miles from Minneapolis, Minnesota. Third person says that you are in 615 miles from Tucson, Arizona. So now you know that you are exactly in Denver, Colorado.

How GPS works continues… The same rule is applied in GPS satellites. The GPS receivers in the earth has to locate at least three satellites in the orbit and figure out the distance between each of those and calculates the location. 1. The basis of GPS is “trilateration" from satellites.trilateration 2. To "triangulate," a GPS receiver measures distance using the travel time of radio signals. 3. To measure travel time, GPS needs very accurate timing which it achieves with some tricks. 4. Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret. 5. Finally you must correct for any delays the signal experiences as it travels through the atmosphere.

How GPS works continues… Each GPS satellite continuously broadcasts a Navigation Message at 50 bits/s giving the time-of-day, GPS week number and satellite health information (all transmitted in the first part of the message), an ephemeris (transmitted in the second part of the message) and an almanac (later part of the message). The ephemeris data gives the satellite's own precise orbit and is output over 18 seconds, repeating every 30 seconds. The almanac consists of coarse orbit and status information for each satellite in the constellation. Each satellite transmits its navigation message with at least two distinct spread spectrum codes: the Coarse / Acquisition (C/A) code, which is freely available to the public, and the Precise (P) code, which is usually encrypted and reserved for military applications.

The SVs transmit two microwave carrier signals. The L1 frequency ( MHz) carries the navigation message and the SPS code signals. The L2 frequency ( MHz) is used to measure the ionospheric delay by PPS equipped receivers. Three binary codes shift the L1 and/or L2 carrier phase. The C/A Code (Coarse Acquisition) modulates the L1 carrier phase. The C/A code is a repeating 1 MHz Pseudo Random Noise (PRN) Code. This noise-like code modulates the L1 carrier signal, "spreading" the spectrum over a 1 MHz bandwidth. The C/A code repeats every 1023 bits (one millisecond). There is a different C/A code PRN for each SV. GPS satellites are often identified by their PRN number, the unique identifier for each pseudo-random-noise code. The C/A code that modulates the L1 carrier is the basis for the civil SPS. The P-Code (Precise) modulates both the L1 and L2 carrier phases. The P-Code is a very long (seven days) 10 MHz PRN code. In the Anti-Spoofing (AS) mode of operation, the P-Code is encrypted into the Y-Code. The encrypted Y-Code requires a classified AS Module for each receiver channel and is for use only by authorized users with cryptographic keys. The P (Y)-Code is the basis for the PPS. The Navigation Message also modulates the L1-C/A code signal. The Navigation Message is a 50 Hz signal consisting of data bits that describe the GPS satellite orbits, clock corrections, and other system parameters.

Frequencies Frequencies used by GPS include L1 ( MHz): Mix of Navigation Message, coarse-acquisition (C/A) code and encrypted precision P(Y) code, plus the new L1Con future Block III satellites. L2 ( MHz): P(Y) code, plus the new L2C code on the Block IIR-M and newer satellites. L3 ( MHz): Used by the Nuclear Detonation (NUDET) Detection System Payload (NDS) to signal detection of nuclear detonations and other high-energy infrared events. Used to enforce nuclear test ban treaties. L4 ( MHz): Being studied for additional ionospheric correction. L5 ( MHz): Proposed for use as a civilian safety-of-life (SoL) signal This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that would provide this signal is set to be launched in 2008.

GPS Augmentations  An augmentation is any system that aids GPS by providing accuracy, integrity, reliability and availability. Nationwide Differential GPS System (NDGPS) Wide Area Augmentation System (WAAS) Continuously Operating Reference Station (CORS) Global Differential GPS (GDGPS) International GNSS Service (IGS)

Evaluation Of GPS The Global Positioning System (GPS) is a U.S. space-based radio navigation system that provides reliable positioning, navigation, and timing (PNT) services to civilian users on a continuous worldwide basis - - freely available to all. GPS provides accurate location and time information for an unlimited number of people in all weather, day and night, anywhere in the world. GPS provides accuracy, integrity, reliability and availability.

References 1. This address has the information of how this GPS receiver is working The basic webpage where you can get the basic information and up to date information in the site This webpage give the information about the system and also how it is used in the applicationhttp:// 4. This webpage makes you understand the working of GPS in a simple manner.