GPS satellite basics
This page covers GPS satellite basics.It mentions GPS satellite application note and GPS satellite manufacturers.
There are three segments in Global Positioning System viz. space segment (GPS satellites), control segment(e.g. ground stations comprised of master control station, monitor stations and ground control stations) and user segment (e.g. GPS users). In this page we will study about GPS space segment. The space segment consists of total 28 operational satellites around the earth in 6 different orbital planes. The orbits are about 20180 Km above the surface of earth and are enclined at 55 degree with the equator. GPS satellite usually takes about 12 hours to complete the one round of its orbit, but due to rotation of earth, it takes about 24 hours for the GPS satellite to come to its originatin place in the orbit. Total four GPS satellites are needed to determine the position in 3D space.
The GPS satellite transmits following information in a navigation message at 50 bps.
• Satellite time and synchronized signals• orbital data• time correction information • approx. orbital data for all satellites • data on ionosphere • correction signals to calculate transit time of signal • information on satellite health
GPS satellite takes 12.5 minutes to transmit all the informations. This navigation message is used by GPS receiver to determine transmission time of all the satellite signals as well as exact position of satellite at the time of transmission.
GPS satellite transmits a unique signature i.e. PN sequence of 1023 zeros and ones. It is of 1ms duration. This unique PN sequence of GPS satellite will help GPS receiver identify the received signal is from which of the satellite in the constellation. It also helps GPS receiver perform transit time measurement.
The figure-1 depicts simple block diagram of GPS satellite.
There will be four accurate atomic clocks. The useful time pulses/frequencies are derived from resonant frequency of
one of the atomic clocks. They are as follows:
• 50Hz pulse for data • C/A Code pulse (Coarse/Acquistion) which modulates data using EX-OR• Frequency of L1 carrier(1575.42MHz)
The data modulated by C/A code does modulation of L1 carrier using BPSK. This results into 180 degree change in L1 carrier phase correspond to change in modulated data.
The figure-2 mentions all the waveforms of GPS satellite at different modules. Atomic clocks need stability greater than about 2 X 10-13. As mentioned basic frequency of value 10.23MHz of GPS satellite is derived from resonant frequency.
GPS Satellite Application Note
GPS satellite based Global Positioning System to determine following:
• One's location(longitude,latitude,height co-ordinates) with accuracy within range of 20meters to 1mm.
• The precise time (world time, Universal Time Coordinated, UTC) with accuracy within range of 60ns to 1ns.
The other parameters which can be derived from above 3D position and exact time are as follows:
• local time
• range measurements
GPS satellite is widely used for following domain areas:
• SCIENCE and RESEARCH
• COMMERCE and Industry
• Agriculture and Forestry
• Communication Technology
• Tourism and Sports
GPS Satellite Manufacturers
Following are the manufacturers of GPS satellite and its parts:
• Lockheed Martin Corporation
• Boeing Space Systems
• AeroAstro L.L.C.
• Alcatel Space
• Alenia Spazio
• COM DEV
• Saab Ericsson Space
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