Satellite Constellation
A group of artificial satellites working in concert. The satellites are:
 Synchronized
 Operate together under shared control
Characteristics of GPS Constellation
 24 Satellites
 6 Orbits
 4 Satellites per orbit
 Orbit Period = approx 12 hrs
 Orbit Radius = 26600 Km (approx)
 Satellites’ Altitude = 20,200 Km (approx)
 Inclination with Earth = 55 degree
 Angle b/w Orbital Planes = 60 degrees
 Satellites do not rotate w.r.t. Earth but w.r.t distant stars
 6 to 11 satellites are always in sight from earth surface
 For this purpose the 4 satellites are not evenly spaced(900)
 They are spaced at 30, 105, 120, 105 degrees
 4 satellites are sufficient for GPS receiver
Working of GPS System
A GPS receiver calculates its position by precisely timing the signals sent by GPS satellites. The receiver uses the messages it receives to determine the transit time of each message and computes the distance to each satellite. The GPS receiver with the help of the satellites determine
 Time (t)
 Longitude (x)
 Latitude (y)
 Altitude (z)
 Let xi, yi, zi, ti be the xcoordinate, ycoordinate, zcoordinate and time respectively of satellite i
 Let tr be the time the GPS receives the message
 Then the transit time of the message = (tr – ti)
 Assume the message travels at a speed of light c
 Then by distance formula;
 Distance b/w the satellite and GPS receiver = (tr – ti)c
 Consider a receiver receives messages from 4 satellites
 Then the position of the receiver is confined by the spheres formed by these 4 satellites
 The center of the sphere is the satellite and
the GPS receiver is at its surface, therefore,
the distance from the receiver to satellite
known as pseudorange is the radius of the
sphere
 The position of the GPS receiver is
somewhere at the intersection of these
spheres
 Greater the number of spheres smaller will
be the intersection space and accurate will
be the position of the receiver

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Sources of Errors in GPS
The sources of errors in GPS, satellite erros, multipath errors and reciever errors.
Scope of GIS Project Management Planning
The scope and implementation of GIS projects including Singlepurpose GIS project, Departmental GIS project and Enterprise GIS project. 