Transcript GPS (ppt)

GPS
How it Works
For a full tutorial on GPS and its applications
visit the Trimble Website
What is GPS?
• Global Positioning
System is a
worldwide radionavigation system
formed from a
constellation of
satellites
• Dependent on line
of sight
GPS Systems
• NAVSTAR (Navigation System Timing and Ranging)
– United States
• GLONASS
– Russian Global Navigation SS
• GALILEO
– European Union
• COMPASS (BeiDou)
– Chinese Navigation System
• IRNSS
– Indian Regional Navigational SS
Components of GPS
• Space
– Satellites
• Control
– Ground control and antennas and
monitoring stations
• User
– Military, civil, commercial, scientific
Satellites (SV’s)
• 32 satellites
• Orbit at 12,600
miles
• Six orbital planes
• Each SV has a ½
sidereal day orbital
period
• ~ Nine satellites are
visible at any given
time
Garmin
Control Segment
Users
• Civilian
– Cartography, Cellular networks, fleet
tracking, navigationetc.
• Military
– Navigation, target tracking, munitions
guidance, and reconnaissance
– Nuclear detonation detectors
How do we determine our position?
• We have to determine our precise
distance to at least 4 GPS satellites
– We need extremely accurate clocks
• In addition to distance we need to
know exactly where the satellites are in
space.
• Lastly, we correct for any delays the
signal experiences as it travels through
the atmosphere.
How GPS works
Information sent in the GPS signal
• GPS timing signal (PRC)
• Ephemeris
– Satellite orbit information
– Updated every 2 hours valid for 4 hours
• Almanac
– Orbit and status for all 32 satellites
– Error corrections
Sync Satellite and Receiver
• We need to have an accurate measure
of the radio signal travel time
• Atomic Clock
– Use oscillations of an atom for timing
• A fourth measurement is used to
determine the position
– The receiver looks for a single correction
factor that it can subtract from all its
timing measurements that would cause
them all to intersect at a single point
Correcting Errors
• Charge particles in the ionoshpere and
water vapor in troposhpere slows the
GPS signal
• Multipath error
• Tiny errors that occur at the satellite
• Selective Availability
Differential GPS
• Relies on the operation of two receivers
– One is stationary and its location is know
precisely
– The second is roving and making
measurements (this is YOU!)
• The reference station “knows” where it
is located
– It uses that information to correct the
timing from the GPS satellite
• This error correction is then sent to the
rover to correct the measurements
Wide Area Augmentation System
• WAAS
– Uses ground stations to measure the
variation in the GPS system
– It corrects for this error and sends the
correction to WAAS satellites
• Must have a specially equipped receiver
to obtain the WAAS signal
• Typically accurate to within 1 meter
Real Time Kinematic
• RTK
– Based on the use of
carrier phase
measurements
– RTK systems use a
base station that
transmits the
measure carrier
phase to mobile
individuals
– Can result in cm
accuracy