GPS TECHNOLOGY
What is GPS
·
Radio-based navigation
system developed by DoD
ü
Initial operation in
1993
ü
Fully operational in
1995
·
System is called
NAVSTAR
ü
NAVigation with Satellite Timing And Ranging
ü
Referred to as GPS
·
Series of 24
satellites, 6 orbital planes, 4 satellite vehicles (SV) on each plane
·
Works anywhere in the
world, 24 hours a day, in all weather conditions and provides:
ü
Location or positional
fix
ü
Velocity
ü
Direction of travel
ü
Accurate time
Global
Navigation Satellite Systems (GNSS)
q NAVSTAR
ü USA
q GLONASS
ü Russians
q Galileo
ü Europeans
GPS involves
5 Basic Steps
q Trilateration
ü Intersection of spheres
q SV Ranging
ü Determining distance from SV
q Timing
ü Why consistent, accurate clocks are
required
q Positioning
ü Knowing where SV is in space
q Correction of errors
ü Correcting for ionospheric and
tropospheric delays
How GPS
works
q Range from each satellite calculated
range = time delay X speed of light
q Technique called trilateration
is used to determine you position or “fix”
ü Intersection of spheres
q At least 3 satellites required for 2D
fix
q However, 4 satellites should always
be used
ü The 4th satellite used to
compensate for inaccurate clock in GPS receivers
ü Yields much better accuracy and
provides 3D fix
Determining
Range
q Receiver and satellite use same code
q Synchronized code generation
q Compare incoming code with receiver
generated code
Signal
Structure
q Each satellite transmits its own
unique code
q Two frequencies used
ü L1 Carrier 1575.42 MHz
ü L2 Carrier 1227.60 MHz
q Codes
ü CA Code use L1 (civilian code)
ü P (Y) Code use L1 & L2 (military
code)
Accurate
Timing is the Key
q SVs have highly accurate atomic
clocks
q Receivers have less accurate clocks
q Measurements made using “nanoseconds”
ü 1 nanosecond = 1 billionth of a
second
q 1/100th of a second error
could introduce error of 1,860 miles
q Discrepancy between satellite and
receiver clocks must be resolved
q Fourth satellite is required to solve
the 4 unknowns (X, Y, Z and receiver clock error)
Satellite
Positioning
q Also required in the equation to
solve the 4 unknowns is the actual location of the satellite.
q SV are in relatively stable orbits
and constantly monitored on the ground
q SV position is broadcast in the
“ephemeris” data streamed down to receiver
Sources of
Errors
q Largest source is due to the
atmosphere
ü Atmospheric refraction
•
Charged
particles
•
Water
vapor
Other Sources
of Errors
q Geometry of satellite positions
q Satellite clock errors
q SV position or “ephemeris” errors
q Quality of GPS receiver
q Multi-path errors
Dilution of
Precision (DOP)
q Geometric location of the satellites
as seen by the receiver
q The more spread out the satellites
are in the sky, the better the satellite geometry
q PDOP (position dilution of precision)
is a combination of VDOP and HDOP
q The lower the PDOP value, the better
the geometric strength
q PDOP value less than 6 is recommended
Selective
Availability
q The intentional introduction of
errors for civilian users is called Selective Availability
q SA was terminated on May 2, 2000
q When SA was on, civilian users
accuracy was ~100 meters
q Military has capability to degrade
signal in certain “theaters of operation” – this is called “spoofing”
Differential
Correction
q Technique used to correct some of
these errors
q Referred to as “differential GPS” or
DGPS
q In DGPS, two GPS receivers are used
q One receiver is located at an
accurately surveyed point referred to as the “base station”
q A correction is calculated by
comparing the known location to the location determined by the GPS satellites
q The correction is then applied to the
other receiver’s (known as the “rover”) calculated position
DGPS Methods
q Post-processing
ü Corrections performed after the
data is collected
ü Special software required
q Real-time
ü Corrections are performed while
the data is being collected
ü Need special equipment to receive the
DGPS signal
Wide Area
Augmentation System – WAAS
q New “real-time” DGPS
q Satellite based
q FAA initiative….now fully operational
q Series of ~25 ground reference
stations relay info to master control station
q Master control station sends
correction info to WAAS satellite
GPS Accuracy
Issues
q Ways to improve the accuracy of your
GPS collected data
ü Standardize data collection methods
ü Establish protocols for your
applications
ü Employ averaging techniques
ü Perform mission planning
ü Utilize DGPS
ü Understand how the selection of
datums and coordinate systems affect accuracy
•
GPS data collected in wrong datum can introduce ~200 meters of error into
your GIS!
Some issues to consider when purchasing GPS devices
q What is the accuracy level required
for your application?
(10 meters or
sub-meter)
q How is unit going to be used in
field?
ü External antenna required, in heavy
canopy, ease of use, durability, data dictionary capability, waterproof…
q Cost…… from $100 to $12K
q Staff expertise..training..support
network
q How well does unit interface with
GIS?
Many drivers or employees may assume that GPS vehicle tracking is only
beneficial for the business owner or fleet manager, but that is simply
not true. GPS tracking can provide benefits for an organization as a
whole. The solution can be a useful tool for drivers, field
representatives, dispatchers, and even human resources.