Transcript GPS Overview April 2004 TEC7132

GPS Overview
TEC7132
April 2004
What is GPS?




Radio-based navigation system developed by DoD
 Initial operation in 1993
 Fully operational in 1995
System is called NAVSTAR
 NAVigation with Satellite Timing And Ranging
 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:




Location or positional fix
Velocity
Direction of travel
Accurate time
Global Navigation Satellite Systems
(GNSS)

NAVSTAR


GLONASS


USA
Russians
Galileo

Europeans
GPS involves 5 Basic Steps

Trilateration


SV Ranging


Why consistent, accurate clocks are required
Positioning


Determining distance from SV
Timing


Intersection of spheres
Knowing where SV is in space
Correction of errors

Correcting for ionospheric and tropospheric
delays
How GPS works?
Range from each satellite calculated
range = time delay X speed of light
 Technique called trilateration is used to
determine you position or “fix”


Intersection of spheres
At least 3 satellites required for 2D fix
 However, 4 satellites should always be used



The 4th satellite used to compensate for
inaccurate clock in GPS receivers
Yields much better accuracy and provides 3D
fix
Determining Range
 Receiver
and satellite use same code
 Synchronized code generation
 Compare incoming code with receiver
generated code
Measure time difference
between the same part of code
From satellite
From receiver
Series of ones
and zeroes repeating
every 1023 bits. So
Complicated alternation
of bits that pattern looks
random thus called
“pseudorandom code”.
Signal Structure
 Each
satellite transmits its own
unique code
 Two frequencies used
 L1
Carrier 1575.42 MHz
 L2 Carrier 1227.60 MHz
 Codes
 CA
Code use L1 (civilian code)
 P (Y) Code use L1 & L2 (military code)
Three SV ranges known
20,000 Km radius
Located at one of these 2 points.
However, one point can easily
be eliminated because it is either
not on earth or moving at impossible
rate of speed.
22,000 Km radius
21,000 Km radius
Accurate Timing is the Key
SVs have highly accurate atomic clocks
 Receivers have less accurate clocks
 Measurements made using “nanoseconds”


1 nanosecond = 1 billionth of a second
1/100th of a second error could introduce
error of 1,860 miles
 Discrepancy between satellite and receiver
clocks must be resolved
 Fourth satellite is required to solve the 4
unknowns (X, Y, Z and receiver clock error)

Satellite Positioning
 Also
required in the equation to solve
the 4 unknowns is the actual location
of the satellite.
 SV are in relatively stable orbits and
constantly monitored on the ground
 SV position is broadcast in the
“ephemeris” data streamed down to
receiver
Sources of Errors
 Largest
source is due to the
atmosphere
 Atmospheric
•
•
refraction
Charged particles
Water vapor
Ionosphere
(Charged Particles)
Troposphere
Other Sources of Errors
 Geometry
of satellite positions
 Satellite clock errors
 SV position or “ephemeris” errors
 Quality of GPS receiver
 Multi-path errors
Dilution of Precision (DOP)
Geometric location of the satellites as seen
by the receiver
 The more spread out the satellites are in
the sky, the better the satellite geometry
 PDOP (position dilution of precision) is a
combination of VDOP and HDOP
 The lower the PDOP value, the better the
geometric strength
 PDOP value less than 6 is recommended

Selective Availability
The intentional introduction of errors for
civilian users is called Selective Availability
 SA was terminated on May 2, 2000
 When SA was on, civilian users accuracy
was ~100 meters
 Military has capability to degrade signal in
certain “theaters of operation” – this is
called “spoofing”

Differential Correction






Technique used to correct some of these errors
Referred to as “differential GPS” or DGPS
In DGPS, two GPS receivers are used
One receiver is located at an accurately surveyed
point referred to as the “base station”
A correction is calculated by comparing the known
location to the location determined by the GPS
satellites
The correction is then applied to the other
receiver’s (known as the “rover”) calculated
position
DGPS Methods

Post-processing



Corrections performed after the data is
collected
Special software required
Real-time


Corrections are performed while the data is
being collected
Need special equipment to receive the DGPS
signal
Wide Area Augmentation System - WAAS
New “real-time” DGPS
 Satellite based
 FAA initiative….now fully operational
 Series of ~25 ground reference stations
relay info to master control station
 Master control station sends correction
info to WAAS satellite


http://gps.faa.gov/programs/waas/howitworks.htm
WAAS Satellites



WAAS satellites are geo-stationary
On east coast, WAAS satellite sits off coast of Brazil over equator
at 53.96° West (#35 on Garmin)
 http://www.lyngsat.com/tracker/inmar3f4.shtm
On west coast, WAAS satellite sits over Pacific ocean at 178.0°
East (#47 on Garmin)

http://www.lyngsat.com/tracker/inmar3f3.shtml
Ability to get signal deteriorates in northern latitudes (satellite is
lower on the horizon)
 If you can get WAAS satellite signal……..~3 meter accuracy
 However, cannot always get signal due to obstructions
 More WAAS satellites becoming available in future
 Europeans (EGNOS)
 Japanese (MSAS)

GPS Accuracy Comparison
Some common GPS devices used by FWS:
GPS Device
Autonomous
WAAS
DGPS
Real-time
Post-process
Garmin GPSMap 76s
~ 10 - 15
~3
3
1-3
Rockwell – PLGR
Federal Users Only
~ 8 - 15
NA
3
NA
Trimble - GeoXT
~ 10
~3
1-3
Sub-meter
DGPS
DGPS
Accuracy given in meters
GPS Accuracy Issues

Ways to improve the accuracy of your GPS
collected data






Standardize data collection methods
Establish protocols for your applications
Employ averaging techniques
Perform mission planning
Utilize DGPS
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

What is the accuracy level required for your application?
(10 meters or sub-meter)

How is unit going to be used in field?

External antenna required, in heavy canopy, ease of use,
durability, data dictionary capability, waterproof…

Cost…… from $100 to $12K

Staff expertise..training..support network

How well does unit interface with GIS?
Latest Technology
Mobile mapping software for WindowsCE devices
TerraSync (Trimble)
ArcPad (ESRI)
Multi-path rejection technology
Trimble GeoXT
Bluetooth
Allows for cable free operation
ArcPad Software
Bring GIS data into the field!
Integrate GPS with GIS
Custom forms for data collection
ArcPad Training
NCTC 3 day course (TEC7133)
 Utilize ArcPad Tools for ArcGIS



Geodatabase - “check out” & “check in”
Design custom forms for data collection

Applets & ArcPad Studio
Utilize GPSCorrect extension
 Customize the ArcPad interface

Next offering: September 1-3, 2004