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55 Cards in this Set
- Front
- Back
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Four components of GPS receiver
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Antenna
Receiver Data Logger Batteries |
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Trimble GeoXT
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Good
-Window CE based -Submeter accuracy possible with differential correction -All day battery Bad -expensive -dim screen -limited map memory |
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Trimble GeoXH
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30cm post processed accuracy
Predicted post processed accuracy displayed in real time |
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Trimble R-6
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Full system including reciever w/ radio link to portable base station
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Terrasync
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Communicates with reciever, controling software
Allows for differential corrections from various sources, ex radio, internet |
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ArcPAd
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lightweight GIS designed for windows CE
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Where doesn't GPS work
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Blocked signal
Around strong radio signals |
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GPS alternatives
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Russian-GLONASS
EU-Galileo China-Beidou |
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Data collection methods
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Primary-vector
Secondary-raster |
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Why use GPS
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High elevation, location accuracy
Global coverage All weather |
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GPS ages
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Stone
Star Radio Loran Satellite |
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Loran Age
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Long range navigation
At least 4 radio towers, 1 tower typically covers 500 miles |
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Transit
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GPS precursor
Used only 6 satellites observable for a brief period every 90 minutes Needed a stationary receiver for a while |
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When did selective availability end
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Current accuracy within 15 meters
Ended selective deniability on may first 2000 |
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Space Segment
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24 active, 3 backup satellites
In 6 orbits inclined 55 degrees from equator 11 hours 58 minutes for one orbit |
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Control Segment
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One master control station in Flacon Air force base colorado springs co
Five unmanned control stations Two backup control stations |
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User Segment
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Navigate
Map Survey -Accuracy -Precision Track |
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Field Reconnaissance
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What features?
Line, Point or Polygon? Attributes? Attribute values? |
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Project Planning
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1) Create a project in PFO
2) Build a data dictionary 3) Transfer data to receiver and Download an "Almanac" 4) Check satellite availability 5)Configure the receiver for data collection |
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Other functions of Pathfinder office
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Data dictionary editor
Import/export Display Quick plan Differential correction Coordinate system editor |
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Transferring procedure
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Connect receiver to computer
Select data transfer On computer select utilities/data transfer On data transfer select data dictionary Select the dictionary you created from the available files dialog and click add Click transfer a "file sent" message should appear |
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Quick planning tools
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Tells you...
Number of visible satellites Where they will travel PDOP(indicator) Uses almanac file collected by receiver |
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Checklist before data collection
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Equipment set up?
Charged batteries? Equipment working? Configure rover receiver, base station |
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GPS in 5 easy steps
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1)The basis of GPS is "trilateration" from satellites
2)To trilaterate, GPS receiver determines time required by satellites to reach receiver 3)To measure travel time needs accurate clock 4)High, stable satellite orbits allow easy tracking 5)Adjust for signal delays |
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Types of GPS receivers
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1)single frequency code receivers
2) single frequency carrier smoothed code receivers 3)Single frequency code and carrier receivers 4)Dual frequency receivers |
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Satellite positions
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checked by DOD using radar
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Ephemeris
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the precise position of a satellite in its orbit
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Configuration
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a configuration file determines how data is collected, entered, stored, displayed and communicated with outside devices
A configuration file can be used to lock some of those settings |
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How to configure GPS receiver
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Using receivers
Using configuration manager in pathfinder office |
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Logging Intervals`
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Point feature 1 second
Line area feature Walking 5 seconds driving 1 second |
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PDOP
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Position Dilution of Precision
-Satellite Geometry -Each quadrant of the sky is ideal -Good PDOP less than 4, 4-6 ideal |
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Types of DOP
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PDOP-precision
HDOP-horizontal VDOP-vertical TDOP-time GDOP-generic |
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Elevation Mask
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Elevation above horizon
Base stations are at 10d Rovers are usually set at 15d 1 degree for every 60 miles from base |
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SNR
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minimum signal strength, usually 4
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WGS 1984
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Official datum of GPS
Developed by former Defense Mapping industry, now NIMA |
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Systematic vs Random errors
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Systematic-operator caused, correctable
Random-incorrectable |
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Systematic errors
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Atmospheric effects
Receiver bias Orbital bias Clock bias Multipath bias Selective availability Geometric dilution of precision GDOP |
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Satellite errors
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Clock bias
orbital errors (ephemris) |
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Upper and lower atmosphere errors
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Ionosphere reaction
Troposphere reaction Satellite mask angle Obstruction |
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Receiver or receiver noise
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Antenne phase center variation
Clock bias Multipath |
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Solution:Laser finder
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use compas, laser rangefinder to determine position
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Solution:Off set positioning
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use compass and set distance
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Multipath
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multiple signals registered due to bouncing from surrounding objects
Trimble everest technology=fix |
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Selective deniability
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errors up to 300 ft
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Differential GPS
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Logging intervals
Position mode Elevation mask SNR mask PDOP mask |
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General solutions
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Accurate antenna height
Clear view of sky 5 satellites in view within 50 km of base station synchronized base station oversurveyed coordinates |
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Differential GPS
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uses equations and two receivers to make more accurate measurements
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What errors can be corrected?
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Satellite receiver clock error
Satellite ephemeris error Atmospheric refraction -iconospheric refraction -tropospheric refraction receiver noise multipath selective availability |
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How does DGPS work?
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Highly accurate base receiver placed at known coordinates, roving receiver collects points
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Differential correction methods
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Autonymous: no correction
Differential correction-post processing Differential GPS-real time |
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Code phase vs Carrier phase
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Advantage of code phase
-Less problem with trees -quicker due to consistent code Carrier -accuracy of 1 cm vs 1 meter |
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CEP, RMS, 2DRMS
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CEP-Circular error probable 50% of measurements will be within 1 meter of position, the rest can be anywhere in the universe
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RMS
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Root mean square, roughly the same as standard deviation
Normal distribution-68% of points within 1 standard deviation of truth, 96 % within 2 standard deviations |
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CORS
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continually operable reference station
allows for signal to be checked |
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Wide area augmentation system
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system of ground stations
many GPS receivers use to check free to use, no special equipment needed |
