Transcript Global Positioning Systems

Global Positioning Systems
OnStar Navigation System
Terms to Know
Antenna – Receives signal from satellites
and differential source.
 Attribute – A characteristic that describes
a Feature. They can be thought of as
questions which are asked about the
Feature.
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Terms
Channel – Consists of the circuitry
necessary to track the signal from a single
GPS satellite.
 Degree – Unit of angular measurement
used to measure latitude and longitude.
 Department of Defense (DoD) – Created
Global Positioning System
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Terms
Differential Correction (DGPS) –
Comparing GPS data collected in the field
to data at a known point. By collecting
data at a known point a correction factor
can be determined and applied.
 Dilution of Precision (DOP) – Positions
tagged with a higher DOP value have
poorer accuracy than those with a lower
DOP
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Terms
Ephemeris – The predicted changes in
the orbit of a satellite
 Ephemeris Errors – Errors that originate
in the data transmitted by a GPS satellite.
Ephemeris errors are removed by
differential correction.
 Feature – The object which is being
mapped for the use in a GIS system.
Features may be points, lines, or areas.
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Terms
Geographic Information System (GIS) –
A mapping system that combines
positional data with descriptive information
to form a layered map.
 Global Positioning System (GPS) – A
system providing precise location which is
based on data transmitted from a
constellation of 24 satellites.
 Raw GPS signal is only accurate to 100
meters.
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Terms
Latitude – The angular distance north or
south of the equator measured by lines
circling the earth parallel to the equator
 Degrees from 0 to 90
 Longitude – The angular distance east or
west of the prime meridian
 Measured by lines perpendicular to the
parallels and converging at the poles
 0 to 180 degrees
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Lines of Latitude
Lines of Longitude
Terms
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Minute – Unit of angular measurement
subdivision of Degrees. Each degree is divided
into 60’ (minutes).
One minute of latitude anywhere on the earth
and one minute or longitude at the equator
equals 1.15 miles or 1 nautical mile.
Second – Unit of angular measurement
subdivision of minutes. Each minute is divided
into 60” (seconds).
One second of latitude anywhere on the earth
and one second of longitude at the equator is
equal to 33.82 yards.
Terms
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Multi-path – The interference to a signal that
has reached the receiver antenna by multiple
paths.
Usually caused by the signal being bounced or
reflected. Signals from satellites low on the
horizon will have high multi-path error.
Precision Farming – Carefully tailoring soil and
crop management to fit the different conditions
found in each field.
Precision Farming Yield Map
Terms
Pseudo range – An uncorrected
measurement of the distance between a
GPS satellite and a GPS receiver
 Receiver – Hardware component needed
to process signals received by antenna.
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Garmin GPS Handheld
Receiver
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GPS receivers
Satellite Constellation – The group of 24 GPS
satellites from which data is used to determine a
position.
Terms
Select Availability (SA) – Governmentinduced signal error.
 SNR – Signal to noise ratio. Measure of
signal quality.
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Terms
Spatial Data – Data pertaining to the
location, shape, and relationship among
geographical features.
 Value – Descriptive information about a
Feature.
 Variable Rate Technology (VRT) –
Instrument used for varying the rates of
application of fertilizer, pesticides, and
seed as one travels across a field.
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Terms
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Yield Monitoring – Regular intervals
where a harvested weight has been
obtained along with a GPS reading. A
display of the weights translated to
bushels/acre or yield provides a yield map.
Examples of Variable Rate
Technology
Harvesting with VRT
Variable Rate Anhydrous
Ammonia
Parallel Tracking
2004 Corn Yield Map
2003 Soybean Yield Map
What is GPS?
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GPS (Global Positioning System)
Created by US Dept. of Defense
 Operates 24 hrs/day
 Started in early 1980’s
 Fully operational April 27, 1995
 24 NAVSTAR satellites (Navigation with
Satellite timing and ranging)
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What is GPS?
Orbit 10,900 miles above earth surface
 Each orbits every 12 hours
 6 orbital paths, 4 satellites per path
 At least 4 satellites always visible
 Transmit radio waves which travel at a rate
of 186,300 miles/sec. (speed of light)
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What are the Satellites Like?
Weight: 1900 lbs.
 Size: 17 feet
 Power: 80 watts – 480 watts
 Signal:
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Send two signals
 Civilian use
 Military use
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How are the Satellites Controlled?
The US Air Force controls Satellites
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Main tracking station: Falcon Air Force Base
Colorado Springs, CO
5 tracking stations around the Earth
 Satellites
are tracked
 New orbits predicted
 Clock errors modeled
 This data is uploaded to satellites to send to user’s
receiver. This info is called the almanac
How do we Communicate with the
Satellites
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Basic Components Required
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Antenna – Receives signal from satellites
 Two
required for certain differentially corrected
systems (may be housed in one unit)
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Receiver – Processes information contained
in signal
 This
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information is refereed to as almanac
PDA – Personal Decision Aid (ex. computer)
 Includes
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software needed to visualize information
Power Source
Factors Affecting GPS Accuracy
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Selective availability
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Technique used by the DoD to reduce
accuracy
Satellite clocks
Each satellite has an atomic clock but they
are not completely accurate
 An error of one millisecond would produce an
error or 186 miles
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Satellite orbits
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Orbit changes due to gravitation from sun and
moon. Known as Ephemeris
Continued…
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Earth’s atmosphere
Slows radio signals
 Occurs mostly in ionosphere
 (50 to 250 miles up) blanket of charged
particles
 Also in troposphere – from water vapor
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Continued….
• Multi-path errors
– Signal bounce off near objects
Improving Accuracy
Differential Correction
Uses a stationary station of a known location
 The base station calculates the pseudo range
and determines the amount of error of each
measurement and then calculates the true
distance
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Applications In Ag
Soil sampling
 Yield mapping
 Variable rate planting/fertilizer application
 Weather data
 Production costs
 Variable rate chemicals
 Field scouting
 Others???
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