GPS Segments

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Global Navigation Satellite System GNSS

• NAVSTAR GPS (USA) Since early 1980s
• Galileo (European Union) by 2008
• GLONASS (Russian) 1993
• Global'naya Navigatsionnaya Sputnikovaya Sistema
• Global Orbiting Navigation Satellite System

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GPS Fundamentals
Introduction the Global Positioning System
components and operation
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GPS Description The Global Positioning
System (GPS) is based on observations of signals
transmitted from satellites
Sourcehttp//www.garmin.com/aboutGPS/
Owned and operated by the Department of Defense
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GPS Description The Global Positioning
System (GPS) is based on observations of signals
transmitted from satellites
Source http//msl.jpl.nasa.gov/QuickLooks/gps1QL.
html
Owned and operated by the Department of Defense
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.
Satellite transmits a signal, along with
information on satellite position
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.
Using the signal, we measure elapsed time, and
hence distance from the satellite to our point..
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By triangulating to several satellites, we can
unambiguously determine our position.
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• Three Main Components of GPS
• Satellite Segment 21 flying radio transmitters
• (really 24 as three are spares)
• Control Segment 5 control stations
• User Segment Receivers collecting satellite
  signals, determining position

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NAVSTAR Rockwell International 20,190 km 1900
lbs 17ft with solar panels
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GPS Control Segment Tracking, Communications,
Data gathering , Integration and
Control Observe, maintain and manage the GPS
satellites and system Master Control in Colorado
Springs Data is synthesized and broadcasts
navigation, timing and other data to each
satellite Corrections made once a day
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GPS User Segment
Users with a device that records data transmitted
by each satellite and processes this data to
obtain three dimensional coordinates
Sourcehttp//www.trimble.com/geoexplorer3.html
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Summary

• Three Main Components of GPS
• Satellite Segment 21 flying radio transmitters
• Control Segment 5 control stations
• User Segment Receivers collecting satellite
  signals, determining position

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Using information on satellite position, we can
determine where we are in space. GPS satellites
in near-circular orbits about 20,200 kilometers
above earth. (12,000 miles) 21 satellites,
three spares. Six different orbital paths, four
satellites in each orbit. (best coverage between
60 and 60 latitude)
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Each satellite travels around the earth twice
daily, visible from any one point for about 10
hours each day. With full constellation, four to
twelve satellites should be visible from any
unobstructed location. Each satellite
continuously broadcasts signals on two carrier
frequencies, L1 and L2. Satellites are 1 ton
Space-borne Radio Stations
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GPS L1 and L2 are carriers. The C/A code
(Course acquisition, psuedo random noise, or PRN
code) is a series of plus and minus 1 values at
1.023 MHz. P code(Precision Code Military
encrypted) is a sequence of plus and minus 1
values at 10.23 MHz.
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Example of FM Station 97.1 with 440 A note
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Satellites transmit coded signals at various
frequencies
1.023
10.23
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GPS L1 carries the 50 Hz Navigation Message each
GPS satellite broadcasts with data about on
satellite status and location. Almanac, data
used to determine the location of every satellite
in the GPS constellation. (weekly ) Ephemeris
data for the broadcasting satellitesatellite
health, clock corrections, etc. (daily) (allows a
GPS receiver to accurately calculate the position
of the broadcasting satellite)
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GPS Receiver has an internal clock so can
determine when the signal was generated and when
the satellite signal was received. Subtraction
yields the time, which can be converted to
distance. Note we assume satellite receiver
clocks are in synch. Must make this assumption
to calculate travel time. Unfortunately, this is
not the case. Basically, receiver clock may be
biased, so we need extra measurements. Satellites
have very accurate atomic clocks, (cost 100k
each). These can be assumed to be in synch with
each other.
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GPS
Range speed of light x travel time Range c(t1
t2) (c 299,792,458 meters per second)
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GPS The coded signals are sometimes called
Pseduo-random code. (appears similar to random
noise) Short segments of the code are unique for
each satellite Positions are based on range
measurements using the carrier or the coded
signal modulated on the carrier. Two types of
receivers C/A or P(coded) and Carrier phase (no
code the phase of each satellite signal is
measured requires long unbroken observations)
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GPS (code receivers)
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Carrier-Phase Range Measurements
Measure integer number of waves received while
observing the carrier signal
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GPS With three satellites we have three
observations and four unknowns (our X, Y, Z, and
clock bias). We must either assume we know Z
(e.g. at sea level or from map), or track extra
satellite. We need at least three satellites for
2-D, four satellites for 3-D positioning.
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Determine Position by Combining
Range Measurements
One satellite a sphere
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GPS
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Two satellites circle of intersection
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GPS
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Three satellites two points
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Four or more satellites one point
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Our Range Measurements Arent Perfect
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• GPS
• Positional Uncertainty
• Errors in range measurements and satellite
  location introduce errors
• Creates a range of uncertainty around the GPS
  receiver position

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GPS Sources of Error Several factors can result
in erroneous location determination with GPS
(besides blunders) Source Typical Range Error
(m) Satellite clock error 1 Satellite
position error 1 Receiver error 1.5 Atmospher
ic/Ionospheric effects 4 Total 7.5
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Atmospheric and Ionospheric Errors
The speed of light is not a constant It is
affected by charge density in the outer
atmosphere (ionosphere) Speed is affected by
atmospheric density (water vapor)
These cant be predicted very well
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System and Receiver Errors
Errors in knowledge of satellite position
(ephemeris) Satellite Clock errors Transmission
delays Imprecision in algorithm for range
decoding
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Multi-path Errors
Signal reflects off of objects before reaching
the receiver
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GPS Example of a Multipath Error
Sourcehttp//www.garmin.com/aboutGPS/
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Causes of Range Uncertainty
Ionospheric effects 3 meter Atmospheric
effects 0.5 meter Satellite/system errors 2
meters Receiver errors 0.5 meter Multipath de
pends Total Range Error 6 meters TOTAL
Positional Error 10 meters
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Positional Error

• If range errors average about 6 meters, why is
  positional error greater?
• Positional accuracy depends on both range
  accuracy and satellite geometry

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PDOP Position Dilution of Precision Figure of
merit that describes the quality of satellite
geometry Varies from 1 (best) to infinity
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• GPS
• Positional Dilution of Precision (PDOP)
• (Various others VDOP, Vertical DOP,
  HDOP,Horizontal DOP often referred as GDOP,
  Geometric Dilution of Precision)
• Range errors create and area of uncertainty
  perpendicular to the GPS signal transmission
  direction.
• The smaller intersection area the more accurate
  the position fixes.
• Signals from widely spaced satellites result in a
  smaller area of uncertainty.
• (To calculate actual error multiply the total
  system errors by the GDOP)

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PDOP - Measure of Satellite Geometry
Low PDOPs Are GOOD!!!!!
Ideal (one overhead and three all at 120
intervals)
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Common Range of PDOPs
Open Sky 1 - 4 about 85 of the time
4 - 6 about 10 above 6 the
remaining Under Canopy (dense conifer) less
than 2.5 rare (lt5 of time) 3 - 6 about 50 of
the time 6 - 10 about 30 of the time above 10
about 15 of the time
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GPS Accuracies Range errors and DOPs combine
to affect GPS position accuracies. Current C/A
code receivers typically provide accuracies
between 3 30 meters for a single
fix. Improvements possible when multiple fixes
can be averaged (reducing the impact of rare
large single error). Accuracies with carrier
phase receivers can be a few centimeters or
millimeters (note all carrier phase is
differential and requires more expensive
equipment and sampling time)
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How Accurate is GPS in Practice?
Carrier-Phase 1 to 50 cm (0.5 inch to 18
inches) C/A Code (single fix) with multiple
fixes Typical clear sky..6-11
m Typical sub-canopy10-25 m Best
case..50 cm
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You Can Affect Accuracy
Collect data with low PDOPs Collect multiple
fixes per point Use high quality external
antennas (reduces multipath signals) Use
more than one receiver (differential
correction)
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Lower PDOPs are Better
PDOP
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More Fixes are Better
Number of fixes
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Receivers May Reduce Errors
receiver type
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A telescoping external antenna helps