Transcript Introduction to GPS

Introduction to GPS
GLY 560:
GIS for Earth Scientists
What is GPS?
• The original intent of the Global
Positioning System was to develop an allweather, 24-hour, truly global navigation
system to support the positioning
requirements for the armed forces of the
U.S. and its allies.
• First satellite launched in 1978
• The total investment by the U.S. military in
the GPS system to date is well over $10
BILLION!
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What is GPS?
• Although the primary goal is to provide
positioning capabilities to the U.S. armed
forces and its allies, GPS is freely
available to all users.
• The number of civilian users is already far
greater than the military users, and the
applications are growing rapidly.
• The U.S. military however still operates
several "levers" with which they control the
performance of GPS.
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How GPS Works
Consider two ways of determining ranges:
One Clock
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Two Clocks
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Advantages of One-Way Ranging
Receiver doesn’t have to generate
signal, which means
• We can build inexpensive portable
receivers
• Receiver cannot be located (targeted)
• Receiver cannot be charged
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Determining Range (Distance)
• Measure time it takes for radio signal
to reach receiver, use speed of light to
convert to distance.
• This requires
• Very good clocks
• Precise location of the satellite
• Signal processing over background
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Determining Position
• To determine position in 3-D, we need
3 satellites for triangulation
Once we have position narrowed to 2 possible points,
we can usually throw one away as “nonsense”
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The Clock Problem
• To measure distance from speed of
light we need a VERY accurate clock
(clock error of 1/100 sec = distance error of 1820 miles!).
• GPS Satellites have very accurate
atomic clocks.
• Our receivers do not have atomic
clocks, so how can we measure time
with necessary accuracy?
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Psuedo-Random Code
• GPS satellites and receivers
communicate via pseudo-randomcode (PRC) signals.
• PRC has three advantages:
1. Enhances signal over background
2. Allows synchronization of satellite and
receiver clocks
3. Military can change the code and switch of
system if necessary
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PRC Signal Amplification
• Uses correlation of
peaks between
generated random
signal and truly
random background
noise to enhance
signal
• Allows receiver to
work without a big
satellite dish!
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PRC Synchronization
• GPS receiver
generates the same
PRC as satellite, i.e.
they start “counting” at
the same time.
• By determining how far off the satellite
and receiver are in their counting,
determines difference in time it took for
signal to reach receiver.
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PRC Synchronization
• How do we assure
satellite and receiver
start counting at same
time, i.e. clocks are
synchronized?
• The trick is to use a 4th satellite to overspecify position. This allow timing to be
corrected by the receiver
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Pseudo-Range (2-D Example)
• If clocks were
perfect, 2
satellites would
locate position
• If clocks are off,
range is off
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Pseudo-Range
• We add a 3rd satellite
to over-specify position
• There is only one
combination of “wrong”
times for which all 3
ranges converge.
• Receiver varies clock
times until all satellites
agree
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Satellite Position
• Must know position of
satellite to determine
receiver location
• Satellites are put in
precise orbit
• Satellite's orbit or
"ephemeris“ is
monitored by DOD and
transmitted to satellite
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Atmospheric Correction
• GPS signal
slowed down
through the
charged particles
of the
ionosphere and
then through the
water vapor in
the troposphere
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• Must correct for
atmospheric effects
with modeling
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GPS Constellation
• Must have at
least 4
satellites
overhead to
determine
position
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NAVSTAR
• Current GPS System is NAVSTAR. There
are 4 GPS satellite constellations in
existence:
• Block I satellites were the experimental satellites
launched between 1978 and 1985 used to test the
system. Eleven (11) were launched, none functioning.
• Block II satellites comprise the first nine spacecraft of
the operational series.
• The Block IIA satellites comprise the second 19
spacecraft of the operational series.
• The Block IIR satellites comprise the replacement
series.
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Where are the Satellites?
• Orbit is high
enough to avoid
earth gravity
perturbations, low
enough to pass
correction stations
1 per day.
• Orbital period of
GPS satellites is
~12 hours
Kepler' s Third Law :
GM e
2

T
a3
a  semi - major axis of orbital ellipse
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Satellite Overhead Schedule
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Orbital Period and Altitude
Orbital period and satellite altitude.
Semi-major axis
Altitude Period
Comment
a ( km )
( km ) ( min )
6700
300
90
7200
800
100 remote sensing satellites
10600
4200
180
12800
6400
240
16800
10400
360
26600
20200
718 GPS satellites
42300
35900
1436* geostationary satellites
* length of a sidereal day
http://liftoff.msfc.nasa.gov/RealTime/JTrack/3D/JTrack3D.html
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GPS Accuracy
"The decision to discontinue Selective Availability is the latest measure in an ongoing
effort to make GPS more responsive to civil and commercial users worldwide…This
increase in accuracy will allow new GPS applications to emerge and continue to
enhance the lives of people around the world.“
President Bill Clinton, May 1, 2000
Between 1st and 3rd May 2000, the National Geodetic
Survey/NOAA compared the accuracy of GPS
determined navigation positions at its Continuous
Reference Station with and without Selective Availability.
• With SA turned on, 95% of solutions were within a radius of 45
meters
• With SA turned off, 95% of the estimated (horizontal) positions were
within 6.3 meters.
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Accuracy of GPS
Autonomous
Accuracy
15 - 100 meters
Differential GPS
(DGPS)
Accuracy
0.5 - 5 meters
Real-Time
Kinematic Float
(RTK Float)
Accuracy
20cm - 1 meter
Real-Time
Kinematic Fixed
(RTK Fixed)
Accuracy
1cm - 5 cm
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GPS Signals
GPS satellites broadcast on three different frequencies, and each
frequency (or career wave) has some information or codes on it. You
can think of it as three different radio stations broadcasting several
different programs. The table below lists the signals and the contents:
L1 Career
L2 Career
19 cm wavelength
24 cm wavelength
1575.42 M Hz
1227.6 M Hz
C/A Code
P Code
Navigation
Navigation Message
L3 Career
Data not available
•P Code : Reserved for direct use only by the military
•C/A Code : Used for rougher positioning
•For Single frequency use only L1 career is used
•For Double frequency, L1/L2/L3 career is used
•The navigation message (usually referred to as the ephemeris) tells us where the satellites are located, in WGS-84.
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Different types GPS locations
• Autonomous Positions
(C/A signal, 5-15 m accuracy)
• Real-Time Differential GPS
(C/A signal, 0.5-5 )
• Real-Time Kinematic (RTK) Float
(C/A and Carrier, 0.2-1 m)
• Real-Time Kinematic (RTK) Fixed
(C/A and Carrier, 1-5 cm)
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Differential GPS (DGPS)
• Error due to signal
transmission through
the atmosphere can be
corrected using DGPS
• Atmospheric errors are
the same over short
distances.
• Error in base station,
can be removed from
remote (roving) receiver
position, and code
phase signal.
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Code vs. Carrier Phase
• Satellites generate Code Phase and
Carrier Phase signals.
• Code phase is used by hand-held
GPS
• Carrier phase used by surveying
instruments, navigational systems
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Where to Get Differential Corrections
• The United States Coast Guard and other international
agencies are establishing reference stations all over
especially around popular harbors and waterways.
• Anyone in the area can receive these corrections and
radically improve the accuracy of their GPS
measurements. Most ships already have radios capable
of tuning the direction finding beacons, so adding
DGPS will be quite easy.
• Many new GPS receivers are being designed to accept
corrections, and some are even equipped with built-in
radio receivers.
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Differential Code GPS (Navigation)
• Differential corrections may be used in
real-time or later, with post-processing
techniques.
• Real-time corrections can be transmitted
by radio link. The U.S. Coast Guard
maintains a network of differential
monitors and transmits DGPS corrections
over radio beacons covering much of the
U.S. coastline.
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RTK (Differential Carrier GPS)
• RTK is based on using many (~5
satellites) to resolve timing.
• Produces very accurate
measurements because using carrier
phase.
• Requires advance tracking of
satellites, and better signal resolution
(bigger antennae and more power)
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Future of GPS
• Soon, the U.S. Federal Communications
Commission will require location determination
technology in cellular phones for use in
emergencies as part of their enhanced 911
service.
• Future plans for improving the accuracy of GPS
include the launching of eighteen additional
satellites that are awaiting launch or are
currently in production.
• Two new signals will be broadcast from the
satellites by 2005, to help bypass any distortion
from the ionosphere.
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