Gps in remote sensing, pk mani

Global positioning system in remote sensing study.

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What is GPS?
The Global Positioning System (GPS) is a
satellite based navigational aid. Essentially
it is a radio positioning navigation and time
transfer system. It provides accurate
information on position, velocity and time
of an object or a platform at any moment,
anywhere in the globe.
A Constellation of Earth-Orbiting Satellites
Maintained by the United States Government for
the Purpose of Defining Geographic Positions
On and Above the Surface of the Earth. It
consists of Three Segments:
Control Segment
Space Segment
User Segment

3. Position and coordinates.
The distance and direction between any two
waypoints, or a position and a waypoint.
Travel progress reports.
Accurate time measurement.
Four Basic Functions of GPS

4. •Control
Segment
•Space
Segment
•User Segment
Three Segments of the GPS
•Monitor Stations
•Ground
Antennas
•Master Station

5. •Kwajalein Atoll
•US Space
Command
Control Segment
•Hawaii
•Ascension Is.
•Diego Garcia
•Cape
Canaveral
•Ground Antenna•Master Control Station•Monitor Station
•Falcon AFB,
Colorado springs

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Control Segment: Maintaining the System
(5) Monitor Stations
• Correct Orbit
and clock
errors
• Create new
navigation message
• Observe
ephemeris
and clock
Falcon AFB
Upload Station

7. 7
Space Segment
24+ satellites
–6 planes with 55°
inclination
–Each plane has 4-5
satellites
–Broadcasting position
and time info on 2
frequencies
–Constellation has spares
Very high orbit
–20,200 km
–1 revolution in approx. 12 hrs
–Travel approx. 7,000 mph
Considerations
–Accuracy
–Survivability
–Coverage

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9. Military.
Search and rescue.
Disaster relief.
Surveying.
Marine, aeronautical and terrestrial navigation.
Remote controlled vehicle and robot guidance.
Satellite positioning and tracking.
Shipping.
Geographic Information Systems (GIS).
Recreation.
User Segment

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How the system works
Space Segment
24+ Satellites
The Current
Ephemeris is
Transmitted to
Users
Monitor
Stations
• Diego Garcia
• Ascension Island
• Kwajalein
• Hawaii
• Colorado Springs
GPS Control
Colorado Springs
End
User

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Triangulation
Satellite 1 Satellite 2
Satellite 3 Satellite 4

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20. Position is Based on Time
•T + 3
•Distance between satellite
and receiver = “3 times the
speed of light”
•T
•Signal leaves satellite
at time “T”
•Signal is picked up by
the receiver at time “T +
3”

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Distance Measuring
Rate = 186,000 miles
per second (Speed of
Light)
Time = time it takes
signal to travel from the
SV to GPS receiver
Distance = Rate x Time
Each satellite carries
around four atomic
clocks
Uses the oscillation of cesium
and rubidium atoms to measure
time
Accuracy?
plus/minus a second over more
than 30,000 years!!
The whole system
revolves around
time!!!

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SV and Receiver Clocks
•SV Clocks
–2 Cesium & 2 Rubidium in each SV
–$100,000-$500,000 each
•Receiver Clocks
–Clocks similar to quartz watch
–Always an error between satellite
and receiver clocks ( D t)
•4 satellites required to solve for
x, y, z, and D t

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•PROBLEM
–Can’t use atomic
clocks in receiver
•SOLUTION
–Receiver clocks
accurate over short
periods of time
–Reset often
–4
th
SV used to
recalibrate receiver
clock
Cesium Clock =
$$$$$$$!!!
Size of PC
4

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PRN code

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•GPS Satellite Signals
•• The SVs transmit two microwave carrier signals. The L1 frequency (1575.42
MHz) carries the navigation message and the SPS code signals. The L2
frequency (1227.60 MHz) is used to measure the ionospheric delay by PPS
equipped receivers.
•• Three binary codes shift the L1 and/or L2 carrier phase. The C/A Code
(Coarse Acquisition) modulates the L1 carrier phase. The C/A code is a
repeating 1 MHz Pseudo Random Noise (PRN) Code. This noise-like code
modulates the L1 carrier signal, "spreading" the spectrum over a 1 MHz
bandwidth.
•The C/A code repeats every 1023 bits (one millisecond). There is a different
C/A code PRN for each SV. GPS satellites are often identified by their PRN
number, the unique identifier for each pseudo-random-noise code. The C/A
code that modulates the L1 carrier is the basis for the civil SPS.
•• The P-Code (Precise) modulates both the L1 and L2 carrier phases. The
P-Code is a very long (seven days) 10 MHz PRN code. In the Anti-Spoofing
(AS) mode of operation, the P-Code is encrypted into the Y-Code. The
encrypted Y-Code requires a classified AS Module for each receiver channel
and is for use only by authorized users with cryptographic keys. The P (Y)-
Code is the basis for the PPS.
The Navigation Message also modulates the L1-C/A code signal. The
Navigation Message is a 50 Hz signal consisting of data bits that describe
the GPS satellite orbits, clock corrections, and other system parameters.

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Breaking the Code
Transmission Time
Receiver
The Carrier Signal...
combined with…
The PRN code...
produces the
Modulated carrier signal
which is transmitted...
demodulated...
And detected by receiver,
Locked-on, but
With a time delay...
Time delay
Satellite

27. Pseudo Random Noise Code
•Receiver PRN
•Satellite PRN
•Time
Difference

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Accuracy and Precision in GPS
•Accuracy
–The nearness of a measurement to the
standard or true value
•Precision
–The degree to which several
measurements provide answers very close
to each other.
What affects accuracy and
precision in GPS?

30. Sources of GPS Error
•Standard Positioning Service (SPS ): Civilian Users
• Source Amount of Error
Satellite clocks: 1.5 to 3.6 meters
Orbital errors: < 1 meter
Ionosphere: 5.0 to 7.0 meters
Troposphere: 0.5 to 0.7 meters
Receiver noise: 0.3 to 1.5 meters
Multipath: 0.6 to 1.2 meters
Selective Availability(see notes)
User error: Up to a kilometer or more
•Errors are cumulative and increased by PDOP.

31. Receiver Errors are Cumulative!
•User error = +- 1 km
•System and other flaws = < 9 meters

32. Sources of Signal Interference
•Earth’s Atmosphere
•Solid
Structures
•Metal
•Electro-magnetic
Fields

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Sources of Error
•Clock Error
– Differences between satellite
clock and receiver clock
•Ionosphere Delays
– Delay of GPS signals as
they pass through the layer
of charged ions and free
electrons known as the
ionosphere.
•Multipath Error
–Caused by local reflections of the
GPS signal that mix with the desired
signal
D
i r e
c
t S
i g
n
a
l
R
e
fle
c
te
d
S
ig
n
a
l
GPS
Antenna
Reflected Signal
Hard Surface
Satellite

34. GPS Satellite Geometry
Satellite geometry can affect the quality of GPS signals and
accuracy of receiver trilateration.
Dilution of Precision (DOP) reflects each satellite’s position
relative to the other satellites being accessed by a receiver.
There are five distinct kinds of DOP.
Position Dilution of Precision (PDOP) is the DOP value used
most commonly in GPS to determine the quality of a receiver’s
position.
It’s usually up to the GPS receiver to pick satellites which
provide the best position triangulation.
Some GPS receivers allow DOP to be manipulated by the user.

35. Ideal Satellite Geometry
•N
•S
•W •E

36. Good Satellite Geometry

37. Good Satellite Geometry

38. Poor Satellite Geometry
•N
•S
•W •E

39. Poor Satellite Geometry

40. Poor Satellite Geometry

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Sources of Error
•Geometric Dilution of
Precision (GDOP)
–Describes sensitivity of receiver
to changes in the geometric
positioning of the SVs
•The higher the DOP value, the
poorer the measurement
QUALITY DOP
Very Good 1-3
Good 4-5
Fair 6
Suspect >6

42. •DGPS Site
•x+30,
y+60
•x+5, y-
3
•True coordinates =
x+0, y+0
•Correction = x-5,
y+3
•DGPS correction = x+(30-5)
and y+(60+3)
•True coordinates = x+25, y+63
•x-5, y+3
Real Time Differential GPS
•DGPS Receiver•Receiver

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Differential GPS
•Method of removing errors that affect GPS
measurements
•A base station receiver is set up on a
location where the coordinates are known
•Signal time at reference location is
compared to time at remote location
•Time difference represents error in
satellite’s signal
•Real-time corrections transmitted to remote
receiver
–Single frequency (1-5 m)
–Dual frequency (sub-meter)
•Post-Processing DGPS involves correcting at a
later time
Reference
location
Remote
location
= Error
www.ngs.noaa.gov/OPUS
Online post-processing

44. •Wide Area Augmentation System
•Geostationary
WAAS satellites
•GPS Constellation
•WAAS Control
Station (West
Coast)
•Local Area System
(LAAS)
•WAAS
Control
Station (East
Coast)

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Wide Area Augmentation System (WAAS)
•System of satellites and
ground stations that provide
GPS signal corrections
•25 ground reference stations
across US
•Master stations create GPS
correction message
•Corrected differential message
broadcast through
geostationary satellites to
receiver
•5 Times the accuracy (3m)
95% of time
•Only requires WAAS enabled
GPS

46. •How good is WAAS?
•+ -
3
meters
•+-15
meters
•With Selective Availability set
to zero, and under ideal
conditions, a GPS receiver
without WAAS can achieve
fifteen meter accuracy most
of the time.*
•Under ideal conditions a
WAAS equipped GPS
receiver can achieve three
meter accuracy 95% of the
time.*
•* Precision depends on good satellite geometry, open sky view, and no user
induced errors.

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54. Differential GPS

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