1. GPS Theory and applications
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2. Atlas Gis Positioning Systems
Positioning Systems Around The World Is Three
Global Positioning System GPS ( USA )
GLONASS (RUSSIAN SYSTEM )
GALILEO (EUROPEAN SYSTEM )
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3. Other Positioning Systems
Russian SYSTEM Know as GLONASS
Around 9 Operational Satellites
Hasn’t Reached Full Operational Capability
European System Known as GALILEO
2 Satellites launched yet in
System Will Start To Operate 2013.
All Systems Will Combine To Form GNSS, Global Navigation Satellite Systems
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4. Global Positioning System GPS
Global Positioning System GPS Developed by the US Dep. of Defense in 1974( Civil Access In 1995 )
Navigational GPS Up to m Accuracy
Deferential GPS Up to 0.1 mm Accuracy
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5. General Characteristics
Navigation System with Time and Ranging -Global Positioning System
Designed to replace existing navigation systems
Developed by the US Dep. of Defense
Development started in 1974
According to military aspects
Worldwide Coverage
24 hour access
Accurate Navigation to 10 m
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6. General Characteristics
Common Coordinate System WGS84
Weather and Sight independent
One-way ranging system
Cheap end-user receivers
Military safe
Accessible by Civil and Military
Full Operational Capability since June 95
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7. Atlas Gis GPS System Components Space Segment Control Segment
NAVSTAR : NAVigation
Satellite Time and Ranging
24 Satellites
20200 Km
Control Segment
1 Master Station
5 Monitoring Stations
User Segment
Receive Satellite Signal
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8. Atlas Gis GPS Constellation Nominal 24 Satellites Today we have 29Sats
6 orbital planes
55 ° Inclination
60 ° right ascension seperation
Almost circular orbits
Orbital radius km
Orbital period 12 h sideral time
Repeated satellite constellation every day, 4 min earlier
4 min/day, 2h/month, 24 h/Year
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9. Fundamental Frequency 10.23 MHz
GPS Signal Structure
Each GPS satellite transmits a number of signals
The signal comprises two carrier waves (L1 and L2) and two codes (C/A on L1 and P or Y on both L1 and L2) as well as a satellite orbit message
Fundamental Frequency MHz
÷ 10
L MHz
C/A Code MHz
P-Code MHz
x 154
L MHz
P-Code MHz
x 120
50 BPS
Satellite Message
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10. Navigation Principle with GPS
Simultaneous measurement of distances to at least 4 sats
Satellites have known coordinates
Receiver position: Intersection of 4 spheres around the sats
Analytical view:
Following unknowns have to be determined
LATITUDE, LONGITUDE, HEIGHT
RECEIVER CLOCK ERROR
4 observations <=> 4 unknowns
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11. Range = Runtime x Velocity of Light
GPS Principle : Range
Range = Runtime x Velocity of Light
Xll Vl Xl
lll l ll lV V
Vll
Vlll X lX
Xll Vl Xl
lll l ll lV V
Vll
Vlll X lX
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12. GPS Principle : Point Positioning
3 Spheres intersect at a point
3 Ranges to resolve for Latitude, Longitude and Height R3
2 Spheres intersect as a circle R2
We are somewhere on a sphere of radius, R1
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13. Outline Principle : Position
The satellites are like “Orbiting Control Stations”
Ranges (distances) are measured to each satellites using time dependent codes
Runtime: Time from the transmission of the signal at the satellite till the reception at the receiver
Multiply the runtime with the velocity of light
Range = Runtime x velocity of light
Notes :

14. Runtime Determination (Code)
One-way ranging system: One clock inside the satellite and another one inside the receiver
Receiver creates a duplicate of the code
Cross correlation between received and generated code sequence
Maximum correlation gives the runtime t
Satellite Signal
Receiver Signal
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15. Range Determination from Code Observation
Pseudoranges (Code)
Each satellite sends a unique signal which repeats itself approx. every 1 msec
Receiver compares self generated signal with received signal
From the time difference (DT) a range observation can be determined
Receiver clock needs to be synchronized with the satellite clock
Received Code
from Satellite
Generated
Code from
Receiver DT
D = V (DT)
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16. Atlas Gis Pseudorange The Pseudorange
Typically GPS receivers use inexpensive clocks. They are much less accurate than the clocks on board the satellites
Runtime measurement is done in the receiver
Receiver time scale has an offset
Consider an error in the receiver clock
1/10 second error = 30,000 Km error
1/1,000,000 second error = 300 m error
A radio wave travels at the speed of light
actual signal velocity differs from theoretical value
Instead of the true distance Satellite - Receiver we obtain the
Pseudorange
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17. Atlas Gis Point Positioning
4 Ranges to resolve for Latitude, Longitude, Height & Time
It is similar in principle to a resection problem
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18. Range Determination from Phase Observation
Phase Observations
Wavelength of the signal is 19 cm on L1 and 24 cm on L2
Receiver compares self-generated phase with received phase
Number of wavelengths is not known at the time the receiver is switched on (carrier phase ambiguity)
As long as you track the satellite, the change in distance can be observed (the carrier phase ambiguity remains constant)
Received Satellite
Phase
Generated
Phase from
Receiver DT
D = c DT + lN
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19. Atlas Gis Error Sources
Like all other Surveying Equipment GPS works in the Real World
That means it owns a set of unique errors
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20. Atlas Gis Satellite Errors Satellite Clock Model
though they use atomic clocks, they are still subject to small inaccuracies in their time keeping
These inaccuracies will translate into positional errors.
Orbit Uncertainty
The satellites position in space is also important as it’s the beginning for all calculations
They drift slightly from their predicted orbit
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21. Atlas Gis Observation Errors
GPS signals transmit their timing information via radio waves
It is assumed that a radio wave travels at the speed of light.
GPS signals must travel through a number of layers making up the atmosphere.
As they travel through these layers the signal gets delayed
This delay translates into an error in the calculation of the distance between the satellite and the receiver
19950 Km
Ionosphere
200 Km
Troposphere
50 Km
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22. Atlas Gis Receiver Error
Unfortunately not all the receivers are perfect. They can introduce errors of their own
Internal receiver noise
Receiver clock drift
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23. Atlas Gis Multipath Error
When the GPS signal arrives at earth it may reflect off various obstructions
First the antenna receives the signal by the direct route and then the reflected signal arrives a little later
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24. Point Positioning Accuracy
Accuracy m
In theory a point position can be accurate to m based on the C/A Code( Navigational)
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25. Point Positioning Accuracy
Accuracy m
Point Positioning under SA :
+/- 100 m Horizontally
+/- 160 m Vertically
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26. Point Positioning Accuracy
As the owner of the GPS System the USA have decided to limit the
access to the full system accuracy for civil users
SPS Standard Positioning Service
Based on the C/A-Code
Worldwide available without limitation
Navigation Accuracy von ±100 m in position and ±160 m in height (~95 % probability)
PPS Precise Positioning Service
Provides the full System accuracy
C/A-Code gives ±10-30 m, P-Code ±20 m (~95 %)
PPS is available for authorized users only
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27. How do I
Improve my Accuracy ?
Use
Differential GPS
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28. Atlas Gis Differential GPS
The position of Rover ‘B’ can be determine in relation to Reference ‘A’ provided
Coordinates of ‘A’ is known
Simultaneous GPS Observations
Differential Positioning
Eliminates errors in the sat. and receiver clocks
Minimizes atmospheric delays
Accuracy 3mm - 5m
Baseline Vector B A
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29. Differential Code / Phase
If using Code only accuracy is in the range of cm This is typically referred to as DGPS
If using Phase or Code & Phase accuracy is in the order of 3 mm + 0.5
Baseline Vector B A
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30. Initial Phase Ambiguity
Initial phase Ambiguity must be determined to use carrier phase data as distance measurements over time
Once the ambiguities are resolved, the accuracy of the measurement does not significantly improve with time
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31. Resolving Ambiguities
The effect of resolving the ambiguity is shown below :
Rapid Static
Accuracy (m)
1.00
0.10
0.01
Static
Time (mins)
Ambiguities
Not resolved
Resolved
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32. Dilution of Precision (DOP)
A description of purely geometrical contribution to the uncertainty in a position fix
It is an indicator as to the geometrical strength of the satellites being tracked at the time of measurement
GDOP (Geometrical), Includes Lat, Lon, Height & Time
PDOP (Positional) Includes Lat, Lon & Height
HDOP (Horizontal) Includes Lat & Lon
VDOP (Vertical) Includes Height only
Poor GDOP
Good GDOP
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33. Summary of GPS Positioning
Point Positioning :
m (1 epoch solution, depends on SA)
m (24 hours)
Differential Code :
cm (P Code)
1 - 5 m (CA Code)
Differential Phase :
3 mm + 0.5
1 cm
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34. Thank You For Your Attention
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