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GLOBAL POSITIONING SYSTEM. GNSS – Global Navigation Satellite System US GPS System (Navstar) Russian GLONASS system European Galileo System. GPS SYSTEM COMPONENTS. (Fully operational since 1993) The Space Segment 24 satellites in six near circular orbits orbits

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GLOBALPOSITIONING SYSTEM

  • GNSS – Global Navigation Satellite System

  • US GPS System (Navstar)

  • Russian GLONASS system

  • European Galileo System


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GPS SYSTEM COMPONENTS

  • (Fully operational since 1993)

  • The Space Segment

    • 24 satellites in six near circular orbits orbits

    • 24 hour coverage anywhere on earth’s surface between Lat. 80°N and 80°S

    • Altitude approx. 20 200km

    • Orbital period approx. 12hrs (speed of satellites about 14000km/hr)

    • Satellites equipped with very precise (and expensive!) atomic clocks

    • Satellites transmit signals with extremely stable frequencies

  • The Control Segment

    • Five monitoring stations (Col. Springs, Hawaii, Ascension, Diego Garcia, Kwajaleni)

    • Satellites monitored and tracked at control stations

    • Data relayed to Master Control Station (Colorado Springs)

    • Orbital parameters and clock corrections computed and uploaded to satellites for transmission to system users (broadcast vs rapid (24 hrs) vs precise ephemeris (2wks))

  • The User Segment

    • GPS receivers

    • Passive devices that record and analyze satellite signals for positioning

    • Various receiver types for different levels of accuracy and applications

    • Equipped with less precise (less expensive) clocks than satellites



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CORE IGS TRACKING NETWORK – late 1998

Source: http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap12/1224.htm#fig1


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A

THE GPS MEASUREMENT PRINCIPLE

·Based on the basic physical relationship:

distance = velocity * time

·Observations (pseudo-ranges) from 4 satellites provide 3 dimensional position (3 positional and 1 time unknown)

·Coordinate system realized by the satellite orbits (ephemeris data) and by the coordinates and physical locations of the control and tracking stations

Trilateration


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A: Geodetic (carrier phase with resolved ambiguities), real-time/post-processed

B: Carrier smoothed C/A Code Phase, post-processed

C: Real-time (RTCM SC104), post-processed C/A Code

D: Real time P-Code (Precise Positioning Service [PPS])

E: Real time C/A-Code (Standard Positioning Service [SPS])

GPS TECHNOLOGY CLASSIFICATION

mapping

geodetic

navigation/

100 m

civilian (SPS)

recreational

grade

grade

(prior to 05/02/00)

grade

20 m

civilian (SPS) – post 05/02/00

APPROXIMATE ACCURACY

10 m

military (PPS)

5 m

1 m

0.5 m

dm

cm

mm

E

C

A

B

D

POINT (ABSOLUTE)

RELATIVE

POSITIONING

POSITIONING

Selective Availability switched off – see http://geography.about.com/library/weekly/aa050400a.htm


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The Geocentric Cartesian Coordinate System real-time/post-processed

Z

Satellite P

Greenwich Meridian

N

ZP

A

Y

XP

YP

Equator

S

X

AP = √(XP-XA)2 + (YP-YA)2 + (ZP-ZA)2


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  • THE GPS SIGNALS real-time/post-processed

  • Each Satellite transmits two carrier waves

  • L1 - frequency of 1575.42 MHz and a wavelength of approx 19cm

  • L2 - frequency of 1227.60 MHz and a wavelength of approx 24cm

  • The following satellite-specific signals, called the pseudo random noise (PRN) codes are modulated on the carrier waves:

  • On L1: C/A (Coarse/Acquisition) code λ = approx 300m

  • - Accessible to civilian users

  • - Consists of a series of 1023 binary digits (called chips) that are unique to each satellite.

  • - The chip pattern is repeated every millisecond

  • P (precise) code λ = approx. 30m

    • - Accessible only to military equipment

  • On L2: P code only

  • Coming on-line: L2C and L5


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    Code Signal Positioning real-time/post-processed

    Subframe of message

    Receiver Signal

    TimeDelay

    Matching Subframe

    DelayedSatellite Signal

    The ‘mis-match’ between the code patterns is a measure of the time the signal has taken to travel from satellite to receiver.


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    Geometric Dilution of Precision real-time/post-processed

    - Measures the effect of geometry on the precision of the observations

    - Multiply GDOP by the Std Error to get actual uncertainty

    - Also HDOP, VDOP

    Position Dilution of Precision (PDOP)

    - This is positional part of GDOP


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    COMMON MISTAKES real-time/post-processed

    • Logistical weaknesses

    • battery power, memory overruns, no inter-party communications, no

    • contingencies in observation schedule

    • Operator mistakes

    • incorrect antenna heights, careless centering, incorrect receiver settings

    • (epoch interval), accidental deletion of raw observations, inadequate field

    • records, careless handling of antenna and power cables

    • Processing mistakes

    • insufficient or incorrect datum definition (e.g. incorrect base station

    • coords), no checks on centering and antenna heights, inclusion of trivial

    • base lines, insufficient redundancy and quality checks


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    • Precautions to minimize errors real-time/post-processed

    • Schedule your survey to fall within periods of good satellite geometry (i.e. low PDOP)

    • Eliminate satellites at low elevation to reduce the length of the signal path through the atmosphere

    • Avoid multi-path conditions near the GPS antenna

    • For precise positioning use differential corrections and/or phase observations of the carrier waves

    15° (Mask Angle)

    Earth

    Atmosphere

    Multipathing


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    GPS POSITIONING ERROR CLASSIFICATION real-time/post-processed


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    Post-processing vs Real Time Correction real-time/post-processed


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    Base Stations real-time/post-processed

    Connected via cable

    Tirana, Albania

    Antenna on Tripod

    Receiver and Laptop logging base station measurements

    Base Station over Known Point – Cajamarca, Peru


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    Differential GPS real-time/post-processed

    (Static)

    Single Differencing:

    One satellite observed from two receivers

    Satellite clock error is eliminated

    Double Differencing:

    Two satellites observed from two receivers

    Receiver clock error is eliminated

    Triple Differencing:

    Two satellites observed from two receivers

    at two different epochs.

    Eliminates integer cycle ambiguity

    Epoch 2

    Epoch 1


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    Real Time Kinematic (RTK) real-time/post-processed

    Differential corrections are broadcast via radio

    Base station over free point

    Base station over known point

    Data latency = 0.05 – 1.0 secs

    Radio limits range between base and rover


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    THIRD PARTY DIFFERENTIAL CORRECTION SERVICE real-time/post-processed

    • Service available commercially (e.g. Omnistar)

    • Sub-meter accuracies possible when used in combination with L1

    • User needs only one receiver

    GPS satellites

    Geostationary

    Communication

    Satellite

    Differential Base

    Station

    Rover

    Footprint of Communication

    Satellite coverage

    See http://www.omnistar.com/


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    Eccentric Points real-time/post-processed

    Geostationary

    Communication

    Satellite

    Useful when Canopy prevents direct occupation of point or when Communication Satellite is blocked


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    STATIC SURVEYS FOR CONTROL NETWORK IN NAMIBIA real-time/post-processed

    Source: Walter Volkmann


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    Fiducial Points for defining real-time/post-processed

    GPS datum in the country








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