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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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slide1

GLOBALPOSITIONING SYSTEM

  • GNSS – Global Navigation Satellite System
  • US GPS System (Navstar)
  • Russian GLONASS system
  • European Galileo System
slide2

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
slide4

CORE IGS TRACKING NETWORK – late 1998

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

slide5

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

slide6

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

slide7

The Geocentric Cartesian Coordinate System

Z

Satellite P

Greenwich Meridian

N

ZP

A

Y

XP

YP

Equator

S

X

AP = √(XP-XA)2 + (YP-YA)2 + (ZP-ZA)2

slide8

THE GPS SIGNALS

  • 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

slide9

Code Signal Positioning

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.

slide10

Geometric Dilution of Precision

- 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

slide11

COMMON MISTAKES

  • 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
slide12

Precautions to minimize errors

  • 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

slide16

Base Stations

Connected via cable

Tirana, Albania

Antenna on Tripod

Receiver and Laptop logging base station measurements

Base Station over Known Point – Cajamarca, Peru

slide17

Differential GPS

(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

slide18

Real Time Kinematic (RTK)

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

slide19

THIRD PARTY DIFFERENTIAL CORRECTION SERVICE

  • 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/

slide20

Eccentric Points

Geostationary

Communication

Satellite

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

slide23

Fiducial Points for defining

GPS datum in the country

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