chapter 2 gps fundamentals

8/7/2019 Chapter 2 GPS Fundamentals

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GPS Fundamentals

GPS Fundamentals Copyright 2009, Trimble Navigation Limited

Chapter Objectives

Identify and describe the 3 segments of

Identify the acceptable number of satellitesfor an accurate position


List and explain the sources ofenvironmental errors that affect GPS dataaccuracy


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GNSS and GPS Acronyms

GNSSGlobal Navigation Satellite

navigation systems

GPSGlobal Positioning System (USDept of Defense)


GLONASSGLObal NAvigation SatelliteSystem (Russian System)

GPS System Segments

Control

Space

User



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Control Segments

Monitors satellites orbits Receives Information, Transmits Information


Space Segment

Min 24 Satellites

Orbit with a 55rotation

20,200 km orbit (approx.)

1 revolution per 12 hours

High Orbit Provides:

Longer Satellite Survival


Higher Accuracy


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User Segment

Multiple Applications

GIS Data Ca ture

Vehicle Tracking

Marine/Vehicle Navigation

Surveying

Emergency Services

Aviation


o ogramme ry

Recreation (hiking etc.)

Many more and more tofollow

Why Satellites for Mapping

Line of site not required Accurate and rapid positioning



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Satellite Ranging

Measuring the distance from a satellite Measuring the travel time of radio signals

Distance D = Speed of light Time


Satellite Ranging

Each satellite transmits a different code The receiver generates matching codes at the exact

same time The receiver compares the satellite and receiver

generated code to determine the travel time. This isused to calculate the distance between the satellite andGPS receiver

measure time difference between same part of code


From satellite

From receiver


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GPS by Trilateration

GPS doesnt provide 3 perfect measurements 4th satellite is used to resolve timing offsets


Clock Offset

Three GPS signal spheres will not intersect:

5 milliseconds out


Need a fourth satellite to eliminate timing offsets


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Alamanac

Gives the general location of all satellites

Stored on GPS receiver

GPS receiver can download a new Almanac in12 minutes (continuous logging)

Useful for:Project planning

Rapid acquisition


so ava a e on www. r m e.com

Ephemeris Transmitted in GPS signal Exact orbit information of individual satellite Re uired to determine the satellite ran e

your location is:

3723.323 N

122 02.162 W



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Summarizing GPS Positioning

GPS signalsget tracked by

Trilateration isused for

Four satellitesrequired for an

and the traveltime is used tocalculate thedistance

but 3 satellitesarentaccurateenough tocreate anintersection

GPS positionto eliminatetiming offsets


Differential GPS

Two receivers track the SAME signals and errorsat the SAME time

They both compute imperfect positions

Base

Base differences applied to Rover



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Sources of error in GPS

Obstruction

Multipath

Atmospheric delay


ObstructionGPS signals cannot travel through physical objects thiswill reduce position yield, and less satellites can mean lessaccuracy



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Multipath

GPS signals are reflected and the wrong signal can beused changing the GPS position significantly


Atmospheric Delay

GPS signals are delayed as they pass throughthe atmosphere

Ionosphere

Troposphere


BaseRoverBase

< 10 km > 10 km


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Carrier Phase

Carrier phase measurements are more accurate

e carr er s gna as a g er requency an etransmitted digital Code, resulting in smallermeasurement errors.

Disadvantages


are more difficult to collect than code measurements.

Also requires Differential techniques (real-time orpostprocessed) to be of use

L1 Wavelength = 19 cm

L2 Wavelength = 24 cm

Satellite Signal Structure

Carrier


CodeCode modulated (0s and 1s)within the L1 Carrier signal


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3 Carrier Phase Types

H-Star technology Decimeter to subfoot accurac in real time or after

postprocessing with as little as two minutes of carrier data.

Carrier postprocessing Ranging from 30 cm down to 1 cm, with carrier data spanning

from 10 minutes up to 45 minutes.


Real-time kinematic (RTK) using Survey equipment Provides accuracies in the centimeter (sub-inch) range, in real

time.

How Carrier Phase WorksUse a Base Station (with known position) to provide itscarrier measurements (real-time or postprocessed)

measure time difference

between same part of code

Rover receiver uses its Code measurement to approximateposition, and therefore Carrier range to within a fewcycles/wavelengths


from satellite

from ground receiver


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Range - Carrier Measurement

Integer Ambiguity (N) -is the unknown number of cycles,

= First Partial Wavelength

Using each tracked satellitesCarrier measurement - searchthe possible combinations tillthe best set is found


N = Integer Ambiguity

Range - Carrier Measurement

Once the matching measurements are calculated that

= First Partial Wavelength


N = Integer Ambiguity


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Carrier Lock

Must maintain carrier signal lock toac eve arge accuracy

Recommended to maintain lock on at least 5satellites (but not required)

Loss of lock is caused by obstructionsblocking signal (e.g. body, tree or building), or


receiver disconnection

In difficult GPS environments Codecorrections may be better accuracy

Accuracy improveswith:

Accuracy Relationships

Duration of carrier lock

Baseline length (proximityto differential source)

Also dependent on GPS Longer baselineError


Time

Shorter baseline


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All data within a

Carrier Blocks

Block Block

Useful Carrier dataarr er oc sused forpostprocessing

All data within a NumberofSVs

3

7

6

5

4

8

A

B C D


Lock lostgiven block willbe the sameaccuracy Time in minutes

5 10 15 20 25 30 35 40 45 50 55

1

2

Review Questions



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Questions and Answers

1. Define GPS.

2. What are the 3 GPS segments?

3. How often do the satellites orbit?

use by GPS receivers to calculate accurate 3D positions

A: Control, Space, User


4. What are 5 GPS applications?

A: Every 12 hours (twice daily)

A:Navigation, surveying, GIS data capture, photogrammetry, vehicletracking, etc.


5. What is the main reason we usesa e es

6. How does the receiver measure itsdistance from individual satellites?

-

A:Line of sight between known and unknown location is not necessary


it takes a radio signal to reach the GPS receiver from a satellite, and using

that time to calculate the distance.


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7. What is a Carrier Block?

8. Carrier phase measurements are moreaccurate, but what is a disadvantage

A:A block of GPS data where 4 or more Carrier Phase signals have been

continuously recorded (maintained)


o carr er p ase measuremen sA:Requires constant lock onto 5 satellites for a period of time, making it moredifficult to achieve than Code measurements in difficult GPS environments.

Also, requires a differential technique (real-time or postprocessed) to be of use.

chapter 2 gps fundamentals

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