Introduction to GPS/GNSS

Introduction to Tidal and Geodetic Vertical DatumsCorbin Training Center

January 7, 2009

Jeff LittleGuest Speaker

678-551-5188, jlittledg@gmail.com

Introduction• What is GPS?

• What is GNSS?

• GPS/GNSS Uses

• Basic Errors

•GPS/GNSS Classes of Errors

• GPS/GNSS Methods

What is GPS?

• GPS is NOTGPS is NOT

• GPS was developed for and byGPS was developed for and by

• GPS IS GPS IS the Global Positioning System. GPS consists of a constellation of satellites that orbit the earth’s surface, monitored by ground stations which enable the user to position or navigate. “Man Made Stars”

What is GPS?The Global Positioning System

• Orbits the earth • 24 hrs a day• 365 days per year• In all weather conditions

• Allows the user to acquire positioning information at any time or any place on or above the earth’s surface.

3 Segments of GPS • Space Segment

• 27+ Satellites “number continues to grow”• 11,000 nautical miles• 6 Planes 55o Rotation

• Control Segment• Primary station is in Colorado Springs, Colorado• Ascension Island (Atlantic Ocean)• Diego Garcia (Indian Ocean)• Kwajalein (Pacific Ocean)• Hawaii (Pacific Ocean)

• User Segment• Equipment used to log GPS Data “ GPS Receiver”

GPS is a Measuring & Navigation Tool

GPS is not a magic box just another tool in the tool box

GPS/GNSS Equipment

GPS/GNSS Equipment

• Just another Tool in the Tool Box

• Different Manufactures

• Different Applications

• Different Precisions/Accuracies

The Macrometer V1000 -- the first GPS receiver owned by NOAA!!

The GPS Pathfinder – puts a whole new spin on WHEN and

WHERE!!

What is GNSS?

GNSS is

• Global Navigation Satellite Systems• Combination of different Satellite Systems

• Current and potential contributing systems• GPS/NAVSTAR – USA• GLONASS – Russia • COMPASS – China • GALILEO – European Union

GPS/GNSS Gives Us

YA

X

Z

Y

A

XA

+ZA

Equator

Gre

enw

ich

Mer

idia

n

EarthMass Center

- X

- Y

- Z

X1

Y1

Z1

X2

Y2

Z2

X3

Y3

Z3

X4

Y4

Z4

Accuracy

The correct position

The known position

The Absolute Gospel

What is the truth?

– The monument we intend to hold as the Absolute Gospel

– Assumed or known Datum

Accuracy VS. PrecisionAccuracy VS. Precision

Precision

Repeatable!

Not accurate

Not true value

Not position

Accuracy VS. PrecisionAccuracy VS. Precision

The average of the measurements close to the known position

Accurate

Accuracy VS. PrecisionAccuracy VS. Precision

Repeatability of the measurement

Precise

Neither Precise or AccurateNeither Precise or Accurate

No repeatability and known values are questionable

Not Accurate or Precise

Accuracy VS. PrecisionAccuracy VS. Precision

Ideal solution

Accurate & Precise

Continuously Operating Reference Stations

http://www.ngs.noaa.gov/CORS

Online Positioning User Service

http://www.ngs.noaa.gov/OPUS/

GPS/GNSS Uses

• Recreation

• Navigation

• Positioning

• Modeling

• Monitoring

• Military

Basic Errors

• Systematic – • The difference between a measured value and its true value or position

• Blunder – • A mistake caused by the user

• Random – • The difference between true and measured values after systematic and blunders have been removed

GPS/GNSS Classes of Errors

• Ephemeris Error• Satellite does not transmit its correct location.

• Satellite Clock Error • Satellite Clock predictability

• Ionosphere Error • Electrons slow down the speed of the signal

• Troposphere Error• Temperature, pressure and humidity change the speed of the signal

• Receiver Error• Software, accuracy and internal channel biased

GPS/GNSS Classes of Errors

• Multipath!!! Multipath!!! Multipath!!! Multipath!!!

•Reflective signals entering the receiver

• The most common error in everyday application• Trees • Buildings• Fences• Vehicles• Road signs • Any object natural or man made

• Affects both horizontal and vertical

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August 1987 -Ionospheric refraction and Multipath Effects in GPS Carrier Phase ObservationsYola Georgiadou and Alfred Kleusberg IUGG XIX General Assembly Meeting, Vancouver, Canada

ø ø

Figure 1Multipath Description

d ø/dt ~ 2 rad/12 hr.h

Signal Multipath

Preventing GPS/GNSS Errors

• Planning

• Proper Equipment

• Avoid Obstructions

• Follow Standards and Guidelines

• Modeling Software

• Continuing Education

GPS/GNSS Methods Autonomous

• Position derived in receiver directly from satellites • +/- 10 Meters • 1 Receiver

Post Processing Static Surveying • mm Precisions• Multiple Receivers “OPUS”• Time consuming• Office software “PAGES”

Kinematic• +/- 2cm • Multiple receivers• Office software • Time consuming

GPS/GNSS Methods Real Time Kinematic (RTK)

• +/- 2cm • Multiple receivers or single receiver in Real Time Network (RTN)• Communication equipment i.e. Radio, Cell Phone, or Data Modem• Limited time • Position in the field

Differential GPS (DGPS) “Mapping Grade”• Cost effective receiver • cm to meter Positioning• Precisions depend on time spent collecting data• Office software

Real Time Differential GPS (RTDGPS) “Mapping Grade”• Cost effective receiver• cm to meter positioning• Precisions depend on time spent collecting data and correction type• Position in the field• May need communication equipment