Introduction to GPS/GNSS Introduction to Tidal and Geodetic Vertical Datums Corbin Training Center January 7, 2009 Jeff Little Guest Speaker ,

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 ISGPS 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 55 o 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 Just another Tool in the Tool Box Different Manufactures Different Applications Different Precisions/Accuracies

The Macrometer V 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 YAYA X Z Y A XAXA +Z A Equator Greenwich Meridian Earth Mass Center - X - Y - Z X1Y1Z1X1Y1Z1 X2Y2Z2X2Y2Z2 X3Y3Z3X3Y3Z3 X4Y4Z4X4Y4Z4

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. Precision Precision Repeatable! Not accurate Not true value Not position

Accuracy VS. Precision The average of the measurements close to the known position Accurate

Accuracy VS. Precision Repeatability of the measurement Precise

Neither Precise or Accurate No repeatability and known values are questionable Not Accurate or Precise

Accuracy VS. Precision Ideal solution Accurate & Precise

Continuously Operating Reference Stations

Online Positioning User Service

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

August Ionospheric refraction and Multipath Effects in GPS Carrier Phase Observations Yola Georgiadou and Alfred Kleusberg IUGG XIX General Assembly Meeting, Vancouver, Canada øø Figure 1 Multipath 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