GNSS 101 Bringing It Down To Earth

Transcription

1 GNSS 101 Bringing It Down To Earth Steve Richter Frontier Precision, Inc. UTM County Coordinates NGVD 29 State Plane Datums Scale Factors Projections Session Agenda GNSS History & Basic Theory Coordinate Systems/Calibrations Setting up a Job MNDOT VRS Recommendations Resources Available Questions? 1

2 Learning Objectives Fundamental understanding of GNSS Confidently set-up a job Increase productivity and reduce down-time Maximize your investment Find resources available What is GNSS? GNSS Global Navigation Satellite System This was originally called GPS Global Positioning System (which most people still refer to this term). Now that there are many satellite vehicles (SV S) and systems in place, we refer to it as GNSS. Systems: GPS (NAVSTAR USA) 31 sv s Glonass (Russian) 24 sv s Galileo (European) 8 sv s BeiDou (Chinese) 10 sv s QZSS (Japan) 1 sv IRNSS (India) 3 sv s GPS/GNSS History Navy built the first satellite navigation system. Designed to locate submarines Aerospace Corporation completes military study laying out the concepts for modern GPS Military launches the first of proposed 24-satellite GPS system (NAVSTAR) Government contracts with private companies man-pack portable GPS receivers Accuracy was decreased using Selective Availability (turned off May 2000) military implications 2

3 GPS/GNSS History 1995 First Revision of the GPS system fully completed. Full Constellation of 24 SV s 1999 Mobile phone GPS introduced in Europe 2000 Selective Availability (SA) turned off 2012 Air Force manages 31 operational GPS sv s plus 3 decommissioned for future use. Phase II 2015 RTX and xfill Technologies March 28, 2015 Galileo launches two SV s (8 total) GPS/GNSS Components : Space segment Control segment User segment Space Segment The space segment consist of a constellation of satellites that transmit one-way signals that give the current satellite position and time. Orbit the earth every 11 hours 58 minutes Altitude of 20,200km (12,552 miles) Six orbits spaced at 60 deg. apart 3

4 Space Segment Current GNSS Constellation GNSS Satellites Launched from 1997 to 2012 Control Segment The control segment consist of worldwide monitor and control stations that maintain the satellites in their proper orbits and adjust satellite clocks. It tracks the GPS satellites, uploads navigational data, and maintains health and status of the satellite constellation. Master control stations GPS: Schreiver AFB Colorado Glonass: Moscow, Russia User Segment The user segment consist of the GPS receiver equipment, which receives the signals from the GPS satellites and uses the transmitted information to calculate the user's three dimensional position and time. 4

5 How does GNSS measure? Minimum of 4 satellites must be observed by both the base and the rover. 5 required for an over-determined solution Satellite Ranging: Radio waves from satellites in conjunction with the exact location of satellites based from atomic clocks and precise orbits (space/control segment) determine distance from satellite to base and rover. Trilateration gives us our distance between the base and rover. Survey Requirements: 10:30 10:35 Code Phase vs. Carrier Phase L1 = 19 cm L2 = 24 cm Carrier Code 5

6 Different Applications Different Precisions DGPS: m VRS:1 2 cm H-Star: cm Standalone GPS: 5 10 m Carrier Phase Interger Ambiguity Initialization Required for cm accuracy solutions What Real-Time sources are Available? SBAS - WAAS Trimble RTX - OMNI-Star Single Base Real-Time VRS 6

7 What Real-Time sources are Available? WAAS -Integrated SBAS Compatible with all current Trimble Handhelds <1 5 m <1 5 m FREE No Subscription Required What Real-Time sources are Available? Trimble RTX and Omni- Star m m Subscription Based What Real-Time sources are Available? Single Base Real-Time Corrections m m 7

8 What Real-Time sources are Available? How can I receive corrections from a single base provider solution? m m What Real-Time sources are Available? How can I receive corrections from a single base provider solution? m m What Real-Time sources are Available? CORS GPS Network m m 8

9 What does GNSS measure? True/False It measures northing, easting and elevation. False GNSS measures latitude, longitude and height of ellipsoid. Ellipsoid A mathematical ellipse approximating the shape of the Earth. World Geodetic System 84 (WGS84) Geoid models convert GNSS ellipsoid heights to North American Vertical Datum 88 (NAVD88) orthometric heights. *Trimble software converts these numbers to northing, easting, elevation based on Projections, Calibrations, Localizations, and Geoid Models. GNSS Coordinate Systems Earth Center Earth Fixed WGS 84 Local Grid Coordinates State Plane (NAD83) MN County Coordinates UTM Local Grid NEE Types of Plane Systems Projections Plane Point of Origin Apex of Cone Ellipsoid Axis of Cone & Ellipsoid Tangent Plane Local Plane Axis of Ellipsoid Line of intersection Axis of Cylinder Intersecting Cone 2 Parallel Lambert Ellipsoid Ellipsoid Intersecting Cylinder Transverse Mercator 9

10 GNSS Heights vs. Elevations GPS Measures Height - I want Elevation (MSL) How do I get the best elevation from GPS? Is it the proper tool to measure Elevation? MN CORS VRS- Recommendations MN CORS Mount Points / Geoid Models Looking further. Whoever thought we would be talking about datum shifts Does WGS84 = NAD83? Only with certain realizations: ie NAD83(1986). The Earth is under constant change and Geodesist are measuring these changes and offering different realizations to accommodate this. Fundamental Awareness required. 10

11 Know your Datums! Definition of Geodetic Datum A geodetic datum is a set of constants specifying the coordinate system used for geodetic control NAD83 is a Horizontal Control Datum With multiple realizations NGS NOAA Tutorial on Datums NAD83 Relizations (1986, HARN, CORS 96, NSRS, 2011) NAD83 (2011) is the most current realization which all NGS CORS sites and NGS OPUS use How about VRS Does it Matter Error Sources In GNSS PDOP satellite geometry < 6 RMS multipath < 30 Ionosphere upper atmosphere Troposphere lower atmosphere Human Error Which Error Source do we have no control over? Trimble Access Job Settings 11

12 Trimble Access Job Settings Coordinate Systems Total Station Only State Plane, County Coordinates, UTM Enter Projection, Datum Tran., H & V adjustments When performing calibration* Coordinate system defined by broadcast Long corridor (many km s) railroads, pipelines, etc. Important Settings 12

13 MN CORS VRS- Recommendations MN CORS Mount Points / Geoid Models Is a Performing a Site Calibration or localization Necessary? With Pre-Defined Coordinate Systems (ie. County Coordinates): Horizontal calibration should be one point only. More could alter the system. Vertical calibration should be either one point or four or more with the geoid model. Without A Pre-Defined Coordinate System: Horizontal Calibration should use three or more points. Vertical calibration should be either one point or four or more with the geoid model. Note: remember that this can only be determined to any degree of certainty by actually measuring trusted control monuments in your area and examining the differences. Site Calibrations are also dependant on Project Accuracy requirements Common Support Questions Important to note in Minnesota when using VRS most counties except the Transverse Mercator counties, grid elevations equal ground level. Sea level corrections should be unchecked as the grid has been inflated to ground level of each county. Orthometric heights grid/ground are essentially the same. 13

14 Resources: Support Blog Sites: Geospatial Survey Geospatial Mapping Webinars, Tech Talks, Procedure Sheets Trimble Support: Software/firmware updates, latest versions, documentation Geodetic Resources: NGS OPUS Datums Youtube Search Certified Training Highly Recommended: Geospatial Survey Geospatial Mapping Web Based watch out website ( or Blogs Additional Resources: IPhone/Android App Product Resources, Support, Training, Events and much more. Questions? Steve Richter 14