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GNSS 101“Bringing It Down To Earth”

Steve Richter – Frontier Precision, Inc.

NGVD 29

Datums

ProjectionsScale Factors

County Coordinates

State Plane

UTM

Session Agenda

GNSS History & Basic Theory

Coordinate Systems/Calibrations

Setting up a Job

MNDOT VRS Recommendations

Resources Available

Questions?

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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

1959 - Navy built the first satellite navigation system.

Designed to locate submarines.

1963 - Aerospace Corporation completes military study

laying out the concepts for modern GPS.

1974 - Military launches the first of proposed 24-satellite

GPS system (NAVSTAR).

1985 - Government contracts with private companies

“man-pack portable” GPS receivers.

1990-2000 – Accuracy was decreased using Selective

Availability (turned off May 2000)

– military implications

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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

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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.

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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

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Different Applications –Different Precisions

Standalone GPS: 5–10 m

DGPS: 0.5–1 m

VRS:1–2 cm

H-Star: 10–30 cm

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

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WAAS -Integrated

SBAS

<1–5 m <1–5 m

FREE – No Subscription Required

Compatible with all current

Trimble Handhelds

What Real-Time sources

are Available?

Trimble RTX and Omni- Star

0.1–5 m 0.1–5 m

Subscription Based

What Real-Time sources

are Available?

What Real-Time sources

are Available?

0.01–5 m 0.01–5 m

Single Base Real-Time

Corrections

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What Real-Time sources

are Available?

0.01–5 m 0.01–5 m

How can I receive corrections from a

single base provider solution?

What Real-Time sources

are Available?

0.01–5 m 0.01–5 m

How can I receive corrections from a

single base provider solution?

What Real-Time sources

are Available?

CORS GPS Network

0.01–5 m 0.01–5 m

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What does GNSS measure?

True/False It measures northing, easting and elevation.

FalseGNSS 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

Ellipsoid

Tangent PlaneLocal Plane

Point of Origin

Intersecting CylinderTransverse Mercator

Axis of Ellipsoid

Ellipsoid

Axis of Cylinder

Line of intersection

Apex of Cone

Intersecting Cone2 Parallel Lambert

Axis of Cone & Ellipsoid

Ellipsoid

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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.

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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

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Trimble Access Job Settings

Total Station Only

State Plane, County Coordinates, UTM

Enter Projection, Datum Tran., H & V adjustments

When performing calibration*

Long corridor (many km’s) railroads, pipelines, etc.

Coordinate system defined by broadcast

Coordinate Systems

Important Settings

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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.

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Resources:

Support Blog Sites:• Geospatial Survey – www.fpisurvey.com

• Geospatial Mapping – www.fpimapping.com

• Webinars, Tech Talks, Procedure Sheets

Trimble Support:• www.trimble.com/support/support_AZ

• Software/firmware updates, latest versions, documentation

Geodetic Resources:• NGS

• OPUS – www.ngs.noaa.gov/OPUS

• Datums – Youtube Search

Certified Training – Highly Recommended:• Geospatial Survey

• Geospatial Mapping

• Web Based – watch out website (www.frontierprecision.com) or Blogs

Additional Resources:

IPhone/Android App – Product Resources,

Support, Training, Events and much more.

Questions?

Steve Richtersrichter@frontierprecision.com