Global Navigation Satellite System

An Overview

For Mappers and

Resource ManagersSource: Peter H. Dana, Department of Geography, The University of Colorado at Boulder.

-Bob Champoux

What is GNSS?

Why is GNSSS Important

to Mappers and Resource Managers?

Mapping

Locate Features That Can be Incorporated into a GIS.

Navigation

Allows User to Visit the Same Location Again.

Objectives

A Quick Overview.

How A GNSS Receiver Determines Its Position.

How Accurate is GNSS?

Factors Affecting Accuracy.

How GNSS relates to GIS.

An Overview

GPS developed by the US Dept. of Defense (1978).

24 Satellites. Each orbits every 12 hours.

Worldwide Coverage

24 hours a day, 7days a week.

Independent of Weather…

Source: GPSCO

Users of GNSS

The Future?

How Does A GNSS Receiver

Determine Its Position?

By measuring distances from satellites.

Each satellite transmits a unique radio signal.

Receiver listens to radio signals.

Receiver measures distance from each satellite.

With three distances, receiver can determine its

position via trilateration.

Source: Understanding the GPS,

Gregory French

What’s Trilateration?

If you know distances from three known points, you

can determine your position.

You are Here…

How Do Receivers Measure Distances?

Distances are determined by signal travel time:

Signal Travels at Speed of Light.

Since satellites are 11,500± miles away,

travel time is 0.06± seconds.

Accurate timing is very important.

(Thus, satellites have atomic clocks.)

Source: Understanding the GPS,

Gregory French

So, Only Three Satellites are Needed?

Actually, a minimum of 4 satellites are needed.

The fourth satellite is required to help minimize

errors caused by inaccurate clock in the receiver.

Source: Peter H. Dana, Department of Geography,

The University of Colorado at Boulder.

What’s the Accuracy?

A receiver will display a position, but:

This position has error.

The displayed coordinates may not even be close!

Many users think the position is absolute.

The accuracy in the position of the receiver is

10 - 20 meters (95% of the time).

The accuracy varies because of error…

Errors and Factors Affecting Accuracy

We Can Control:

Geometry of the satellites.

Multipath.

Atmospheric Delay.

Geometry of the Satellites

PDOP (Position Dilution of Precision).

Geometry of the Satellites

Poor PDOP (Satellites are closely spaced).

Geometry of the Satellites

Source: Understanding the GPS,

Gregory French

Geometry of the Satellites

PDOP (Position Dilution of Precision).

Less than 4 = Good Geometry (Very Good Position)

4 – 6 = Good Position

Greater than 6 = Poor Geometry (Poor Position)

Receiver will indicate PDOP.

PDOP can be determined prior to fieldwork.

Checking Satellite Coverage and PDOP Prior

to Fieldwork

Source: QKPlan,

by Trimble Corp.

Multipath

Satellite signal is reflected before reaching receiver.

Not always obvious that this is occurring. Source: Understanding the GPS,

Gregory French

Atmospheric Delay

Source: Understanding the GPS,

Gregory French

Atmospheric Delay

Source: Understanding the GPS,

Gregory French

Minimizing the Errors

and Improving the Accuracy

Or: What do I really need to know to use a GNSS receiver in the field?

4 Satellites Solid objects and power lines block satellite signals.

Good Satellite Geometry Collect data when the PDOP is 6 or less.

(Low PDOP indicates good satellite geometry)

Eliminate Multipath Avoid positioning near buildings, cars, ledges, etc.

Minimize Atmospheric Delay Set the receiver mask angle = 15º

More Ways to Improve Accuracy

Collect numerous positions for each point.

When in the woods:

GNSS works under open tree canopies.

Problematic with softwoods, dense canopies and canopies with numerous branches.

Move slightly to improve signal reception and accuracy.

Wet leaves can interfere with signal reception.

Differential Corrections…

Differential Corrections

Source: Understanding the GPS,

Gregory French

Differential Corrections

Corrections are made while in the field (Real Time)

or afterwards (Post Processing).

Government agencies broadcast real time

corrections.

Can download corrections from agencies,

via the Internet, for post processing.

How Do Differential Corrections Help?

Eliminates or reduces numerous errors.

Plus, corrections improve accuracy of position:

Without Differential Corrections: 10-20 meters.

Using Real Time Corrections: 1-5 meters.

Using Post Processed Corrections: 1-3 meters.

Advanced mapping receivers: 0.1 meters.

(But only 95% of the time…)

The GNSS/GIS Link GNSS receivers can locate point, line and polygon

features and collect attribute data.

Software easily converts GNSS points and attributesinto GIS geodatabases or shapefiles and tablesto be incorporated in a map.

Need to confirm that the coordinate systemand projection matches the those of the map.

Source: ARCView,

ESRI

Summary

GNSS is used to locate feature positions or navigate.

Works by measuring distances from satellites.

Several factors affect accuracy:

PDOP (The lower the number, the better).

Avoid proximity to solid objects (Can cause multipath).

Set mask angle to 15º (to limit atmospheric delay).

Differential Correction

Second receiver used.

Further improves accuracy.

GNSS can be used to collect point, line and polygon

features for inclusion in a GIS map.

Backup the GNSS Data…

Resources

These serve as great introductions to GNSS:

Hurn, Jeff (1989). GPS A Guide to the Next Utility, Sunnyvale, California: Trimble Navigation.

NOAA Coastal Services Center (2001). Introduction to Global Positioning Systems.

Quickplan is free software used for GNSS planning. Graphics show projected number of satellites available, locations and PDOP.

(www.trimble.com/support/files/qp.htm)

References

French, Gregory T. (1996). Understanding the GPS. An Introduction to the

Global Positioning System. What it is and How it Works. Bethesda,

Maryland: GeoResearch Inc.

Hurn, Jeff (1989). GPS A Guide to the Next Utility. Sunnyvale,

California: Trimble Navigation.

McElroy, Simon ( 1992). Getting Started with GPS Surveying. Bathurst,

New South Wales, Australia: Global Positioning System Consortium

(GPSCO).