GPS Overview Matt Rowbotham, GIS Coordinator North Country Trail Association

Matt Rowbotham, GIS Coordinator

North Country Trail Association

Introduction Global Positioning System (GPS) consists of three parts. The Space Segment The Control Segment The User Segment

Global Positioning System (GPS) consists of three parts.

The Space Segment

The Control Segment

The User Segment

Global Positioning System Control Segment Space Segment User Segment

Space Segment A network of 24 – 32 satellites orbiting Earth. These satellites continually transmit time-marked radio (microwave) signals back to Earth.

A network of 24 – 32 satellites orbiting Earth.

These satellites continually transmit time-marked radio (microwave) signals back to Earth.

Control Segment A number of tracking stations located around the world. These stations monitor and send corrective data via the satellites.

A number of tracking stations located around the world. These stations monitor and send corrective data via the satellites.

User Segment The user segment consists of the people and receivers on the ground.

The user segment consists of the people and receivers on the ground.

How does it work?

How does it work The signal received on the ground from a satellite contains information on that satellite’s position as well as the time the signal was sent.

The signal received on the ground from a satellite contains information on that satellite’s position as well as the time the signal was sent.

How does it work Speed of Light x Travel Time Distance Time signal left satellite Time current

How does it work Once a GPS receiver has locked onto the signals from two different satellites it can begin to estimate a position

Once a GPS receiver has locked onto the signals from two different satellites it can begin to estimate a position

How does it work One on the surface of the earth and one in deep space (which of course is not accurate).

One on the surface of the earth and one in deep space (which of course is not accurate).

How does it work With three different satellites it can use this information to calculate (trilateration) a position

With three different satellites it can use this information to calculate (trilateration) a position

How does it work With the addition of a fourth satellite signal the GPS receiver can calculate a three-dimensional position (3D), including altitude.

With the addition of a fourth satellite signal the GPS receiver can calculate a three-dimensional position (3D), including altitude.

Accuracy

Accuracy The level of accuracy we expect varies between recreational grade units (Garmin, Magellan, etc) and resource grade units (Trimble, Lecia, etc.) According to Garmin their receivers are on average accurate to 15 meters (~ 50 feet). The Trimble ProXT receiver owned by the NCTA is capable (with user correction) of measurements accurate down to submeter (< 3 feet), nearing a 1 centimeter accuracy in ideal conditions.

The level of accuracy we expect varies between recreational grade units (Garmin, Magellan, etc) and resource grade units (Trimble, Lecia, etc.)

According to Garmin their receivers are on average accurate to 15 meters (~ 50 feet).

The Trimble ProXT receiver owned by the NCTA is capable (with user correction) of measurements accurate down to submeter (< 3 feet), nearing a 1 centimeter accuracy in ideal conditions.

What level of accuracy do we need? In the US, national accuracy standards for large scale maps (~1:20,000) can be considered to be roughly +/- 3 feet. But… Location of information on the North Country Trail is fairly obvious. Trimble GPS is cumbersome and has a steep learning curve, as well as limited access. There is widespread availability of recreational grade GPS units.

In the US, national accuracy standards for large scale maps (~1:20,000) can be considered to be roughly +/- 3 feet.

But…

Location of information on the North Country Trail is fairly obvious.

Trimble GPS is cumbersome and has a steep learning curve, as well as limited access.

There is widespread availability of recreational grade GPS units.

What level of accuracy do we need? To balance high demand for up to date information with the need for accurate information, accept the lower accuracy potential of recreational grade GPS receivers. Strive to push these units to the maximum accuracy potential. This can be accomplished by better understanding this technology and incorporating best practices and error correction procedures whenever possible.

To balance high demand for up to date information with the need for accurate information, accept the lower accuracy potential of recreational grade GPS receivers.

Strive to push these units to the maximum accuracy potential.

This can be accomplished by better understanding this technology and incorporating best practices and error correction procedures whenever possible.

Reduced Accuracy and Errors

Reduced Accuracy There are many factors at play that can affect the accuracy of position estimates being made by a GPS receiver. These can be grouped into two generalized categories Arrangement of satellites in the sky at a given time in your area (satellite geometry). Signal interference between the satellite and receiver.

There are many factors at play that can affect the accuracy of position estimates being made by a GPS receiver.

These can be grouped into two generalized categories

Arrangement of satellites in the sky at a given time in your area (satellite geometry).

Signal interference between the satellite and receiver.

Satellite Geometry Not only is the number of satellites that your GPS receiver has locked onto important, but the arrangement of these satellites to one another and to your receiver can affect the precision of your measurement as well as amplify other errors.

Not only is the number of satellites that your GPS receiver has locked onto important, but the arrangement of these satellites to one another and to your receiver can affect the precision of your measurement as well as amplify other errors.

Satellite Geometry Satellite geometry is considered good when one satellite is directly above the receiver, and three or more satellites are equally spaced around the horizon .

Satellite geometry is considered good when one satellite is directly above the receiver, and three or more satellites are equally spaced around the horizon .

Satellite Geometry Satellite geometry is considered good when one satellite is directly above the receiver, and three or more satellites are equally spaced around the horizon .

Satellite geometry is considered good when one satellite is directly above the receiver, and three or more satellites are equally spaced around the horizon .

Signal Interference A reduction in accuracy will occur when the length of time it takes a signal to travel between a satellite and GPS receiver is affected. The factors that can slow down the speed of the signal include Particles and weather systems in the atmosphere. Buildings and other landscape and surface features.

A reduction in accuracy will occur when the length of time it takes a signal to travel between a satellite and GPS receiver is affected.

The factors that can slow down the speed of the signal include

Particles and weather systems in the atmosphere.

Buildings and other landscape and surface features.

Signal Interference Ionosphere Solid Structures Metal Electro-magnetic Fields

Errors You will have slightly different readings for the same point each time you take a measurement, including trying to navigate back to the point.

You will have slightly different readings for the same point each time you take a measurement, including trying to navigate back to the point.

Wide-Area Augmentation System (WAAS)

WAAS WAAS is a system of satellites and ground stations that provide GPS signal corrections. The system was developed for aviation use, but is available to everyone. The two WAAS satellites show up on Garmin GPS units as satellite numbers 48 and 51. Reception of a WAAS corrected signal is indicated on the Garmin 76CSx by the letter “D” on the signal strength bar for a given satellite on the Satellite Page of the unit.

WAAS is a system of satellites and ground stations that provide GPS signal corrections.

The system was developed for aviation use, but is available to everyone.

The two WAAS satellites show up on Garmin GPS units as satellite numbers 48 and 51.

Reception of a WAAS corrected signal is indicated on the Garmin 76CSx by the letter “D” on the signal strength bar for a given satellite on the Satellite Page of the unit.

WAAS If WAAS signal intermittent- WAAS correction for SV 19 No WAAS correction for SV 11

WAAS WAAS satellites GPS Constellation WAAS Control Station (Position known) GPS receiver (Position with errors) 25 Reference Stations (Position known)

WAAS In certain instances Garmin units may select a 2D WAAS corrected signal over a more accurate 3D non-WAAS corrected signal, which is potentially less accurate. Unfortunately for the NCTA, WAAS reception seems to be rather limited in the more remote areas of the northern tier of the continental US. WAAS should be disabled if if you’re getting poor WAAS reception.

In certain instances Garmin units may select a 2D WAAS corrected signal over a more accurate 3D non-WAAS corrected signal, which is potentially less accurate.

Unfortunately for the NCTA, WAAS reception seems to be rather limited in the more remote areas of the northern tier of the continental US.

WAAS should be disabled if if you’re getting poor WAAS reception.

Enhancement External Antenna Map76S more accurate under canopy Hat/ Helmet Mounts Sleeve Mounts Vehicle Brackets

External Antenna

Map76S more accurate under canopy

Hat/ Helmet Mounts

Sleeve Mounts

Vehicle Brackets

Taking Measurements

Taking Measurements For the North Country Trail we are interested in both points and lines. GPS units must be able to connect to a computer to transfer large amounts of data If no connection, you can still work with a small number of points (no lines), using the G ood P encil S ystem.

For the North Country Trail we are interested in both points and lines.

GPS units must be able to connect to a computer to transfer large amounts of data

If no connection, you can still work with a small number of points (no lines), using the G ood P encil S ystem.

Taking Measurements Tips Always clear Track log before you start working (line) Use Average feature for taking points If just recording with GPS for NCTA you can work in WGS 84 Backup and save often!

Tips

Always clear Track log before you start working (line)

Use Average feature for taking points

If just recording with GPS for NCTA you can work in WGS 84

Backup and save often!

Transferring Information

MN DNR Garmin The Minnesota DNR has put together a freely available software package that makes sharing data between our mapping system and GPS units extremely easy. Step by step instructions in handout Note: Be aware of datum and projects when sending data to NCTA. I prefer unprojected, WGS 84.

The Minnesota DNR has put together a freely available software package that makes sharing data between our mapping system and GPS units extremely easy.

Step by step instructions in handout

Note: Be aware of datum and projects when sending data to NCTA. I prefer unprojected, WGS 84.

Submission Forms

Questions?