GPS OverviewGPS OverviewMatt Rowbotham, GIS CoordinatorMatt Rowbotham, GIS Coordinator

North Country Trail AssociationNorth Country Trail Association

Introduction Global Positioning System (GPS) consists of

three parts. The Space Segment

The Control Segment

The User Segment

Control Segment

Space Segment User

Segment

Global Positioning System

Space Segment

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.

User Segment The user segment consists of the people and

receivers on the ground.

How does it work?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.

How does it workSpeed of Light x Travel Time

Distance

Time signal left satelliteTime signal left satellite

Time currentTime 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

How does it work 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

How does it work With the addition of a fourth satellite signal

the GPS receiver can calculate a three-dimensional position (3D), including altitude.

AccuracyAccuracy

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.

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.

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.

Reduced Accuracy Reduced Accuracy and Errorsand 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.

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.

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 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.

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.

Wide-Area Wide-Area Augmentation Augmentation

System System (WAAS)(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

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

WAAS correction for SV 19No WAAS correction for SV 11

WAAS

WAAS satellitesWAAS satellites

GPS GPS ConstellationConstellation

WAAS Control WAAS Control StationStation

(Position known)(Position known)GPS receiverGPS receiver(Position with (Position with errors)errors)

25 Reference Stations 25 Reference Stations (Position known)(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.

Enhancement External Antenna

Map76S more accurate under canopy

Hat/ Helmet Mounts

Sleeve Mounts

Vehicle Brackets

Taking Taking MeasurementsMeasurements

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 Good Pencil System.

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!

Transferring Transferring InformationInformation

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.

Submission FormsSubmission Forms

Questions?Questions?