Transcript An Introduction to Global Positioning Systems

Global Positioning
Systems
in the Classroom
3 Key Elements
Without adequate understanding and skill with all
three of these you should not consider leaving
civilization…
MAP
or overestimate your ability to return
to it!
GPS
COMPASS
Today we use a constellation of artificial
stars
provided by the
Global Positioning System.
3 clear signals will provide you with
accurate 2 dimensional position.
A 4th will give reasonably
accurate elevation
information.
Sources of GPS Errors
Sources of GPS Errors

Ionosphere and troposphere delays —
The satellite signal slows as it passes through the
atmosphere. The GPS system uses a built-in model that
calculates an average amount of delay to partially
correct for this type of error.
Sources of GPS Errors

Signal multipath —
This occurs when the GPS
signal is reflected off
objects such as tall
buildings or large rock
surfaces before it reaches
the receiver. This increases
the travel time of the signal,
thereby causing errors.
Sources of GPS Errors

Receiver clock errors — A receiver's built-in
clock is not as accurate as the atomic clocks onboard
the GPS satellites. Therefore, it may have very slight
timing errors.
Sources of GPS Errors

Orbital errors — Also known as ephemeris errors,
these are inaccuracies of the satellite's reported location.
Sources of GPS Errors

Number of satellites visible — The more
satellites a GPS receiver can "see," the better the
accuracy. Buildings, terrain, electronic interference, or
sometimes even dense foliage can block signal
reception, causing position errors or possibly no position
reading at all.

GPS units typically will not work indoors, underwater or
underground.
Sources of GPS Errors

Satellite geometry— This refers to
the relative position of the satellites at any
given time.

Poor geometry results when the satellites
are located in a line or in a tight grouping.

Ideal satellite geometry exists when the
satellites are located at wide angles relative
to each other.
Sources of GPS Errors

Intentional degradation of the satellite
signal — Selective Availability (SA) is an intentional
degradation of the signal once imposed by the U.S.
Department of Defense.

SA was intended to prevent military adversaries from
using the highly accurate GPS signals.

The government turned off SA in May 2000, which
significantly improved the accuracy of civilian GPS
receivers.
Three things to keep in mind….
Avoid buildings and trees…
They will weaken or block your signals
When trying to navigate…
you must be moving to get accurate information…
?
Always remember that the GPS does not replace common sense.
It will ALWAYS SHOW YOU THE STRAIGHT LINE
ROUTE…
not the BEST ROUTE or the SAFEST ROUTE!
A
B
Maps

The map will always be necessary.



The digital computer map, the cartography on the
GPS screen, or the traditional paper map.
A map can be abstract. Even a highway map can
be confusing when you first unfold it.
You must become comfortable with the map and
your abilities before you will trust it.
3 Important Map Questions
Where are you on the map?
Current Location
Where are you going?
Destination
How am I going to get there?
Map Study
The Main Pages
Every GPS displays its information on a set
of main pages. These pages include:
Main Menu Page
Satellite Page
Map Page
Compass Page
Position Page
MAIN MENU PAGES
Garmin
etrex
Garmin
12
Garmin
etrex Vista
Garmin
Etrex LEGEND
SATELLITE PAGES
Garmin
Etrex
Garmin
12
Garmin
etrex Vista
Garmin
60
Garmin
Etrex LEGEND
•This is where we check if we have an adequate signal to use the GPS
•The outer circle is the horizon, the inner circle is a 45o angle in the sky,
and the dot in the centre is straight up.
•The numbers in the circle correspond to the numbers in the bar diagram
MAP PAGES
Garmin
etrex
Garmin
Etrex LEGEND
Garmin
72
Garmin
60
•Some GPS units come with maps, others require a map to be uploaded to the unit.
•Use the zoom option to change the scale of your map
•Use the pan option to look at the off-screen parts of the map *quit or esc gets you back
•Set your map to North Up or Track Up
TRAK
MAPS
COMPASS PAGES
Garmin
etrex
Garmin
12
Garmin
Etrex LEGEND
Garmin
60
•This is the screen we use to navigate to a given waypoint
•On newer units, (including the LEGEND) you can manipulate the
fields to show what’s important to you
•The arrow will show you the direction to travel (straight line)
•The arrow is usually wrong if you are standing still –
you have to be moving!
Garmin
60c
data
POSITION PAGES
Garmin
etrex
Garmin
12
Garmin
etrex Vista
Garmin
60
Garmin
Etrex LEGEND
•On newer units, including the LEGEND your position is given right on the
Satellite Page
•The accuracy of these coordinates will depend on satellite reception
•Position will likely be wrong if your map datum isn’t set properly
And finally….
Pick up your GPS!!
GARMIN ETREX
GARMIN ETREX LEGEND
POWER BUTTON…press and hold to turn on.
GARMIN ETREX
GARMIN ETREX LEGEND
PAGE BUTTON…press to move through 5 screen
GARMIN ETREX
GARMIN ETREX LEGEND
SCROLL BUTTONS…move up or down menus
Note: On LEGEND,
they are called ZOOM
BUTTONS, since they
also allow you to
zoom into the map.
GARMIN ETREX
GARMIN ETREX LEGEND
ENTER BUTTON…press to select an item.
Note: On LEGEND, it
is also called the
FIND button …it takes
you to the FIND
menu
GARMIN ETREX
GARMIN ETREX LEGEND
THUMB STICK….press to enter, scroll, hold to mark.
GARMIN ETREX
GARMIN ETREX LEGEND
Press and hold the power
button until your GPS
turns on…
then press the PAGE
button to get to the MAIN
MENU.
Setup Menus
The Setup Menu can usually be found on the
Main Menu page.
The setup menu is where you access
all the various settings which you need
to know about to properly operate your GPS.
Newer units have more options.
Lets Set our units…..(next slide)
Garmin
etrex
Garmin
Etrex LEGEND
Setting your GPS
Be sure to set the map datum to WGS 84 or
the specific datum shown on the map you
are using
Set your Position Format (UTM or Lat/Long)
Set the Units (metric or imperial)
See the next slide for an example….
Navigation Setup
Position format:
Set to
UTM/UPS
Etrex
Legend
Navigation Setup
Map Datum:
Set to
?????
Etrex
Legend
Navigation Setup
Units:
Set to
Metric
Etrex
Legend
Navigate to the
MAIN MENU
ETREX:
LEGEND:
Go to WAYPOINTS
Go to FIND,
Then WAYPOINTS
Then NEAREST…
Then DELETE ALL
Navigate to the
MAIN MENU
ETREX:
LEGEND:
Go to MARK
Go to MARK,
Then ENTER
Then press the thumb stick.
Navigate to the
MAIN MENU
ETREX:
LEGEND:
Go to WAYPOINTS
Go to FIND,
Then ENTER…
Go to FIND,
Then WAYPOINTS
Then NEAREST….
Edit Waypoint 001
UTM Coordinates are as follows:
Take both GPS units….
lets go outside and try this…