Introduction%20to%20GPS

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Introduction to GPS

• SHSM Certificate

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Welcome!

• The purpose of this tutorial is to introduce you
  to the theory behind Global Positioning Systems
  and their safe use.
• Although the information provided here is
  essential for anyone using a GPS in the field,
  there is no substitute for hands on training.
• Please obtain training from a qualified
  instructor before entrusting your safety to a GPS
  unit!

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Even in relatively mild weather conditions,
becoming lost overnight can result
potentially deadly hypothermia, not to mention
panic, dehydration, increased chance of injury,
bumping into unfriendly animalsor even paying
the cost of the rescue operation.
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There is a reason why this is the first message
you see when you turn a GPS on! The Global
Positioning System provides convenient and
accurate information, as long as you remember
the basics and are aware of the
limitations. Like all technology GPS is not
perfect. Always be prepared (and able!) to use
alternate means to find your way home!
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A Bit of Background
Global Positioning Systems were first established
by the United States Department of
Defense. While initially a military system, it
was always intended to be used in part by
civilians. The project became fully active in
the mid 1990s, but with degraded accuracy for
civilian users (about 100 metres) On May 1,
2000, the accuracy of signals for civilian users
was upgraded and practical applications for the
technology were quickly found. Some of these
applications are field navigation map
making surveying search and
rescue geo-caching
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3 Key Technologies
The safest and most effective way to use GPS is
to integrate it with older technologies. Map
reading skills and the ability to navigate with a
magnetic compass are critical. And dont forget
to take them along with you!
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Magnetic Compass vs GPS
Compass Compass
Pro Con
No batteries Magnetic field is not uniform
Easy to use with training Unreliable in polar areas
Magnetic pole moves
Minerals in the ground cause deflections
Not as accurate as GPS
GPS GPS
Pro Con
Accuracy Batteries die
Easy to use with training Several types of error
Multiple functions (course storage, speed read out, altitude) Requires more training
On board map
Not affected by environment
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An Artificial Constellation
The GPS constellation consists of 24 satellites,
orbiting at about 20,000 km above the earth.
At any time, on any point on the surface of the
earth, an average of 9 satellites are visible to
GPS receivers.
A series of ground stations keep track of the
satellites and ensure the signals are
accurate. Most GPS receivers use signals from 4
satellites to generate their position information.
http//www.oceanservice.noaa.gov/topics/navops/pos
itioning/welcome.html
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An Alarming Analogy
Imagine being in a very large room with an alarm
clock. When the alarm goes off, the sound
travels to your ear.
If you were able to measure the time it takes for
the sound to reach your ear, you could calculate
how far away the clock was.
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An Alarming Analogy
With a single alarm clock, the distance that you
calculate is the radius of a circle you are
standing on. The problem is that you have no
idea where exactly on the circle you are.
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An Alarming Analogy
Add a second alarm clock and you can narrow your
location to 2 possible points where the circles
intercept (see below). Good, but not perfect.
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An Alarming Analogy
Adding a third alarm clock to the equation
produces only one possible location. The
precision of that position will depend on a few
things, such as the accuracy of the clocks, how
carefully you measure the time, or how well you
can do the calculations.
Since few things are perfect, you will not be
able to calculate the exact position. BUT, you
can get to within a reasonable (and useable)
distance from the exact location.
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An Alarming Analogy
Although it is a bit simplistic, the analogy is
actually closer than you may think. GPS relies
on a series of very accurate atomic clocks.
Distances are calculated by measuring how long it
takes for a signal to get from the satellites to
the receiver.
The GPS unit in your hand receives signals from
the satellites and performs the calculations. In
general it will pick the best signals to generate
the most accurate location.
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The Reality
Satellites transmit a radio signal with precise
time information.
Three clear signals will provide accurate
two dimensional position.
A fourth signal will give reasonably
accurate elevation information.
The GPS will show you which satellites it is
tracking and how strong the signals are.
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The Reality
A typical handheld GPS unit used in field
navigation will provide the user with 3 to 5
metres of accuracy under ideal conditions. The
WRDSB currently uses the Garmin ETREX and Garmin
ETREX Legend GPSs. They are typical entry
level units and allow a trained user to navigate
safely. They will be discussed in more detail in
the third section of this tutorial. They are
subject to the errors and limitations that will
be outlined in the next section.
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Quiz 1 here
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GPS Errors
GPS provides rapid and accurate position
information with some limitations. You must be
aware of these limitations If you are going to
navigate effectively. The following slides
outline the major sources of error encountered
when using GPS technology.
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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.
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GPS Errors
Receiver clock errors A receiver's built-in
clock is not as accurate as the atomic clock
onboard the GPS satellites. Therefore, it may
have very slight timing errors. This will affect
the accuracy of the calculated position.
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GPS Errors
Orbital errors Also known as ephemeris errors,
these are inaccuracies of the satellite's
reported location.
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GPS Errors
Ionosphere and troposphere delays The satellite
signal slows as it passes through the atmosphere.
The GPS system uses software that calculates an
average amount of delay to partially correct for
this type of error.
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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.
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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.

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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.

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GPS Errors FIX
Wide Area Augmentation System (WAAS)
Think of WAAS as an electronic tune up for
GPS. Newer GPS units (including our Garmin ETREX
Legend) have software enabling them to use
WAAS. Accuracy can be boosted to 1 meter!
Some systems (ETREX Legend included) experience
software glitches when WAAS is enabled! Be
careful.
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Quiz 2 here
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Features and Operation of a Typical GPS
There are a multitude of GPS receivers available.
The cost of an entry level unit is around the
100 mark. Common accessories include download
cables, computer mapping software, and various
unloadable map packages. Regardless of the model
you use most will have the same general features
described on the next few slides.
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Features and Operation of a Typical GPS

• The Garmin ETREX Legend is typical of the entry
  level GPS
• unit you may encounter.
• Some of its features include
• 22 hours operation on 2 AA batteries
• water resistant to 1 metre for 30 minutes
• can track up to 12 satellites
• contains a North America base map
• can hold up to 500 waypoints

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The Main Pages
Most GPS units display information on a set of
main pages. These pages include Main Menu Page
Satellite Page Map Page Compass Page Position
Page
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MAIN MENU PAGES
The main menu is the starting point when you turn
your GPS on.
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SATELLITE PAGES
This is where you check to see if you have an
adequate signal to use the GPS. The outer circle
is the horizon, the inner circle is 45o up,
and the dot in the centre is straight up. The
numbers in the circle correspond to the numbers
in the bar diagram.
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MAP PAGES
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. Set your map to North Up or Track Up.
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COMPASS PAGES
This is the screen you use to navigate to a
chosen waypoint. You can usually manipulate the
data fields to show what you want. The arrow
shows you the direction to travel (straight
line) You have to start moving to get the arrow
to point the right way.
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POSITION PAGES
On many units, 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 isnt set properly.
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Features and Operation of a Typical GPS
The WRDSB currently uses two types of GPS. Both
are entry level units that have proven to be very
reliable and accurate.
GARMIN ETREX Legend
GARMIN ETREX
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Features and Operation of a Typical GPS
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Features and Operation of a Typical GPS
PAGE BUTTONpress to move through 5 screens.
SCROLL BUTTONSmove up or down menus
ENTER BUTTONpress to select an item.
POWER BUTTON
THUMB STICK.press to enter, scroll, hold to mark.
GARMIN ETREX Legend
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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.
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GPS Settings
POSITION FORMAT Allows you to chose the
co-ordinate system you want to use.
MAP DATUM Make sure this matches the map you
are using!.
DISTANCE, SPEED, DEPTH Set this to whatever
matches your map.
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Navigating
Theory is a poor substitute for hands on
experience when it comes to GPS. Navigation with
the GPS is fairly simple and can be learned
easily. In its simplest terms, navigation is
simply a case of following the needle on the GPS
to a WAYPOINT.
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Navigating
WAYPOINTS can either be marked with the GPS unit,
or uploaded from some other source such as an
electronic database or a map. To MARK a
WAYPOINT, select Mark on the appropriate menu,
then verify it by clicking OK. The WAYPOINT
can also be edited (values changed) to make it a
new location. If you can text, this will be a
snap. To navigate to a chosen point, select it
and click on Goto. Start walking and follow
the arrow!
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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!
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Congratulationsyoure done!
The preceding slides have introduced you to the
theory and some of the features of a basic GPS
unit. Remember that the GPS unit is only one
part of the equipment you should carry for field
navigation. You should seek qualified hands on
instruction with a GPS unit prior to using it in
the field. And always remember that GPS is a
navigation AID, not a substitute for common
sense and proper training.