Getting to know Garmin: GPS theory and use

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Getting to know Garmin GPS theory and use
March 2004
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Course Outline

• Welcome and Introductions
• What is GPS?
• Sources of GPS error
• Correcting GPS error
• Getting to know Garmin 12 and 72/76
• Setting up your Garmin GPS
• Collecting and using Waypoints
• Collecting and using Tracks
• Definition of Routes
• Using the DNRGarmin program

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What is GPS?

• Worldwide radio-navigation system formed
  from a constellation of 31 satellites and ground
  stations. These "man-made stars" are used as
  reference points to calculate positions.
• Created by DOD in early 1980s
• Scrambling of signals (selective availability)
  turned off in 2001

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What is GPS?
Space Segment
Control Segment
(Base Station)
User Segment
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What is GPS? - Control Segment
Master control station
Monitor station

Ground control
Colorado Springs
Diego Garcia
Hawaii
Kwajalein
Ascension
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What is GPS? Space Segment
Physical Characteristics
Name NAVSTAR
Manufacturer Rockwell International
Weight 900 kg
Size 5 meters wide (panels extended)
Lifespan 8 years
Orbital Parameters
31 total satellites, 28 operational
Altitude 20,200 km
Orbital Period 12 hr
Orbital Plane 55 degrees to equator
Signal Parameters
L1 (civilian code) and L2 (military code)
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What is GPS? User Segment
Project planning
Post processing
Data collection
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How does GPS work?

• Triangulation. At least 3 known points are
  required to determine an unknown location.
• GPS receivers measure distance using the travel
  time of radio signals.
• Along with distance, must know exactly where the
  satellites are in space.
• Finally, correct for any delays the signal
  experiences as it travels through the atmosphere.
  

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How does GPS Work?
How to know when the signal left the satellite?

• Receiver and satellite use same code
• Synchronized code generation
• Measure time difference for the same code

From satellite
From receiver
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GPS Sources of error

• Type of GPS Receiver
• Atmospheric Refraction
• Multipath Errors- objects in the way
• Satellite clock and ephemeral (inserted) error
• Magnetism
• Mask Angles

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Sources of Error Receiver type

• How many channels?
• Internal antenna strength?
• External antenna capability?
• WAAS (internal) or DGPS (add-on) compatible?
• Post-processing compatible?
• Three user levels recreation (Garmin), mid
  (Trimble Geo II/III), survey (Trimble ProXR)

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Sources of Error Receivers

• Clock Error - Slight time difference in clocks
• Ephemeral Error - Actual position vs. predicted
  position of satellites
• Receiver Noise - Internal operational noise
  that interferes with signal

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Sources of Error Atmospheric
Satellite Orbit
Ionosphere
(Charged Particles)
Troposphere
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Sources of Error Multipath
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Sources of Error Constellation
15 degree mask
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Sources of Error Constellation
Poor DOP
Good DOP
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Sources of Error Measurement

• PDOP Complete Positional Dilution of Precision
• VDOP - Vertical...
• GDOP - Geometric...
• HDOP - Horizontal...
• TDOP - Time...

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Avoiding Error

• Mission Planning check satellite coverage before
  GPSing
• Avoid from magnets, power lines, large metal
  structures
• Heavy cover/canopy will decrease signal
  regularity use external antenna, travel more
  slowly, and watch GPS Satellite Page

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Avoiding Error

• Weather reports
• Travel faster when going straight, slower when
  going around curves
• External antenna securely fastened to GPS
• Trimble Planning software
• www.trimble.com/planningsoftware.html
• Download the almanac (ephemeris) file from
  the site every 30 days

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Avoiding Error

• Set FILE STATION to your location
• Most important graphs to view include
• DOP-Position
• Visibility
• Sky plot

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Correcting Error

• Real-time correction (DGPS) requires add-on (1-2
  meter accuracy for both Trimbles and Garmins)
• Add-on talks to a network of Beacon Towers
• Most new GPS units are WAAS-compatible (1-2 meter
  accuracy for both Trimbles and Garmins). WAAS
  network of towers for aviation GPS complete
  2008
• Differential correction (post-processing) for
  Trimble using Pathfinder Office and beacon files
• Editing data in ArcView

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Capabilities of Handheld Garmins

• Records specific locations (waypoints) and tracks
  you (bread-crumb trail)
• Helps you direct others to a location
• Measure distances and area
• Integrates with Landview and ArcView
• 20 ft accuracy without differential correction
  add-on, 6 ft with add-on

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Garmin 12 Series GPS
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Garmin 12 Series GPS Pages
Page Button moves forward through pages
Satellite Status Page
Position Page
Map Page
Compass Page
Highway Page
Quit Button moves backward through pages
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Garmin 72/76 Series GPS
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Garmin 72/76 Series GPS Pages
Page Button moves forward through pages
Quit Button moves backward through pages
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Satellite Page

• The Satellite Page shows what the GPS is doing at
  any given time, including
• Sky View of visible satellites
• (More evenly spaced better accuracy)
• Signal Strength of satellites
• (Stronger signals better accuracy)
• Current Position Fix (2D, 3D, Acquiring etc)
• Estimated Positional Error (EPE, a measure of
  accuracy)

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Satellite Page
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Power Supply

• Always check your power supply!
• Batteries installed in the bottom or back
• External power cord (cigarette lighter or Lithium
  pack) can also be used.
• Garmin 12 battery life (4 AA alkaline) 24 hrs
• Garmin 72/76 battery life (2 AA alkaline) 16 hrs
  
• Cold weather drains alkalines - use lithium.
• All Garmins contain an internal backup battery to
  preserve data when AAs/power cord are removed.
• 3 month lifespan but is rechargeable from
  AAs/power cord. Always keep AAs installed.

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The Position Page
When the GPS gets a position fix, the page
changes to the Position Page. This Page shows you
where you are, what direction you are going, and
how fast you are going. This Page is
customizable to show information you find most
useful. Current Position Coordinates and
Current Time and Date are also customizable
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The Map Page
The Map Page shows your movement using a
real-time track log (a bread-crumb trail that
appears on the screen as youre traveling). Your
present position is a pointer icon in the
center of the map. Main lakes, rivers, highways,
and towns are also shown. Use the Zoom Keys to
zoom in and out on the map to increase or
decrease detail.
Default setting is to keep the top of the Page
always pointing North.
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The Compass Page
The Compass Page shows the direction youre
traveling. This compass arrow is pointing to
S, so to travel south you would simply walk
forward. If you want to travel West, hold the GPS
in your hand and turn your entire body until the
compass arrow points to W, then walk
forward. Remember there is a lag time with all
Garmin models except the MAP76-S.
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The Highway Page
The Highway Page gives a different perspective of
your travel from the Map Page. For Garmin 12
series, the Highway Page is its own entity. For
the 72/76 series, you can select either the Map
or the Highway Page by pressing MENU when you are
on that page. When navigating, you want the
pointer to remain in the center of the highway
and be pointing forward.
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Collecting Waypoints

• Used to store locations of interest
• Must be named Garmin gives default integers
  (001, 002), or user can specify a name
• Stores the date and time the waypoint was
  recorded
• Can be averaged to improve accuracy 30-60
  seconds is adequate
• Garmin 12 series holds 500, Garmin 72/76 hold
  500, Garmin MAP76 holds 5000

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Collecting Waypoints

• Press and hold ENTER/MARK. The waypoint entry
  form will appear.
• The location you were at the moment you pressed
  the button is saved while you enter additional
  information.
• A unique default name is automatically assigned
  but can be changed at any time.
• Highlight SAVE and press ENTER to save the
  waypoint.

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Navigating to a Waypoint

• Press GOTO, select a waypoint, then use the
• Compass, Map, or Highway Page to navigate

GPS 12 12XL
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Navigating to a Waypoint

• All Garmins except the MAP76-S have a lag time
  between when you change direction and when the
  compass changes.
• Navigate as you would with a traditional compass
  site a landmark, travel to it, then check and
  realign your line of travel
• In most cases the GPS compass will be of little
  help once you get within 50 ft of your
  destination.

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Track Logs

• Setting a track log records a bread crumb trail
  of movement.
• Garmin 12s hold 1024 hits, Garmin 72/76s hold
  2048, Garmin MAP76 holds 10,000
• 72/76 and newer units can save 10 track logs, but
  they are compressed to the 256 points of largest
  change and do not contain time data.
• Track logs need NOT be contiguous. Turn the GPS
  off, travel, then turn it back on to create a new
  segment not connected to the old.

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Setting up the Track Log

• Must turn track log recording on, either from the
  Setup Page (12s) or Main Menu (72/76s).
• 3 choices for how a track log will record
• Off - Use to avoid erasing a track log that you
  are using or saving for a later
• WRAP - Keeps the latest information in history
  and will overwrite oldest data first if track log
  becomes full.
• FILL - Stops collecting data when the track log
  fills up. GPS will flash a message to indicate
  such.

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Setting up the Track Log

• 3 choices for frequency of recording hits
• Time Specify an interval in seconds. Use when
  needing to guarantee a time before filled.
• Distance Specify a distance traveled before a
  new hit is recorded. (N/A in Garmin 12 series)
• Automatic New hit is recorded when youve
  turned gt25 meters (82 feet) from your last point
  or have changed speed significantly from the last
  hit. Use when GPS memory is an issue, especially
  with MAP12.
• Can adjust this sensitivity to distance on 72/76
  series to LESS or MORE OFTEN.

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Viewing the Track Log

• Zoom into the Map Page far enough to see your
  movements along the track.
• To navigate using the Track log you need to see
  when you deviate from the course this requires
  you to be zoomed in close.
• If viewing in Landview, must zoom into location
  manually Landview doesnt zoom to tracks as it
  does to a current waypoint.

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Routes

• Routes are assembled, not recorded simply
  waypoints strung together by the user.
• Not awfully useful unless you need to navigate
  back to the same place often.
• Routes cant contain enough waypoints to be
  smooth, detailed line data
• Use tracks for collecting lines and areas,
  waypoints for collecting points.

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DNRGarmin Program

• Can be used stand-alone or from within GIS
  software (Landview or ArcView)
• DNRGarmin acts as abridge for data between a
  Garmin and a computer
• Items put into the bridge are not removed from
  their source (GPS or computer) must delete from
  the source separately
• Easiest, most efficient way to name, organize,
  and share waypoints and tracks

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DNRGarmin Program

• Utilizes the following formats
• Comma-delimited text file Easiest and smallest
  to archive if sharing with others who have
  DNRGarmin
• DBF file Best for importing to Access or Excel
• ArcView/Landview shapefile Best for sharing with
  other GIS users
• ArcView/Landview graphic/measure tool Best for
  quick drawing on screen, then uploading
• All formats can be opened and re-saved as other
  formats.

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Using DNRGarmin

• GPS must be recognized by computer
• Step 1 Get data from source into bridge
• WAYPOINT/TRACK DOWNLOAD
• FILE LOAD FROM
• Step 2 Edit data as you need to (names,
  selecting records to delete, etc)
• Step 3 Get data from bridge to destination
• FILE SAVE TO
• WAYPOINT/TRACK UPLOAD

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GPS Use Reminders

• Mission planning makes a difference! Weather,
  satellite constellation, location of GPSing,
  model of GPS all count
• Secure external antennas and other cords before
  GPSing
• Download existing data to a computer, then in
  most cases clear memory before beginning a new
  GPS session
• Always bring extra batteries
• Very cold and very hot weather can affect
  satellite signals and the GPS screen. Keep GPS
  tucked near your chest in cold temps, dont leave
  exposed on dashboard, etc in hot temps.
• No data is perfect there will always be
  post-processing!

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Where do we go from here?

• Handheld Devices
• http//thoreau.dnr.state.mn.us/mis/gis/tools/arcvi
  ew/Training/WebHelp/Training.htmUser_Meetings/200
  4Meeting/handheldcomp.doc
• Pendragon
• ArcPad
• Hotlinking Images to Waypoints
• Real-Time Tracking

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This concludes Getting to Know Garmin GPS
Theory and Use