Introduction to GPS
Mark Francek
Central Michigan University
1. Pair up
2. Origins of GPS
3. How GPS Works: go to http://www.pbs.org/wgbh/nova/longitude/gps/ “GPS-the
new navigation” and complete the short exercise how GPS works from a “hands
on” approach.
4. Sources of Error: (Source: Trimble)
Typical Error in Meters Standard GPS Differential
(per satellite)
(m)
GPS (m)
Satellite Clocks
1.5
0
Orbit Errors
2.5
0
Ionosphere
5.0
0.4
Troposphere
0.5
0.2
Receiver Noise
0.3
0.3
Multipath
0.6
0.6
5. Determining the best time to collect data: Trimble, for example, has a proprietary
“Mission Planning” program that will indicate the best time to collect GPS data
for any where in the world.
6. Tracking a GPS satellite: Go to Real Time Satellite Tracking
http://www.n2yo.com/ and report on the current location of NAVSTAR 57
7. Collecting GPS data in the field
a. Acquiring a signal and signal strength
b. Unit configuration
c. Creating a waypoint, record UTM coordinates for at least 2 waypoints
d. Navigating to a waypoint
e. Unit limitations
Introduction to GPS Field Exercise
Configuring the Unit
a) Setting the same coordinate system (we’ll use UTM (primary) and DMS (alternate))
and datum (WGS 84)
--Menu button, GPS Setup, Position Format, UTM, Exit
--Menu button, GPS Setup, Alternate Format, DMS, Exit
--Menu button, GPS Setup, Select Datum, WGS84 (on very top of menu), Exit
b) No sound when pressing buttons
--Menu button, System Setup, Audio/Screen, off, Exit
c) Using standard versus metric units
--Menu button, System Setup,Change Units, Statute, Exit
d) True north rather than magnetic north
--Menu button, System Setup,Change Bearing, Tru, Exit
e) Deleting previous waypoints
--Menu button, System Setup, Delete all WPTS, Yes, Exit
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f) Deleting previous routes
--Pages Button, Map 1, Menu Button, Map 1 Setup, Trail Options, Clear Trail, Yes, Exit
Getting our Bearings
--Which way is north?
--Cardinal direction vs. azimuth, N=0, NE=45, E=90, SE=135, S=180, SW=225, W=270,
NW=315,
Position/Navigation Screens
This unit has four screens: status, navigation, map, and window screens. Use the PAGES button
to switch between screens, ARROW keys to move between screens, EXIT button to acquire
erase a screen menu.
Explanation of Status Screen
--Satellite location
--Signal Strength: Limitations imposed by terrain, time of day, battery drain
--Position Acquired Signal
--PS
--3D vs 2D
Explanation of Navigation Screen 2
There are two different modes: Nav 2 shows all navigation details in large digital numbers. Nav
1 screen shows a graphical view of your trip.
To acquire the Nav 2 view, press the PAGES key, press the up and down arrows until nav is
highlighted. Press the right or left arrow key to view the Nav 2 view. Eight digital boxes appear.
--TRK: current direction you are travelling BRG: direction to destination (waypoint)
These are the two most important boxes. Adjust course so as to minimize deviation between
these two boxes
Other boxes:
--GS: ground speed (if you are not navigating to a waypoint, TRK and GS will be the only boxes
that are active
--DIS: distance to waypoint
--ETE: estimated time en route (ETA)
--XTK: cross track error (distance you are off-course to the side of the desired course
line)
--CDI: course deviation indicator, shows both the direction and distance you are off course.
Always steer toward the center line to get back on course
Creating a Waypoint
A named location representing a point on earth is called a “waypoint.” Coordinates and
waypoints are datum dependent. You can save your current position, a cursor position, or a
coordinate location. We will create 2 waypoints and then navigate to our first waypoint.
1. Press the WPT button, highlight the WPT label at the top of the screen using arrow key, use
left or right arrow until you get to waypoint 1.
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2, Highlight “create wpt” option, press right arrow, a menu appears, highlight “current position”,
press right arrow, a “waypoint
created” message briefly appears.
Navigating to a Waypoint
1. Press the WPT button. Highlight
the WPT button at the top of the
screen, use the right arrow to select
wpt.
2. Scroll down to the “Go to WPT”
label, press right arrow. You will
automatically be placed in the NAV2
screen mode. We will navigate to
WPT. Record the number of feet
“off” the receiver is to the actual
waypoint location. What accounts for error?
Checking our accuracy with the Acme Mapper Web Site
http://mapper.acme.com/?
Converting Between Formats: Menu-GPS Setup-Position Format-DM, DMS, or UTM. Example
of DM=43 36.206’ N Example of DMS=4336’12.4” N, Example of UTM 4830260 N
For these examples: converting DM to DMS multiply .206 by 60. Converting DMS to DM
divide .206 by 60
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Appendices
The following three excerpts are taken verbatim, from the work of Joseph Kerski, USGS
and “How Things Work”
Appendix 1
GIS Course at Sinte Gleska University:
Lakota Studies 400/600: Special Topics: Introduction to Geographic Information
Systems and Science
Instructor: Joseph J. Kerski, USGS, jjkerski@usgs.gov, 303-202-4315
http://rockyweb.cr.usgs.gov/outreach/sgu/coordinatenotes.html
Week 7 Notes: Coordinate Systems
Referencing to Real-World Coordinates, Part 1: Coordinate Systems
As you are aware by this time in the course, everything within a GIS is geo-referenced.
That is, rather than being a simple map in a computer paint program or graphics program,
all GIS data is referenced to positions on the Earth's surface. But, how do we know
where things are located on the Earth's surface in the first place? The science of geodesy
is concerned with these topics, and geodesy is therefore one of the sciences associated
with geographic information sciences. Why should we care about such topics? We want
the data for a particular project to overlay, or "match up." Not paying attention to
coordinate systems and datums could mean errors and frustration with your GIS projects
in the future.
We know where things are located on the Earth by using coordinate systems and geodetic
datums. This week's notes will describe coordinate systems. Next week's notes will
discuss geodetic datums. Both will be important in the lab exercises and throughout your
future journey with GIS. The most commonly used coordinate system today is the
latitude, longitude, and height system. The Prime Meridian and the Equator are the
reference planes used to define latitude and longitude. The geodetic latitude of a point is
the angle between the equatorial plane and a line normal to the reference ellipsoid. Here,
the "ellipsoid" refers to the shape of the Earth--not a sphere, but an ellipse, because it is
slightly flattened at the poles. The geodetic longitude of a point is the angle between a
reference plane and a plane passing through the point, both planes being perpendicular to
the equatorial plane. The geodetic height at a point is the distance from the reference
ellipsoid to the point in a direction normal to the ellipsoid.
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We use several coordinate systems to reference positions on the Earth's surface. These
positions have been used on maps, and therefore, they find their way into digital map data
and GIS. The most common ones in North America are latitude/longitude, UTM, and the
State Plane Coordinate System.
Latitude/Longitude
Latitude/longitude begins at the Prime Meridian, at 0 degrees longitude, which runs
through Greenwich, England. The equator is 0 degrees latitude. Here in North America,
longitude is measured west of the Prime Meridian and north of the equator. West
Longitude is given in negative units in all GIS software. North Latitude is given in
positive units. Latitude/longitude is a geographic coordinate system; it is not tied to any
particular map projection.
Universal Transverse Mercator System
The UTM (Universal Transverse Mercator) coordinate system is a rectangular coordinate
system tied to the Transverse Mercator projection. It divides the Earth into 60 zone
numbers of 6° wide longitudinal strips extending from 80° South latitude to 84° North
latitude. UTM coordinates define two dimensional, horizontal, positions. Each zone has
a central meridian. For example, zone 14, for much of South Dakota, has a central
meridian of 99° west longitude. The zone extends from 96 to 102° west longitude.
Locations within a zone are measured in meters eastward from the central meridian and
northward from the equator. Eastings increase eastward from the central meridian which
is given a false easting of 500000 meters so that only positive eastings are measured
anywhere in the zone. Northings increase northward from the equator with the equator's
value differing in each hemisphere. In the Northern Hemisphere, the Equator has a
northing of 0. In the Southern Hemisphere, the Equator is given a false northing of
10,000 km.
State Plane Coordinate System
State plane systems were developed in order to provide local reference systems that were
tied to a national datum. In the USA, the State Plane System 1927 was developed in the
1930s and was based on the North American horizontal Datum of 1927. The coordinates
are in English units (feet). The State Plane System 1983 is based on the North
American Datum of 1983 and the coordinates are metric. While the NAD27 State Plane
System has been superceded by the NAD83 System, maps and digital data in NAD27
coordinates are still in widespread use. For example, most USGS 7.5 Minute topographic
maps show State Plane coordinates from 1927. Each state has its own State Plane system
with specific parameters and projections. Some smaller states use a single state plane
zone while larger states are divided into several zones. State plane zone boundaries often
follow county boundaries.
Appendix 2
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GIS Course at Sinte Gleska University:
Lakota Studies 400/600: Special Topics: Introduction to Geographic Information
Systems and Science
Instructor: Joseph J. Kerski, USGS, jjkerski@usgs.gov, 303-202-4315
Week 8 Notes: Datums
Referencing to Real-World Coordinates, Part 2: Datums
http://rockyweb.cr.usgs.gov/outreach/sgu/datumnotes.html
As you know, everything within a GIS is geo-referenced. All GIS data is referenced to
positions on the Earth's surface. But, how do we know where things are located on the
Earth's surface in the first place? As you read last week, we have been dealing with
topics in the science of geodesy. Because we want our data for a particular project to
overlay, or "match up," we need to pay attention to coordinate systems (last week) and
datums (this week).
Datums
We know where things are located on the Earth by using geodetic datums. These datums
define the reference systems that describe the size and shape of the Earth, and the
orientation of the coordinate systems used to map the Earth. These can be "flat-earth"
models used in plane surveying on a local scale to complex models for applications that
span continents, and can describe the size, shape, orientation, gravity, and angular
velocity of the planet. Just like there is no "one best map projection," or "one best
coordinate system," there is no "one datum is best" model. Different nations and
organizations use different datums for GIS and navigation systems, depending on their
needs.
True geodetic datums were employed only after the late 1700s, when measurements
showed that the Earth was an ellipse; close to a sphere, but the circumference is longer
around the equator than from pole to pole. The slight flattening of the earth at the poles
results in about a twenty kilometer difference at the poles between an average spherical
radius and the measured polar radius of the Earth. If the GIS user is not aware of the
issues surrounding datums, the positions on the map can be off by hundreds of meters.
Reference ellipsoids are usually defined by the semi-major axis (equatorial radius) and
flattening (the relationship between equatorial and polar radii). Some geodetic datums
are based on ellipsoids that touch the surface of the earth at a defined point. For our
purposes, the North American Datum 1927 (NAD27) is commonly used. It is tangent to
the mean sea level surface at Meades Ranch in Kansas. NAD27 is not a global datum,
but only for North America. Some USGS data is referenced to NAD 27. Other datums
are "topocentric" datums with a reference ellipsoid that has its center at the center of mass
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of the earth. The World Geodetic System 1984 (WGS-84) is an example of a global
datum. These global datums can be better fits to the gravity surface for the entire earth
but can be less accurate in specific areas. WGS 84 is the basis for that used in Global
Positioning Systems (GPS) units.
Two Types of Datums
Two major types of datums exist -- horizontal and vertical. Horizontal datums define the
relationship between the physical Earth and horizontal coordinates such as latitude and
longitude. The North American Datum of 1927 is an example. Vertical datums define
the height of surfaces, such as the National Geodetic Vertical Datum of 1929 (based on
sea-level measurements and leveling networks) and the North American Vertical Datum
of 1988, based on gravity measurements. Some USGS and other commonly used data is
in the 1929 datum, and others are in the 1988 datum. The WGS 84 datum describes both
vertical and horizontal coordinates. It considers the rotation rate of the Earth and various
physical constants such as the angular velocity of the earth and the Earth's gravitational
constant. We will be using several datums here in this course.
Metadata
Metadata files are reference files about your spatial data, including who created the data,
when it was created, what the data contains, and other information. These files should
indicate the horizontal and vertical datums for the spatial data.
Back to SGU GIS Course Home
Appendix 3
How your GPS rivals the accuracy of an atomic clock?
Every satellite contains an expensive atomic clock, but the receiver itself uses an ordinary
quartz clock, which it constantly resets. In a nutshell, the receiver looks at incoming
signals from four or more satellites and gauges its own inaccuracy. When you measure
the distance to four located satellites, you can draw four spheres that all intersect at one
point. Three spheres will intersect even if your numbers are way off, but four spheres will
not intersect at one point if you've measured incorrectly. Since the receiver makes all its
distance measurements using its own built-in clock, the distances will all be
proportionally incorrect. In order to make this measurement, the receiver and satellite
both need clocks That can be synchronized down to the nanosecond. To make a satellite
positioning system using only synchronized clocks, you would need to have atomic
clocks not only on all the satellites, but also in the receiver itself. But atomic clocks cost
somewhere between $50,000 and $100,000, which makes them a just a bit too expensive
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for everyday consumer use. The receiver can easily calculate the necessary adjustment
that will cause the four spheres to intersect at one point. Based on this, it resets its clock
to be in sync with the satellite's atomic clock. The receiver does this constantly whenever
it's on, which means it is nearly as accurate as the expensive atomic clocks in the
satellites. http://electronics.howstuffworks.com/gps5.htm
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