Mark Francek-Introduction to GPS



Introduction to GPS

Mark Francek

Central Michigan University

1. Pair up

2. Origins of GPS

3. How GPS Works: go to “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 (m) |Differential GPS (m) |

|(per satellite) | | |

|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 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

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.

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

?

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

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@, 303-202-4315



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.  

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

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@, 303-202-4315

Week 8 Notes:  Datums

Referencing to Real-World Coordinates, Part 2:  Datums



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 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 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.

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