GPS Exercise

A Tour of the Status Screen

 This GPS unit acquires fixes on up to twelve satellites orbiting at around 10,000 miles above the earth’s surface.  There are 24 satellites operational at any time.  Once outdoors, a series of numbers with horizontal boxes appear on the status screen below the satellite tracking circle.  These numbers identify individual satellites (satellite i.d.).  The horizontal boxes next to the numbers represent satellite signal strength. 

            If the signal strength is sufficient, a black box will appear around the satellite number, meaning that a signal fix has been established between the GPS unit and the satellite.  When at least four signal fixes are established, a “Position Acquired” window will flash across the screen, accompanied by a beep. It can take up to three minutes for the unit to get a fix on enough satellites to make it operational.   If battery strength is low, if there are a lot of obstructions like buildings and trees, or if satellites are all bunched in one part of the sky the unit might not be able get a satellite fix.  As long as numbers are flashing on the screen, the unit isn’t operational.

            The “Status” screen on the GPS unit also displays a satellite tracking circle. The outer most circle represents the horizon.  The inner circle represents an imaginary circle 45° above the horizon.  Also note line segments converging upon the center of the 45° circle.  These line segments converge on the sky’s zenith which is 90° above the horizon. 

            From the line segments one can also infer cardinal direction (for example, N, SE, W) at 45° increments.  The line pointing to the top of the display represents (if properly oriented) the cardinal direction of north (0°azimuth), the next line northeast (45°azimuth), the next line east (90°azimuth), etc...  Thus, the numbers on the screen identify individual satellites (satellite i.d.), their height above the horizon (satellite height), and their direction (whether cardinal direction or azimuth). 

            If you watch carefully you'll see satellites moving slowly across the screen.  This represents the actual orbital path of each satellite.  Note how some satellites leave the screen while others enter on to it, representing satellites dipping below and entering above the horizon, respectively. 

            Once position is acquired, look at the number located on the extreme left hand corner of the display.  This number represents the positional accuracy of the unit.  Before selective availability (designed to make GPS less accurate) was disabled in May, 2000, positional accuracy was usually around 100 m.  Now it is nearly always under 30 m and accuracies of under 10 m.  are not unusual.  What helps create positional accuracies of under 10 m.? 

       1.  A constellation of many satellites widely and evenly distributed.
2.      Few obstructions.
3.      Adequate battery strength
4.   Moving with the receiver rapidly. A GPS signal is more reliable if the user is covering a lot of ground.  This creates a bigger “signature” that satellites can interpret.

            Finally, the bar located on the lower right hand corner of the display represents battery strength.  When the black bar approaches “E” the unit will begin giving inaccurate readings.

Procedure:

 1.     Take the GPS unit out of its case and turn it on by pressing two seconds on the “PWR” button found on the lower right hand corner of the unit.  
2.     Press the “Pages” button located on the unit’s upper left hand corner.  A window with four options--”Status,” “Nav,” “Plot” and “Grp”--appears.  Scroll to the “Status” option by using the upward pointing white arrow key located in the center of the unit.  To view the “Status” screen, press the “Exit” button located above the “PWR” button.
3.     Read the “Background” discussion above.
4.     Assuming that your position has been acquired, answer the following questions.

a) Date and exact time of observation

Date:____________________            Exact Time: ____________________

b) On the answer key, complete the table below.  

 Just before turning off the unit answer these questions for the next quiz (this does not need to be answered on the exercise sheet):

What factors contribute to good GPS accuracy?

____________________            ____________________            ____________________

Navigating with GPS

1. Acquire a GPS signal fix at the manhole cover located on the northeast corner of Dow Science Building. This will be called the "Dow site."
2. Switch the datum of the unit to WGS 1984.  To do this, Press MENU, SYSTEM SETUP, SELECT DATUM, scroll to "WGS 1983."
3. Before navigating, delete all waypoints.  How?  Press Menu, System Setup, then scroll to Delete All WPTS.  A message, "Are you sure you want to delete wpts?" will appear.  Choose YES. This will clear any previous waypoints.  Press Exit to return to the Status Screen.
4. Press the WPT button.  A WPT menu will appear.  Scroll to the top of the screen, and use the arrow keys until accessing waypoint 1. 
5. Scroll to CREATE WPT.  Press right arrow key.  A four item menu appears.  Scroll to CURRENT POS.  Press arrow key. A message "WPT 1 CREATED" will appear. 
6. Press the EXIT key and access the STATUS screen.  Press the PAGES button and scroll to the bottom of the screen accessing the GROUP item.  Use the arrow key until accessing GROUP F.
7. Record the latitude and longitude plus UTM of WPT 1 at Dow:  Latitude and Longitude=___________ºN, ___________ºW  __________  UTM:  _________ m E, ______________ m N  
8. Walk quickly to the waypoint pointed out to you during class.  Watch the coordinates change as you walk.  Record position immediately upon arriving at the base of this waypoint:  ___________ºN, ___________ºW  __________   Record UTM:  _________ m E, ______________ m N    What happened to longitude?  _____________________________  What happened to the E reading of UTM? __________________________.  Are you closer or more distant from the prime meridian?  ________________.  In regard to UTM, are you closer or farther from this zone's principal meridian? _______________. Consult the UTM Grid Zones of the World site http://www.dmap.co.uk/utmworld.htm to see the position of this zone's principal meridian.
9. Navigate back to WPT 1 by Dow using the Nav 1 screen.  To do this, press the WPT button.  Go to the top of the screen and scroll to WPT 1, then scroll to GO TO WPT.   A message, "Navigating to WPT 1" will appear.  Note the following when navigating to WPT 1.  What is your average Ground Speed (GS)? _________.  What is the average direction you are traveling (TRK)?  _______________ º

Testing the Accuracy of GPS

1. Access the Acme Map Site.  http://mapper.acme.com/
2. Enter your zip at the bottom of the page.  Zoom (1 m/p) into a landmark close to your home (street corner, driveway intersection-someplace that a signal can easily be acquired) using the "image" theme.  Record the UTM value of this landmark.  This will be called the "Photo Waypoint."_________ m E, ______________ m N.  Be sure the resolution is 1 m/p for maximum accuracy.  
3. Acquire a GPS signal, make sure the GPS datum is set to WGS 1984.  Create a waypoint at this landmark.  This will be called the "Single Waypoint."  Record the UTM value. _________ m E, ______________ m N.
4. Create a second waypoint using the AVERAGE POSITION option.  Average10 positions. Record the UTM value. _________ m E, ______________ m N.
5. Create a third waypoint using the AVERAGE POSITION option.  Average100 positions. Record the UTM value. _________ m E, ______________ m N.
6. Of the three GPS waypoints recorded by your home, which value recorded in 3-5 above was most accurate? Record accuracy this way:  My closest waypoint was _________ m east/west (circle one) of the photo waypoint and _____________ m north/south of the photo waypoint.

Place all answers on the GPS answer sheet.  This exercise is due Monday, 2/28

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