G. P. S. BASICS

CHAPTER ONE How GPS Works & Basic Navigation Operating Principles: The basis of the GPS technology is a set of 24 satellites that are continuously orbiting the earth. These satellites are equipped with atomic clocks and send out radio signals as to the exact time and their location. These radio signals from the satellites are picked up by the GPS receiver. Once the GPS receiver locks on to four or more of these satellites, it can triangulate its location from the known positions of the satellites. This is a very simple explanation, but unless you are a surveyor or engineer who needs to understand how to use GPS to locate within fractions of an inch, this is all you really need to know. Regarding the issue of time, UTC time is the basis of all GPS time functions and calculations. If nothing else, in owning a GPS receiver, you have in your possession the most accurate time piece available. Your receiver updates itself from the atomic clocks on the satellites. It is also very important for you to understand that your receiver must know the time difference between your location and of Greenwich England or UTC time. This is a function in the set-up of all GPS receivers. With many GPS manufacturers, this is referred to as Offset which is referring to the offset or difference in time zones from the present location to UTC time. The functionability of a receiver is dependent on the ability to receive signals from the satellites. Certain locations such as under very thick foliage or down in the bottom of a slot canyon will preclude your receiver from getting a good signal from enough satellites to determine your location. With many of the newer receivers however, these problems are minimal. All receivers have warning messages when they are not getting sufficient signal to properly navigate. Accuracy: The accuracy of the receivers is dependent on the number and quality of the signals it is getting from the satellites and from a factor called Selected Availability. Selected Availability is an intentional error that is introduced into the signals coming from the Satellites that create readings that can be off as much as 300 feet. Even so, the accuracy levels with Selected Availability turned on, is usually within 100 ft. or better. Other factors effect accuracy such, quality of GPS signals from the satellites, and operator error. Operator error can include such things as inputting the wrong values for position coordinates and as using the wrong datum for the map being referenced. These issues will be covered in Chapter two. Page -1- GPS Receiver Set-Up: To be able to properly use a GPS receiver, it needs to be set-up and initialized. Set-up establishes the basic information about the units of distance, speed, Map Datum, Navigation Grid system, time difference from Greenwich England or UTC time, and other basics. The user’s manual that comes with each GPS receiver gives detailed instructions on the process of selecting the options for initialization and set-up. This must be done to be able to use the unit for navigation. Most Common Set-up Components: 1. Initialization Initialization is the process of telling the receiver your approximate location on the surface of the earth. This must be done the first time you use the receiver or if it has moved more than 300 miles from the last location where it was being used. Otherwise, it will take an unreasonable amount of time for the receiver to establish what is called a Position Fix. 2. Units, for speed, distance etc. Self explanatory, units of feet, meters, miles. 3. Grid System Latitude & Longitude or UTM can be selected. Lat./Long usually has choices of Degrees, Minutes, & seconds or Degrees, Minutes, and ,oo Minutes (instead of seconds). 4. Datum This references the map that coordinates will be plotted on or taken from. A common datum is NAD27 (see glossary). 5. North reference Either Magnetic North (same as compass) or True North. 6. Time Offset. From UTC Time zone difference from Greenwich England. Once the set-up has been completed and the unit has been initialized, it will then lock on the signals of three or more satellites and establish a Position Fix. The Position Fix is the calculated position of the receiver’s current location. Page -2- GPS Receiver Basic Use : Once the receiver is initialized and set-up, the most useful and immediate function is to save the current position as a waypoint. Saving Current Position as a Waypoint: To save the current position as a named or numbered waypoint you must access the function for your unit that does this. On many of the Garmin units, the "MARK" button is specifically for this purpose. Other units may access a menu first where "Create Waypoint" or some other related option is available. Usually, the current position coordinates are then displayed and can be edited if you choose to create a waypoint that is not the current location. If you are saving the current position, you then proceed with the menu choices to name and save the waypoint. With many units available, the waypoint will automatically be assigned a sequential number that can be changed to a name of your choosing. This is so a waypoint can be saved quickly and the name noted. You can go back later and rename it if you choose or you can rename it as you are saving it initially. The naming process is usually simple using the up and down arrows of your keypad to choose various letters and the left and right arrows being used to move to the different characters. This process will be adequately described in the user’s manual. GOTO Once a location has been saved as a waypoint, the next obvious activity will be to navigate to that waypoint when you are away from it. In almost all GPS units this is called the "GOTO" function. A classic example would be if you were going hiking or camping and acquired a position fix at your camp and named it something like "CAMP399". Try and use something descriptive enough so as not to be confused with other names. In our example, we put the month and year at the end of the name so we will know more about it. It is important for you to understand that you will get confusing headings and distances using the GOTO, if you don’t get more that about ½ mile away first. If you activate the GOTO right after saving the waypoint and are essentially in the same location, you are very likely to get indications that it is .2 to .3 miles away. This is primarily due to Selected Availability errors, but may be confusing if you don’t understand the problem. When you activate the GOTO, the receiver will then go into the navigation mode and you will have on your display any of a number of "Navigation Screens" available. There are options to select various Navigation Screens and a default one can usually be established in Set-up. The main types of screens available are: Highway - This screen looks like a highway and shows the direction you are progressing toward the destination. You will have values displayed for Heading, Distance, and Speed. CDI - Course Deviation Indicator: This has a horizontal graph usually towards the bottom of the screen with the center representing being on course. If you deviate to the left or right, a pointer or vertical line will indicate that you are to the left or right of course and a numeric value will usually indicate by how much. This screen also has values displayed for Heading, Distance, and Speed. Page -3- Compass Card - This display shows a set of compass values with a pointer indicating what direction you are traveling. This screen also has values displayed for Heading, Distance, and Speed. There is some variety in Navigation Screens, but the essential information on Bearing, Heading, Distance and Speed are always displayed. It is important to understand the difference between Bearing and Heading when navigating to a waypoint. Bearing - This is the compass heading (When Magnetic North is in Set-up) to the waypoint. Heading - This is the direction you are traveling. Ideally if terrain were not a consideration, the Heading would be the same as the Bearing. Notes on Navigation: Compass Most GPS receivers do not have an internal compass although a very few have that as an added feature. Therefore, the receiver does not act as a compass and only indicates Bearing and the direction you are traveling no matter which way it is being pointed. A compass is a very handy tool to have with you. When you choose a GOTO and get a Bearing to the waypoint, you can check your compass so you can start out on the right direction. Otherwise, you can spend some time walking in different directions until your receiver can pick up your Heading which you will be likely to need to correct significantly. Battery Life Battery life of the receivers varies drastically from one model to another so conserving batteries may or may not be a concern to you. Also, there may be a large discrepancy between what the manufacturer states as battery life and what you really get. Therefore, in many instances, you may want to turn your unit on at a given location, get a position fix and save it as a waypoint and then turn the unit back off. This is good when you want to save a waypoint such as camp or someplace you later want to return to. When navigating however, unless you have large distances or blocks of time where you know you will be on the same course, you probably will leave the unit on the whole time. Page -4- Backtrack Most receivers have a plotting function which plots your course of travel if you leave the unit turned on while you are traveling. By choosing the Backtrack function, the unit will create multiple waypoints from your route of travel and automatically navigate you from one to the next in reverse order that they were originally traveled. These Backtrack waypoints are automatically numbered by the receiver in sequential order so when you get to the first one, you will be back at your starting position when the unit first established a Position Fix for that session. Summary of Basic GPS Use: For Basic GPS use, you only need to understand the most basic of the receiver’s functions. Set-up and initialization functions, Saving a Waypoint, and using the GOTO feature. For this level of use, you don’t need to understand the coordinate grid systems, datums or how to use maps. Therefore, a new GPS user can make good use of the unit with the minimal amount of experience reading the manual and working the menus and keypad. Those who need to refer to maps and use them to plot and read position coordinates must understand at least one of the coordinate grid systems. Once the grid system is understood and its coordinate references can be identified on maps, it is a simple matter to learn to use the tools and techniques to properly identify coordinates. Page -5- Top Introduction | Contents | Chapter 1 | Chapter 2 | Chapter 3 | Glossary ©1997-2007 Waypoint Enterprises. All Rights Reserved If you surfed in, via search engine, etc., This is a remote page Please click the GPS Scale for our main index page.