GPS Overview - Lecture Material - Completely GPS, GIS dan Remote Sensing tutorial -
GPS Overview

What Is GPS? 
The Global Positioning System (GPS) is a space age navigational system that can pinpoint your position anywhere on the globe, usually within a few yards or meters. This amazing technology is available to everyone, everywhere, day and night, and best of all, at no cost for use of the navigational data. GPS uses a constellation of 24 satellites in precise orbits approximately 11,000 miles above the earth. The satellites transmit data via high frequency radio waves back to Earth and, by locking onto these signals, a GPS receiver can process this data to triangulate its precise location on the globe.

GPS operates 24 hours a day, in all weather conditions, and can be used worldwide for precise navigation on land, on water and even in the air. Some of its many current applications include: boating, fishing, hunting, scouting on land or from the air, hiking, camping, biking, rafting, pack trips by horseback, hot air ballooning, general aviation, snowmobiling and skiing, search and rescue, emergency vehicle tracking, 4 wheeling, highway driving and a host of other outdoor activities where accurate positioning is required.

How GPS Determines Your Position
GPS uses satellite ranging to triangulate your position. In other words, the GPS unit simply measures the travel time of the signals transmitted from the satellites, then multiplies them by the speed of light to determine exactly how far the unit is from every satellite it's sampling.

By locking onto the signals from a minimum of three different satellites, a GPS receiver can calculate a 2D (two-dimensional) positional fix, consisting of your latitude and longitude. By locking onto a fourth satellite, the GPS can compute a 3D (three-dimensional) fix, calculating your altitude as well as your latitude/longitude position.

In order to do this Lowrance uses a 12 parallel-channel receiver in all of its current products. Three of the channels lock on to satellites for triangulation. Another channel locks on to a fourth satellite for 3D navigation, which lets the unit calculate altitude in addition to latitude and longitude. These four channels continuously and simultaneously track the four satellites in the best geometrical positions relative to you. The additional eight channels track all other visible satellites, then add this data to the data from the original four satellites. The unit then over-resolves a solution, creating an accuracy-enhanced reading. The additional channels also ensure reliable, continuous and uninterrupted navigation, even in adverse conditions such as valleys or dense woods.

GPS Accuracy
GPS was conceived in the 1970s, and is controlled by the United States Department of Defense. Although GPS was initially envisioned for military use, the Government realized early on that there would be numerous civilian applications as well. Subsequently, the Department of Defense (DOD) created two transmission codes; the P code (Precision code) for military use, and the C/A code (Civilian Access code) for civilian use.

The highest accuracy levels were to be reserved for the military so as to prevent hostile enemy attacks against the U.S. using our own navigational system. However, once in operation, the civilian GPS receivers using the C/A code proved to be more accurate than the DOD had intended. Consequently, the military developed a system for randomly degrading the accuracy of the signals being transmitted to civilian GPS receivers. This intentional degradation in accuracy is called Selective Availability or S/A. This reduced the civilian GPS accuracy levels to being within 100 meters or less, 95% of the time. However, typical accuracy for most users averaged between 20 and 50 meters the majority of the time. You could easily see the effects of S/A on a GPS receiver when you were not moving. Typically, there would be random movements in speed, altitude and position readings, along with slow position "wandering" on the plotter trail. This was easily seen when you were on a .1 or .2 mile zoom range and not moving. For example, while parked at the dock in your boat, you would see unexplainable changes in your digital speed readings up to a few miles per hour, even though you were not moving.

Plot of position accuracy using standard Lowrance GPS receiver (stationary). Note the differences in scale. 5.5 hour period immediately prior to shutoff of selective availability 8 hour period immediately after shutoff of selective availability longitude meters | longitude meters 

Effective May 2, 2000 selective availability (S/A) has been eliminated. The United States Department of Defense now has the technology to localize the control system to deny GPS signals to selected areas. It is not often that your electronics products increase in value after you've purchased them. Now boaters, aviators, drivers, hikers, hunters, and outdoor enthusiasts of all types can locate their position up to ten times more precisely (within 10 to 20 meters) and navigate their way through unfamiliar terrain. Anglers can now return to their favorite spot on a lake or river instead of just their favorite area. A Lowrance GPS receiver in combination with advanced technology of today's GPS management will take you anywhere you want to go.

The decision to allow civilians so much accuracy in location information was finally made because GPS is continually playing a more important role in the lives of people around the world - it's becoming a national utility. GPS is the global standard in navigation because it is completely free of charge to the public.

Differential GPS (DGPS)
Differential GPS, or DGPS, has been developed to improve GPS accuracy to within a few meters. DGPS was originally initiated by the U.S. Coast Guard to counter the accuracy degradation caused by Selective Availability. Even with S/A now eliminated, DGPS continues to be a key tool for highly precise navigation on land and sea. DGPS technology adds a land-based reference receiver – located at an accurately surveyed site – to the other GPS components. This non-moving DGPS reference station knows where the satellites are located in space at any given moment, as well as its own exact location. This allows the station to compute theoretical distance and signal travel times between itself and each satellite. When those theoretical measurements are compared to actual satellite transmissions, any differences represent the error in the satellite's signal. All the DGPS reference station has to do is transmit the error factors to your DGPS receiver, which gives the information to the GPS receiver so it can use the data to correct its own measurements and calculations.

The two most common sources of corrective DGPS signals currently are: (1) Coast Guard, land-based beacon transmitters, broadcasting the data at no charge to the public, covering all coastal areas and much of the inland USA as well; and (2) FM radio sub carrier transmissions available both in coastal and inland areas, but limited to paid subscribers. In order to receive DGPS correction data from Coast Guard beacon transmitters, a mobile GPS unit requires a separate beacon receiver. And to receive FM sub carrier DGPS signals from local subscriber radio stations, the GPS unit requires a separate FM receiver, normally the size of a pager. Naturally, your GPS unit must have the capability to both receive and process DGPS data.

Wide Area Augmentation System (WAAS)
GPS is plenty accurate for route navigation, but the U.S. Federal Aviation Administration has special need for aircraft traffic control that go beyond basic GPS. The FAA has a plan under way to boost GPS performance even further with its Wide Area Augmentation System, or WAAS. This GPS add-on will include a time control element that will help airliners fly closer together while avoiding collisions. In addition to carefully spacing airplanes along travel corridors, WAAS will eventually make instrument landings and takeoffs more accurate as it replaces existing aviation navigation systems.

Non aviators can use WAAS signals to make their GPS navigation even more accurate. However, WAAS has some limits you should know about.

First, the U.S. government has not completed construction of the WAAS system, so it is not yet fully operational. The ground stations are in place, but only a few of the needed WAAS satellites have been launched.

WAAS can boost the accuracy of land GPS navigation, but the system is designed for aircraft. The satellites are in a fixed orbit around the Equator, so they appear very low in the sky to someone on the ground in North America. Aircraft and vessels on open water can get consistently good WAAS reception, but terrain, foliage or even large man-made structures frequently block the WAAS signal from ground receivers.

You'll find that using your GPS receiver without WAAS is both easy and amazingly accurate. It's easily the most accurate method of electronic navigation available to the general public today. Remember, however, that this receiver is only a tool. Always have another method of navigation available, such as a map or chart and a compass.

GPS Navigation 101 Power
To turn on your Lowrance GPS unit, press the PWR key. Read the message which appears on the screen, then press the EXIT key to erase it. Your Lowrance GPS unit is now ready for use. To turn your Lowrance GPS unit off, press and hold the PWR key for three seconds. A countdown until shutoff will appear on the screen.

New GPS Receiver Initialization
When your unit is turned on for the first time, it does not know where it is, the date or the time. To initialize a GPS unit is to basically tell the receiver where it is, what the date is, and what the time is. This allows it to know which satellites should be overhead, so it can start searching for them to lock onto them. When a new GPS receiver is first powered-up, even if it is not initialized, it can still determine its position after a few minutes. It will however, achieve a much faster satellite lock-on if it is initialized.

To lock onto satellites, a GPS receiver must first find them. If you simply turn on the GPS receiver and wait, it may take more than two minutes to find and lock onto the appropriate satellites. That's referred to as a cold start. In contrast, when initialized by the user, the GPS receiver typically takes only a few seconds to lock onto the satellites. The GPS receiver should have a clear view of the sky during initialization.

Initialization requires that you provide the GPS receiver up to 3 pieces of information:

  1. Your approximate present position in latitude/longitude;
  2. Your approximate elevation, or altitude: and
  3. The current local time and date. Normally, initialization is necessary only once, provided each subsequent time the GPS receiver is turned on it's within approximately 300 miles of where it was last turned off. Regardless of which start up method is chosen, initialization or cold start, once the GPS receiver has achieved satellite lock on, it will typically begin tracking much faster the next time it's turned on, often within seconds.

The Satellite Information Screen
A simplified screen displays this satellite information by putting it into a graphical format (see screen). For each of the 12 channels a SAT number (satellite number) is shown along with a bar graph showing the relative strength of the signal. A circular overhead view of the satellite position in the sky is above this information. The center of the circle corresponds to a satellite position directly overhead. The edges of the circle are at the horizon. The top of the circle is North. If the satellite number is highlighted, it is being tracked and data is being measured from it. The display also shows the EPE (estimated position error) in feet or meters. This will be in the upper right hand corner of the screen once it locks on. This is an estimate of the accuracy of your position. It depends on the geometry of the location of the satellites tracked, and other factors.

The vertical bar on the bottom of the screen is the battery life left (on portable models only).

Saving Waypoints in Memory
A waypoint is a position you wish to save and return to later. GPS receivers typically offer two methods to store waypoints in memory: 1. The Quick Save method, which uses the coordinates from either your present position, or those from the cursor position in the plotter mode. In this method, the waypoint is automatically identified with the next available waypoint number in the list; 2. The View & Save method, which lets you pick the specific waypoint number under which you want to store the new waypoint. You can also name the waypoint during the same procedure.

Using GPS to Navigate to a Waypoint
There are three basic methods you can use to navigate to a waypoint: 1. If it's already stored in memory, the waypoint can simply be recalled and the unit instructed to navigate to the waypoint; or 2. If it's determined from a navigational chart or communicated by some other means, the waypoint can be entered using the unit's keypad, then navigated to; and 3. On the plotter, the cursor can be used to pinpoint the location of a waypoint, then the unit instructed to navigate to the cursor position. All three techniques employ easy-to-understand, on-screen menus, guiding the user through every step.

Straight Line Navigation
GPS products use what is called "straight line" navigation. The units, when commanded to navigate to a waypoint, draw a straight line from their present position to the destination waypoint. The straight line represents the shortest, most direct route to the waypoint.

One very important point must be made about "straight line" navigation: It does not take into account any obstacles in the path (on land, in the air or in the water). Consequently, it may be necessary in some situations to record interim waypoints that alter the course to navigate around obstacles. These additional minisegments of the journey will each represent straight line routes. New GPS users should be cautioned to take these considerations seriously, and to never rely solely on a single navigation aid.

Using a Route to Bypass Obstacles
Since GPS products use straight line navigation, it is necessary to use a waypoint at each place you need to turn when you are navigating around an obstacle such as a cliff, or navigating down a highway or river channel. By connecting each of these waypoints in a chain, you form a "Route". This provides the automatic capability to navigate through several waypoints in order, without having to manually recall another waypoint in the unit. Once programmed into a GPS unit, a route provides the option of navigating forward through the waypoints, or navigating in reverse in order to go either direction through the route.

Using Plot Trails to Find Your Way Back
One of the most important features in a Lowrance GPS unit is the ability to display, save and navigate plot trails. This is the feature that allows you to retrace your steps or repeat a journey at a later date, and it's especially useful when navigating in roadless areas. A plot trail is a line plotted or drawn on the screen tracing the path you've taken, from your starting point to your present position. 

A plot trail appears on your GPS screen as a line that flashes once per second, for easy viewing. Your GPS plots a trail by placing a position marker dot on the screen every three seconds as you travel. (This can be adjusted from one dot per second to one dot per 30 minutes, or you can update your trail by distance instead of time.) With menu commands, you can save, recall and navigate a trail (forward or backward) just as you would a route. Navigation steering information is provided in either a compass rose display (with an arrow pointing out the correct travel direction) or as a dotted line on a unit's map display. Or, you can skip the navigation commands and simply retrace your path following the flashing plot trail on the map display.

GPS NMEA Interface with Other Electronics Devices
NMEA is an abbreviation for the National Marine Electronics Association, the group that establishes the data protocol and wiring standards for the marine electronics industry. As previously discussed, some GPS units can receive DGPS data from beacon and FM receivers. GPS receivers must also be able to send standard positioning and navigational information to a variety of listener devices such as charting instruments, autopilots and others. Most quality built GPS products permit their users to select from two different NMEA data protocols that transmit data output sentences. The first protocol is NMEA 0180, which is reserved strictly for sending steering information, primarily to marine auto pilots. The second protocol, NMEA 0183, sends latitude/longitude position, steering, speed and other navigational data. Depending on the specific GPS product, these NMEA protocols are in code versions 1.5 and/or 2.0.