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Navigation 101

Author: Lowrance

Learn the many aspects of navigating with a GPS unit starting with the basics and moving step by step to a full understanding of this amazing technology.

GPS Navigation 101


To turn on your Lowrance GPS unit, press the PWR key. Read the message on the screen that appears, 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 power 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 it's 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

There are two ways Lowrance products show satellite information.

Products such as the LMS-350 and GlobalMap 12 show you in a chart format which satellites the GPS is locked onto and using for navigation, how well it is receiving them, and their positions relative to your position. The following information is shown on this screen:

CHN: The channel of the receiver. (One for each of the twelve channels.)

PRN: Pseudo Random Number or the number of the Satellite.

TRK: S for Searching, and T for Tracking the satellite.

ELV: The Satellite's elevation above the horizon.

AZM: The Azimuth or compass bearing to the satellite.

SNR: Signal to Noise Ratio, the higher the number, the clearer the signal.

DOPS: Dilution of Precision, The lower the number, the more accurate the position fix.
GPS screen.
Other products such as the GlobalMap 100 and LMS-160 have simplified the display of this 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.

On the screen shown, 9 satellites are in view. The unit is tracking satellites 25 and 29. Thus it is not locked on yet as only 2 satellites are being tracked. Satellite 14 is on the horizon to the NorthWest. Satellite 30 is about 30 degrees up from the horizon to the East. Satellite 29 is 45 degrees up from the horizon to the East NorthEast.

The display also shows the EPE (setimated position error) in feet or meters. This will be in the upper left hand corner of the screen once it locks on. This is an estimate of the accuracy of your position depending on the geometry of the location of the satellites tracked and other factors.

The vertical bar on 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 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 already stored in memory, the waypoint can simply be recalled and the unit instructed to navigate to waypoint; or 2. If plotted 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 utilize 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 mini segments 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 utilize 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 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, without having to recall another waypoint in the unit. Once programmed into the GPS unit, the route provides the option of navigating forward through the waypoints, or reverse in order to go either direction through the route.

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. And 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

© 2000 Lowrance Electronics, Inc