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Learning About GPS
The Global Positioning System (GPS) is a satellite-based navigation
system made up of a network of 24 satellites placed into orbit by the
United States Department of Defense.
GPS was originally intended for military applications, but in the
1980s, the government made the system available for civilian use.
GPS works in any weather conditions, anywhere in the world, 24
hours a day. There are no subscription fees or setup charges to use
GPS. For more information refer to the Garmin Web site at
www.garmin.com and also read the GPS Guide for Beginners, which
can be found on the Web site at
Common GPS Terms
Initialize —the ﬁrst time a GPS receiver orients itself to its current
location and collects data. After the receiver is initialized, it
remembers its location and acquires a location more quickly.
Location—an exact, unique location based on geographic
coordinates (Latitude and Longitude).
Route—A group of waypoints entered into the GPS receiver in the
sequence you want to navigate them.
—a location you store in your GPS.
What is Differential GPS (DGPS)?
The United States and Canadian governments (among others) have
set up Differential GPS (DGPS) stations to transmit correction
signals. They are operational in coastal areas and on many navigable
The DGPS system is available for use without a fee, but you do need
additional equipment to receive DGPS signals: A beacon receiver
compatible with the RTCM format sentences supported by your
Garmin GPS is needed to use DGPS.
Refer to the United States Coast Guard’s Web site (
.uscg.gov/) for locations and status of the differential stations.
Differential receiver status is indicated in the bottom-left ﬁeld of the
GPS Page and shows one of the following conditions:
• None—no optional beacon receiver is attached or enabled
on the Interface sub tab on the Main Menu or WAAS is
• Searching for WAAS—WAAS is enabled and the receiver is
searching for WAAS signal.
• Using WAAS—WAAS capability is enabled and the unit is
receiving WAAS corrections.
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• Check Beacon Wiring—the DGPS setting is enabled in the
Interface sub tab on the Main Menu but no DGPS device is
• No Beacon Signal—DGPS receiver is attached, but not
transmitting RTCM data to GPS.
• Tuning Beacon—the receiver is tuning manually to a DGPS
• Using Differential—the unit is receiving DGPS corrections.
• Scanning for Beacon—the DGPS receiver is scanning for an
What is WAAS/EGNOS?
The Wide Area Augmentation System (WAAS) is an FAA-funded
service to improve the overall integrity of the GPS signal and
increase position accuracy for users in North America. In Europe,
WAAS is referred to as EGNOS.
The system is made up of satellites and approximately 25 ground
reference stations positioned across the United States that monitor
GPS satellite data. Two master stations, located on either coast,
collect data from the reference stations and create a GPS data
correction message. According to the FAA’s Web site, testing in September 2002, WAAS
conﬁrmed an accuracy performance of 1–2 meters horizontal and
2–3 meters vertical throughout the majority of the continental
United States and portions of Alaska.
WAAS is just one service provider that adheres to the Minimum
Operational Performance Standard (MOPS) for global Satellite
Based Augmentation Systems (SBAS). Eventually, there will
be several services of worldwide geostationary communication
satellites and ground reference stations.
All SBAS systems use the same receiver frequency; therefore any
operational SBAS system should be capable of providing your GPS
unit with increased accuracy at any location in the world.
Currently, enabling WAAS on your Garmin GPSMAP 496 in regions
that are not supported by ground stations may not improve accuracy
even when receiving signals from an SBAS satellite. In fact, it can
degrade the accuracy to less than that provided by GPS satellites
alone. For this reason, when you enable WAAS on your Garmin
GPS receiver, the receiver automatically uses the method that
achieves the best accuracy. To enable WAAS, refer to
For more information, go to
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LORAN TD Setup
LORAN is a radio navigation aid operated and maintained in the
United States by the United States Coast Guard. The name LORAN
is an acronym for “LOng RAnge Navigation”. The LORAN
system covers the entire United States and the United States
Coastal Conﬂuence Zone. From the perspective of a mariner, the
system is used for ocean and coastal navigation. It can be used as a
supplemental system for harbor and harbor approach navigation, and
it is used for inland navigation by recreational vehicles.
LORAN TD Feature
The LORAN TD (Time Delay) feature eases the transition from
using LORAN to using GPS. The GPSMAP 496 unit automatically
converts GPS coordinates to LORAN TDs for those who have a
collection of LORAN ﬁxes for favorite ﬁshing spots and other
waypoints recorded as TDs. You can show your location as a TD
or enter waypoints as TDs. The accuracy to be expected from this
conversion is approximately thirty (30) meters. When the unit is
placed in the LORAN TD format mode, it simulates the operation
of a LORAN receiver. Location coordinates can be showed as TDs,
and all navigation functions can be used as if the unit was actually
receiving LORAN signals.
Using the LORAN TD Format
When creating new waypoints using LORAN TD coordinates,
you must set the correct LORAN chain number and secondary
stations in the Setup TD ﬁeld before storing the waypoint. After the
waypoint is stored in unit memory, it always reference the LORAN
chain number and secondary stations currently selected in the
Setup TD ﬁeld. If you enter a different LORAN chain number, or
change the secondary stations or offsets in the Setup TD ﬁeld, the
active waypoint information does reﬂect those changes. Since the
GPSMAP 496 does not rely on the LORAN signal for navigation,
it can reference a different GRI chain and/or secondary stations and
still navigate to the location stored in memory.
The LORAN Location Format ﬁeld is located under the Units sub
tab in the Main Menu. The LORAN TD Setup window contains
the ﬁelds to select the Loran GRI-Chain Number, Primary and
Secondary Stations, and TD Offsets.
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To set up LORAN TD from the Main Menu:
1. Press MENU twice to show the Main Menu.
2. Use the
ROCKER to highlight Setup on the vertical tab list.
Then select Location from the row of horizontal tabs.
3. Using the
ROCKER, highlight the Location Format ﬁeld and
Location Sub Tab
4. Highlight Loran TD, and press ENTER.
5. Highlight the
Setup button located on the right side of the
Location Format ﬁeld, and press ENTER. The Loran TD
Setup window appears.
6. To change the settings of any of the ﬁve ﬁelds, highlight the ﬁeld, press
ENTER, enter the setting using the ROCKER,
and press ENTER.
7. When ﬁnished, highlight the
Save button, and press ENTER.
LORAN TD Setup
If the active GRI Chain, secondary stations, or offsets were changed
since the waypoint was created, the waypoint now references
the active GRI chain and secondary stations and adjust the TD
coordinates accordingly. Remember that the GPS is not relying
on the LORAN signal for navigation and actually converts the
TD coordinate to a useful latitude and longitude coordinate before
storing the waypoint to memory or using it for navigation. Because
of this, the unit can navigate to a TD coordinate anywhere in the
You must know your GRI chain number and/or secondary stations
to create a LORAN TD location. For more information read our
LORAN TD Position Format Handbook, available at the Garmin
Web site, at
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Map Datums and Location Formats
Map Datums are based on a mathematical model of the Earth.
What are Map Datums?
A datum is a mathematical model of the Earth that approximates
the shape of the Earth and enables calculations to be carried out in a
consistent and accurate manner. The datum is physically represented
by a framework of ground monuments (such as trig. stations) whose
locations have been accurately measured and calculated on this
reference surface. Lines of latitude and longitude on a map or chart
are referenced to a speciﬁc map datum. Every chart has a map datum
reference and the GPSMAP 496 can be set to match most of those
If you are navigating and comparing the GPS coordinates to a map,
chart, or other reference, the map datum in the GPS unit should
be set to the same datum as the map to ensure the most accurate
What is a Location Format?
Your current location can be viewed on the GPS in the form of
coordinates. Since different maps and charts use different location
formats, Garmin GPS units allow you to choose the correct
coordinate system for the type of map you are using. The most
common format is latitude and longitude, which is used by all
Garmin units. You can change the location format to use with other
coordinate systems in Location Preferences. UTM/UPS (Universal
Transverse Mercator/Universal Polar Stereographic) are easy-to-use
metric grids that are found on most USGS topographic quadrangle
maps. Several other grids, including a user-deﬁnable grid (for the
advanced user), are available.
For more information about using paper maps with your Garmin
unit, refer to Using a Garmin GPS with Paper Land Maps, available
for download at www.garmin.com/manuals/UsingaGarminGPSwith
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