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GENERAL INFORMATION
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What is WAAS?
Wide Area Augmentation System (WAAS) is a GPS-based
navigation system that provides precision above and beyond
what GPS can do on its own. WAAS was designed to improve
the accuracy and ensure the integrity of information coming
from GPS satellites.
WAAS is a network of 25 ground reference stations that
cover the entire U.S. plus parts of Canada and Mexico.
Implemented by the Federal Aviation Administration (FAA)
for aviation users, these reference stations are located at
precisely surveyed spots and compare GPS distance
measurements to known values. When the WAAS signal is
available, WAAS-capable receivers typically have a much
higher accuracy than non-WAAS units.
What is a Map Datum?
Maps and charts are essentially grids created from a starting
reference point called a “datum.” Many maps being used
today were originally created decades ago. Over the years,
technology has allowed us to improve our surveying skills
and create more accurate maps. However, there is still a
need to adapt GPS receivers to use with older maps.
Most navigational charts and maps will have the datum listed.
Cobra Electronics Corporation™
GPS receivers include up to 100
map datums that allow you to switch to a setting that matches
your map. The most common US map datums are World
Geodetic System 1984 (WGS 84), North American Datum
1983 (NAD 83) and North American Datum 1927 (NAD 27)
NOTE: Using a map datum that does not match the chart
you are using can result in significant differences in position
information.
About North Reference
The vertical grid lines on any map can be aligned in one of
three different ways.
“True North” means that the vertical lines are aligned with
the geographic north pole. This is the most common method
of orienting maps.
“Magnetic North” means that the vertical lines are aligned
in the direction indicated as north by a compass, which can
differ from True North to varying amounts, depending on
where you are on the earth. (This difference is called
“inclination”and “declination.”)
“Grid North” refers to the direction in which the straight lines
on a map are actually aligned, which may be slightly different
from True North, because the rounded surface of the earth
can not be accurately depicted on the flat surface of a map.
GENERAL INFORMATION
Sources of Interference / Errors:
GPS receivers have the potential for position errors due to
interference, primarily from the following sources:
Ionosphere and troposphere delays
The satellite signal slows as it passes through the atmosphere.
The system uses a built-in model that calculates an average
– though not exact – amount of delay.
Signal multi-path
This occurs when the GPS signal is reflected off objects such
as tall buildings before it reaches the receiver. This increases
the travel time of the signal, causing errors.
Receiver clock errors
The built-in clock can have very slight timing errors.
Orbital errors
Also known as ephemeris errors, these are inaccuracies of the
satellite’s reported location.
Most common error
Most common error is not having a clear view of the sky, or
being under very cloudy conditions, when a satellite signal
may not be received.
Number of satellites visible
The more satellites the receiver can see, the better the
accuracy. Buildings, terrain, electronic interference or even
dense foliage can block signal reception, resulting in position
errors or possibly no position reading at all. In general, the
clearer the view of the sky is at your location, the better
the reception will be. GPS receivers will not work indoors,
underwater or underground.
Due to the errors that might occur, it is recommended that
the serious navigator use a second navigational tool, such as
a magnetic compass or map.
When satellite lock is lost
Whenever satellite lock is lost, the following animation
sequence appears onscreen and continues to loop until
satellite lock is re-acquired.
When GPS is turned Off
Whenever GPS is turned Off, the following animation
sequence occurs and continues to loop until search is
activated by user.
Also, all data fields (except for power status data
fields) appear blank until GPS is turned On and
satellite lock is re-acquired.