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Receiving GPS Satellite
Signals
This section briefly explains the GPS satellite reception.
Guidelines for reception
To ensure the best possible satellite reception, follow these
guidelines when setting up and using the unit:
¢ Place the antenna horizontally, with no line-of-sight
obstructions. The fewer the obstructions. the better the unit
can receive satellite signals.
* Keep in mind that certain objects, such as dense tree
coverage, buildings, your body, etc., may interfere with the
GPS satellite signal reception.
¢ The satellite signals can be received through certain
materials such as glass, fiberglass, or acrylic.
* As the satellites move low on the horizon. reception may
become poor.
¢ The satellite constellation is not yet complete: not all
satellites are in orbit, and the existing satellites undergo
periodic testing. Therefore, there may be times when there
are not enough satellites to determine your position.
About the positioning time -
The GPS satellites transmit orbital and timing information.
This information is called a "navigation message" and is
transmitted in 30 seconds of data row. The orbital
information included in the navigation message is called the
"broadcast ephemeris." 22 satellites (and two operational
spares) transmit the navigation message with their own
orbital information and timing information. The common
information such as orbital information for all satellites are
also included in the transmitted information, which is called
an "almanac." Almanac data is used to determine the
positions of each satellite when acquiring the current
position.
If the GPS receiver has the orbital information for all
satellites in its memory and knows the approximate
allocation of the satellites, and then it receives 30 seconds of
data row once and stores them in its memory, it can
continuously calculate its current position. The data is valid
for a few hours.
.
The unit can calculate your current position a few minutes
after you have input your approximate current position on
initial setting. After turning on the unit, if you wait for about
one minute to continuously receive the orbital! information in
a location with no line-of-sight obstructions, you can acquire
your current position quickly. It may take a little longer to
acquire your current position while moving because the unit
receives the orbital information intermittently.
Because this unit is an 8-channel parallel receiver, it can
search for the satellites that are not used for calculating the
current position, and gather the orbital information.
Under the following conditions, the unit will take about 20
minutes (or more than one hour, depending on the reception
condition) to acquire the current position:
¢ When you execute the "GPS Reset" option
¢ When you replace the built-in lithium batteries
GPS Coordinates
GPS determines latitude, longitude and altitude by defining
the earth as an ellipsoid that revolves on its axis. This
system is called the WGS84 (World Geodetic System 1984)
coordinate system. Conventional maps and charts use
different coordinate systems, depending on the location.
The unit calculates the latitude, longitude and altitude as
determined by the WGS84 coordinate system, which uses a
single coordinate system for the world. The unit displays the
WGS84 coordinates by default. Therefore, the coordinates
displayed by the unit may not be identical with those found in
an atlas. You must set the appropriate Geodetic System in
the Position Setting menu to have the unit display
coordinates which correspond to the map or chart you may
be using. Instructions for setting up the unit — including
selecting your Geodetic System — are discussed in Chapter
1s
Introduction
9
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