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such as that included with many Humminbird® products, receives the signals from
these satellites and is able to determine position based on very slight differences in
the time each signal is received and the receiver’s knowledge of the location of each
of the satellites.
Heading:
Heading describes the direction the boat is pointing and is measured in
degrees (i.e. 321°, where 000° is North, 090° East, 180° is South, 270° is West). Due
to wind and waves, the boat is often traveling in a slightly different direction than it
is heading, and therefore Heading is often confused with Course Over Ground. See
Course Over Ground.
Map Datum:
A Map Datum refers to a particular survey of the earth’s surface that was
referenced when creating a chart (i.e. WGS84). Since the earth is not flat, and not even
consistently round, a mathematical model must be used to translate coordinates from
the sphere of the earth to the flat surface of a chart. This mathematical model is the
Map Datum. Historically, many different Map Datums have been used around the world
to create maps; often a reference to the Map Datum can be found in the legend of a
paper map. It is important to select the correct Map Datum setting in your fishfinder
when comparing GPS position to a paper chart to avoid slight position inaccuracies.
Almost all electronic charts use the WGS84 Map Datum and therefore, they do not
require setting changes.
MMC:
MMC is an acronym for Multi Media Card. An MMC is a postage stamp-sized
memory device used to store electronic data such a maps, waypoints, routes and other
information. MMC is a very rugged format suitable for the marine environment, but it is
not waterproof. The MMC is removable from Humminbird® products, and can be used
in a PC that is equipped with an appropriate card reader. MMC is the same format that
many digital cameras use. See
SD
.
North, Magnetic:
North is the principle reference point of the compass (i.e. North is
000°). Magnetic North relies on the earth’s magnetic field to align a metal pointer
generally towards True North. However, due to local variations in the earth’s magnetic
field around the globe, the difference between True North and Magnetic North can be
greater than 10°. See
True North
.
North, True:
North is the principle reference point of the compass (i.e. North is 000°).
True North uses the axis of the rotation of the earth as the reference for North and is
constant for all points on the earth. Most GPS units default to True North as the setting.
The GPS heading may vary from the boat compass heading because the compass uses
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Cold Start:
Cold Start refers to the procedure that a GPS receiver must perform when it
does not have historical information regarding its current location. This procedure
typically lasts for several minutes, and may be experienced upon first power up of the
receiver, or if the receiver has been moved a great distance.
Course Over Ground (COG):
Course Over Ground is the current direction the boat is
traveling measured in degrees from North (i.e. 321°, where 000° is North, 090° East, 180°
is South, 270° is West). When the Course Over Ground is equal to Bearing, the boat is
said to be “On Course” and will arrive at the destination in the most efficient manner.
COG is often confused with Heading. See
Heading
.
Cross Track Error (XTE):
Cross Track Error is the straight-line distance of the boat from
the intended Track. XTE measures how far the boat is off course, and also triggers the
Off Course Alarm. See
Track
.
Differential GPS (DGPS):
Differential GPS is a system to improve the position accuracy
of the Department of Defense GPS through the use of secondary “correction signals”
broadcast from ground stations around the coastal United States and on some inland
waterways. These correction signals are provided by the USCG and received by an
auxiliary Differential Beacon Receiver (DBR) that connects to the GPS receiver. The
advent of WAAS has largely negated the need for the added expense of a DBR in the
United States.
Distance To Go:
Distance To Go is the straight-line distance between Present Position
and the Destination Waypoint. DTG and SOG (Speed Over Ground) are used to calculate
Time To Go. See
Time to Go
and
Speed Over Ground
.
Estimated Position
Error (EPE): Estimated Position Error is a calculation that indicates the
potential position inaccuracy the user may experience due to a variety of GPS factors
which include satellite position in the sky, signal strength and others factors.
Fix Type:
Fix Type indicates whether the GPS receiver is providing 2D fix or 3D fix. A 2D
fix requires only three satellites and provides only latitude and longitude. A 3D fix
requires four or more satellites, and provides latitude, longitude and altitude.
GPS:
GPS is an acronym for Global Positioning System. GPS provides accurate position
(latitude, longitude, altitude) information virtually anywhere on the earth through
satellite technology and personal receivers on the ground. A series of geosynchronous
satellites broadcast a unique signal toward the earth once per second. A GPS receiver,
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