This page contains answers to common questions, along with some tips and tricks
that we have found useful and presented here as questions.
New questions and answers will be added in the future.
|What is NMEA ?|
|How to connect a GPS receiver to a computer ?|
|What is a datum or WGS84 ?|
|What is the difference between NMEA 0183 version 1.5 and 2.0 ?|
|What Is NMEA ?|
0183 (or NMEA for
short) is a combined electrical and data specification for communication
between marine electronics and also, more generally, GPS receivers.
The NMEA 0183
protocol is a means by which marine instruments and also
data is transmitted as plain ASCII
text at 480 characters/second in a sequence of sentences
NMEA is not the same as
RS232. The voltage levels are different, which can cause data transmitted
compatability does not guarantee that
systems can talk to each other.
|How to Connect a GPS Receiver to a Computer?|
the difference in signal levels between NMEA and RS232C we recommend
the use of a NMEA/RS232C interface. If the computer is powered by the ships AC
outlet you will need an isolated type of interface to avoid ground voltages.
|What is a Datum or WGS84?|
|When an area
is surveyed, the measurements taken have to be made with respect to
an overall framework defining the size and shape of the earth, at least in the area being surveyed.
A number of elements are present in defining a datum, and in converting between one datum and another.
GPS uses the WGS84 ellipsoid (GRS80), with a major axis of 6378137 metres and a minor axis of 6356752m.
The geoid is the model of the mean sea level of the earth, passing through all land masses as well as
the sea areas. It is not a surface of revolution, as the earth's mass distribution is not uniform.
The geoid is often visualised as a contour map of heights above or below the ellipsoid.
What About Grids?
An unfortunate fact of
life is that maps are flat and the earth is not. To help solve this
To go beyond this, a
method is developed at the Geodetic Research Division of the National Land
|What Is the Difference Between NMEA 0183 Version 1.5 and 2.0 ?|
and data valid flags have been added to some of the sentences in version
One commonly used sentence is the position sentence GLL, which has an UTC time field
and a data valid flag added at the end of the sentence.
The compatability with older equipment can be maintained by a NMEA 0183 filter.
mail to: MARIN ELEKTRONIK AB
with questions or product information requests.