FAQ


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 ?

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
most GPS receivers can communicate with each other. It has been defined by,
and is controlled by, the US based National Marine Electronics Association.

NMEA 0183 data is transmitted as plain ASCII text at 480 characters/second in a sequence of sentences
of up to 80 characters in length each. These are of the form $aabbb,ccc*dd where $ marks the beginning
of a sentence, aa is the type of device sending the data (e.g. GPS, echo sounder etc.), bbb is the tag that
marks the sentence type (e.g. GLL or VTG), and ccc is any number of comma delimited data fields.
The dd at the end is an optional checksum field, that can be used to check for corrupted sentences.

... and is not

NMEA is not the same as RS232. The voltage levels are different, which can cause data transmitted by
the computer's serial port not to be understood by the receiving equipment, and also for the PC either
not to understand the incoming NMEA sentences, or in some cases for the PC's serial port to be damaged.
In addition, NMEA 0183 is designed to protect devices and to minimize interference problems
by having the data input opto-isolated (further interference protection is provided by specifying the use
of shielded twisted pair cabling). For this reason, we strongly recommend the use of an NMEA/RS232 converter.

NMEA 0183 compatability does not guarantee that systems can talk to each other.
There are many sentences available, so the same data can be transmitted in many different ways.
Also, within a sentence, not all of the data fields need to be filled in, so you cannot always check
compatability by looking at the lists of sentences transmitted/received.
In some cases a  NMEA 0183 sentence filter is necessary to establish compatability.

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   How to Connect a GPS Receiver to a Computer?

Because of 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.

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   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 problem,
a number of grid systems have been produced that give the user easy, constant scale grid squares
within which to reference his position. Examples of this include the Swedish RT90, 2.5 gon W grid,
and in the US the UTM and State Plane grids.

To go beyond this, a method is developed at the Geodetic Research Division of the National Land Survey
in Sweden, to compute projektion parameters which maps the latitudes and longitudes of the WGS84
geodetic datum to Transverse Mercator northing and eastings of RT90 plane coordinates X/Y.

Our filter Ferret 8RS uses this method.
This filter extracts the position from one of the NMEA 0183 sentences GLL, GGA or RMC
and converts it to RT90 X/Y coordinates.
The calculation errors in transforming from geodetic to grid are less than 1m in most parts of Sweden.

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What Is the Difference Between NMEA 0183 Version 1.5 and 2.0 ?

        
Data fields and data valid flags have been added to some of the sentences in version 2.0.
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.

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