OmniSTAR 5220HP, User Manual

Page: 57 / 83

OmniSTAR 5220HP User Manual
Issue 1.32 12/2010
57
NMEA Messages
The NMEA messages screens are used to control the NMEA output of the
5220HP. The 5220HP is capable of outputting a number of NMEA messages
on different output rates, which may not all be needed as inputs for the
attached device.
For detailed descriptions of NMEA messages see Appendix B.
Furthermore, the number of NMEA messages that can be sent by the 5220HP
also depends on the baudrate of the communication port and the set output
rate of the NMEA messages.
For example: Selecting GGA, GSV and RMC to be output at a 5 Hz. output
rate, with an average of 8 satellites in view (so 2 GSV messages for every
GGA message), the baudrate (bits per second) can be calculated as follows:
1 GGA-message = 89 bytes * (8+1) bits/byte =
801 bits
2 GSV messages = 2*72 bytes * (8+1) bits/byte = 1296 bits
1 RMC message = 77 bytes * (8+1) bits/byte = 693 bits
Total 2790 bits
5 Hz. output = 5 * 2790 = 13950 bits/sec.
In the above example, the baudrate should be set to the next higher standard
baudrate, which is 19,200, to prevent a buffer overflow of the COM-port.
Warning: The above calculation is indicative, based on an average
message length count. Actual values may differ, yielding
different outcomes.
Warning: The initial NMEA standard called for a baud rate of only
4800. As a result, many external devices are still set to
communicate with a GPS receiver at 4800 baud. This is an
average of only 6 NMEA messages per second. Therefore,
be aware not to select too many NMEA messages or a
very high output rate for NMEA output, unless you have
set the baudrate on both the communication port of the
5220HP and the communication port of the external device
to a (much) higher value first.