Airmar PB200 - Manual (Page 140)

PB200 WeatherStation User Manual revision 1.000
Proprietary and Confidential
134
The GPS receiver in the PB200 can calculate a position fix when it has achieved a
lock on at least three GPS satellites. If it has achieved a lock on four or more
satellites, the unit may be able to calculate a 3D position fix, meaning it can
determine latitude, longitude, and altitude. But if it has achieved a lock on only
three satellites, it may only be able to calculate a 2D position fix, meaning it can
determine latitude and longitude, but not altitude.
By default, the unit will automatically calculate a 3D position fix if it is able to,
but will fall back to 2D position fixing if there are not enough satellites being
tracked.
It is possible to restrict the operation of the GPS to only allow 3D position fixes
by sending the unit the command $PAMTC,OPTION,SET,2,2. If this is done,
then the unit will not calculate a position fix until it has achieved a lock on at least
four satellites.
Note that sometimes the GPS may be tracking at least four satellites, but is still
unable to calculate a position fix. This is because the positions of the satellites in
the sky must be satisfactory in order to achieve good geometry in the fix
calculation, as determined by the HDOP, VDOP, and PDOP values.
It is also possible to restrict the operation of the GPS to only allow 2D position
fixes by sending the unit the command $PAMTC,OPTION,SET,2,1.
To re-enable the automatic selection of 2D versus 3D fix calculations, send the
unit the command $PAMTC,OPTION,SET,2,3.
This setting is stored in EEPROM within the unit, and retrieved on power up.
The current setting may be queried by issuing the $PAMTC,OPTION,Q,2
command. The unit will respond with the sentence $PAMTR,OPTION,Q,2,<4>,
where the value of <4> is 1, 2, or 3, corresponding to 2D, 3D, or Automatic
mode, respectively.
This setting is mirrored in the NMEA 2000® transmitted GNSS Control Status
PGN 129538, field 5.
When calculating a 2D position fix, the latitude/longitude values may have a
relatively large error if the altitude of the GPS receiver is not set appropriately.
This can be corrected by programming a fixed altitude offset into the unit. See
the $PAMTC,ALT command on page 108.
Note that sometimes the GPS may be tracking at least four satellites, but is still
unable to calculate a position fix. This is because the positions of the satellites in
the sky must be satisfactory in order to achieve good geometry in the fix
calculation, as determined by the HDOP, VDOP, and PDOP values.