Q: How does NaviPac interpret the GPS quality, and what happens to the position and height when the GPS is not in RTK Fixed?

A: NaviPac reads the fix quality from the GGA string and converts it to its own quality level. The settings under Project settings → GPS then decide what NaviPac does with GPS data below the highest level.

GGA fix quality and NaviPac quality level

For NMEA GGA input (e.g. the GPS1/GPS2 NMEA drivers), NaviPac converts the GGA fix quality as follows:


GGA fix qualityMeaningNaviPac quality level
1, 6, 7GPS fix (autonomous), dead reckoning, manual inputOrdinary GPS
2, 3, 5, 8DGPS, PPS, RTK Float, simulationDifferential (DGPS)
4, 9RTK Fixed (9: SBAS/WAAS in NMEA 4.x)Highest (RTK)
0 or otherInvalid / unknownUnknown


Note: RTK Float (5) counts as differential, not as RTK. Only RTK Fixed (4) is the highest quality.


Other strings read by the same driver:


StringNaviPac quality level
GGK (Trimble $PTNL,GGK)The GGK quality code is used directly: 1 → ordinary, 2 (RTK float) → differential, 3 and higher → highest.
Note: GGK codes 4 (DGPS), 5 (SBAS), 6/8 (network RTK float), 10/11 (OmniSTAR), 13 (beacon DGPS), 14 (RTX), 15 (xFill) and 16 (INS dead reckoning) are therefore also treated as highest quality. This is a known issue and has been reported to development.
LLQ (Leica)1, 3 → ordinary; 2, 5 → differential; 4 → highest
PSBGO (Leica local grid)Always highest
GLLAlways differential
Trimble 4000 formatI → highest; D → differential; otherwise ordinary


Known issue: with Trimble GGK input, the quality settings below do not work reliably (DGPS, SBAS, network RTK float, INS dead reckoning etc. are accepted as RTK Fixed). Until this is fixed, use GGA if you depend on GPS position, Flag if not highest quality or Discard height if not highest quality. Other GPS drivers (e.g. POS MV) set the quality from their own status information.


Settings (Project settings → GPS)


SettingWhat NaviPac does
GPS position
All (Autonomous) / Differential / Highest/Fixed RTK		Which quality level is accepted for the position. A GPS below the selected level gets weight 0 in the position calculation, and an alarm is raised.
Default: All.
Flag if not highest qualityThe navigation status is shown as yellow when the GPS used is not at the highest level (RTK Fixed), i.e. also for RTK Float.
Default: off.
Discard height if not highest qualityThe GPS height is ignored when the quality is not RTK Fixed (also RTK Float), and an alarm is raised. This affects everything that uses the GPS height, e.g. GPS tide: the tide pauses while the quality is lower.
With the setting off, the height is used at any quality.
Default: off.
HDOP limitA GPS with HDOP above the limit gets weight 0 in the position calculation, and an alarm is raised.


The position and the height are controlled separately: GPS position does not discard the height, and Discard height if not highest quality does not discard the position.


Alarms

  • When a GPS drops out of RTK Fixed, NaviPac raises a non-RTK alarm, also when the height and position are still used.
  • When a GPS changes between differential and non-differential, NaviPac raises a differential / non-differential alarm.
  • When a GPS is weighted out (quality below the GPS position level, or HDOP above the limit), NaviPac raises a "weighted to 0" alarm.

Recommendations

  • For work that depends on the GPS height (GPS tide, height-aided positioning of objects), use RTK Fixed. RTK Float heights can be off by several decimetres.
  • To make sure only RTK Fixed heights are used, enable Discard height if not highest quality. Note that the GPS tide then pauses while the GPS is not in RTK Fixed.
  • Enable Flag if not highest quality to make a drop out of RTK Fixed visible to the operator.
  • Check the GGA fix quality in Raw data or in the logged NPD file (G file) if the position or height behaves unexpectedly.

See also: GPS tide calculation in NaviPac and NaviPac 4 Configuration – menus and window description.