Q: How does NaviPac calculate the GPS tide eg also applying a reduction file? 


A: See the steps below for calculating the GPS Tide in NaviPac:


  1. RAW GPS Height received from GPS
  2. This is compensated for antenna offsets (3D roll/pitch) -> CRP height
  3. Based on position a geoidal separation (eg VORF or DNN) is found and subtracted from the tide
  4. If a heave sensor exists then the heave is taken out of the value

Eg an Octans MRU is giving vessel HPR and gyro with XYZ of 0,0,0. Does this MRU require lever arms set for CRP XYZ? 

Yes that is needed as NaviPac needs to calculate the heave value in CRP – and to do so it need the offsets from MRU to CRP.

  5. We have now CRP height where short terms waves are taken away – this is GPS tide

  6. If there is defined a draught sensor then

  1. Use the pressure value (has to be in dB)
  2. Subtract pressure from surface.
    The pressure comes from the NaviPac Global settings Depth calculation parameters.
  3. Convert to depth. NP estimates the depth from pressure based on the Saunders and Fofonoff - Simple method.
  4. Compensate for offset of draught sensor
  5. Now we have draught in CRP
  6. Subtract that from the tide