Instrument error correction

The FAI logger allow instrument error correction to adjust for instrument error and drift of the sensors. The instrument error adjustment is configured using a separate config file (inst_cal.cfg) which is placed on the SD card. I’ve written many emails telling officials how to do this. I think it is time to post it…
I suggest to do the correction whenever possible. If you don’t use it, make sure the inst_cal.cfg file doesn’t exist on the loggers.

  1. Make sure the devices don’t have any inst_cal.cfg on the SD card and that the altitude is set to barometric. Probably best is to not use any SD card at all.
  2. Switch all devices on with the correct QNH. Let them heat up for 10-15 minutes
  3. Note the indicated altitude for all loggers.
  4. Use the Excel sheet to calculate the instrument error. If the reference altitude is known, it should be entered. If it is unknown, the median will be used. In that case the loggers are corrected among each other, but the absolute altitude might have some offset.
  5. Use the Excel sheet to create the inst_cal.cfg file. This file has the format XXX=123 with XXX being the 3-digit serial number and 123 being the altitude correction in decimeter. This value will be added to the measured altitude to get the indicated altitude. A single inst_cal.cfg file can contain the instrument error for all loggers used in the event.
  6. Copy the inst_cal.cfg to all SD cards or use BLM to copy it when initializing the SD card.
  7. Optional: Start all loggers again and check if the indicated altitude is within a small offset (Usually within 8m).

Important Note: The instrument error file on the logger is only used to adjust the indicated altitude. The altitude stored in the IGC file is not modified. Thus the same instrument error must be added during scoring. Do not use the header information in the IGC file, but use a separate database to prevent cheating possibility.

Remark: Pre 1.3.246 firmware release the file was named insterr.cfg. Based on feedback the file was renamed to inst_cal.cfg. Versions 1.3.246 and later automatically rename insterr.cfg to inst_cal.cfg if no inst_cal.cfg exists yet.

Download Excel sheet