The pressure altitude system in an approved recorder has the same function as a barograph and must be calibrated to the ICAO ISA (International Civil Aviation Organisation International Standard Atmosphere). Re-calibrations to check any errors from the ICAO ISA are carried out at regular intervals in the same way as an analogue barograph that uses an aneroid pressure sensor rather than an electronic pressure transducer. Recorded GNSS fix data replaces the need for photography to certify the track over ground and in particular whether a particular turn points has been reached. GNSS altitude data can be compared after flight with the pressure altitude data from the independent sensor in the recorder and this is a valuable check that both systems are working correctly.