FAI Gliding Commission - GNSS Flight Recorders

GNSS Flight Recorders

With the advent of satellite navigation equipment at an affordable price, the International Gliding Commission of FAI developed a technical specification for approved flight recorders. The IGC-approved flight recorders provide precise evidence of position for competitions, world records and FAI awards and other activities. They replace earlier methods of observation that used photographic evidence or ground-based observers to record aircraft position. The IGC-approved recorders include a pressure altitude sensor and a Global Navigation Satellite System (GNSS) receiver. They also include data output in a standard ASCII-based format, the "IGC flight data format". This format is used in files with the suffix "IGC" that is specified in detail in Appendix A of the document "Technical Specification for IGC-approved GNSS Flight Recorders". It should also be mentioned that organisations outside IGC also use IGC-approved flight recorders and the IGC flight data file format.

In IGC-approved recorders, GNSS and pressure altitude data is continuously recorded during flight in the form of regular fixes stored in non-volatile memory inside the recorder. Typical fix intervals, set by the pilot before flight, are between 5 and 15 seconds for "cruising" flight between turn points, and between 1 and 2 seconds at or near turn points or other points of interest.

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.

In March 1995, the IGC created their "GNSS FR Approval Committee" (IGC GFAC) to test recorders for compliance with the Commission's rules, the FAI Sporting Code Section 3 (Gliders and Motor Gliders). GFAC also issues IGC-approval documents for approved types of recorder and these documents are posted on the IGC GNSS web site.

For IGC-approval, the recorder design includes adding a security code to the downloaded file of flight data. This code and the file data itself can be checked ("validated") at any time later using a validation program originating from the recorder manufacturer that is posted on the IGC GNSS web site for general use. This validation program checks three things. (1) That the file has properly originated from an approved type of recorder, (2) That the recorder has not been altered from its IGC-approved state, (3) That the data in the downloaded file that is being validated is identical to when it was originally downloaded form the recorder. This allows the data to be used for the validation of flight performances up to and including world records.

There are three levels of IGC-approval and at the higher levels a public/private key encryption system such as RSA or equivalent is used to achieve the validation process above. Recorders at the higher approval levels also have a security device such as a microswitch that operates if the recorder is opened. This enables unauthorised modification to the recorder to be detected and protects the integrity of its output data.