GPS Map Datums

One of the mandatory requirements for a FR to receive IGC approval is that it is fixed on the WGS84 map datum.   Briefly, map datums are assumptions made about the actual shape of the earth when early mapmakers transferred the earth's oblate spheroid shape onto a flat sheet of paper.   In order that a GPS can give a correct position on a particular map, the GPS map datum and the datum used in the map construction have to be the same.   There is at last some standardization in datums and the preferred setting is World Geodetic Survey 1984 (WGS84).   Accordingly, this setting is required to be permanently fixed in the GPS engines of all "All Flights" recorders. 

The map datum of an Off the Shelf (OTS) GPS unit is user selectable.   However, the selected datum only appears in the NMEA output of certain Garmin GPS units.   It is for this reason that the EW Models A, B and D recorders can only be used with certain specified Garmin models.   All GPS engines output positional data in a ASCI (plain text) stream of characters.   This ouput stream conforms to an original nautical standard derived by the National Maritime Electronics Authority, or NMEA, and is the basis of the data that is recorded in the FR.   Normally, the NMEA data output does not contain details of the map datum that is set in the unit; except for the later Garmin units.  Therefore, when an approved Garmin unit is connected to an EW, the map datum set on the GPS is stored in the secure flight record and can be subsequently verified.

Engine Recording

Most IGC FRs have some method of recording engine running in a motor glider or turbo.  The usual arrangement is to have an integral microphone inside the unit, the output of which is filtered, amplified and then recorded in the FRs memory.  The arrangement works well and the engine running signature is easily recognisable on the subsequent analysis.  A print of the output of the Engine Noise Level (ENL) is shown below.  This recording came from a turbo and the noise signatures of the tug during the launch, the retraction and extension of the landing gear, the running of the turbo engine and the landing can all be identified.  The ENL system requires no action on the part of the pilot; does not require specific mounting arrangements; is active immediately the flight starts; and any engine running during flight is easily identified.

Other systems used to detect engine operation are a vibration recorder which also works well.  However, such a system requires that the FR be mounted in direct contact with the glider structure; if the FR is wrapped in a sweater and then stowed in the baggage compartment, any subsequent engine operation can be difficult to analyse.

Other FRs use an external switch which detects when the engine doors are opened, the pylon raised or the ignition switch operated.   Such systems rely on external wiring from the FR to the sensor or switch and such wiring needs to be checked by an Official Observer (OO) both before and after flight if the FR is going to used for flight evidence.  This can sometimes be difficult to arrange as an OO is not always present before launch.  One FR uses a slight variation of this system in that it measures an electrical voltage which is proportional to engine RPM coming from the Ilec engine control unit used with Schempp Hirth gliders.

GPS Accuracy

When a different GPS is being used for task navigation, it is possible that there will be a small positional difference between the co-ordinates being shown on the nav GPS when compared with those of the FR GPS.  This difference, which can amount to tens of metres, is caused by each GPS engine navigating by using a different mix of satellites or constellation.  If the turn point sector is only penetrated by a few metres as recorded by the nav GPS, then it possible that the FR GPS may not actually record a fix in the zone.  The only sure way to combat this potential problem is to ensure that the FR actually indicates a fix in the TP zone.  Most FR manufacturers have solved the problem by using the same GPS engine to provide positional information for both recorder and the navigation system.

Memory Capacity and Fast Fixing

It is very difficult to compare the memory capacity of individual FRs unless each is quoted using the same parameters.  Later in this series, the memory capacity of each FR is quoted in hours and assumes that logging is being carried out at 10 second intervals.  Use of the PEV (pilot event marker) or fast fix capability causes the record of fix recording to speed up considerably for a pre-determined period of time.  Excessive use of the PEV facility can cause memory to be used up more quickly.

Memory Roll Over

FRs have a finite memory capacity and it is important that should the memory become full, say by excessive use of the fast fixing facility, then in most FRs the memory 'rolls over' and the oldest information is deleted to make room for current fixes rather than the unit simply stop recording. Usually, the old information relates to previous flights and it is therefore not critical when it is erased.  However, at least one FR, the Volkslogger, will stop recording when the memory is full.