This section has been contributed by David Mills as a guide for the use of GPS track-log for turnpoint verification.
INTRODUCTION
Traditionally turn point verification in Hang Gliding competitions has been achieved by means of photographs, or by manual sightings from a turn point marshal on the ground. For the first time in Australia, 2 competitions in the 1998/99 season will trial the use of GPS track-logs for turn point verification.
This document provides help and guidelines for pilots intending to compete in those competitions and highlights some of the pitfalls to be avoided.
Remember that in the event of any discrepancy, protest, or inconsistency the organiser will discard a pilot’s track-log and ask the pilot to submit a film for flight verification. Failure to provide a film will result in the pilot only being awarded bomb-out points for the round.
PROBLEMS WITH GPS
The following problems must be considered when using a GPS track-log for flight verification.
Track-log Time Interval
Most GPS units have a track-log function. The number of points available for a track-log varies from unit to unit. For instance most Garmin units can store up to 768 track-log points with some of the newer models having 1024 points available. The number of points available has a significant impact on the use of the track-log for turn point verification.
For instance, if the track-log interval was set to 10 seconds, then only 128 minutes or just over 2 hours of flight can be recorded with 768 points or 170 minutes with a track-log of 1024 points. If the pilot takes longer than this to complete the task or flight, then the remainder of the flight will not be logged and any turn points rounded during this remaining time cannot be verified. Care must be taken to set a suitable track-log interval so that the entire flight can be verified.
Only Points Are Logged
Remember that only individual points are logged. This is especially important when rounding a turn point. For instance, consider the following example if the track-log interval is set to 30 seconds. A pilot flies straight towards a turn point, rounds the turn point and then flies away again along a line similar to the line of entry. If the GPS has logged a point 10 seconds prior to the pilot reaching the sector or turn point, then the next point logged will not be till 30 seconds later, by which time the pilot will have rounded the turn point and flown away again.
At trim speed most gliders fly at about 35km/h. This is approximately 10 m/s. So in 30 seconds a glider can have flown 300m. In the above example, the track-log would therefore only show the pilot having flown within 100-150m of the turn point but not around it. In this case the competition organiser would have to discard the track-log and ask the pilot to submit their film.
GPS "Selective Availability" And Error
The GPS system suffers from 2 types of error:
"Selective availability" is the error intentionally added to the system by the U.S. government (in an attempt to prevent the use of GPS for missile guidance). This error can be anything up to 300m but will be 50m on average and can occur in any direction. Thus it is possible for a reading taken at a given location to be up to 600m different from another reading taken at the same location at a different time (e.g. the first reading was take when SA was 300m north and the second when SA was 300m south). Readings this far apart are extremely rare in practice.
Calculation and geometric error are the error resulting from:
You don’t need to know why any of these occur, but the result of all these errors can be a further position error on top of the "Selective availability". Most GPS units calculate an estimate of these errors and display it on one of the screens. On the Garmin GPS this error is called EPE (estimated position error). This error will typically be in tens of meters, but if your unit is not currently receiving many satellites may be more than 100m.
It may be that this error is in alignment with SA in which case your measured position may be as much as 400m from your actual position. In general this error will not be in alignment with SA and may consequently result in a more accurate position reading than if no error existed.
The result of these errors may work in a pilots favour (by logging the pilot as having rounded a turn point when in-fact the turn point has not been reached) or may work against a pilot (possibly requiring the pilot to fly hundreds of meters past the turn point in order to get the track-log to indicate that the turn point has been rounded.
COPING WITH GPS ERRORS
The rules governing the use of GPS track-log for flight verification have been formulated with these errors in mind. It is hoped that in general these rules will allow a pilot to fly a course in the normal manner, rounding turn-points and taking a photo in FAI sector and then have their flight verified using the track-log without requiring a film to be developed.
The rules therefore contain some special measures to cope with the GPS problems noted above.
As noted a GPS does not remember everywhere that a pilot has flown, just positions every interval (as configured in the GPS settings). To make allowances for this, the track-log need not show a point within the sector. A point either side of the sector where a straight line between these points passes through the sector will be accepted.
To make some allowance for the errors in the system, the standard sector has been extended for the purposes of GPS track-log flight verification only. If a pilots track-log is rejected, the photo taken of the turn point must be in FAI sector.
An extra part being a cylinder of radius 100m from the turn point has been added to the FAI sector for GPS use. This new sector has been called the "beer can" sector. Thus if a pilot flies around the turn point, but the GPS error shows them as being 100m short, the beer can component of the sector will usually result in the turn point being accepted.
Marked points will be allowed as part of the track-log provided their position and recorded time are consistent with the track-log. When mark is pressed on a GPS the current position is saved into a way point. When enter is pressed after mark, the current time in UTC (Greenwich mean time) is added to the way points comment field (unless the pilot has entered a comment manually). If enter is pressed 1 minute after mark, the time recorded will be 1 minute after mark was pressed. This would make the way point inconsistent with the track log and the organiser would have to discard the way point.
Even a delay of a few seconds in pressing enter after mark may result in the way point being discarded. The rules state that the calculated speed for a pilot to fly from the last track-log point to the way point or from the way point to the next track-log point must not exceed the maximum speed logged between any 2 track-log points.
To illustrate, assume a pilot flies at 36km/h in a straight line for 1 minute. During that time the 2 track-log points are recorded. If between them the pilot presses mark and enter (almost simultaneously) then the speed calculated between either of the track-log points and the marked point would also be 36km/h. If however, the pilot had delayed 2 seconds between pressing mark and enter, the calculated speeds would be very different. Let’s say that the position is equidistant between the points. Thus the time shown to get to the marked point would be 17 seconds, and the time shown to fly to the track-log point would be 13 seconds. This results in calculated speeds of 32 and 42km/h. If the marked point was 4/5 of the way to the second track-log point with the same delay the calculated speeds would be 33 and 54km/h. If 54km/h is faster than any other logged speed then the way point will be discarded.
GPS SETTINGS
The following are the recommended settings for the GPS:
Operation or System
Standard time offset: +10
Daylight savings offset: +11
Navigation
Position FRMT: UTM/UPS
Map datum: Astrln Geod ‘66
Units: Metric
Heading: True
Angle: Degrees
Track-log
Record: fill or yes
Method or Criteria: Time interval
00:00:30
Remember that most Australian maps use a map datum of "Astrln Geod ‘66". If a position is read off an Australian map and entered into the GPS when the GPS is set to another map datum such as "WGS ‘84" then the point entered may be as much as 2km from the actual point.
The above settings will log 6.4 hours if your GPS can log 768 points and 8.5 hours if it can log 1024 points. Setting a GPS track-log interval to 00:00:20 on a GPS with 1024 points will allow it to log about 5.7 hours. This may be a setting that requires less compensation by the pilot, but remember on a very long task it may result in some turn points being missed.
Hours for
Interval setting 768 points 1024 point
00:00:60 12.8 17.0
00:00:50 10.6 14.2
00:00:40 8.5 11.3
00:00:30 6.4 8.5
00:00:20 4.2 5.6
00:00:10 2.1 2.8
When your competition organiser provides turn point co-ordinates s/he will also provide the map datum to set when entering those co-ordinates. Once the position is programmed into the GPS, the map datum can be changed without loss of position accuracy.