On Tue, 22 Aug 2006 03:53:42 -0700, Ian Strachan wrote:


From Ian Strachan, the other side of "the Pond":
Very interesting, Papa 3. Further questions a to what zero-altitude
datum are the altitude figures:
(1) displayed on screen and
(2) recorded for later download.

But with unknown algorithms applied before download, you do not know
exactly what you are dealing with. This is not a problem in the
recreational area, but may be in ours if accurate altitude is to be
used in a performance claim or to defend a pilot against a possible
airspace violation or other situation where a flight record wiuld be
useful. Then there is the issue of the general reliability of pressure
altitude compared to the sometimes erratic figures for GPS altitude
recorded in a proportion of IGC flight data files ...

Is this really a problem?

In the old days we were happy to accept a paper baragraph trace without
any attempt to "zero" the altitude. The procedure required the pilot to
scribe a "base line" on the chart at the known local airfield altitude
before start and the official observer measured all other significant
altitudes (start height etc) relative to this this height. Of course
for height gain tasks you only need to measure the difference between a
"low point" and a "high point".

So all we are left with is knowing whether the recorded altitude is from a
GPS engine, a pressure sensor or a mangled proprietary combination of
the two.

Pressure altitude is obviously acceptable. I believe GPS altitude, without
SA, is probably just as useful but it does not comply with the current
rules. The combination is probably the most accurate, but as the algorithm
is undefined and hence not reproducible, the data may not be acceptable.

The are only two significant difference between these GPS units and an
approved Flight Recorder:

1) The the mechanical tamper proof switch is missing. This the security of
the "private key" required to encrypt the trace before it is downloaded
is not protected. I am sure the only reason Garmin and others don't
include encrypted downloads in their software is because we would not
accept them without being assured of the security of their keys.

2) The price difference. Approved flight recorders being about 5x more
costly.


Regards

Ian

(another Ian, from the other side of the equator)