Re: [Flightgear-devel] altimeter, encoder, and kap140

[John D. <sf...@av...>]

On 02/25/2007 07:06 PM, Dave Perry wrote:

>> Summary proposed compromise:
>> 1) Use an encoder instantiation (altimeter[1]) with quantum = 10 as well
>> as altimeter[0] with quantum = 0.

Yes, that seems entirely reasonable.

>> 2) Leave the 5 new lines to allow unambiguous service to autopilot use
>> of the encoder.

How about
2')  Have the autopilot calculate the Kollsman shift on its own.


> At the start of this discussion, I thought you wanted just one model of
> the atmosphere.  So it is a bad idea to have each autopilot use it's own
> model.  Users are going to want to ignore different returned values for
> different instantiations for "realism" reasons.

But a real autopilot *does* have its own model ... not a model
of the real atmosphere, but an ISA-model atmosphere (or, more
precisely, a Kollsman-model atmosphere).

Ditto for real altimeters.

If you want to start with the canonical Kollsman model and then
depart from it as a model of realistic instrumental imperfection,
that's entirely reasonable.

What was not reasonable was the previous altimeter, that used an
algorithm bearing no known resemblance to reality, and was off
by hundreds of feet under some perfectly ordinary conditions.

> .... means the
> kap140.nas has to retrieve the value it cannot have in reality
> (indicated altitude) and wants to approximate from values it does have
> (PA and baro shift), 

The real KAP does not have access to an oracle of any kind that
will perform the Kollsman calculation for it.  Arguing about which
type of oracle is "more realistic" is an argument about members
of the null set.  Not the sort of discussion we ought to be having.

The exact FAA-approved Kollsman calculation is well understood
and easily implemented in nasal code.

    I know I previously pointed out that it was possible to use
    the altimeter as an oracle ... but that was before I realized
    how seriously people were worried about values "it cannot have
    in reality".  If you care about reality at that level, doing
    the Kollsman calculation within the autopilot code is the
    only realistic option.