> The way I understand this, the issues are to some degree separate (?).
>
> I believe (correct me if I'm wrong) that what Jon is largely talking about
> is the altitude extrapolation of atmosphere conditions, i.e. given that I
> specify pressure, temperature and dew point at sea level, how will these
> change as a function of altitude. Within some reasonable set of
> assumptions, you can compute these by solving physics equations.
>
> What the weather system does is specify these values *at sea level* as a
> function of (lat, lon) - the altitude interpolation is still done by the
> built-in Flightgear atmosphere model, and as far as I know it'd be no
> problem to use the JSBSim atmosphere for that purpose instead. As far as
> the properties in /environment/ are concerned (I assume they're used by
> the FDM...), one'd probably just switch from one function calculating
> the altitude dependence of pressure from the sea level value to a
different
> one.
> Cheers,
> * Thorsten
Yes, Thorsten understands what I am asking. :-)
In the new JSBSim standard atmosphere model - which is based on the
information and algorithms in the U.S. Standard Atmosphere 1976 ("USSA"
Document: NASA TM-X-74335) the atmospheric properties are "propagated"
upwards to 86 kilometers in altitude based on temperature and pressure
conditions at sea level or at altitude (in the case of temperature). If the
default standard pressure and temperature are assumed, what results is the
U.S. Standard Atmosphere. However, sea level temperature or pressure can be
specified (in one of several units), and then the entire set of key
atmospheric properties is recomputed for the entire altitude range.
Additionally, not only can a simple temperature bias at sea level be
specified, but an additional temperature gradient can be applied, allowing
one to alter the standard lapse rates. This latter feature is an
experimental one.
Another feature I'd like to add at some point is one where some "patchiness"
affects the atmospheric properties, so they are not 100% homogenous at an
altitude and over time - sort of like adding some noise. I'm looking at the
NASA Global Reference Atmosphere Model to gain some insight on how they do
it.
As part of this effort on the JSBSim side, the FGAtmosphere class has been
modified as a standard interface for other atmosphere models which may
follow.
So, currently in JSBSim as it exists in our cvs repository, the temperature,
pressure, density, viscosity, speed of sound, etc. are provided by the
JSBSim atmosphere model. No other sources are used internally for those
parameters, no matter what FlightGear does. The interface between FlightGear
and JSBSim does (or will, when it is finished) modify the sea level
temperature and pressure - although there is some facility for permitting
conditions to be specified at an elevation (or altitude), which JSBSim then
would "transform" into sea level conditions, and propagate properties for
the rest of the altitude profile algorithmically. What is possible, but
highly discouraged, is to permit FlightGear to specify a temperature and
pressure at every timestep. A sea level temperature and pressure should be
specified only at initialization or periodically as one moves geographically
to a new area where weather is changed. Perhaps changing sea level
conditions once every five or ten seconds would be often enough?
Another thing that the new JSBSim standard atmosphere model does is to allow
properties to be queried for a specified altitude, not just the altitude one
is at.
I guessed that on the FlightGear side there was a capability to express sea
level (or ground level) conditions, and that atmospheric properties were
determined from those. It would be good, however, to make sure that
FlightGear and JSBSim are using very similar models. One thing that JSBSim
does not ]yet] do is to model effects of water vapor in the air. According
to the USSA reference document, water vapor plays a relatively minor role in
atmospheric pressure and density calculations. The approach used in the
reference document is to slightly alter the temperature used in calculating
pressure and density, depending on the level of humidity present, IIRC. I'd
be interested to see your calculations for air pressure when humidity is
specified.
Jon
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