Setting/updating atmospheric properties

[benbusby]

Hi,

I've been working on developing a good way to dynamically set and update atmospheric properties through the Python API, but have been running into a couple of issues.

It seems that at least some of the documented atmospheric properties in JSBSim (see here) might be immutable, since attempts to set new a value for atmospheric properties don't seem to actually update the value. Here's an excerpt of how I'm attempting to set temperature, for example:

self.flight_model.set_property_value('atmosphere/T-R', 540.27)
print(self.flight_model.get_property_value('atmosphere/T-R'))

# Prints '518.655...'

Are these atmospheric properties intended to accept user modification or are they meant to be immutable?

In addition, I'd also like to be able to set more properties than what I've come across in the sourceforge documentation, namely things like gravity, wind speed, etc, and eventually integrate global reference atmospheric models like the Venus-GRAM into my use case, but the properties listed don't seem to include these. Is there a separate reference for atmospheric properties that I'm overlooking or is this the full list?

Thanks again for all of your work on this project!

[seanmcleod]

Take a look at this set of properties, which all have setters and getters.

jsbsim/src/models/atmosphere/FGStandardAtmosphere.cpp

Lines 671 to 698 in 76411ec

	void FGStandardAtmosphere::bind(void)
	{
	typedef double (FGStandardAtmosphere::*PMFi)(int) const;
	typedef void (FGStandardAtmosphere::*PMF)(int, double);
	PropertyManager->Tie("atmosphere/delta-T", this, eRankine,
	(PMFi)&FGStandardAtmosphere::GetTemperatureBias,
	(PMF)&FGStandardAtmosphere::SetTemperatureBias);
	PropertyManager->Tie("atmosphere/SL-graded-delta-T", this, eRankine,
	(PMFi)&FGStandardAtmosphere::GetTemperatureDeltaGradient,
	(PMF)&FGStandardAtmosphere::SetSLTemperatureGradedDelta);
	PropertyManager->Tie("atmosphere/P-sl-psf", this, ePSF,
	(PMFi)&FGStandardAtmosphere::GetPressureSL,
	(PMF)&FGStandardAtmosphere::SetPressureSL);
	PropertyManager->Tie("atmosphere/dew-point-R", this, eRankine,
	(PMFi)&FGStandardAtmosphere::GetDewPoint,
	(PMF)&FGStandardAtmosphere::SetDewPoint);
	PropertyManager->Tie("atmosphere/vapor-pressure-psf", this, ePSF,
	(PMFi)&FGStandardAtmosphere::GetVaporPressure,
	(PMF)&FGStandardAtmosphere::SetVaporPressure);
	PropertyManager->Tie("atmosphere/saturated-vapor-pressure-psf", this, ePSF,
	(PMFi)&FGStandardAtmosphere::GetSaturatedVaporPressure);
	PropertyManager->Tie("atmosphere/RH", this,
	&FGStandardAtmosphere::GetRelativeHumidity,
	&FGStandardAtmosphere::SetRelativeHumidity);
	PropertyManager->Tie("atmosphere/vapor-fraction-ppm", this,
	&FGStandardAtmosphere::GetVaporMassFractionPPM,
	&FGStandardAtmosphere::SetVaporMassFractionPPM);
	}

Then the following set only have getters.

jsbsim/src/models/FGAtmosphere.cpp

Lines 271 to 286 in 76411ec

	void FGAtmosphere::bind(void)
	{
	PropertyManager->Tie("atmosphere/T-R", this, &FGAtmosphere::GetTemperature);
	PropertyManager->Tie("atmosphere/rho-slugs_ft3", this, &FGAtmosphere::GetDensity);
	PropertyManager->Tie("atmosphere/P-psf", this, &FGAtmosphere::GetPressure);
	PropertyManager->Tie("atmosphere/a-fps", this, &FGAtmosphere::GetSoundSpeed);
	PropertyManager->Tie("atmosphere/T-sl-R", this, &FGAtmosphere::GetTemperatureSL);
	PropertyManager->Tie("atmosphere/rho-sl-slugs_ft3", this, &FGAtmosphere::GetDensitySL);
	PropertyManager->Tie("atmosphere/a-sl-fps", this, &FGAtmosphere::GetSoundSpeedSL);
	PropertyManager->Tie("atmosphere/theta", this, &FGAtmosphere::GetTemperatureRatio);
	PropertyManager->Tie("atmosphere/sigma", this, &FGAtmosphere::GetDensityRatio);
	PropertyManager->Tie("atmosphere/delta", this, &FGAtmosphere::GetPressureRatio);
	PropertyManager->Tie("atmosphere/a-ratio", this, &FGAtmosphere::GetSoundSpeedRatio);
	PropertyManager->Tie("atmosphere/density-altitude", this, &FGAtmosphere::GetDensityAltitude);
	PropertyManager->Tie("atmosphere/pressure-altitude", this, &FGAtmosphere::GetPressureAltitude);
	}

[benbusby]

Thanks for the quick response! So despite FGAtmosphere properties not having setters, could I not just use the atmosphere/override/<property name> nodes to achieve the same effect? I'm referencing this section (although it still looks like a lot of the control I was hoping for is not available):

jsbsim/src/models/FGAtmosphere.cpp

Lines 107 to 131 in 76411ec

	void FGAtmosphere::Calculate(double altitude)
	{
	FGPropertyNode* node = PropertyManager->GetNode();
	if (!PropertyManager->HasNode("atmosphere/override/temperature"))
	Temperature = GetTemperature(altitude);
	else
	Temperature = node->GetDouble("atmosphere/override/temperature");
	if (!PropertyManager->HasNode("atmosphere/override/pressure"))
	Pressure = GetPressure(altitude);
	else
	Pressure = node->GetDouble("atmosphere/override/pressure");
	if (!PropertyManager->HasNode("atmosphere/override/density"))
	Density = GetDensity(altitude);
	else
	Density = node->GetDouble("atmosphere/override/density");
	Soundspeed = sqrt(SHRatio*Reng*Temperature);
	PressureAltitude = CalculatePressureAltitude(Pressure, altitude);
	DensityAltitude = CalculateDensityAltitude(Density, altitude);
	Viscosity = Beta * pow(Temperature, 1.5) / (SutherlandConstant + Temperature);
	KinematicViscosity = Viscosity / Density;
	}

[seanmcleod]

I guess so, I didn't spot those. I've only used atmosphere/delta-T in the past.

If they still don't offer the level of control you require then I guess your other option is to implement your own atmosphere model. In addition to the standard atmosphere model you'll see there are 2 other atmosphere models currently in JSBSim, MSIS and Mars.

[beltzjr]

How can the MSIS or Mars models be chosen for a run? Haven't been able to find any example.

[bcoconni]

Unfortunately at the moment the Mars and MSIS atmosphere models are dead code, you cannot select them.

[beltzjr]

Can you give more background in this? Does someone need to just fix it? Is there a master plan in work for how to better implement it?

[bcoconni]

This feature could be added with a reasonable amount of effort (C++ coding involved). At the moment we are processing an element <planet/> which allows to alter the planet geometry and gravity:

jsbsim/src/FGFDMExec.cpp

Lines 801 to 812 in a35be5c

	// Process the planet element. This element is OPTIONAL.
	element = document->FindElement("planet");
	if (element) {
	result = Models[eInertial]->Load(element);
	if (!result) {
	cerr << endl << "Planet element has problems in file " << aircraftCfgFileName << endl;
	return result;
	}
	// Reload the planet constants and re-initialize the models.
	LoadPlanetConstants();
	InitializeModels();
	}

In order to allow modifying the atmosphere itself, you'd need to process an subelement <atmosphere/> in <planet/> and modify the atmosphere accordingly. The class FGAtmosphere would need a new method Load that would select the atmosphere.

However rather than using the FGMars class which design is quite out fashioned with respect to FGStandardAtmosphere, I'd rather inherit from FGStandardAtmosphere and allow customization of the table StdAtmosTemperatureTable and the likes.

[bcoconni]

Properties atmosphere/override/* are meant to bypass the ISA1976 atmospheric model, rather than letting JSBSim compute the pressure and temperature from the altitude and the atmospheric properties at sea level. When using these properties, JSBSim assumes your program is taking over the atmospheric properties computations.

As @seanmcleod mentioned, the temperature can be modified by the following properties:

• property atmosphere/delta-T which basically shifts the temperature by the amount specified (in Rankines)
• property atmosphere/SL-graded-delta-T which modifies the slope of the temperature as a function of altitude.

When fed with these 2 properties, JSBSim assumes the ISA1976 atmospheric model must be used to assess the pressure and temperature.

[bcoconni]

This issue has been moved to "Discussions". Now that this new feature is available in GitHub, it has been decided to move issues related to support requests and questions to "Discussions" and restrict "Issues" to bugs and feature requests.
This change is only motivated by project organization and will not affect the support provided. We'll gladly continue to answer your questions 😄.