Calculating Altitude AGL From Pressure

[fred 0]

I have a pressure reading on the ground in mbar as well as a series of measurements taken during the skydive (this is actually raw data from a protrack, if that helps). I'd like to convert this into an altitude. I know the software does it, but I don't know the math involved, and I'm hoping somebody here does.

Anybody have any idea? I can give some sample values if anybody would like.

[councilman24 36]

Try this http://www.nfinity.com/~exile/howhigh.htm

convert units to those needed in the equation.

Two google searches away.

I'm old for my age.
Terry Urban
D-8631
FAA DPRE

[pilotdave 0]

Won't be very accurate, but you could just match up the pressures you found to a standard atmosphere table. It will give the standard pressure at each altitude. Just check what altitude (MSL) is associated with the nearest pressure to each data point.

Doesn't take into account all the variables, but it might be as close as you can come if the pressure and temperature that day was somewhere close to standard.

Dave

[kallend 1,647]

aero.stanford.edu/StdAtm.html

This may help - an online calculator.

...

The only sure way to survive a canopy collision is not to have one.

[fred 0]

Quote

Doesn't take into account all the variables, but it might be as close as you can come if the pressure and temperature that day was somewhere close to standard.

I wonder what the protrack itself does. It doesn't seem to record temperature.

Thanks, Terry aka Councilman... That's a great link.

[pilotdave 0]

I know the neptune uses temperature as part of its calculation. I'd be surprised if the protrack didn't work the same way. It probably just doesn't display or record it.

Dave

[kelpdiver 2]

Quote

I wonder what the protrack itself does. It doesn't seem to record temperature.

I imagine it just works with the base pressure when you take off. Probably ignores the drop in temp as you go to the exit altitude.

This probably means it is a bit less accurate at DZs well above sea level, or like dive computers, perhaps it presumes that a low enough reading to register in the 1000s means that you really are up and not just in a hurricane.

[pilotdave 0]

Somewhere in the neptune manual it explains the need for a temperature reading... the solid state pressure transducer's output varies with temperature. It really has nothing to do with atmospheric conditions, but just calibration of the electronics. Protrack might not work the same way...

Dave

[IGOLOW 0]

Depending on the pressure sensor that they use it could be temperature compensated. I made a digital altimeter that was very accurate without the need to adjust for temperature.

[mjosparky 3]

You need to know the altitude first and then get the pressure. When you are flying and reset your altimeter the ground station you call knows their altitude, msl and then they give you a pressure reading and you set your altimeter to that. Pressure is always changing, a know altitude is not.
And Pro-track does give temp. in the Jump-track software.
Sparky

My idea of a fair fight is clubbing baby seals

[vidiot 0]

Quote

And Pro-track does give temp. in the Jump-track software.

This value is calculated from the ICAO Standard Atmosphere.

My Logbook

[Otterpilot 0]

What your trying to do is calculate "True Altitude" If you set your altimeter to zero on the ground then you have already corrected for pressure and the only thing you need to do is correct for temperature. Otherwise you need to take your indicated altitude from when you jumped and correct it first for pressure. Use the pressure reading on the ground and adjust it against standard pressure (29.92mb) If you were at 10,000ft and surface pressure was 30.32 you had a pressure altitude of 9600ft.
You would then require the surface temperature and the temperature at altitude to convert this to True Altitude. I forget the calculation but if the temperatures are below standard than you will be lower than your pressure altitude. Standard temperatures are 15 degrees Celsius at sea level and drop 2 degrees per thousand feet.
Generally speaking, skydiving on hot days with warm upper temps will mean you are jumping from a higher altitude even ithough your altimeter will not say so. Pressure levels are closer together when its cold so you would actually be lower when it's unusually cold.

[ryoder 1,398]

Quote

I know the neptune uses temperature as part of its calculation. I'd be surprised if the protrack didn't work the same way. It probably just doesn't display or record it.

Dave

Some years back I was doing research to build an altimeter/AAD device. I found that many solid-state absolute pressures sensors were available with temperature-compensating circuitry built in.

"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

[Jonsmann 0]

Try altitude = 8000 meters * ln(Pground/Paltitude)
That is a base e logarithm if you are in doubt.
Accurate within say 50 m.

Jacques

[MrBrant 0]

most electronic pressure sensor "chips" you buy are already temperature-compensated.

I'm accually in the middle of building one right now (digital), ready for testing, accually. Used a Motorolla MPX-blah-blah-something-something as my sensor - already sugar-coated and temeperature compensated.

I would imagine the pro-trac has something similar.

[kallend 1,647]

Quote

most electronic pressure sensor "chips" you buy are already temperature-compensated.

I'm accually in the middle of building one right now (digital), ready for testing, accually. Used a Motorolla MPX-blah-blah-something-something as my sensor - already sugar-coated and temeperature compensated.

I would imagine the pro-trac has something similar.

It can only compensate for its own temperature, not the temperature of the atmosphere which may well be different if the chip is inside a case with other components that generate heat, and the device is inside, say, a helmet with a head to keep it warm.

...

The only sure way to survive a canopy collision is not to have one.

[kbell 0]

I worked on this same problem a few years ago trying to analyze pro-track data on a non-PC platform. I remember it being much harder than I expected to find a formula to convert from pressure to altitude. Here's what I finally came up with, combining several formulas I found and solving for altitude. It seems to work pretty well (agrees within a few feet with the PC Jump-Track software)

const float kDryAirGasConstant     = 287.04;

const float kGravity               = 9.80665;

const float kSeaLevelTemperatureK  = 288.15;

const float kSeaLevelPressure      = 1013.250;

const float kTempLapseRatePerMeter = -0.0065;

const float kMetersToFeet          = 3.28084;

// calculate the height above sea level for test pressure

float a = kDryAirGasConstant / kGravity * log(kSeaLevelPressure / testPressure);

float meters = 2 * a * kSeaLevelTemperatureK / (2 - kTempLapseRatePerMeter * a);

// optional: float feet = meters * kMetersToFeet;	

This calculates height above sea-level, so to get height above ground, you need to calculate the ground height above sea level and subtract that from all the altitudes you calculate. I find that the absolute altitudes vary a bit, especially in Texas on a hot day (100+ F), but the relative height above ground is pretty accurate.