Re: JBM altitude hangup
Sorry - I'm afraid that turns out not to be the case.
The airport doesn't report the station pressure. The altimeter setting it reports is the setting required to get a pressure altimeter to read the airport elevation - which means it's basically the sea-level-referenced barometric pressure. (Not quite - but the difference is not relevant here.)
See:
Barometric Pressure and Ballistic Software
Also note:
This applies to the standard trajectory screen,
http://www.jbmballistics.com/cgi-bin/jbmtraj-5.0.cgi
If you check "Standard Atmosphere at Altitude", you get the ICAO temperature and pressure at whatever altitude you specify, with the humidity set to zero. That means that whatever temperature, pressure, and relative humidity you might have entered in the input screen are completely ignored. See the chart of the ICAO Standard Atmosphere below. The temperature and pressure listed for the appropriate altitude in that chart are what the program will use.
If "Standard Atmosphere at Altitude" is not checked, temperature and RH entered in the appropriate boxes always affect the air density calculation.
If "Pressure is corrected" is checked, the density is calculated using the pressure corrected for whatever altitude is entered in the altitude box. It means that the pressure you entered is from a source which corrected the station pressure to the sea-level-referenced barometric pressure, and the program corrects <span style="font-style: italic">that</span> pressure for altitude.
That's confusing to me, but I didn't write the program. It still uses the temperature and relative humidity entered in the appropriate places.
If "Pressure is corrected" is not checked, the density is calculated ignoring the contents of the altitude box. In other words, the programs assumes that you have entered the station pressure. It still uses the entered temperature and relative humidity in the air density calculation.
The most accurate way to get any ballistic program to work is to have an instrument like a Kestrel or many watches which measures local atmospheric pressure, which we call the station pressure. Enter the station pressure, the temperature, and the relative humidity, and make sure that you're not using a program option to calculate the "standard atmosphere".