Similar to ISA which has been defined for aircraft and instruments another definition has been made for high level flights and spacecraft testing. Although most of the definitions are same there are some variations between JSA and ISA.
In JSA air is considered to be dry and mean sea level its temperature is +15 degrees centigrade just like that of ISA. The mean sea level pressure continues to remain 1013.25 hecta pascals.
Air density at sea level is 1225 grams per meter cube. The acceleration due to gravity is 9.80665 meters per second square.
The difference lies in the lapse rate and tropopause. Lapse rate is considered to be 2 degrees centigrade per thousand feet. In addition, Tropopause is considered to be absent in Jet Standard Atmosphere.