Latitude is the latitude of the launch site. To get into orbit with the least expenditure of energy, it is desirable to launch from the equator of a rotating body because the rotation boost is maximized there. When launching from a rotating body, the orbit achievable with the least energy expenditure will range between the latitude of the liftoff site and the mirror latitude. To explain that by example, say you launch from Cape Kennedy, which has a latitude of twenty eight degrees, the lowest energy orbit achievable from there will reach up to twenty eight degrees latitude and half an orbit later it will be at minus twenty eight degrees latitude.

When trying achieve an orbit which ranges to a high latitude, the lowest energy route there would start at that high latitude. So when trying to launch a polar orbiting satellite, the launch will take place from as high a latitude as possible. It is not economic to launch from a pole because of the harsh environment there, so as a compromise, the launch will start at typically fifty degrees latitude. Latitude is used with radius and rotation period to calculate rotation boost.

rotation boost = 2 * pi * radius * rotation period * cos( latitude )
 

This is used in astropolis, atmospheric spacecraft, multi stage spacecraft, and spacecraft cost.
 

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