A method for calculating the straight length and the shortest arc line length between two points according to the longitude and latitude information given by satellite positioning data is presented. The operation range between an infrared detection system and a target is the length of a straight line. It can be calculated according to the longitude and latitude information of the infrared detection system and the target. If either of the infrared detection system and the target is moving (e.g. ground-to-air or air-to-ground) or both of them are moving (e.g. air-to-ground), the operation range of the infrared detection system can be calculated by using this method.