Description of the GNSS found on the ATR called the HT1000 and explanation of the basic functions such as planning a route, SID, STARS, approaches and general ground preparation.
ISA is international standard atmosphere. It is used to standardize aircraft instruments and performance calculations. It states 3 guidelines 1) temperature on ground is 15°C at 0 feet MSL 2) pressure on ground is 1013 hpa or 29.92 inches of mercury 3) lapse rate is 1.98°/1000 ft. Performance for the aircraft depends on the outside air temperature and pressure. What I showed in the QRH was performance according to the outside environment. Since ISA is a standard it can't always be 15° on ground it can be hotter or colder. In the East African airspace it is usually ISA+15 that means it is 15° hotter than ISA temperatures. At 15,000 feet at ISA it would be -15°C outside air temperature but since for example when you check the temperature it is 0°.. it is 15° hotter than expected ISA. Therefore ISA+ 15. For example in the temperate climates in the winter I can sometimes be ISA-20! Hope this cleared it for you
