The International Geomagnetic Reference Field (IGRF) 1965 was the first of such reference fields and was adopted by the International Association of Geomagnetism and Aeronomy (IAGA) in 1968 [IAGA Commission 2 Working Group 4, 1969]. It consists of a model of the main field at 1965.0 along with a model of secular variation for use in extending the main field model in time, both backward (not earlier than 1955.0) and forward (not later than 1975.0). IGRF 1975, adopted later, consists of IGRF 1965 extended to 1975.0, along with a revised model of secular variation for use in extending the main field model up to 1980.0 [IAGA Division 1 Study Group, 1976].
By the late 1970s, the cumulative effect of the inevitable uncertainties in the secular variation models had led to unacceptable inaccuracies in the IGRF. To satisfy the need for an accurate international geomagnetic reference field, this working group recommended the following additions: