This data set contains the 448 coefficients of the GSFC (11/87) geomagnetic field model. The spherical harmonics series is of degree and order 13 in the constant and first derivative terms. GSFC (11/87) is the first model making use of the large DE data base. Data from the DE 2 spacecraft for September 30, 1981, through January 6, 1983, are combined with data from MAGSAT (11/79-4/80), from Project MAGNET (1981-1983), from 158 magnetic observatories (1979-1983), from marine surveys (1980-1983.5), and from land surveys (1979.5- 1983.5). Vector data were obtained from MAGSAT and MAGNET (aircraft: 1.5 km to 8 km). From all other data sources only scalar data were considered. Poleward of 50 latitude, only scalar data were used from MAGSAT to avoid the contribution from the field-aligned currents in the polar ionosphere. For all observatory data the anomaly biases from non-core fields were taken into account. The GSFC (11/87) model is virtually identical to the GSFC (12/83) model at 1980 and may be regarded as an extension of that model in time to 1983.5. Its mean epoch is 1982. The model differs significantly from the IGRF model. Langel et al. (1988) also determined the first degree coefficients resulting from external sources and their dependence on the magnetic disturbance index Dst. The software package includes a subroutine for calculation of magnetic field strength and vector components.