Drifter data from the Gulf of Mexico are used to assess and enhance a primitive 
equation, general circulation model. The analysis is made in a 450 km by 450 km 
open subdomain encompassing a warm core Loop Current ring. The model velocity 
field is compared with position data from four drifters at the drogue depth of 
50m using geometrical orthogonal functions (GOFs). An Eulerian velocity field is 
reconstructed from the model velocity field and drifter velocities. This 
reconstructed velocity improves the accuracy of numerical trajectories relative 
to the model field by at least an order of magnitude, as quantified by two 
Lagrangian error metrics referenced to the real drifter paths. Enhancement of 
the model velocity field is determined by two tests: the ability of the GOF 
velocity field to: 1) improve the forecast of drifter positions using only a 
posteri data and 2) improve the forecast of withheld drifter data.