It is known that improvements in target tracking can be achieved by us
ing multiple sensors. Most commonly, the individual measurement sequen
ces are merged using a variant of linear algorithms. The approach prop
osed here differs from the conventional one in that nonlinear methods
of data fusion are proposed to account for the peculiarities of the di
fferent measurement categories. This technique, called complementary f
usion, is illustrated with the problem of tracking an agile target. It
is shown that complementary fusion not only leads to higher fidelity
tracking, but it also permits the more efficient utilization of the pr
imary sensor.