Frequency difference of arrival (FDOA) or differential Doppler (DD), is a technique analogous to TDOA for estimating the location of a radio emitter based on observations from other points.
TDOA and FDOA are sometimes used together to improve location accuracy and the resulting estimates are somewhat independent.
It differs from TDOA in that the FDOA observation points must be in relative motion with respect to each other and the emitter.
This relative motion results in different doppler shifts observations of the emitter at each location in general.
A disadvantage of FDOA is that large amounts of data must be moved between observation points or to a central location to do the cross-correlation that is necessary to estimate the doppler shift.