1. A method of estimating the closest point of approach of an object, comprising: tracking in a first tracker a range rate of the object based on Doppler measurements of the object obtained at sequential measurement times; computing for each measurement time a first closest point of approach estimate based on the tracked range rate; tracking in a second tracker separate from and independent of the first tracker a position of the object based on position measurements of the object obtained at the sequential measurement times; computing for each measurement time a second closest point of approach estimate based on the tracked position; computing in a processor a fused closest point of approach estimate based on the first and second closest point of approach estimates; and reporting, via a user interface, an output closest point of approach estimate based on the fused closest point of approach estimate.