Multi-platform precision passive location of continuous wave emitters

This invention relates to methods and apparatus for locating radio frequency emitters, and more particularly to such methods and apparatus for locating continuous waveform emitters.

In order to meet the requirements of Network Centric Warfare, existing sensing platforms, such as the E-2 early warning, command and control aircraft, will have to be leveraged through the use of emerging architectures and technologies. Specifically, they will be called upon to perform fast, accurate location of targets, including traditional threat emitters such as radars, which use waveforms that have widely separated pulses with well-defined leading edges. In addition, fast, accurate location of an emerging set of target emitters, which have more continuous waveforms, is becoming increasingly important. These non-traditional target emitters include communications terminals that might be associated with terrorists, drug dealers, urban combatants, and emergency-first-response rescue personnel. Such communications terminals may include cell phones, PDAs, laptop computers, and other devices.

To address these target emitters, legacy platforms will have to be equipped with the sensor and sensor-management capability, and the communications infrastructure to support multi-platform collaborative targeting. In a collaborative targeting system, participating sensor platforms can contribute multiple measurements that are dependant on the target emitter\'s location, such as the Angle of Arrival (AOA). By taking advantage of the geometries available with multiple platforms, the geometric dilution of precision (GDOP) resulting from finite measurement accuracy can be avoided, and fast yet accurate location can be obtained from measurements such as AOA.

Although AOA can generally be measured for most emitter types, other precision measurements, such as Time Difference of Arrival (TDOA), are traditionally applied to radar emitters, because radars emit easy-to-distinguish pulses with leading edges that enable time of arrival to be measured. Applying techniques to continuous waveform emitters, such as communications terminals, is less straightforward, however, due to the absence of a well-defined event, such as the leading edge of a pulse that would enable measurement of the time of arrival. There is a need, then, for a method for passively determining the location of continuous wave emitters in multi-platform network centric systems.

This invention provides an apparatus comprising a first sensor mounted on a first platform for sampling a first portion of a continuous waveform occurring in a time window and for producing a first signal sample, a second sensor mounted on a second platform for sampling a second portion of the continuous waveform occurring in the time window for producing a second signal sample, and a processor for determining time difference of arrival measurements and for applying a maximum likelihood estimation process to combine multiple time difference of arrival measurements between multiple pairs of platforms, to estimate the location of an emitter of the continuous waveform.

In another aspect, the invention provides a method comprising the steps of: using a first sensor mounted on a first platform to sample a first portion of a continuous waveform occurring in a time window to produce a first signal sample, using a second sensor mounted on a second platform to sample a second portion of the continuous waveform occurring in the time window to produce a second signal sample, time shifting the first signal sample with respect to the second signal sample, correlating the first and second signal samples to determine a time difference of arrival measurement, and applying a maximum likelihood estimation process to combine multiple time difference of arrival measurements between multiple pairs of platforms, to estimate the location of an emitter of the continuous waveform.

In yet another aspect, the invention provides an apparatus comprising a first sensor mounted on a first platform for sampling a first portion of a continuous waveform occurring in a time window and for producing a first signal sample, a second sensor mounted on a second platform for sampling a second portion of the continuous waveform occurring in the time window for producing a second signal sample, and a processor for determining frequency difference of arrival measurements and for applying a maximum likelihood estimation process to combine multiple frequency difference of arrival measurements between multiple pairs of platforms, to estimate the location of an emitter of the continuous waveform.

In still another aspect, the invention provides a method comprising the steps of using a first sensor mounted on a first platform to sample a first portion of a continuous waveform occurring in a time window to produce a first signal sample, using a second sensor mounted on a second platform to sample a second portion of the continuous waveform occurring in the time window to produce a second signal sample, determining frequency difference of arrival measurements from the first and second signal samples, and applying a maximum likelihood estimation process to combine multiple frequency difference of arrival measurements between multiple pairs of platforms, to estimate the location of an emitter of the continuous waveform.