The invention relates, generally, to a system for passively locating electromagnetic signal sources. In particular, the invention relates to a system for measuring rotational Doppler frequencies imposed on an electromagnetic signal by the motion of an aircraft. More particularly, the invention relates to using these rotational Doppler frequencies to determine the location of the source of the radar signal impinging on the aircraft.
Many military aircraft today have electronic warfare (EW) systems including some form of radar warning and/or targeting capability. However, because of opponent counter-measures and the need for increased accuracy in targeting, it is desirable to develop improved systems and methods for military aircraft to passively locate enemy radar systems. Furthermore, in order to be cost effective it is desirable to develop upgrades to current systems that utilize existing antenna/cable assemblies on existing aircraft without the need for costly and difficult calibration procedures.
A number of radar detection systems for aircraft are known in the art. U.S. Pat. No. 5,870,056 to Fowler discloses an air-to-air passive location system comprising two antennae on a moving aircraft that determines Doppler frequency and long baseline interferometer (LBI) measurements. These are based on certain equations which are then used to provide initial estimates of the position of the emitter parameters. The initial estimates are then used in least squares calculations to estimate the position and velocity of the transmitter. However, this system is air-to-air specific, and requires phase coherent pulse trains and a phase calibrated interferometer.
U.S. Pat. No. 5,572,427 to Link et al. discloses a Doppler position bearing angle locator, comprising determining the location of a radio frequency source relative to an antenna on a moving aircraft by measuring the Doppler shift of the frequency of the source induced by motion of the antenna/aircraft. The system uses multiplexing, demultiplexing, modulation, and demodulation. The Doppler detection is based on active cooperation of the signal source (e.g., a navigation system), and thus the system cannot be used for passive location of non-cooperative or hostile signal sources.
U.S. Pat. No. 4,942,404 to Kefer discloses a passive Doppler differential ranging system and method. Kefer provides two antennae for receiving radar signals over time as the aircraft moves, and a system for calculating two differential Doppler phase shifts and the differential therebetween for determining the range of the signal source. This system assumes as negligible the rotation of the antenna baseline induced by the aircraft angular velocity.
U.S. Pat. No. 5,708,443 to Rose discloses a method and apparatus for using signal Doppler change to resolve long baseline interferometer ambiguous phase change measurements for locating a radar emitter. Rose provides a single antenna to measure the Doppler change in signal angle of arrival caused by the aircraft motion relative to the signal emitter as a means of passively locating the emitter. The Doppler effect from the lateral velocity is then used to remove ambiguities in an LBI instead of using antenna-array-derived SBI measurements. The system requires a coherent pulse train and assumes angular velocity is negligible.
U.S. Pat. No. 5,406,291 to Guerci et al. discloses a passive emitter location system. A multiplex system for an aircraft is provided for non-simultaneous measurement of the signal bearing and the Doppler induced frequency shifts using extended Kalman filters, single-measurement delayed initialization techniques, and the aircraft""s inertial navigational system. The system provides a single antenna design but requires accurate (0.5 degrees) direction finding capability and a highly stable (3 Hz) signal source, which conditions are rarely if ever found on military fighter aircraft.
U.S. Pat. No. 5,218,361 to Avila et al. discloses interferometer ambiguity resolution using missile roll, comprising a receiver that measures signal parameters received by interferometer antenna elements and determines the bearing to a transmitting antenna by using the missile and antenna roll motion to reorient the interferometer baseline. Avila et al. considers only phase-calibrated interferometer methods.
U.S. Pat. No. 5,969,677 to Herrmann et al. discloses a direction-finding method for determining the incident direction of a high-frequency electromagnetic signal. The method provides using at least two spatially separated antennae. Each antenna receives a signal with the phase difference between the signals determined through a phase-sequence analysis, and the incident angle of the signal (relative to the antenna) determined from these values. The direction finding is based on phase calibrated antenna pairs, with no apparent consideration of any platform motion.
U.S. Pat. No. 5,936,575 to Azzarelli et al. discloses an apparatus and method for determining angles-of-arrival and polarization of incoming RF signals. There is disclosed a non-planar array of at least two antennae and amplitude measurements of the incoming signals processed to provide the required polarization induced phase correction which allows for the determination of the angles-of-arrival. Signal polarization and phase are combined to correct anomalies in a 3 (or more) antenna azimuth/elevation direction-finding system, with no apparent consideration of any platform motion.
U.S. Pat. No. 5,724,047 to Lioio et al. discloses a phase and time-difference precision direction-finding system comprising at least two antennae for receiving signals. There are determined multiple ambiguous estimates of the angle of arrival based on the phase difference and frequency values, a coarse estimate of the angle of arrival based on a time difference of arrival calculation, and a precision angle of arrival estimate which is the ambiguous estimate closest to the coarse estimate based on phase interferometry. Time differences of arrival measurements are used to resolve ambiguities in a phase calibrated interferometer.
U.S. Pat. No. 5,526,001 to Rose et al. discloses precise bearings only geolocation in systems with large measurements bias errors. Antennae on aircraft are used to measure from multiple aircraft platforms the bearing rate of change and/or bearing differences, which are associated with circles on which the emitters must lie. There is provided bearings only geolocation requiring that the angular velocity term be ignored or removed.
These systems and methods may be suitable in some applications. However, these systems and methods leave room for improvement in the areas of more accurate and reliable radar location determination at a low cost.
For example, several of these systems are based on phase calibrated antenna methods. Accomplishing such phase calibration of existing antennae and cables on current military aircraft is very difficult and costly.
Additionally, the patents disclosing non-calibrated phase methods explicitly or inferentially assume that the lateral velocity term in the Doppler equation is the dominant source of the desired emitter location information. For these systems the angular velocity term is assumed to be either negligible or a source of confusion to be removed prior to the main computation.
Accordingly, what is needed but not found in the prior art are systems and methods exploiting both the lateral and angular velocity of military aircraft to improve their capability to passively locate enemy radar systems. Further, these systems and methods should accommodate existing antenna/cable assemblies on existing aircraft without the need for costly and difficult calibration procedures.
The present invention is a system for locating electromagnetic signal sources via the rotational Doppler effect. A radar station generating electromagnetic signals, such as but not limited to, radar frequency signals, scans the sky for aircraft. An aircraft having the present invention operationally installed thereon and utilizing apparatus or navigational systems already present on the aircraft receives those signals. The apparatus may, if desired, incorporate a computer, a multi-channel receiver, an inertial navigation system, and at least two receiving antennae.
The present invention is programmed to capture a time series of buffers of sampled signals from the signal source with an appropriately designed and tuned receiver through two or more spatially separated antennae. It then analyzes the sampled signals in each buffer, measures the rotational Doppler frequency that the aircraft motion has imparted to the signals of each buffer, and identifies the messages from the aircraft navigation system that describe the aircraft motion during the signal sampling process. A vector formula, given below, describing. the physics of the rotational Doppler effect predicts the series of rotational Doppler frequencies that would be measured from any hypothetical emitter location. A least squares algorithm applies this formula to a large grid of hypothetical emitter locations surrounding the aircraft sub-point. At each grid point every hypothetical rotational Doppler frequency is compared to the corresponding measured rotational Doppler frequency and the square of the discrepancy between the two is accumulated over the selected time interval. Finally, the least squares prediction of the emitter location is (by definition) the location of that grid point with the lowest accumulated squared discrepancy.
In a preferred embodiment, the present invention provides a system for determination of the location of emitters generating electromagnetic signals, the system being operationally disposed on an aircraft, the system having incorporated therein a computer, a multi-channel receiver, an inertial navigation system, and at least two receiving antennae, comprising the steps of:
a) receiving selected emitter signals via the receiving antennae;
b) formulating an emitter location grid;
c) deriving a hypothetical Delta Phase Rate, or rotational Doppler, for each grid point based upon the dynamics of the host platform;
d) deriving an actual Delta Phase Rate from said received emitter signals;
e) comparing said hypothetical Delta Phase Rate to said actual Delta Phase Rate for hypothetical emitter locations and calculating the discrepancy between them;
f) accumulating over a selected interval of time said compared squared discrepancies for a plurality of points in said grid; and
g) determining a prediction of the emitter location via said accumulated discrepancies.
When taken in conjunction with the accompanying drawings and the appended claims, other features and advantages of the present invention become apparent upon reading the following detailed description of embodiments of the invention.