United States of America As Represented by the Secretary of the Air Force

Rudy L Shaw

James B Tsui

Nicholas A Pequignot

David L Sharpin

James N Hedge

Joseph Caschera

Disclosed is a method to extract the periodic properties (or Pulse Repetition Intervals (PRIs)) of radar signals whose time-of-arrival at an airborne platform have been time tagged by an Electronic Warfare (EW) receiver. The PRIs are determined using a modified Discrete Fourier Transform (DFT) written as ##EQU1## where k represents the frequency components, the T.sub.i are the individual Time-Of-Arrival (TOA) data values and N is the last TOA value collected. Disclosed are three possible methods of reducing the number of computations involved with the modification of the Discrete Fourier Transform (DFT) equation to facilitate its use with Radar Time-Of-Arrival (TOA) to extract periodic properties of radar signals. This is particularly applicable to radar pulse trains which are interleaved in time and where each individual pulse train may be staggered or jittered in time so that a time domain deinterleaving scheme must be employed. A spectral estimator can be computed by taking the modulus of the DFT of the time series data and extending the periodogram estimator by making a slight modification of the DFT equation.

Spectral estimation of radar time-of-arrival periodicities

The technique allows precise angle-of-arrival (AOA) and radio frequency measurements on non-cooperative radar signals, by exploiting precise phase measurement capabilities of frequency measurement receivers. It is assumed that the intercept receivers with this capability are on board an aircraft and there are at least two antennas available. By utilizing the phase measurements, the incident frequency and incident phase angle are calculated using formulae derived under the disclosed technique. By taking multiple samples and averaging, angle measurement errors can be reduced.

Doppler frequency angle measurement technique

The method provides the capability to estimate the angle-of-arrival (AOA) of radar pulses with respect to a ground or airborne based platform. It utilizes a spectral estimation technique which extracts the periodic properties of radar signals whose time-of-arrivals (TOAs) have been tagged by an Electronic Warfare receiver. The method can be used in a multiple signal environment and can separate and individually measure AOA of emitters that are spatially very close but have incommensurate Pulse Repetition Intervals (PRIs).

Angle-of-arrival measurement via spectral estimation of radar

To allow measurement of the angle-of-arrival (AOA) of the radar pulses from an uncooperative ground-based emitter, the method exploits the time doppler shift resulting from the velocity of a high performance aircraft and the high maneuver ability available to such aircraft to form initial angle calculations. These initial angle calculations along with inherent radar stability are then used for subsequent AOA measurements. The aircraft is flown at a constant velocity V.sub.B along successive legs with an angle .phi..sub.K between successive legs, and the velocity V.sub.B and the angles .phi..sub.K are found using a navigation system such as GPS or inertial navigation. The time difference T.sub.n '-T.sub.1 ' is measured between the arrival of the first and last of n pulses for each sample. A general approximation equation of T.sub.n '-T.sub.1 ' is normalized such that for any K samples the following equation applies,

Angle-of-arrival measurement via time doppler shift

To passively measure the range to a target, GPS signals which are scattered by the target are used to determine the distance from the target to an observation station whose position P.sub.o is determined by a GPS technique. If the delay time from the reflected signal of the target can be measured, the position P.sub.t of the target can be calculated. Four simultaneous nonlinear equations from the four satellites can be written as

Passive ranging through global positioning system

The apparatus detects the presence of long or short pulse phase shift keyed (PSK) modulated signals and determines the underlying clock frequency and modulation code associated with such signal, in real time, using an instantaneous frequency measurement (IFM) receiver as its signal source, without affecting the band-width or sensitivity of this receiver. It monitors the output of the IFM correlators for transients which occur in the correlator's outputs when a PSK signal is present. Logic circuits are used to declare the PSK signal is present, determine its clock rate, a record the modulation or total number of pulses.

PSK detection using an IFM receiver

The instantaneous frequency measurement (IFM) receiver is highly susceptible to erroneously reporting frequency if two or more signals are overlapped in time. This problem is extremely important when the receiver is presented with synchronized signals. A method is presented to detect simultaneous signals which overlap in the critical frequency encode strobe. In essence, during the pulse overlap condition, a transient will occur on the video output lines of the receiver. These lines are monitored during the transient period and compared with predetermined values which are obtained from the single signal condition. If a transient is detected, a simultaneous signal detect flag is raised and this flag is sent to an associated processor. In this special design, the receiver will measure the frequency again at approximately 200 ns from the first measurement. The transient detection is also performed on the second measurement.

Simultaneous signal detection for IFM receivers by transient detection

An apparatus for use in conjunction with an instantaneous frequency measurement (IFM) receiver, for detecting the presence of two or more RF pulse signals, differing in frequency, between the onset of the first RF signal pulse and the completion of the frequency encode strobe. A low pass filter branching off from the output of the limiting amplifier of the IFM receiver will allow passage of only the low frequency intermodulation products formed from the presence of simultaneous signals. These intermodulation products are then detected and sampled during the time period of interest with a flag set if simultaneous pulses are present. This flag, which indicates to the receiver that the data associated with the received pulse may contain erroneous information, is maintained until reset by the onset of a succeeding RF pulse.

Simultaneous signal detection for IFM receivers by detecting

An apparatus for use in conjunction with an instantaneous frequency measurement (IFM) receiver, for detecting the presence of two or more RF signals, differing in frequency, between the onset of the first RF signal pulse and the completion of the frequency encoding strobe. High frequency sample and hold circuits detect the level of the video upon stabilization of the leading edge of the first received pulse. Thereafter, comparators monitor both the video and sampled levels to detect defined differences therebetween. Monitoring ceases at the termination of the frequency encode strobe. Ambiguities in the encode frequency are detected by comparator unbalances, which actuate the inputs to a logic OR gate. The output of the OR gate is connected to a logic AND gate, whose other input changes in level upon the termination of the encode strobe. The output of the AND gate latches an alarm, signalling the IFM receiver that the frequency measurement is ambiguous.

Growing community of inventors

Huber Heights, OH, United States of America

Rudy L Shaw