Autonomous synchronization of a bistatic synthetic aperture radar (SAR) system

• US 5,113,193 A
• Filed: 11/12/1985
• Issued: 05/12/1992
• Est. Priority Date: 11/12/1985
• Status: Expired due to Fees

• Current Assignee: United States Department Of Health And Human Services (Government of the United States of America)
• Original Assignee: United States Of America As Represented By The Secretary Of The Air Force
• Inventors: Mallean, Henry G., Powell, Norman F.
• Primary Examiner(s): Tubbesing, T. H.
• Application Number:
  US06/795,571
• Time in Patent Office: 2,373 Days
• Field of Search: 343/5 CM, 343/9 R, 342/25, 342/109
• US Class Current: 342/25.00F
• CPC Class Codes: G01S 13/003 Bistatic radar systems; Multistatic radar systems
• CPC Class Codes: G01S 13/9058 Bistatic or multistatic SAR
• CPC Class Codes: G01S 13/9092 combined with monopulse techniques

Abstract

An airborne bistatic radar synchronization apparatus and process is presented which receives clutter point echo returns from the bistatic radar receiver on a penetrator aircraft, and determines therefrom the location of the illuminator aircraft without the need of receiving direct path signals from the radar transmitter on board the illuminator aircraft. Once the state of the illuminator aircraft (position and velocity) has been determined, the estimate of the illuminator state may be refined using direct path data. Finally, a target location algorithm is applied to bistatic target echo returns to calculate an equivalent monostatic range for a desired target, and provide an estimate of the location of the target.

Claims

1. In combination with an airborne bistatic radar system with a radar transmitter on an illuminator aircraft which transmits radar signals, and a bistatic radar receiver on a penetrator aircraft which receives said radar signals, a process of determining a location of a target, said location containing a measure of range from said target to said penetrator aircraft, denoted as R_T, and a measure of elevation and azimuth from said penetrator aircraft to said location of said target, denoted respectively as φ

_T and θ

_T, said process comprising the steps of;

calibrating said bistatic receiver on said penetrator aircraft by using an active radar transmitter and said bistatic radar receiver to determine an initial location of said illuminator aircraft, and therefrom a calibration of timing of when said radar signals are transmitted from said illuminator aircraft;

continually collecting clutter data on a plurality of clutter points, each of said clutter data containing a measure of a clutter point target range sum, denoted by R_cs, range sum rate, denoted by R_cs, and clutter point elevation and azimuth with respect to the penetrator aircraft, denoted respectively as φ

_c and θ

_c ;

continually calculating an illuminator state for said illuminator aircraft by applying an inverse bistatic transformation to said clutter point range sum and range sum rate measured by said bistatic receiver in said penetrator aircraft, said illuminator state being a measure of a position and velocity of said illuminator aircraft;

collecting bistatic target echo return signals by said bistatic radar receiver on said penetrator aircraft as said reflected off of said target, a measure of a target range sum, denoted by R_s, a target range sum rate, denoted by R_s, and target elevation and azimuth denoted by φ

_T and θ

_T ;

calculating a target range difference value for said target location; and

calculating an equivalent monostatic range for said location of said target, denoted R_T, using a bistatic transformation on said target range difference value.

2. A process as defined in claim 1, wherein said penetrator aircraft acquires direct path measurements from said illuminator aircraft prior to said calculating an equivalent monostatic range step, said direct path measurements including a measure of range between said penetrator aircraft and said illuminator aircraft and a measure of range rate between said penetrator aircraft and said illuminator aircraft, said direct path measurements serving to refine and improve estimates made on said illuminator state.

3. In combination with an airborne bistatic radar system with a radar transmitter on an illuminator aircraft which transmits radar signals, and a bistatic radar receiver on a penetrator aircraft which receives said radar signals un the form of clutter point echo return signals, target echo return signals, and direct path data signals, a bistatic radar synchronization system, said bistatic radar synchronization system being located on said penetrator aircraft and receiving radar signals from said bistatic radar receiver, said bistatic radar synchronization system being capable of determining the position and velocity of said illuminator aircraft using sad clutter point echo return signals, said bistatic radar synchronization system comprising:

an inertial navigation system producing an output signal containing the position and velocity of said penetrator aircraft; and

a doppler estimation means receiving said clutter point echo return signals from said bistatic radar receiver, said clutter point echo return signals indicating a measure of a clutter point target range sum which equals a sum of;

illuminator-to-clutter point range plus clutter point-to-receiver range, said doppler estimation means outputting a clutter point range sum rate which represents an amount of change observed in measures of said clutter point target range sum;

a filter means receiving;

said clutter point echo return signals from said bistatic radar receiver, said clutter point target range sum rate from said doppler estimation means and said output signal from said inertial navigation system, said filter means, producing an output signal which indicates the position and velocity of said illuminator aircraft.

4. A bistatic radar synchronization system, as defined in claim 3, wherein said filter means comprises:

a sum filter which indicates the position and velocity of said illuminator aircraft using an illuminator locator algorithm on;

said clutter point target range sum received from said bistatic radar receiver, said clutter point target range sum rate received from said doppler estimation means, and said output signal from said inertial navigation system, said illuminator locator algorithm, for regular terrain, being given by;

space="preserve" listing-type="equation">X.sub.k,k =X.sub.k,k-1 +G.sub.k (Y.sub.k -h(X.sub.k,k-1, Z.sub.k)),

where;

X

X_k,k-1 =Φ

(k,k-1)X_k-1,k-1, predicted value of a k^th state given k-1 measurements such that each solution for X_k yields said illuminator state as shown in this matrix;

##EQU38## where (X_I,Y_I,Z_I)=inertial position coordinates of said illuminator aircraft, and

(X_I,Y_I,Z_I), (X_I, Y_I, Z_I)=velocity and acceleration components of said illuminator aircraft;

G_k =P_k,k-1 H_k^T (R_k +H_k, P_k,k-1 H_k^T)^-1, filter gain matrix

h(X_k,k-1, Z_k)=nonlinear transformation evaluated at

X_k,k-1, Z_k, and is presented in this matrix;

##EQU39## Φ

(k,k-1)=state transition matrix from state k-1 to state k which is expressed in this matrix;

##EQU40## {t_k,k-1 =time interval between state k-1 and state k;

P_k,k-1 =Φ

(k,k-1)P_k-1,k-1 Φ

^T (k,k-1)+Q_k,k-1, predicted covariance matrix;

R_k =data noise covariance matrix given by;

##EQU41## Y_k =filter data matrix for a k^th sample where Y_k =Y'"'"'_k -N_k, and ##EQU42## R_s.sbsb.i,R_s.sbsb.i =range sum and range sum rate of an i^th clutter cell, and ##EQU43## Z_k =clutter position data and receiver position and velocity data for a k^th sample

Z_ik =variable representing the receiver and clutter geometry data for i^th clutter cell at k^th filter sample or scan (i.e., (X_R,Y_R,Z_R)_k, (X_R,Y_R,Z_R)_k and (X_c,Y_c,Z_c)_ik).

5. A bistatic synchronization system as defined in claim 4 including a synchronization means, said synchronization means receiving said output signal from said sum filter and any direct path data signals from said bistatic radar receiver, said direct path data signals being radar signals received by said bistatic radar receiver in a direct path from said radar transmitter on said illuminator aircraft and indicating measure of the position and velocity of said illuminator aircraft, said synchronization means producing an illuminator state signal which indicates the position and velocity of said illuminator aircraft by outputting said direct path data signals when they are available, and outputting said output signal of said sum filter when said direct path data signals are not received by said bistatic radar receiver.

6. In combination with an airborne bistatic radar system with a radar transmitter on an illuminator aircraft which transmits radar signals, and a bistatic radar receiver on a penetrator aircraft which receives said radar signals in the form of clutter point echo return signals, target echo return signals, and direct path data signals, a bistatic radar synchronization system being located on said penetrator aircraft and receiving radar signals from said bistatic radar receiver, said bistatic radar synchronization system being capable of determining the position and velocity of said illuminator aircraft, said bistatic radar synchronization system comprising:

a summing junction which outputs a target range difference signal which it produces by subtracting an illuminator range signal from a target range sum signal, said illuminator range signal being a measure of distance between said illuminator aircraft and said penetrator aircraft, said illuminator range signal being contained in said direct path data signals which are received by said summing junction from said bistatic radar receiver, said target range sum signal being a measure of range between said illuminator and said clutter point plus range between a clutter point and said penetrator aircraft, said target range sum signal being contained in said clutter point echo return signals which are received by said summing junction from said bistatic radar receiver;

a doppler estimation means receiving said clutter point echo return signals and said direct path data signals from said bistatic radar receiver and outputting a target range difference rate signal which indicates a rate of change in said target range difference signal; and

a filter means receiving said target range difference signal from said summing junction, and said target range difference rate signal from said doppler estimation means, said filter means producing an output signal which indicates the position and velocity of said illuminator aircraft.

7. A bistatic radar synchronization system as defined in claim 6 wherein said filter means comprises a difference filter which indicates the position and velocity of said illuminator aircraft using an illuminator locator algorithm on:

said target range difference signal received from said summing junction, said target range difference rate signal received from said doppler estimation means, and said output signal from said inertial navigation system, said illuminator locator algorithm, for regular terrain, being given by;

space="preserve" listing-type="equation">X.sub.k,k-1 =X.sub.k,k-1 +G.sub.k (Y.sub.k -h(X.sub.k,k-1, Z.sub.k)),

where;

X_k,k-1 =Φ

(k,k-1)X_k-1,k-1, predicted value of a k^th state given k-1 measurements such that each solution for X_k yields said illuminator state in the following matrix;

##EQU44## where (X_I,Y_I,Z_I)=inertial position coordinates of said illuminator aircraft, and

(X_I,Y_I,Z_I), (X_I, Y_I, Z_I)=velocity and acceleration components of said illuminator aircraft;

G_k =P_k,k-1 H_k^T (R_k +H_k, P_k,k-1 H_k^T)^-1, filter gain matrix h(X_k,k-1, Z_k)=nonlinear transformation evaluated at X_k,k-1, Z_k, and is presented in the following matrix;

##EQU45## Φ

(k,k-1)=state transition matrix from state k-1 to state k which is expressed in the following matrix;

##EQU46## Δ

t_k,k-1 =time interval between state k-1 and state k;

P_k,k-1 =Φ

(k,k-1)P_k-1,k-1 Φ

^T (k,k-1)+Q_k,k-1, predicted covariance matrix;

R_x =data noise covariance matrix given by;

##EQU47## Y_k =filter data matrix for a k^th sample where Y_k =Y'"'"'_k -N_k, and

σ

_W.sbsb.1², σ

_W.sbsb.i² =measurement variances of differential bistatic range and range rate of an i^th clutter cell, and

Q_k,k-1 =deweighting matrix ##EQU48## Δ

t_k,k-1 `time interval between state k-1 and d state k y_k =filter data matrix for a k^th sample where Y_k =Y'"'"'_k -N_k, and ##EQU49## W_i,W_i =differential bistatic range and range rate of an i^th clutter cell, and

N_W.sbsb.i and N_W.sbsb.i are noise terms occurring in W_i and W_i ##EQU50## Z_k =clutter position data and receiver position and velocity data for a k^th sample where

Z_ik -variable representing receiver and clutter geometry data for i^th clutter cell at k^th filter sample or scan (i.e., (X_R,Y_R,Z_R)_k, (X_R,Y_R,Z_R)_k (X_c,Y_c,Z_c)_ik.