Passive monopulse ranging to a non-cooperative emitter and non-emitting object

A radar system for passive determination of range to a non-cooperative scanning radar, to a non-emitting object illuminated by the scanning emitter and between scanning emitter and illuminated object entirely from the passive location. The radar equipment at the passive location includes a passive array with beam forming and switching matrix to provide output signals separately on the basis of angular discrimination, for each of the object and emitter. Resolver circuits respond to the angle between emitter and object vector, the incremental time between direct emitter reception and reflected echo from the object as well as to emitter scan rate and instantaneous pointing angle. Algorithms for the emitter range, object range and range between emitter and object are given.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates generally to radar system passive ranging, and more 
specifically to systems providing substantially instantaneous ranging from 
parameters determinable entirely at a passive location. 
2. Description of the Prior Art 
Known methods for passive location of objects include concepts generally 
identified as triangulation, tri-lateration and bistatic radar. All of 
these rely on prior knowledge of two or more points in space to resolve 
the location of an object in question. In this description, the terms 
"object" and "target" are both used to refer to a non-transmitting 
(non-emitting) object such as a friendly or non-cooperating aircraft. 
The triangulation concept presumes two known receiver locations for 
separate measurement of angle to an emitting object, and resolvers are 
computers or determining location from those separate measurements. 
The tri-lateration method uses three or more time-of-arrival measurements 
from separate known positions to locate an emitting object. 
The classical bistatic radar employs knowledge of the transmitter and 
receiver locations, pointing angle of the transmitter and 
time-difference-of-arrival along a direct path, vis-a-vis that along an 
indirect path, to locate a non-emitting object. 
It is highly desirable to have the capability for location of both emitting 
and non-emitting objects having only knowledge of the measurement platform 
instantaneous location and the measurements which can be made internally 
and passively at any given instant in tune. 
The manner in which the invention provides a novel combination for passive 
rapid measurements not requiring a priori knowledge of positions except 
for the passive measurement equipment itself is disclosed. 
The manner in which the invention addresses the shortcomings of the prior 
art and the aforementioned needs will be apparent as this description 
proceeds. 
SUMMARY OF THE PRESENT INVENTION 
It may be said to have been the general objective of the invention to 
provide ranging information in respect to non-cooperating scanning pulse 
radar equipment and objects or targets illuminated thereby wholely 
passively from a passive measurement radar system. The apparatus according 
to the invention includes resolvers for measuring the angle to a 
non-emitting object from said passive location using reflected energy from 
the non-cooperating pulse scanning transmitter (emitter) and also by 
determine the angle to the said emitter from said passive location. An 
angle between emitter and a line from the emitter to the reflecting object 
is calculated by measuring the emitter beam passage, determining its 
period and projecting the pointing angle with respect to the line 
connecting the radar emitter and the passive measurement antenna. These 
angles and the time difference of arrival obtained by measuring arrival 
times of energy via the direct path and the indirect path is the only 
information required to locate the radar emitter and/or the non-emitting 
object. 
The details of two conceptually equivalent embodiments will be understood 
as this description proceeds. Although conceptually equivalent, the 
implementation of the two embodiments differs as will be realized from the 
description following.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, the geometry of ranging according to the invention 
is illustrated. A passive measurement radar system 10 according to the 
invention is located at point B and has an angular receiving capability at 
least in excess of the angle .alpha.. A line 15 is the angular reference 
and may be, for example the boresight of the antenna system associated 
with radar 10. The scanning, pulsed surveillance radar (emitter) 11 is 
located at point C. This radar has a beam antenna pattern 12 with a usual 
plurality of side lobes 13. This beam is shown directed along the range 
line R.sub.E/t, i.e., the range from the emitter to the non-emitting 
object. The range line R.sub.1 will be seen to be the range to the 
non-emitting object A from the passive measurement radar system 10 and, 
similarly, the range line R.sub.E represents the bearing of emitter 11 as 
see from point B and with respect to the angle reference line 15. 
It will be seen that the angular values capable of measurement directly 
from the passive measurement radar system lo at point B are the angles 
.alpha. and .beta.. From those measurements, certain relationships become 
immediately obvious as follows: 
EQU .phi.=.vertline..alpha.-.beta..vertline. 
EQU .theta.=180.degree. -.phi.-.OMEGA. 
EQU .OMEGA.=180.degree. -.theta.-.phi. 
or 
EQU .OMEGA.=180.degree. -.theta.-.vertline..alpha.-.beta..vertline. 
EQU .DELTA.t=1/C(R.sub.E/t +R.sub.t -R.sub.E) 
where C is the speed of light. 
Directly emitted energy from emitter 11 facilitates the measurement of the 
angle .beta., however, the measurement of the angle .alpha. depends on 
reflected energy from a non-emitting object 14. The angle .theta. is 
calculated by measuring the passage of emitter beam 12 to determine its 
period and by projecting the pointing angle with respect to R.sub.E. Those 
angles and the time-difference-of-arrival .DELTA.t obtained by measuring 
arrival times of energy arriving via the direct path R.sub.E and the 
indirect path R.sub.t is the only information required to locate the radar 
emitter and/or the non-emitting object (points C and A, respectively). 
There are essentially two principal variations under which the passive 
measurement radar system 10 according to the invention may be required to 
operate. One of these will be identified as Case I, in which the scan rate 
of the non-cooperating emitter 11 is substantially constant through 
360.degree. as would be the case with a conventional Pulse Position 
Indicator (PPI) surveillance radar. Derivation of the location algorithms 
for Case I is as follows: 
From the Law of Sines: 
##EQU1## 
Solving for R.sub.E/t and R.sub.t in terms of R.sub.E 
##EQU2## 
The definition of time-difference-of-arrival is: 
##EQU3## 
Substituting for R.sub.E/t and R.sub.t and collecting terms: 
##EQU4## 
Thus, R.sub.E, R.sub.E/t, and R.sub.t may be found: 
##EQU5## 
In Case II, we will be concerned with a non-constant scan rate or sector 
scanning and agile beam emitters at point C. 
The passive location algorithm may be expanded to obtain range to emitters 
or non-emitters when the emitter scan rate is non-determinable by 
utilizing knowledge of any illuminated object's locations with respect to 
the measurement platform. The angle .beta. and the range to the emitter is 
first established as in Case I. Once the emitter location is known, 
locations of all non-emitters of interest may be established. The 
derivation of algorithms for determining the location of the emitter and 
non-emitters for Case II is as follows: 
##EQU6## 
Relating to FIG. 1: 
EQU 2S=R.sub.E/t +R.sub.t +R.sub.E 
EQU 2S-2R.sub.E =R.sub.E/t +R.sub.t -R.sub.E =.DELTA.R=.DELTA.tC 
##EQU7## 
Equation (2) may be solved for R.sub.E or R.sub.t. Solving for R.sub.E 
(emitter location). 
##EQU8## 
Solving for R.sub.t (illuminated object location): 
##EQU9## 
Referring now to FIG. 2, a schematic block diagram shows a simple 
implementation for obtaining monopulse type operation for location of 
emitting or non-emitting objects using a plurality of two located and 
mechanically driven antennas 101 and 102. These co-located receiver 
antennas may be in vertical juxtaposition, for example, such that their 
point of azimuth origin is the same. Independent emitter antenna 102 and 
target search antenna beams are thereby generated. The emitter beam is 
steered to dwell on the emitter during the location process. Data 
collected by the emitter pulse receiver is used to calculate the emitter 
pointing angle .theta. of the radar emitter 11 as a function of time 
identified as signal A.sub.E. This is accomplished by accurately 
determining the times when main beam passage of an emitter beam 103 
occurs. This establishes the emitter scan period letting .tau.=T.sub.early 
-T.sub.late. The scan rate (360.degree..div..tau..times..omega.) may be 
computed and the emitter pointing angle .theta. which is a function of 
time (.theta.=T.sub.late +.omega.t) may be projected. Knowing the emitter 
pointing angle .theta., the target antenna 101 may be scanned through the 
space illuminated by the emitter 11. An emitter receiver and encoding 
mechanism 110 shown measures each direct path pulse event and also 
measures the bearings of targets illuminated by the location of the target 
antenna beam. Accordingly, the indicated angles and a target pulse arrival 
time signal t.sub.t and an emitter pulse arrival time signal t.sub.E and 
.alpha., .DELTA.t, .DELTA.tC, .phi., sin .phi., sin .theta., and sin 
(.phi.+.theta.) may be calculated the ultimately range to the emitter 
R.sub.e and the range to the target R.sub.t from the passive measurement 
radar system 10 as shown in FIG. 1 may be calculated and displayed. The 
implementation represented in the labeled blocks of FIG. 2 are 
instrumentable per se by those of skill in this art. The outputs are the 
values of R.sub.E, .beta., R.sub.t, and .alpha.. 
Referring now to FIG. 3, an electronic implementation employing an array 
antenna 301 with a beam forming and switching matrix 302 take the place of 
the mechanically positioned pencil beam antennas 101 and 102 from FIG. 2. 
The manner of forming a plurality of pairs of monopulse beams, such as 
beams 303 and 304 for an emitter directed receiving channel 307 and a 
target location beam pair 305 and 306 (also known as target channel 
radiation patterns), is also conventional in this art. The beam forming 
network 302 may be of the "Rotman lens" type providing a number of output 
ports each representative of a discrete receiving angle. A switching 
arrangement can provide for the connection of each of the receivers 307 
and 308. The receiver outputs T.sub.t, .alpha., A.sub.e, T.sub.e and 
.beta. are connected as, shown among the resolvers 309, 310 and 311. These 
resolvers make the calculations indicated on FIG. 3, the resolver 309 
providing the angles .phi., .alpha., and .beta. as an output, while 
resolver 310 calculates .DELTA.t and resolver 311 calculates the angle 
.theta.. Those resolver outputs are all fed to range resolver 312. The 
algorithms hereinbefore described are then applied to provide the range 
values R.sub.E, R.sub.t and R.sub.E/t. 
From the understanding of the concepts of the invention, those of skill in 
this art will realize that using, optical techniques in a comparable way 
can make the algorithms according to the invention applicable to 
surveying. Furthermore, using acoustic techniques, the algorithms may be 
applied for underwater usage. 
From an understanding of the basic concepts of the invention, it will be 
realized by those of skill in this art that other applications as well as 
variations and modifications in the implementations of the invention are 
possible. Accordingly, it is not expected that the scope of the invention 
should be regarded as limited to the drawings or this description, these 
being intended to be typical and illustrative only.