Two-channel array for moving target indications

Abstract

A radar antenna system includes a single transmitter for creating pulses from a wideband waveform. A splitter divides each pulse into half-power pulses, and sends them along respective paths. On one path, successive half-power pulses are alternately modulated with a phase shift .sub.A or .sub.F. On the other path, the half-power pulses are not modulated. Each modulated half-power pulse is then combined with an un-modulated half-power pulse to transmit pulses of a full aperture beam with either .sub.A or .sub.F. This establishes two degrees of freedom for the system. Two separate receivers then simultaneously receive the pulse echoes and a signal processor uses the consequent four degrees of freedom to create a radar indicator with mitigated clutter and useable azimuth estimation. A coherent processing interval can then be selected for multi-mode operation of the system.

Claims

1. A radar antenna system which comprises: a generator for creating pulses of a wideband waveform; a splitter for dividing each individual pulse into a first half-power pulse and a second half-power pulse; circuitry for directing the first half-power pulse along a first path, and for directing the second half-power pulse along a second path; a switch positioned on the first path to alternately modulate, in sequence, a first phase shift (.sub.A), and a second phase shift (.sub.F) on successive half-power pulses on the first path; a means for combining the modulated half-power pulses from the first path with the un-modulated half-power pulses from the second path to alternately create a first full aperture beam having a unique steering direction determined by the phase shift (.sub.A), and a second full aperture beam having a unique steering direction determined by the phase shift (.sub.F); a first receiver for receiving respective pulse echoes from the first full aperture beam and from the second full aperture beam to establish a first signal; a second receiver for receiving respective pulse echoes from the first full aperture beam and second full aperture beam to establish a second signal; and a signal processor for using the first and second signals to create a radar indicator.

2. A system as recited in claim 1 wherein the switch is a Single Pole Double Transmit (SPDT) switch.

3. A system as recited in claim 1 wherein a slow-time modulation is employed on half-power pulses on the first path.

4. A system as recited in claim 3 wherein the slow-time modulation is bi-phase.

5. A system as recited in claim 1 wherein a coherent processing interval is selected to mitigate clutter and minimize endo-clutter azimuth estimation.

6. A system as recited in claim 1 wherein the first receiver includes a means for reordering the first signal into a bifurcation, wherein the bifurcation comprises: a first subset of pulses with each pulse having the first phase shift (.sub.A); and a second subset of pulses with each pulse having the second phase shift (.sub.B), wherein the first subset combines with the second subset to establish two receive degrees of freedom for the first signal.

7. A system as recited in claim 6 wherein the second receiver includes a means for reordering the second signal into a bifurcation, wherein the bifurcation comprises: a first subset of pulses with each pulse having the first phase shift (.sub.A); and a second subset of pulses with each pulse having the second phase shift (.sub.B), wherein the first subset combines with the second subset to establish two receive degrees of freedom for the second signal.

8. A system as recited in claim 7 wherein the two receive degrees of freedom of the first signal, and the two receive degrees of freedom of the second signal, are used together to provide four degrees of freedom for creation of the radar indicator.

9. A method for operating a radar antenna system, wherein use of a transmitter in the system comprises the steps of: generating a wideband waveform; creating pulses of the waveform; splitting each pulse into a first half-power pulse and a second half-power pulse; directing a plurality of the first half-power pulse along a first path, and a plurality of the second half-power pulse along a second path; alternately modulating, in sequence, a first phase shift (.sub.A) and a second phase shift (.sub.F)on successive first half-power pulses on the first path; sequentially combining a modulated first half-power pulse from the first path with an un-modulated second half-power pulse from the second path to alternately create pulses for a first full aperture beam having a unique steering direction, and pulses for a second full aperture beam having a unique steering direction; and alternately transmitting a respective pulse of the first full aperture beam and of the second full aperture beam.

10. A method as recited in claim 9 wherein receiving return signals (echoes) from the first full aperture beam and from the second full aperture beam comprises the steps of: receiving the pulse echoes with respective phase shifts (.sub.A) and (.sub.F) at a first receiver to establish a first signal having two degrees of freedom; receiving pulse echoes with respective phase shifts (.sub.A) and (.sub.F) at a second receiver to establish a second signal having two degrees of freedom; and combining the first and second signals to use four degrees of freedom for creation of a radar indicator.

11. A method as recited in claim 10 further comprising the steps of: reordering the first signal into a bifurcation having a first subset of pulses wherein each pulse has the first phase shift (.sub.A), and a second subset of pulses wherein each pulse has the second phase shift (.sub.B); and separating the first subset from the second subset to establish two degrees of freedom for the first signal.

12. A method as recited in claim 11 further comprising the steps of: reordering the second signal into a bifurcation having a first subset of pulses wherein each pulse has the first phase shift (.sub.A) , and a second subset of pulses wherein each pulse has the second phase shift (.sub.B); and separating the second subset from the first subset to establish two degrees of freedom for the second signal.

13. A method as recited in claim 12 wherein the two degrees of freedom of the first signal, and the two degrees of freedom of the second signal, are used together in the combining step to provide four degrees of freedom for creation of the radar indicator.

14. A method as recited in claim 9 wherein the alternately modulating step is accomplished using a Single Pole Double Transmit (SPDT) switch.

15. A method as recited in claim 9 further comprising the step of selecting a coherent processing interval to mitigate clutter and minimize endo-clutter azimuth estimation.

16. A method as recited in claim 9 further comprising the step of selecting a coherent processing interval to selectively accommodate Synthetic Aperture Radar (SAR) and Ground Moving Target Indications (GMTI) operations.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:

(2) FIG. 1 is a depiction of a radar antenna system for the present invention as it is intended to be used in a typical operational environment;

(3) FIG. 2A is a functional schematic, for the transmit side of the radar system of the present invention, showing how a single waveform generator can be used to synthesize two different transmit beams to thereby establish two different transmit degrees-of-freedom;

(4) FIG. 2B is a functional schematic, for the receive side of the radar system of the present invention, showing how echoes from each of the two synthesized transmit beams, as illustrated in FIG. 2A, can be individually received by different receivers and then further synthesized to establish four degrees-of-freedom;

(5) FIG. 3A is an operational schematic, for the system of the present invention, showing how a single transmit wideband waveform can be manipulated to create two different transmit beams; and

(6) FIG. 3B is an operational schematic, for the system of the present invention, showing how the two different transmit beams shown in FIG. 3A can each be further manipulated to establish four degrees of freedom (perspectives) for presenting an image on a radar indicator with minimized clutter and useable azimuth estimation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(7) Referring initially to FIG. 1 a radar antenna system in accordance with the present invention is shown in an operational environment and is designated 10. As shown, the system 10 is mounted on an Unmanned Aerial Vehicle (UAV) 12 to generate a pulsed radar beam 14 that can be used to detect a slow moving target 16 (e.g. troop movements). As envisioned for the system 10, this will be so, even though the target 16 may be partially obscured by ground clutter 18. For purposes of the present invention, the radar beam 14 can be any wideband waveform known in the pertinent art that can be pulsed in a manner as disclosed below. In operation, there will be a pulse interval between successive pulses of the radar beam 14. For the system 10, will preferably be less than about one second.

(8) With reference now to FIG. 2A, it will be seen that the system 10 includes a single transmitter 20 (T.sub.x), but it has two different receive antennas (receivers). These receive antennas are respectively designated as a first receiver 22 (R.sub.x1) and a second receiver 24 (R.sub.x2). Further, it is to be appreciated with reference to FIG. 2A, that the transmitter 20 is capable of alternately directing the radar beam 14 between a beam field 26.sub.F and a beam field 26.sub.A. More specifically, in accordance with more detailed disclosure below, the radar beam 14 is to be electronically steered to have a forward (F) inclination in the beam field 26.sub.F, and an aft (A) inclination in the beam field 26.sub.A. For the purpose of distinguishing one steering inclination from the other, the designations of F and A are completely arbitrary and are interchangeable.

(9) As envisioned for the system 10, the target 16 will be covered by the pulsed radar beam 14 in both the beam field 26.sub.F and the beam field 26.sub.A. Thus, as shown in FIG. 2B, both the first receiver 22 and the second receiver 24 will each individually and separately receive a sequence of same return signals (echoes) 28 and 28 et seq. from the target 16. Furthermore, both receivers (R.sub.x1 22 and R.sub.x2 24) will simultaneously receive echoes 28.sub.F which are generated when the radar beam 14 has the forward (F) inclination and they will also simultaneously receive echoes 28.sub.A which are generated when the radar beam 14 has the aft (A) inclination.

(10) In FIG. 3A, the transmitter 20 is shown to include a generator 30 that will create a wideband waveform that is pulsed, with a pulse interval . The transmitter 20 also includes a splitter 32 that splits each pulse of the wideband waveform into two, substantially same half-power pulses. The half-power pulses are then directed from the splitter 32 onto either a first path 34 or a second path 36. While the half-power pulses on the second path 36 are not modulated, and are sent directly from the splitter 32 to a combiner 38, the half-power pulses on the first path 34 are sequentially modulated with different phase shifts.

(11) After leaving the splitter 32, succession of half-power pulses on the first path 34 are alternately directed by a switch 40 to a modulator 42. Preferably, the switch 40 is a Single Pole Double Transmit (SPDT) switch. More specifically, the switch 40 operates at the pulse interval to separate sequential half-power pulses from each other in order for them to be differently modulated. At the modulator 42, these half-power pulses from the first path 34 will then be given either a phase shift .sub.A or a phase shift .sub.F. The modulated half-power pulses are then transferred from the modulator 42 to the combiner 38. There they are sent respectively to either a summing point 44 (.sub.A) or a summing point 46 (.sub.F). FIG. 3A also shows that the un-modulated half-power pulses on the second path 36 are also sent by the combiner 38 to either the summing point 44 or the summing point 46.

(12) At the summing point 44, a .sub.A-modulated half-power pulse is combined with an un-modulated half-power pulse from the second path 36. Similarly, at the summing point 46, a .sub.F-modulated half-power pulse is combined with an un-modulated half-power pulse from the second path 36. As a consequence of these manipulations, a full aperture radar beam 14.sub.A is established with an aft-inclination, and a different full aperture radar beam 14.sub.F is established with a forward-inclination. The full aperture radar beam 14.sub.A is then radiated into the beam field 26.sub.A and the full aperture radar beam 14.sub.F is alternately radiated into the beam field 26.sub.F (see FIG. 2A).

(13) As indicated in FIG. 3B, the echoes (return signals) 28.sub.F from target 16 are simultaneously received by both the first receiver (R.sub.x1) 22 and the second receiver (R.sub.x2) 24. Likewise, the echoes (return signals) 28.sub.A from target 16 are simultaneously received by both the first receiver (R.sub.x1) 22 and the second receiver (R.sub.x2) 24. The echoes 28.sub.F and 28.sub.A, however, are alternately received, in sequence. It is therefore possible, in each receiver, to sort them out by separating the odd number echoes (e.g. echoes 28.sub.F) from the even number echoes (e.g. echoes 28.sub.A). In the event, after being separated, they are all sent to a signal processor 48 for processing. Thus, the signal processor 48 receives four subsets of signals, with each subset having a different perspective, degree of freedom (DoF). Specifically, within this structure, the first receiver 22 and the second receiver 24, respectively provide their own DoF. Further, the different echoes 28.sub.A and 28.sub.F cause each receiver to establish two DOFs. Thus, in combination, a single transmitter 20 and two receivers (R.sub.x1 and R.sub.x2) provide a system 10 with four DoFs for use in creating a radar indicator 50.

(14) While the particular Two-Channel Array for Moving Target Indications as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.