High-Resolution Synthetic Aperture Side View Radar System Used By Means of Digital Beamforming

• US 20090109086A1
• Filed: 05/04/2007
• Published: 04/30/2009
• Est. Priority Date: 05/13/2006
• Status: Active Grant

• Current Assignee: Deutsches Zentrum Fã¼R Luft- Und Raumfahrt E.V.
• Original Assignee: Deutsches Zentrum Fã¼R Luft- Und Raumfahrt E.V.
• Inventors: Krieger, Gerhard, Moreira, Alberto, Gebert, Nicolas
• Granted Patent: US 7,944,390 B2
• Time in Patent Office: Days
• US Class Current: 342/25.F
• CPC Class Codes: G01S 13/904 SAR modes

Abstract

The transmission antenna (10) of the high-resolution synthetic aperture side view radar system comprises a plurality of sub-apertures (7, 8, 9). In each individual transmission pulse, said sub-apertures are controlled in such a manner that a spatiotemporally non-separable multi-dimensional high-frequency waveform is produced as an transmission signal pulse form, such that the modulation of each transmission pulse has a spatiotemporal diversity which is not described by the product having functions which are independent from each other and which are dependent on, respectively, only one spatial dimension. The thus produced transmission pulse form is combined to a capture-sided spatial filtering by means of digital beamforming adapted to said transmission signal pulse form.

Claims

1-21. -21. (canceled)

22. A high-resolution synthetic aperture side view radar system on a carrier moving above the earth surface at a constant speed, the carrier being provided with a transmission antenna (10) directed obliquely downward orthogonal to the direction of movement and formed by spatially separated sub-apertures (7, 8, 9), with a receiving antenna correspondingly directed obliquely downward orthogonal to the direction of movement and divided into a plurality of spatially separated sub-antennas, and with a coherently operating radar device, which emits high-frequency signals of independent waveforms via the sub-apertures that form transmission elements and which receives echo signals of the emitted high-frequency signals in a plurality of receiving channels via the spatially separated sub-antennas of the receiving antenna, characterized in that the waveforms emitted from the sub-apertures (7, 8, 9) of the transmission antenna (10), which periodically emits high-frequency pulses at a pulse repetition frequency, are all respectively emitted within a single high-frequency pulse and, as a transmission signal pulse form, result in a spatiotemporally non-separable multidimensional high-frequency waveform per high-frequency pulse, and that, to achieve a spatial filtering, the echo signals received in the receiving channels by the receiving antenna, which is subdivided into a plurality of sub-antennas in the azimuth and/or elevation directions, are digitally processed according to the so-called “

digital beamforming”

in the form of additional directional information and the transmission signal pulse form generated on the transmission side is combined with the receiving-side spatial filtering by means of the “

digital beamforming”

adapted to this transmission signal pulse form.

23. The radar system of claim 22, wherein the transmission antenna (10) formed by a plurality of sub-apertures (7, 8, 9) is structurally combined with the receiving antenna that comprises a plurality of sub-antennas and allows for “

digital beamforming”

.

24. The radar system of claim 22, wherein, within a transmission pulse, the individual transmission elements, and thus sub-apertures (7, 8, 9), of the transmission antenna (10) are controlled such in the azimuth and/or the elevation by varying the respective phase coefficients that radar transmission pulses with a spatiotemporally non-separable waveform are obtained.

25. The radar system of claim 22, wherein, within a transmission pulse, the individual transmission elements, and thus sub-apertures (7, 8, 9), of the transmission antenna (10) are controlled in the azimuth and/or the elevation by separate function generators such that radar transmission pulses with a spatiotemporally non-separable waveform are obtained.

26. The radar system of claim 22, wherein, within a transmission pulse, individual or, at the same time, a plurality of transmission elements, and thus sub-apertures (7, 8, 9), of the transmission antenna (10) are controlled sequentially in the azimuth and/or the elevation such that radar transmission pulses with a spatiotemporally non-separable waveform are obtained.

27. The radar system of claim 22, wherein the transmission antenna is configured as a multi-aperture transmission antenna for multi-dimensional spatiotemporal pulse coding, including n sub-elements (12) in the azimuth, each supplied by a function generator of its own, and m sub-apertures (13) in the elevation per azimuth column, which are controlled through separate controllable phase shifters (14) to allow the forming and controlling of the beam in the elevation direction.

28. The radar system of claim 22, wherein, for an improved suppression of azimuth ambiguities, spatiotemporally non-separable multi-dimensional waveforms are used in the azimuth, which are designed such that the radar echoes from different spatial angles, with otherwise similar scattering properties, lead to different received signals and are therefore separable from each other in the receiver.

29. The radar system of claim 28, wherein a spatiotemporal coding of the emitted waveform is provided, where the radar echoes from scattering objects at the same distance but at different spatial angles arrive at the receiver chronologically offset so that also a chronological offset between the desired signal and the azimuth ambiguities corresponding thereto is obtained, which is equivalent to the fact that, at a given time, with a radar directed obliquely downward, the ambiguities arrive under another elevation angle than the desired signal, so that the energy of the ambiguities can be significantly reduced or even be completely suppressed by the receiving-side spatial filtering by means of “

digital beamforming on receive”

in the elevation.

30. The radar system of claim 28, wherein the transmission pulse is generated by the coherent superposition of the signals of all transmission elements which all transmit at the same time, that each partial pulse comprises a different amplitude distribution in the azimuth direction (“

qAz”

) and a plurality of narrow azimuth beams are thus transmitted within a pulse which each only illuminate a part of the azimuth spectrum and therefore limit the azimuth bandwidth in each of the partial pulses in the manner of a band pass splitting in the azimuth, which, depending on which of the azimuth beams causes the echo, materializes in the receiving branch of the radar system as a relative time-shift of the echo from a target, which time-shift shows in the recorded data as a difference in the range direction and causes a distribution of the energy of the azimuth ambiguities in the range direction, that, by “

beamforming”

on the receiving side, the echoes of different azimuth beams are separated by the time-shift and the energy distributed in the range direction is suppressed, and that the echoes of the different azimuth beams are combined in the manner of a signal reconstruction of a multi-channel band pass splitting, in order to restore the complete azimuth bandwidth and to thus allow for the full geometric resolution in the azimuth direction.

31. The radar system of claim 22, wherein the transmission beam is controlled in the elevation direction within one transmission pulse, so that the antenna elements form a narrow transmission beam in the elevation that “

wanders”

across the target area during the duration of the transmission pulse, thereby, on the whole, illuminating a wide ground swath which results in a time-delayed illumination of different range areas with narrow transmission beams.

32. The radar system of claim 31, wherein the transmission beam control is effected such that the echoes received from different directions coincide chronologically, wherein the transmission beam dwells on each target area long enough to illuminate it with the entire band width of the transmission signal, and that the signals from different directions that arrive simultaneously are separated from each other using receiving-side “

digital beamforming”

in the elevation.

33. The radar system of claim 31, wherein a receiving window is provided that is greatly reduced in duration as compared to the duration of the radar transmission pulse.

34. The radar system of claim 33, wherein the duration of receiving window is calculated to be so long that all radar echoes are received within its opening time interval.

35. The radar system of one of claim 22, wherein the transmission pulse is generated by sequentially controlling the individual transmission elements according to a sequential code, wherein all transmission elements transmit within a total pulse, but only one at a time in each partial pulse, so that after an appropriate spatial filtering of the received signal by means of “

digital beamforming”

, each transmission element can be interpreted as a quasi-independent transmitter.

36. The radar system of claim 35, wherein the reflected echoes of a certain target, which result from a partial pulse of the respective transmission pulse and are received at different times, are separated by an appropriate processing on the receiving side.

37. The radar system of claim 35, wherein a detection or determination of moving objects is achieved by using the so-called base line that corresponds to the maximum distance between two sampled values that are obtained for the reception of an emitted pulse with a plurality of transmission elements, and which is extended by the use of the special transmission pulse form.

38. The radar system of claim 22, wherein a purposeful illumination of the target area is provided, so that a better spatial resolution and/or a higher radiometric sensitivity is achieved by different dwelling times of the transmission beam in certain partial areas and/or by a variation in the system resources, e.g. the bandwidth.

39. The radar system of claim 38, wherein partial areas of greater importance are allocated more system resources that areas of lesser importance.

40. The radar system of claim 39, wherein an operation in hybrid modes, wherein, at the same time, large areas are mapped and detail pictures of limited partial areas are made.

41. The radar system of claim 39, wherein areas which have been allocated more resources, can be selected adaptively.

42. The radar system of claim 39, wherein an optimization of the signal-to-noise ratio SNR by a dynamic adaptation of the spatiotemporal waveform to the evaluation of the received signal power for individual partial areas.