High-resolution synthetic aperture side view radar system used by means of digital beamforming

• US 7,944,390 B2
• Filed: 05/04/2007
• Issued: 05/17/2011
• Est. Priority Date: 05/13/2006
• Status: Expired due to Fees

• 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
• Primary Examiner(s): Gregory; Bernarr E
• Application Number:
  US12/226,785
• Publication Number:
  US 20090109086A1
• Time in Patent Office: 1,474 Days
• Field of Search: 342 25 R- 25 F, 342 73- 81, 342/175, 342/195, 342368-377, 342/118, 342134-145, 342/176, 342/179
• US Class Current: 342/25.00F
• 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. A high-resolution synthetic aperture side-looking radar system on a vehicle moving above the earth surface at a constant speed, the vehicle being provided with a coherently operating radar device comprising a transmission antenna directed obliquely downward orthogonal to the direction of movement and formed by spatially separated sub-apertures, and a receiving antenna correspondingly directed obliquely downward orthogonal to the direction of movement and divided into a plurality of spatially separated sub-antennas, the coherently operating radar device emitting radio frequency signals via the spatially separated sub-apertures of the transmission antenna, a respective radio frequency signal of the radio frequency signals being emitted through a respective spatially separated sub-aperture of the spatially separated sub-apertures of the transmission antenna and forming a respective transmission element, the coherently operating radar device receiving echo signals of the emitted radio frequency signals in a plurality of receiving channels via the spatially separated sub-antennas of the receiving antenna, wherein the transmission antenna periodically emits radio frequency pulses at a pulse repetition frequency, wherein the radio frequency signals emitted from the sub-apertures of the transmission antenna are all respectively emitted within a single radio frequency pulse of the radio frequency pulses emitted periodically by the transmission antenna, and, as a transmission signal pulse form, result in a spatiotemporally non-separable multidimensional radio frequency waveform per radio frequency pulse, and wherein, 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 at least one of the azimuth and elevation directions, are digitally processed by 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 via the digital beamforming adapted to this transmission signal pulse form.

2. The radar system of claim 1, wherein the transmission antenna formed by a plurality of sub-apertures is structurally combined with the receiving antenna that comprises a plurality of sub-antennas and allows for digital beamforming.

3. The radar system of claim 1, wherein, within a transmission pulse, the individual transmission elements, and thus sub-apertures, of the transmission antenna are controlled such in at least one of the azimuth and the elevation by varying the respective phase coefficients that radar transmission pulses with a spatiotemporally non-separable waveform are obtained.

4. The radar system of claim 1, wherein, within a transmission pulse, the individual transmission elements, and thus sub-apertures, of the transmission antenna are controlled in at least one of the azimuth and the elevation by separate function generators such that radar transmission pulses with a spatiotemporally non-separable waveform are obtained.

5. The radar system of claim 1, wherein, within a transmission pulse, individual or, at the same time, a plurality of transmission elements, and thus sub-apertures, of the transmission antenna are controlled sequentially in the azimuth and the elevation such that radar transmission pulses with a spatiotemporally non-separable waveform are obtained.

6. The radar system of claim 1, wherein the transmission antenna is configured as a multi-aperture transmission antenna for multi-dimensional spatiotemporal pulse coding, including n sub-elements in the azimuth, each supplied by a function generator of its own, and m sub-apertures in the elevation per azimuth column, which are controlled through separate controllable phase shifters to allow the forming and controlling of the beam in the elevation direction.

7. The radar system of claim 1, 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.

8. The radar system of claim 7, wherein a spatiotemporal coding of the emitted radio frequency signal 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 digital beamforming on receive in the elevation.

9. The radar system of claim 7, wherein the transmission pulse is generated by the coherent superposition of the signals of all transmission elements which all transmit at the same time, wherein each partial pulse comprises a different amplitude distribution in the azimuth direction 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, wherein, by digital 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 wherein 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.

10. The radar system of claim 1, 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.

11. The radar system of claim 10, 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 each target area with the entire band width of the transmission signal, and wherein the signals from different directions that arrive simultaneously are separated from each other using receiving-side digital beamforming in the elevation.

12. The radar system of claim 10, wherein a receiving window is provided that is greatly reduced in duration as compared to the duration of the radar transmission pulse.

13. The radar system of claim 12, wherein the duration of receiving window is calculated to be so long that all radar echoes are received within its opening time interval.

14. The radar system of claim 1, 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.

15. The radar system of claim 14, 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.

16. The radar system of claim 14, wherein a detection or determination of moving objects is achieved by using a 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.

17. The radar system of claim 1, wherein a purposeful illumination of the target area is provided, so that at least one of a better spatial resolution and a higher radiometric sensitivity is achieved by at least one of different dwelling times of the transmission beam in certain partial areas and a variation in the system resources.

18. The radar system of claim 17, wherein an operation in hybrid modes, wherein, at the same time, large areas are mapped and detail pictures of limited partial areas are made.

19. The radar system of claim 17, wherein the radar system is adapted to allocate differing amounts of resources to different areas to be mapped.

20. The radar system of claim 17, wherein the radar system performs an optimization of the signal-to-noise ratio by a dynamic adaptation of the spatiotemporal waveform to the evaluation of the received signal power for individual partial areas.