Synthetic aperture radar process

• US 8,134,490 B2
• Filed: 08/08/2008
• Issued: 03/13/2012
• Est. Priority Date: 08/30/2007
• Status: Active Grant

• Current Assignee: Deutsches Zentrum Fã¼R Luft- Und Raumfahrt E.V.
• Original Assignee: Deutsches Zentrum Fã¼R Luft- Und Raumfahrt E.V.
• Inventors: Gebert, Nicolas, Krieger, Gerhard
• Primary Examiner(s): Brainard, Timothy A
• Application Number:
  US12/675,879
• Publication Number:
  US 20100164785A1
• Time in Patent Office: 1,313 Days
• Field of Search: 342/25
• US Class Current: 342/25.R
• CPC Class Codes: G01S 13/904 SAR modes
• CPC Class Codes: G01S 2013/0245 Radar with phased array antenna
• CPC Class Codes: H01Q 1/28 Adaptation for use in or on aircraft, missiles, satellites, or balloons
• CPC Class Codes: H01Q 3/26 varying the relative phase or relative amplitude of energisation between two or more active radiating elements; varying the distribution of energy across a radiating aperture H01Q3/12, H01Q3/22, H01Q3/24 take precedence

Abstract

A continually adapted pulse-to-pulse shift, performed in the azimuth direction, of the phase center which is electrically active on the side of the transmitting antenna (Tx; Tx₁, Tx₂, Tx₃), in connection with the SAR antenna control of a multi-aperture SAR system is designed such that, in the case of an existing pulse repetition frequency (PRF) due to the likewise shifted position of the effective phase center of the entire antenna (Tx; Tx₁, Tx₂, Tx₃; Rx; Rx₁, Rx₂, Rx₃), a compensation or complete correction of non-equidistant scanning in the azimuth direction is achieved. The principle of the pulse-to-pulse shift of the position of the effective phase center of the antenna for achieving the best possible equidistant scanning can be expanded to the side of the receiving antenna (Rx; Rx₁, Rx₂, Rx₃) and to multi-aperture antennas. The technological solution proposed by the invention can be advantageously combined with a subsequent digital beam formation on the receiving antenna side.

Claims

1. A synthetic aperture radar method for remote surveillance of the surface of the earth using one or a plurality of carrier platforms moving at a constant speed over the surface of the earth, wherein said one or plurality of carrier platforms comprise a multi-aperture antenna system beam-controllable in the azimuth direction and directed obliquely downwards orthogonally to the moving direction and thus to the azimuth direction and consisting of a transmitting and receiving antenna system, and further comprise a coherently operating radar device periodically transmitting high-frequency transmission pulses at a pulse-repetition frequency via the transmitting antenna system and receiving echo signals via the receiving antenna system, wherein effective azimuth phase centers which are assigned to the received echo signals referred to as scan values, are arranged at a spatial site in the azimuth direction, which spatial site corresponds to the geometric center between the phase center of the transmitting antenna system that is actually electrically active during the transmission of the high-frequency pulses and the phase center of the receiving antenna system that is actually electrically active during the reception of the echo signals,

wherein, by a position shift effected from transmission pulse to transmission pulse in the azimuth direction or opposite thereto of the electrically active phase center of the transmitting antenna system designed as a multi-aperture antenna which is beam-controllable in the azimuth direction, and/or of the electrically active phase center of the receiving antenna system designed as a multi-aperture antenna which is beam-controllable in the azimuth direction, the position of the effective azimuth phase center is continuously adapted in such a manner that, at the existing pulse repetition frequency, a substantially equidistant scanning is effected in the azimuth direction.

2. The radar method according to claim 1, wherein, when using a transmitting antenna system consisting of a plurality of separate antenna elements and a receiving antenna system consisting of a plurality of separate antenna elements, the antenna elements are individually controlled for beam control of the respective antenna system.

3. The radar method according to claim 2, wherein the controlling of the antenna elements consists in switching on and switching off these antenna elements.

4. The radar method according to claim 2, wherein the controlling of the antenna elements consists in assigning to each antenna element a variable phase and/or amplitude assignment for achieving a “

tapering”

of the antenna characteristic.

5. The radar method according to claim 2, wherein, in case that common antenna elements are used for the transmitting and the receiving antenna systems, the antenna elements are switched between the transmitting and the receiving operation through T/R technology by use of transmitting/receiving switches.

6. The radar method according to claim 2, wherein control of the antenna elements is performed in such a manner that only selected areas of the transmitting and receiving antenna system are used, so that the electrically active phase center of the respective antenna system is arranged at the geometric center of gravity of the antenna elements used.

7. The radar method according to claim 2, wherein, in a network architecture of filters following the antenna elements comprising the transmitting and receiving antenna system, the signals are weighted and then combined to a plurality of apertures, wherein, on the reception side, this processing is performed on the digital echo signal with the received and amplified high-frequency echo signal prior to scanning, but also after a digital/analog conversion.

8. The radar method according to claim 7, wherein said network architecture is operative in such a manner that the apertures overlap each other on the transmission and/or reception sides so that the high-frequency pulses of individual transmitting antenna elements and respectively the echo signals of individual receiving antenna elements are used in a plurality of apertures.

9. The radar method according to claim 7, wherein the weighting coefficients in the network architecture are selected as frequency-independent filter functions.

10. The radar method according to claim 1, wherein pre-processing the received and digitized signals in the azimuth direction is performed prior to the actual SAR processing in the form of a so-called “

digital beamforming on the receiving side”

which is based on a Doppler-frequency-depending weighting and a subsequent coherent summation of the individual received signals and which serves primarily for suppressing azimuth ambiguities occurring due to non-equidistant scanning.

11. The radar method according to claim 10, wherein, by the shifting of the electrically active phase centers of the transmitting and respectively receiving antenna system and thus of the effective phase center of the whole antenna system, there is performed a rough setting of the equidistant scanning and, through the pre-processing by means of digital beamforming on the receiving side, a fine correction.

12. The radar method according to claim 1, wherein the receiving antenna system in comparison to the transmitting antenna system is given a longer dimension in the azimuth direction.