LAYERED DUAL FREQUENCY ANTENNA ARRAY AND SATELLITE IMAGING METHOD

Abstract: A dual frequency array antenna comprises an upper layer including an array of antennas operating at a first frequency, a first ground layer disposed below the upper layer at a predetermined distance and a dielectric material disposed between the first ground layer and the upper layer. An array of slot radiator apertures are provided in the first ground layer. The apertures are spaced from the array of antennas and radiate at a second frequency. An array of feed lines is disposed at a predetermined distance below the first ground layer with one of the feed lines below each one of the apertures. A second ground plane layer is disposed at a predetermined distance below the array of feed lines. The two frequencies are, for example, from two distinct microwave bands.

Description: LAYERED DUAL FREQUENCY ANTENNA ARRAY AND SATELLITE IMAGING METHOD TECHNICAL FIELD The present invention relates to a dual frequency array antenna radiator or antenna. More particularly, the present invention relates to such an array antenna in which a first ground layer for an upper array of antennas is provided with an array of slots spaced from the array of antennas. The upper array of antennas can operate at a different frequency from the slot antennas for dual frequency operation. The invention also relates to a method of radar satellite imaging and a method of obtaining such images from a ground station. BACKGROUND ART In radar microwave imaging of the earth's surface by satellite antennas, a phased array of antennas operating at frequencies in the range of 5 - 10 GHz form images with very good accuracy. Such antennas and imaging systems are known in the art. In radar imaging and generally in the field of remote sensing, it is known to use dual frequency and dual polarization transceivers in order to be able to have different sensing characteristics, namely, different qualities of resolution and penetration. In particular, higher frequency X-band signals at 9.6 GHz are known for good resolution whereas C-band signals at 5.3 GHz are more capable of penetrating cloud although the imaging resolution is of lower quality. A constraint of incorporating dual frequency operation into radar satellite antennas for remote imaging is largely physical space and weight. Lightweight and compact dual frequency radar antennas of good efficiency are not known in the art. U.S. Patent 5,241,321 (Tsao) illustrates a microwave antenna in which a radiating patch is placed over a ground plane having a coupling slot for dual frequency circularly polarized radiation. U.S. Patent 4,097,868 (Borowick) describes an antenna for combined surveillance and foliage penetration radar which operates at both X-band and L-band frequencies by providing L-band stripline dipole antennas on plates perpendicular to a flat plate waveguide slot array including broadwall slots for X- band transmission. DISCLOSURE OF THE INVENTION It is an object of the present invention to provide a dual polarization dual frequency antenna array (herein, an antenna array includes both a direct radiating array and a feed array for use in single or dual reflector systems) which is capable of operating at two frequencies with minimal interference between the two transmission systems. It is furthermore an object of the present invention to provide a flat array antenna in which two separate arrays operate at different frequencies with minimal interference between the arrays. It is yet another object of the present invention to provide such an antenna which is compact and lightweight. These objects are provided in part by providing a ground plane for an upper antenna array operating at a first frequency with the slot apertures of a lower antenna array, the slot apertures being spaced laterally from the upper antenna elements. According to the invention, there is provided a dual frequency antenna array comprising an upper layer comprising an array of antennas operating at a first frequency, a first ground layer disposed below the upper layer at a predetermined distance, a dielectric material disposed between the first ground layer and the upper layer, an array of slot apertures provided in the first ground layer, the slot apertures being spaced from the array of antennas, an array of feed lines disposed at a predetermined distance below the first ground layer providing one of the feed lines below each one of the slot apertures, and a second ground layer disposed at a predetermined distance below the array of feed lines. Preferably, the array of antennas are square patch antennas connected to radiate with dual polarization. Also, the slot apertures may comprise two sets of slots arranged to radiate with orthogonal polarization. Of course, the array of antennas can also be an array of dipole antennas, and more particularly, a dual layer dipole array is preferred. The slot apertures are preferable radiating apertures, although they may alternatively be coupling slots for feeding a patch antenna disposed above the slots. Preferably, the first frequency is C-band (5.3 GHz) and the second frequency is X-band (9.6 GHz), although other frequencies in the microwave range will also work well. The invention also provides a method for obtaining radar earth images from a space satellite equipped with a radar imaging antenna, comprising the steps of: transmitting from a dual frequency array antenna on the satellite two different frequencies of electromagnetic imaging radiation towards the earth; receiving a reflected signal for each of the said frequency using the dual frequency antenna; and transmitting the reflected signal to an earth station for earth image production. The invention further provides a method for obtaining radar earth images from a space satellite equipped with a radar imaging antenna, comprising the steps of: transmitting from an earth station a control command for selecting an area of the earth to be imaged to the satellite, the antenna being a dual frequency array antenna able to radiate and receive at two different frequencies of electromagnetic imaging radiation; receiving from the satellite image signals for two different imaging frequencies; and processing the image signals to produce earth surface images. BRIEF DESCRIPTION OF DRAWINGS The invention will be better understood by way of the following detailed description of a preferred embodiment with reference to the appended drawings in which:- FIGURE 1 is a plan view of an array antenna element in accordance with the preferred embodiment; FIGURE 2 is a vertical cross-section showing a detail of the layered construction of the array antenna of the preferred embodiment; FIGURE 3 is a detailed view illustrating the feed lines supplying a group of four slot apertures of one polarization; and Figure 4 is a vertical cross-section showing a detail of the layered construction of a dual layer dipole array/slot coupled patch array antenna. DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 illustrates an element of a microstrip array antenna according to the preferred embodiment. An array of patch antennas 14 are provided for transmission of C-band radiation (5.3 GHz). As illustrated, the patches of antennas 14 are square and are fed by two separate orthogonal lines for operating the patch antennas in one of two orthogonal polarizations. The patch antennas 14 are provided in a matrix which is spaced from a matrix of slot apertures provided in the first ground plane 16 shown in Figure 2. As is known in the art, the dielectric constant of material 18 (about 2.9) and the distance d^ between patches 14 and ground layer 16 in accordance with the dimensions of patches 14 determine the transmission characteristics. Slots 20 are provided at a sufficient distance from patches 14 in order to interfere as little as possible with the transmission and reception between the different arrays. The slots 20 comprise two sets arranged in two different orthogonal directions parallel to the orthogonal directions of patches 14. The slot antennas 20 operate at 9.6 Hz (X-band) in a fashion well known in the art, namely feed lines 22 are provided below apertures 20 at a given distance d2• To direct radiation, a second ground plane 24 is placed at a given distance d3 from feed lines 22. The feed lines 22 illustrated in greater detail in Figure 3 are conductive strips provided on the lower surface of a dielectric material 25. The dimensions and shape of feed lines 22 are important for efficient operation. At the resonant frequency of operation, the end portions 22a are of a given length so that the maximum electric field coupling occurs under apertures 20. This means that the length of portions 22a are a quarter wavelength long between the apertures and the ends, as is known in the art. The lengths of the feed lines between the various end portions in a feed line group fed by a trunk line 22c are selected to feed each feed line end portion 22a in phase with one another. Additionally, detour bend portions 22b are provided so that the feed lines circumnavigate an area where the feed lines pass for the orthogonal set of slots in the slot array. As can be appreciated from Figure 1, the lines feeding patches 14 are of a predetermined path length in order to feed all patches 14 in phase at the resonant frequency. As can also be appreciated, the two sets of orthogonal lines for array 14 or array 20 are selectively powered for the purposes of transmitting in one of the two polarizations. The antenna thus provides a dual polarization and dual frequency array. The ground layer 16 also effectively shields radiation generated by the two signal distribution networks (SDN) . This has the advantage that minimal interference between the two different frequency array antennas occurs during operation. In the embodiment of the invention shown in Fig. 4, a variant arrangement is provided. The upper antenna array comprises a dipole array 14 on an upper surface 12, with SDN 14' of the dipole array 14 being provided on an intermediate layer 12'. The slot apertures 20 do not directly radiate, but rather they are coupling slots for patches 20' . The SDN 22 is arranged to feed the coupling slots 20. As illustrated, the dielectric used at 18, 18', 18" and 25 is Duroid (TM) and the dielectric used between the patch SDN 22 and the ground plane 24 is Nomex (TM). As can be appreciated, a lower radiating slot array can also be arranged with the upper dipole array. It is to be understood that the present invention is not to be limited by the above description of the preferred embodiment. By way of example only, some of the preferred dimensions and materials for constructing the array antenna panel of the preferred embodiment are set out herein below. As can be appreciated, when the antenna is used for radar imaging, the earth image may be based on both of the imaging signals from the two frequencies. This will yield better quality images under varying conditions of could cover. As set out above, a first one of the frequences is suited to obtain good resolution images under clear sky conditions and a second one of the frequencies is suited to obtain good resolution images under cloudy sky conditions, the images obtained using the first frequency under clear sky conditions having a better resolution than the images obtained using the second frequency under clear sky conditions, and the images obtained using the first frequency under cloudy sky conditions having a poorer resolution than the images obtained using the second frequency under cloudy sky conditions. As an example, the first frequency may be X-band, while the second frequency may be C-band. While it is safer to obtain images using both frequencies, it is also possible to use the antenna according to the invention for imaging separate areas of the earth to image the quantity of imaging. Of course, X-band imaging would be used for clear sky areas, while C-band imaging can be reserved for cloudy areas.

Claims: CLAIMS 1. A dual frequency array antenna comprising: an upper layer comprising an array of antennas operating at a first frequency; a first ground layer disposed below said upper layer at a predetermined distance; a dielectric material disposed between said first ground layer and said upper layer; an array of slot apertures provided in said first ground layer, said apertures being spaced from said array of antennas; an array of feed lines disposed a predetermined distance below said first ground layer, one of said feed lines being at lest in part in registration with each one of said apertures; and a second ground layer disposed a predetermined distance below said array of feed lines. 2. The antenna array as claimed in claim 1, wherein said slot apertures are radiating apertures. 3. The antenna array as claimed in claim 1, wherein said upper layer array comprises dipole antennas. 4. The antenna array as claimed in claim 3, wherein said dipole antennas are fed by a signal distribution network provided on a layer below said upper layer. 5. The antenna array as claimed in claim 1, wherein said slot apertures are coupling slots for coupling with patch antennas provided on a layer disposed at a predetermined distance above said slots. 6. The antenna array as claimed in claim 1, wherein said antenna radiates microwave radiation. 7. The antenna array as claimed in claim 2, wherein said antenna radiates microwave radiation. 8. The antenna array as claimed in claim 6, wherein said first frequency is about 5.3 GHz and a second frequency of said slot apertures is about 9.6 GHz. 9. The antenna array as claimed in claim 1, wherein said antenna is dual polarization. 10. The antenna array as claimed in claim 2, wherein said antenna is dual polarization, said slot apertures comprising slots in two orthogonal directions. 11. The antenna array as claimed in claim 1, wherein said upper layer array comprises square patch antennas, and said antenna is dual polarization, said patch antennas each being fed by two orthogonal feed lines on sides of said patch antennas. 12. The antenna array as claimed in claim 2, wherein said upper layer array comprises square patch antennas, and said antenna is dual polarization, said patch antennas each being fed by two orthogonal feed lines on sides of said patch antennas, said slot apertures comprising slots in two orthogonal directions parallel to said orthogonal feed lines. 13. The antenna array as claimed in claim 12, wherein said antenna radiates microwave radiation. 14. The antenna array as claimed in claim 13, wherein said first frequency is about 5.3 GHz and a second frequency of said slot apertures is about 9.6 GHz. 15. The antenna array as claimed in claim 13, wherein said antenna array is a microstrip antenna array. 16. A method for obtaining radar earth images from a space satellite equipped with a radar imaging antenna, comprising the steps of: transmitting from a dual frequency array antenna on said satellite two different frequencies of electromagnetic imaging radiation towards the earth; receiving a reflected signal for each said frequency using said dual frequency antenna; and transmitting said reflected signal to an earth station for earth image production. 17. A method for obtaining radar earth images from a space satellite equipped with a radar imaging antenna, comprising the steps of: transmitting from an earth station a control command for selecting an area of the earth to be imaged to said satellite, said antenna being a dual frequency array antenna able to radiate and receive at two different frequencies of electromagnetic imaging radiation; receiving from said satellite image signals for two different imaging frequencies; and processing said image signals to produce earth surface images. 18. The method as claimed in claim 17, wherein said antenna comprises: an upper layer comprising an array of antennas operating at a first frequency; a first ground layer disposed below said upper layer at a predetermined distance; a dielectric material disposed between said first ground layer and said upper layer; an array of slot apertures provided in said first ground layer, said apertures being spaced from said array of antennas; an array of feed lines disposed a predetermined distance below said first ground layer, one of said feed lines being at lest in part in registration with each one of said apertures; and a second ground layer disposed a predetermined distance below said array of feed lines. 19. The method as claimed in claim 16, 17 or 18, wherein a first one of said frequencies is suited to obtain good resolution images under clear sky conditions and a second one of said frequencies is suited to obtain good resolution images under cloudy sky conditions, said images obtained using said first frequency under clear sky conditions having a better resolution than said images obtained using said second frequency under clear sky conditions, and said images obtained using said first frequency under cloudy sky conditions having a poorer resolution than said images obtained using said second frequency under cloudy sky conditions. 20. The method as claimed in claim 19, wherein said earth images produced are based on images signals from both said frequencies for a same area of the earth's surface. 21. The method as claimed in claim 19, wherein said images signals from said first and said second frequencies are for two different areas of the earth.

Inventors: POKULS RALPH, UHER JAROSLAV, POKULS, RALPH, UHER, JAROSLAV
Applicants: SPAR AEROSPACE LTD, SPAR AEROSPACE LIMITED
Application year: 1995
Main classification: H01Q5
IPC codes: G01S13, H01Q5