WO2013037701A1

WO2013037701A1 - Interferometric scanning system and method

Info

Publication number: WO2013037701A1
Application number: PCT/EP2012/067464
Authority: WO
Inventors: Michel Sarkis; Miquel Testar; Richard Stirling-Gallacher
Original assignee: Sony Corporation; Sony Deutschland Gmbh
Priority date: 2011-09-12
Filing date: 2012-09-07
Publication date: 2013-03-21
Also published as: CN103782191A; CN103782191B

Abstract

The present invention relates to an interferometric scanning system (1000) for scanning an object (1) with electromagnetic waves, the object (1) having a longitudinal axis (A) defining an axis of a coordinate system having a first, second, third and fourth quadrant (I,II, III, IV). The system comprises a first antenna assembly (301) comprising a first antenna block (201) comprising at least one antenna line comprising at least one transmitter element (101) for transmitting electromagnetic waves as a transmission signal (103), the first antenna block (201) arranged in the first quadrant (I), and a second antenna block (202) comprising at least one antenna line comprising only receiver elements (102) for receiving reflections of the transmitted electromagnetic waves from the object (1) as reception signals (105), the second antenna block (202) arranged in the second quadrant (II) next to the first quadrant (I). The system (1000) further comprises a second antenna assembly (302) comprising a third antenna block (203) comprising at least one antenna line comprising at least one transmitter element (101) for transmitting electromagnetic waves as a transmission signal (103), the third antenna block (203) arranged in the third quadrant (III), and a fourth antenna block (204) comprising at least one antenna line comprising only receiver elements (102) for receiving reflections of the transmitted electromagnetic waves from the object (1) as reception signals (105), the fourth antenna block (204) arranged in the fourth quadrant (IV) next to the third quadrant (III). The system (1000) further comprising at least one signal processing unit (100) for processing baseband signal (105a, 105b) down-converted from the reception signals (105) using an interferometric method comprising determining at least one phase difference (Δφ1) between specific receiver elements (102).

Description

INTERFEROMETRIC SCANNING SYSTEM AND METHOD

FIELD OF THE INVENTION

[0001] The present invention relates to an interferometric scanning system and method for scanning an object with electromagnetic waves, in particular in the range between 1 GHz and 10 THz, preferably between 30 GHz and 300 GHz (e.g. millimeter waves or sub-millimeter- waves). The system and method can in particular additionally provide an image of the object (e.g. 3D image). The present invention further relates to a computer program and a computer readable non-transitory medium for implementing such method. BACKGROUND OF THE INVENTION

[0002] The scanning system according to the present invention can for example be a portal security scanner. Such portal security scanners are becoming widely employed at public locations, for example airports, to detect whether an object, such as a person, is hiding any suspicious item. The employment of such portal scanners at public locations, such as airports, is in particular becoming necessary these days due to the increase in the amount of security threats. Such a portal security scanner can generate an image of the object (person) by for example either using the technique of frequency modulated continuous waves (FMCW) to resolve depth or by employing reflector antenna arrays to focus the electromagnetic waves on different points in space. Both these techniques lead to limitations in extracting the (3D) image of the object.

[0003] For example, for the FMCW technique a very high bandwidth is required to achieve a fine depth resolution. The standard depth accuracy AR of an FMCW system is defined by

where c is the speed of light and BW is a bandwidth of the FMCW transmission signal or waveform. Just as an example, to achieve a 1 cm depth accuracy or resolution, a bandwidth of 15 GHz would be required. Such a large bandwidth is challenging to achieve at lower frequencies (for example 35 GHz) and the use of higher frequencies (for example 140 GHz and beyond) can cause the scanning system or portal security scanner to be very expensive.

[0004] For example, for the technique using reflector antenna arrays, such as for example disclosed in US 6,965,340 Bl, it is necessary to perform a very fine electronic scanning by focusing the beam on many fine voxels in space. This is usually very time- consuming and leads to a long capture time, resulting in images with a low frame rate. [0005] US 7,583,221 B2 discloses a device comprising at least one transmitting and receiving system which successively illuminates a test object with millimeter waves along a circumference of said test object and which receives the scattered waves. The device further comprises an analysis system, which produces an image of the test object from the received waves. A transmitting antenna and a receiving antenna of each transmitting and receiving system are oriented such that the direction of view of the transmitting antenna and the direction of reception of the receiving antenna are inclined at an angle of 15° to 70° to a longitudinal axis of the test object. A pulse radar or FMCW radar principle is used by the analysis system to analyze radiation scattered in the direction of the longitudinal axis of the test object and the SAR principle is used for analysis perpendicular to the longitudinal axis. The transmitting and receiving system can have an additional receiving device with an offset antenna for the scattered field in order to interferometrically analyze the scattered field information. In the example shown in US 7,583,221 B2, the transmitting and receiving systems rotate about the stationary test object in order to successively illuminate it along its circumference with millimeter waves. Another proposed alternative can include arranging individual transmitting and receiving antennas in an antenna array over the entire circumference of the test object.

[0006] A problem of such system could be that the scanning is slow (long scanning time) due to the rotation about the object. A problem of such a system could also be that its hardware is complex and thus expensive. Another problem of such a system could be a high probability of having dark (missing data) spots in the image due to a high probability that no signals are obtained at the receivers. An additional problem of such a system could be that it cannot resolve objects at multiple depth layers, a fact relevant for detecting hidden objects at different depth layers.

BRIEF SUMMARY OF INVENTION

[0007] It is an object of the present invention to provide a scanning system and method having a less complex (thus more cost effective) and/or a faster or shorter scanning time. It is a further object of the present invention to provide a computer program and a computer readable non-transitory medium for implementing such a method.

[0008] According to an aspect of the present invention there is provided an inter- ferometric scanning system for scanning an object with electromagnetic waves, the object having a longitudinal axis defining an axis of a coordinate system having a first, second, third and fourth quadrant. The system comprises a first antenna assembly comprising a first antenna block comprising at least one antenna line comprising at least one transmitter element for transmitting electromagnetic waves as a transmission signal, the first antenna block arranged in the first quadrant, and a second antenna block comprising at least one antenna line comprising only receiver elements for receiving reflections of the transmitted electromagnetic waves from the object as reception signals, the second antenna block arranged in the second quadrant next to the first quadrant. The system comprises a second antenna assembly comprising a third antenna block comprising at least one antenna line comprising at least one transmitter element for transmitting electromagnetic waves as a transmission signal, the third antenna block arranged in the third quadrant, and a fourth antenna block comprising at least one antenna line comprising only receiver elements for receiving reflections of the transmitted electromagnetic waves from the object as reception signals, the fourth antenna block arranged in the fourth quadrant next to the third quadrant. The system further comprising at least one signal processing unit for processing the baseband signal down-converted from the reception signals using an interferometric method comprising the determination of at least one phase difference between specific receiver elements.

[0009] According to a further aspect of the present invention there is provided an interferometric scanning method for scanning an object with electromagnetic waves, the object having a longitudinal axis defining an axis of a coordinate system having a first, second, third and fourth quadrant. The method comprises transmitting electromagnetic waves as a transmission signal by means of a first antenna block comprising at least one antenna line having at least one transmitter element, the first antenna block arranged in the first quadrant, and receiving reflections of the transmitted electromagnetic waves from the object as reception signals by means of a second antenna block comprising at least one antenna line comprising only receiver elements for, the second antenna block arranged in the second quadrant next to the first quadrant. The method further comprises transmitting electromagnetic waves as a transmission signal by means of a third antenna block comprising at least one antenna line comprising at least one transmitter element, the third antenna block arranged in the third quadrant, and receiving reflections of the transmitted electromagnetic waves from the object as reception signals by means of a fourth antenna block comprising at least one antenna line comprising only receiver elements, the fourth antenna block arranged in the fourth quadrant next to the third quadrant. The method further comprises processing baseband signal down-converted from the reception signals using an interferometric method comprising determining at least one phase difference between specific receiver elements.

[0010] According to a further aspect of the present invention there is provided an interferometric scanning and imaging system for scanning an object with electromagnetic waves and for providing an image of the object. The system comprises a first antenna assembly comprising a first antenna block comprising at least one antenna line comprising at least one transmitter element for transmitting electromagnetic waves as a transmission signal, and a second antenna block comprising at least one antenna line comprising at least three receiver elements for receiving reflections of the transmitted electromagnetic waves from the object as reception signals. The at least one transmitter element of one antenna line and the receiver elements of one corresponding antenna line forming one scanline. The system further comprises at least one signal processing unit for processing baseband signal down-converted from the reception signals using an interferometric method comprising determining, for each scanline, at least three significant receiver elements at which a main signal portion of the received signals is received, and determining, for each scanline in which significant receiver elements are determined, at least a first phase difference between a first pair of significant receiver elements and a second phase difference between a second pair of significant receiver elements. The at least one signal processor is further configured to provide an image of the object using the at least first phase difference and second phase difference.

[0011] According to a further aspect there is provided an interferometric scanning and imaging method for scanning an object with electromagnetic waves and for providing an image of the object. The method comprises transmitting electromagnetic waves as a transmission signal by means of a first antenna block comprising at least one antenna line comprising at least one transmitter element, and receiving reflections of the transmitted electromagnetic waves from the object as reception signals by means of a second antenna block comprising at least one antenna line comprising at least three receiver elements. The at least one transmitter element of one antenna line and the receiver elements of one corresponding antenna line form one scanline. The method further comprises processing baseband signal down-converted from the reception signals using an interferometric method comprising determining, for each scanline, at least three significant receiver elements at which a main signal portion of the received signals is received, and determining, for each scanline in which significant receiver elements are determined, at least a first phase difference between a first pair of significant receiver elements and a second phase difference between a second pair of significant receiver elements. The method further comprises providing an image of the object using the at least first phase difference and second phase difference.

[0012] According to still further aspects a computer program comprising program means for causing a computer to carry out the steps of one of the methods according to the present invention, when said computer program is carried out in a computer, as well as a computer readable non-transitory medium having instructions stored thereon which, when carried out on a computer, cause the computer to perform the steps of one of the methods according to the present invention are provided.

[0013] Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed method, the claimed computer program and the claimed computer readable medium have similar and/or identical preferred embodiments as the claimed system and as defined in the dependent claims. Further, it shall be understood that the claimed scanning and imaging system has similar and/or identical embodiments as the claimed scanning system and as defined in the dependent claims.

[0014] The present invention is based on the idea to provide a scanning system having a specific antenna arrangement that provides a faster scanning time and/or less antenna elements, while still providing a good and/or entire image of the object. The first antenna assembly comprises a first antenna block (having at least one transmitter element) and a second antenna block (having only receiver elements, thus no transmitter elements). In the same way also the second antenna assembly comprises a third antenna block (having at least one transmitter element) and a fourth antenna block (having only receiver elements, thus no transmitter elements). The antenna blocks are distributed in the four quadrants. The first antenna assembly and the second antenna assembly are "non-co-located", meaning that they are arranged on opposite sides of the object (such as front and back). In this way both the front and the back of the object can be efficiently scanned. For example, the front of the object can be scanned using the transmitter element(s) of the first antenna block (frontal scan) and the back of the object can be scanned using the transmitter element(s) of the third antenna block (back scan). Thus, at least two (more than one) antenna assemblies for transmitting and receiving electromagnetic waves are used. Compared to a system using mechanical rotation of an antenna about or around the object which takes a longer time, the present invention thus provides a faster scanning time. Compared to a system using an antenna over the entire circumference of the object, the present invention thus typically requires a reduced number of antenna elements. Furthermore, the present invention can provide more space for the object, because the antenna blocks can be spaced at a sufficient distance from the object. Compared to a system using mechanical rotation of an antenna about or around the object or a system using an antenna over the entire circumference of the object, where the antenna would need to be arranged at a very small radius to achieve a fast scanning time, the system according to the present invention thus provides more or sufficient space for the object. [0015] The present invention is based on the idea to use an interferometric method for processing baseband signal down-converted from the reception signals, the interferometric method comprising determining at least one phase difference between specific ones of the receiver elements, in particular between significant receiver elements at which a main signal portion of the received signals is received. In particular, at least a first phase difference between a first pair of significant receiver elements and a second phase difference between a second pair of significant receiver elements can be determined. A pair of such significant receiver elements (two receiver elements) can also be called a baseline. Using multiple (more than one) baselines is also called "Multi-Baseline" concept. In this way a lower probability of having dark (missing data) spots in the image is achieved because there is a high or higher probability that signals are obtained at the receiver elements. Compared to a system determining only one baseline, objects at several depth layers can be resolved.

[0016] The scanning system or method of the present invention can provide less complex and thus more cost effective hardware, in particular it can provide less limitations in the hardware implementation (e.g. using lower bandwidth components which are correspondingly cheaper). The scanning system or method of the present invention can provide a faster or shorter scanning time and/or can result in higher resolution (3D) images, in particular with finer depth resolution. The scanning system or method of the present invention can provide a lower probability of having dark (missing data) spots in the image and/or resolving objects at several depth layers.

BRIEF DESCRIPTION OF DRAWINGS

[0017] These and other aspects of the present invention will be apparent from and explained in more detail below with reference to the embodiments described hereinafter. In the following drawings Fig. 1 shows a perspective view of a scanning system for scanning an object with electromagnetic waves according to a first embodiment;

2 shows a top view of a basic embodiment of the scanning system;

3a, 3b each shows a top view of a scanning system according to a second embodiment;

4a, 4b each shows a top view of a scanning system according to a third embodiment;

5a, 5b each shows a top view of a scanning system according to the first embodiment of Fig. 1;

6 shows a front view of the first or second antenna assembly of the scanning system, in particular of the first embodiment shown in Fig. 1;

7 shows a front view of the first or second antenna assembly of a scanning system according to an alternative embodiment;

8 shows a flow diagram of an interferometric method according to an embodiment;

Fig. 9a, 9b show two alternative embodiments of the step of determining phase differences and depth values of Fig. 8;

Fig. 10 shows a schematic block diagram of an exemplary receiver unit which is part of or attached to each antenna block 1; Fig. 11a shows a schematic block diagram of an antenna block comprising at least one antenna line having only receiver elements according to a first example;

Fig. 1 lb shows a schematic block diagram of an antenna line having only receiver elements according to a second example;

Fig. 12a shows an antenna line having a single transmitter element, in particular an antenna line of the embodiment shown in Fig. 3a and Fig. 3b;

Fig. 12b shows a schematic block diagram of an antenna line having a single transmitter element and multiple receiver elements, in particular an antenna line of the embodiment shown in Fig. 4a and Fig. 4b;

Fig. 12c shows a schematic block diagram of an antenna line having transmitter/receiver elements, in particular an antenna line of the embodiment shown in Fig. 5a and Fig. 5b; and

Fig. 12d shows a schematic block diagram of the first or second antenna assembly, in particular the antenna assembly of the embodiment shown in Fig. 6.

DETAILED DESCRIPTION OF THE INVENTION

[0018] Fig. 1 shows a perspective view of a scanning system 1000 for scanning an object 1 with electromagnetic waves according to a first embodiment. Additionally, the system 1000 can provide an image of the object 1. The scanning system 1000 is here a portal security scanner for inspection of the object 1, which is a subject or person as indicated in Fig. 1. The system can comprise a signal generator (not shown in Fig. 1) for generating an FMCW transmission signal 103 or a pulse transmission signal 103. [0019] The scanning system 1000 can in particular scan the object 1 with electromagnetic waves in the range between 1 GHz and 10 THz, preferably between 30 GHz and 300 GHz. Thus, the electromagnetic waves can be millimeter-waves (abbreviated mm- waves) or sub-millimeter- waves (abbreviated sub-mm- waves). Such frequencies and wavelengths are transparent to most clothing materials. This allows the scanning system 1000 working in this frequency or wavelength range to detect hidden objects or items beneath the clothes. Thus, a suspicious object or item can be automatically located in the image provided by the scanning (and imaging) system 1000. Another advantage of using mm- waves or sub-mm-waves is that these electromagnetic waves are safe when applied to humans, as compared to X-rays for example. Mm-waves or sub-mm-waves are nonionizing radiations and therefore they do have reduced impacts on health.

[0020] As indicated in Fig. 1, the object 1 has a longitudinal axis A defining an axis of a coordinate system having a first quadrant I, a second quadrant II, a third quadrant III and a fourth quadrant IV. The coordinate system has an origin O at which the object 1 is placed. The coordinate system has a first (x-) axis and a second (z-) axis. The first, second, third and fourth quadrant I, II, III, IV are defined or separated by the first (x-) axis and the second (z-) axis. The coordinate system further comprises a third axis (y-) which is in the direction of the longitudinal axis A. Thus, the third (y-) axis is the axis defined by the longitudinal axis A of the object or subject 1.

[0021] The scanning system 1000 comprises a first antenna assembly 301 and a second antenna assembly 302. The first antenna assembly 301 comprises a first antenna block 201 comprising at least one antenna line (arranged horizontally in Fig. 1) comprising at least one transmitter element 101 for transmitting electromagnetic waves as a transmission signal 103. The first antenna block 201 is arranged in the first quadrant I in Fig. 1. The first antenna assembly 301 further comprises a second antenna block 202 comprising at least one antenna line comprising only receiver elements 102 for receiving reflections of the transmitted electromagnetic waves from the object 1 as reception signals 105. The second antenna block 202 is arranged in the second quadrant II next to the first quadrant I in Fig. 1. The second antenna assembly 302 comprises a third antenna block 203 comprising at least one antenna line comprising at least one transmitter element 101 for transmitting electromagnetic waves as a transmission signal 103. The third antenna block 203 is arranged in the third quadrant III. The second antenna assembly 302 further comprises a fourth antenna block 204 comprising at least one antenna line comprising only receiver elements 102 for receiving reflections of the transmitted electromagnetic waves from the object 1 as reception signals 105. The fourth antenna block 204 is arranged in the fourth quadrant IV next to the third quadrant III.

[0022] As shown in Fig. 1, the third quadrant III is diagonally opposite to (not next to) the first quadrant I. By providing the transmitter elements 101 in diagonally opposite quadrants, minimum interference of the transmitted signals 103 can be achieved. Each antenna line is a stationary antenna line. Each of the first and second antenna assembly 301 and 302 comprises multiple antenna lines in the direction of the longitudinal axis A such that a two-dimensional array of transducer elements is formed. Each antenna line of the first antenna block 201 and each antenna line of the third antenna block 203 comprises multiple transmitter elements 101. Each antenna line is arranged in a different transmitter/receiver-plane P perpendicular to the longitudinal axis A of the object 1. For each of the first and second antenna assembly 301 and 302 the at least one transmitter element 101 of one antenna line and the receiver elements 102 of one corresponding antenna line are arranged in the same transmitter/receiver-plane P, which is perpendicular to the longitudinal axis A of the object 1. For each of the first and second antenna assembly 301 and 302, the transmitter elements 101 of one antenna line and the receiver elements 102 of one corresponding antenna line (arranged in the same transmitter/receiver-plane P) form one scanline.

[0023] As can be see in Fig. 1, the front of the object 1 can be scanned using the transmitter elements 101 of the first antenna block 201 (frontal scan). The back of the object 1 can be scanned using the transmitter elements 101 of the third antenna block 203 (back scan). Thus, two antenna assemblies 301 and 302 for transmitting and receiving electromagnetic waves are used. By using such frontal scan and back scan, a complete scan of the object or subject 1 from each side can be performed. In each of the frontal scan and the back scan, each transmitter antenna 101 of each antenna line (or scanline) of the first or third antenna block 201 or 203 can send a transmission signal 103, in particular a (wideband) mm-wave or sub-mm-wave transmission signal.

[0024] As can be seen in Fig. 1, the scanning system 1000 further comprises a (or at least one) signal processing unit, depicted by the reference numeral 100, for processing baseband signal 105 a, 105b down-converted from the reception signals 105. The at least one signal processing unit 100 (e.g. processor or microprocessor) configured to process the baseband signal 105a, 105b down-converted from the reception signals 105 using an interferometric method comprising determining at least one phase difference Δφι between specific receiver elements 102, as will be explained in more detail below. In particular, the at least one phase difference can be between significant receiver elements at which a main signal portion of the received signals is received. A pair of such significant receiver elements (two receiver elements) can also be called a baseline. Using multiple (more than one) baselines is also called "Multi-Baseline" concept.

[0025] It will be understood that the at least one signal processing unit can be a single processing unit or can be multiple processing units. In the embodiment of Fig. 1 , the signal processing unit 100 comprises a signal acquisition unit (e.g. a controller) for acquiring and/or controlling the transmission signals 103 and the baseband signals 105a, 105b down-converted from the reception signals 105. It will be understood that the signal acquisition unit 109 can also be a separate unit. The signal processing unit 100 further comprises an imaging unit 110 for proving an image of the object 1. Optionally, as indicated by the dashed lines, the signal processing unit 100 can comprise a post-processing unit 11 1. Also optionally, as indicated by the dashed lines, the system can further comprise a display 112 for displaying the image and/or a storage unit for storing the image. [0026] As mentioned above, the scanning system 1000 of the embodiment of Fig. 1 is configured for providing an image of the object 1. After acquiring the baseband signals 105 a, 105b, the signal acquisition unit 109 passes them to the imaging unit 110 which constructs the (3D) image of the object or subject 1. The image is constructed using the at least one determined phase difference Acpithus using an interferometric method. Optionally, a suspicious object or item can be automatically located in the image.

[0027] As mentioned above, the signal processing unit 100 comprises signal acquisition unit 109 to acquire and/or control the transmission signals 103 and receptions signals 105. The signal acquisition unit 109 communicates with each of the antenna blocks 201, 202, 203, 204 via connections or communication channels 108. As the object 1, here subject or person, enters the scanning system 1000 or portal security scanner, the acquisition unit 109 can then start the scanning process. The task of the acquisition unit 109 is to communicate through the connections or communication channels 108 with the transmitter elements 101 to transmit electromagnetic waves towards the object or subject 1 as a transmission signal 103, and to receive reflections of the transmitted electromagnetic waves (reflected from the object 1) from the receiver elements 102 as reception signals through the connections or communication channels 108.

[0028] The signal acquisition unit 109 can send the transmission signal(s) 103, in particular wideband transmission signals, to the transmitter element(s) 101 of each of the antenna blocks 201, 202, 203, 204. The signal generator mentioned before is synchronised to signal acquisition unit 109. The signal generator can be arranged in each antenna block. A synchronization unit can be arranged in the signal acquisition unit 109. A receiver unit or receiver units for providing the baseband signals down-converted from the reception signal can be arranged in or attached to each antenna block.

[0029] The transmission signals 103 of the transmitter elements 101 of each antenna line can be transmitted sequentially or in parallel using a signal multiplexing concept. In the case of sequential transmission of the transmission signals 103, the signal acquisition unit 109 communicates with each transmitter element 101 of a specific antenna line of the antenna block 201 or 203, so that one transmitter element 101 transmits at a time, while the signal acquisition unit 109 also communicates with all the receiver elements 102 to receive the reception signals 105. This process is repeated for each of the transmitter elements 101 in the specific antenna line sequentially, until all transmitter elements 101 in the antenna line have sent the signals. In the case of parallel transmission of the transmission signals 103, the signal acquisition unit 109 communicates with the transmitter elements 101 of one antenna block 201 or 202, so that the transmitter elements 101 (in all antenna lines or scanlines) send the transmission signals 103 in parallel using different transmission signals 103 which can be separated. These transmission signals 103 can for example use different frequency bands or can for example use a form or coding to enable the signals to be separated and received at the same time.

[0030] A receiver unit in or attached to each antenna block can receive the electromagnetic waves reflected from the object or subject 1 as reception signals 105 from receiver elements 102. The receiver unit can then down-convert the reception signals 105 into baseband signal 105a, 105b. The baseband signals 105a, 105b down-converted from 105 are then passed to the imaging unit 110. The imaging unit 110 can apply the "Multi- baseline interferometry" concept described herein to provide partial images. The imaging unit 110 can then combine the partial images to construct the complete image of the object or subject 1. Optionally, the image can be post-processed using post-processing unit 111, for example to locate a suspicious item automatically in the image (e.g. using a machine learning algorithm). Further optionally, the (post-processed or non-post-processed) image can be visualized on a display 112 and/or stored on a storage unit (e.g. a memory).

[0031] After the transmitter elements 101 of one antenna line or scanline transmit the electromagnetic waves, the electromagnetic waves then hit the object 1 and scatter in many directions. The strongest reflections scattered from the object 1 hits back the second antenna block 202 or third antenna block 204 having the receiver elements 102 and can be received as reception signal 105. The received signal can be converted to Interme- diate Frequency (IF). In this way, the (IF) reception signal can be acquired by the signal acquisition system 109. The signal acquisition unit 109 repeats this process for each antenna line or scanline, until the entire object 1 is scanned (in the third (y-) direction in Fig. 1). The (IF) reception signals are then passed from the signal acquisition unit 109 to the imaging unit 110 where the (3D) image is constructed. The image can be then directly displayed on the display 112 and/or stored in the storage unit. Alternatively, the image can first be post-processed using the post-processing unit 1 1 1, and then the final image can be displayed on the display 112 and/or stored in a storage unit. In Fig. 1 the post-processing unit 111 is a separate unit. However, it will be understood that the post-processing unit 111 can also be integrated into another unit, such as e.g. the imaging unit 110.

[0032] In general, a scanning method, according to one aspect described herein, for scanning the object 1 with electromagnetic waves comprises transmitting electromagnetic waves as a transmission signal 103 by means of the first antenna block 201, and receiving reflections of the transmitted electromagnetic waves from the object 1 as reception signals 105 by means of (at least) the second antenna block 202 (frontal scan). The method further comprises transmitting electromagnetic waves as a transmission signal 103 by means of the third antenna block 203, and receiving reflections of the transmitted electromagnetic waves from the object 1 as reception signals 105 by means of (at least) the fourth antenna block 204 (back scan). The method further processing the baseband signals 105 a, 105b down-converted from the reception signals 105 using an interfero metric method comprising determining the at least one phase difference Δφι between specific receiver elements 102.

[0033] The concept described herein is called " Multi-Baseline-Interferometry". The main point is to assess the depth dimension (second (z-) direction in Fig. 1), using the "Multi-Baseline-Interferometry" concept. This requires some considerations on the antenna arrangement, in particular the setup of the transmitter element(s) 101 and the receiver elements 102 with respect to the object or subject to be scanned. The resolution of the obtained image depends on three factors. The resolution in the direction of the first (x-) axis depends on the bandwidth of the transmission signal. The resolution in the direction of the second (z-) axis depends on the signal processing using the interferometric method. The resolution in the direction of the third (y-) axis depends on the specific method used to obtain this dimension, as will be explained in further detail below. It should be noted that the antenna configuration disclosed herein in connection with the figures illustrate specific embodiments. However, it should be understood that the resolutions in the first (x-) direction and the third (y-) direction can be easily interchanged by changing the antenna configuration, for example by flipping the left and right antenna blocks shown in Fig. 1 to up and down.

[0034] Fig. 2 shows a top view of a basic embodiment of the scanning system 1000. A single transmitter/receiver-plane P perpendicular to the longitudinal axis of the object 1, as explained above, is illustrated in Fig. 2 (plane of projection). For example, the scanning system can be the scanning system 1000 of Fig. 1. In this case, for simplification purposes, only the first antenna assembly 301 is depicted in Fig. 2. Furthermore, in this case, it will be understood that the statements made for the first antenna assembly 301 in the same way apply for the second antenna assembly 302.

[0035] However, it will be understood that the scanning system 1000 of Fig. 2 can also comprise only one single antenna assembly 301, thus not a first and a second antenna assembly as shown in Fig. 1. In this case, the first antenna block 201 and the second antenna block 202 can be placed next to each other (as shown in Fig. 2) or can be placed opposite to each other. The system of Fig. 2 can in particular be an interferometric scanning and imaging system 1000 for scanning an object 1 with electromagnetic waves and for providing an image of the object 1. As can be seen in Fig. 2, the system comprises a first antenna assembly 301 comprising a first antenna block 201 comprising at least one antenna line comprising at least one transmitter element 101 for transmitting electromagnetic waves as a transmission signal 103. The first antenna assembly 301 further comprises a second antenna block 202 comprising at least one antenna line comprising at least three receiver elements 102a, 102b, 102c arranged in a scanline for receiving reflections of the transmitted electromagnetic waves from the object 1 as reception signals 105. The at least one transmitter element (101) of one antenna line and the receiver elements (102) of one corresponding antenna line form one scanline. The system further comprises at least one signal processing unit 100 (not shown in Fig. 2) for processing a baseband signal 105a, 105b down-converted from the reception signals 105 using an interferometric method. The interferometric method can in particular comprise (as will be explained in more detail with respect to Fig. 8 and Fig. 9) determining, for each scanline, at least three significant receiver elements at which a main signal portion of the received signals is received, and determining, for each scanline in which significant receiver elements are determined, at least a first phase difference Δφι between a first pair of significant receiver elements and a second phase difference Δφ₂ between a second pair of significant receiver elements. The at least one signal processor 100 can be configured to provide an image of the object 1 using the at least first phase difference Δφι and second phase difference Δφ₂.

[0036] In Fig. 2, each antenna line of the second antenna block 202 comprises at least three receiver elements 102a, 102b, 102c (exactly three in Fig. 2 for simplification purposes). In Fig. 2, the receiver elements 102 are uniformly arranged, which means that they are spaced apart at one constant or uniform distance d. Alternatively, the receiver elements 102 could also be arranged non-uniformly, which means spaced apart at varying distances. One pair of receiver elements (thus two receiver elements) are called a baseline. As can be seen in Fig. 2, a first baseline Bi is defined between the first receiver element 102a and the second receiver element 102b, and a second baseline B₂ is defined between the first receiver element 102a and the third receiver element 102c. Because there are multiple (here two) baselines, this describes a "Multi-Baseline" concept. Based on the baseband signals 105a, 105b the phase differences are then determined between the different receiver elements 102. A first phase difference Δφι is determined for the first baseline Bi (between first receiver element 102a and second receiver element 102b) and a second phase difference Δφ₂ is determined for the second baseline B₂ (between first receiver element 102a and third receiver element 102b). The first phase difference Δφι is determined using the reception signal at the first receiver element 102a and the reception signal at the second receiver element 102b. The second phase difference Δφ₂ is determined using the reception signal at the first receiver element 102a and the reception signal at the third receiver element 102c.

[0037] As can be seen in Fig. 2, each transmitter element 101 and each receiver element 102 is arranged in the coordinate system at a non-normal and/or non-zero angle Θ. This means that the angle Θ is not 0° and/or not 90° to any axis. If the angle Θ would be 0° or 90°, the interfero metric method used to compute the second (z-) dimension and the bandwidth which it is relied on to compute the first (x-) dimension both will be resolving the same dimension. Each transmitter element 101 and/or receiver element 102 will have a certain half power beam width (HPBW). A typical suitable value of HPBW in the first (x-) direction can for example be between 10° and 50°. In this way the antenna blocks can cover the whole object 1 (e.g body of the person) to be scanned in the first (x-) direction. In order to use the principle of interferometry, the transmitter element 101 can be transmitting at a look angle Θ with respect to the horizontal line in the x-direction. This may be necessary for the interferometry principle to be applied and to compute the depth value in the third (z-) direction. In one example, the antennas can be directed so that the transmit and receive signal are transmitted along an angle Θ from the horizontal directions x. In another example, the first antenna block 201 can be tilted by an angle Θ clockwise, and the second antenna block 202 by an angle Θ anti-clockwise with respect to the horizontal line x. In this case, the transmitted and received signal will be along the normal direction to the horizontal line x. The object or subject 1 to be scanned should be standing at nominal distance R from the first antenna block 201 and the second antenna block 202. Under these assumptions, the cross-range resolution dx of the system can be approximately calculated as follows:

dx (1),

2 x BW x cos(e) where c is the speed of light and BW is the bandwidth of the transmission signal 103. The transmission signal with a certain bandwidth can be generated in any suitable way. For example, one possibility is using frequency modulated continuous waves (FMCW) as the transmission signal 103, where the bandwidth is defined by the frequency span of the chirp pulse. Another possibility is a pulse based approach for the transmission signal 103 (pulse transmission signal), where the bandwidth is defined by the duration of the pulse. It will be understood that also any other suitable transmission signal can be used.

[0038] The depth sensitivity (in the second (z-) direction) can be defined with respect to the phase difference Δφ between the different receivers elements 102 (receiving the same part of the image). The depth sensitivity in terms of phase accuracy can be defined as follows:

In the above equation (Equation (2)), /is the central frequency of the transmission signal 103 and B is the baseline or distance between two receiver elements 102. This equation (Equation (2)) gives the depth change or resolution per radian of phase detection. The maximum possible baseline length (or distance between two receiver elements) can also be called "critical baseline" B_c. The "critical baseline" B_c can be defined as:

/ x sin 0

The critical baseline B_c defines the largest baseline length (or distance between two receiver elements) that can be used and where still the interferometry principle can be performed. Usually, by taking the maximum baseline length (or maximum distance between the receivers elements) to be less than the critical baseline B_c, the interferometric principle can be safely operated. [0039] Using the above mentioned equations for cross-range resolution dx (Equation (1)), depth sensitivity in terms of phase accuracy (Equation (2)), and critical baseline B_c< (Equation (3)), a suitable design for the scanning system using the "Multi-Baseline-Interferometry" concept can be made. To take all of factors of the above mentioned equations (Equations (1), (2), and (3)) into account, the nominal range value R can for example vary between 0.5 m to 3 m, depending on the operation frequency. A typical value for the look angle Θ can for example be between 20 to 80 . The bandwidth can be chosen depending on the cross-range resolution desired in the system. The actual- depth accuracy of the system will be governed by the phase detection accuracy designated by ψ . The variable ψ is governed by the hardware implementation.

[0040] The maximum baseline length B_max used for the interferometric method, and hence the maximum number of receivers elements 102 used to resolve each pixel in the image, should be less than the critical baseline B_c, in particular much less. In particular a factor of 20% to 30% of the critical baseline B_c is recommended for optimal performance. However, the actual number of receiver elements 102 in a scanline or antenna line can be more than the number set by the critical baseline B_c to ensure that the signals reflected back from the object or subject 1 are captured by the receiver elements 102. Thus, the number of receiver elements 102 in a scanline (or antenna line) can be higher than the number of significant receiver elements. In this way the probability of having dark spots in the final image is reduced.

[0041] Merely for a better understanding of this concept an exemplary, non- limiting setup will be given. Assuming that the nominal range R is 1.5 m, the look angle Θ is 45 and the transmission signal frequency/ is 94 GHz. For a cross range resolution dx of 2 cm, the required bandwidth B W needed would be around 10 GHz. The critical baseline B_c is thus 0.45 m and therefore the maximum baseline B_max used in interferometry should be less than this value. Assuming that a lower value for the maximum baseline B_max should be used, for example maximum baseline B_max of 0.1 m, and that at the same time a effective number N_e of receiver elements should be 5 in each antenna line (uniformly arranged), then the distance between the two consecutive receiver elements is set to 2 cm. In a practical implementation, to ensure that 5 receiver elements receive a signal simultaneously in each antenna line, there can be more than 5 receiving antennas physically available. This means, it is necessary to install a number N of more than 5 receiver elements in this specific example to ensure that a received signal is captured in each line. How many number of elements should be eventually installed in a practical implementation depends on the configuration used, cost and complexity. For example, a factor of 2 x N_e to 5 x N_e can be used as a guideline for practical implementations to reduce the probability of having dark spots in the final image.

[0042] Fig. 3a and Fig. 3b each shows a top view of a scanning system according to a second embodiment. Fig. 4a and 4b each shows a top view of a scanning system according to a third embodiment. Fig. 5a, 5b each shows a top view of a scanning system according to the first embodiment of Fig. 1. It will be understood that even though Fig. 1 shows an embodiment corresponding to the top view of Fig. 5, any other suitable embodiment, such as the embodiment of Fig. 3 or Fig. 4, could be implemented in the same way as explained with reference to Fig. 1. Further, it will be understood that any of the embodiments of Fig. 3 to Fig. 5 can be used for the system as explained with reference to Fig. 2, in particular the interferometric scanning and imaging system. For example, if the system uses only one single antenna assembly 301, the configuration of the first and second antenna block 201 and 202 (which is the same configuration as the configuration of the third and fourth antenna block 203 and 204) shown in any of the embodiments of Fig. 3 to Fig. 5 can be used for the one single antenna assembly. Further, it will be understood that same reference numerals in the Figures depict the same elements (in particular the elements as explained in connection with Fig. 1 or Fig. 2). Each of Fig. 3a, Fig. 4a and Fig. 5a shows a frontal scan, and each of Fig. 3b, Fig. 4b and Fig. 5b shows a back scan, as explained above.

[0043] In the embodiment of Fig. 3a and Fig. 3b there is only one transmitter element (Tx) in an antenna line of the first antenna block 201 or the third antenna block 203, and no receiver elements. The transmit-only antenna elements Tx are designated by white circles with a cross. Thus, each the first antenna block 201 and the third antenna block 203 is designed to transmit only. In the embodiment of Fig. 3a and Fig. 3b, part of the reflected signals will not be captured. This can still be partly remedied by the signal processing unit or imaging unit, as will be discussed below. In Fig. 3a and Fig. 3b only the second antenna block 202 and the fourth antenna block 204 have receiver elements (Rx). Thus, each of the second antenna block 202 and the fourth antenna block 204 is designed to receive only. The receiver-only antenna elements Rx are designated by black circles.

[0044] In the embodiment of Fig. 4a and Fig. 4b only the middle element in an antenna line of the first antenna block 201 or the third antenna block 203 is a transmitter element (Tx), while the other elements in the antenna line are receiver elements (Rx). Thus, each of the first antenna block 201 and the third antenna block 203 are designed to transmit and to receive. The reference sign Tx&Rx indicates that each antenna line comprises transmitter element(s) and receiver elements that cannot be switched. Each of the second antenna block 202 and the fourth antenna block 204 are designed to receive only.

[0045] In the embodiment of Fig. 5a and Fig. 5b each element in an antenna line of the first antenna block 201 or the third antenna block 203 is a transmitter/receiver element (Tx/Rx) for transmitting electromagnetic waves as a transmission signal and receiving reflections of the transmitted electromagnetic waves from the object 1. The transmitter/receiver elements Tx/Rx are designated by plain white circles (with no cross). Thus, each of the first antenna block 201 and the third antenna block 203 are designed to transmit and to receive. The reference sign Tx/Rx means that all the transmitter/receiver elements in the antenna line can be used as transmitter and receiver elements. Each the second antenna block 202 and the fourth antenna block 204 are designed to receive only.

[0046] During the frontal scan (see e.g. Fig. 3a, 4a or 5a) the front part of the object or subject 1 is scanned. Each transmitter element in an antenna line sends a transmission signal. Due to the geometric shape of the object or subject 1 to be scanned, there are multiple possibilities where the reflected signal will be received. In the embodiment of Fig. 3 a, there are two possibilities where the reflected signal will be received, namely the second antenna block 202 and the fourth antenna block 204. In each of the embodiment of Fig. 4a and the embodiment of Fig. 5a, there are three possibilities where the reflected signal will be received, namely the first antenna block 201, second antenna block 202, and the fourth antenna block 204. The left part of the object or subject 1 will most probably be received by the fourth antenna block 204. The rest of the front-part of the object or subject 1 will be most probably be received by the second antenna block 202 while other parts will received with less probability by the first antenna block 201. It will be understood that a similar explanation can be applied for the back scan in connection with Fig. 3b, 4b or 5b, when scanning the back side of the object or subject 1.

[0047] Fig. 6 shows a front view of the first antenna assembly 301 or the second antenna assembly 302 of the scanning system 1000, in particular of the first embodiment shown in Fig. 1. It will be understood that Fig. 6 could also show the scanning system of the embodiment of Fig. 3a, 3b, or Fig. 4a, 4b, or Fig. 5a, 5b. The antenna assembly 301 or 302 comprises multiple antenna lines or scanlines in the direction of the longitudinal axis A (direction of third (y-) axis) such that a two-dimensional array of transducer elements or antenna elements is formed. Each antenna line or scanline is arranged in a different transmitter/receiver-plane P perpendicular to the longitudinal axis A of the object 1 (direction of third (y-) axis). In Fig. 6 each antenna line 201a or 203a, 201b or 203b, 201c or 203c, as well as each antenna line 202a or 204a, 202b or 204b, 202b or 204c, is a stationary antenna line.

[0048] Fig. 7 shows a front view of the first antenna assembly 301 or second antenna assembly 302 of a scanning system 1000 according to an alternative embodiment. In Fig. 7 the antenna assembly 301 or 302 is movable in the direction of the longitudinal axis A (direction of third (y-) axis) such that multiple scanlines can be formed by moving the antenna assembly 301 or 302. [0049] To obtain a full (3D) image it is also required that the third dimension in the third (y-) direction is scanned and sufficient resolution provided. There are several possibilities for achieving this and some examples will be explained in the following. The main aspect described herein is however how to achieve resolution in the first (x-) direction and second (z-) direction. This can then be combined with any suitable method to achieve resolution in the third (y-) direction.

[0050] In particular, image points for multiple antenna lines or scanlines in the direction of the longitudinal axis A (third (y-) direction) for providing a three-dimensional image of the object are determined. In a first example, image points can be determined using a Synthetic Aperture Radar (SAR) concept. In a second alternative example, these image points can be determined using a Beamforming concept. The determination of these image points in the direction of the longitudinal axis A (third (y-) direction), e.g. using SAR or Beamforming concept, can be performed before determining the at least one phase difference. This provides for a better resolution image. However, alternatively the determination of these image points in the direction of the longitudinal axis A (third (y-) direction), e.g. using SAR or Beamforming concept, could also be performed after determining the at least one phase difference.

[0051] As mentioned, the first example to achieve resolution in the third (y-) direction is to use the Synthetic Aperture Radar (SAR) concept in which a large aperture in the third (y-) direction is created by transmitting and receiving the reflected signal (from the object) at different positions in the third (y-) direction. In one example this can be achieved by physically moving the antenna blocks in the third (y-) direction, as shown in the embodiment of Fig. 7. The reception signals can then be collected at every sample position in the third (y-) direction, thus in every scanline. Another example to achieve resolution in the third (y-) direction using the SAR concept is to use a so-called "Stop-Go" SAR technique, in which instead of moving the antenna blocks in the third (y-) direction, the transmit and receive signals are multiplexed to a different line at a time. A complete two-dimensional array of transducer elements or antenna elements is thus needed, as shown in the embodiment of Fig. 6. The signals can be multiplexed for such a two- dimensional array or panel.

[0052] As mentioned, in the second alternative example to provide resolution in the third (y-) direction, beamforming for the transmission signal and/or the reception signal can be performed. Such a Beamforming approach also requires a (fully populated) two- dimensional array or panel of transducer elements or antenna elements, as shown in the embodiment of Fig. 6. Additionally, it can also require a dedicated receiver unit and/or transmitter unit for each antenna line of the two-dimensional array or panel, and can therefore potentially be the more expensive alternative for providing resolution in the third (y-) direction.

[0053] In the embodiment of Fig. 6 the antenna block is fully populated in the third (y-) direction, while in the embodiment of Fig. 7 the scanline mechanically moves. The main difference between these two embodiments is the scanning time and cost. The embodiment of Fig. 6 is faster, but more expensive due to the increase in the hardware components. The embodiment of Fig. 7 is cheaper, but slower due to the movement. The height of the scan (in the third (y-) direction) depends on the height of the object or subject 1 to be scanned or checked. When scanning persons, a maximum height of 2.5 m can in general be sufficient to also take into account tall persons.

[0054] In general, a scanning and imaging method according to one aspect described herein comprises transmitting electromagnetic waves as a transmission signal 103 by means of the first antenna block 201 (and/or the third antenna block 203), and receiving reflections of the transmitted electromagnetic waves from the object 1 as reception signals 105 by means of the second antenna block 202 (and/or the fourth antenna block 204). The at least one transmitter element 101 of one antenna line and the receiver elements 102 of one corresponding antenna line form one scanline. The method further comprises processing the baseband signals 105a, 105b down-converted from the reception signals 105 using an interferometric method comprising determining, for each scanline, at least two signifi- cant receiver elements, in particular at least three, at which a main signal portion of the received signals is received, and determining, for each scanline in which significant receiver elements are determined, at least a first phase difference Δφι between a first pair of significant receiver elements and a second phase difference Δφ₂ between a second pair of significant receiver elements. The method further comprises providing an image of the object 1 using the at least first phase difference Δφι and second phase difference Δφ₂.

[0055] Fig. 8 shows a flow diagram of an interferometric method according to an embodiment. In step S10 a transmitter element in each antenna line or scan line sends the transmission signal 103 and the signal acquisition unit 109 obtains the reception signals 105, as explained above. Then the obtained baseband signal 105a, 105b down-converted from the reception signals 105 is processed. In step S20, for each antenna assembly 301 or 302 and for each scanline, at least two significant receiver elements at which a main signal portion of the received signals is received is determined. The main received signals can be located in each scanline. In other words, it determines which receiver elements will be processed to generate the image(s) in each of the scanlines using the interferometric method. This determination can be done in each scanline or antenna line having receiver elements. The significant baselines can be determined in only one antenna block having receiver elements, in only part of or in each of the antenna blocks having receiver elements. For example, in each of the embodiments of in Fig. 4a, 4b or 5a, 5b this determination can be done in each antenna line of the three antenna blocks receiving signals, as explained above.

[0056] In one example, this determination can be done by performing an adaptive threshold. In this example, the total energy received is calculated throughout all the receiver elements, and then the receiver elements corresponding to the highest measured energy are the ones that contain the significant received signals (hence the baselines needed for interfere metry). In another example, this determination can be done using a sub-space analysis method, like for example a Principal Component Analysis (PCA). In this example, the covariance matrix of the received signals from all the receiver elements is computed, the computed covariance matrix is decomposed using Principal Component Analysis (PCA), and then the receiver elements corresponding to the significant principal components are determined. It will be understood that any other suitable example or method can also be used for this determination.

[0057] In step S30, after the significant receiver elements (baselines) have been located, for each scanline in which significant receiver elements are determined, a phase difference Ac i between each pair of the significant receiver elements is determined. The phase differences Ac i can for example be determined using a Direction of Arrival (DOA) estimation algorithm. The phase differences (interferometric phases) Ac i are determined between the different "co-located" receiver elements. "Co-located" receivers are here defined to be the neighbouring, significant receiver elements 102 in at least one of the antenna blocks having receiver elements. If the distance would be higher than the critical baseline B_c, the interferometric method could not be applied, as mentioned above. In particular a factor of 20% to 30% of B_c is recommended for optimal performance. Each of the "co-located" receiver elements or baselines can receive different parts of the overall image corresponding to different part of the object or subject 1, for example the body of a person. The reception signal at each of the receiver elements could be a mixture between different points of the object or subject 1. These may create a problem in the calculation of the phase differences and this is why this should be taken into account. This problem can especially occur when several objects of different material properties and different depth levels would be hidden by the object or subject 1. To take this that into account, Direction of Arrival (DOA) estimation can for example be used.

[0058] The overall received signals Υτ at one of the significant interferometric baselines for each transmitted signal from a transmitter element T can be written as

where the variable τ . depicts the reflectivity contribution from each point of the object (e.g. subject or body) related to the mixture, it is assumed that the maximum number of the different points in the mixture is L (number of different points of the object contributing to the total received reflectivity), the maximum baseline of the significant baselines is B_max, Δφ is the phase difference (interferometric phase) corresponding to the maximum baseline B_max, , and 5. is one of the baselines from the significant baselines smaller than B_max and η is additive noise. It will be understood that also more complicated noise models could be assumed, which however are not incorporated in the above equation (Equation (4)) above for simplicity purposes. Just as an example, the multiplicative noise can be modelled by multiplying the left addend of the equation above (Equation (4)) by an entity representing the noise. Using the equation of the overall signal Y_T above (Equation (4)), a received signal for each transmission signal transmitted in the antenna line or scanline can be constructed and placed in a single matrix. These received signals can then be used in a DOA algorithm to estimate the corresponding reflectivity and phase contribution from each of the different points of the body. In the above equation (Equation (4)), the distance d between the receiver elements is assumed to be non-uniform to make the description of the problem more general. In case the distance are uniform, thus the values of B_i in the equation above (Equation (4)) being equal, the received overall signal can be written as:

1 expyA(¾ / N_e -l · · · expyA(¾ / N_e -/z · · · expyA(¾

where N_e is the effective number of receiver elements available in the maximum baseline B_l

[0059] The Direction of Arrival (DOA) algorithm can be performed by using a an algorithm to simultaneously solve for τ . and Αφ with uniform or non-uniform baselines. Exemplary methods that can be used for this purpose are Maximum Likelihood (ML), ESPRIT, MUSIC, CAPON Beamforming (such as for example disclosed in Riib- samen and Greshman, "Direction-of-Arrival-Estimation for Nonuniform Sensor Arrays: From Manifold Separation to Fourier Domain Music Methods", IEEE Trans. Signal Processing, vol. 57, no. 2, Feb. 2009, which is incorporated by reference herein) and variations thereof. In each of these methods, for each of the transmission signals, the corresponding radar reflectivities τ . and phase differences Δφ are obtained, in a similar manner as described in the equation of the overall signal Yr above (Equation (4)). These have to be estimated in the (baseband) reception signals of each scanline where significant baselines are found.

[0060] Fig. 9a and Fig. 9b show two alternative embodiments of the step S30 of determining phase differences and depth values of Fig. 8. Any of these two embodiments can be realized before the determination of image points in the y-direction, e.g. using a SAR or Beamforming concept as explained above, or after the determination of image points in the y-direction. In other words, as explained above, the determination of the image points in the y-direction can be performed before or after determining the at least one phase difference.

[0061] In step S31 of Fig. 9a or Fig. 9b registering or alignment is performed for the scanlines in the different antenna blocks. In particular the determined phase differences between each combination of pairs of significant receiver elements for scanlines in at least two different antenna blocks can be aligned or registered. The registration step tries to find which pixels in the constructed partial images in the different antenna blocks, e.g. 201 and 202, belong to each other. This is because the reception signals at each of the receiver elements in the different antenna blocks correspond to different parts of the body. In other words, direct reflections and diffuse reflections will be received from all body parts at the different antenna blocks and the corresponding parts need to be found in order to reconstruct the whole image from all the parts at the different antenna blocks. Since, however, the DOA is calculated along the phase differences (interferometric phases) and the nominal range R is also known, it is possible to differentiate them and place all the received signals into their corresponding locations in the image, for example using a simple search and place algorithm. This means that given the geometric locations of the receiver elements with respect to each other and the estimated DOA of each of the signals, the corresponding locations are joined together in a single image that contains all of the information. The resulting image after registration can, nevertheless, have some empty locations where the signal-to-noise-ratio (SNR) of the reception signal was for example too low.

[0062] To solve this issue, an interpolation algorithm can be used to estimate the missing phases from the received ones. In Fig. 9a and Fig. 9b in a step S33 missing phase differences are determined from the determined phase differences using an interpolation algorithm. Any suitable interpolation algorithm can be used, such as for example applying bilinear, bi-cubic, splines or total variation to interpolate the missing locations.

[0063] The example of Fig. 9b differs from the example of Fig. 9a by a further step S34 of determining if the determined phase difference is greater than 2π, and if the phase difference is determined to be greater than 2π, to perform a phase unwrapping algorithm before calculating a depth value z. An ambiguous depth z _A at which a phase difference (interferometric phase) Δφ reaches the value of 2π can be defined. The ambiguous depth z_A for a given baseline B can be as defined in the following equation:

For a given baseline B, the ambiguous depth z_A defines the value after which the depth values cannot be resolved anymore. It is defined mainly by the maximum size of baseline B_max used, the look angle θ , the nominal range R of the object, and the frequency / of operation.

[0064] Two possibilities exist for the imaging algorithm depending on the scanning setup designed. Firstly, the ambiguous depth z_A might not be reached at all in some setups depending on the parameters chosen according to the above mentioned equation of the ambiguous depth (Equation (6)). In this case, after the interpolation the depth value z can be calculated from the phase differences (interferometric phases) Δφ based on the above mentioned equation for the depth sensitivity (Equation (2)). Secondly, in some parameters of the system, the ambiguous depth z_A can be trespassed which means that the phase difference Δφ (interferometric phase) wraps around the 2π length interval again. In this case, it is necessary before calculating the depth values, to perform a phase unwrapping algorithm, as shown in step S34 in Fig. 9b, after the interpolation step. The idea of phase unwrapping is to determine the absolute phase value from a modulo- 2π phase obtained when the ambiguous depth is trespassed. Any suitable algorithms can be used for the phase unwrapping algorithm, for example a method based on Least Squares (LS) fit, branching techniques, graph-based optimization techniques (like belief propagation and graph cuts) and many other ones. Examples of such algorithms can for example be found in Rosen, Hensley, Joughin, Li, Madsen, Rodriguez and Goldstein, "Synthetic Aperture Interferometry", Proc. of the IEEE, vol. 88, no. 3, Mar. 2000, which is incorporated by reference herein.

[0065] In Fig. 9a after the interpolation step S33, or in Fig. 9b after the phase unwrapping step S34, a depth value z based on the determined phase differences Δφ is calculated. In particular, for each scanline, the determined phase differences Δφ and depth values z can be used to create image points of the image of the object 1. The algorithm can compute the depth value z for example based on the above mentioned equation for the depth sensitivity (Equation (2)).

[0066] Performing this process for the frontal scan and the back scan of the person (see e.g. Fig. 1, 3, 4 and 5) and processing the reception signals using the imaging algorithm described herein, results in a 2D image (in the first (x-) and second (z-) dimensions). To resolve the third (y-) dimension other algorithms, such as for example Synthetic Aperture Radar (SAR) Concept or a Beamforming concept can be used. The results of the 2D image can then be combined with the results of the third (y-) dimension algorithm, leading to full 3D image of the object or subject 1. This 3D image can for example be used for inspecting the object or subject 1. Optionally, an image enhancement algorithm can be used to further enhance the image, for example implemented in the post-processing unit 111 shown in Fig. 1. For example, schemes used for super-resolution imaging or noise reduction techniques can be easily combined.

[0067] In other examples, e.g. where the privacy is an issue, the signal processing unit(s) can be amended with some other algorithms, for example with the goal to detect some hidden object or item and to display the sketches of the object or item without the necessity to display the actual image of the subject (person). In such a case, a suspicious object or item can be automatically located on the image using an object detection algorithm which then allows displaying the location of the suspicious object or item in the image of the subject or person. Such an algorithm can for example also be implemented in the post-processing unit 111 of Fig. 1.

[0068] Fig. 10 shows a schematic block diagram of an exemplary receiver unit which is part of or attached to each antenna block, for example as shown in Fig. 1. In this example the system 1000 further comprises a receiver mixer in the form of an I-/Q-mixer for generating a mixed reception signal based on the transmission signal 103 and the reception signal 105. The signal acquisition unit 109 comprises here a typical FMCW receiver. The reception signal 105 received in a receiver element or antenna line is amplified via a Low Noise Amplifier (LNA) 906 and mixed with the reference transmission signal 103 via the I-/Q-mixer 905 to obtain the beat signals after an isolation stage 904, for both signals. Both I&Q outputs are filtered using a Band Pass Filter (BPF) 903 via an Automatic Gain Control Unit (AGC) 902 to be sampled with the full dynamic range via an Analog to Digital Converter (ADC) 901. The output are baseband signals 105 a, 105 in form of an I-output (I branch) and a Q-output (Q branch). The I-output and the Q-output (I/Q signal) can in general also be obtained using a single branch non-I/Q-mixer followed by a digital method. The receiver mixer can for example be a Digitial Quadrature Demodulator. [0069] Fig. 11a shows a schematic block diagram of an antenna block comprising at least one antenna line having only receiver elements (but no transmitter elements) according to a first example. Fig. 1 lb shows a schematic block diagram of an antenna line having only receiver elements (but no transmitter elements) according to a second example. The two examples are two different solutions for one antenna line. In the first example of Fig. 11a each receiver element 102 is connected to one receiver block 1004. In the second example of Fig. l ib the receiver elements are switched using a switch 1005 to a single receiver 1004. The selection of the solution is independent of the scanning solution selected in the first (x-) direction depicted in Fig. 3a, 3b or 4a, 4b or 5a, 5b for example.

[0070] In the first example of Fig. 1 la of the antenna line with the receiver elements (Rx) a receiver 1004 for each of the N receiver elements 102 is provided. For simplification purposes only two receivers 1004 of the N receivers are illustrated in Fig. 11a. In this example of Fig. 11a the reference signal is driven to the N receivers 1004 via a power divider 1001. The I-output and the Q-output of the N receiver elements 102 are collected in a data bus 1002. In the second example of Fig. 1 lb of the antenna line with the receiver elements (Rx) a single receiver 1004 is switched to the N receiver elements 102 sequentially using a switch 1005. In this case the signal acquisition unit 109 can define the signal of which receiver element 102 needs to be driven to the receiver 1004 via the (N: l) switch 1005. An advantage of the first example of Fig. 11a is that the scanning time is N times faster, because the reception does not need to be done sequentially, but can be done in parallel. An advantage of the second example of Fig. l ib, in comparison to the first example, is a lower cost, because N receiver elements are needed for each antenna line.

[0071] Fig. 12a shows an antenna line having a single transmitter element, in particular an antenna line of the embodiment shown in Fig. 3a and 3b. In the example of Fig. 12a, each antenna line (of the first antenna block 201 or the third antenna block 203) comprises only a single transmitting antenna 101. The transmission signal (Tx signal) is delivered to the single transmitter element 101. [0072] Fig. 12b shows a schematic block diagram of an antenna line having a single transmitter element and multiple receiver elements, in particular an antenna line of the embodiment shown in Fig. 4a and 4b. In the example of Fig. 12b, each antenna line comprises a single transmitting antenna 101 and N receiving antennas. In this example shown in Fig. 12b, the Tx signal is derived to a transmitting antenna and to a Rx line 1104 via a power divider 1102. This is made to derive the signal to the transmit antenna and to simultaneously make the signal as a reference to the reception line. Both of the options explained for Rx line can be applied here, with the same implications that were described in the Rx line section.

[0073] Fig. 12c shows a schematic block diagram of an antenna line having transmitter/receiver elements, in particular an antenna line of the embodiment shown in Fig. 5a and 5b. In the example of Fig. 12b, each antenna line comprises M transmitter/receiver elements that can both transmit and receive. In this example, one transmitter/receiver elements element of the M transmitter/receiver elements can be sequentially selected to transmit the transmission signal, while the other M-l transmitter/receiver elements are receiving. In this example shown in Fig. 12c, the signal acquisition unit 109 can define to which antenna the transmission signal is connected via a (M: l) switch 1105. A number M of circulators 1107 can define if the transmitter/receiver elements are connected to the transmitting or receiving line. The circulators 1107 allow the usage of transmit/receive models simultaneously in case it is desired. The I output and the Q output of the M receivers 1106 are collected in a data bus 1108. For simplification purposes only two receivers 1106 of the M receivers are illustrated in Fig. 12c.

[0074] Fig. 12d shows a schematic block diagram of the first or second antenna assembly, in particular the antenna assembly of the embodiment shown in Fig. 6. With reference to Fig. 12d, an option to extend a single antenna line solution to the third (y-) direction of the antenna block (thus forming an array or panel) is explained. The first or third antenna block 201 , 203 having the transmitter elements (Tx/Rx panel) and the second or fourth antenna block 202, 204 having only the receiver elements (Rx panel) are ex- tended from the described antenna lines depending on the scanning technology used in the third (y-) direction.

[0075] Fig. 6 shows a first example how to solve the scanning in the third (y-) direction, in which the Tx and Rx panels are populated with several Tx/Rx and Rx lines respectively. Such a solution could be used to for the "Stop and Go" SAR and for a Beamforming concept for the third (y-) direction, as explained above. Fig. 12d shows a general embodiment of Tx/Rx and Rx panels with K lines.

[0076] In the example of Fig. 12d a digital platform 1201 directs how the scanning is performed via the control outputs and collects the data from the receiver elements via the data input. A baseband chirp and a reference signal are generated separately from the panels in a Direct Digital Synthesizer (DDS) 1203 and local oscillator 1202, respectively. These signals are driven to both the Tx/Rx panel and the Rx panel with power dividers 1204.

[0077] In the Tx/Rx panel, the transmission signal is generated in baseband by a DDS 1203 in a first stage. Then, it is up-converted and mixed (via a mixer 1205) with the reference signal which is generated by the oscillator 1202. The band of the transmission signal is then expanded by using a multiplier 1206. Subsequently the signal is filtered with a band pass filter (BPF) 1207 to suppress unwanted frequency components. The next stage is to switch the transmission signal to each of the K antenna lines where the scanning needs to be performed. To achieve this, the transmission signal is amplified before the switch

1209 via a power amplifier (PA) 1208, in order to compensate the losses introduced by the switch 1209. The Tx line 1212 used for this example is the solution shown in Fig. 12c. The transmission signal is used for both transmission and as reference in the reception process. The I-outputs and the Q-outputs of the M receiver elements are collected with a data bus

1210 and directed to the digital platform 1201. [0078] For the Rx panel, the reference signal used in reception to obtain the beat frequency is generated in the same way as for the Tx/Rx panel. There is a difference in how the signal derived to the K lines, as in this case a power divider 1213 is used because the signal can be present in all the Rx lines during the complete scanning process and because the power level of this signal is not an issue in reception. The I-outputs and the Q- outputs of the N receivers are collected with a data bus 1210 and directed to the digital platform 1201 in the same ways as with the Tx/Rx panel.

[0079] The invention has been illustrated and described in detail in the drawings and foregoing description, but such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

[0080] In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0081] A computer program may be stored / distributed on a suitable non- transitory medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

[0082] Any reference signs in the claims should not be construed as limiting the scope. [0083] It follows a list of further embodiments:

1. An interferometric scanning and imaging system (1000) for scanning an object (1) with electromagnetic waves and for providing an image of the object (1), the system comprising:

- a first antenna assembly (301) comprising:

- a first antenna block (201) comprising at least one antenna line comprising at least one transmitter element (101) for transmitting electromagnetic waves as a transmission signal (103), and

- a second antenna block (202) comprising at least one antenna line comprising at least three receiver elements (102a, 102b, 102c) arranged in a scanline for receiving reflections of the transmitted electromagnetic waves from the object (1) as reception signals (105), the at least one transmitter element (101) of one antenna line and the receiver elements (102) of one corresponding antenna line forming one scanline,

the system further comprising at least one signal processing unit (100) for processing baseband signal (105a, 105b) down-converted from the reception signals (105) using an interferometric method comprising determining, for each scanline, at least three significant receiver elements at which a main signal portion of the received signals is received, and determining, for each scanline in which significant receiver elements are determined, at least a first phase difference (Δφι) between a first pair of significant receiver elements and a second phase difference (Δφ₂) between a second pair of significant receiver elements, wherein the at least one signal processor (100) is configured to provide an image of the object (1) using the at least first phase difference (Δφι) and second phase difference (Δφ₂).

2. The system of embodiment 1, wherein the first antenna block (201) and the second antenna block (202) are arranged next to each other.

3. The system of embodiment 1, wherein the first antenna block (201) and the second antenna block (202) are arranged opposite to each other. 4. The system of one of embodiments 1 to 3, wherein the first antenna assembly (301) is the single antenna assembly of the system.

5. The system (1000) of one of embodiments 1 to 3, further comprising a second antenna assembly (302) comprising:

- a third antenna block (203) comprising at least one antenna line comprising at least one transmitter element (101) for transmitting electromagnetic waves as a transmission signal (103), and

- a fourth antenna block (204) comprising at least one antenna line comprising only receiver elements (102) for receiving reflections of the transmitted electromagnetic waves from the object (1) as reception signals (105).

6. The system (1000) of one of the preceding embodiments, the object (1) having a longitudinal axis (A) defining an axis of a coordinate system having a first, second, third and fourth quadrant (I, II, III, IV), the first antenna block (201) arranged in the first quadrant (I) and/or the second antenna block (202) arranged in the second quadrant (II) next to the first quadrant (I).

7. The system (1000) of embodiment 5 and 6, the third antenna block (203) arranged in the third quadrant (III) and/or the fourth antenna block (204) arranged in the fourth quadrant (IV) next to the third quadrant (III).

8. The system of one of embodiments 6 to 7, the third quadrant (III) being diagonally opposite to the first quadrant (I).

9. The system of one of embodiments 6 to 8, wherein the coordinate system has an origin (O), a first axis (x), and a second axis (z), wherein a third axis (y) is in the direction of the longitudinal axis (A). 10. The system of embodiment 9, wherein, for each antenna assembly (301, 302), the at least one transmitter element (101) of one antenna line and the receiver elements (102) of one corresponding antenna line are arranged in the same transmitter/receiver-plane (P) which is perpendicular to the longitudinal axis (A) of the object (1).

11. The system of embodiment 9 or 10, wherein each antenna line is arranged parallel to the first axis (x).

12. The system of one of embodiments 6 to 11, wherein each transmitter element (101) and/or each receiver element (102) are arranged in the coordinate system at a non-normal and/or non-zero angle (Θ).

13. The system of one of the preceding embodiments, wherein, for each antenna assembly (301, 302), the at least one transmitter element (101) of one antenna line and the receiver elements (102) of one corresponding antenna line form one scanline.

14. The system of one of the preceding embodiments, each antenna line of the first antenna block (201) and/or each antenna line of the third antenna block (203) comprising multiple transmitter elements (101).

15. The system of embodiment 14, wherein the transmission signals of the transmitter elements (101) of each antenna line are transmitted sequentially.

16. The system of embodiment 14, wherein the transmission signals of the transmitter elements (101) of each antenna line are transmitted in parallel using a signal multiplexing concept.

17. The system of one of the preceding embodiments, each antenna line of the second antenna block (202) and/or each antenna line of the fourth antenna block (204) comprising at least three receiver elements (102a, 102b, 102c). 18. The system of one of the preceding embodiments, wherein the at least one transmitter element (101) of the first antenna block (201) and/or the third antenna block (203) is a transmitter/receiver element for transmitting electromagnetic waves as a transmission signal and receiving reflections of the transmitted electromagnetic waves from the object (1).

19. The system of one of the preceding embodiments, wherein each antenna line is a stationary antenna line.

20. The system of one of the preceding embodiments, each antenna assembly (301, 302) comprising multiple antenna lines or scanlines in the direction of the longitudinal axis (A) such that a two-dimensional array of transducer elements is formed.

21. The system of embodiment 20, wherein each antenna line or scanline is arranged in a different transmitter/receiver-plane (P) perpendicular to the longitudinal axis of the object (1).

22. The system of one of the preceding embodiments, wherein each antenna assembly (301, 302) is movable in the direction of the longitudinal axis (A) such that multiple scanlines can be formed by moving the antenna assembly (301, 302).

23. The system of one of the preceding embodiments, further comprising a signal generator for generating a FMCW transmission signal (103) or a pulse transmission signal (103).

24. The system of one of the preceding embodiments, the system further comprising a receiver mixer for generating a mixed reception signal based on the transmission signal (103) and the reception signal (105). 25. The system of embodiment 24, wherein the receiver mixer is an I-/Q-mixer.

26. The system of one of the preceding embodiments, wherein the number of receiver elements (102) in a scanline is higher than the number of significant receiver elements.

27. The system of one of the preceding embodiments, the at least one signal processor (100) configured to determine, for each scanline in which significant receiver elements are determined, a phase difference between each pair of the significant receiver elements.

28. The system of one of the preceding embodiments, the at least one signal processor (100) configured to determine the at least two phase differences (Δφι and Δφ₂) using a Direction of Arrival (DOA) estimation algorithm.

29. The system of one of the preceding embodiments, the at least one signal processor (100) configured to register or align the determined phase differences between each combination of pairs of significant receiver elements for scanlines in at least two different antenna blocks.

30. The system of one of the preceding embodiments, the at least one signal processor (100) configured to determine image points for multiple antenna lines or scanlines in the direction (y) of the longitudinal axis (A) for providing a three-dimensional image of the object.

31. The system of embodiment 30, wherein the image points for multiple antenna lines or scanlines in the direction (y) of the longitudinal axis (A) are determined using a Synthetic Aperture Radar Concept or using a Beamforming Concept.

32. The system of embodiment 30 or 31, the at least one signal processor (100) configured to determine the image points for multiple antenna lines or scanlines in the direction (y) of the longitudinal axis (A) before determining the at least one phase difference (Δφι). 33. The system of embodiment 30 or 31, the at least one signal processor (100) configured to determine the image points for multiple antenna lines or scanlines in the direction (y) of the longitudinal axis (A) after determining the at least one phase difference (Δφι).

34. The system of one of the preceding embodiments, the at least one signal processor (100) configured to estimate missing phase differences from the determined phase differences using an interpolation algorithm.

35. The system of one of the preceding embodiments, the at least one signal processor (100) configured to calculate a depth value (z) based on the determined phase differences.

36. The system of embodiment 35, the at least one signal processor (100) configured to determine if the determined phase difference is greater than 2π, and if the phase difference is determined to be greater than 2π, to perform a phase unwrapping algorithm before calculating a depth value (z).

37. The system of one of the preceding embodiments, wherein, for each scanline, the determined phase differences and depth values (z) are used to create image points of the image of the object (1).

38. The system of one of the preceding embodiments, the at least one signal processor (100) configured to perform an image enhancement algorithm to enhance the image.

39. The system of one of the preceding embodiments, the at least one signal processor (100) configured to locate a suspicious object or item automatically in the image.

40. The system of one of the preceding embodiments, wherein the electromagnetic waves are in the range between 1 GHz and 10 THz.

41. The system of one of the preceding embodiments, wherein the electromagnetic waves are mm-waves.

Claims

1. An interferometric scanning system (1000) for scanning an object (1) with electromagnetic waves, the object (1) having a longitudinal axis (A) defining an axis of a coordinate system having a first, second, third and fourth quadrant (I, II, III, IV), the system comprising:

- a first antenna assembly (301) comprising:

- a first antenna block (201) comprising at least one antenna line comprising at least one transmitter element (101) for transmitting electromagnetic waves as a transmission signal (103), the first antenna block (201) arranged in the first quadrant (I), and

- a second antenna block (202) comprising at least one antenna line comprising only receiver elements (102) for receiving reflections of the transmitted electromagnetic waves from the object (1) as reception signals (105), the second antenna block (202) arranged in the second quadrant (II) next to the first quadrant (I),

- a second antenna assembly (302) comprising:

- a third antenna block (203) comprising at least one antenna line comprising at least one transmitter element (101) for transmitting electromagnetic waves as a transmission signal (103), the third antenna block (203) arranged in the third quadrant (III), and

- a fourth antenna block (204) comprising at least one antenna line comprising only receiver elements (102) for receiving reflections of the transmitted electromagnetic waves from the object (1) as reception signals (105), the fourth antenna block (204) arranged in the fourth quadrant (IV) next to the third quadrant (III),

the system further comprising at least one signal processing unit (100) for processing baseband signal (105a, 105b) down-converted from the reception signals (105) using an interferometric method comprising determining at least one phase difference (Δφι) between specific receiver elements (102).

2. The system of claim 1, the third quadrant (III) being diagonally opposite to the first quadrant (I).

3. The system of claim 1 or claim 2, wherein the coordinate system has an origin (O), a first axis (x), and a second axis (z), wherein a third axis (y) is in the direction of the longitudinal axis (A).

4. The system of claim 3, wherein, for each antenna assembly (301, 302), the at least one transmitter element (101) of one antenna line and the receiver elements (102) of one corresponding antenna line are arranged in the same transmitter/receiver-plane (P) which is perpendicular to the longitudinal axis (A) of the object (1).

5. The system of claim 3 or 4, wherein each antenna line is arranged parallel to the first axis (x).

6. The system of one of claims 3 to 5, wherein each transmitter element (101) and/or each receiver element (102) are arranged in the coordinate system at a non-normal and/or non-zero angle (Θ).

7. The system of one of the preceding claims, wherein, for each antenna assembly (301, 302), the at least one transmitter element (101) of one antenna line and the receiver elements (102) of one corresponding antenna line form one scanline.

8. The system of one of the preceding claims, each antenna line of the first antenna block (201) and/or each antenna line of the third antenna block (203) comprising multiple transmitter elements (101).

9. The system of claim 8, wherein the transmission signals of the transmitter elements (101) of each antenna line are transmitted sequentially.

10. The system of claim 8, wherein the transmission signals of the transmitter elements (101) of each antenna line are transmitted in parallel using a signal multiplexing concept.

11. The system of one of the preceding claims, each antenna line of the second antenna block (202) and/or each antenna line of the fourth antenna block (204) comprising at least three receiver elements (102a, 102b, 102c).

12. The system of one of the preceding claims, wherein the at least one transmitter element (101) of the first antenna block (201) and/or the third antenna block (203) is a transmitter/receiver element for transmitting electromagnetic waves as a transmission signal and receiving reflections of the transmitted electromagnetic waves from the object (1).

13. The system of one of the preceding claims, wherein each antenna line is a stationary antenna line.

14. The system of one of the preceding claims, each antenna assembly (301, 302) comprising multiple antenna lines or scanlines in the direction of the longitudinal axis (A) such that a two-dimensional array of transducer elements is formed.

15. The system of claim 14, wherein each antenna line or scanline is arranged in a different transmitter/receiver-plane (P) perpendicular to the longitudinal axis of the object (1).

16. The system of one of the preceding claims, wherein each antenna assembly (301, 302) is movable in the direction of the longitudinal axis (A) such that multiple scanlines can be formed by moving the antenna assembly (301, 302).

17. The system of one of the preceding claims, further comprising a signal generator for generating a FMCW transmission signal (103) or a pulse transmission signal (103).

18. The system of one of the preceding claims, the system further comprising a receiver mixer for generating a mixed reception signal based on the transmission signal (103) and the reception signal (105).

19. The system of claim 18, wherein the receiver mixer is an I-/Q-mixer.

20. The system of one of the preceding claims, the at least one phase difference comprising a first phase difference (Δφι) and second phase difference (Δφ₂).

21. The system of one of the preceding claims, further configured for providing an image of the object (1).

22. The system of claim 21, the at least one signal processor (100) configured to provide the image of the object (1) using the at least one phase difference (Δφι).

23. The system of one of the preceding claims, the at least one signal processor (100) configured to determine, for each scanline, at least two significant receiver elements at which a main signal portion of the received signals is received.

24. The system of claim 23, wherein the number of receiver elements (102) in a scanline is higher than the number of significant receiver elements.

25. The system of claim 23 or 24, the at least one signal processor (100) configured to determine, for each scanline in which significant receiver elements are determined, a phase difference between each pair of the significant receiver elements.

26. The system of one of claims 23 or 25, the at least one signal processor (100) configured to determine, for each scanline in which significant receiver elements are determined, a first phase difference (Δφι) between a first pair of significant receiver elements and a second phase difference (Δφ₂) between a second pair of significant receiver elements.

27. The system of one of claims 23 to 26, the at least one signal processor (100) configured to determine the at least two phase differences (Δφι and Acp₂) using a Direction of Arrival (DOA) estimation algorithm.

28. The system of one of claims 23 or 27, the at least one signal processor (100) configured to register or align the determined phase differences between each combination of pairs of significant receiver elements for scanlines in at least two different antenna blocks.

29. The system of one of the preceding claims, the at least one signal processor (100) configured to determine image points for multiple antenna lines or scanlines in the direction (y) of the longitudinal axis (A) for providing a three-dimensional image of the object.

30. The system of claim 29, wherein the image points for multiple antenna lines or scanlines in the direction (y) of the longitudinal axis (A) are determined using a Synthetic Aperture Radar Concept or using a Beamforming Concept.

31. The system of claim 29 or 30, the at least one signal processor (100) configured to determine the image points for multiple antenna lines or scanlines in the direction (y) of the longitudinal axis (A) before determining the at least one phase difference (Δφι).

32. The system of claim 29 or 30, the at least one signal processor (100) configured to determine the image points for multiple antenna lines or scanlines in the direction (y) of the longitudinal axis (A) after determining the at least one phase difference (Δφι).

33. The system of one of the preceding claims, the at least one signal processor (100) configured to estimate missing phase differences from the determined phase differences using an interpolation algorithm.

34. The system of one of the preceding claims, the at least one signal processor (100) configured to calculate a depth value (z) based on the determined phase differences.

35. The system of claim 34, the at least one signal processor (100) configured to determine if the determined phase difference is greater than 2π, and if the phase difference is determined to be greater than 2π, to perform a phase unwrapping algorithm before calculating a depth value (z).

36. The system of claim 34 or 35, wherein, for each scanline, the determined phase differences and depth values (z) are used to create image points of the image of the object (1).

37. The system of one of the preceding claims, the at least one signal processor (100) configured to perform an image enhancement algorithm to enhance the image.

38. The system of one of the preceding claims, the at least one signal processor (100) configured to locate a suspicious object or item automatically in the image.

39. The system of one of the preceding claims, wherein the electromagnetic waves are in the range between 1 GHz and 10 THz.

40. The system of one of the preceding claims, wherein the electromagnetic waves are mm- waves.

41. An interferometric scanning and imaging system (1000) for scanning an object (1) with electromagnetic waves and for providing an image of the object (1), the system comprising:

- a first antenna assembly (301) comprising:

42. An interferometric scanning method for scanning an object (1) with electromagnetic waves, the object (1) having a longitudinal axis (A) defining an axis of a coordinate system having a first, second, third and fourth quadrant (I, II, III, IV), the method comprising:

- transmitting electromagnetic waves as a transmission signal (103) by means of a first antenna block (201) comprising at least one antenna line having at least one transmitter element (101, the first antenna block (201) arranged in the first quadrant (I), and - receiving reflections of the transmitted electromagnetic waves from the object (1) as reception signals (105) by means of a second antenna block (202) comprising at least one antenna line comprising only receiver elements (102) for, the second antenna block (202) arranged in the second quadrant (II) next to the first quadrant (I),

- transmitting electromagnetic waves as a transmission signal (103) by means of a third antenna block (203) comprising at least one antenna line comprising at least one transmitter element (101), the third antenna block (203) arranged in the third quadrant (III) , and

- receiving reflections of the transmitted electromagnetic waves from the object (1) as reception signals (105) by means of a fourth antenna block (204) comprising at least one antenna line comprising only receiver elements (102), the fourth antenna block (204) arranged in the fourth quadrant (IV) next to the third quadrant (III),

- processing baseband signal (105 a, 105b) down-converted from the reception signals (105) using an interferometric method comprising determining at least one phase difference (Δφι) between specific receiver elements (102).

43. An interferometric scanning and imaging method for scanning an object (1) with electromagnetic waves and for providing an image of the object (1), the method comprising:

- transmitting electromagnetic waves as a transmission signal (103) by means of a first antenna block (201) comprising at least one antenna line comprising at least one transmitter element (101), and

- receiving reflections of the transmitted electromagnetic waves from the object (1) as reception signals (105) by means of a second antenna block (202) comprising at least one antenna line comprising at least three receiver elements (102a, 102b, 102c), the at least one transmitter element (101) of one antenna line and the receiver elements (102) of one corresponding antenna line forming one scanline,

- processing baseband signal (105 a, 105b) down-converted from the reception signals (105) using an interferometric method comprising: - determining, for each scanline, at least three significant receiver elements at which a main signal portion of the received signals is received, and

- determining, for each scanline in which significant receiver elements are determined, at least a first phase difference (Δφι) between a first pair of significant receiver elements and a second phase difference (Δφ₂) between a second pair of significant receiver elements,

- providing an image of the object (1) using the at least first phase difference (Δφι) and second phase difference (Δφ₂).

44. A computer program comprising program code means for causing a computer to perform the steps of said method as claimed in claim 42 or 43 when said computer program is carried out on a computer.

45. A computer readable non-transitory medium having instructions stored thereon which, when carried out on a computer, cause the computer to perform the steps of the method as claimed in claim 42 or 43.