阅读说明：本技术 Gnss天线附件 (GNSS antenna accessory ) 是由 N·卡佩 F-X·马尔梅 于 2018-11-14 设计创作，主要内容包括：本发明涉及被配置为降低到达GNSS接收器的天线的GNSS信号的反射路径(612、613)的功率电平的附加设备、相关联的GNSS接收器组、嵌入该附加设备的一些装备以及降低反射的GNSS信号的功率电平的相关联的方法。GNSS信号根据第一极化进行发射,并且附加设备包括材料(601),该材料被配置为对所述第一极化透明,并且反射根据与所述第一极化正交的第二极化而极化的GNSS信号。(The invention relates to an add-on device configured to reduce the power level of a reflected path (612, 613) of a GNSS signal reaching an antenna of a GNSS receiver, to an associated set of GNSS receivers, to some equipment embedded in the add-on device and to an associated method of reducing the power level of a reflected GNSS signal. The GNSS signals are transmitted according to a first polarization, and the add-on device comprises a material (601) configured to be transparent to said first polarization and to reflect GNSS signals polarized according to a second polarization orthogonal to said first polarization.)

1. An add-on device (401, 501, 601, 701, 801, 901) configured to reduce a power level of a reflected path (111, 112, 113, 211, 212, 213) of a GNSS signal reaching an antenna (101, 202) of a GNSS receiver, the GNSS signal being transmitted according to a first polarization, the add-on device comprising a material (601, 701, 801, 901) configured to be transparent to the first polarization and to reflect GNSS signals polarized according to a second polarization orthogonal to the first polarization.

2. The supplemental device of claim 1, wherein the first polarization is right-hand circular polarization and the second polarization is left-hand circular polarization.

3. An add-on device according to any of the preceding claims, wherein the material is configured to operate in at least one of the following frequency bands:

·1164MHz-1214MHz，

·1215MHz-1254MHz，

·1260MHz-1300MHz，

·1559MHz-1610MHz，

2483.5MHz-2500MHz, and

·5010MHz-5030MHz。

4. a GNSS receiver set comprising:

-a GNSS receiver (101, 201) comprising an antenna (202) configured to receive GNSS signals, an

-at least one additional device (601, 701, 801, 901) according to any one of claims 1 to 3.

5. The GNSS receiver set of claim 4, wherein the material is a polarization selective surface.

6. The set of GNSS receivers of claim 5, wherein the material is placed in front of the antennas of the GNSS receivers so as to reflect at least some of the reflected paths (612, 613, 712, 713, 711, 812) of GNSS signals before they reach the antennas of the GNSS receivers.

7. The set of GNSS receivers of any of claims 5 and 6, wherein the material is optically transparent.

8. The GNSS receiver set of any of claims 5 and 6, wherein the material is a metalized textile.

9. A GNSS receiver housing (901) comprising an add-on device according to any of claims 1 to 3.

10. A windscreen (103) comprising the add-on device (601) according to any one of claims 1 to 3, wherein the material is optically transparent.

11. A method for reducing the power level of a reflected path (111, 112, 113, 211, 212, 213) of GNSS signals reaching an antenna (101, 202) of a GNSS receiver, said GNSS signals being transmitted according to a first polarization, said method comprising placing at least one additional device (601, 701, 801, 901) configured to reduce the power level of said reflected path, said additional device comprising a material (601, 701, 801, 901) configured to be transparent to said first polarization and to reflect GNSS signals polarized according to a second polarization orthogonal to said first polarization.

Technical Field

The invention is applicable to the field of Global Navigation Satellite System (GNSS) receivers. More specifically, the present invention describes an accessory for a GNSS receiver that increases the robustness of the GNSS receiver to multipath reflections.

Background

GNSS positioning technology has been used and improved over the years today. Two Global Navigation Satellite Systems (GNSS) have been fully deployed for many years (the United states Global Positioning System (GPS) and Russian GLONASS)^TM) And also two are being deployed (china beidou navigation satellite system and european Galileo)^TM)。

These systems rely on the same principle to provide accurate and accurate positioning measurements: microwave Radio Frequency (RF) signals are broadcast on a common carrier frequency from many satellites in orbit around our planet; the signal carrying the navigation message is propagated using PRN (pseudo random noise) sequences specific to each transmitter. At the receiver side, the various transmissions are identified from their PRN code sequences. For at least four satellites in view, the time of arrival is used together with information retrieved from the navigation message about the time of transmission and the position of the satellite to calculate a pseudorange measurement relating to the distance between the receiver and the satellite. When four or more pseudorange measurements are computed from different satellites, the receiver computes position, velocity, and time measurements (PVT), for example, by trilateration.

Among the various phenomena that affect the accuracy of a positioning system are signal reflections. In fact, in order to establish a relationship between the distance separating the satellite and the receiver (pseudorange) and the propagation time, the satellite in question must be in line of sight (LOS) without signal reflection. At the receiver side, the direct propagation path may be received along with multipath reflections of the positioning signal caused by reflections of the signal on various elements of the propagation environment. These paths are delayed versions of the direct paths, which typically attenuate and phase shift.

Multipath reflections create artifacts that affect the pseudorange measurements and, therefore, the accuracy of the GNSS receiver. These multipath reflections may have power levels that are close to, and sometimes greater than, the power level of the direct path, depending on the propagation environment and/or antenna performance.

Multipath reflections can be divided into two categories:

-multipath reflections that delay more than one PRN chip from the direct propagation path and thus create inter-symbol interference. This condition is known as "selective fading". Implementing signal processing techniques (equalizers) in GNSS receiver software may mitigate selective fading to some extent. However, this technique is complex to implement, does not cancel the interference well, and therefore requires a hardware solution to supplement to reduce the power level of the multipath reflections;

-multipath reflections delayed from the direct propagation path by less than one PRN chip. When the reflected paths travel a greater distance than the direct path, they recombine in a constructive or destructive manner at the level where the receiver antennas have different phases. When they recombine in a destructive manner, i.e. when they are in phase opposition, the amplitude of the received signal decreases and the signal may eventually disappear. This condition is known as "flat fading". Software algorithms cannot mitigate flat fading and therefore must be processed upstream of the GNSS receiver by hardware solutions.

Selective fading is a common situation in urban environments. It comes from reflections of the positioning signal off various elements of the propagation environment, such as buildings, cars, trees, etc. … …. Selective fading may be advantageous for most telecommunication standards, where an equalizer recombines the direct and reflected paths in time and phase, thereby increasing the signal-to-noise ratio. This is not the case, however, for positioning purposes, where the direct path is the only path to consider when computing pseudorange measurements. The reflected path is only a source of inaccuracy and should be suppressed. Flat fading is related to signal reflections due to elements of the propagation environment located near the receiver. Flat fading can result in signal loss and/or reduced accuracy of the receiver.

The GNSS signals transmitted by the satellites are Right Hand Circularly Polarized (RHCP). To enhance the robustness of GNSS receivers, it is known to use antennas configured to receive RHCP signals and to suppress left-hand circularly polarized (LHCP) signals. In effect, when an electromagnetic signal is reflected, its polarization becomes orthogonal (in this case, the reflected signal is left-hand circularly polarized or LHCP). Using the RHCP antenna in a GNSS receiver, the power level of the LHCP signal (i.e., the signal that reflects an odd number of times) is greatly reduced and the accuracy of the receiver is reduced to a lesser degree than would be the case without multipath reflections. Therefore, the use of RHCP antennas at the receiver side is sufficient to handle a large number of multipath propagation scenarios. However, the efficiency of this technique depends to a large extent on the quality of the receive antenna pattern, and some propagation environments (especially in dense urban environments) may still be a problem.

One of these problems is applicable to mobile GNSS receivers, such as GNSS chips embedded in smartphones, or handheld GNSS receivers. In fact, these receivers are designed to be steered and have no a priori condition as to their orientation. It is therefore not possible to use directional antennas in such receivers, and they are usually equipped with omnidirectional antennas which radiate over a solid angle of 4 pi steradians. It is difficult for such antennas to be polarization selective, and therefore the LHCP signal is attenuated more or less efficiently depending on its angle of arrival.

Furthermore, the portable device may be located in a place where direct view to the satellites is not guaranteed. A portion of the direct path propagates through some materials and is therefore attenuated. This is the case, for example, for a smartphone transported in a bag or a mobile GNSS receiver in a car, where the attenuation comes from the bag and its contents, possibly from the user's body, the roof … … of the car. Conversely, the reflected path may only propagate in free space and, although attenuated by the antenna gain, may reach the receiver at a power level equal to or greater than the power level of the direct path. To overcome this problem, it is known to place the receiver in an advantageous position, for example on a support close to the windscreen of the car, so that the free-space field of view of the receiver is maximized. Fig. 1 shows such a device, wherein a GNSS receiver 101 is placed on a support 102, for example suspended by suction cups on the windscreen 103 of a car. In the illustration of fig. 1, when the satellites 104 emit GNSS positioning signals, the GNSS positioning signals reach the receiver through a direct path 110, a path 111 reflected on top of the dashboard 106, a path 112 reflected on the hood 107 of the car, and a path 113 reflected on an element 105 of the propagation environment (e.g., a building). Such a device helps to maximize the number of satellites in direct view of the receiver. Thus, most direct paths do not have to pass through the material and show higher power levels than reflected paths. However, they are not optimal because the antenna pattern of such a receiver may not be able to efficiently suppress LHCP reflected paths 111, 112 and 113, particularly from paths 111 and 112 under the antenna. In Khosravi, Moghadas and Mousavi, "A GNSS Antenna with a polarization selective surface for the orientation of low-angle multipath interference" (IEEE transactions on antennas and propagation, Vol. 63, No. 12, month 12 of 2015), a polarization selective surface is disposed on the GNSS patch Antenna. The purpose of the surface is to multiply the electric field by a factor to promote multi-path suppression of the antenna. However, the polarization selective surface must be designed with consideration to the antenna pattern and the LHCP reflection path modified but not suppressed.

It is also known to connect GNSS receivers to external antennas with an elevation gain towards the sky, for example patch antennas hung on the windshield or mounted on the roof of a vehicle, or patch antennas mounted on poles in a backpack. It is also known to hang a GNSS receiver with a patch antenna on the windshield of a car and connect via a wired connection or wirelessly (Bluetooth)^TM、Wi-Fi^TM… …) to connect the GNSS receiver to a display. Fig. 2 shows a device in which a GNSS receiver 201 is connected to an antenna 202, which antenna 202 is placed on top of a pole 203 hanging on a backpack 204. In the illustration of fig. 2, when a satellite 104 transmits a positioning signal, the positioning signal reaches the receiver through a direct path 210, a path 211 reflected on an element of the propagation environment (e.g., a metal helmet 205), a path 212 reflected on a metal element stored in a backpack, and a path 213 reflected on an element 105 of the propagation environment. Such an arrangement helps to maximise the number of satellites in the direct field of view of the receiver and uses the patch antenna to efficiently reduce the power level of the LHCP signals (211, 212, 213). However, some of the antenna patterns may not be able toThe direction of the LHCP signal is suppressed efficiently. This is the case, for example, with reflected paths 211 and 212 from below the antenna (i.e., having a high off-axis angle), or in an automobile, such reflected paths may be from reflections on the dashboard or hood of the automobile. In the case of fig. 2, the reflected path may be from a reflection on the backpack. These reflections are even more disadvantageous because they produce flat fading, which cannot be mitigated by software, due to the short distance between the reflecting surface and the antenna.

As is known from the prior art, fig. 3 shows the radiation pattern of an omnidirectional patch antenna configured to receive GNSS signals in a solid angle of 2 pi steradians. In fig. 3, the elevation gain of the antenna with respect to off-axis angle (OBA) is shown in right-hand circular polarization and left-hand circular polarization. The off-axis angle is expressed with respect to the antenna toward the zenith direction, and thus an off-axis angle of 0 ° corresponds to the vertically upward direction of the antenna pattern. The antenna diagram is omnidirectional in azimuth in both RHCP and LHCP.

Line 301 represents the gain of the co-polarized electromagnetic wave, i.e., when the received signal is RHCP. It can be seen that the gain is greatest when the signal comes from above the receiver (off-axis angle is zero). This is very suitable for satellite communication. The gain decreases with increasing off-axis angle and approaches zero when the signal comes from the back of the antenna.

Conversely, line 302 represents the gain of the cross-polarized electromagnetic wave, i.e., the gain when the received signal is LHCP. When the off-axis angle is zero, the gain is quite low and does not reach high values except when the signal comes from below the antenna.

In fig. 3, the gain difference between the right-hand circularly polarized signal and the left-hand circularly polarized signal is about 40dB when the satellite is located above the receiver (position 303), which brings about natural protection of the reflected propagation path. This difference decreases as opposed to the off-axis angle. When the off-axis angle is about ± 90 ° (meaning that the signal arrives almost horizontally, position 304 on the graph), the difference is about 10dB, which is not enough to significantly attenuate the reflected propagation path. This difference is zero when the off-axis angle is about ± 140 ° (position 305 in the figure), and even negative when the signal comes from higher angles. Therefore, problems arise when the signal has a high off-axis angle (i.e., reflected signal from the back of the antenna).

Therefore, suppressing GNSS reflected signals based solely on the use of polarized antennas may not be sufficient to ensure efficient operation of GNSS receivers, and a complementary solution is needed to suppress these multipath reflections upstream of GNSS receivers, especially when they come from behind the antennas.

Disclosure of Invention

It is an object of the present invention to provide an improvement over the prior art using a device that is placed close to a GNSS antenna, helping to mitigate multipath reflections without any consideration regarding the GNSS antenna pattern. Thus, the device shows full interoperability with any GNSS receiver.

To this end, the invention discloses an add-on device configured to reduce the power level of a reflected path of a GNSS signal reaching an antenna of a GNSS receiver. The GNSS signals are transmitted according to a first polarization. The add-on device includes a material configured to be transparent to the first polarization and to reflect GNSS signals polarized according to a second polarization orthogonal to the first polarization.

The first polarization may be right-hand circular polarization and the second polarization may be left-hand circular polarization.

Advantageously, the material comprised in the additional device is configured to operate in at least one of the following frequency bands:

·1164MHz-1214 MHz，

·1215MHz-1254 MHz，

·1260MHz-1300 MHz，

·1559MHz-1610 MHz，

2483.5MHz-2500MHz, and

·5010MHz-5030 MHz，

they are the GNSS bands currently used for GNSS communications. Of course, the frequency bands in which the present invention is easily operable are not limited to these frequency bands and may include any additional frequency bands that may be used for future GNSS communications.

The invention further discloses a GNSS receiver group, which comprises:

a GNSS receiver comprising an antenna configured to receive GNSS signals, an

At least one additional device according to any embodiment of the invention.

According to another embodiment of the GNSS receiver set according to the invention, the material is a polarization selective surface. Advantageously, the material is placed in front of an antenna of a GNSS receiver so as to reflect at least some of the reflected paths of GNSS signals before they reach the antenna of said GNSS receiver. In front of the antenna of the GNSS receiver it is understood that: the add-on device is disposed somewhere on the reflected signal path between the location of the reflected signal and the location of the antenna of the GNSS receiver. Since the course of the reflected path is hardly known in advance, the add-on device according to the present embodiment is preferably placed close to the antenna (close relative to the distance between the transmitter and the receiver, e.g. several meters or less from the antenna of the GNSS receiver) on a surface that is prone to produce GNSS signals of the highest power level. For example, for a car or a house, these surfaces are windows/windshields. For a smartphone, one of these surfaces is the smartphone case. Advantageously, the material may surround the antenna.

Advantageously, the material may be optically transparent and/or may be a metallized textile.

The present invention further comprises:

a GNSS receiver housing comprising an add-on device according to any embodiment of the invention; and

a windscreen for a vehicle (e.g. car, truck, boat or aircraft) comprising an add-on device according to any embodiment of the invention, wherein the material of the add-on device is optically transparent.

The invention further comprises a method of reducing the power level of a reflected path of a GNSS signal reaching an antenna of a GNSS receiver. The GNSS signals are transmitted according to a first polarization. The method includes placing at least one additional device configured to reduce a power level of a reflected path of a GNSS signal, the additional device comprising a material configured to be transparent to the first polarization and to reflect GNSS signals polarized according to a second polarization orthogonal to the first polarization. Advantageously, the material may be arranged in front of an antenna of a GNSS receiver so as to reflect at least some of the reflected paths of GNSS signals before they reach the antenna of said GNSS receiver.

Drawings

The present invention may be better understood, and its numerous features and advantages made apparent from the following description of various exemplary embodiments, which are provided for purposes of illustration only, and the accompanying drawings of which:

fig. 1, already described, shows a use case in which a GNSS receiver is embedded in a car, as known from the prior art;

fig. 2, already described, shows a use case in which a GNSS receiver is connected to a remote antenna placed on top of a pole, as known from the prior art;

fig. 3, already described, shows a radiation pattern for an omnidirectional patch antenna configured to receive GNSS signals, as known from the prior art;

figures 4 and 5 show two exemplary embodiments of the device according to the invention, in which a material configured to absorb RHCP GNSS signals is used;

figures 6, 7 and 8 show various exemplary embodiments of the device according to the invention, in which materials configured to suppress LHCP GNSS signals are used;

fig. 9 shows another embodiment of the invention in the form of a smartphone case.

The examples disclosed in this specification are merely illustrative of some embodiments of the invention and may be combined as appropriate.

Detailed Description

Fig. 4 shows an embodiment of the device according to the invention. The illustration is made with respect to fig. 1, considering a GNSS receiver 101 attached by a support 102 to the windscreen 103 of a car, but is equally applicable to many other scenarios, in particular the use case where the GNSS receiver is connected to an external antenna attached to the windscreen of a car, or any use case where the GNSS receiver easily receives multipath reflections of a propagating signal due to reflections occurring in the closed environment of the receiver. In fig. 4, the dashboard represented is a motorcar dashboard, but the invention applies in exactly the same way to any unit of a vehicle or vehicle, for example the dashboard of a truck, a boat, an airplane, but also to the tank of a motorcycle or to the frame of an electric bicycle. The invention is equally applicable if the receiver is not attached to the windscreen, but to any other part of the vehicle, or placed above or on top of the dashboard.

In the exemplary embodiment of fig. 4, the dashboard of the automobile is coated with a material 401, which material 401 is configured to absorb the RHCP GNSS signals (i.e., GNSS signals) before they are reflected.

Such absorbing materials are already known from transceiver antennas. Such absorbing materials are used to minimize the parasitic reflections of the signal on the ground plane or mast so that they do not distort their radiation pattern. Such absorbing materials are also used to protect certain areas from radiation, for example, to protect hospitals from electromagnetic interference. Finally such absorbing materials are used in anechoic chambers to characterize the electromagnetic radiation of electronic equipment. But such absorbing materials have never been used in an operating environment because by definition all the reflection sources (i.e. the entire propagation environment) have to be covered by absorbing materials, which may not be technically feasible.

These absorbent materials, usually in the form of printed patterns or substantially periodic structures made of activated carbon foam, are designed to operate within a certain frequency band. Finer materials are known, for example, from WO2015/136121A1 filed by the same applicant. These absorbing materials exhibit higher absorption characteristics than conventional absorbing materials, since they are specifically designed in consideration of at least one of the frequency, polarization, and angle of arrival of signals.

The present invention assumes that the most harmful reflection paths occur at the rear of the antenna in a known and limited area (body and interior) mostly under the antenna, and that this area can be easily covered by an absorbing material to reduce the power level of these reflection paths. Indeed, by covering the dashboard of the car with an absorbing material configured to absorb RHCP GNSS signals, a large portion of the path 411 is absorbed rather than reflected. When a large portion of the signal is absorbed, the power level of the reflected signal that still reaches the receiver is significantly reduced, and therefore the reflected signal is no longer the source of flat fading. The absorbing material can be tuned to absorb signals in the GNSS frequency band exclusively and can be made very efficient using the teachings of WO2015/136121a1 when the antenna of the receiver shows some elevation gain towards the sky, since the polarization (RHCP) of the signal intended to be absorbed is known. Indeed, in this case, absorption of LHCP GNSS signals (e.g. signals that are reflected once before the signals encounter the absorbing material) is not necessary, as the reflected left-hand circularly polarised signals reaching the rear of the antenna are attenuated by the antenna gain. Furthermore, the range of angles of arrival of GNSS signals is likely to be given by the size and location of the window and windshield openings.

The device according to the invention (made of an absorbing material configured to absorb GNSS signals) may thus advantageously be configured to exclusively absorb right-handed polarized signals. The device may be placed on the dashboard of a car (as a complementary layer) to cover the dashboard completely or to cover a limited area under the GNSS receiver. Alternatively, the dashboard itself may also be manufactured, comprising an upper layer made of said absorbent material.

In an alternative embodiment, which may be combined with the previous embodiment, the present invention may be directed to attenuating the path 412 reflected on the vehicle body. For this purpose, the hood of the vehicle is covered with an absorbent material 402. As for path 413, it will not be absorbed by the material and will reach the receiver, which path will have to be correlated with its antenna gain to mitigate it.

The device according to the invention may be selected for its good absorption characteristics in one or more GNSS frequency bands, which are currently mainly the following:

·1164MHz-1214 MHz，

·1215MHz-1254 MHz，

·1260MHz-1300 MHz，

·1559MHz-1610 MHz，

2483.5MHz-2500MHz, and

·5010MHz-5030 MHz。

the present invention can be readily adapted to a variety of use cases, including the use case shown in figure 2, in which the patch antenna 202 is mounted on a pole 203 in a backpack 204. Fig. 5 illustrates one example of such a use case. Since the most harmful reflections come from under the antenna, i.e. from the contents of the backpack 204 and/or the user's helmet 205, these areas can easily be covered with absorbing material (501, 502) to absorb the signal before it is reflected, thereby reducing the flat fading phenomenon and improving the accuracy of the receiver. The material should be placed around the GNSS receiver in a way that does not interfere with the direct propagation path, i.e. does not hide the GNSS receiver's direct view of the satellites. Similarly, the invention may be implemented for other use cases, for example, on a motorcycle or bicycle, or on any helmet (for use as a motorcycle helmet, heavy duty helmet, etc. … …).

By placing the absorbing material below the antenna (on top of the reflection source), the direct propagation path 210 and the path 213 reflected on elements of the propagation environment 105 away from the GNSS receiver are not attenuated. Conversely, when reflected on the absorbing material layer, the propagation paths 511 and/or 512 lose power levels, thus reducing the occurrence of flat fading.

In this example, the invention can be implemented by dropping a layer of absorbing material on top of the backpack or arranging it inside the backpack (it is important that the absorbing material is located above the metallic elements of the backpack that produce the signal reflection), or by designing the backpack with the top containing the absorbing material. The absorbing material is configured to exhibit good absorption properties in the GNSS band and may advantageously be configured to exclusively absorb RHCP signals.

The present invention processes GNSS signal reflections even before they occur by covering a limited area prone to reflections of the path to the rear of the antenna with an additional device independent of the GNSS receiver and configured to absorb RHCP GNSS signals upstream of the antenna. Implementation of the present invention is possible because the present invention does not attempt to mitigate every source of multipath reflections, but instead focuses on specific multipath reflections that are particularly harmful because they are not properly handled by polarized antennas. These multipath reflections are a source of flat fading and cannot be mitigated by software. Furthermore, the present invention is not dependent on any particular GNSS receiver architecture or antenna characteristics and fully conforms to any type of receiver without regard to any antenna diagram.

Alternatively, or in addition to power level reduction of the reflected path by using absorbing materials, the present invention proposes to reduce the source of selective fading by filtering the reflected LHCP signal before it is received by the receiver's antenna. To this end, the invention proposes to use reflective materials (for example, Polarized Reflective Surfaces (PRS) or Polarization Selective Surfaces (PSS), which are two designations of electromagnetic devices having the same characteristics) in order to reflect most of the LHCP GNSS signals (i.e. signals reflected by the propagation environment) upstream of the antenna, while being transparent to the RHCP GNSS signals. Transparent to the RHCP GNSS signals means that most of the signals can pass through it. The surface should be placed in front of the antenna of the GNSS receiver so as to be located on the propagation path of the various reflected paths, preferably close to the antenna (or to the antenna of the multi-antenna receiver) so as to maximize the number of multi-path reflections through the reflective material.

Such PRS/PSS electromagnetic structures have been known for many years for other purposes. They are basically periodic structures made of periodic conducting wires arranged according to the frequency band and polarization of interest, with the purpose of being transparent to one particular signal polarization and reflecting the orthogonal polarization.

For example, PSS Surfaces designed to reflect circularly polarized signals are known, for example, from satellite dish reflectors, as described in Cappellin et al, "Design and Analysis of a Reflector Antenna System Based on guided Circular Polarization selected Surfaces" (Proc. EuCAP'2016,10th European Conference on Antennas and Propagation (EuCAP), 2016, 4 months). Such surfaces may be made of a plurality of metal bent wires separated by λ₀8 and rotated 45 deg. relative to the adjacent plate, where lambda₀Is the center wavelength of operation. Many other modes are known for implementing circularly polarized selective surfaces using, for example, coupled open-loop resonators or resonant spirals. Such PSS surfaces may be designed as reciprocal surfaces in order to reflect a particular circular polarization.

According to an embodiment, such a surface must also be optically transparent to some portion of the visible spectrum and/or infrared. Optically transparent PRS/PSS surfaces operating in the visible spectrum are used on sunglasses to reduce brightness by accounting for polarization of the optical signal. Other PRS/PSS surfaces are used on receiver antennas configured to receive signals in a frequency band shared by two orthogonally polarized signals.

Various techniques are known to design optically transparent PRS/PSS filters. One of these techniques is the use of glass or Plexiglass^TMA transparent dielectric substrate of the type and wires are formed on a plastic film (e.g., a polyester film) using an optically transparent conductive material (e.g., tin-doped indium oxide ITO or silver-doped tin oxide AgHT).

Another technique is to use a transparent dielectric substrate of glass or Plexiglass type, and the wires are usually implemented in a grid in the form of a metal mesh (e.g. silver or copper). The transparency level is then defined by the size of the openings in the grid (relative to the width of the conductors).

By reflecting/filtering the LHCP GNSS signals before they reach the antenna of the receiver, the present invention provides additional reduction in the power level of the signals reflected by the propagation environment. Fig. 6 shows an embodiment of the device according to the invention. Again, this illustration is made with respect to fig. 1. In this embodiment, the PRS/PSS filter 601 is placed above or below the windshield 103 of a vehicle (e.g., an automobile, truck, airplane, or any other vehicle). Alternatively, the PRS/PSS filter may be inserted into the windshield during its production, similar to what is done for automotive defrost systems.

Windshields having particular characteristics are known, for example, non-conductive windshields designed to limit the passage of sunlight. They may also unintentionally reflect GNSS signals without making any consideration about the polarization of the GNSS signals. The filter or windshield according to the invention takes into account the polarization of the signals and reflects a large portion of the LHCP GNSS signals while letting as much RHCP GNSS signals pass as possible.

Thus, the direct path 110 reflected inside the car and the path 111 pass through the PRS/PSS surface unaltered and both signals reach the receiver at high power levels. Paths 612 and 613 reflected from outside the vehicle are again reflected by the LHCP filter and a large portion of these signals do not reach the antenna of the receiver. Thus, when these paths (613) are reflected in a propagation environment far from the receiver, which is prone to selective fading, the power level of the reflected paths reaching the GNSS receiver is significantly reduced. They are also significantly reduced when the path is reflected (612) close to the receiver but upstream of the LHCP PRS/PSS filter, which is prone to flat fading.

By combining this embodiment with the previous embodiment, the reflected path 612 can be easily overcome, wherein the car hood is covered by a material that absorbs GNSS signals, and advantageously absorbs RHCP GNSS signals. In this case, GNSS reflections occurring before the PRS/PSS surface are attenuated by the reflective surface, and GNSS reflections occurring after the PRS/PSS surface are attenuated by the absorbing material. Thus, the GNSS receiver receives a non-attenuated GNSS signal direct path and an attenuated GNSS signal reflected path, which improves its reception capability.

According to another embodiment, shown in fig. 7, which is suitable in the case of GNSS receivers provided on supports, the material made of PRS/PSS is not placed on the windscreen of the vehicle, but is arranged in the form of a casing 701, the casing 701 surrounding the whole GNSS receiver 101 or external antenna. This situation does not necessarily cover the entire GNSS receiver, but may be limited to the side of the receiver directed towards the satellites. The PRS/PSS material does not necessarily have to be optically transparent unless the receiver comprises a display unit. In this case, the optically transparent portion may be limited to the portion of the housing facing the display.

The present embodiments may be advantageously applied to any type of handheld GNSS receiver, to an external GNSS antenna or to a portable device comprising a GNSS chip, e.g. a smartphone, etc.

The direct path 110, which is the RHCP, is not affected by the receiver housing 701, while the reflected paths 711, 712, and 713 are reflected by the PRS/PSS material and do not reach the antenna of the GNSS receiver. Thus, flat fading and selective fading are prevented.

Figure 8 shows another implementation example with respect to the use case of figure 2, where the patch antenna 202 is mounted on top of a pole 203.

In this embodiment, the antenna 202 is covered by PRS/PSS material 801 configured to suppress LHCP GNSS signals. As shown in the example shown in fig. 7, the PRS/PSS cover may be in the form of a hard shell or in the form of a cover 801 made of a metallized textile that is placed over the antenna or incorporated into an existing cover, such as a boat cover or soft top of an automobile. Indeed, it is known to impart certain textile electromagnetic properties, in particular polarization properties, by printing PRS or PSS patterns directly on the textile. The textile acts as a dielectric substrate and the pattern can be printed (using, for example, conductive ink) or stitched (using conductive thread) inside and/or over the textile. It may also be included in a radome, protecting the antenna from rain and dust.

The reflected paths 811, 812, and 813 are advantageously suppressed by the PRS/PSS metalized textile 801, while the direct propagation path 210 is unaffected by polarization.

Fig. 9 shows another embodiment of the invention in the form of a smartphone case. In fig. 9, a flip cover case 901 is designed to protect the smart phone from impact. The flip cover housing includes PRS/PSS material configured to suppress LHCP GNSS signals. The filter may be inserted into the rear portion 903, the front portion 904, or the entire housing of the clamshell. This embodiment may be advantageously used in a vehicle in combination with the windscreen configuration described above. This embodiment is particularly useful because it helps to mitigate the power level of reflected GNSS signals reaching smartphones that are not typically equipped with polarized antennas. The smartphone case according to the present invention is not limited to a clamshell case, but may be any type of case that covers all or part of the smartphone, or it may be part of the case of the smartphone itself.

The invention also includes a method for reducing the power level of a reflected GNSS signal path in a GNSS receiver. The method includes placing one or more materials configured to absorb GNSS signals polarized according to a polarization of the transmitted GNSS signals or reflect GNSS signals polarized according to a polarization orthogonal to the polarization of the transmitted GNSS signals.

Such material may be one or more absorbing materials placed under the antenna of the GNSS device (e.g., above the dashboard of the automobile), as well as electromagnetic materials designed to reflect GNSS signals according to a given polarization between GNSS satellites and GNSS receivers.

While embodiments of the invention have been illustrated by a description of various examples and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. In particular, combinations between the various embodiments described may be performed, for example, using both a material configured to absorb right-hand circularly polarized signals and a material configured to suppress left-hand circularly polarized signals.

The invention in its broader aspects is therefore not limited to the specific details, representative method, and illustrative examples shown and described. The invention is applicable not only to right-hand circularly polarised GNSS signals but also to left-hand circularly polarised GNSS signals and, more broadly, to any type of signal transmitted according to the first polarisation, provided that reflection of said signal results in a change of orthogonal polarisation.