Patent Application
20240353572
The present invention relates to the field of global navigation satellite systems (GNSS). More particularly, the present invention relates to the field of mitigating multipath interference in GNSS.
GNSS is a satellite navigation system which utilizes signals transmitted from one or more constellations of satellites orbiting the earth. GNSS receivers are configured to receive the satellite signals and process them to determine a navigation solution. Such a navigation solution generally includes the position and velocity of the receiver as well as precise time. This can be referred to as Position, Velocity and Time or PVT. GNSS has proven to be highly accurate. However, because GNSS signals are electromagnetic, they can be blocked by objects such as mountains, tall buildings, and dense foliage.
Multipath is a major error source for GNSS receivers and can occur in situations where the direct GNSS signal is blocked and only a reflected satellite signal is received by the antenna of a GNSS receiver. However multipath effects can also occur when GNSS signals transmitted by a particular satellite travel by two or more paths to the GNSS receiver's antenna. In such situations, known as multipath interference, the GNSS receiver intercepts a direct or line-of-sight (LOS) component together with one or more non-line-of-sight (NLOS) components. The latter can be caused by the signals being reflected from objects such as mountains or buildings. The signals travelling different paths can experience fading and phase shifting which can cause measurement errors and/or compromise the correlation function shape used to estimate the time delay when processing the GNSS signals. In some environments, such as urban canyon environments, the GNSS signals can be subjected to a significant amount of multipath interference which can result in significant GNSS measurement errors.
A typical approach for mitigating multipath interference in GNSS involves detection and correction of individual affected navigation measurements. Typically, this is done during a signal processing stage. Such an approach may not always perform well. For example, heavy multipath interference tends to distort measurements in ways that are difficult to model reliably. Therefore, if the signals are severely degraded or if assumptions about properties of the degraded signals, such as uniformity of phase error, are not accurate, detection and correction of individual measurements may not yield accurate results.
What is needed, therefore, is an improved technique for mitigating multipath interference in GNSS.
It would be desirable to provide a method of mitigating a multipath signal. There is a plurality of different GNSS signals, each with different characteristics based on, inter alia, their chipping rate and their spread spectrum. Due to the differing characteristics, some signals are more resilient to multipath interference because it is easier to distinguish between different portions of the signal.
Thus, some signals may be more resilient to multipath interference: if there is a large difference between data from these signals and data from those signals known (due to their characteristics) to be less resilient to multipath interference it may indicate that multipath interference is occurring. In these situations, one or more of the signals less resilient to multipath interference may be excluded from the calculation of the position.
The method uses data from a previous epoch (or time period) to predict the position during the current epoch. The range from the predicted position is compared to the pseudo-range based on an individual signal from a space vehicle: this is known as the residual. If a signal is affected by multipath interference there may be a greater discrepancy between the predicted position (or range based on the predicted position) and the measured position (or pseudo-range) than there would be if the signal is not affected by multipath.
By comparing the residuals of signals known to be more resilient to multipath interference and those known to be less resilient to multipath interference it is possible to infer the likelihood that there is multipath interference.
L5 signals are known to be resilient to multipath interference due to their faster chipping rate. On the other hand, L1 and L2 signals are more susceptible to multipath interference because of their slower chipping rates.
The invention is defined by the claims. According to a first aspect, there is provided a method of mitigating multipath interference in a global navigation satellite system, GNSS, receiver, the method comprising:
The present invention therefore provides a method to identify the likelihood of a location being affected by multipath interference. If it is determined that a location is likely to be affected by multipath interference, then a position can be determined based on signals known to be less affected by multipath interference. Thus, the position determination may be made using only a subset of signals, excluding at least one L1 and/or L2 signal. In this way, the effects of multipath interference are mitigated and improved GNSS positioning is achieved.
In this matter, the term “statistic” refers to mathematically generated result/s, e.g. distribution/s, wherein said result/s may be presented as absolute or discrete value/s or value/s relative to the predetermined limit or threshold (or to another one). The result/s may also be dependent on other variables, which are not part of the comparison between the previously mentioned residuals and/or which are variables of said residuals.
Optionally, L1 and/or L2 signals from space vehicles from which there is also an L5 signal are excluded from the subset used to determine position. L5 signals from these space vehicles will already be used in the position determination so the exclusion of the corresponding L1 and/or L2 signal does not reduce the number of space vehicles used in the position determination.
Alternatively, L1 and/or L2 signals from space vehicles from which there is also a L5 signal are excluded from the subset if the difference between the L1 and/or L2 signal residual and the L5 residual is greater than a predetermined individual distance threshold.
There are some space vehicles which only transmit L1 and/or L2 signals, known to be less resilient to multipath interference, and no L5 signal. Optionally, these signals may be excluded from the subset used to determine the position.
If a multipath environment is detected using the method above, L1 and/or L2 signals from space vehicles without an L5 signal are excluded from the subset when their residual is above a predetermined individual residual threshold. Thus, only signals for which there is a large discrepancy between the predicted position and the determined position are excluded. The other signals, for which there is not a large discrepancy, may be used. Generally, the more space vehicles that are used in the position determination the greater the resulting accuracy.
If a multipath environment is detected the method may further comprise determining an elevation angle for L1 and/or L2 signals from space vehicles without an L5 signal are excluded from the subset if the angle of elevation of the corresponding space vehicle is below a predetermined elevation threshold. Lower angles of elevation are more likely to be affected by multipath interference, so this provides a way to identify which L1 and/or L2 signals (for which there is no corresponding L5 signal) are more likely to be affected by multipath interference.
Determining the statistic may comprise:
Thus, if the over a predetermined percentage of the signals have been deemed likely to be affected by multipath interference, then the position is deemed to be a position affected by multipath interference. Mitigation, by determining the position based only on a subset of signals can then occur.
Additionally or alternatively the statistic may be based on an aggregate across all the signals of a particular type. An example of this is where determining the statistic may comprise:
The statistic may not be a direct comparison between residuals and may comprise other data. An example of this is the use of the standard deviation of the residuals. According to an embodiment determining the statistic may comprise:
The method described above may be used if the number of space vehicles exceeds a predetermined threshold. If the number of space vehicles is less than a predetermined threshold, then there may be insufficient space vehicles for the method described above to be statistically significant. According to the invention there is a method of determining an overall position comprising:
Thus, if the number of space vehicles is above a predetermined space vehicle number threshold the overall position is determined as described above; and
Optionally, a method of determining an overall position comprises:
If the dilution of precision is below a predetermined dilution of precision threshold the overall position is determined according to a method as described above; and
If the residuals are very large i.e., there is a large position error then the method described above may not be performed as there may be other underlying reasons causing the large residuals and the method would therefore not be suitable. According to the invention there is a method according to any one of the preceding claims further comprising: comparing each residual to a maximum threshold;
Thus, if the average of all residuals is above a maximum average residual value the overall position is determined based on the estimated position from all the signals; and
Also provided is a GNSS receiver comprising:
The static GNSS receiver may further comprise an RF front-end. The RF front-end may be configured to receive GNSS signals via an antenna. The signal processing unit may be configured to make GNSS measurements (pseudorange measurements, and optionally carrier range measurements) on the GNSS signals received by the RF front-end.
Optionally, the processor of the GNSS receiver is further configured to perform the steps of the previously mentioned methods of the invention.
Also provided is a base station for a communications network, comprising the GNSS receiver as described above.
Also provided is a computer program comprising computer program code configured to cause one or more processors to perform all the steps of the method as described above when said computer program is executed on said one or more processors. The one or more processors may comprise or consist of one or more processors of a GNSS receiver. The computer program may be stored on a computer-readable storage medium (optionally non-transitory).
The invention will now be described by way of example with reference to the accompanying drawings, in which:
It should be noted that these figures are diagrammatic and not drawn to scale.
Reference will now be made in detail to examples according to the present disclosure, which are illustrated in the accompanying drawings. The scope of the present disclosure should not be construed as being limited to the descriptions given in this section; other examples may have different forms. These and other features, aspects, and advantages of the apparatus, systems and methods of the present disclosure will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the same reference numerals are used throughout the figures to indicate the same or similar parts.
The present invention provides a method of mitigating multipath interference in a global navigation satellite system (GNSS). A GNSS receiver can be configured to perform a method of mitigating multipath interference in accordance with an embodiment of the present invention. Accordingly, the present invention also provides a GNSS receiver which is configured to mitigate multipath interference in a global navigation satellite system (GNSS).
Currently there are four fully operational GNSSs: the United States' Global Positioning System (GPS), Russia's Global Navigation Satellite System (GLONASS), China's BeiDou Navigation Satellite System (BDS) and the European Union's Galileo. A GNSS receiver which can acquire and track two or more GNSSs concurrently is known as a multi-GNSS receiver.
GNSS satellites of these various systems typically transmit GNSS signals in more than one frequency band. By way of example, the frequency bands which are currently available under GPS include the L1, L2 and L5 bands. For the L1 signal band, the signal carrier frequency is generally in the range of 1.559 GHz to 1.610 GHz. For the L2 signal band, the signal carrier frequency is generally in the range of 1.18845 GHz to 1.258 GHz. And, for the L5 signal band, the signal carrier frequency is generally in the range of 1.164 GHz to 1.189 GHz. Additional or different frequency bands may become available in the future for GPS.
Different signal bands have different characteristics. L1, L2 and L5 have different chipping rates: L1 has a chipping rate of 1.023 MHz, L2 has a chipping rate of 511.5 kHz whereas L5 has a faster chipping rate of 10.23 MHz. As L5 has a faster chipping rate the peak of the correlation functions become narrower than those of L1 and L2. For example, a correlation peak of L1 corresponds to 293 m whereas a correlation peak of L5 corresponds to 29.3 m. Due to the narrower correlation functions, it is easier to distinguish between L5 signals transmitted by different paths (for example due to reflections in a multipath environment) than it is to distinguish between L1 or L2 signals transmitted by different paths. Thus, L5 signals are more resistant in multipath environments than L1 signals.
A GNSS receiver may operate in a single-band mode in which it performs signal processing to determine navigation measurements on signals in only one band, for example, the L1 band only or, as another example, the L5 band only. Alternatively, the GNSS receiver may operate in a dual- or multi-band mode in which it performs signal processing using signals in multiple bands within a particular constellation, for example, L1 and L5 dual-band mode in GPS. As another example, a GNSS receiver may be configured to operate in a multi-band mode in which three or more signal bands are utilized (e.g., L1, L2 and L5). It is generally expected that multi-band processing will achieve more accurate navigation measurements than single-band processing.
The satellites 106a-d are each configured to transmit GNSS signals 104a-h in two or more frequency bands. The two or more frequency bands can be selected from among the currently available bands. For example, if the GNSS receiver 100 comprises equipment for receiving and processing GPS, then GNSS receiver 100 may be configured to receive satellite signals in any two bands chosen from the L1, L2 and L5 frequency bands. Alternatively, GNSS receiver 100 may be configured to receive all three frequency bands L1, L2 and L5 of GPS. However, it will be apparent that additional or different frequency bands may become available, in which case, the satellites 106a-d can be configured to transmit GNSS signals 104a-h in any or all such frequency bands and the receiver 100 can be configured to receive and process GNSS signals 104a-h in any or all such bands. Accordingly, changes to the available signal frequency bands are presently contemplated as being within the scope of the present invention.
The receiver 100 is configured to receive the navigation satellite signals from the satellites 106a-d and to process those signals to determine a navigation solution from the received signals. In the example illustrated in
The receiver 200 is further configured to process the received signals using a processor 204 to extract observables and navigation data from each channel of the baseband processing unit and to generate a navigation solution 206. The processor 204 may include an internal or external memory 205 for storing and/or retrieving data associated with the processing.
The navigation solution 206 generated by the receiver 200 may include the calculated position and velocity of the receiver 200 as well as a determination of precise GNSS time. The navigation solution may be derived from anyone, or multiple ones of, the GNSS frequency band signals received at the antenna 201, which may include GNSS signals from different GNSS constellations. The navigation solution may also take into account discounting of any one or more of the GNSS frequency band signals received at the antenna 201. More particularly, the receiver 200 receives satellite signals from one or more GNSS constellations and the processor 204 processes these signals to provide GNSS measurements such as pseudo-ranges, carrier phases, rates of pseudo-ranges, etc. to a navigation filter. The processor 204 may, for example, implement the filter, e.g., a Kalman filter or a least-squares (LSQ) filter, which is used to determine the navigation solution 206 based on the GNSS measurements.
The processor 204 may also determine whether multipath interference is likely to be present.
In a second step 302 a pseudo-range is determined for each signal for the current epoch (time period). To determine a pseudo-range, a GPS receiver compares the time a signal was transmitted by a satellite with the time it was received by the GPS receiver. The time difference tells the GPS receiver how far away that particular satellite is i.e., the pseudo-range.
In each epoch, or time period a position is determined based on a combination of pseudo-ranges from a plurality of satellites. Thus, for the previous epoch (time period) there is a determined position and velocity. Based on at least the determined position and velocity in the previous epoch an estimated measurement to the space vehicle in the current epoch can be made. The estimate includes an estimate of the current position based on the position and velocity in a previous epoch but may also include corrections due to the transmission. For each satellite, there is a corresponding estimated measurement to the space vehicle in the current epoch. In a third step 303 of the invention the difference between the estimated measurement to the space vehicle and the pseudo-range of the corresponding space vehicle can be determined.
Once residuals for all the signals have been determined a comparison can be made between the residuals of the L1 and/or L2 signals and the residuals of the L5 signals. As discussed above, the L5 signals are more resilient to multipath interference so a difference between the residuals of the L5 signals and the L1 and/or L2 signals may indicate multipath interference. In a fourth step of the invention 304 a statistic is determined based on a comparison between residuals of the L5 signals and residuals of the L1 and/or L2 signals. This statistic may be the difference in average residuals between the L5 signals and the L1 and/or L2 signals. Alternatively, it could be a ratio of the L1 and/or L2 signals to a corresponding L5 signal.
A determination is made about whether the statistic is above a predetermined threshold in step 305. If the statistic is not above a predetermined threshold, then the position is determined (306) based on both the L1 and/or L2 signals and the L5 signals. If the statistic is above the predetermined threshold, then the position is determined, 307, based on a subset of signals, the subset comprising the L5 signals with at least one of the L1 and/or L2 signals excluded from the subset. As an example, L1 and/or L2 signals from space vehicles from which there is an L5 signal may be excluded from the subset.
Some space vehicles transmit both an L5 signal and an L1 and/or L2 signal. However, other space vehicles transmit only L1 and/or L2 signals. These signals may or may not be included within the subset. The advantage of including signals from space vehicles not transmitting an L5 signal in the subset is that such a position may be more accurate as it includes information from more space vehicles. The disadvantage of including signals from space vehicles not transmitting an L5 signals in the subset is that the subset may include data heavily affected by multipath interference. In one example, L1 and/or L2 signals from space vehicles without an L5 signal are excluded from the subset. In another example, L1 and/or L2 signals from space signals without an L5 signal are excluded from the subset if their residual is above a predetermined individual threshold. Thus, signals which do not deviate significantly from the prediction are included as they are less likely to be affected by multipath interference. Signals which deviate significantly from the prediction are excluded because they are more likely to be affected by multipath interference.
If there are sufficient space vehicles transmitting L5 signals that a good position can be determined from signals from those satellites alone then it may not be necessary to include the L1 and/or L2 signals from space vehicles not transmitting an L5 signal in the subset. As an example, if the number of space vehicles transmitting L5 signals is above a predetermined threshold then signals from space vehicles not transmitting L5 signals are excluded from the subset. As a further example, the dilution of precision may be determined based on signals from the space vehicles transmitting a L5 signal and if the dilution of precision is below a threshold, then L1 and/or L2 signals from space vehicles not transmitting a L5 signal can be excluded from the subset.
In one example, the elevation angle of space vehicles transmitting only an L1 and/or L2 signal (and not a L5 signal) is determined. If the angle of elevation of the corresponding space vehicle is below a predetermine elevation threshold, then the L1 and/or L2 signal is excluded from the subset.
The statistic may relate to a combination of L5 signals and a combination of L1 and/or L2 signals or alternatively may compare specific L5 signals with specific L1 and/or L2 signals. Depending on the elevation angle of individual space vehicles signals from different space vehicles may be affected by multipath interference to a greater or lesser extent.
An example of a comparison between individual signal is a comparison between a residual of a L5 signal from a specific space vehicle and a residual of a L1 and/or L2 signal from the same specific space vehicle to determine a difference between the two residuals. The determination step 306 then comprises determining whether the difference in residuals is greater than a predetermined difference threshold. The percentage of space vehicles for which the ratio is over the predetermined difference threshold is then determined. If the percentage is greater than a predetermined percentage threshold, then the environment is deemed to be affected by multipath interference and the position is determined based on a subset of signals comprising the L5 signals excluding at least one of the L1 and/or L2 signals. A further statistic, based on signals from individual space vehicles may be determined and individual L1 and/or L2 signals excluded on the basis of the statistic based on signals from the respective space vehicle.
A further example of comparison between individual signals is a comparison between a residual of a L5 signal from a specific space vehicle and a residual of a L1 and/or L2 signal from the same specific space vehicle to determine a ratio of the two residuals. The determination step 306 then comprises determining whether the difference is greater than a predetermined ratio threshold. The percentage of space vehicles for which the ratio is over the predetermined ratio threshold is then determined. If the percentage is greater than a predetermined percentage threshold, then the environment is deemed to be affected by multipath interference and the position is determined based on a subset of signals comprising the L5 signals excluding at least one of the L1 and/or L2 signals. A further statistic, based on signals from individual space vehicles may be determined and individual L1 and/or L2 signals excluded on the basis of the statistic based on signals from the respective space vehicle.
Alternatively, the statistic may relate to the group properties of the L5 signals as a whole compared the group properties of the L1 and/or L2 signals as a whole (from satellite vehicles transmitting both a L5 signal and a L1 and/or L2 signal). In one example, an average residual for all the L5 signals is determined and an average residual for all the L1 and/or L2 signals determined. The difference between the residual for the L5 signals and the residual from the L1 and/or L2 signals is compared and if it is greater than a predetermined average distance threshold the environment is considered affected by multipath interference and the position is calculated based on a subset excluding at least on L1 and/or L2 signal.
The statistic can be any statistic which compares the residuals of one or more L5 signals with one or more L1 and/or L2 signals. An example of an alternative statistic is the standard deviation of all the residuals. In the statistic determining step the standard deviation of all the results is determined. The standard deviation indicates the spread of results so a large spread may indicate a large amount of variation in the residuals and therefore a multipath environment. The standard deviation is compared to a predetermined standard deviation threshold to determine whether it is a multipath environment. Alternatively, in another favourable embodiment the statistic to be determined may be based on or is a combination of one or more of the same and/or different statistics previously mentioned.
Alternatively, a multipath environment may be detected using a statistic based on signals from a plurality of signals. Then, if a multipath environment is detected a further statistic evaluated (using the methods above) based on signals from a single space vehicle and individual L1 and/or L2 signals excluded based on the statistic from the single space vehicle.
This method may only be applicable if the number of space vehicles transmitting an L5 and an L1 and/or L2 signal is sufficiently significant. Therefore, a test may be performed in which the number of space vehicles transmitting a L5 and a L1 and/or L2 signal is determined. The method of mitigating a multipath environment described above is only carried out if the number of space vehicles transmitting a L5 and a L1 and/or L2 signal is above a predetermined threshold. If it is not above a predetermined threshold, then a conventional position determination (without the multipath mitigation method described above) is carried out.
The dilution of precision refers to the dilution in precision of the determined position if one or more signals are removed from the position determination. If the dilution of precision is large, then the method described herein may not be suitable because the loss of precision would be too significant. Thus, the dilution of precision of the space vehicles may be determined. If (and only if) the dilution of precision is below a predetermined dilution of precision threshold the method described above is carried. If the dilution of precision is not below the predetermined dilution of precision threshold, then the position is determined using all the available signals.
If the there is a significant discrepancy between the predicted position and the position based on the pseudo-range(s) then there may be another underlying error and it may not be suitable to use this method. Thus, the average of all residuals may be determined. If the average of all the residuals is below a maximum average residual value, the method may be carried out. However, if the average of all the residuals is not below a maximum average residual the position may be determined using all the available signals.
A GNSS receiver comprises a signal processing unit 110 and a processor configured to carry out the steps described above. The GNSS receiver may form part of a base station for a communications network.
It should be understood that when the phrase “number of space vehicles” is used it refers to the number of space vehicles from which signals are received at the given position at that point in time.
It should be understood that the scope of the present disclosure is not limited to the examples described above. Many variations will be apparent to those skilled in the art, based on the foregoing description.
In general, in the flowcharts of
A further method is depicted in
In step 403 the quality of the constellation as a whole is assessed. This can be by comparing the number of satellites transmitting an L5 and L1 and/or L2 signal. It can also, or alternatively be by assessing the maximum dilution of precision.
If the assessment of the constellation is passed, then an assessment about the environment is performed at step 404. This can be by one or both of the following methods:
If one or both of these conditions is fulfilled then the environment is dominated by multipath.
At step 405 L1 and/or L2 signals are rejected. This may be all L1 and/or L2 signals, or only L1 and/or L2 signals from a space vehicle with an L5 signal or only L1 and/or L2 signals with residuals larger than a threshold.
At step 406 a PVT (position, velocity, time) fix is determined using signals not excluded in step 405.
If, at step 404, the environment is determined not to be dominated by multipath then the signals flagged at step 402 can be excluded as these are the signals for which there is a discrepancy between the residuals from the L5 signal and the L1 and/or L2 signal. A PVT fix can be determined using the signals except for these.
It should be noted that the term “above” could equally be replaced with “below” for a particular threshold.
In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim. However, where the word “comprising” is used, this also discloses as a special case the possibility that the elements or steps listed are exhaustive—that is, the apparatus or method may consist solely of those elements or steps. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The embodiments may be implemented by means of hardware comprising several distinct elements. In a device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. 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. Furthermore, in the appended claims lists comprising “at least one of: A; B; and C” should be interpreted as (A and/or B) and/or C.
In flowcharts, summaries, claims, and descriptions relating to methods, the sequence in which steps are listed is not, in general, intended to be limiting on the order in which they are carried out. The steps may be performed in a different order to that indicated (except where specifically indicated, or where a subsequent step relies on the product of a preceding step). Nevertheless, the order in which the steps are described may in some cases reflect a preferred sequence of operations.
Furthermore, in general, the various embodiments may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. For example, some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software, which may be executed by a controller, microprocessor or other computing device, although these are not limiting examples. While various aspects described herein may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.
The embodiments described herein may be implemented by computer software executable by a data processor of the apparatus, such as in the processor entity, or by hardware, or by a combination of software and hardware. Further in this regard it should be noted that any blocks of the logic flow as in the Figures may represent program steps, or interconnected logic circuits, blocks and functions, or a combination of program steps and logic circuits, blocks and functions. The software may be stored on such physical media as memory chips, or memory blocks implemented within the processor, magnetic media such as hard disk or floppy disks, and optical media such as for example DVD and the data variants thereof, or CD.
The memory may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor-based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The data processors may be of any type suitable to the local technical environment, and may include one or more of general-purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASIC), gate level circuits and processors based on multi-core processor architecture, as non-limiting examples.
Embodiments as discussed herein may be practiced in various components such as integrated circuit modules. The design of integrated circuits is generally a highly automated process. Complex and powerful software tools are available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate.
| Number | Date | Country | Kind |
|---|---|---|---|
| 23169058.7 | Apr 2023 | EP | regional |
