GB2529370A

GB2529370A - Radio frequency attitude sensor

Info

Publication number: GB2529370A
Application number: GB1405113.0A
Authority: GB
Inventors: Paul Robertson; Sean Thomas George Maguire
Original assignee: Individual
Current assignee: Individual
Priority date: 2014-03-21
Filing date: 2014-03-21
Publication date: 2016-02-24
Anticipated expiration: 2034-03-21
Also published as: GB2529370B; GB201405113D0

Abstract

The invention discloses a method of measuring the dominant axis or axes of time-varying magnetic/electric fields caused by one or more radio frequency electromagnetic waves. It provides for a method to use the measurements of these axes in combination with knowledge of the axes' orientation with respect to a reference frame in the vicinity of the receiver to determine the orientation of the receiver with respect to the reference frame, otherwise known as the attitude of the receiver. This derived measurement of attitude can be formed solely from measurements of electromagnetic field properties or from such measurements in combination with other sensors. The system comprises a receive antenna array 1 constructed from an array of two or more antennas, a set of receiver circuits 2, an analogue to digital converter 3 and a digital processing element 4 that produces an output 5. It also preferably includes a circuit 6 for varying the frequency of the antennas that comprise the receiver array, and it preferably includes a circuit 7 for driving the receive antenna array with locally generated signals in order to tune each of its constituent antennas and in order to characterise the receive antenna array as a whole.

Description

Field of the Invention

This invention relates to the measurement of the orientation of a platform with respect to a reference frame.

The invention provides for a method of measuring the dominant axis or axes of time-varying magnetic and/or electric fields caused by one or more radio frequency electromagnetic waves. It further provides for a method to use the measurements of these axes in combination with knowledge of the axes' orientation with respect to a reference frame in the vicinity of the receiver to determine the orientation of the receiver with respect to the reference frame, otherwise known as the attitude of the receiver. This derived measurement of attitude can be formed solely from measurements of electromagnetic field properties or from such measurements in combination with other sensors.

Background to the Invention

The problem of determining the attitude of a platform is encountered in a large number of fields. In many cases the platform is a vehicle which may be in motion. Many systems have been devised to measure attitude with varying degrees of accuracy. Relatively small and low cost systems generally include one or more of a three-axis gyroscope, a three-axis accelerometer, a three-axis magnetometer and a GPS receiver. Systems used in spacecraft also often include a sun sensor and/or star trackers.

It is commonly recognised in the prior art that if measurements of two or more vectors are taken with respect to a platform, and those vectors are also known in a reference frame, then the orientation of the platform may be determined with respect to the reference frame. Thus if an accelerometer is assumed to measure the known acceleration due to gravity and a magnetometer is assumed to measure the known magnetic field vector due to the earth's magnetic field, a solution for the attitude of a platform equipped with both sensors can be determined (relative to the ground,

for example).

It is also commonly recognised that these measurements can be unreliable over short time periods.

Both sensors can suffer from noise, distortion and interference that can introduce errors, but a moving platform may also experience an acceleration that is not due to gravity, and therefore the measurement of the accelerometer can deviate from the expected measurement of the acceleration due to gravity, introducing additional errors.

Numerous examples of solutions to these problems have been presented in the prior art. Many focus on using accelerometers and/or magnetometers in combination with gyroscopes. Modern rate gyroscopes can be suitably small and low cost, and their output may be integrated with respect to time to give the change in the sensor's attitude. The relatively large drift of low cost gyroscopes renders these measurements inaccurate over long time periods, but reliable for short periods of time. Existing low cost systems thus often use a complementary filter or a Kalman filter to combine the high bandwidth rate gyroscope output with the low bandwidth estimate from the accelerometer and/or magnetometer.

Such systems have found wide application despite exhibiting large errors when used on a platform that experiences significant accelerations due to motion over long periods of time, such as a fixed-wing aircraft.

Further schemes have been presented in the prior art to use the velocity measurement of a GPS receiver to derive an estimate of the platform's acceleration due to its motion, for example by differentiation with respect to time. This estimate can then be subtracted from the acceleration measured by the accelerometer before its use in one of the above schemes to produce a better estimate of a platform's attitude. This step can also be built into the attitude estimation algorithm, or may be accomplished using a dynamic model of the platform and its motion in some cases.

The invention presented here provides an alternative means of estimating the attitude of a platform, or can be combined with some or all of the elements commonly used in existing systems to produce a superior attitude estimate, which in particular is much less sensitive to the accelerations experienced by the platform and which is thus particularly suitable for use in high acceleration environments such as fixed-wing aircraft, and in particular unmanned aerial vehicles. It can also offer improved robustness by introducing an additional, independent source of attitude information.

This can be accomplished without adding significantly to the overall system cost.

In order to achieve this, the invention includes a means of measuring the dominant axis or axes of the time-varying electric and/or magnetic components of the electromagnetic field or fields due to one or more incident radio waves. If the orientation of these axes is known relative to a reference frame, then measurements of two or more such axes can be used to provide an estimate of the attitude of a platform to which the invention is attached.

The invention further includes a means of processing these electromagnetic field measurements, alone or in combination with measurements from other sensors, to produce an estimate of the platform's attitude. This may be achieved by first processing the electromagnetic field measurements to produce vectors representing one or more of the relevant axes, and subsequently processing these vectors alone or in combination with other sensor data to produce an estimate of the platform's attitude. For example, such vector measurements may be combined with vector measurements from other sensors such as accelerometers and/or magnetometers using methods common in the prior art. These measurements may further be combined with measurements from other sensors such as gyroscopes and GPS receivers using additional methods common in the prior art.

Description of the Invention

The invention, including the radio frequency sensor, interface electronics and processing systems will now be described.

The radio frequency sensor includes a receive antenna array which comprises an array of two or more antennas orientated along two or more different axes. The array may be comprised of antennas sensitive primarily to the magnetic component of an electromagnetic field (known as the H-field) such as loop antennas. The array may alternatively be composed of antennas sensitive primarily to the electric component of an electromagnetic field (known as the E-field) such as dipole antennas. Or it may be comprised of a combination of different types of antenna, with some sensitive to the electric field and some to the magnetic field -for example an array of dipole and loop antennas.

In the case of a receive antenna array composed of loop antennas, each antenna can be implemented as one or more coils of conducting material connected together, where each coil comprises one or more turns of the conductor. The conductor may be provided with a core made from a material with a high electromagnetic permeability in order to increase its sensitivity. In the case of a receive antenna array composed of dipole antennas) each antenna may be implemented as a pair of collinear conducting elements. A person skilled in the art will appreciate that many other antenna geometries would also be suitable.

The array preferably consists of at least three similar antennas orientated along at least three axes, and it is preferable that these axes are orthogonal. It is further preferable that the array is symmetrical in order to reduce cross-coupling of signals between the antennas. In the case of loop antennas, this may be implemented, for example, by constructing each loop antenna from two coils of wire connected together in series. In the case of a dipole, the antenna is naturally suitable for a simple symmetrical arrangement.

Non-magnetic metal elements may also be included in the receiver array to reduce unwanted electrostatic coupling.

Such an antenna array can provide measurements of a radio frequency electric and/or magnetic field in several directions. If three or more differently orientated and non-coplanar H-field antennas are used then enough information is present in their outputs to determine the instantaneous orientation of a measured time-varying magnetic field. Similarly if three or more differently orientated and non-coplanar [-field antennas are used then enough information is present in their outputs to determine the instantaneous orientation of a measured time-varying electric field.

In order to enhance the receiver's sensitivity and selectivity, each antenna in the array may be made resonant at a particular frequency using one or more capacitors and/or inductors. By including a variable capacitance and/or inductance, the resonant frequency of the antennas may be independently varied.

The time-varying voltage output from each of the two or more antennas comprising the receive antenna array is fed to a radio receiver circuit. Such circuits are also very common in the prior art.

They may consist of some combination of amplifiers and filters to increase the signal level and narrow the signal bandwidth. They may include mixers and one or more local oscillators in order to convert the measured signal to an intermediate frequency using a superheterodyne architecture,

which is very common in the prior art.

An analogue to digital converter is used to produce a digital representation of the signal measured by the antenna array. This may be achieved by directly producing a digital representation of the time-varying voltage waveforms at the output of the receiver circuits, which represent the received electric or magnetic field measured by each antenna in the array. Alternatively, the analogue to digital conversion may be performed on derived signals such as the relative phases of the voltage waveforms. This digital representation can be further processed, for example using embedded software running on a microcontroller or microprocessor. This processing may include digital filtering, re-sampling and trigonometric functions in order to produce measurements of the orientation over time of the electric and/or magnetic field at the receiver due to a particular radio signal.

These measurements can then be further processed, either alone or in combination with data from other sensors, to produce an estimate of the platform's attitude. For example, this could be achieved by first processing the digital output describing the received radio frequency signal to produce measurements of the dominant axis of the measured electric and/or magnetic field over a short period of time. The secondary and tertiary axes of an ellipse or ellipsoid which is described by the time varying electric and/or magnetic field can also be derived, as can the strength of the electric and/or magnetic field in the direction of each of those axes. There are many algorithms in the prior art that may be suitable for this processing step, such as singular value decomposition.

A similar process can be performed on more than one radio signal if provision for tuning the receiver and/or varying the analogue filter properties and/or varying the digital processing is provided.

The output of this part of the system thus consists of a digital representation of the received signals which is available for further processing, and an example of such a digital representation is a set of one or more vectors each representing a significant axis of the electric and/or magnetic field due to an incident radio signal, and preferably also the signal strength in that direction.

It is preferable that some method is provided for varying the tuned frequency of each antenna. It is further preferable that this variation can be carried out electronically by a digital controller, which may be implemented as embedded software on a microcontroller or microprocessor.

It is preferable that some means of exciting each antenna with a locally generated signal is provided in order to allow the frequency, phase and amplitude response of each loop antenna to be characterised by a digital controller, which may also be implemented as embedded software running on a microcontroller or microprocessor.

Assuming the dominant axis or axes of the radio frequency electric and/or magnetic field or fields due to the measured signal or signals are known relative to a reference frame) and further assuming that at least one vector measurement derived from those fields is available from the system described above) and finally assuming that one or more known vectors have been measured by additional sensors such as accelerometers or magnetometers, additional digital processing can therefore derive an estimate of the orientation of a platform to which the invention is attached, relative to the reference frame.

Assuming the dominant axis or axes of the radio frequency electric and/or magnetic field or fields due to the measured signal or signals are known relative to a reference frame) and further assuming that at least two different vector measurements derived from those fields are available from the system described above, additional digital processing can therefore derive an estimate of the orientation of a platform to which the invention is attached, relative to the reference frame, without relying on any other sensors.

The use of vector measurements may be explicit, or may alternatively be implicit to the algorithm concerned.

Either of the above methods may additionally combine the vector measurements described with measurements from other sensors such as gyroscopes and GPS receivers to produce an improved solution.

The radio frequency signals measured by this invention may originate from any source as long as they satisfy the condition that the orientation of the electric and/or magnetic field in the vicinity of the receiver is known relative to a reference frame. The fact that externally generated signals are used has the benefit that there is no requirement for the platform whose attitude is being measured to be in communication with an operator, ground station or other element of fixed infrastructure.

Existing radio signals transmitted for other purposes, such as commercial radio broadcasts, may be measured by the invention in order to determine attitude. Using such existing commercial broadcasts has the advantage that no additional infrastructure would need to be constructed and no additional signals would need to be broadcast in order to use the invention.

It has been observed that radio signals with a frequency below 1MHz tend to exhibit relatively constant electric and magnetic field orientations over large distances due to their propagation mechanism. This makes them particularly suitable for measurement by the invention. In particular, radio signals in the longwave frequency band (those with frequencies up to approximately 300kHz) generally exhibit the property of constant electric and magnetic field orientation and in addition can be received over a wide area even if transmitted from a small number of antennas or a single antenna. The invention may therefore be particularly useful when operating on signals of this type.

A number of suitable commercial signals are already broadcast in this frequency range, and thus the benefits of using existing infrastructure, as detailed above, could also be obtained.

Illustrative examples of the invention will now be described. Some details of these particular embodiments have been omitted for clarity, while alternative embodiments are possible that do not include all of the features described below or depicted in the figures. The examples will be described with reference to the following figures, where: Figure 1 shows a block diagram of an example implementation of the invention.

Figure 2 shows an illustrative example of a receiver antenna array according to this invention. In this example the array consists of three orthogonal loop antennas and each loop antenna is formed from two series-connected coils of copper wire wound on non-magnetic formers.

Figure 3 shows an illustrative example of a receiver antenna array according to this invention. In this example the array consists of three orthogonal loop antennas and each loop antenna is formed from two series-connected coils of conductor patterned on an insulating substrate.

Figure 4 shows an illustrative example of a receiver antenna array according to this invention. In this example the array consists of three orthogonal dipole antennas and each dipole antenna is formed from a pair of collinear conducting rods.

FigureS shows a schematic diagram of an example of a variable resonant circuit that could be used with a 1oop antenna forming part of an antenna array according to this invention if each loop antenna is implemented as a single coil.

Figure 6 shows a schematic diagram of an example of a variable resonant circuit that could be used with a loop antenna forming part of an antenna array according to this invention if each loop antenna is implemented as a series-connected pair of coils.

Figure7 shows a schematic diagram of an example of a variable resonant circuit that could be used with a loop antenna forming part of an antenna array according to this invention if each loop antenna is implemented as a series-connected pair of coils.

Figure 8 shows a schematic diagram of an example of a variable resonant circuit that could be used with a dipole antenna forming part of an antenna array according to this invention.

Figure 9 shows a schematic diagram of an example of a variable resonant circuit that could be used with a loop antenna forming part of an antenna array according to this invention if each loop antenna is implemented as a series-connected pair of coils.

Figure 10 shows a schematic diagram of an example of a variable resonant circuit that could be used with a loop antenna forming part of an antenna array according to this invention if each loop antenna is implemented as a series-connected pair of coils.

Figure 11 shows a block diagram of an example radio receiver circuit that could be used to amplify and filter the signal measured by one of the loop antennas forming part of the antenna array.

Figure 12 shows a block diagram of an example radio receiver circuit that could be used to amplify and filter the signal measured by one of the loop antennas forming part of the antenna array.

Figure 13 shows a block diagram of an example processing scheme that could be used to produce an estimate of the attitude of a platform to which the invention is attached) given vector measurements of the dominant axis or axes of one or more incident radio waves and measurements from other sensors.

An example of an embodiment of this invention is shown in overview in figure 1. The system comprises a receive antenna array 1 constructed from an array of two or more E-and/or H-field antennas, a set of receiver circuits 2, an analogue to digital converter 3 and a digital processing element 4 that produces an output 5. It also preferably includes a circuit 6 for varying the tuned frequency of the antennas that comprise the receiver array, and it preferably includes a circuit 7 for driving the receive antenna array with locally generated signals in order to tune and characterise each of its constituent antennas and in order to characterise the receive antenna array as a whole.

The receive antenna array 1 comprises an array of two or more antennas orientated along two or more axes. The array preferably consists of at least three similar antennas orientated along at least three axes. It is further preferable that the array is symmetrical to reduce cross-coupling between the antennas. In the case of an array constructed from loop antennas, this may be achieved by constructing each loop antenna from two coils of wire connected together in series or parallel.

Additional metal elements may also be included in the receiver array to reduce unwanted electrostatic coupling. In the case of an antenna array including H-field antennas, this shielding should preferably have a thickness which is less than the skin depth of signals at the frequency of interest in the shielding material in order to reduce electrostatic coupling between elements of the array, and/or between other parts of the invention and the receiver antenna array, without significantly attenuating the magnetic field which is to be measured.

A particular embodiment of a suitable receiver antenna array is illustrated in figure 2. Here there are three loop antennas each orientated along orthogonal axes and each consisting of two coils of copper wire connected in series. Each of the six coils is wound on a separate non-magnetic structure (the former) and mounted symmetrically to the faces of a cube 8 constructed primarily of non-magnetic materials. A suitable material for the former would be ABS plastic, but many others are possible. A coil lOis wound on a former 9. This coil is connected in series with the coil 12, wound on former 11, to form a loop antenna with its axis orientated in the Z direction. Similarly coil 14, wound on former 13, and coil 16, wound on former 15, are connected in series to form a loop antenna with its axis orientated in the X direction and coil 18, wound on former 17, is connected in series with a coil on the opposite face of cube 8, which is not visible in figure 2, to form a loop antenna with its axis orientated in theY direction. In this way three orthogonal loop antennas are formed from the six coils.

The loops antennas used in the array of figure 2 do not have high-permeability cores. However an alternative embodiment of such an array may use high permeability core materials, such as ferrite.

Another embodiment of a suitable receiver antenna array is shown in figure 3. Here there are three loop antennas each orientated along orthogonal axes and each consisting of two coils of copper connected in series. The coils are patterned on the surface of an insulating substrate 19 and mounted symmetrically on the faces of a cube constructed primarily of non-magnetic materials.

Such patterned coils can easily be constructed in practice on printed circuit boards. The coil 20 is connected via one of its ends 22 to a similar coil on the opposite face of the cube to form a loop antenna with its axis orientated in the Z direction. The other end 21 of the coil 20 forms one of the two connections to the resulting loop antenna, while the other connection is made to the coil on the opposite face (not visible in figure 3). The coil 23 is connected via one of its ends 25 to a similar coil on the opposite face of the cube to form a loop antenna with its axis orientated in the X direction.

The other end 24 of the coil 23 forms one of the two connections to the resulting loop antennas, while the other connection is made to the coil on the opposite face (not visible in figure 3). The coil 2 is connected via one of its ends 28 to a similar coil on the opposite face of the cube to form a loop antenna with its axis orientated in the V direction. The other end 27 of the coil 26 forms one of the two connections to the resulting loop antenna, while the other connection is made to the coil on the opposite face (not visible in figure 3). In this way three orthogonal loop antennas are formed from the six coils.

Another embodiment of a suitable receiver antenna array is shown in figure 4. Here the array consists of three orthogonal dipole antennas, joined together by insulating mechanical elements (not shown). Vertical conductors 29 and 34 form a dipole antenna orientated in the Z direction. An electrical connection is made to the conductors at point 39 via wires 40. Conductors 30 and 37 form a dipole antenna orientated in theY direction. An electrical connection is made to the conductors at point 36 via wires 35. Conductors 31 and 38 form a dipole antenna orientated in the X direction. An electrical connection is made to the conductors at point 32 via wires 33.

Each antenna in the receiver antenna array is preferably combined with one or more capacitors and/or inductors to form a resonant circuit. A method is provided for varying the resonant a frequency of the circuit, in a way that permits control of the frequency and phase response of the antenna by a control element. For example, varactors could be used in the resonant circuit and a means could be provided for varying the bias voltage applied to the varactors.

With reference to figures, a simple embodiment of such a resonant circuit for use with a single loop antenna formed from a single coil is formed by placing the coil 45 in parallel with a fixed capacitor 46 and a varactor 41. The varactor's capacitance is varied by means of a voltage applied at point 42.

The DC blocking capacitor 43 prevents this voltage appearing across the coil. The output and ground reference are taken at point 44.

With reference to figure 6, an embodiment of such a resonant circuit for use with a single loop antenna formed from a pair of coils connected in series is formed by placing the series-connected coils 50 and 52 in parallel with a fixed capacitor 53 and a varactor 47. The varactor's capacitance is varied by means of a voltage applied at point 48. The DC blocking capacitor 49 prevents this voltage appearing across the coil. The output and ground reference are taken at point 51. If, for example, the antenna array is constructed as indicated in the example of figure 2, the coil 50 in figures could correspond to the coil 10 in figure 2 and the coil 52 in figures could correspond to the coil 12 in figure 2. A similar arrangement would be needed for the other two loop antennas depicted in figure 2.

With reference to figure 7, an embodiment of such a resonant circuit for use with a single loop antenna formed from a pair of coils connected in series is formed by placing the series-connected coils 57 and 60 in parallel with a fixed capacitor 61 and a varactor 64. The varactor's capacitance is varied by means of a voltage applied at point 54. The resistors 55 and 63 isolate the antenna from the low impedance voltage source and ground connections. The DC blocking capacitors 56 and 62 couple the varactor to the coil. The differential output voltage is taken at point 58.

With reference to figure 8, an embodiment of such a resonant circuit for use with a single dipole antenna 75 is formed by connecting the antenna to a resonant circuit consisting of the fixed capacitor 65, inductor 66 and varactor 68. The varactor is coupled to the antenna and other elements of the resonant circuit by means of the DC blocking capacitors 67 and 74. The varactor's capacitance is varied by means of a voltage applied at point 70, with reference to the ground at point 72. The resonant circuit and antenna are isolated from the low impedance voltage source applied to point 70 and the low impedance ground connection 72 by means of resistors 69 and 73.

The differential output voltage is taken at point 71.

With reference to figure 9, an embodiment of such a resonant circuit for use with a single loop antenna formed from a pair of coils connected in series is formed by placing each of the series-connected coils 79 and 81 in parallel with separate fixed capacitors 78 and 82 respectively and separate varactors 76 and 84 respectively. The varactors' capacitance values are varied by means of a voltage applied at point 86 and connected to the varactors by means of resistors 85 and 87. The DC blocking capacitors 77 and 83 prevent this voltage appearing across the coils. The differential output voltage is taken at point 80.

With reference to figure 10, an embodiment of such a resonant circuit for use with a single loop antenna formed from a pair of coils connected in series is formed by placing each of the series-connected coils 90 and 92 in parallel with separate fixed capacitors 89 and 93 respectively and separate varactors 98 and 95 respectively. The varactors' capacitance values are varied by means of two separate voltages applied at points 97 and 96 from high impedance voltage sources. The DC blocking capacitors 88 and 94 prevent these voltages appearing across the coils. The differential output voltage is taken at point 91.

A person skilled in the art will appreciate that many other circuit arrangements may also be used in place of those outlined above.

A method of driving each antenna with a locally generated signal is preferably provided in order to allow the frequency, phase and amplitude response of each antenna to be measured and/or controlled. In the case of a receive array consisting of loop antennas, this can be achieve by placing a small coil close to each receive antenna in the array that can be driven with digital signals generated by a control/processing element, or by directly coupling such signals into each antenna via resistors and/or capacitors.

The output from each antenna in the receive antenna array is connected to the input of a radio receiver circuit that may allow the signal level to be increased and may also increase the selectivity of the receiver. The connection between the receive antenna and the receiver circuit may be made directly) as in the examples below, or may be made indirectly. For example inductive and/or capacitive coupling may be used.

In some embodiments it may be necessary to use differential, shielded or optical coupling of the signals from the receive antenna array in order to reduce feedback and cross-coupling.

A suitable radio receiver circuit is illustrated in figure 11, where the input is taken at point 99, amplifiers 100 and 102 are used to increase the signal level and a band pass filter 101 is used to increase selectivity prior to digitisation by the analogue to digital converter 103. The digitised output is provided at point 104. If an array of loop antennas is used as the receiver array, and one of the loop antennas comprising the array is made resonant using the circuit of figure 6, then the output 51 could be connected to the input 99 of the radio receiver circuit of figure 11, for example.

The circuit of figure 12, which uses a superheterodyne architecture that is also common in the prior art, would also be suitable. It comprises an amplifier 106 operating on the input voltage taken at point 105 to increase the received signal level, a mixer 107 and local oscillator 112 that change the frequency of the received signal to an intermediate frequency (IF), a band pass filter 108 to increase selectivity and an amplifier 109 to further increase the signal level prior to digitisation by the analogue to digital converter 110. The digitised output is then available at point 111. The local oscillator 112 used in the superheterodyne architecture is preferably common to all channels of the receiver.

It should be noted that the receiver circuit may at various stages operate on differential or single-ended signals, and may include additional circuit stages beyond those described in the above

examples.

A person skilled in the art will appreciate that many other receiver circuits, including those based on alternative architectures, may also be used.

With reference to figure 1, the output of the receiver circuit for each channel 2 is used as the basis for an analogue to digital conversion by the analogue to digital converter 3. The analogue to digital converter takes simultaneous samples at regular intervals of the voltage output from each receiver circuit, where the voltage from each circuit corresponds to the signal level at one of the antennas comprising the receiver array. This is consistent with the circuit of figures 11 and 12. However, the analogue to digital converter may operate on any signals that lead to a complete or partial digital representation of the received electromagnetic wave of waves. For example, it may operate on the relative phase of the received signals as measured by one or more phase comparators.

Whichever form of the signal is digitised, the analogue to digital converter provides a digital representation of the received signal measured. This digital representation can be processed by a processing/control element 4, which may be implemented as embedded software run on a microcontroller or microprocessor, to produce measurements over time of the orientation of the radio frequency magnetic and/or electric field at the receive antenna array 1 due to an incident radio wave. The processing performed may include filtering, re-sampling and the use of a process such as singular value decomposition to estimate the dominant axis or axes of the received signal, amongst other possible steps.

These measurements can then be further processed independently or in conjunction with measurements from other sensors in order to produce an estimate of the attitude of the platform to which the invention is attached. A person skilled in the art will appreciate that many combinations of sensor data and many processing schemes can be used to produce an estimate of platform attitude, and that many such methods have been presented in the prior art.

An example embodiment of a suitable processing scheme to produce a full attitude solution is presented in figure 13. Digital representations of the measured radio frequency magnetic vectors 113, which are measured and digitised as described above, are combined with a measured acceleration vector 114 from a three-axis accelerometer, a measurement of the earth's magnetic field 115 from a three-axis magnetometer and stored values 117 of all of these vectors with respect to the local ground, using the QUEST algorithm 118, which is widely cited in the prior art.

Measurements of rotation rate 116 from a three axis rate gyroscope are also taken. They are numerically integrated with respect to time 119 using a simple rigid body kinematic model before being combined with the output of the QUEST algorithm in a complementary filter 120 to produce a high bandwidth attitude estimate as a final output 121. A person skilled in the art will appreciate that suitable alternative schemes include those based on Kalman filters, extended Kalman filters, unscented Kalman filters, particle filters, recursive estimators and many others.

The various measurements available in such a system can also be used for other purposes, including radio direction finding, ranging and positioning. These concepts are all extensively covered in the prior art. However this invention provides a new way to implement them, in particular to produce a combined attitude and/or radio direction finding and/or ranging and/or positioning system using a single sensor system.

It will be appreciated by those skilled in the art that many other configurations of the receive antenna array, receiver tuned circuit, radio receiver electronics, additional sensor combinations, processing schemes and overall system architecture are possible beyond those illustrated by way of example herein. The examples described herein are intended for illustration only and not to limit the scope of this patent specification, which is defined by the following claims.

Claims

CLAIMS: 1. A system for fully or partially determining the orientation of a platform or vehicle relative to a reference coordinate frame (i.e. the attitude of the platform or vehicle) using measurements of the polarisation of the electric and/or magnetic field of one or more Radio Frequency electromagnetic waves, where these electromagnetic waves are broadcast from one or more external sources and are measured at the platform or vehicle in a coordinate frame whose orientation is known relative to the platform or vehicle) and where the polarisation of the signals in the vicinity of the platform or vehicle is known in the reference coordinate frame.
2. An attitude determination system according to claim 1, where the system is capable of fully determining the orientation of the platform or vehicle in three dimensions, for example by determining the pitch, roll and yaw of the platform or vehicle.
3. An attitude determination system according to either of claims 1 or 2, where the system comprises an array of two or more receiving antennas, a set of radio receiver circuits, one or more Analogue to Digital Converters and a digital processing and control module.
4. An attitude determination system according to claim 3, where there are three or more receiving antennas in the array, where these antennas are oriented along three or more axes and where the array is used to make electric field measurements in three or more axes.
5. An attitude determination system according to claim 3, where there are three or more LI') receiving antennas in the array, where these antennas are oriented along three or more axes and where the array is used to make magnetic field measurements in three or more axes.
6. An attitude determination system according to either of claims 4 and 5, where the O measurements are made in three substantially orthogonal axes.
7. An attitude determination system according to any of the preceding claims, where the Cv') measurements of the polarisation of the one or more Radio Frequency signals are combined with data from one or more additional sensors in order to fully determine attitude.
8. An attitude determination system according to any of claims 2 to 6, where two or more Radio Frequency signals are measured in order to provide measurements of two or more known vectors in order to fully determine the attitude of the platform or vehicle.
9. An attitude determination system according to claim 8, where the polarisation vector measurements are combined with data from one or more additional sensors in order to improve the accuracy of the attitude solution.
10. An attitude determination system according to either of claims 7 and 9, where the additional sensors include a magnetometer.
11. An attitude determination system according to any of the preceding claims, where the accuracy and/or bandwidth of the resulting attitude solution is further improved by being combined in a filter with measurements of angular velocity made using a gyroscope.
12. An attitude determination system according to any of the preceding claims, where the Radio Frequency signals being measured have a frequency below 1MHz.
13. An attitude determination system according to any of the preceding claims, where one or more of the Radio Frequency signals being measured are broadcast primarily for purposes other than attitude determination.
14. An attitude determination system according to any of the preceding claims, where one or more of the Radio Frequency signals are linearly polarised.
15. An attitude determination system according to any of the preceding claims, where one or more of the Radio Frequency signals are elliptically polarised.
16. An attitude determination system according to any of the preceding claims, where the direction and, optionally, magnitude of the major axis of the polarisation ellipse of each signal is measured or estimated, and subsequently used in an attitude solution.
17. An attitude determination system according to claim 6, where the receiving antenna array consists of three orthogonal loop antennas.
18. An attitude determination system according to claim 17, where each loop antenna is made resonant by use of one or more capacitors and/or one or more varactors.
19. An attitude determination system according to either of claims 17 and 18, where the receiving antenna array additionally includes an antenna for measuring the electric field of incident Radio Frequency electromagnetic waves.
20. An attitude determination system according to any of claims 17 to 19, where each loop antenna is constructed from a pair of coils connected in series.
21. An attitude determination system according to claim 20, where each coil is air-cored.
22. An attitude determination system according to claim 21, where the two coils comprising each loop antenna are mounted to opposite faces of a cube.
23. An attitude determination system according to claim 22, where each coil is patterned on an insulating substrate.
24. An attitude determination system according to claim 6, where the receiving antenna array consists of three orthogonal dipole antennas.
25. An attitude determination system according to claim 24, where each loop antenna is made CV) resonant by use of one or more capacitors and/or one or more varactors in combination 0 with one or more inductors.
26. An attitude determination system according to any of the preceding claims, where electrostatic shielding is applied to the array of receiver antennas.
27. An attitude determination system according to claim 26, where the electrostatic shielding has a thickness less than the skin depth, in the shielding material, of the signals being measured.
28. An attitude determination system according to claim 3, where the output of each receiving antenna is used as the input to a radio receiver circuit.
29. An attitude determination system according to claim 28, where there is one radio receiver circuit for each antenna in the receiving antenna array.
30. An attitude determination system according to claim 29, where the radio receiver circuits are substantially separate from each other.
31. An attitude determination system according to any of claims 28 to 30, where the radio receiver circuits amplify the output of each receiving antenna and/or provide frequency selectivity.
32. An attitude determination system according to claim 31, where the radio receiver circuits use a superheterodyne architecture.
33. An attitude determination system according to any of claims 3,28, 29, 30,31 and 32, where the output of each radio receiver circuit is digitised at regular intervals using one or more Analogue to Digital Converters.
34. An attitude determination system according to claim 33, where a number of Analogue to Digital Converters greater than or equal to the number of radio receiver circuits are used to sample the signals from each radio receiver circuit substantially simultaneously at each sampling instant.
35. An attitude determination system according to either of claims 33 and 34, where a digital processing module is used to process the output of the Analogue to Digital Converters in order to determine the polarisation of the one or more measured Radio Frequency signals.
36. An attitude determination system according to claim 35, where the digital processing module is further used to calculate an attitude solution using the Radio Frequency polarisation data alone, or using the Radio Frequency polarisation data in addition to data from other sources.
37. An attitude determination system according to claim 3, where means is provided for the processing and control module to apply locally-generated signals to each antenna in the receiving antenna array.
38. An attitude determination system according to claim 37, where means is further provided for the processing and control module to measure the gain and/or phase shift of each receiving antenna and/or radio receiver circuit.
39. An attitude determination system according to claim 38, where means is further provided for the processing and control module to vary the phase shift of each receiving antenna and/or radio receiver circuit.
40. An attitude determination system according to claim 38, where means is further provided for the processing and control module to vary the gain of each receiving antenna and/or radio receiver circuit.
41. An attitude determination system according to claim 37, where means is further provided CV) for the processing and control module to measure the frequency response of each receiving 0 antenna and/or radio receiver circuit.
42. An attitude determination system according to claim 37, where means is further provided for the processing and control module to measure the sensitivity of each receiving antenna to signals applied to the other antennas in the array, in order to determine the cross-axis sensitivity.