Patent Description:
Many systems on modern aircraft, particularly those that provide control over aircraft position and/or trajectory, utilise speed data. During flight, speed data that may be utilised include, for example, an indicated airspeed (IAS), a calibrated airspeed (CAS), an equivalent airspeed (EAS), and a true airspeed (TAS). However, during ground manoeuvres, it is common to use a speed value which accurately indicates the speed of the aircraft relative to the ground, referred to hereinafter as groundspeed.

Modern aircraft typically have multiple systems for determining groundspeed. The most accurate groundspeed measurements may be provided by global positioning system (GPS) data.

However, in some circumstances, while aircraft are manoeuvring around an airfield, particularly in larger airports in which there are multiple large buildings, GPS signals may be temporarily lost or may otherwise become unreliable. In such circumstances, speed values determined on the basis of GPS may accordingly become unreliable. Accordingly, modern aircraft have backup systems for determining groundspeed.

One such system is an inertial reference system (IRS). An IRS determines speed by integrating acceleration values provided by an accelerometer. Another such system is a wheel speed system that determines the groundspeed based on a frequency of rotation of one or more wheels of the undercarriage of the aircraft. However, each of these systems may be less accurate than systems that utilising GPS data. Therefore, it is useful to be able to determine when speed data determined on the basis of GPS data becomes unreliable.

The present invention mitigates the above-mentioned problems and accordingly may provide a more reliable speed measurement system. <CIT> discloses a system for use with an inertial reference system and a global position receiver for calculating a position error after a loss of integrity by utilizing the global position system values for position and velocity at a time just before the loss of integrity and by utilizing the inertial reference system position modified by the known error in inertial reference system position as it varies with time and the position error as calculated by the global position system velocity extrapolated over the time since integrity loss. <CIT> discloses a second-order filter for use with an airborne navigation system that blends accurate position information from a one source, and velocity information from another source, both suitably scaled, in a second-order complementary filter. <CIT> discloses a filtering mechanization method for integrating a global positioning system receiver with an inertial measurement unit to produce mixed global positioning system and inertial measurement unit position, velocity and attitude information of a carrier.

A first aspect of the present invention provides a monitoring system for a speed determination system in an aircraft, the speed determination system comprising a first speed measurement system and a second speed measurement system, the second speed measurement system being associated with a predetermined behaviour characteristic, the monitoring system comprising a processor arranged to: receive speed data provided by the speed determination system; perform a correspondence determination process comprising processing the received speed data to determine whether a correspondence condition is satisfied, the correspondence condition comprising a correspondence between the received speed data and the predetermined behaviour characteristic of the second speed measurement system; and in response to determining that the correspondence condition is satisfied, determine that the first speed measurement system is in an error condition.

Optionally, the first speed measurement system is arranged to determine a time-varying speed measurement based on global positioning system data.

Optionally, the second speed measurement system is arranged to determine a time-varying speed measurement based on accelerometer data.

Optionally, the second speed measurement system comprises an integrator and the predetermined behaviour characteristic comprises an error value that is amplified over time by the integrator.

Optionally, the monitoring system is arranged to monitor the predetermined behaviour characteristic to determine whether the behaviour characteristic corresponds with a behaviour characteristic exhibiting amplification.

Optionally, the predetermined behaviour characteristic of the second speed measurement system comprises a first value, the first value being a value of a rate of change of acceleration, and the correspondence determination process comprises: determining a second value, the second value being a value of a rate of change of acceleration indicated by the received speed data, and wherein the correspondence condition comprises a correspondence between the second value and the first value.

Optionally, the correspondence condition comprises that the second value lies within a predetermined range, the predetermined range including the first value.

A second aspect of the present invention provides a method of monitoring a speed determination system for an aircraft, the speed determination system being configured to determine the speed of an aircraft based on time-varying data provided by a plurality of speed measurement systems including a first speed measurement system and a second speed measurement system, the second speed measurement system being associated with a predetermined behaviour characteristic, the method comprising: receiving speed data provided by the speed determination system; performing a correspondence determination process comprising processing the received speed data to determine whether a correspondence condition is satisfied, the correspondence condition comprising a correspondence between the received speed data and the predetermined behaviour characteristic of the second speed measurement system; and in response to determining that the correspondence condition is satisfied determining that the first speed measurement system is in an error condition.

Optionally, the speed data indicates a series of speed values corresponding to a respective series of time intervals, and the correspondence condition comprises that a correspondence between the first value and the second value persists over at least a predetermined number of the time intervals.

Optionally, the predetermined number of time intervals is greater than or equal to five.

Optionally, a duration of the time intervals is less than or equal to <NUM>.

Optionally, the first speed measurement system is arranged to determine a time-varying speed measurement based on global positioning system data and the second speed measurement system is arranged to determine a time-varying speed measurement based on accelerometer data, and wherein the speed determination system is arranged to provide speed data based on the first speed measurement system when the first measurement system is not in an error condition.

Optionally, the method comprises: in response to determining that the first speed measurement system is not in an error condition, determining a speed of the aircraft from the first speed data and determining one or more correction values for the second speed measurement system; and in response to determining that the first speed measurement system is in an error condition, determining a speed of the aircraft from the second speed data and the determined one or more correction values.

According to a third aspect of the present invention, there is provided an aircraft comprising a monitoring system according to the first aspect.

According to a fourth aspect of the present invention there is provided a computer program which, when executed by a processor in a speed determination system for an aircraft, the speed determination system being configured to determine the speed of an aircraft based on time-varying data provided by a plurality of speed measurement systems including a first speed measurement system and a second speed measurement system, the second speed measurement system being associated with a predetermined behaviour characteristic, causes the processor to: receive speed data provided by the speed determination system; perform a correspondence determination process comprising processing the received speed data to determine whether a correspondence condition is satisfied, the correspondence condition comprising a correspondence between the received speed data and the predetermined behaviour characteristic of the second speed measurement system; and in response to determining that the correspondence condition is satisfied, determine that the first speed measurement system is in an error condition.

<FIG> is schematic diagram of a monitoring system <NUM> for a speed determination system for an aircraft, according to an example.

In the example shown in <FIG>, the monitoring system <NUM> is a computerized device implemented by a processor <NUM> executing software instructions stored in a memory <NUM> on the basis of inputs received via an interface <NUM>. For example, the monitoring system <NUM> may be an avionics unit installed in an aircraft such as the aircraft <NUM> described below with reference to <FIG>.

The memory <NUM> may also store behaviour characteristics of one or more speed measurement systems as described below in relation to the example described with reference to <FIG>.

The interface <NUM> is arranged to receive data including speed data from a speed determination system.

The interface <NUM> is also arranged to output an error signal indicating that the speed determination system is in an error condition or state. For example, the interface <NUM> may provide a signal indicating that one or more speed measurement systems comprising the speed determination system is not operating correctly. This may mean that data from the incorrectly operating speed measurement system is unreliable and should therefore be ignored or discounted by the speed determination system. Such information may, for example, be used to provide a speed indication (for example via a speed indication display in the cockpit) or may be used by other aircraft systems that use speed data provided by the speed determination system. In other examples, data provided to the monitoring system may be provided to other aircraft systems that use the same sources of data used by the speed determination system to determine the aircraft speed, and the monitoring system may provide information regarding the validity of that information to such systems.

The error signal output from the interface <NUM> may be provided to these other systems which, in response may determine to use a different speed measurement system to the speed measurement system giving rise to the error condition.

The inputs received at the interface <NUM> may correspond to speed data received from one or more speed measurement systems including a first speed measurement system arranged to provide first speed data and a second speed measurement system arranged to provide second speed data, as described below with reference to <FIG>. The first speed measurement system is arranged to provide the first speed data based on a first speed measurement and the second speed measurement system is arranged to provide the second speed data based on a second speed measurement. The second speed measurement system is associated with a predetermined behaviour characteristic such as that described below with reference to <FIG>.

Although, in the example shown in <FIG>, the monitoring system <NUM> is implemented in software executed by hardware (processor <NUM>) in some examples, the monitoring system <NUM> may be implemented entirely in hardware.

<FIG> is a schematic diagram illustrating an interaction between a monitoring system (MS) <NUM> as described above with reference to <FIG> and a speed determination system (SDS) <NUM> for an aircraft, according to an example. The speed determination system <NUM> may, for example, be as described below with reference to <FIG>.

As shown in <FIG>, the processor <NUM> implements a speed data receipt (SDR) function <NUM>, which receives speed data (SD) <NUM>. For example, the SDR function <NUM> may receive speed data <NUM>, provided by one or more speed measurement systems of the speed determination system <NUM>, via the interface <NUM> of the monitoring system <NUM>.

The monitoring system <NUM> comprises a correspondence determination process (CDP) function <NUM>, arranged to receive speed data <NUM> (for example from the SDR function <NUM>, or directly from the SDS <NUM>) and to receive a predetermined behaviour characteristic (PBC) <NUM>. The PBC <NUM> is a predetermined behaviour characteristic of the second speed measurement system. Specifically, the CDP function <NUM> performs a correspondence determination process comprising processing the received speed data <NUM> to determine whether a correspondence condition is satisfied. The correspondence condition is satisfied, for example, when there is a correspondence between the received speed data <NUM> and the predetermined behaviour characteristic <NUM> of the second speed measurement system.

In response to the CDP <NUM> determining that the correspondence condition is satisfied (that is, that in response to determining that there is a correspondence between the speed data <NUM> and the predetermined behaviour characteristic <NUM> of the second speed measurement system), an error condition recognition (ECR) function <NUM> is arranged to determine that the first speed measurement system is in an error condition (EC) <NUM>. The ECR <NUM> may be arranged, as shown in <FIG>, to provide a signal indicating the error condition <NUM> to the speed determination system <NUM>.

<FIG> is a flow diagram illustrating a method <NUM> of monitoring a speed determination system of an aircraft. For example, the method <NUM> may operate under the control of the monitoring system <NUM> described above with reference to <FIG> and <FIG>.

At block <NUM> speed data, such as the speed data <NUM> data provided by the speed determination system <NUM> described above with reference to <FIG> is received. For example, the speed data may be received by the SDR described above with reference to <FIG>. The speed data may include speed data from a first speed measurement system and from a second speed measurement system. As described below with reference to <FIG> below, the first speed measurement system may determine a speed measurement based on GPS data and the second speed measurement system may determine speed based on IRS data; however, one or both of the first and second speed measurement systems may determine a speed measurement based on different speed measurement techniques.

At block <NUM>, a correspondence determination process comprising processing the received speed data to determine whether a correspondence condition is satisfied is performed. The correspondence determination process may be performed by the CDP function <NUM> described above with reference to <FIG>, for example. The correspondence condition comprises a correspondence between the received speed data and the predetermined behaviour characteristic of the second speed measurement system. For example, the speed data received from the speed determination system <NUM> includes speed data generate by or provided by the first speed measurement system (which may correspond, for example, to a speed measurement generate based on GPS data) and the second speed measurement system (which may generate speed values based on data not utilising GPS, such as inertial reference data determined based on values generated by an accelerometer in the aircraft and/or wheel speed data determined based on values generated based on a frequency of rotation of one or more wheels of the aircraft).

At block <NUM>, a determination that the first speed measurement system is in an error condition is made in response to determining that the correspondence condition is satisfied. Such a determination may be made, for example, by the ECR <NUM> described above with reference to <FIG>.

For example, in response to determining that the correspondence condition is satisfied, the monitoring system <NUM> may determine that the first speed measurement system is in an error condition because the speed determined by the speed determination system <NUM> has a characteristic similar to or the same as that which would be expected if the speed determined by the speed determination system <NUM> was determining a speed measurement based on a measurement provided by the second speed measurement system alone.

As described above, the memory <NUM> of the monitoring system <NUM> may store predetermined behaviour characteristics of one or more speed measurement systems <NUM>. For example, each speed measurement system from which the speed determination system can obtain speed data may have an associated predetermined behaviour characteristic <NUM> that is stored in the memory <NUM>, which the monitoring system can retrieve from the memory <NUM> to perform the correspondence determination process described above with reference to block <NUM> of the method <NUM> of <FIG>. That is, the monitoring system <NUM> may monitor speed data <NUM> determined by a speed determination system <NUM> to determine whether that speed data <NUM> has a characteristic corresponding with a predetermined behaviour characteristic <NUM> stored in the memory <NUM>.

If the monitoring system <NUM> determines that there is a correspondence between speed data <NUM> determined by a speed determination system <NUM> and one or more of the predetermined behaviour characteristics <NUM> stored in the memory <NUM> (i.e. characteristics relating to a second speed measurement system), the monitoring system <NUM> may determine that there is an error in the speed data <NUM> provided by a first measurement system <NUM>.

<FIG> is a graph showing an illustrative example of a behaviour characteristic <NUM> which may correspond with a second speed measurement system. In the example shown in <FIG>, the behaviour characteristic relates to a speed measurement system arranged to measure a speed of the aircraft based on values determined from an accelerometer. The monitoring system may receive speed data (e.g. the speed data <NUM>) and using, for example, the correspondence determination function <NUM>, may determine a correspondence between the speed data <NUM> and a predetermined behaviour characteristic <NUM>.

For example, the speed determined by the speed determination system may utilise speed data from a first speed measurement system (for example a GPS system) and a second speed measurement system (for example, an IRS system), which may be compared with a predetermined behaviour characteristic associated with the second speed measurement system, to determine an error condition of the first speed measurement system.

The predetermined behaviour characteristic <NUM> shown in <FIG> comprises a series of speed values corresponding to a respective series of time intervals, n. A duration of each of the time intervals may be equal. For example, the duration of each of the time intervals may be less than or equal to <NUM>.

In the example shown in <FIG>, the predetermined behaviour characteristic <NUM> is that a rate of change of speed doubles every speed interval. That is, for a time interval, n:
<MAT>
where v is a speed of the aircraft determined by the speed determined by the speed determination system in a given time interval tn, such that <MAT> is a rate of change of speed of the aircraft in time interval n, and <MAT> is a rate of change of speed of the aircraft in time interval n+<NUM>.

That is, the rate of change of acceleration is constant, with a constant of proportionality approximately equal to <NUM>.

Accordingly, if the correspondence determination process <NUM> of the monitoring system <NUM> determines that received speed data <NUM> corresponds with a predetermined behaviour characteristic <NUM> such as that shown in <FIG> (i.e. that the rate of change of acceleration is approximately constant), the monitoring system <NUM> may determine that the received speed data <NUM> is characteristic of speed data from a speed measurement system arranged to determine speed based on values generated by an accelerometer, and in turn infer that expected data from a primary or first speed measurement system that would be used in preference to speed values determined based on accelerometer data (e.g. GPS data) is unavailable or unreliable, to determine an error condition of the first speed measurement system.

In some examples, the predetermined behaviour characteristic <NUM> against which the rate of change of speed determined by the speed determination system is processed comprises a first value, which is a first rate of change of acceleration. In such examples, the correspondence determination process may comprise determining a second value (that is a second rate of change of acceleration indicated by the received speed data), and the correspondence condition may be determined if there is a correspondence between the second rate of change of acceleration (second value) and the first rate of change of acceleration (first value).

In some examples, the correspondence may be determined if the second value is within a predetermined range of rates of change of acceleration that includes the first value. For example, the correspondence may be determined if the second value is within, for example, ± <NUM> % of the first value.

In some examples, the correspondence condition may be that there is a correspondence between the first value and the second value persists over at least a predetermined number of the time intervals. For example, the correspondence condition may be that there is a correspondence between the first value and the second value for at least five time intervals.

In some examples, the first speed measurement system is arranged to determine a time-varying speed measurement based on global positioning system data and the second speed measurement system is arranged to determine a time-varying speed measurement based on accelerometer data, and the speed determination system is arranged to provide speed data based on the first speed measurement system when the first measurement system is not in an error condition. For example, in response to determining that the first speed measurement system (i.e. the GPS system) is not in an error condition, a speed of the aircraft and/or one or more correction values for the second speed measurement system may be determined from a speed measurement from the first speed measurement system and in response to determining that the first speed measurement system is in an error condition, a speed of the aircraft and one or more correction values may be determined from the second speed data.

For example, <FIG> is a schematic diagram illustrating the operation of a speed determination system <NUM> which is an example of the speed determination system <NUM> described above with reference to <FIG>. In this example, the speed determination system <NUM> is arranged to determine a speed value based on data provided by two speed measurement systems: a first speed measurement system which is a global positioning system (GPS) <NUM>, and a second speed measurement system which is an inertial reference system (IRS) <NUM>.

The GPS system <NUM> uses global positioning data received from global positioning satellites to determine a position of the aircraft and determines a measure of the speed of the aircraft based on a rate of change of position of the aircraft. The accuracy of the determined position of the aircraft and therefore the accuracy of the speed of the aircraft determined by the GPS system <NUM> is related to the number of satellites from which the GPS system <NUM> can receive positioning data.

In some circumstances, such as when the aircraft is near large buildings, GPS signals may be temporarily lost or may otherwise become unreliable. In such circumstances, speed values determined on the basis of GPS data may accordingly become unreliable. In such circumstances, the speed determination system <NUM> shown in <FIG> can determine the speed of the aircraft based on the second speed measurement system, the IRS <NUM>.

The IRS <NUM> may determine speed by integrating acceleration values provided by one or more accelerometers located within the aircraft. The accelerometers may be calibrated (e.g. "zeroed") while the aircraft is stationary and may measure the acceleration of the aircraft over time. By integrating values of acceleration, the IRS <NUM> determines a value corresponding to the speed of the aircraft. However, because errors in the determined acceleration values accumulate, over time speed values determined by the IRS <NUM> may become less accurate and are typically less accurate than the GPS <NUM>.

The speed determination system shown in <FIG> comprises a filter <NUM> which is arranged to determine which of the speed measurements to use; i.e. whether to use speed measurements determined by the GPS <NUM> or the IRS <NUM>. The filter <NUM> comprises an algorithm arranged to process speed data from the GPS <NUM> and IRS <NUM> to determine whether to use a speed measurement provided by the GPS <NUM> or the speed measurement provided by the IRS <NUM> (or some combination of the two). For example, the filter <NUM> may comprise a Kalman filter.

In other examples, the speed determination system <NUM> may not include a filter but the monitoring system <NUM> described above may nevertheless determine that reliable speed data provided by the GPS <NUM> is unavailable; for example, when no GPS data is available, the SD <NUM> may exhibit the behaviour characteristic of the IRS <NUM>.

The filter <NUM> outputs speed data (SD) <NUM> which, depending on the inputs to and parameters of the algorithm, may be based on speed data provided by the GPS <NUM>, speed data provided by the IRS <NUM>, or some combination of the two. In an example, the filter <NUM> filters out speed data provided by the GPS <NUM> when the GPS <NUM> is determined to be in error. For example, when there is no available GPS data or when the GPS data is otherwise determined to be unreliable (for example where there is insufficient or unreliable GPS data to determine an accurate speed measurement), the filter <NUM> may filter out speed measurements provided by the GPS <NUM> and provide speed data <NUM> provided by the IRS <NUM>. Accordingly, when the SD <NUM> exhibits the behaviour characteristic of the IRS <NUM>, it may be inferred that the GPS <NUM> is in an error condition.

The speed data <NUM> may be used by other systems of the aircraft. For example, the speed data <NUM> may be used to provide a groundspeed indication to the pilot via a display in the cockpit. In other examples, the speed data <NUM> may be used as an input to other aircraft navigation and/or control systems. For example, in-flight, the speed data <NUM> may be used by flight control systems or other hydraulic systems (such as ground spoilers, for example) or, for example, in-flight entertainment systems. At lower speeds (e.g. when the aircraft is on the ground) control systems of the aircraft may use the speed data <NUM> to determine, for example, whether to steer the aircraft using the control surfaces of the aircraft (e.g. the rudder) or by changing the angle of the nose wheel.

The speed data <NUM> provided to other systems of the aircraft by the speed determination system (i.e. that processed by the filter <NUM>) typically does not provide any indication of the source of the speed data <NUM>. That is, when other aircraft systems receive and use the speed data <NUM>, they receive no indication as to whether the speed data <NUM> was provided based on a speed measurement from the GPS <NUM> or based on a speed measurement from the IRS <NUM> (or some combination of the two).

In some circumstances, it may be useful for an aircraft system to know the source of the speed data <NUM> provided by the speed determination system <NUM>. For example, when the GPS <NUM> is in receipt of signals from an appropriate number of satellites speed measurements determined by the GPS <NUM> are typically more accurate than speed measurements determined by the IRS <NUM>, because the speed measurements determined by the GPS <NUM> are not subject to the same cumulative errors as those of the IRS <NUM>. In an example, it may be useful for a navigation system that uses the speed data <NUM> to know the source of the speed data <NUM> in order to determine an accuracy of navigation data that is generated on the basis of the speed data <NUM>. In another example, it may be useful for a control system that uses the speed data <NUM> to know the source of the speed data <NUM> in order to determine a degree of control authority.

In some embodiments, the monitoring system <NUM> described above with reference to <FIG> and <FIG> may be installed in an aircraft. Referring to <FIG>, there is shown a schematic side view of an example of an aircraft <NUM> according to an embodiment of the invention. The aircraft <NUM> may comprise one or more monitoring systems such as the monitoring system <NUM> described above with reference to <FIG> and <FIG>.

It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claim 1:
A system comprising a monitoring system (<NUM>) for a ground speed determination system (<NUM>) in an aircraft and the ground speed determination system,
the ground speed determination system (<NUM>) comprising a first speed measurement system (<NUM>) and a second speed measurement system (<NUM>), the first speed measurement system (<NUM>) arranged to determine a first time-varying ground speed measurement based on global positioning system data, the second speed measurement system (<NUM>) arranged to determine a second time-varying ground speed measurement based on accelerometer data, the second speed measurement system comprising an integrator and being associated with a predetermined behaviour characteristic comprising an error value that is amplified over time by the integrator, the error value being a first value of a rate of change of acceleration;
the monitoring system (<NUM>) comprising a processor arranged to:
receive ground speed data provided by the ground speed determination system (<NUM>);
perform a correspondence determination process comprising
processing the received ground speed data to determine a second value of a rate of change of acceleration indicated by the received ground speed data, and
determining whether a correspondence condition is satisfied, the correspondence condition comprising a correspondence between the first value and the second value; and
in response to determining that the correspondence condition is satisfied, determine that the first speed measurement system (<NUM>) is in an error condition.