Methods and systems for displaying backup airspeed of an aircraft

The disclosed embodiments relate to an aircraft that includes system for selecting an airspeed reference that is displayed within a cockpit of an aircraft. The system includes a processor and a display located in the cockpit. The processor is configured to determine whether primary airspeed is valid or invalid, and, to select a backup airspeed as the airspeed reference that is output to the display when it is determined that the primary airspeed is invalid. The display is configured to display the backup airspeed as the airspeed reference. The backup airspeed is generated based on Global Position System (GPS) information that is received by the aircraft.

TECHNICAL FIELD

The present invention generally relates to aircraft, and more particularly relates to methods and systems for displaying backup airspeed of an aircraft.

BACKGROUND

When an aircraft is in flight, availability of airspeed data is critical and therefore it is necessary to have systems that can be used to measure and/or provide an indication of airspeed of the aircraft. To measure airspeed data that is needed to determine airspeed, many aircraft employ a pitot-static system.

A pitot-static system generally has a pitot tube, a static port, and pitot-static instruments. The pitot-static system is used to obtain pressures for interpretation by the pitot-static instruments. For example, this equipment measures the forces acting on a vehicle as a function of the temperature, density, pressure, and viscosity of the fluid in which it is operating. For instance, an airspeed indicator (ASI) is connected to both the pitot tube and static port. The difference between the pitot pressure and the static pressure is called “impact pressure.” The greater the impact pressure, the higher the indicated airspeed (IAS) is that will be reported.

Other instruments that might be connected can include air data computers, flight control computers, autopilot systems, flight data recorders, altitude recorders, cabin pressurization controllers, and various airspeed switches. For example, many modern aircraft use an air data computer (ADC) to calculate airspeed, rate of climb, altitude, and Mach number. In some aircraft, two ADCs receive total and static pressure from independent pitot tubes and static ports, and the aircraft's flight data computer compares the information from both computers and checks one against the other.

Failure of Pitot-Static Measurement Equipment

Although pitot-static equipment is normally reliable, in some situations pitot-static systems and apparatus can fail. Information obtained from the pitot static system, such as airspeed or altitude, is often critical to a successful and safe flight. As such, errors in pitot-static system readings (or the absence thereof) can be extremely dangerous.

For example, one type of pitot-static system malfunction occurs when a pitot tube is blocked or clogged for some reason, but the static port remains clear. A blocked pitot tube will cause the airspeed indicator to register a faulty or incorrect airspeed. In some cases, this can result in a reading of zero airspeed.

Another type of pitot-static system malfunction occurs when a static port is blocked. A blocked static port is a more serious situation because it affects all pitot-static instruments. One of the most common causes of a blocked static port is airframe icing. A blocked static port will cause the altimeter to freeze at a constant value, the altitude at which the static port became blocked. The vertical speed indicator will freeze at zero and will not change at all, even if vertical airspeed increases or decreases. The airspeed indicator will reverse the error that occurs with a clogged pitot tube and result in an airspeed that is less than it is actually is as the aircraft climbs. When the aircraft is descending, the airspeed will be over-reported. In most aircraft with unpressurized cabins, an alternative static source is available and toggled from within the cockpit of the airplane.

Inherent errors can affect different pitot-static equipment. For example, density errors affect instruments metering airspeed and altitude. This type of error is caused by variations of pressure and temperature in the atmosphere. Therefore, modern pitot-static systems will automatically correct for temperature and pressure variances from standard atmospheric conditions to ensure accurate airspeed data is presented.

Need for Backup Airspeed Measurement Sources

Many modern aircraft implement redundant pitot-static airspeed measurement equipment that can serve as a backup when the primary pitot-static measurement equipment experiences a fault condition or fails. For example, many large transport category aircraft include three very similar or identical pitot-static systems for redundancy.

While the FAA permits this configuration, one drawback of this approach is that the two redundant pitot-static airspeed measurement systems are susceptible to failing for the same reasons that caused the primary pitot-static measurement system to fault or fail. For instance, all three pitot-static measurement systems can fall prey to a common mode failure (e.g., blockage failure due to contamination by ice, volcano ash, bird strikes and/or pitot heater failure, etc.) and experience a fault or failure at the same time. Unfortunately, in such systems, no other backup airspeed measurement system is available.

There is a need for improved backup/redundant systems and apparatus that can be used to provide airspeed measurements during flight of an aircraft in the event that the pitot-static airspeed measurement equipment experiences a fault or fails.

It would be desirable to provide a backup airspeed measurement source for use in emergencies (e.g., when a partial or complete failure of the primary airspeed measurement occurs). It would also be desirable if such backup airspeed measurement sources are not susceptible to the same modes of failure as the primary and secondary pitot-static system(s). Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.

BRIEF SUMMARY

In one embodiment, a method is provided for selecting an airspeed reference that is displayed on a display in a cockpit of an aircraft. The method comprises: determining whether primary airspeed is invalid at a processor, and when it is determined that the primary airspeed is invalid: selecting, at the processor, a backup airspeed as the airspeed reference that is output to the display, and displaying, at the display, the backup airspeed as the airspeed reference. The backup airspeed is generated based on Global Position System (GPS) information that is received by the aircraft.

In another embodiment, a system is provided for selecting an airspeed reference that is displayed within a cockpit of an aircraft. The system comprises: a processor and a display. The processor is configured to determine whether primary airspeed is valid or invalid, and, to select a backup airspeed as the airspeed reference that is output to the display when it is determined that the primary airspeed is invalid. The display is configured to display the backup airspeed as the airspeed reference. The backup airspeed is generated based on Global Position System (GPS) information that is received by the aircraft.

In another embodiment, an aircraft is provided including a cockpit, at least one display located in the cockpit, and a processor. The processor is configured to determine whether primary airspeed is valid or invalid, and to select a backup airspeed as an airspeed reference that is output to the display when it is determined that the primary airspeed is invalid. The display is configured to display the backup airspeed as the airspeed reference. The backup airspeed is generated based on Global Position System (GPS) information that is received by the aircraft.

DETAILED DESCRIPTION

As used herein, the word “exemplary” means “serving as an example, instance, or illustration.” The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. All of the embodiments described in this Detailed Description are exemplary embodiments provided to enable persons skilled in the art to make or use the invention and not to limit the scope of the invention, which is defined by the claims. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description.

FIG. 1is a perspective view of an aircraft100that can be used in accordance with one non-limiting implementation of the disclosed embodiments. The aircraft100includes two main wings101-1,101-2, a fuselage105, two jet engines111-1,111-2, a vertical stabilizer112, and an elevator109that includes two horizontal stabilizers113-1and113-2in a T-tail stabilizer configuration. The aircraft100also includes, among other things, a Global Position System (GPS) module130that includes a GPS receiver-processor module as will be described below with reference toFIG. 2.

FIG. 2is a block diagram of a system200implemented within an aircraft100for selecting airspeed to be used at the aircraft and displaying the selected airspeed in accordance with an exemplary implementation of the disclosed embodiments. As will be explained below, the airspeed that is selected to be used and displayed can be either a primary airspeed or a backup airspeed when the primary airspeed is invalid (e.g., unavailable or unreliable). As will be explained below, the backup airspeed is derived from GPS information that is received by the aircraft.

The system200includes an onboard air data computer (ADC) system210, a data bus215, a Global Position System (GPS) receiver-processor module230, a GPS antenna235that is communicatively coupled to external GPS satellites240over satellite radio frequency communication links245, other aircraft instrumentation250, cockpit output devices260(e.g., display units262such as control display units, multifunction displays (MFDs), etc., audio elements264, such as speakers, etc.), and input devices270(e.g., including pitot, static and temperature probes).

The onboard air data computer (ADC) system210includes a processor220, and system memory223. It is noted that although the system memory223and the processor220are illustrated as two blocks for purposes of illustration, in some implementations, the system memory223and the processor220may be distributed across several different on-board computers that illustrated collectively as blocks of the on-board ADC system210.

The data bus215serves to transmit data, status and other digital information between the various elements ofFIG. 2. For example, the data bus215is used to carry information communicated between the processor220, the GPS receiver-processor module230, aircraft instrumentation250, and cockpit output devices260. The data bus215can be implemented using any suitable physical or logical means of connecting the onboard ADC system210to at least the external elements mentioned above. This includes, but is not limited to, direct hard-wired connections, fiber optics, and infrared and wireless bus technologies.

The processor220performs the computation and control functions of the onboard ADC system210, and may comprise any type of processor220or multiple processors220, single integrated circuits such as a microprocessor, or any suitable number of integrated circuit devices and/or circuit boards working in cooperation to accomplish the functions of a processing unit.

It should be understood that the system memory223may be a single type of memory component, or it may be composed of many different types of memory components. The system memory223can include non-volatile memory (such as ROM224, flash memory, etc.), memory (such as RAM225), or some combination of the two. The RAM225can be any type of suitable random access memory including the various types of dynamic random access memory (DRAM) such as SDRAM, the various types of static RAM (SRAM). The RAM225includes an operating system226, and programs (e.g., computer processor-executable code or instructions) including an airspeed processor program228.

The other aircraft instrumentation250can include, for example, flight control computers, sensors, transducers, autopilots, elements of an Inertial Reference System (IRS), proximity sensors, switches, relays, video imagers, etc. In general, the IRS is a self-contained navigation system. The IRS can include inertial detectors, such as accelerometers, and rotation sensors (e.g., gyroscopes) to automatically and continuously calculate the aircraft's position, orientation, heading (direction) and velocity (speed of movement) without the need for external references once the IRS has been initialized.

The Air Data Computer (ADC) system210can process inputs supplied from pitot-static system components, such as those described above, to generate a signal that indicates a primary airspeed and altitude of the aircraft. When the primary airspeed signal is valid, the primary airspeed can be displayed on displays262within the cockpit, and provided to various other avionics systems and their processors (e.g., a flight controls system (FCS) processor, an autopilot system processor, a flight management system (FMS) processor, an auto-throttle system processor, traffic alerting system (TCAS), warning systems, flight data recorders (FDR), among others, and used by those avionics systems to perform their various functions. As used herein, the term “primary airspeed” refers to airspeed of an aircraft that is determined by an air data system (ADS) based on information obtained or derived from pitot-static system sensors, temperature probes and barometric correction input devices. Examples of primary airspeed include indicated airspeed (IAS) and calibrated airspeed (CAS), where CAS is the IAS with corrections for errors, such as instrument and installation errors. In one embodiment, the primary airspeed is an indicated airspeed (IAS) that is read directly from an airspeed indicator on an aircraft that is driven by the pitot-static system. In aircraft that operate below transonic or supersonic speeds, IAS is typically displayed as the pilot's primary airspeed reference. IAS uses the difference between total pressure and static pressure, provided by the pitot-static system, to either mechanically or electronically measure dynamic pressure. The dynamic pressure includes terms for both density and airspeed. Since the airspeed indicator cannot know density, it is by design calibrated to assume the sea level standard atmospheric density when calculating airspeed. It is noted that IAS can vary considerably from true airspeed (TAS), which is the relative velocity between the aircraft and the surrounding air mass.

The GPS receiver-processor module230is operatively and communicatively coupled to a GPS antenna235that can be external to the on-board ADC system210. The GPS receiver-processor module230and GPS antenna235can be used to receive information from GPS satellites240over satellite radio frequency links245. The GPS receiver-processor module230can then use the GPS information to determine (e.g., compute or derive) a backup airspeed (or aircraft speed).

As noted above, the RAM225stores computer processor-executable code or instructions of the airspeed processor program228. When the airspeed processor program228is loaded from system memory223and executed at processor220, the processor220can execute an airspeed processor module222to perform various steps of a method that will be described below with reference toFIG. 3. As will be explained below, the processor220executes the airspeed processor program228to select and output either a primary airspeed that is provided from an air data computer (ADC) processor, or a backup airspeed of the aircraft100that is provided from the GPS receiver-processor module230.

The GPS receiver-processor module230is communicatively coupled to at least one GPS antenna235. The GPS receiver-processor module230includes at least one GPS receiver and at least one GPS processor along with other elements of a GPS chipset that are not illustrated. The GPS receiver-processor module230is illustrated as a single block inFIG. 2, but can be implemented as separate microchips at block230. The GPS receiver-processor module230can be used to receive GPS signals from GPS satellites240over radio frequency link245. The GPS signals can include a variety of GPS information.

As will be described below, the GPS information can be processed at the GPS receiver-processor module230to determine an indication of airspeed of the aircraft that is referred to herein a “backup airspeed” since it can be used by the aircraft in the event a primary airspeed of the aircraft becomes invalid (e.g., unreliable or unavailable). In some cases, the primary airspeed can be determined to be invalid when the primary airspeed is determined to be unavailable. For example, primary airspeed can be determined to be unavailable when the air data computer system220losses electrical power, a processor of the air data computer system210or the data bus215fails, etc. In other cases, the primary airspeed can be determined to be invalid when the primary airspeed is determined to be unreliable. For example, the primary airspeed can be determined to be unreliable when components of the pitot static system fail or do not operate correctly. Examples of such situations would include clogging or icing of pitot and/or static probes or sensors, electrical faults (e.g., open or short circuits), physical damage to the pitot and/or static probes or sensors or pneumatic lines, loss of electrical power, etc. Further details that describe backup airspeed will be described below.

The primary airspeed is determined to be valid when systems are operating normally (e.g., monitors and warning systems do not indicate an ADS failure or degraded system status). For example, parameters that contain the airspeed values will include status flags that convey the health of the data (valid, invalid, etc.) to other user systems that can be used to determine when the primary airspeed is valid or invalid. When the primary airspeed is determined to be valid, the airspeed processor module222can then select and output the primary airspeed to at least one of the cockpit output devices260, and can also provide the primary airspeed to other avionics systems, sub-systems or processors.

In accordance with the disclosed embodiments, the GPS signals can include a variety of GPS information including positional data that indicates position of the aircraft, and other information that can be processed by the GPS receiver-processor module230to determine or compute an indication of airspeed of the aircraft that is referred to herein a “backup airspeed.” When the airspeed processor module222determines that the primary airspeed is invalid (e.g., unavailable or unreliable), the airspeed processor module222can then select and output a backup airspeed to at least one of the cockpit output devices260, and can also provide the primary airspeed to other avionics systems, sub-systems or processors. This way, the backup airspeed can be displayed within the aircraft and used by the aircraft's avionics systems in the event a primary airspeed (e.g., indicated airspeed (IAS)) of the aircraft becomes invalid (e.g., unreliable or unavailable).

As used herein, the term “backup airspeed” refers to airspeed of an aircraft that provided from a GPS receiver-processor module that is located on-board the aircraft. The GPS receiver-processor module is typically part of a GPS microprocessor chipset that also includes a GPS processor and a GPS receiver. The GPS receiver can simultaneously monitor a number of channels and receives GPS data from a number of GPS satellites (e.g., between four and twenty GPS satellites). The GPS receiver can receive GPS data from each GPS satellite over one of the channels, and can then provide this GPS data to the GPS processor. Based on the GPS data provided from each of the satellites, the GPS processor can then compute, derive or determine backup airspeed. This GPS data can include GPS positional and/or speed data and/or data that can be processed to compute or derive backup airspeed of the aircraft. Depending on the implementation, the backup airspeed can be determined in a number of different ways.

Backup Airspeed Computation Based on Position Tracking

In some embodiments, a GPS receiver-processor module can receive GPS signal(s) from one or more GPS satellites. Each GPS signal includes GPS positional data along with corresponding time stamps that indicate when that GPS positional data was measured. The GPS receiver-processor module can then use GPS positional data and time stamp information to compute backup airspeed based on distance traveled over a particular time. For example, the GPS receiver-processor module can compute backup airspeed indirectly based on positional data differences and their corresponding time differences (as indicated between successive time stamps).

Backup Airspeed Computation Based on Doppler Frequency Shift

In some embodiments, the GPS receiver-processor module includes a digital phase-locked loop (PLL) that can be used to continuously track carrier frequencies of a number of satellites. Due to relative motion between a GPS satellite and a GPS receiver-processor module, the nominal GPS carrier frequency of the GPS satellite will appear to be “offset” or shifted in frequency (i.e., Doppler shifted) from the frequency of GPS signals received at the GPS receiver-processor module. A Doppler frequency shift refers to a “frequency offset” or difference between a known/nominal GPS satellite carrier frequency (e.g., the L1 carrier frequency of the GPS satellites) and corresponding carrier frequencies of the GPS signals that are received at the GPS receiver-processor module. This Doppler frequency shift is directly proportional to velocity of the GPS receiver-processor module along the direction to the satellite regardless of the distance to this satellite. This can also be referred to as Doppler velocity or instantaneous Doppler-speed since it is the velocity component along a line of sight of the GPS receiver-processor module relative to the satellite. Because Doppler-speed is determined directly from the Doppler-shift of the GPS satellite carrier frequencies, it is completely independent from the positional computations.

Thus, based on the Doppler frequency shift, the GPS receiver-processor module can compute speed of the GPS receiver-processor module that can then be used as a backup airspeed. When the Doppler frequency shifts of signals received from multiple GPS satellites are tracked, the computed “backup airspeed” becomes even more accurate. For example, the GPS receiver-processor module can receive GPS signals from each of a plurality of different GPS satellites, and the GPS receiver-processor module can measure Doppler frequency shift information associated with each of the GPS signals and then use this Doppler frequency shift information to compute speed of the GPS receiver-processor module. In one embodiment, when the Doppler frequency shifts associated with multiple satellites are tracked, a three-dimensional (3D) velocity vector of the GPS receiver-processor module can be determined and used as a backup airspeed.

FIG. 3illustrates a method300for selecting an airspeed reference to be used by avionics systems of an aircraft100and for displaying the selected airspeed reference on a display262in the cockpit of the aircraft100. The method300ofFIG. 3will be described with reference toFIGS. 1 and 2.

Preliminarily it is noted that the aircraft100includes a GPS module130that is located on-board the aircraft100, and, as described above, the GPS module130can include a GPS receiver-processor module230. The GPS receiver-processor module230receives GPS signals transmitted from each of a plurality of GPS satellites240over radio frequency links. Each GPS signal is modulated at a particular carrier frequency and thus includes carrier frequency information associated with that particular GPS signal. In addition, each GPS signal is encoded with GPS positional data and corresponding time stamps that indicate when that GPS positional data was measured. This GPS information that is includes in each GPS signal can be processed by the GPS receiver-processor module230to generate a backup airspeed. For example, in one embodiment, the GPS receiver-processor module230can compute the backup airspeed based on the GPS positional data and corresponding time stamps. In another embodiment, the GPS receiver-processor module230can compute the backup airspeed by determining a frequency offset for each of the GPS signals, and then computing the backup airspeed based on the frequency offsets. As explained above, the GPS receiver-processor module230can determine the frequency offset for each particular one of the GPS signals by determining a difference between the carrier frequency information for that particular one of the GPS signals and a nominal GPS carrier frequency of the GPS satellites that transmitted that particular one of the GPS signals.

Referring again toFIG. 3, method300begins at310when a processor220loads an airspeed processor program228and executes it as an airspeed processor module222. At320, the airspeed processor module222determines whether a primary airspeed is valid or invalid. As explained above, the primary airspeed can be determined to be invalid when the primary airspeed is determined to be unavailable or unreliable for a number of different reasons.

When it is determined (at320) that the primary airspeed is valid, the method300proceeds to330where the airspeed processor module222selects the primary airspeed as the airspeed reference that is to be output, and outputs the primary airspeed to a display262and other output devices264in the cockpit of the aircraft100, as well as to other avionics systems in the aircraft100. The primary airspeed can then be displayed on or at the display262as the airspeed reference, and can be used for various purposes by the other avionics systems in the aircraft100. In addition, in some implementations, an indication can be provided to indicate that the airspeed reference is the primary airspeed so that the pilot or others in the cockpit have an indication that the airspeed reference is the primary airspeed. As described above, the primary airspeed is an airspeed that is determined by an air data system (ADS) processor based on information supplied from a pitot-static system and temperature probes. As also described above, the pitot-static system includes a pitot tube, a static port, and an airspeed indicator (ASI) connected to the pitot tube and static port. In one embodiment, the primary airspeed is an indicated airspeed (IAS) that is read directly from an airspeed indicator (ASI) of the aircraft100. The airspeed indicator (ASI) is driven by the pitot-static system. In another embodiment, the primary airspeed is a calibrated airspeed (CAS). The CAS is a version of the IAS that has been adjusted or corrected to compensate for errors. The ASI generates a signal indicative of the primary airspeed, and so long as the primary airspeed is valid, provides the IAS (or alternatively the CAS) to avionics systems in the aircraft100, as well as one or more aircraft display(s).

By contrast, when it is determined (at320) that the primary airspeed is invalid, method300proceeds to340where the airspeed processor module222selects a backup airspeed as the airspeed reference that is to be output, and outputs the backup airspeed to a display262and other output devices264in the cockpit of the aircraft100, and to other avionics systems in the aircraft100. The backup airspeed can then be displayed on or at the display262as the airspeed reference, and can be used for various purposes by the other avionics systems in the aircraft100. In addition, in some implementations, an indication can be provided to indicate that the airspeed reference is the backup airspeed so that the pilot or others in the cockpit have an indication that the airspeed reference is the backup airspeed. This indication can be any visual, audible or textual indication that indicates that the airspeed reference has a backup status.

FIG. 4is a block diagram that illustrates one non-limiting, exemplary implementation of a system for providing backup airspeed as an airspeed reference. The system ofFIG. 4will be described with reference toFIG. 2to the extent thatFIG. 4uses reference numerals that correspond toFIG. 2.

The GPS receiver-processor module230provides a backup airspeed that is derived from GPS information from GPS signals, and is referred to inFIG. 4as GPS-derived airspeed (GPSDAS).

Backup airspeed function222-1represents a portion of computer executable code from the airspeed processor program228that is executing at processor220as the airspeed processor module222when it has been determined (at320) that a primary airspeed is invalid.

As illustrated inFIG. 4, the backup airspeed function222-1outputs backup airspeed to graphics generator module228that generates signals that communicate the backup airspeed as an airspeed reference to various displays262-1,262-2,262-3.

In one implementation, display262-1can be, for example, a Primary Flight Display (PFD) and can display the backup airspeed in appropriate units to the pilot or others in the cockpit of the aircraft. Display262-2can be a Multi-Function Display (MFD) and can display a visual indication (e.g., a warning light or a flashing message) that provides a signal to the pilot that the airspeed reference displayed is a backup airspeed, and not the primary airspeed so that the pilot or others in the cockpit have an alert or are notified that the airspeed reference is the backup airspeed. Display262-3can be the equivalent of any of the two above as a redundant display that provides redundant and simultaneous awareness that the airspeed reference displayed is a backup airspeed, and not the primary airspeed so that the pilot or others in the cockpit have another indication that the airspeed reference is the backup airspeed. The various indications provided on displays262-2,262-3provide an alert to the pilot that that the airspeed reference has a backup status.

As is also illustrated inFIG. 4, the backup airspeed function222-1outputs the backup airspeed to various avionics system processors224-227. In this non-limiting example that is illustrated inFIG. 4, the avionics system processors224-227include an air data system (ADS) processor224, an autopilot system processor225, a flight management system (FMS) processor226, and an auto-throttle system processor227; however, it will be appreciated that the backup airspeed can be provided to other avionics system processors that are not illustrated, but that also process airspeed as an input. Although not illustrated inFIG. 4, the backup airspeed function222-1can also output the backup airspeed to other output devices264in the cockpit of the aircraft100.

FIG. 5illustrates a logic diagram of a system for selecting an airspeed reference to be displayed. The logic diagram ofFIG. 5will be described with reference toFIGS. 1 and 2. Further it is noted that although the logic that is illustrated viaFIG. 5can be implemented by executing software that comprises processor-executable instructions at a processor, such as by a processor220that loads an airspeed processor program228and executes it as an airspeed processor module222, in other implementations, the logic can be executed using semiconductor logic gates that are implemented as a standalone processing unit.

The system includes a two-input AND gate520, a three-input AND gate530and a two-input OR gate540. This is one exemplary implementation and other equivalent logic schemes can be implemented that provide equivalent logical processing.

In the logic diagram ofFIG. 5, four inputs are received. The inputs include a primary airspeed input502that indicates the primary airspeed, a first validity input504that indicates whether the primary indicated airspeed input502is valid, a backup airspeed input506that indicates the backup airspeed, and a second validity input508that indicates whether the backup airspeed input506is valid. As explained above, the primary airspeed502can be invalid when the primary airspeed is determined to be unavailable or unreliable for a number of different reasons, but otherwise should be valid. When this happens status flags will be set to indicate that the primary indicated airspeed input502is invalid. The backup airspeed input506can become invalid, for example, when the system does not receive enough (or any) data from GPS signals from the satellites to compute position and/or speed. When this happens status flags will be set to indicate that the backup airspeed input506is invalid.

The two-input AND gate520receives the primary airspeed input502and the first validity input504, and performs a logical AND operation on those inputs to generate a first output. The three-input AND gate530receives an inverted version of the first validity input504, the backup airspeed input506, and the second validity input508, and performs a logical AND operation on those inputs to generate a second output. The second output is used to control a second display element560that indicates whether or not the airspeed reference displayed at the first display element550has a primary or backup status.

The two-input OR gate540receives the first output and the second output, and generates an airspeed reference output that is displayed at a first display element550as an airspeed reference. The airspeed reference output is determined based on the first output or the second output, whichever is active.

For example, when the first validity input504indicates that the primary airspeed is valid, then the primary airspeed input502will be selected and displayed (as the airspeed reference) at the first display element550, and the second display element560will not be activated. When the first validity input504indicates that the primary airspeed is invalid, then the backup airspeed input506will be selected and displayed as the airspeed reference at the first display element550, and the second display element560will be activated. As noted above, airspeed reference that is output from two-input OR gate540can also be provided to other avionics systems in the aircraft100, and can be used for various purposes by the other avionics systems in the aircraft100.

As such, the second display element560can be used to indicate whether the airspeed reference is the primary airspeed or the backup airspeed so that those in the cockpit have some indication of whether the airspeed reference is the primary airspeed or a backup airspeed. This way, the pilot or others in the cockpit will have an indication (visual, audible or textual) to notify them when the airspeed reference that is being displayed is the backup airspeed and has a backup status.

One of the benefits of the disclosed embodiments is that they can be used to acquire and provide an indication of backup airspeed when pitot-static measurement devices are unavailable or unreliable. In one implementation, the systems and methods in accordance with the disclosed embodiments can be employed in an aircraft as a secondary or backup airspeed measurement source for use in emergency situations when primary pitot-static airspeed measurement systems experience a partial or complete failure. For example, in the event pitot sensors fail due to blockage or other reasons, the GPS system can be used to provide backup airspeed data. The use of GPS to provide backup airspeed is not subject to the same failure modes that the primary pitot-static airspeed measurement systems are subject to (e.g. a blocked pitot tube or pitot heater failure) since they do not rely on data from pitot-static probes.