Navigation device and method using asynchronous pre-integrated data in remote IMU

Abstract

The invention relates to a navigation device (1) comprising an inertial measurement unit (100) and an electronic navigation computing unit (200) interconnected by a data link (300), the inertial measurement unit (100) comprising an inertial sensor (110, 120) providing a first signal comprising first data representative of a change in speed and second data representative of a change in angle, and the electronic navigation calculation unit (200) is arranged to calculate navigation from the signal provided by the inertial measurement unit (100), characterized in that the inertial measurement unit (100) comprises an electronic processing circuit (130), the electronic processing circuit (130) is connected to the inertial sensor (110, 120) and is arranged to time-dependent at least one first integration of the first data and the second data over an integration period (the period begins with a single integration start time and is measured), to generate first processed data and second processed data included in the second signal together with time information representing the integration time; and the electronic navigation calculation unit (200) is arranged to extract the processed data and the time information from the second signal and to use them to calculate navigation while taking into account two successive extracts of separate integral times.

Description

[0001] The present invention relates to the field of inertial measurement units, and in particular to an inertial navigation system for performing navigation based on measurements provided by at least one inertial measurement unit. Background Art

[0002] Inertial navigation systems are well known. Figure 6 An example is shown at 1000 in FIG, where the same housing contains an inertial measurement unit 1100 connected to an electronic navigation computation unit 1200 via a data link.

[0003] The inertial measurement unit 1100 includes accelerometers and angle sensors arranged along the axes of the measurement reference system [m] to provide a specific force vector and an angular velocity vector relative to the inertial reference system [i] at a time step (from time t i-1 To time t i ). Therefore, the consecutive signals represent the integration of the specific force vector and the angular velocity vector from time t0 to t1, t1 to t2, t2 to t3, and so on, and these signals are therefore usually called increments.

[0004] Specific force ("gravity" or "mass specific force") represents the sum of acceleration and the Earth's gravity relative to an inertial reference frame.

[0005] Electronic navigation computation unit 1200 includes a processor and memory, with a navigation computer program stored in the memory. This navigation computer program, executed by the processor, processes the signals provided by inertial measurement unit 1100 to determine the trajectory of the vehicle (vehicle) carrying the navigation system. Because the signals provided by inertial measurement unit 1100 are incremental, rather than absolute, signals indicating changes in the vehicle's position, navigation computations must be performed at a high frequency, typically 50 to 200 Hz, to ensure accurate position reconstruction in a manner that is insensitive to vehicle dynamics. Clock 1001 allows synchronization of inertial measurement unit 1100 and electronic navigation computation unit 1200.

[0006] It is understood that even a brief loss of signal between the inertial measurement unit and the electronic navigation computation unit can be very detrimental because some of the incremental data is not used.

[0007] However, in aircraft, it is envisioned that the computers, including the navigation system's electronic computing units, may be centrally located in one or more avionics bays at the same location on the aircraft. As for the inertial measurement unit, its location should preferably be as close as possible to the aircraft's center of gravity. In this case, however, the inertial measurement unit would be connected to the electronic computing units located in the avionics bays via an Ethernet link (e.g., compliant with the ARINC 664 standard).

[0008] Any disturbance in signal transmission (such as a lightning strike) may result in data loss and thus be detrimental to navigation accuracy.

[0009] To mitigate this risk, fiber optic links can be used, but this is costly. Alternatively, specific data exchange protocols can be implemented to ensure no data loss through redundant data transmission. However, this can lead to link overload and increased computational load.

[0010] Purpose of the Invention

[0011] The present invention aims in particular to overcome at least some of the above-mentioned disadvantages. Summary of the Invention

[0012] To this end, a navigation device is provided, comprising an inertial measurement unit and an electronic navigation calculation unit interconnected via a data link. The inertial measurement unit includes an inertial sensor that provides a first signal containing first data representing a change in linear velocity and second data representing a change in angular attitude. The electronic navigation calculation unit is configured to calculate navigation based on the signal provided by the inertial measurement unit.

[0013] According to the invention, the inertial measurement unit comprises an electronic processing circuit connected to the inertial sensor and arranged to perform at least one first integration of the first and second data as a function of time over an integration period (the period being measured from a single integration start time) to generate first and second processed data included in a second signal together with time information representative of the integration period. The electronic calculation unit is arranged to extract the processed data and the time information from the second signal and to process them to calculate navigation, taking into account the integration period between two consecutive extractions.

[0014] Therefore, instead of transmitting the first signal (the increment generated by the inertial sensor) to the electronic navigation calculation unit at a high frequency as in the prior art, the integrated values ​​over an unbounded integration period starting from a single initialization time (the same for all first data, for example, corresponding to system startup or receipt of an integration start command) are transmitted, and these integrated values ​​can be transmitted at the same frequency or a lower frequency. It can be understood that the second signals sequentially transmitted by the electronic processing circuit represent the integration from time t0 to time t1, from time t0 to time t2, from time t0 to time t3, and so on. This way, no matter at which point the second signal is received, it represents the integration starting from the single integration start time (i.e., t0 in the above example). This transmission method has fewer constraints and a limited risk of transmission errors. Therefore, processing the data to form the transmitted second signal improves the reliability of both data transmission and navigation calculations based on that data. This transmission method is advantageous not only over short distances but also over relatively long distances (several meters).

[0015] According to a particular feature of the invention, the electronic processing circuit (130) is arranged to compare the first integrated data with at least one first threshold value and, when a current value of the first integrated data is above the first threshold value, to apply a first predetermined offset to the first integrated data so as to bring the first integrated data back to an offset value below the first threshold value.

[0016] Preferably, the electronic processing circuit is arranged to perform two consecutive integrations of the first data and, when a current value of the first doubly integrated data exceeds a second threshold, apply a second predetermined offset to the first doubly integrated data so as to bring the first doubly integrated data back to an offset value below the second threshold.

[0017] Thus, according to a particular embodiment, the electronic navigation calculation unit is arranged to, upon each receipt of data from said inertial measurement unit:

[0018] ● Collect the inertial posture at the current receiving moment and store the inertial posture at the previous receiving moment,

[0019] reconstructing the variation of the inertial pseudo-velocity in said inertial frame between two receptions, said variation having been corrected for the effect of the first offset,

[0020] Calculating a compensation term from the position evolution in the inertial coordinate system corrected for the influence of the second offset, the compensation term being used to compensate for the fact that the acceleration in the inertial coordinate system is not constant during the time period between two receptions;

[0021] Calculating the evolution in time between the current reception and the previous reception.

[0022] Preferably, therefore, the electronic navigation computation unit is arranged to, upon each receipt of data from said inertial measurement unit:

[0023] - The position evolution since the last reception is calculated according to:

[0024] The currently received inertial attitude, the previously received inertial attitude, the pseudo-velocity change, the compensation term used to compensate for the fact that the acceleration in the inertial reference frame is not constant during the time between two receptions, and the evolution in time;

[0025] The last calculated position;

[0026] Assuming that the apparent acceleration remains constant in the navigation reference frame during the time between receptions, the navigation algorithm provides a position that integrates attitude, velocity, and information about the Earth's geographic position.

[0027] The present application also relates to a method for navigation by means of a navigation device comprising an inertial measurement unit and an electronic navigation calculation unit connected to each other via a data link. The method comprises the following steps:

[0028] -In the inertial measurement unit:

[0029] measuring the linear velocity change and the angular attitude change by means of an inertial sensor, said inertial sensor providing a first signal comprising first data representative of the linear velocity change and second data representative of the angular attitude change;

[0030] performing processing including performing at least a first integration of the first data and the second data within an integration period starting from a single integration time;

[0031] • generating first processed data and second processed data included in the second signal together with time information representing the integration period; and

[0032] - In electronic computing units:

[0033] - extracting processed data and time information from the second signal,

[0034] • It is processed to calculate the navigation, taking into account the integration period between two consecutive extractions.

[0035] Finally, the invention relates to a vehicle having a navigation device according to the invention.

[0036] Other characteristics and advantages of the invention will become clear on reading the following description of particular and non-limiting embodiments of the invention.

[0037] BRIEF DESCRIPTION OF THE DRAWINGS

[0038] With reference to the accompanying drawings, in which:

[0039] [ Figure 1 ] Figure 1 is a schematic partial view of an aircraft having a navigation device according to the invention;

[0040] [ Figure 2 ] Figure 2 is a schematic diagram of an apparatus according to the present invention;

[0041] [ Figure 3 ] Figure 3 is a flow chart illustrating data exchange when executing the method according to the present invention;

[0042] [ Figure 4 ] Figure 4 is a flow chart showing the implementation of the method according to the present invention on the inertial measurement unit side;

[0043] [ Figure 5 ] Figure 5 is a flow chart showing the implementation of the method of the present invention on the electronic computing unit side;

[0044] [ Figure 6 ] Figure 6 is a flow chart illustrating data exchange in a navigation device according to the prior art. DETAILED DESCRIPTION

[0045] With reference to the accompanying drawings, the present invention will be described below in an aviation application scenario. The navigation device of the present invention is mounted on an aircraft A, which has a structure including a fuselage and wings and has a center of gravity G.

[0046] The navigation device of the present invention (generally designated 1) comprises an inertial measurement unit 100 and an electronic navigation calculation unit 200 interconnected via a data link 300. Here, the inertial measurement unit 100 is located approximately at the center of gravity G of aircraft A, while the electronic navigation calculation unit 200 is located at the front of aircraft A, within an avionics bay B, which houses a computer that processes data used to pilot aircraft A. Thus, the inertial measurement unit 100 is positioned at a first distance from the center of gravity G, while the electronic navigation calculation unit 200 is positioned at a second distance from the center of gravity. In this case, the first distance is less than the second distance. Here, the difference between the first and second distances is several meters.

[0047] The inertial measurement unit 100 comprises a first housing 101, which houses inertial sensors, namely linear inertial sensors (more specifically accelerometers 110) arranged along the axes of a measurement reference frame [m] for measuring the "gravity velocity" of that reference frame (i.e., the time integral of the specific force at the center of the reference frame); and angular inertial sensors (here, gyroscopes 120) arranged along the axes of said reference frame for measuring the rotation of the measurement reference frame [m] relative to the inertial reference frame [i]. Inertial sensors do not provide absolute values, but rather increments representing the change in the measured quantity compared to the previous measurement. The inertial reference frame [i] is, for example, the measurement reference frame when the inertial measurement unit 100 is powered on, or any other inertial reference frame angularly offset relative to it. Thus, the increment in the specific force integral represents the change in the components of the gravity velocity along the three axes of the reference frame [m]. The rotation increment represents the time-integrated change in the angular rotational velocity of the measurement reference frame [m] relative to the inertial reference frame [i] and is provided in the form of a quaternion, Euler angles, rotation matrix, or Boltz vector. Thus, the inertial sensor provides a first signal containing first data representing the change in the velocity due to gravity (accelerometer measurement) and second data representing the change in angle (gyroscope measurement). Typically, these signals are provided at a rate between 100 Hz and 400 Hz.

[0048] First housing 101 is housed within second housing 102 of inertial measurement unit 100. Second housing 102 also houses electronic processing circuitry 130, whose inputs are connected to the outputs of inertial sensors 110 and 120 via electrical conductors (e.g., traces or cables). Electronic processing circuitry 130 comprises at least one processor and a memory containing a first computer program executable by the processor, the program comprising instructions configured to perform the method of the present invention. This first program will be further described below.

[0049] Electronic navigation computing unit 200 is known per se and comprises a housing 201 housing at least one processor and a memory containing a second computer program executable by the processor, the second computer program comprising instructions arranged to perform the method of the present invention. Typically, electronic navigation computing unit 200 is arranged to calculate inertial navigation based on signals provided by inertial measurement unit 100. This second computer program will also be described further below.

[0050] The inertial measurement unit 100 and the electronic navigation calculation unit 200 are each equipped with a clock, which can time-stamp the transmitted signals on the one hand and time-stamp the reception time on the other hand.

[0051] Inertial measurement unit 100 and electronic navigation calculation unit 200 are physically separated from each other but are connected to exchange signals. Thus, at least one output of electronic processing circuit 130 is connected to at least one input of electronic navigation calculation unit 200 via data link 300. Data link 300 is, for example, an Ethernet link conforming to the ARINC 664 standard.

[0052] The first program executed by the electronic processing circuit 130 receives as input a first signal comprising first data and second data. It is arranged to perform:

[0053] - During the integration period measured from the single integration start time t0 (the integration period is Figure 3 and Figure 4 Indicated as t e -t0), performing a first integration on the second data to generate second integrated data;

[0054] - projecting the first data into the inertial reference frame [i] to obtain first projected data;

[0055] - performing a first integration on the first projected data within an integration period measured from a single integration start time t0 to generate first integrated data;

[0056] - performing a first shift (Shift V) on the first integrated data to obtain first processed data (the first shift is performed when the value of the first integrated data exceeds a value range acceptable for subsequent processing of these data);

[0057] - performing a second integration on the first processed data within an integration period measured from the single integration start time t0 to generate first double-integrated data;

[0058] - performing a second shift (Shift P) on the first doubly integrated data to obtain first doubly processed data (the second shift is performed when the value of the first doubly integrated data exceeds a value range acceptable for subsequent processing of these data).

[0059] The first program executed by the electronic processing circuit is arranged to calculate the floating point or fixed point integral by using a number of mantissa bits that enables the required position accuracy to be achieved with a minimum time between two consecutive offsets of 20 seconds. 64-bit double precision calculations (of which 48 bits are mantissa bits) enable a precision of, for example, 0.001 ms to be achieved. -1 , 0.001m, 0.001 / h and 1μrad accuracy targets.

[0060] It is understandable that:

[0061] - the second integrated data represents an angular position (or orientation);

[0062] - the first integrated data and the first processed data represent linear velocity;

[0063] - The first doubly integrated data and the first doubly processed data represent line positions.

[0064] The first offset operation includes a step of comparing the absolute current value of each component of the first integrated data with at least one first threshold value, SVelocity. When the absolute current value of a component of the first integrated data is below the first threshold value, the first procedure maintains the current value unchanged, which is equivalent to applying a zero offset. When the absolute current value of a component of the first integrated data is above the first threshold value, the first procedure applies a predetermined offset to that component of the first integrated data to bring that component of the first integrated data to an offset value below the first threshold value. In other words, a first offset value SHV (here equal to the first threshold value SVelocity) is subtracted from (or added to, depending on the sign of) the current value of that component of the first integrated data to obtain the offset value. This ensures that the value of each component of the first data remains within the value range [-Svelocity; +SVelocity]. The first threshold value, Svelocity, is determined based on the desired velocity resolution required for navigation. The offset-free integration period is obviously dependent on the dynamics of aircraft A. Here, the first integrated data and the first offset value SHV are expressed in meters per second.

[0065] The second offset operation includes comparing the absolute current value of the first doubly integrated data with at least one second threshold value, SPosition. When the absolute current value of a component of the first doubly integrated data is below the first threshold value, the first procedure maintains the current value unchanged, which is equivalent to applying a zero offset. When the value of a component of the first doubly integrated data exceeds the second threshold value, the first procedure applies a predetermined offset to that component of the first doubly integrated data to bring that component of the first doubly integrated data to an offset value below the second threshold value. In other words, the second offset value SHP (here equal to the second threshold value SPosition) is subtracted from the current value of the first doubly integrated data (or added to it, depending on the sign of the current value of the component) to obtain the offset value. This ensures that the value of each component of the first doubly integrated data remains within the value range [-Sposition; +SPosition]. The second threshold value Sposition is determined based on the desired position resolution required for navigation. The offset-free integration period depends on the dynamics of aircraft A and the threshold value SPosition. Here, the first doubly integrated data and the second offset value SHP are expressed in meters.

[0066] First processed data (corresponding to three velocity components, including the integral of the gravity effect, which may be affected by an offset and is therefore called pseudo-velocity PV or inertial pseudo-velocity PVI), first double processed data (corresponding to three position components, including the double integral of the gravity effect, which may be affected by one offset (velocity or position) or two offsets (velocity and position), and is therefore called pseudo-position PP), second processed data (corresponding to three components of the rotation representing the attitude of the aircraft A), and time information (integration step or time step counter of the inertial measurement unit between the sampling time and the single integration start time, here t e -t0) is converted into a packet which is incorporated into a second signal transmitted via the data link 300 to the electronic navigation calculation unit 200.

[0067] The second program executed by the electronic navigation calculation unit 200 is arranged to extract processed data and time information from the second signal and process it to calculate navigation taking into account the evolution of the integration period since the last extraction of the second signal.

[0068] More precisely, refer to Figure 5 The second program receives as input the second signal in the form of two data packets, which are sent by the first program at each t e The inertial measurement unit transmits the first processed data, the first doubly processed data, the second processed data and the time information corresponding to the sampling time of the pseudo navigation data in the inertial measurement unit, together with any internal drift of the clock of the inertial measurement unit (the time information is the number of integration steps of the inertial measurement unit since the single integration start time t0, associated with the data packet transmitted by the inertial measurement unit).

[0069] The interval between the two instants t1 and t2 is less than half the minimum time between two consecutive excursions of a component of the inertial pseudo-velocity or pseudo-position.

[0070] To simplify the description, assume that the data received at time t1 has been processed. The second program needs to perform operations based on the data received at time t2 and calculate the following:

[0071] - three components of the change in inertial pseudo-velocity from t1 to t2 in the measurement reference frame [m], denoted DVm(t1->t2), which have been corrected for the effects of the offset SHV;

[0072] - three components of the change in inertial pseudo-velocity from t1 to t2 in the inertial reference frame [i], denoted DVi(t1->t2), which have been corrected for the effects of the offset SHV;

[0073] - three components of the change in the inertial pseudo-position from t1 to t2 in the inertial reference frame [i], denoted DP(t1->t2), which have been corrected for the effects of the offset SHP;

[0074] - the three correction components of the inertial pseudo-position from t1 to t2 in the inertial reference frame [i], denoted as CorrPPi(t1->t2),

[0075] - a logical indicator ShiftPVdetected for detecting a shift of at least one pseudo velocity component between t1 and t2.

[0076] Note that both the data received at time t1 and the data received at time t2 are integrated starting from the integration start time t0. Therefore, the data corresponding to the time interval t2 - t1 can be obtained by subtracting the data received at time t1 from the data received at time t2.

[0077] In addition, the second program has an offset value and is configured to analyze the values ​​of the transmitted processed data to detect whether an offset exists. This analysis involves comparing each component of the most recently received processed data with each component of the previously received processed data, and detecting any inconsistencies therein, taking into account the potential dynamics and physical laws of the vehicle. If any such inconsistency exists, it indicates that an offset has occurred, and the second program sets the indicator ShiftPVdetected to true and compensates for the applied offset using the corresponding offset value. Otherwise, the second program sets the indicator ShiftPVdetected to false.

[0078] The second program calculates the inertial position evolution from t1 to t2 based on the following information:

[0079] - inertial attitude at times t1 and t2 (transmitted by the electronic processing circuit 130),

[0080] - the change in inertial pseudo-velocity in the inertial reference frame from t1 to t2,

[0081] - duration t2-t1,

[0082] - Position at time t1.

[0083] In a manner known per se, the second program is also arranged to process the following information:

[0084] - the curvature matrix MC, used to determine the local curvature of the navigation reference ellipsoid relative to the Earth,

[0085] - local apparent gravity GravApp (locally perpendicular to the ellipsoid),

[0086] - External correction factor Corr ext, used to correct the calculation of apparent gravity by stabilization altitude.

[0087] The second procedure makes these calculations of the evolution of the inertial position from t1 to t2 using the assumption H1, i.e. the apparent acceleration γ in the navigation reference frame p Remains constant from t1 to t2.

[0088] These calculations are then corrected by taking into account the three correction components of the inertial position from t1 to t2 in the inertial reference frame [i].

[0089] The three correction components CorrPPi(t1->t2) of the inertial position from t1 to t2 in the inertial reference frame [i] are calculated as follows:

[0090] - If ShiftPVdetected is false (no velocity shift), then

[0091] CorrPPi(t1->t2)=DP(t1->t2)-(PV(t1)

[0092] +PV(t2))*(t1-t2) / 2

[0093] -If ShiftPVdetected is true (a velocity shift occurred between t1 and t2), the value of CorrPPI cannot be calculated and is therefore arbitrarily set to 0, i.e.

[0094] CorrPPi(t1->t2)=0

[0095] The calculation is based on the assumption H2, that is, the acceleration f in the inertial reference frame [i] [i] is constant. It is assumed that the difference between H1 and H2 is mainly due to the apparent gravitational rotation observed in the inertial reference frame [i] from the navigation reference frame p. For horizontal motorized navigation on the Earth's free orientation reference ellipsoid, this difference has a negligible effect on the calculated positioning bias correction term CorrPPi(t1->t2). It is understood that the indicator ShiftPVdetected is determined in order to refine the assessment of the change in the inertial position error according to the underlying dynamics of the vehicle at that time.

[0096] The second procedure corrects the inertial position using the deviation CorrPi calculated as follows:

[0097] - Project the components of CorrPPi(t1->t2) in the inertial reference frame [i] to the navigation reference frame [p] using the following terms to obtain the correction components CorrPPp(t1->t2):

[0098] The inertial attitude transmitted by the electronic processing circuit 130 at time t1 (and possibly at time t2),

[0099] ●Navigation posture at time t1;

[0100] - Calculate the equivalent rotation correction of the attitude and calculate the horizontal position on the Earth by using the apparent gravity (GravApp) and the local curvature matrix MC(2x2) of the Earth ellipsoid in the horizontal navigation reference frame [p] and the two horizontal correction components CorrPPp(t1->t2);

[0101] - take this additional rotation into account when calculating the evolution of the inertial position from t1 to t2;

[0102] - Add the vertical correction component CorrPPp(t1->t2) to the altitude at time t2, calculated as the evolution of the inertial position from t1 to t2, to obtain CorrPi.

[0103] A second program then uses these corrected data by means of a navigation algorithm known per se to provide the position of the aircraft A (this position integrating information on attitude, speed and geographical position of the Earth).

[0104] It should be noted that, preferably, the duty cycle of the electronic navigation unit is less than half of the minimum time between two consecutive offsets.

[0105] The invention relates to the integration of these increments by an inertial measurement unit in an inertial reference frame (within the range of sensor failures) in order to enable navigation calculations to be performed more frequently and asynchronously on the Earth reference ellipsoid. The use of the output of the inertial measurement unit by a second procedure is based on the knowledge and processing of the inertial pseudo-velocities and pseudo-position offsets before calculating the evolution of the inertial position in the event of a possible offset.

[0106] It goes without saying that the invention is not limited to the embodiments described, but covers any variant coming within the ambit of the invention as defined by the claims.

[0107] In particular, the structure of the device may differ from that described.

[0108] For example, the structure of the electronic processing circuit and the electronic navigation calculation unit may be different from that described and may include, for example, a co-processor, a dedicated ASIC type processor, a microcontroller, an FPGA type programmable circuit, etc.

[0109] While the present invention is particularly advantageous when the inertial measurement unit and the electronic processing unit are far apart (e.g., several meters apart), the present invention is also applicable when the inertial measurement unit and the electronic processing unit are relatively close (e.g., less than one meter apart). The second distance is not limited by the first distance; depending on the needs and the configuration of the vehicle, the second distance can be less than, greater than, or equal to the first distance.

[0110] Computer programs can be configured differently and perform calculations at different levels of precision.

[0111] When the integration period is such that the values ​​of the processed data provided are compatible with the expected resolution required for navigation, no offset operation is necessary.

[0112] The output of the electronic processing circuit may be provided at a fixed rate (possibly configurable via circuit initialization commands) or on external request; in the latter case, the program making the request needs to ensure that the request rate is sufficient to avoid the risk of the same data being offset multiple times between two data supplies.

[0113] Preferably, the attitude output (gyro measurement data), velocity output, and position output (from accelerometer data) are all limited to 22 or 24 bits per output, with sufficient resolution to ensure that the degradation in navigation accuracy using the method of the present invention is negligible compared to the accuracy of navigation based directly on sensor increments. The described offset operation allows the number of bits of the velocity and position outputs to be reduced.

[0114] The output of the electronic processing circuitry enables the inertial navigation system to:

[0115] - synchronous or asynchronous;

[0116] - Accurate even when output is processed at rates exceeding several seconds and for dynamic trajectories;

[0117] - Robust to multiple interruptions that may last up to several seconds, and also robust to interruptions of more than one minute when static;

[0118] - No overload of the electronic navigation computing unit in the event of data loss.

[0119] Preferably, the first program executed by the electronic processing circuit has an initialization mode, through which all or part of the following parameters can be modified:

[0120] - Orientation of the measurement reference system;

[0121] - Clock-controlled output or on-demand output;

[0122] - output rate;

[0123] - Type of velocity output (in the measurement reference frame or in the inertial reference frame);

[0124] - offset threshold;

[0125] -Offset value;

[0126] - Scale factor bias or error, etc.

[0127] Preferably, in order to minimize projection errors in the navigation reference system, inertial or horizontal free bearing mechanized navigation on the Earth ellipsoid (and the corresponding navigation reference system) is used. However, this is not required.

[0128] It is preferred to ignore t0 to t e The effects of gravity changes, local curvature of the ellipsoid, and Coriolis acceleration during this period.

[0129] In the case that multiple shifts have to be performed after a single integration, it is necessary to add a shift counter to the data packet so that the electronic calculation unit can retrieve the number of shifts that have been performed.

[0130] The time between two offsets may vary from a few seconds to tens of seconds depending on the dynamics of the vehicle carrying the navigation device.

[0131] Preferably, to maintain good navigation accuracy, the number of offsets per hour is limited. For example, for a 24-bit encoded output, the maximum number of offsets per 400-second period is advantageously 3; for a 32-bit encoded output, the maximum number of offsets per 28-hour period is advantageously 3.

[0132] The device may comprise one or more inertial measurement units, which may be arranged close to or far from each other at any point of the aircraft, and in particular not necessarily close to the centre of gravity.

[0133] In the basic form of navigation, the deltas are reconstructed at the normal navigation rate by taking the difference between two consecutive sets of output data from the inertial measurement unit. Inertial navigation can be maintained at this rate, just as in normal navigation. In this case, the correction vector CorrPPi is systematically zero.

[0134] The present invention is applicable to any type of vehicle, whether land, water or air.

Claims

1. A navigation device (1), comprising an inertial measurement unit (100) and an electronic navigation calculation unit (200) connected to each other via a data link (300), the inertial measurement unit (100) comprising an inertial sensor (110, 120) providing a first signal, the first signal comprising first data representing a change in speed and second data representing a change in angle, and the electronic navigation calculation unit (200) being arranged to calculate navigation based on the signal provided by the inertial measurement unit (100), characterized in that: The inertial measurement unit (100) comprises an electronic processing circuit (130) connected to the inertial sensors (110, 120) and arranged to perform at least a first integration of the first data and the second data as a function of time over an integration period to generate first processed data and second processed data included in a second signal together with time information representative of the integration period, the integration period starting at a single integration start time and being measured; and the electronic navigation calculation unit (200) is arranged to extract the processed data and the time information from the second signal and process them to calculate navigation while taking into account the integration period between two consecutive extractions.

2. The device according to claim 1, wherein The electronic processing circuit (130) is arranged to perform a second integration on a result of a first integration of the first data, the processed data comprising a result of the first integration and a result of the second integration.

3. The device according to any of the preceding claims, characterized in that The electronic processing circuit (130) is arranged to compare the first integrated data with at least one first threshold value and, when a current value of the first integrated data is above the first threshold value, apply a first predetermined offset to the first integrated data so as to bring the first integrated data back to an offset value below the first threshold value.

4. The device according to claim 3, characterized in that The electronic processing circuit (130) is arranged to perform two consecutive integrations of the first data and, when a current value of the first doubly integrated data exceeds a second threshold, apply a second predetermined offset to the first doubly integrated data so as to bring the first doubly integrated data back to an offset value below the second threshold.

5. The device according to claim 4, characterized in that The electronic navigation computation unit (200) is arranged to, upon receipt of data from the inertial measurement unit (100): Collect the inertial posture at the current receiving moment and store the inertial posture at the previous receiving moment, reconstructing the variation of the inertial pseudo-velocity in said inertial frame between two receptions, said variation having been corrected for the effect of said first offset, Calculating a compensation term from the evolution of the position in the inertial reference frame corrected for the influence of the second offset, the compensation term being used to compensate for the acceleration (f [i] ) is not constant; · Compute the temporal evolution between the current reception and the previous reception.

6. The device according to claim 5, characterized in that The electronic navigation computation unit (200) is arranged to, upon receipt of data from the inertial measurement unit (100): - The position evolution since the last reception is calculated according to: The currently received inertial attitude, the previously received inertial attitude, the pseudo-velocity change, the acceleration (f [i] ) is non-constant and evolves in time; The last calculated position; Under the assumption that the apparent acceleration remains constant in the navigation reference frame during the time between two receptions, a position is provided by a navigation algorithm known per se, said position integrating information on attitude, velocity and geographical position of the Earth.

7. The device according to any one of claims 4 to 6, characterized in that The first threshold and the second threshold are determined depending on an expected resolution of navigation.

8. The device according to any one of claims 3 to 7, characterized in that The duty cycle of the electronic navigation unit is less than half of the minimum time between two consecutive offsets.

9. The device according to any of the preceding claims, characterized in that The electronic processing circuit (130) is arranged to transmit the second signal at the request of the electronic navigation computation unit.

10. The device according to any one of claims 1 to 8, characterized in that The electronic processing circuit (130) is arranged to transmit the second signal at predetermined time intervals.

11. Apparatus according to any preceding claim, characterised in that The electronic processing circuit (130) is arranged to calculate a 64-bit double precision floating point integral, of which 48 bits are mantissa bits.

12. Apparatus according to any preceding claim, characterised in that The inertial measurement unit (100) and the electronic navigation calculation unit (200) are located in two separate housings, and the data link (300) is of Ethernet type.

13. A method for navigation by means of a navigation device (1), said navigation device comprising an inertial measurement unit (100) and an electronic navigation calculation unit (200) connected to each other via a data link (300), said method comprising the following steps: - In the inertial measurement unit (100): measuring the acceleration variation and the gyroscopic measurement variation by means of an inertial sensor (110, 120), said inertial sensor providing a first signal comprising first data representative of the acceleration variation and second data representative of the gyroscopic measurement variation, performing a process comprising at least a first integration of the first data and the second data within a predetermined integration period, generating a second signal comprising the first processed data, the second processed data and time information; and - In said electronic navigation computing unit (200): extracting processed data and time information from the second signal, • Process it to calculate navigation, taking the integration period into account.

14. A vehicle comprising at least one device according to any one of claims 1 to 12.

Images